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  • richardmitnick 2:09 pm on February 14, 2017 Permalink | Reply
    Tags: , An exceptional result on a very rare decay of a particle called Bs0, , , Standard Model   

    From CERN: “The Standard Model stands its ground” 

    Cern New Bloc

    Cern New Particle Event

    CERN New Masthead


    14 Feb 2017
    Stefania Pandolfi

    Event display of a typical Bs0 decay into two muons. The two muon tracks from the Bs0 decay are seen as a pair of green tracks traversing the whole detector. (Image: LHCb collaboration)

    Today, in a seminar at CERN, the LHCb collaboration has presented an exceptional result on a very rare decay of a particle called Bs0. This observation marks yet another victory for the Standard Model (SM) of particle physics – the model that explains, to the best of our knowledge, the behaviour of all fundamental particles in the universe – over all its principal theoretical alternatives.


    The LHCb collaboration has reported the observation of the decay of the Bs0 meson – a heavy particle made of a bottom anti-quark and a strange quark – into a pair of muons. This decay is extremely rare, the rarest ever seen: according to the theoretical predictions, it should occur about 3 times in every billion total decays of that particle.

    Event display from the LHCb experiment shows examples of collisions that produced candidates for the rare decay of the Bs0 meson. Image credit: LHCb Collaboration.

    The decay of the Bs0 meson has been long regarded as a very promising place to look for cracks in the armour of the Standard Model, which, despite being our best available description of the subatomic world, leaves some questions unanswered. Therefore, over time, physicists came up with many alternatives or complementary theories. A large class of theories that extend the Standard Model into new physics, such as Supersymmetry, predicts significantly higher values for the Bs0 decay probability. Therefore, an observation of any significant deviation from the SM predicted value would suggest the presence of new, yet unknown, physics.

    The experimental value found by the LHCb collaboration for this probability is in an excellent agreement with the one predicted by the theory, and the result is confirmed to a very high level of reliability, at the level of 7.8 standard deviations: that is, the scientists are extremely sure that it hasn’t occurred just by chance. The LHCb collaboration obtained the first evidence of this phenomenon in November 2012, with a significance of 3.5 standard deviations. Three years later, together with the CMS collaboration, LHCb obtained the first confirmed observation in May 2015, with a significance of 6.2 standard deviations (for more information read the CERN Press release and the paper published on Nature ).

    This new finding limits the room for action of other models of physics beyond the SM: all candidate models will have to demonstrate their compatibility with this important result.

    Further reading on the LHCb website.

    See the full article here.

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    Meet CERN in a variety of places:

    Quantum Diaries

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    CERN LHC particles

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  • richardmitnick 8:28 am on September 24, 2016 Permalink | Reply
    Tags: , , , Leptons, Standard Model   

    From FNAL: “Lepton flavor violation: the search for mismatched Higgs boson decays at CMS” 

    FNAL II photo

    FNAL Art Image
    FNAL Art Image by Angela Gonzales

    Fermilab is an enduring source of strength for the US contribution to scientific research world wide.

    September 23, 2016
    Bo Jayatilaka

    The Standard Model allows for the Higgs boson to decay to identically flavored pairs of , such as electrons and muons, but not to mixed pairings of lepton flavors. Evidence of the latter would be a sign of new physics.

    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.
    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.

    For a moment, forget everything you know about twins and imagine you were told “the only way two siblings could have been born on the same day is if they were identical twins.” You would go about life assuming that the only twins in the world were siblings who were the same age and looked exactly alike. Of course, in reality, there are fraternal twins, and the first time you encountered a pair of nonidentical siblings born on the same day, you’d have to assume that your initial information was at least incomplete. Physicists are trying to test a principle of the Standard Model by looking for a particle version of fraternal twins, or lepton flavor violation.

    The fundamental particles known as fermions that make up ordinary matter all seem to come in multiple flavors or generations. For example, the electron has a heavier cousin called the muon. Apart from its mass, a muon behaves much the same way as an electron in terms of having similar properties and interacting with the same forces. One key exception is the flavor itself, a quantity unique to a given flavor of particle. For example, an electron has an “electron number” of +1 while its antiparticle, the positron, has a corresponding number of -1. Muons, on the other hand, have an electron number of 0 but have corresponding “muon numbers.” The Standard Model requires that certain types of interactions, say the decay of a Higgs boson, always conserve lepton flavor. This means that a Higgs boson can decay into an electron and a positron (which would sum to an electron flavor of zero) or a muon and an antimuon (again, a muon flavor sum of zero), but not to an electron and an antimuon, the latter being an example of lepton flavor violation. In short, the Standard Model requires that identical twins of particles emerge from Higgs boson decays and expressly forbids fraternal twins.

    Thus, observing decays of Higgs boson into fraternal twins of lepton pairs, say an electron and a muon, would be a strong sign of physics beyond the Standard Model. CMS physicists searched for evidence of such decays, specifically for Higgs boson decays to electron-muon and electron-tau lepton pairs. The search, performed in the dataset accumulated by CMS in 2012 and reported in a paper submitted to Physics Letters B, yielded no evidence of either type of decay. The results did place the tightest bounds yet on the possible rates of such decays and allowed physicists to place constraints on some models of physics beyond the Standard Model.

    See the full article here .

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    Fermilab Campus

    Fermi National Accelerator Laboratory (Fermilab), located just outside Batavia, Illinois, near Chicago, is a US Department of Energy national laboratory specializing in high-energy particle physics. Fermilab is America’s premier laboratory for particle physics and accelerator research, funded by the U.S. Department of Energy. Thousands of scientists from universities and laboratories around the world
    collaborate at Fermilab on experiments at the frontiers of discovery.

