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  • richardmitnick 11:19 am on August 8, 2019 Permalink | Reply
    Tags: , , , , , , Higgs, , ,   

    From Johns Hopkins University via Science Alert: “Fascinating New Study Claims Dark Matter May Be Older Than The Big Bang” 

    Johns Hopkins
    From Johns Hopkins University



    Science Alert

    8 AUG 2019

    A simulated map of dark matter. (Tom Abel & Ralf Kaehler/KIPAC/SLAC/AMNH)

    Dark matter might well be the biggest mystery in the Universe. We know there’s something out there making things move faster than they should. But we don’t know what it is, and we sure as heck don’t know where it came from.

    According to a new paper [below], the origins of dark matter may be more peculiar than we know. Perhaps, they were particles that appeared in a very brief period of time, just fractions of fractions of a second, before the Big Bang.

    This doesn’t just suggest a new connection between particle physics and astronomy; if this hypothesis holds, it could indicate a new way to search for the mysterious stuff.

    “If dark matter consists of new particles that were born before the Big Bang, they affect the way galaxies are distributed in the sky in a unique way,” said astronomer and physicist Tommi Tenkanen of Johns Hopkins University.

    “This connection may be used to reveal their identity and make conclusions about the times before the Big Bang too.”

    It’s all tangled up with the order of events at the beginning of the Universe, which in itself is a pretty murky period of time.

    We think there was something called the Big Bang – although precisely what that entailed is still being debated. And we think there was something called cosmic inflation, a very brief period of time – a fraction of a second so small we don’t have a name for it – in which the Universe blew up like a balloon.


    Alan Guth, from Highland Park High School and M.I.T., who first proposed cosmic inflation

    HPHS Owls

    Lambda-Cold Dark Matter, Accelerated Expansion of the Universe, Big Bang-Inflation (timeline of the universe) Date 2010 Credit: Alex MittelmannColdcreation

    Alan Guth’s notes:

    Alan Guth’s original notes on inflation

    (Drbogdan/Yinweichen/Wikimedia Commons)

    It seems more generally accepted that this occurred between around 10^-36 and 10^-32 seconds after the Big Bang. That model of inflation looks like the image above.

    But some scientists think it happened just before the Big Bang, in which case the Big Bang is the name given to the conditions in the Universe right at the end of inflation.

    At this stage we just have no way of knowing. As Harvard-Smithsonian theoretical physicist Avi Loeb said earlier this year, “the current situation for inflation is that it’s such a flexible idea, it cannot be falsified experimentally.” He was talking about whether or not cosmic inflation actually happened (also a matter of debate), but the statement works for the timing of the whoompf, too.

    Dark matter – which, according to our calculations, makes up around 80 percent of the matter in the Universe – is sometimes considered to be a product of the Big Bang.

    But “if dark matter were truly a remnant of the Big Bang, then in many cases researchers should have seen a direct signal of dark matter in different particle physics experiments already,” Tenkanen states.

    Instead, his mathematical modelling suggests that dark matter could have been a product of cosmic inflation. It’s not the first time this idea has been proposed, but Tenkanen has provided the maths that support it.

    And, if cosmic inflation occurred before the Big Bang, dark matter could have been around before the rest of the stuff in the primordial Universe Soup.

    This suggests that scalar particles could lead us to dark matter. These are particles with a spin of zero, and the inflaton theory – whereby a scalar field drove cosmic inflation – suggests that they were produced in abundance during this eyeblink of time.

    So far, we’ve only ever detected one scalar particle, the Higgs boson.

    Peter Higgs

    CERN CMS Higgs Event

    CERN ATLAS Higgs Event

    But that wouldn’t be able to tell us much about dark matter in and of itself anyway.

    “While this type of dark matter is too elusive to be found in particle experiments, it can reveal its presence in astronomical observations,” Tenkanen said.

    “We will soon learn more about the origin of dark matter when the Euclid satellite is launched in 2022.

    ESA/Euclid spacecraft

    It’s going to be very exciting to see what it will reveal about dark matter and if its findings can be used to peak into the times before the Big Bang.”

    It’s all highly theoretical stuff, but it’s about as good a lead as any on the mysterious matter that’s playing a key role in shaping our Universe. It’ll be fascinating to see how the search for dark matter plays out in the coming decade.

    The research has been published in Physical Review Letters.

    See the full article here .


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    Johns Hopkins Campus

    The Johns Hopkins University opened in 1876, with the inauguration of its first president, Daniel Coit Gilman. “What are we aiming at?” Gilman asked in his installation address. “The encouragement of research … and the advancement of individual scholars, who by their excellence will advance the sciences they pursue, and the society where they dwell.”

    The mission laid out by Gilman remains the university’s mission today, summed up in a simple but powerful restatement of Gilman’s own words: “Knowledge for the world.”

    What Gilman created was a research university, dedicated to advancing both students’ knowledge and the state of human knowledge through research and scholarship. Gilman believed that teaching and research are interdependent, that success in one depends on success in the other. A modern university, he believed, must do both well. The realization of Gilman’s philosophy at Johns Hopkins, and at other institutions that later attracted Johns Hopkins-trained scholars, revolutionized higher education in America, leading to the research university system as it exists today.

  • richardmitnick 10:45 am on January 22, 2019 Permalink | Reply
    Tags: , CERN Compact Linear Collider, , China-Circular Electron Positron Collider, , , Higgs, International Linear Collider in northern Japan, , ,   

    From Science News: “Physicists aim to outdo the LHC with this wish list of particle colliders” 

    From Science News


    January 22, 2019
    Emily Conover

    CERN Future Circular Collider artist’s rendering

    If built, the accelerators could pump out oodles of Higgs bosons.

    If particle physicists get their way, new accelerators could one day scrutinize the most tantalizing subatomic particle in physics — the Higgs boson.

    CERN CMS Higgs Event

    CERN ATLAS Higgs Event

    Six years after the particle’s discovery at the Large Hadron Collider, scientists are planning enormous new machines that would stretch for tens of kilometers across Europe, Japan or China.

    The 2012 discovery of the subatomic particle, which reveals the origins of mass, put the finishing touch on the standard model, the overarching theory of particle physics (SN: 7/28/12, p. 5).

    The Standard Model of elementary particles (more schematic depiction), with the three generations of matter, gauge bosons in the fourth column, and the Higgs boson in the fifth.

    And it was a landmark achievement for the LHC, currently the world’s biggest accelerator.


    CERN map

    CERN LHC Tunnel

    CERN LHC particles

    Now, physicists want to delve further into the mysteries of the Higgs boson in the hope that it could be key to solving lingering puzzles of particle physics. “The Higgs is a very special particle,” says physicist Yifang Wang, director of the Institute of High Energy Physics in Beijing. “We believe the Higgs is the window to the future.”

    But the LHC — which consists of a ring 27 kilometers in circumference, inside which protons are accelerated to nearly the speed of light and smashed together a billion times a second — can take scientists only so far. That accelerator was great for discovering the Higgs, but not ideal for studying it in detail.

    So particle physicists are clamoring for a new particle collider, specifically designed to crank out oodles of Higgs bosons. Several blueprints for powerful new machines have been put forth, and researchers are hopeful these “Higgs factories” could help reveal solutions to glaring weak spots in the standard model.

    “The standard model is not a complete theory of the universe,” says experimental particle physicist Halina Abramowicz of Tel Aviv University. For example, the theory can’t explain dark matter, an unidentified substance whose mass is necessary to account for cosmic observations such as the motions of stars in galaxies. Nor can it explain why the universe is made up of matter, while antimatter is exceedingly rare.

    Carefully scrutinizing the Higgs boson might point scientists in the direction of solutions to those puzzles, proponents of the new colliders claim. But, among scientists, the desire for new, costly accelerators is not universal, especially since it’s unclear what exactly the machines might find.