  • richardmitnick 11:36 am on August 7, 2016 Permalink | Reply
    Tags: , , , , , Standard Model   

    From physicsworld.com: “And so to bed for the 750 GeV bump” 


    Aug 5, 2016
    Tushna Commissariat

    No bumps: ATLAS diphoton data – the solid black line shows the 2015 and 2016 data combined. (Courtesy: ATLAS Experiment/CERN)

    Smooth dips: CMS diphoton data – blue lines show 2015 data, red are 2016 data and black are the combined result. (Courtesy: CMS collaboration/CERN)

    After months of rumours, speculation and some 500 papers posted to the arXiv in an attempt to explain it, the ATLAS and CMS collaborations have confirmed that the small excess of diphoton events, or “bump”, at 750 GeV detected in their preliminary data is a mere statistical fluctuation that has disappeared in the light of more data. Most folks in the particle-physics community will have been unsurprised if a bit disappointed by today’s announcement at the International Conference on High Energy Physics (ICHEP) 2016, currently taking place in Chicago.

    The story began around this time last year, soon after the LHC was rebooted and began its impressive 13 TeV run, when the ATLAS collaboration saw more events than expected around the 750 GeV mass window. This bump immediately caught the interest of physicists world over, simply because there was a sniff of “new physics” around it, meaning that the Standard Model of particle physics did not predict the existence of a particle at that energy. But also, it was the first interesting data to emerge from the LHC after its momentous discovery of the Higgs boson in 2012 and if it had held, would have been one of the most exciting discoveries in modern particle physics.

    According to ATLAS, “Last year’s result triggered lively discussions in the scientific communities about possible explanations in terms of new physics and the possible production of a new, beyond-Standard-Model particle decaying to two photons. However, with the modest statistical significance from 2015, only more data could give a conclusive answer.”

    And that is precisely what both ATLAS and CMS did, by analysing the 2016 dataset that is nearly four times larger than that of last year. Sadly, both years’ data taken together reveal that the excess is not large enough to be an actual particle. “The compatibility of the 2015 and 2016 datasets, assuming a signal with mass and width given by the largest 2015 excess, is on the level of 2.7 sigma. This suggests that the observation in the 2015 data was an upward statistical fluctuation.” The CMS statement is succinctly similar: “No significant excess is observed over the Standard Model predictions.”

    Tommaso Dorigo, blogger and CMS collaboration member, tells me that it is wisest to “never completely believe in a new physics signal until the data are confirmed over a long time” – preferably by multiple experiments. More interestingly, he tells me that the 750 Gev bump data seemed to be a “similar signal” to the early Higgs-to-gamma-gamma data the LHC physicists saw in 2011, when they were still chasing the particle. In much the same way, more data were obtained and the Higgs “bump” went on to be an official discovery. With the 750 GeV bump, the opposite is true. “Any new physics requires really really strong evidence to be believed because your belief in the Standard Model is so high and you have seen so many fluctuations go away,” says Dorigo.

    And this is precisely what Colombia University’s Peter Woit – who blogs at Not Even Wrong – told me in March this year when I asked him how he thought the bump would play out. Woit pointed out that particle physics has a long history of “bumps” that may look intriguing at first glance, but will most likely be nothing. “If I had to guess, this will disappear,” he said, adding that the real surprise for him was that “there aren’t more bumps” considering how good the LHC team is at analysing its data and teasing out any possibilities.

    It may be fair to wonder just why so many theorists decided to work with the unconfirmed data from last year and look for a possible explanation of what kind of particle it may have been and indeed, Dorigo says that “theorists should have known better”. But on the flip-side, the Standard Model predicted many a particle long before it was eventually discovered and so it is easy to see why many were keen to come up with the perfect new model.

    Despite the hype and the eventual letdown, Dorigo is glad that this bump has got folks talking about high-energy physics. “It doesn’t matter even if it fizzles out; it’s important to keep asking ourselves these questions,” he says. The main reason for this, Dorigo explains, is that “we are at a very special junction in particle physics as we decide what new machine to build” and some input from current colliders is necessary.”Right now there is no clear direction,” he says. In light of the fact that there has been no new physics (or any hint of supersymmetry) from the LHC to date, the most likely future devices would be an electron–positron collider or, in the long term, a muon collider. But a much clearer indication is necessary before these choices are made and for now, much more data are needed.

    See the full article here .

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    PhysicsWorld is a publication of the Institute of Physics. The Institute of Physics is a leading scientific society. We are a charitable organisation with a worldwide membership of more than 50,000, working together to advance physics education, research and application.

    We engage with policymakers and the general public to develop awareness and understanding of the value of physics and, through IOP Publishing, we are world leaders in professional scientific communications.
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  • richardmitnick 10:32 am on July 25, 2016 Permalink | Reply
    Tags: , , , , Possible fifth force?, Standard Model   

    From Don Lincoln of FNAL on livescience: “A Fifth Force: Fact or Fiction” 


    FNAL Icon

    FNAL Don Lincoln
    Don lincoln

    July 5, 2016

    Has a Hungarian lab really found evidence of a fifth force of nature? Credit: Jurik Peter / Shutterstock.com

    Science and the internet have an uneasy relationship: Science tends to move forward through a careful and tedious evaluation of data and theory, and the process can take years to complete. In contrast, the internet community generally has the attention span of Dory, the absent-minded fish of Finding Nemo(and now Finding Dory) — a meme here, a celebrity picture there — oh, look … a funny cat video.

    Thus people who are interested in serious science should be extremely cautious when they read an online story that purports to be a paradigm-shifting scientific discovery. A recent example is one suggesting that a new force of nature might have been discovered. If true, that would mean that we have to rewrite the textbooks.

    A fifth force

    So what has been claimed?

    In an article submitted on April 7, 2015, to the arXiv repository of physics papers, a group of Hungarian researchers reported on a study in which they focused an intense beam of protons (particles found in the center of atoms) on thin lithium targets. The collisions created excited nuclei of beryllium-8, which decayed into ordinary beryllium-8 and pairs of electron-positron particles. (The positron is the antimatter equivalent of the electron.)

    The Standard Model is the collection of theories that describe the smallest experimentally observed particles of matter and the interactions between energy and matter. Credit: Karl Tate, LiveScience Infographic Artist

    They claimed that their data could not be explained by known physical phenomena in the Standard Model, the reigning model governing particle physics. But, they purported, they could explain the data if a new particle existed with a mass of approximately 17 million electron volts, which is 32.7 times heavier than an electron and just shy of 2 percent the mass of a proton. The particles that emerge at this energy range, which is relatively low by modern standards, have been well studied. And so it would be very surprising if a new particle were discovered in this energy regime.

    However, the measurement survived peer review and was published on Jan. 26, 2016, in the journal Physical Review Letters, which is one of the most prestigious physics journals in the world. In this publication, the researchers, and this research, cleared an impressive hurdle.

    Their measurement received little attention until a group of theoretical physicists from the University of California, Irvine (UCI), turned their attention to it. As theorists commonly do with a controversial physics measurement, the team compared it with the body of work that has been assembled over the last century or so, to see if the new data are consistent or inconsistent with the existing body of knowledge. In this case, they looked at about a dozen published studies.

    What they found is that though the measurement didn’t conflict with any past studies, it seemed to be something never before observed — and something that couldn’t be explained by the Standard Model.

    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.
    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

    New theoretical framework

    To make sense of the Hungarian measurement, then, this group of UCI theorists invented a new theory.

    The theory invented by the Irvine group is really quite exotic. They start with the very reasonable premise that the possible new particle is something that is not described by existing theory. This makes sense because the possible new particle is very low mass and would have been discovered before if it were governed by known physics. If this were a new particle governed by new physics, perhaps a new force is involved. Since traditionally physicists speak of four known fundamental forces (gravity, electromagnetism and the strong and weak nuclear forces), this hypothetical new force has been dubbed “the fifth force.”

    Theories and discoveries of a fifth force have a checkered history, going back decades, with measurements and ideas arising and disappearing with new data. On the other hand, there are mysteries not explained by ordinary physics like, for example, dark matter. While dark matter has historically been modeled as a single form of a stable and massive particle that experiences gravity and none of the other known forces, there is no reason that dark matter couldn’t experience forces that ordinary matter doesn’t experience. After all, ordinary matter experiences forces that dark matter doesn’t, so the hypothesis isn’t so silly.

    There is no reason dark matter couldn’t experience forces that ordinary matter doesn’t experience. Here, in the galaxy cluster Abell 3827, dark matter was observed interacting with itself during a galaxy collision. Credit: ESO

    There are many ideas about forces that affect only dark matter and the term for this basic idea is called “complex dark matter.” One common idea is that there is a dark photon that interacts with a dark charge carried only by dark matter. This particle is a dark matter analog of the photon of ordinary matter that interacts with familiar electrical charge, with one exception: Some theories of complex dark matter imbue dark photons with mass, in stark contrast with ordinary photons.

    If dark photons exist, they can couple with ordinary matter (and ordinary photons) and decay into electron-positron pairs, which is what the Hungarian research group was investigating. Because dark photons don’t interact with ordinary electric charge, this coupling can only occur because of the vagaries of quantum mechanics. But if scientists started seeing an increase in electron-positron pairs, that might mean they were observing a dark photon.

    The Irvine group found a model that included a “protophobic” particle that was not ruled out by earlier measurements and would explain the Hungarian result. Particles that are “protophobic,” which literally means “fear of protons,” rarely or never interact with protons but can interact with neutrons (neutrophilic).

    The particle proposed by the Irvine group experiences a fifth and unknown force, which is in the range of 12 femtometers, or about 12 times bigger than a proton. The particle is protophobic and neutrophilic. The proposed particle has a mass of 17 million electron volts and can decay into electron-positron pairs. In addition to explaining the Hungarian measurement, such a particle would help explain some discrepancies seen by other experiments. This last consequence adds some weight to the idea.

    Paradigm-shifting force?

    So this is the status.

    What is likely to be true? Obviously, data is king. Other experiments will need to confirm or refute the measurement. Nothing else really matters. But that will take a year or so and having some idea before then might be nice. The best way to estimate the likelihood the finding is real is to look at the reputations of the various researchers involved. This is clearly a shoddy way to do science, but it will help shade your expectations.

    So let’s start with the Irvine group. Many of them (the senior ones, typically) are well- regarded and established members of the field, with substantive and solid papers in their past. The group includes a spectrum of ages, with both senior and junior members. In the interest of full disclosure, I know some of them personally and, indeed, two of them have read the theoretical portions of chapters of books I have written for the public to ensure that I didn’t say anything stupid. (By the way, they didn’t find any gaffes, but they certainly helped clarify certain points.) That certainly demonstrates my high regard for members of the Irvine group, but possibly taints my opinion. In my judgment, they almost certainly did a thorough and professional job of comparing their new model to existing data. They have found a small and unexplored region of possible theories that could exist.

    On the other hand, the theory is pretty speculative and highly improbable. This isn’t an indictment … all proposed theories could be labeled in this way. After all, the Standard Model, which governs particle physics, is nearly a half century old and has been thoroughly explored. In addition, ALL new theoretical ideas are speculative and improbable and almost all of them are wrong. This also isn’t an indictment. There are many ways to add possible modifications to existing theories to account for new phenomena. They can’t all be right. Sometimes none of the proposed ideas are right.

    However, we can conclude from the reputation of the group’s members that they have generated a new idea and have compared it to all relevant existing data. The fact that they released their model means that it survived their tests and thus it remains a credible, if improbable, possibility.