    Next in line

    Closest to inception is the International Linear Collider in northern Japan. Unlike the LHC, in which particles zip around a ring, the ILC would accelerate two beams of particles along a straight line, directly at one another over its 20-kilometer length. And instead of crashing protons together, it would collide electrons and their antimatter partners, positrons.

    But, in an ominous sign, a multidisciplinary committee of the Science Council of Japan came down against the project in a December 2018 report, urging the government to be cautious with its support and questioning whether the expected scientific achievements justified the accelerator’s cost, currently estimated at around $5 billion.

    Supporters argue that the ILC’s plan to smash together electrons and positrons, rather than protons, has some big advantages. Electrons and positrons are elementary particles, meaning they have no smaller constituents, while protons are made up of smaller particles called quarks. That means that proton collisions are messier, with more useless particle debris to sift through.


    THIN LINE An accelerator planned for Japan, the International Linear Collider (design illustrated), would slam together electrons and positrons to better understand the Higgs boson.

    Additionally, in proton smashups, only a fraction of each proton’s energy actually goes into the collision, whereas in electron-positron colliders, particles bring the full brunt of the accelerator’s energy to bear. That means scientists can tune the energy of collisions to maximize the number of Higgs bosons produced. At the same time, the ILC would require only 250 billion electron volts to produce Higgs bosons, compared with the LHC’s 13 trillion electron volts.

    For the ILC, “the quality of the data coming out will be much higher, and there will be much more of it on the Higgs,” says particle physicist Lyn Evans of CERN in Geneva. One in every 100 ILC collisions would pump out a Higgs, whereas that happens only once in 10 billion collisions at the LHC.

    The Japanese government is expected to decide about the collider in March. If the ILC is approved, it should take about 12 years to build, Evans says. The accelerator could also be upgraded later to increase the energy it can reach.

    CERN has plans for a similar machine known as the Compact Linear Collider.

    Cern Compact Linear Collider

    It would also collide electrons and positrons, but at higher energies than the ILC. Its energy would start at 380 billion electron volts and increase to 3 trillion electron volts in a series of upgrades. But to reach those higher energies, new particle acceleration technology needs to be developed, meaning that CLIC is even further in the future than the ILC, says Evans, who leads a collaboration of researchers from both projects.

    Running in circles

    Two other planned colliders, in China and Europe, would be circular like the LHC, but would dwarf that already giant machine; both would be 100 kilometers around. That’s a circle big enough that the country of Liechtenstein could easily fit inside — twice.

    At a location yet to be determined in China, the Circular Electron Positron Collider, or CEPC, would collide electrons and positrons at 240 billion electron volts, according to a conceptual plan officially released in November and championed by Wang and the Institute of High Energy Physics.

    China Circular Electron-Positron collider depiction

    China Circular Electron Positron Collider (CEPC) map

    The accelerator could later be upgraded to collide protons at higher energies. Scientists say they could begin constructing the $5 billion to 6 billion machine by 2022 and have it ready to go by 2030.

    And at CERN, the proposed Future Circular Collider, or FCC, would likewise operate in stages, colliding electrons and positrons before moving on to protons. The ultimate goal would be to reach proton collisions with 100 trillion electron volts, more than seven times the LHC’s energy, according to a Jan. 15 report from an international group of researchers.

    FCC Future Circular Collider at CERN

    Meanwhile, scientists have shut down the LHC for two years, while they upgrade the machine to function at a slightly higher energy (SN Online: 12/3/18). Further down the line, a souped-up version known as the High-Luminosity LHC could come online in 2026 and would increase the proton collision rate by at least a factor of five (SN Online: 6/15/18).

    Portrait of the Higgs

    When the LHC was built, scientists were fairly confident they’d find the Higgs boson with it. But with the new facilities, there’s no promise of new particles. Instead, the machines will aim to catalog how strongly the Higgs interacts with other known particles; in physicist lingo, these are known as its “couplings.”

    Measurements of the Higgs’ couplings may simply confirm expectations of the standard model. But if the observations differ from expectations, the discrepancy could indirectly hint at the presence of something new, such as the particles that make up dark matter.

    Some scientists are hopeful that something unexpected might arise. That’s because the Higgs is an enigma: The particles condense into a kind of molasses-like fluid. “Why does this fluid do that? We have no clue,” says theoretical particle physicist Michael Peskin of Stanford University. That fluid pervades the universe, slowing particles down and giving them heft.

    Another puzzle is that the Higgs’ mass is a million billion times smaller than expected (SN Online: 10/22/13). Certain numbers in the standard model must be fine-tuned to extreme precision make the Higgs less hefty, a situation physicists find unnatural.

    The weirdness of the Higgs suggests other particles might be out there. Scientists previously thought they had an answer to the Higgs quandaries, via a theory called supersymmetry, which posits that each known particle has a heavier partner (SN: 10/1/16, p. 12). “Before the LHC started, there were huge expectations,” says Abramowicz: Some scientists claimed the LHC would quickly find supersymmetric particles. “Well, it didn’t happen,” she says.

    The upcoming colliders may yet find evidence of supersymmetry, or otherwise hint at new particles, but this time around, scientists aren’t making promises.

    BIG SMASH In the new accelerators, collisions would produce showers of exotic particles (illustrated), including the Higgs boson, which explains how particles get mass.

    “In the past, some people have clearly oversold what the LHC was expected to deliver,” says theoretical particle physicist Juan Rojo of Vrije University Amsterdam. When it comes to any new colliders, “we should avoid making the same mistake if we want to keep our field alive for decades to come,” he says.

    Researchers around the world are now hashing out priorities, making arguments for the new colliders and other particle physics experiments. European physicists, for example, will meet in May to discuss options, working toward a document called the European Particle Physics Strategy Update, to guide research there in 2020 and beyond.

    One thing is certain: The proposed accelerators would explore unknown territory, with unpredictable results. The unanswered questions surrounding the Higgs boson make it the most obvious place to look for hints of new physics, Peskin says. “It’s the place that we haven’t looked yet, so it’s really compelling.”


    CERN. Future Circular Collider Conceptual Design Report. Published online January 15, 2018.

    European Particle Physics. Strategy Update 2018–2020.

    Linear Collider Collaboration. Executive Summary of the Science Council of Japan’s Report. LC Newsline. Published online December 21, 2018.

    The Institute of High Energy Physics of the Chinese Academy of Sciences. CEPC Conceptual Design Report Volume I – Accelerator. November 14, 2018.

    The Institute of High Energy Physics of the Chinese Academy of Sciences. CEPC Conceptual Design Report Volume II – Physics & Detector. November 14, 2018.

    See the full article here .


    Please help promote STEM in your local schools.

    Stem Education Coalition

  • richardmitnick 10:22 am on December 21, 2018 Permalink | Reply
    Tags: , , , Higgs, ,   

    From Nature via ILC: “Plans for world’s next major particle collider dealt big blow” 

    From ILC.

    19 December 2018
    Elizabeth Gibney


    Plans to build a particle smasher in Japan to succeed the Large Hadron Collider have suffered a significant setback. An influential report by Japanese scientists concluded that they could not support plans to build the International Linear Collider (ILC) in the country.

    ILC schematic, being planned for the Kitakami highland, in the Iwate prefecture of northern Japan

    The facility has been decades in design and would study the Higgs boson, which was discovered in 2012 and is the last puzzle piece in particle physicists’ ‘standard model’.

    The discoveries predicted to come out of the ILC would not fully warrant its nearly US$7-billion cost, said a committee of the Science Council of Japan in a report released on 19 December, according to press reports. As host, Japan might be expected to pay as much as half of the total. The committee, which advises the government, added that uncertainty about whether international partners would share the project’s costs increased its concerns.