    What about the Hungarian group? I know none of them personally, but the article was published in Physical Review Letters — a chalk mark in the win column. However, the group has also published two previous papers in which comparable anomalies were observed, including a possible particle with a mass of 12 million electron volts and a second publication claiming the discovery of a particle with a mass of about 14 million electron volts. Both of these claims were subsequently falsified by other experiments.

    Further, the Hungarian group has never satisfactorily disclosed what error was made that resulted in these erroneous claims. Another possible red flag is that the group rarely publishes data that doesn’t claim anomalies. That is improbable. In my own research career, most publications were confirmation of existing theories. Anomalies that persist are very, very, rare.

    So what’s the bottom line? Should you be excited about this new possible discovery? Well…sure…possible discoveries are always exciting. The Standard Model has stood the test of time for half a century, but there are unexplained mysteries and the scientific community is always looking for the discovery that points us in the direction of a new and improved theory. But what are the odds that this measurement and theory will lead to the scientific world accepting a new force with a range of 12 fm and with a particle that shuns protons? My sense is that this a long shot. I am not so sanguine as to the chances of this outcome.

    Of course, this opinion is only that…an opinion, albeit an informed one. Other experiments will also be looking for dark photons because, even if the Hungarian measurement doesn’t stand up to scrutiny, there is still a real problem with dark matter. Many experiments looking for dark photons will explore the same parameter space (e.g. energy, mass and decay modes) in which the Hungarian researchers claim to have found an anomaly. We will soon (within a year) know if this anomaly is a discovery or just another bump in the data that temporarily excited the community, only to be discarded as better data is recorded. And, no matter the outcome, good and better science will be the eventual result.

    See the full article here .

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  • richardmitnick 5:11 pm on June 17, 2016 Permalink | Reply
    Tags: , , , , , Standard Model   

    From Don Lincoln at FNAL: “The triumphant Standard Model” 

    FNAL II photo

    FNAL Art Image
    FNAL Art Image by Angela Gonzales

    Fermilab is an enduring source of strength for the US contribution to scientific research world wide.

    June 17, 2016

    FNAL Don Lincoln
    Don Lincoln

    In high-end research, there are a couple of deeply compelling types of data analyses that scientists do. There are those that break the existing scientific understanding and rewrite the textbooks. Those are exciting. But there are also those in which a highly successful theory is tested in a regime never before explored. There can also be two types of outcome. If the theory fails to explain the data, we have a discovery of the type I mentioned first. But it is also possible that the theory explains the data perfectly well. If so, that means that you’ve proven that the existing theory is even more successful than was originally known. That’s a different kind of success. It means that predictions made in one realm taught scientists enough to understand far more.

    In the LHC, pairs of protons are collided together with the unprecedented energy of 13 trillion electronvolts of energy.

    CERN/LHC Map
    CERN LHC Grand Tunnel
    CERN LHC particles
    LHC at CERN

    Before 2015, when the data in this analysis was recorded, the highest energy ever studied by humanity was only 8 trillion electronvolts. So, already we know that the new data is 63 percent higher in terms of energy reach as compared to the old data. To get a visceral sense of what that means, imagine that your bank told you that they made a mistake and that for every dollar you thought you had in your account, you actually had $1.63. I’m guessing you’d start planning for an awesome vacation or perhaps an earlier retirement.

    When the protons collide, most commonly, a quark or gluon from each proton hits a quark or gluon from the other proton and knocks them out of the collision area into the detector. As the quarks and gluons leave the collision area, they convert into sprays of particles that travel in roughly the same direction. These are called jets. Physicists study the location and energy of the jets in the detector and compare them to the predicted distribution.

    CMS scientists studied the production patterns of jets at a collision energy of 13 trillion electronvolts and found that they agreed with the predictions of the Standard Model with the same level of precision seen at lower energy measurements.

    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.
    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

    This result comes with a small sadness because this means that new physics hasn’t been discovered. On the other hand, it is a resounding endorsement of the theory of quantum chromodynamics, or QCD, which is the portion of the Standard Model that deals explicitly with quark and gluon scattering. QCD, first worked out nearly half a century ago, continues its decades-long track record of success.

    Scientists are constantly exploring the universe, seeing what happens when existing theories are tested in new realms. In today’s analysis, scientists put the leading theory of quark scattering to the test, studying what happens when it is compared to data taken at energies over 60 percent higher than ever before achieved.

    See the full article here .

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    Fermilab Campus

    Fermi National Accelerator Laboratory (Fermilab), located just outside Batavia, Illinois, near Chicago, is a US Department of Energy national laboratory specializing in high-energy particle physics. Fermilab is America’s premier laboratory for particle physics and accelerator research, funded by the U.S. Department of Energy. Thousands of scientists from universities and laboratories around the world
    collaborate at Fermilab on experiments at the frontiers of discovery.

  • richardmitnick 4:47 pm on May 9, 2016 Permalink | Reply
    Tags: , , , Standard Model   

    From COSMOS: “Particle physics: a primer to the theory of (almost) everything” 

    Cosmos Magazine bloc


    9 May 2016
    Cathal O’Connell

    Are you a boson bozo? Do quarks leave you quizzical? Do gluons get you unstuck? Cathal O’Connell has a guide to the zoo of particles, known as the Standard Model of particle physics.

    Graphic of a transverse section through a detector showing one of the numerous particle collision events recorded during the search for the Higgs boson.Credit: ATLAS COLLABORATION/CERN

    CERN ATLAS Higgs Event

    Around the turn of the 4th century BC, the Greek philosopher Democritus caught the smell of baking and thought that little bits of bread must be floating through the air and into his nose. He called the little bits “atoms” (meaning “uncuttable”) and imagined them as tiny spherical balls.

    But atoms are not little solid spheres. They are made of even smaller bits, called particles.