    The proposed accelerator — which would be more than 20 kilometres long — would enable physicists to detect the products of precise collisions between electrons and their antimatter counterparts, positrons.

    Government advice

    The government will now use the report, which reflects the views of the academic community in Japan and not just those of high-energy physicists, to guide its decision on whether to host the facility. A decision is expected by 7 March, when the international group overseeing the ILC’s development, the Linear Collider Board, meets in Tokyo.

    Physicists expressed concern at the committee’s conclusions. “This is very bad news, as this makes it very unlikely that the #ILC will be build in Japan — and probably at all,” tweeted Axel Maas, a theoretical physicist at the University of Graz in Austria.

    However, the committee did state that the scientific case for building the ILC was sound, says Hitoshi Yamamoto, a physicist at Tohoku University in Sendai and a member of the ILC collaboration. It also acknowledged that the collider is seen in the particle-physics community as the top priority among possible future projects, he adds.

    The project now needs some good news, says Yamamoto. With funding tight around the world, “the situation for the ILC is getting worse rapidly”, he says. “A positive announcement by the Japanese government will reverse the trend and suddenly bring the ILC as the top item on the table,” says Yamamoto.

    Any concern that other areas of science in Japan could suffer if the costly project goes ahead is understandable, says Brian Foster, a physicist at the University of Oxford, UK, and part of the team designing the facility. But he says the council’s pessimistic take does not necessarily mean the government will not support the project. “If the government wants to do it, it will,” he says.

    Sole nation

    Japan is the only nation so far to show interest in the collider, and a decision on whether it will host the facility is long overdue. Japanese physicists pitched to the international community to build the facility in Japan in 2012, after scientists at the LHC — based at CERN, Europe’s particle-physics lab near Geneva — discovered the Higgs boson, a particle involved in the mechanism by which all others get mass.

    Physicists wanted to use the new facility to study any phenomena that the LHC might discover. They know that the standard model is incomplete and hope that unknown higher-energy particles could help explain long-standing mysteries such as the nature of dark matter.

    But plans for the collider have stagnated because no nations have offered funding, and because of the LHC’s failure to find any new phenomena beyond the Higgs. In 2017 physicists scaled back their ambitions for the ILC, proposing a shorter, lower-energy design that would focus on the Higgs alone.

    To physicists, a ‘Higgs factory’ would still be hugely valuable. As electron and positrons are fundamental particles, their collisions would be cleaner than the proton–proton collisions at the LHC. By targeting collisions at the right energy, the planned collider would produce millions of Higgs bosons for studies that could reveal new physics indirectly, by exploring how the Higgs boson interacts with other known particles.

    Researchers in China, who recently proposed to build a 100-kilometre ring-shaped Higgs factory, will also examine the report carefully. They need funding from both Chinese and foreign governments to build the facility. Although particle physicists would like to see both experiments built, international partners are likely to fund only one Higgs factory. If the ILC receives the backing of the high energy physics community, that may shorten the odds on the Chinese collider being built, although the country could also go it alone.

    See the full article here .


    Please help promote STEM in your local schools.

    Stem Education Coalition

    The International Linear Collider (ILC) is a proposed linear particle accelerator.It is planned to have a collision energy of 500 GeV initially, with the possibility for a later upgrade to 1000 GeV (1 TeV). The host country for the accelerator has not yet been chosen and proposed locations are Japan, Europe (CERN) and the USA (Fermilab). Japan is considered the most likely candidate, as the Japanese government is willing to contribute half of the costs, according to a representative for the European Commission on Future Accelerators.Construction could begin in 2015 or 2016 and will not be completed before 2026.

  • richardmitnick 2:32 pm on September 25, 2018 Permalink | Reply
    Tags: , , , , , Higgs, , ,   

    From ALICE at CERN: “What the LHC upgrade brings to CERN” 

    CERN New Masthead

    From From ALICE at CERN

    25 September 2018
    Rashmi Raniwala
    Sudhir Raniwala

    Six years after discovery, Higgs boson validates a prediction. Soon, an upgrade to Large Hadron Collider will allow CERN scientists to produce more of these particles for testing Standard Model of physics.

    FNAL magnets such as this one, which is mounted on a test stand at Fermilab, for the High-Luminosity LHC Photo Reidar Hahn

    Six years after the Higgs boson was discovered at the CERN Large Hadron Collider (LHC), particle physicists announced last week that they have observed how the elusive particle decays.

    CERN CMS Higgs Event

    CERN ATLAS Higgs Event

    The finding, presented by ATLAS and CMS collaborations, observed the Higgs boson decaying to fundamental particles known as bottom quarks.

    In 2012, the Nobel-winning discovery of the Higgs boson validated the Standard Model of physics, which also predicts that about 60% of the time a Higgs boson will decay to a pair of bottom quarks.

    The Standard Model of elementary particles (more schematic depiction), with the three generations of matter, gauge bosons in the fourth column, and the Higgs boson in the fifth.

    Standard Model of Particle Physics from Symmetry Magazine

    According to CERN, “testing this prediction is crucial because the result will either lend support to the Standard Model — which is built upon the idea that the Higgs field endows quarks and other fundamental particles with mass — or rock its foundations and point to new physics”.

    The Higgs boson was detected by studying collisions of particles at different energies. But they last only for one zeptosecond, which is 0.000000000000000000001 seconds, so detecting and studying their properties requires an incredible amount of energy and advanced detectors. CERN announced earlier this year that it is getting a massive upgrade, which will be completed by 2026.

    Why study particles?

    Particle physics probes nature at extreme scales, to understand the fundamental constituents of matter. Just like grammar and vocabulary guide (and constrain) our communication, particles communicate with each other in accordance with certain rules which are embedded in what are known as the ‘four fundamental interactions’. The particles and three of these interactions are successfully described by a unified approach known as the Standard Model. The SM is a framework that required the existence of a particle called the Higgs boson, and one of the major aims of the LHC was to search for the Higgs boson.

    How are such tiny particles studied?

    Protons are collected in bunches, accelerated to nearly the speed of light and made to collide. Many particles emerge from such a collision, termed as an event. The emergent particles exhibit an apparently random pattern but follow underlying laws that govern part of their behaviour. Studying the patterns in the emission of these particles help us understand the properties and structure of particles.

    Initially, the LHC provided collisions at unprecedented energies allowing us to focus on studying new territories. But, it is now time to increase the discovery potential of the LHC by recording a larger number of events.

    No image credit or caption

    So, what will an upgrade mean?

    After discovering the Higgs boson, it is imperative to study the properties of the newly discovered particle and its effect on all other particles. This requires a large number of Higgs bosons. The SM has its shortcomings, and there are alternative models that fill these gaps. The validity of these and other models that provide an alternative to SM can be tested by experimenting to check their predictions. Some of these predictions, including signals for “dark matter”, “supersymmetric particles” and other deep mysteries of nature are very rare, and hence difficult to observe, further necessitating the need of a High Luminosity LHC (HL-LHC).

    Imagine trying to find a rare variety of diamond amongst a very large number of apparently similar looking pieces. The time taken to find the coveted diamond will depend on the number of pieces provided per unit time for inspection, and the time taken in inspection. To complete this task faster, we need to increase the number of pieces provided and inspect faster. In the process, some new pieces of diamond, hitherto unobserved and unknown, may be discovered, changing our perspective about rare varieties of diamonds.

    Once upgraded, the rate of collisions will increase and so will the probability of most rare events. In addition, discerning the properties of the Higgs boson will require their copious supply. After the upgrade, the total number of Higgs bosons produced in one year may be about 5 times the number produced currently; and in the same duration, the total data recorded may be more than 20 times.

    With the proposed luminosity (a measure of the number of protons crossing per unit area per unit time) of the HL-LHC, the experiments will be able to record about 25 times more data in the same period as for LHC running. The beam in the LHC has about 2,800 bunches, each of which contains about 115 billion protons. The HL- LHC will have about 170 billion protons in each bunch, contributing to an increase in luminosity by a factor of 1.5.

    How will it be upgraded?

    The protons are kept together in the bunch using strong magnetic fields of special kinds, formed using quadrupole magnets. Focusing the bunch into a smaller size requires stronger fields, and therefore greater currents, necessitating the use of superconducting cables. Newer technologies and new material (Niobium-tin) will be used to produce the required strong magnetic fields that are 1.5 times the present fields (8-12 tesla).

    The creation of long coils for such fields is being tested. New equipment will be installed over 1.2 km of the 27-km LHC ring close to the two major experiments (ATLAS and CMS), for focusing and squeezing the bunches just before they cross.

    CERN crab cavities that will be used in the HL-LHC

    FNAL Crab cavities for the HL-LHC

    Hundred-metre cables of superconducting material (superconducting links) with the capacity to carry up to 100,000 amperes will be used to connect the power converters to the accelerator. The LHC gets the protons from an accelerator chain, which will also need to be upgraded to meet the requirements of the high luminosity.

    Since the length of each bunch is a few cm, to increase the number of collisions a slight tilt is being produced in the bunches just before the collisions to increase the effective area of overlap. This is being done using ‘crab cavities’.

    The experimental particle physics community in India has actively participated in the experiments ALICE and CMS. The HL-LHC will require an upgrade of these too. Both the design and the fabrication of the new detectors, and the ensuing data analysis will have a significant contribution from the Indian scientists.

    See the full article here .

    Please help promote STEM in your local schools.

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    Meet CERN in a variety of places:

    Cern Courier

    CERN/ATLAS detector


    CERN/CMS Detector




    CERN map

    CERN LHC Grand Tunnel

    CERN LHC particles

    Quantum Diaries

  • richardmitnick 1:17 pm on September 5, 2018 Permalink | Reply
    Tags: , , , Higgs, , ,   

    From CERN ATLAS: “ATLAS searches for double Higgs production” 

    CERN ATLAS Higgs Event


    5th September 2018

    Upper limits at 95% confidence level on the cross-section of the non-resonant Higgs boson pair production as a function of κλ. The allowed range of κλ is derived from the interval where the theoretical prediction is found below the experimental upper limits on the cross-section. (Image: ATLAS Collaboration/CERN)

    The Brout-Englert-Higgs (BEH) mechanism is at the core of the Standard Model, the theory that describes the fundamental constituents of matter and their interactions. It introduces a new field, the Higgs field, through which the weak bosons (W+/- and Z) become massive while the photon remains massless. The excitation of this field is a physical particle, the Higgs boson, which was discovered by the ATLAS and CMS collaborations in 2012.

    The Standard Model of elementary particles (more schematic depiction), with the three generations of matter, gauge bosons in the fourth column, and the Higgs boson in the fifth.

    Standard Model of Particle Physics from Symmetry Magazine

    CERN CMS Higgs Event

    CERN ATLAS Higgs Event

    The BEH mechanism also predicts that the Higgs field can interact with itself; in other words, a single (virtual) Higgs boson can decay into two Higgs bosons. Observing and measuring this self-interaction, or “Higgs self-coupling”, would be the ultimate validation of the theory of mass generation, while any deviation from Standard Model predictions would open a window on new physics.

    Unfortunately, Higgs boson pairs are predicted to be very rare in proton–proton collisions, with a production rate roughly a thousand times smaller than the single Higgs boson. To make matters worse, not all di-Higgs boson production occurs through Higgs self-coupling. Vast amounts of data are therefore needed for this to be probed, making it a flagship analysis for the high-luminosity upgrade of the LHC (HL-LHC).

    It is nevertheless important to explore di-Higgs production also with smaller datasets as new physics beyond the Standard Model might enhance the production rate.

    The ATLAS collaboration has searched for Higgs boson pairs (HH) in the dataset collected in 2015 and 2016 using various decay channels. The most sensitive of these involve one Higgs boson decaying into a pair of b-quarks and one decaying into either another pair of b-quarks (HH→bbbb), two tau-leptons (HH→bbττ) or two photons (HH→bbγγ). These three searches were recently statistically combined and, as a result, the production rate of HH pairs could be excluded beyond 6.7 times the Standard Model prediction, at a 95% confidence level.

    New physics could be indicated by a Higgs self-coupling which differs from the Standard Model prediction by a factor κλ. This would affect the production rate and the kinematic distributions of the Higgs boson pairs and, as such, is an excellent probe for new physics. The recent statistical combination of the HH searches in ATLAS constrains the value of κλ to be between –5.0 and +12.1, at a 95% confidence level (see figure). It is the world’s most stringent constraint on the anomalous Higgs self-coupling to date.

    Higgs boson pairs are also a key signature of heavy new particle decays in several scenarios beyond the Standard Model. These might include an additional Higgs boson in models that extend the Higgs sector of the Standard Model, or the excitation of a graviton in models with extra spatial dimensions. The combined HH searches performed by ATLAS with the 2015-2016 dataset impose stringent constraints on the production rates of such resonances at the LHC.

    See the full article here .

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    CERN map

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    LHC at CERN

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  • richardmitnick 12:49 pm on August 8, 2018 Permalink | Reply
    Tags: , Higgs, , ,   

    From Ethan Siegel: “What Was It Like When The Higgs Gave Mass To The Universe?” 

    From Ethan Siegel
    Aug 8, 2018

    A candidate Higgs event in the ATLAS detector. Note how even with the clear signatures and transverse tracks, there is a shower of other particles; this is due to the fact that protons are composite particles. This is only the case because the Higgs gives mass to the fundamental constituents that compose these particles. (THE ATLAS COLLABORATION / CERN)

    CERN CMS Higgs Event

    CERN ATLAS Higgs Event

    One moment, every particle in the Universe was massless. Then, they weren’t anymore. Here’s how it happened.

    In the earliest stages of the hot Big Bang, the Universe was filled with all the particles, antiparticles, and quanta of radiation it had the energy to create. As the Universe expanded, it cooled: the stretching fabric of space also stretched the wavelengths of all the radiation within it to longer wavelengths, which equates to lower energies.

    If there are any particles (and antiparticles) that exist at higher energies that are yet to be discovered, they were likely created in the hot Big Bang, so long as there was enough energy (E) available to create a massive (m) particle via Einstein’s E = mc². It’s possible that a slew of puzzles about our Universe, including the origin of the matter-antimatter asymmetry and the creation of dark matter, are solved by new physics at these early times. But the massive particles we know today are foreign to us. At these early stages, they have no mass.

    The particles and antiparticles of the Standard Model are easy to create, even as the Universe cools and the fractions-of-a-second ticked by. The Universe might start of at energies as large as 10¹⁵ or 10¹⁶ GeV; even by time it’s dropped to 1000 (10³) GeV, no Standard Model particle is threatened. At the energies achievable by the LHC, we can create the full suite of particle-antiparticle pairs that are known to physics.

    But at this point, unlike today, they’re all massless. If they have no rest mass, they have no choice but to move at the speed of light. The reason particles are in this strange, bizarre state that’s so different from how they exist today? It’s because the fundamental symmetry that gives rise to the Higgs boson — the electroweak symmetry — has not yet broken in the Universe.