    Scientists’ best description of those particles and the forces that govern their behaviour is called the Standard Model of particle physics, or just “The Standard Model”.

    The Standard Model categorises all of the particles of nature, in the same way that the periodic table categorises the elements. The theory is called the Standard Model because it is so successful it has become “standard”.

    And no, there is no Economy Model, nor a Deluxe one.

    There are, however, still a few kinks to be ironed out (as well as a couple of whopping omissions). That’s why it is sometimes called the “Theory of Almost Everything”.

    How did it all kick off?

    Back in the early 20th century, scientists thought there were only three fundamental particles in nature: protons and neutrons, which make up the nucleus of an atom, and electrons that whizz round it.

    But in the 1950s and 1960s physicists started smashing these particles together and some of them broke. It turned out the protons and neutrons had even smaller particles inside them.

    Many dozens of new particles were discovered – and for a while, nobody could explain them. Physicists called it the “particle zoo”.

    In the 1970s, physicists such as Murray Gell-Mann found an order amongst the chaos. The step they took was similar to the one Russian chemist Dmitri Mendeleev took to find an order to the chemical elements in his periodic table.

    The new ordering of the particles explained many of the properties of the newly discovered particles, as well as correctly predicting some new ones.

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

    Meet the family

    The particles of the Standard Model make up one big family. Your first introduction can be daunting, a bit like attending a gathering with a lot of distant cousins you’ve never heard of. No matter how strange these cousins are, it is important to remember that they are all related.
    The basics

    Gell-Mann and others placed the particles in two main categories: fermions and bosons.

    Fermions, such as the electron, make up the stuff we call matter. Bosons, such as the photon, transmit forces.

    Fermions are subdivided again into two kinds of particles, depending on the forces they feel. These are the quarks and the leptons (see below).
    Forces of nature

    Particles communicate with one another via four forces: electromagnetism, the strong force, the weak force and gravity.

    The Standard Model describes the first three (gravity does not feature in the Standard Model, as explained below).

    Different particles communicate through different forces, similar to the way people can communicate in different languages. For example, only the quarks speak “gluon”. While electrons can speak “photon” as well as “W boson” and “Z boson”.

    Electromagnetism is the force that holds electrons in an atom. It is communicated by photons.

    The strong force keeps the nuclei of atoms together. Without it, every atom in the universe would spontaneously explode. It is communicated by gluons.

    The weak force causes radioactive decay. It’s transmitted by W and Z bosons.

    The fundamental particles

    All matter is made of two types of particles known as quarks and leptons.

    Quarks: (the purple particles in the figure) come in six “flavours”, all with weird names. It’s useful to see them as coming in pairs to make three generations. These are “up” and “down” (first generation), “charmed” and “strange” (second generation) and “top” and “bottom” (third generation).

    Only the up and down quarks are important in day-to-day life because they make protons and neutrons.

    The others make only “exotic” matter, which is too unstable to form atoms. Physicists can create exotic matter in particle accelerators, but it usually only lasts a fraction of a second before decaying.

    Leptons: there are six leptons, the best known of which is the electron, a tiny fundamental particle with a negative charge.

    The muon (second generation) and tau (third generation) particles are like fatter versions of the electron. They also have negative electric charge, but they are too unstable to feature in ordinary matter.

    And each of these particles has a corresponding neutrino, with no charge.

    Neutrinos deserve a special mention because they are perhaps the least understood of all the particles in the Standard Model.

    They are fast but interact only through the weak force, which means they can easily zip straight through a planet. They are created in nuclear reactions, such as those powering the Sun’s core.

    Hadrons: the composite particles

    Now that we know the fundamental particles of nature, we can begin to stack them together in different ways to make bigger particles.

    The most important composite particles are the baryons, made of three quarks. Protons and neutrons are both kinds of baryon.

    The European Organisation for Nuclear Research’s (CERN) biggest particle collider smashes protons together. Because protons are a kind of hadron, it’s called the Large Hadron Collider, or LHC.

    CERN/LHC Map
    CERN LHC Grand Tunnel
    CERN LHC particles
    LHC at CERN

    Antimatter: double or nothing?

    As far as we know, all quarks and leptons have twin particles of antimatter. Antimatter is like matter except it has the opposite charge. For example, the electron has a counterpart that’s exactly the same mass, except with positive charge instead of negative. When a particle of matter meets its antimatter twin, they both annihilate in a burst of pure energy.

    Antimatter is incredibly rare in the Universe, although it does have some important roles in technology. Positron emission tomography (PET) scanners, for instance, use the annihilation of positrons to see inside the body.

    One of the great mysteries of physics is why the Universe is made almost entirely of matter. Many particle physicists are striving to answer it.

    Atoms: composites of composites

    The bread that Democritus sniffed is made of only the first generation of fundamental particles.

    Up and down quarks bind together through the strong force to make protons and neutrons, and the strong force also sticks them together to form the nucleus of an atom.

    Electrons orbit the nucleus in arrangements determined by quantum mechanics (see our primer Quantum physics for the terminally confused).
    The Higgs: the god particle

    You probably noticed the loner off to the right side of particle table – the Higgs boson. The Higgs is a special kind of particle that gives the other fundamental particles their mass.

    The idea is that there is a field existing everywhere in space. And when particles move through space, they tend to bump into this field, and this interaction slows them down (similar to how it’s more difficult to move through water than air). This interaction is what gives fundamental particles their mass.

    Some particles such as photons and gluons don’t interact with the Higgs field, so are massless.

    Just as photons communicate the electromagnetic force, the Higgs Boson communicates the Higgs Field.

    The Higgs Boson was a theoretical particle until 2013 when CERN announced it had been discovered at last, although scientists are still uncovering its properties.
    What’s missing?


    The biggest hole in the Standard Model is the lack of gravity. The fourth force of nature just does not fit into the current picture.