    The particles and antiparticles of the Standard Model have now all been directly detected, with the last holdout, the Higgs Boson, falling at the LHC earlier this decade. Today, only the gluons and photons are massless; everything else has a non-zero rest mass. (E. SIEGEL / BEYOND THE GALAXY)

    When we look at the Standard Model today, it’s arranged as follows:

    six quarks, each of which come in three colors, and their antiquark counterparts,
    three charged leptons (e, μ, τ) and three neutral ones (ν_e, ν_μ, ν_τ), and their antimatter counterparts,
    the eight massless gluons that mediate the strong force between the quarks,
    the three heavy, weak bosons (W+, W-, and Z_0) that mediate the weak nuclear force,
    and the photon (γ), the massless mediator of the electromagnetic force.

    But there’s a symmetry that’s broken at today’s low-energy scale: the electroweak symmetry. This symmetry was restored in the early days of the Universe. And when it’s restored versus when it’s broken, it fundamentally changes the Standard Model picture.

    The Standard Model of elementary particles (more schematic depiction), with the three generations of matter, gauge bosons in the fourth column, and the Higgs boson in the fifth.

    Standard Model of Particle Physics from Symmetry Magazine

    Instead of the weak and electromagnetic bosons (W+, W-, Z_0, γ), where the first three are very massive and the last is massless, we have four new bosons for the electroweak force (W_1, W_2, W_3, B), and all of them have no mass at all. The other particles are all the same, except for the fact that they, too, have no mass yet. This is what’s floating around in the early Universe, colliding, annihilating, and spontaneously being created, all in motion at the speed of light.

    As the Universe expands and cools, all of this continues. So long as the energy of your Universe is above a certain value, you can think about the Higgs field as floating atop the liquid in a soda (or wine) bottle. As the level of the liquid drops, the Higgs field remains atop the liquid, and everything stays massless. This is what we call a restored-symmetry state.

    When a wine bottle is either completely or partially filled, a drop of oil or a ping pong ball will float on the wine’s surface inside the bottle. At any location, the wine-level, and hence what’s floating atop it, will remain at the same level. This corresponds to a restored-symmetry state. (EVAN SWIGART FROM CHICAGO, USA)

    But below a certain liquid level, the bottom of the container starts to show itself. And the field can no longer remain in the center; more generally, it can’t take on simply any old value. It has to go to where the liquid level is, and that means down into the divot(s) at the bottom of the bottle. This is what we call a broken-symmetry state.

    When this symmetry breaks, the Higgs field settles into the bottom, lowest-energy, equilibrium state. But that energy state isn’t quite zero: it has a finite, non-zero value known as its vacuum expectation value. Whereas the restored-symmetry state yielded only massless particles, the broken symmetry state changes everything.

    When a wine bottle is completely empty, any ball or drop of oil inside will slide all the way down to the lowest-level ‘ring’ at the bottom. This corresponds to a broken symmetry state, since all values (i.e., locations) are no longer equivalent. (PATRICK HEUSSER, X8ING.COM)

    Once the symmetry breaks, the Higgs field has four mass-containing consequences: two are charged (one positive and one negative) and two are neutral. Then, the following things all happen at once:

    The W_1 and W_2 particles “eat” the charged, broken-symmetry consequences of the Higgs, becoming the W+ and W- particles.
    The W_3 and B particles mix together, with one combination eating the uncharged broken-symmetry consequence of the Higgs, becoming the Z_0, and with the other combination eating nothing, to remain the massless photon (γ).
    The last neutral broken-symmetry consequence of the Higgs gains mass, and becomes the Higgs boson.
    At last, the Higgs boson couples to all the other particles of the Standard Model, giving mass to the Universe.

    This is the origin of mass in the Universe.

    When the electroweak symmetry is broken, the W+ gets its mass by eating the positively charged Higgs, the W- by eating the negatively charged Higgs, and the Z_0 by eating the neutral Higgs. The other neutral Higgs becomes the Higgs boson, detected and discovered earlier this decade at the LHC. The photon, the other combination of the W3 and the B boson, remains massless. (FLIP TANEDO / QUANTUM DIARIES)

    This whole process is called spontaneous symmetry breaking. And for the quarks and leptons in the standard model, when this Higgs symmetry is broken, every particle gets a mass due to two things:

    The expectation value of the Higgs field, and
    A coupling constant.

    And this is kind of the problem. The expectation value of the Higgs field is the same for all of these particles, and not too difficult to determine. But that coupling constant? Not only is it different for every particle, but — in the standard model — it’s arbitrary.

    The Higgs boson, now with mass, couples to the quarks, leptons, and W-and-Z bosons of the Standard Model, which gives them mass. That it doesn’t couple to the photon and gluons means those particles remain massless. (TRITERTBUTOXY AT ENGLISH WIKIPEDIA)

    We know that the particles have mass; we know how they get mass; we’ve discovered the particles responsible for mass. But we still have no idea why the particles have the values of the masses they do. We have no idea why the coupling constants have the couplings that they do. The Higgs boson is real; the gauge bosons are real; the quarks and leptons are real. We can create, detect, and measure their properties exquisitely. Yet, when it comes to understanding why they have the values that they do, that’s a puzzle we cannot yet solve. We do not have the answer.

    The masses of the fundamental particles in the Universe, once the electroweak symmetry is broken, spans many orders of magnitude, withe the neutrinos being the lightest massive particles and the top quark being the heaviest. We do not understand why the coupling constants have the values they do, and hence, why the particles have the masses they do. (FIG. 15–04A FROM UNIVERSE-REVIEW.CA)

    Before the breaking of the electroweak symmetry, everything that is known to exist in the Universe today is massless, and moves at the speed of light. Once the Higgs symmetry breaks, it gives mass to the quarks and leptons of the Universe, the W and Z bosons, and the Higgs boson itself. Suddenly, with huge mass differences between light particles and heavy ones, the heavy ones spontaneously decay into the lighter ones on very short timescales, especially when the energy (E) of the Universe drops below the mass equivalent (m) needed to create these unstable particles via E = mc².

    A visual history of the expanding Universe includes the hot, dense state known as the Big Bang and the growth and formation of structure subsequently. Without the Higgs giving mass to the particles in the Universe at a very early, hot stage, none of this would have been possible. (NASA / CXC / M. WEISS)

    Without this critical gauge symmetry associated with electroweak symmetry breaking, existence wouldn’t be possible, as we do not have stable, bound states made purely of massless particles. But with fundamental masses to the quarks and charged leptons, the Universe can now do something it’s never done before. It can cool and create bound states like protons and neutrons. It can cool further and create atomic nuclei and, eventually, neutral atoms. And when enough time goes by, it can give rise to stars, galaxies, planets, and human beings. Without the Higgs to give mass to the Universe, none of this would be possible. The Higgs, despite the fact that it took 50 years to discover, has been making the Universe possible for 13.8 billion years.


    Please help promote STEM in your local schools.

    Stem Education Coalition

    See the full article here .

    “Starts With A Bang! is a blog/video blog about cosmology, physics, astronomy, and anything else I find interesting enough to write about. I am a firm believer that the highest good in life is learning, and the greatest evil is willful ignorance. The goal of everything on this site is to help inform you about our world, how we came to be here, and to understand how it all works. As I write these pages for you, I hope to not only explain to you what we know, think, and believe, but how we know it, and why we draw the conclusions we do. It is my hope that you find this interesting, informative, and accessible,” says Ethan

  • richardmitnick 1:35 pm on July 4, 2018 Permalink | Reply
    Tags: , , , , Higgs, , ,   

    From CERN: “We need to talk about the Higgs” 

    Cern New Bloc

    Cern New Particle Event

    CERN New Masthead

    From CERN

    4 Jul 2018
    Anais Schaeffer

    François Englert (left) and Peter Higgs at CERN on 4 July 2012, on the occasion of the announcement of the discovery of a Higgs boson (Image: Maximilien Brice/CERN)

    It is six years ago that the discovery of the Higgs boson was announced, to great fanfare in the world’s media, as a crowning success of CERN’s Large Hadron Collider (LHC).

    CERN/CMS Detector

    CERN CMS Higgs Event

    CERN/ATLAS detector

    CERN ATLAS Higgs Event

    The excitement of those days now seems a distant memory, replaced by a growing sense of disappointment at the lack of any major discovery thereafter.

    While there are valid reasons to feel less than delighted by the null results of searches for physics beyond the Standard Model (SM), this does not justify a mood of despondency. A particular concern is that, in today’s hyper-connected world, apparently harmless academic discussions risk evolving into a negative outlook for the field in broader society. For example, a recent news article in Nature led on the LHC’s “failure to detect new particles beyond the Higgs”, while The Economist reported that “Fundamental physics is frustrating physicists”. Equally worryingly, the situation in particle physics is sometimes negatively contrasted with that for gravitational waves: while the latter is, quite rightly, heralded as the start of a new era of exploration, the discovery of the Higgs is often described as the end of a long effort to complete the SM.

    The Standard Model of elementary particles (more schematic depiction), with the three generations of matter, gauge bosons in the fourth column, and the Higgs boson in the fifth.

    Let’s look at things more positively. The Higgs boson is a totally new type of fundamental particle that allows unprecedented tests of electroweak symmetry breaking. It thus provides us with a novel microscope with which to probe the universe at the smallest scales, in analogy with the prospects for new gravitational-wave telescopes that will study the largest scales. There is a clear need to measure its couplings to other particles – especially its coupling with itself – and to explore potential connections between the Higgs and hidden or dark sectors. These arguments alone provide ample motivation for the next generation of colliders including and beyond the high-luminosity LHC upgrade.

    So far the Higgs boson indeed looks SM-like, but some perspective is necessary. It took more than 40 years from the discovery of the neutrino to the realisation that it is not massless and therefore not SM-like; addressing this mystery is now a key component of the global particle-physics programme. Turning to my own main research area, the beauty quark – which reached its 40th birthday last year – is another example of a long-established particle that is now providing exciting hints of new phenomena (see Beauty quarks test lepton universality). One thrilling scenario, if these deviations from the SM are confirmed, is that the new physics landscape can be explored through both the b and Higgs microscopes. Let’s call it “multi-messenger particle physics”.

    How the results of our research are communicated to the public has never been more important. We must be honest about the lack of new physics that we all hoped would be found in early LHC data, yet to characterise this as a “failure” is absurd. If anything, the LHC has been more successful than expected, leaving its experiments struggling to keep up with the astonishing rates of delivered data. Particle physics is, after all, about exploring the unknown; the analysis of LHC data has led to thousands of publications and a wealth of new knowledge, and there is every possibility that there are big discoveries waiting to be made with further data and more innovative analyses. We also should not overlook the returns to society that the LHC has brought, from technology developments with associated spin-offs to the training of thousands of highly skilled young researchers.

    The level of expectation that has been heaped on the LHC seems unprecedented in the history of physics. Has any other facility been considered to have produced disappointing results because only one Nobel-prize winning discovery was made in its first few years of operation? Perhaps this reflects that the LHC is simply the right machine at the right time, but that time is not over: our new microscope is set to run for the next two decades and bring physics at the TeV scale into clear focus. The more we talk about that, the better our long-term chances of success.

    See the full article here.

    Please help promote STEM in your local schools.

    Stem Education Coalition

    Meet CERN in a variety of places:

    Quantum Diaries

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    CERN CMS New

    CERN LHCb New II

  • richardmitnick 1:02 pm on July 4, 2018 Permalink | Reply
    Tags: , , , , Higgs, , ,   

    From CERN ATLAS: “The Higgs boson: the hunt, the discovery, the study and some future perspectives” 

    CERN ATLAS Higgs Event


    Figure 1: A candidate Higgs to ZZ to 4-lepton event as seen in the ATLAS detector. The four reconstructed muons are visualised as red lines. The green and blue boxes show where the muons passed through the muon detectors. (Image: ATLAS Collaboration/CERN)

    The origins of the Higgs boson

    Many questions in particle physics are related to the existence of particle mass. The “Higgs mechanism,” which consists of the Higgs field and its corresponding Higgs boson, is said to give mass to elementary particles. By “mass” we mean the inertial mass, which resists when we try to accelerate an object, rather than the gravitational mass, which is sensitive to gravity. In Einstein’s celebrated formula E = mc2, the “m” is the inertial mass of the particle. In a sense, this mass is the essential quantity, which defines that at this place there is a particle rather than nothing.

    In the early 1960s, physicists had a powerful theory of electromagnetic interactions and a descriptive model of the weak nuclear interaction – the force that is at play in many radioactive decays and in the reactions which make the Sun shine. They had identified deep similarities between the structure of these two interactions, but a unified theory at the deeper level seemed to require that particles be massless even though real particles in nature have mass.

    In 1964, theorists proposed a solution to this puzzle. Independent efforts by Robert Brout and François Englert in Brussels, Peter Higgs at the University of Edinburgh, and others lead to a concrete model known as the Brout-Englert-Higgs (BEH) mechanism. The peculiarity of this mechanism is that it can give mass to elementary particles while retaining the nice structure of their original interactions. Importantly, this structure ensures that the theory remains predictive at very high energy. Particles that carry the weak interaction would acquire masses through their interaction with the Higgs field, as would all matter particles. The photon, which carries the electromagnetic interaction, would remain massless.

    In the history of the universe, particles interacted with the Higgs field just 10-12 seconds after the Big Bang. Before this phase transition, all particles were massless and travelled at the speed of light. After the universe expanded and cooled, particles interacted with the Higgs field and this interaction gave them mass. The BEH mechanism implies that the values of the elementary particle masses are linked to how strongly each particle couples to the Higgs field. These values are not predicted by current theories. However, once the mass of a particle is measured, its interaction with the Higgs boson can be determined.

    The BEH mechanism had several implications: first, that the weak interaction was mediated by heavy particles, namely the W and Z bosons, which were discovered at CERN in 1983. Second, the new field itself would materialize in another particle. The mass of this particle was unknown, but researchers knew it should be lower than 1 TeV – a value well beyond the then conceivable limits of accelerators. This particle was later called the Higgs boson and would become the most sought-after particle in all of particle physics.

    The accelerator, the experiments and the Higgs

    The Large Electron-Positron collider (LEP), which operated at CERN from 1989 to 2000, was the first accelerator to have significant reach into the potential mass range of the Higgs boson.

    CERN LEP Collider

    Though LEP did not find the Higgs boson, it made significant headway in the search, determining that the mass should be larger than 114 GeV.

    In 1984, a few physicists and engineers at CERN were exploring the possibility of installing a proton-proton accelerator with a very high collision energy of 10-20 TeV in the same tunnel as LEP. This accelerator would probe the full possible mass range for the Higgs, provided that the luminosity[1] was very high. However, this high luminosity would mean that each interesting collision would be accompanied by tens of background collisions. Given the state of detector technology of the time, this seemed a formidable challenge. CERN wisely launched a strong R&D programme, which enabled fast progress on the detectors. This seeded the early collaborations, which would later become ATLAS, CMS and the other LHC experiments.

    On the theory side, the 1990s saw much progress: physicists studied the production of the Higgs boson in proton-proton collisions and all its different decay modes. As each of these decay modes depends strongly on the unknown Higgs boson mass, future detectors would need to measure all possible kinds of particles to cover the wide mass range. Each decay mode was studied using intensive simulations and the important Higgs decay modes were amongst the benchmarks used to design the detector.