    Gravity is also incredibly weak compared to the other forces (the strong force is 100,000,000,000,000,000,000,000,000,000,000,000,000 times stronger than gravity, for example).

    Some physicists think gravity is also transmitted by a kind of particle, called a graviton, but so far there is no evidence that this particle exists.

    Neutrino mass

    The neutrino is so tiny compared to all the other particles that it really begs an explanation. It’s possible that the neutrino doesn’t get its mass from the Higgs in the same way other particles do.

    Dark matter: For observing the Universe, it looks like a huge portion of it is made of Dark Matter – a new kind of stuff that doesn’t interact with regular matter and so is probably missing from the Standard Model entirely.


    Some physicists are looking for extensions to the Standard Model to explain these mysteries. Supersymmetry is one extension where every particle has another twin with higher mass.

    Some of these particles would interact very weakly with ordinary stuff and so could be good candidates for Dark Matter.

    See the full article here .

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  • richardmitnick 10:57 am on August 5, 2015 Permalink | Reply
    Tags: , , Standard Model,   

    From Symmetry: “The mystery of particle generations” 


    August 05, 2015
    Matthew R. Francis

    Why are there three almost identical copies of each particle of matter?

    Artwork by Sandbox Studio, Chicago

    The Standard Model of particles and interactions is remarkably successful for a theory everyone knows is missing big pieces.

    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.

    It accounts for the everyday stuff we know like protons, neutrons, electrons and photons, and even exotic stuff like Higgs bosons and top quarks. But it isn’t complete; it doesn’t explain phenomena such as dark matter and dark energy.

    The Standard Model is successful because it is a useful guide to the particles of matter we see. One convenient pattern that has proven valuable is generations. Each particle of matter seems to come in three different versions, differentiated only by mass.

    Scientists wonder whether that pattern has a deeper explanation or if it’s just convenient for now, to be superseded by a deeper truth.
    The next generations

    The Standard Model is a menu listing all of the known fundamental particles: particles that cannot be broken down into constituent parts. It distinguishes between the fermions, which are particles of matter, and the bosons, which carry forces.

    The matter particles include six quarks and six leptons. The six quarks are called the up, down, charm, strange, top and bottom quark. Quarks typically don’t exist as single particles but lump together to form heavier particles such as protons and neutrons. Leptons include electrons and their cousins the muons and tau particles, along with the three types of neutrinos.

    All of these matter particles fall into three “generations.”

    “The three generations are literally copy-paste of the first generation,” says Carleton University physicist Heather Logan. The up, charm and top quarks have the same electric charge, along with the same weak and strong interactions—they primarily differ in the mass, which comes from the Higgs field. The same thing holds for the down, strange and bottom quarks, along with the electron, muon and tau leptons.

    “The fact that the three generations couple differently to the Higgs sector is maybe telling us something, but we don’t really know what yet,” Logan says. Most of the generations differ in mass by a lot. For example, the tau lepton is roughly 3600 times more massive than the electron, and the top quark is nearly 100,000 times heavier than the up quark. That difference manifests itself in stability: The heavier generations decay into the lighter generations, until they reach the lightest, which are (as far as we can tell) stable forever.

    The generations play a big role in experiments. The Higgs boson, for instance, is an unstable particle that decays into a variety of other particles, including tau leptons. “Since the tau is the heaviest, the Higgs [boson] prefers to change into taus more than electrons or muons,” says Clara Nellist, an experimental particle physicist at the Laboratoire de l’Accélérateur Linéaire (LAL) in Orsay, France. “The best way to study how the Higgs interacts with leptons is by looking at a Higgs changing into two taus.”

    That sort of observation is the heart of Standard Model physics: Crash two or more particles together, watch what new particles are born, look for patterns in the detritus, and—if we’re really lucky—see what doesn’t fit into the map we have.
    Roads outward

    While some stuff like dark matter obviously lies outside the charts, the Standard Model itself has a few problems. For example, neutrinos should be massless according to the Standard Model, but real-world experiments show they have very tiny masses. And unlike quarks and electrically charged leptons, the mass differences between the generations of neutrinos are very small, which is why we see them oscillating from one type to another.

    Without mass, neutrinos are exactly identical; with the mass, they’re different. And that generational difference is puzzling to theorist Richard Ruiz of the University of Pittsburgh. “There is a pattern here staring at us but we cannot quite figure out how to make sense of it.”

    Even if there is only the one Standard Model Higgs, we can learn a lot by how it interacts and decays. For instance, Nellist says, “by studying how often the Higgs boson changes into taus compared to other particles, we can test the validity of the Standard Model and see if there are hints of other generations.”

    It’s unlikely, since any fourth generation quark would need to be far more massive even than the top quark. But any anomaly in Higgs decay could tell us a lot.

    “Nobody knows why there are three generations,” Logan says. However, the structure of the Standard Model is a clue to what might be beyond, including the theory known as Supersymmetry: “If there are supersymmetric partners of the fermions, they should also fall into the three generations. How their masses are set might give us clues to understanding how the masses of the Standard Model fermions are set and why we have those patterns.”

    No matter how many there are, nobody knows why there are generations to begin with. “‘Generations’ is just a conventional organization of the Standard Model’s matter content,” Ruiz says. That organization might survive in a deeper theory (for instance, theories in which quarks are made up of smaller particles called “preons”, which are unlikely based on present data), but new ideas would have to explain why the quarks and leptons seem to fall into the patterns they do.

    Ultimately, even though the Standard Model is not the final description of the cosmos, it’s been a good guide so far. As we look for the edges of the map it provides, we get closer to a true and accurate chart of all the particles and their interactions.

    See the full article here.

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    Symmetry is a joint Fermilab/SLAC publication.