    Meanwhile, at the Fermi National Accelerator Laboratory (Fermilab) outside of Chicago, Illinois, the Tevatron collider was beginning to have some discovery potential for a Higgs boson with mass around 160 GeV. Tevatron, the scientific predecessor of the LHC, collided protons with antiprotons from 1986 to 2011.

    Tevatron Accelerator

    FNAL/Tevatron CDF detector

    FNAL/Tevatron DZero detector

    In 2008, after a long and intense period of construction, the LHC and its detectors were ready for the first beams. On 10 September 2008, the first injection of beams into the LHC was a big event at CERN, with the international press and authorities invited. The machine worked beautifully and we had very high hopes. Alas, ten days later, a problem in the superconducting magnets significantly damaged the LHC. A full year was necessary for repairs and to install a better protection system. The incident revealed a weakness in the magnets, which limited the collision energy to 7 TeV.

    When restarting, we faced a difficult decision: should we take another year to repair the weaknesses all around the ring, enabling operation at 13 TeV? Or should we immediately start and operate the LHC at 7 TeV, even though a factor of three fewer Higgs bosons would be produced? Detailed simulations showed that there was a chance of discovering the Higgs boson at the reduced energy, in particular in the range where the competition of the Tevatron was the most pressing, so we decided that starting immediately at 7 TeV was worth the chance.

    The LHC restarted in 2010 at 7 TeV with a modest luminosity – a luminosity that would increase in 2011. The ATLAS Collaboration had made good use of the forced stop of 2009 to better understand the detector and prepare the analyses. In 2010, Higgs experts from experiments and theory created the LHC Higgs Cross-Section[2] Working Group (LHCHXSWG), which proved invaluable as a forum to accompany the best calculations and to discuss the difficult aspects about Higgs production and decay. These results have since been regularly documented in the “LHCHXSWG Yellow Reports,” famous in the community.

    Figure 2: The invariant mass from pairs of photons selected in the Higgs to γγ analysis, as shown at the seminar at CERN on 4 July 2012. The excess of events over the background prediction around 125 GeV is consistent with predictions for the Standard Model Higgs boson. (Image: ATLAS Collaboration/CERN)

    The discovery of the Higgs boson

    As Higgs bosons are extremely rare, sophisticated analysis techniques are required to spot the signal events within the large backgrounds from other processes. After signal-like events have been identified, powerful statistical methods are used to quantify how significant the signal is. As statistical fluctuations in the background can also look like signals, stringent statistical requirements are made before a new signal is claimed to have been discovered. The significance is typically quoted as σ, or a number of standard deviations of the normal distribution. In particle physics, a significance of 3σ is referred to as evidence, while 5σ is referred to as an observation, corresponding to the probability of a statistical fluctuation from the background of less than 1 in a million.

    Eager physicists analysed the data as soon as it arrived. In the summer of 2011, there was a small excess in the Higgs decay to two W bosons for a mass around 140 GeV. Things got more interesting as an excess at a similar mass was also seen in the diphoton channel. However, as the dataset increased, the size of this excess first increased and then decreased.

    By the end of 2011, ATLAS had collected and analysed 5 fb-1 of data at a centre-of-mass energy of 7 TeV. After combining all the channels, it was found that the Standard Model Higgs boson could be excluded for all masses except for a small window around 125 GeV, where an excess with a significance of around 3σ was observed, largely driven by the diphoton and four lepton decay channels. The results were shown at a special seminar at CERN on 13 December 2011. Although neither experiment had strong enough results to claim observation, what was particularly telling was the fact that both ATLAS and CMS had excesses at the same mass.

    In 2012, the energy of the LHC was increased from 7 to 8 TeV, which increased the cross-sections for Higgs boson production. The data arrived quickly: by the summer of 2012, ATLAS had collected 5 fb-1 at 8 TeV, doubling the dataset. As quickly as the data arrived it was analysed and, sure enough, the significance of that small bump around 125 GeV increased further. Rumours were flying around CERN when a joint seminar between ATLAS and CMS was announced for 4 July 2012. Seats at the seminar were so highly sought after that only the people who queued all night were able to get into the room. The presence of François Englert and Peter Higgs at the seminar increased the excitement even further.

    At the famous seminar, the spokespeople of the ATLAS and CMS Collaborations showed their results consecutively, each finding an excess around 5σ at a mass of 125 GeV. To conclude the session, CERN Director-General Rolf Heuer declared, “I think we have it.”

    The ATLAS Collaboration celebrated the discovery with champagne and by giving each member of the collaboration a t-shirt with the famous plots. Incidentally, only once they were printed was it discovered that there was a typo in the plot. No matter, these t-shirts would go on to become collector’s items.

    ATLAS and CMS published the results in Physics Letters B a few weeks later in a paper titled “Observation of a New Particle in the Search for the Standard Model Higgs Boson with the ATLAS Detector at the LHC.” The Nobel Prize in Physics was later awarded to Peter Higgs and François Englert in 2013.

    What we have learned since discovery

    After discovery, we began to study the properties of the newly-discovered particle to understand if it was the Standard Model Higgs boson or something else. In fact, we initially called it a Higgs-like boson as we did not want to claim it was the Higgs boson until we were certain. The mass, the final unknown parameter in the Standard Model, was one of the first parameters measured and found to be approximately 125 GeV (roughly 130 times larger than the mass of the proton). It turned out that we were very lucky – with this mass, the largest number of decay modes are possible.

    Standard Model of Particle Physics from Symmetry Magazine

    In the Standard Model, the Higgs boson is unique: it has zero spin, no electric charge and no strong force interaction. The spin and parity were measured through angular correlations between the particles it decayed to. Sure enough, these properties were found to be as predicted. At this point, we began to call it “the Higgs boson.” Of course, it still remains to be seen if it is the only Higgs boson or one of many, such as those predicted by supersymmetry.

    The discovery of the Higgs boson relied on measurements of its decay to vector bosons. In the Standard Model, different couplings determine its interactions to fermions and bosons, so new physics might impact them differently.

    The Standard Model of elementary particles (more schematic depiction), with the three generations of matter, gauge bosons in the fourth column, and the Higgs boson in the fifth.

    Therefore, it is important to measure both. The first direct probe of fermionic couplings was to tau particles, which was observed in the combination of ATLAS and CMS results performed at the end of Run 1. During Run 2, the increase in the centre-of-mass energy to 13 TeV and the larger dataset allowed further channels to be probed. Over the past year, the evidence has been obtained for the Higgs decay to bottom quarks and the production of the Higgs boson together with top quarks has been observed. This means that the interaction of the Higgs boson to fermions has been clearly established.

    Perhaps one of the neatest ways to summarise what we currently know about the interaction of the Higgs boson with other Standard Model particles is to compare the interaction strength to the mass of each particle, as shown in Figure 4. This clearly shows that the interaction strength depends on the particle mass: the heavier the particle, the stronger its interaction with the Higgs field. This is one of the main predictions of the BEH mechanism in the Standard Model.

    We don’t only do tests to verify that the properties of the Higgs boson agree with those predicted by the Standard Model – we specifically look for properties that would provide evidence for new physics. For example, constraining the rate that the Higgs boson decays to invisible or unobserved particles provides stringent limits on the existence of new particles with masses below that of the Higgs boson. We also look for decays to combinations of particles forbidden in the Standard Model. So far, none of these searches have found anything unexpected, but that doesn’t mean that we’re going to stop looking anytime soon!


    2018 is the last year that ATLAS will take data as part of the LHC’s Run 2. During this run, 13 TeV proton-proton collisions have been producing approximately 30 times more Higgs bosons than those used in the 2012 Higgs boson discovery. As a result, more and more results have been obtained to study the Higgs boson in greater detail.

    Over the next few years, analysis of the large Run 2 dataset will not only be an opportunity to reach a new level of precision in previous measurements, but also to investigate new methods to probe Standard Model predictions and to test for the presence of new physics in as model-independent a way as possible. This new level of precision will rely on obtaining a deeper level of understanding of the performance of the detector, as well as the simulations and algorithms used to identify particles passing through it. It also poses new challenges for theorists to keep up with the improving experimental precision.

    In the longer term, another big step in performance will be brought by the High-Luminosity LHC (HL-LHC), planned to begin operation in 2024. The HL-LHC will increase the number of collisions by another factor of 10. Among other measurements, this will open the possibility to investigate a very peculiar property of the Higgs boson: that it couples to itself. Events produced via this coupling feature two Higgs bosons in the final state, but they are exceedingly rare. Thus, they can only be studied within a very large number of collisions and using sophisticated analysis techniques. To match the increased performance of the LHC, the ATLAS and CMS detectors will undergo comprehensive upgrades during the years before HL-LHC.

    Looking more generally, the discovery of the Higgs boson with a mass of 125 GeV sets a new foundation for particle physics to build on. Many questions remain in the field, most of which have some relation to the Higgs sector. For example:

    A popular theory beyond the Standard Model is “supersymmetry”, which presents attractive features for solving current issues, such as the nature of dark matter. The minimal version of supersymmetry predicts that the Higgs boson mass should be less than 120-130 GeV, depending on some other parameters. Is it a coincidence that the observed value sits exactly at this critical value, hence still marginally allowing for this supersymmetric model?
    Several models have been recently proposed where the only link of Dark Matter with regular matter would be through the Higgs boson.
    Stability of the universe: the value of 125 GeV is almost at the critical boundary between a stable universe and a meta-stable universe. A meta-stable system possesses another baseline state, into which it can decay anytime due to quantum tunnelling.[3] Is this also a coincidence?
    The phase transition: the details of this transition may play a role in the process which led our universe to be entirely matter and not contain any anti-matter. Present calculations with the Standard Model Higgs boson alone are inconsistent with the observed matter-antimatter asymmetry. Is this a call for new physics or only incomplete calculations?
    Are fermion masses all related to the Higgs boson field? If yes, why is there such a huge hierarchy between the fermion masses spanning from fractions of electron-volts for the mysterious neutrinos up to the very heavy top quark, with a mass on the order of hundreds of billions of electron-volts?

    From what we’ve learned about it so far, the Higgs boson seems to play a very special role in nature… Can it show us the way to answer further questions?

    See the full article here .

    Please help promote STEM in your local schools.

    Stem Education Coalition

    CERN LHC Map
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    LHC at CERN

    CERN Courier


    Quantum Diaries

  • richardmitnick 2:42 pm on April 10, 2018 Permalink | Reply
    Tags: , , , Higgs, Now the question is what if there is a whole sector of undiscovered particles that cannot communicate with our standard particles but can interact with the Higgs boson?, , , , , Theorists predict that about 90 percent of Higgs bosons are created through gluon fusion   

    From Symmetry: “How to make a Higgs boson” 

    Symmetry Mag

    Sarah Charley

    CERN CMS Higgs Event

    CERN ATLAS Higgs Event

    It doesn’t seem like collisions of particles with no mass should be able to produce the “mass-giving” boson, the Higgs. But every other second at the LHC, they do.

    Einstein’s most famous theory, often written as E=mc2, tells us that energy and matter are two sides of the same coin.

    The Large Hadron Collider uses this principle to convert the energy contained within ordinary particles into new particles that are difficult to find in nature—particles like the Higgs boson, which is so massive that it almost immediately decays into pairs of lighter, more stable particles.


    CERN/LHC Map

    CERN LHC Tunnel

    CERN LHC particles

    But not just any collision can create a Higgs boson.

    “The Higgs is not just created from a ‘poof’ of energy,” says Laura Dodd, a researcher at the University of Wisconsin, Madison. “Particles follow a strict set of laws that dictate how they can form, decay and interact.”

    One of these laws states that Higgs bosons can be produced only by particles that interact with the Higgs field—in other words, particles with mass.

    The Higgs field is like an invisible spider’s web that permeates all of space. As particles travel through it, some get tangled in the sticky tendrils, a process that makes them gain mass and slow down. But for other particles—such as photons and gluons—this web is completely transparent, and they glide through unhindered.

    Given enough energy, the particles wrapped in the Higgs field can transfer their energy into it and kick out a Higgs boson. Because massless particles do not interact with the Higgs field, it would make sense to say that they can’t create a Higgs. But scientists at the LHC would beg to differ.

    The LHC accelerates protons around its 17-mile circumference to just under the speed of light and then brings them into head-on collisions at four intersections along its ring. Protons are not fundamental particles, particles that cannot be broken down into any smaller constituent pieces. Rather they are made up of gluons and quarks.

    As two pepped-up protons pass through each other, it’s usually pairs of massless gluons that infuse invisible fields with their combined energy and excite other particles into existence—and that includes Higgs bosons.


    We know that particles follow strict rules about who can talk to whom.

    How? Gluons have found a way to cheat.

    “It would be impossible to generate Higgs bosons with gluons if the collisions in the LHC were a simple, one-step processes,” says Richard Ruiz, a theorist at Durham University’s Institute for Particle Physics Phenomenology.

    Luckily, they aren’t.

    Gluons can momentarily “launder” their energy to a virtual particle, which converts the gluon’s energy into mass. If two gluons produce a pair of virtual top quarks, the tops can recombine and annihilate into a Higgs boson.

    To be clear, virtual particles are not stable particles at all, but rather irregular disturbances in quantum mechanical fields that exist in a half-baked state for an incredibly short period of time. If a real particle were a thriving business, then a virtual particle would be a shell company.

    Theorists predict that about 90 percent of Higgs bosons are created through gluon fusion. The probability of two gluons colliding, creating a top quark-antitop pair and propitiously producing a Higgs is roughly one in 2 billion. However, because the LHC generates 10 million proton collisions every second, the odds are in scientists’ favor and the production rate for Higgs bosons is roughly one every two seconds.

    Shortly after the Higgs discovery, scientists were mostly focused on what happens to Higgs bosons after they decay, according to Dodd.

    “But now that we have more data and a better understanding of the Higgs, we’re starting to look closer at the collision byproducts to better understand how frequently the Higgs is produced through the different mechanisms,” she says.

    The Standard Model of particle physics predicts that almost all Higgs bosons are produced through one of four possible processes.

    The Standard Model of elementary particles (more schematic depiction), with the three generations of matter, gauge bosons in the fourth column, and the Higgs boson in the fifth.

    Standard Model of Particle Physics from Symmetry Magazine

    What scientists would love to see are Higgs bosons being created in a way that the Standard Model of particle physics does not predict, such as in the decay of a new particle. Breaking the known rules would show that there is more going on than physicists previously understood.

    “We know that particles follow strict rules about who can talk to whom because we’ve seen this time and time again during our experiments,” Ruiz says. “So now the question is, what if there is a whole sector of undiscovered particles that cannot communicate with our standard particles but can interact with the Higgs boson?”

    Scientists are keeping an eye out for anything unexpected, such as an excess of certain particles radiating from a collision or decay paths that occur more or less frequently than scientists predicted. These indicators could point to undiscovered heavy particles morphing into Higgs bosons.

    At the same time, to find hints of unexpected ingredients in the chain reactions that sometimes make Higgs bosons, scientists must know very precisely what they should expect.

    “We have fantastic mathematical models that predict all this, and we know what both sides of the equations are,” Ruiz says. “Now we need to experimentally test these predictions to see if everything adds up, and if not, figure out what those extra missing variables might be.”

    See the full article here .

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    Symmetry is a joint Fermilab/SLAC publication.

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