  • richardmitnick 7:24 am on July 16, 2015 Permalink | Reply
    Tags: , , Helmholtz Association, , Standard Model,   

    From Helmholtz via DESY: “What is supersymmetry?” 



    Kristine August

    Using huge particle accelerators, physicists are searching for supersymmetry.

    Supersymmetry standard model
    Standard Model of Supersymmetry

    Their existence could help us to understand the composition of dark matter. But is it possible for something to be more symmetrical than symmetrical? Wilfried Buchmüller from the Deutsches Elektronen-Synchrotron facility (DESY) explains:

    “We usually associate symmetry with spatial symmetry – in connection with an image or a form, for example. But in the standard model of physics, when we think about symmetries we are thinking about something else – the forces between particles. When, for example, the force between two matter particles remains the same after reversal of the electrical charges, we are referring to “a symmetry”.

    The various forces in the standard model possess a number of such symmetries. According to the standard model, it is valid that the smaller the gaps between the matter particles, the greater the similarity becomes between the mathematical formulas that describe the forces there. We would say here that the theory becomes more symmetrical.

    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.

    Expanding on this concept, the last remaining differences are likely to cancel each other out at some point. It is our goal to describe all forces – gravity as well – and all particles on the basis of one unified principle of symmetry – supersymmetry (“SUSY”).

    But the fundamental difference still exists between matter particles and the particles that transfer forces. Although there are different types of particles, the supersymmetry theory is nevertheless able to interconnect them mathematically. We suspect that every particle has an attendant partner, a hidden supersymmetrical partner, i.e. a “superpartner”; in other words, one half of all matter is completed by its mirror image. Such a superpartner, in supersymmetrical theories, comprises the cornerstone of dark matter. Whenever the different types of particles then appear together, all of the forces become more similar to one another due to the superpartners. It is our ambition that we can also finally prove the existence of “SUSY” in reality. Namely, by finding the superpartners. They would play a key role in helping us to understand the origins of our universe.”

    See the full article here.

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    DESY is one of the world’s leading accelerator centres. Researchers use the large-scale facilities at DESY to explore the microcosm in all its variety – from the interactions of tiny elementary particles and the behaviour of new types of nanomaterials to biomolecular processes that are essential to life. The accelerators and detectors that DESY develops and builds are unique research tools. The facilities generate the world’s most intense X-ray light, accelerate particles to record energies and open completely new windows onto the universe. 
That makes DESY not only a magnet for more than 3000 guest researchers from over 40 countries every year, but also a coveted partner for national and international cooperations. Committed young researchers find an exciting interdisciplinary setting at DESY. The research centre offers specialized training for a large number of professions. DESY cooperates with industry and business to promote new technologies that will benefit society and encourage innovations. This also benefits the metropolitan regions of the two DESY locations, Hamburg and Zeuthen near Berlin.

  • richardmitnick 2:00 pm on November 20, 2014 Permalink | Reply
    Tags: , , , , Standard Model   

    From phys.org: “Gravity may have saved the universe after the Big Bang, say researchers” 


    Nov 18, 2014
    No Writer Credit

    New research by a team of European physicists could explain why the universe did not collapse immediately after the Big Bang.

    Studies of the Higgs particle – discovered at CERN in 2012 and responsible for giving mass to all particles – have suggested that the production of Higgs particles during the accelerating expansion of the very early universe (inflation) should have led to instability and collapse.

    Time Line of the Universe. Credit: NASA/WMAP Science Team

    Scientists have been trying to find out why this didn’t happen, leading to theories that there must be some new physics that will help explain the origins of the universe that has not yet been discovered. Physicists from Imperial College London, and the Universities of Copenhagen and Helsinki, however, believe there is a simpler explanation.

    In a new study in Physical Review Letters, the team describe how the spacetime curvature – in effect, gravity – provided the stability needed for the universe to survive expansion in that early period. The team investigated the interaction between the Higgs particles and gravity, taking into account how it would vary with energy.

    They show that even a small interaction would have been enough to stabilise the universe against decay.

    “The Standard Model of particle physics, which scientists use to explain elementary particles and their interactions, has so far not provided an answer to why the universe did not collapse following the Big Bang,” explains Professor Arttu Rajantie, from the Department of Physics at Imperial College London.

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

    “Our research investigates the last unknown parameter in the Standard Model – the interaction between the Higgs particle and gravity. This parameter cannot be measured in particle accelerator experiments, but it has a big effect on the Higgs instability during inflation. Even a relatively small value is enough to explain the survival of the universe without any new physics!”

    The team plan to continue their research using cosmological observations to look at this interaction in more detail and explain what effect it would have had on the development of the early universe. In particular, they will use data from current and future European Space Agency missions measuring cosmic microwave background radiation and gravitational waves.

    “Our aim is to measure the interaction between gravity and the Higgs field using cosmological data,” says Professor Rajantie. “If we are able to do that, we will have supplied the last unknown number in the Standard Model of particle physics and be closer to answering fundamental questions about how we are all here.”

    See the full article here.

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    About Phys.org in 100 Words

    Phys.org™ (formerly Physorg.com) is a leading web-based science, research and technology news service which covers a full range of topics. These include physics, earth science, medicine, nanotechnology, electronics, space, biology, chemistry, computer sciences, engineering, mathematics and other sciences and technologies. Launched in 2004, Phys.org’s readership has grown steadily to include 1.75 million scientists, researchers, and engineers every month. Phys.org publishes approximately 100 quality articles every day, offering some of the most comprehensive coverage of sci-tech developments world-wide. Quancast 2009 includes Phys.org in its list of the Global Top 2,000 Websites. Phys.org community members enjoy access to many personalized features such as social networking, a personal home page set-up, RSS/XML feeds, article comments and ranking, the ability to save favorite articles, a daily newsletter, and other options.

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  • richardmitnick 3:17 pm on November 11, 2014 Permalink | Reply
    Tags: , , , , Standard Model   

    From Symmetry: “The November Revolution” 


    November 11, 2014
    Amanda Solliday

    Forty years ago today, two different research groups announced the discovery of the same new particle and redefined how physicists view the universe.

    On November 11, 1974, members of the Cornell high-energy physics group could have spent the lulls during their lunch meeting chatting about the aftermath of Nixon’s resignation or the upcoming Big Red hockey season.

    But on that particular Monday, the most sensational topic was physics-related. One of the researchers in the audience stood up to report that two labs on opposite sides of the country were about to announce the same thing: the discovery of a new particle that heralded the birth of the Standard Model of particle physics.

    Ting and Richter

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

    “Nobody at the meeting knew what the hell it was,” says physicist Kenneth Lane of Boston University, a former postdoctoral researcher at Cornell. Lane, among others, would spend the next few years describing the theory and consequences of this new particle.

    It isn’t often that a discovery comes along that forces everyone to reevaluate the way the world works. It’s even rarer for two groups to make such a discovery at the same time, using different methods.

    One announcement would come from a research group led by MIT physicist Sam Ting at Brookhaven National Laboratory in New York. The other was to come from a team headed by physicist Burton Richter at SLAC National Accelerator Laboratory, then called the Stanford Linear Accelerator Center, in California. Word traveled fast.

    “We started getting all sorts of inquiries and congratulations before we even finished writing the paper,” Richter says. “Somebody told a friend, and then a friend told another friend.”

    Ting called the new particle the J particle. Richter called it psi. It became known as J/psi, the discovery that sparked the November Revolution.

    Independently, the researchers at Brookhaven and SLAC had designed two complementary experiments.

    Ting and his team had made the discovery using a proton machine, shooting an intense beam of particles at a fixed target. Ting was interested in how photons, particles of light, turn into heavy photons, particles with mass, and he wanted to know how many of these types of heavy photons existed in nature. So his team—consisting of 13 scientists from MIT with help from researchers at Brookhaven—designed and built a detector that would accept a wide range of heavy photon masses.

    “The experiment was quite difficult,” Ting says. “I guess when you’re younger, you’re more courageous.”

    In early summer 1974, they started the experiment at a high mass, around 4 to 5 billion electronvolts. They saw nothing. Later, they lowered the mass and soon saw a peak near 3 billion electronvolts that indicated a high production rate of a previously unknown particle.

    At SLAC, Richter had created a new type of collider, the Stanford Positron Electron Asymmetric Rings (SPEAR). His research group used a beam of electrons produced by a linear accelerator and stored the particles in a ring of magnets. Then, they would generate positrons in a linear accelerator and inject them in the other direction. The detector was able to look at everything produced in electron-positron collisions.

    The goal was to determine the masses of known elementary particles, but the researchers saw strange effects in the summer of 1974. They looked at that particular region with finer resolution, and over the weekend of November 9-10, discovered a tall, thin energy peak around 3 billion electronvolts.

    At the time, Ting visited SLAC as part of an advisory committee. The laboratory’s director, Pief Panofsky, asked Richter to meet with him.

    “He called and said, ‘It sounds like you guys have found the same thing,’” Richter says.

    Both researchers sent their findings to the journal Physical Review Letters. Their papers were published in the same issue. Other labs quickly replicated and confirmed the results.

    At the time, the basic pieces of today’s Standard Model of particle physics were still falling into place. Just a decade before, it had resembled the periodic table of the elements, including a wide, unruly collection of different types of particles called hadrons.

    Theorists Murray Gell-Mann and George Zweig were the first to propose that all of those different types of hadrons were actually made up of the same building blocks, called quarks. This model included three types of quark: up, down and strange. Other theorists—Sheldon Lee Glashow, James Bjorken, and then also John Iliopoulos and Luciano Maiani—proposed the existence of a fourth quark.

    On the day of the J/psi announcement, the Cornell researchers talked about the findings well into the afternoon. One of the professors in the department, Ken Wilson, made a connection between the discovery and a seminar given earlier that fall by Tom Appelquist, a physicist at Harvard University. Appelquist had been working with his colleague David Politzer to describe something they called “charmonium,” a bound state of a new type of quark and antiquark.

    “Only a few of us were thinking about the idea of a fourth quark,” says Appelquist, now a professor at Yale. “Ken called me right after the discovery and urged me to get our paper out ASAP.”

    The J/psi news inspired many other theorists to pick up their chalk as well.

    “It was clear from day one that J/psi was a major discovery,” Appelquist says. “It almost completely reoriented the theoretical community. Everyone wanted to think about it.”

    Less than two weeks after the initial discovery, Richter’s group also found psi-prime, a relative of J/psi that showed even more cracks in the three-quark model.

    “There was a whole collection of possibilities of what could exist outside the current model, and people were speculating about what that may be,” Richter says. “Our experiment pruned the weeds.”

    The findings of the J/psi teams triggered additional searches for unknown elementary particles, exploration that would reveal the final shape of the Standard Model. In 1976, the two experiment leaders were awarded the Nobel Prize for their achievement.

    In 1977, scientists at Fermilab discovered the fifth quark, the bottom quark. In 1995, they discovered the sixth one, the top.

    Today, theorists and experimentalists are still driven to answer questions not explained by the current prevailing model. Does supersymmetry exist? What are dark matter and dark energy? What particles have we yet to discover?

    Supersymmetry standard model
    Standard Model of Supersymmetry

    “If the answers are found, it will take us even deeper into what we are supposed to be doing as high-energy physicists,” Lane says. “But it probably isn’t going to be this lightning flash that happens on one Monday afternoon.”

    Ting and Richter
    Courtesy of: SLAC National Accelerator Laboratory

    See the full article here.

    Symmetry is a joint Fermilab/SLAC publication.

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