Tagged: CERN LHCb Toggle Comment Threads | Keyboard Shortcuts

  • richardmitnick 12:14 pm on August 8, 2018 Permalink | Reply
    Tags: , , CERN LHCb, , New measurement of particle’s lifetime intrigues physicists, , ,   

    From CERN: “New measurement of particle’s lifetime intrigues physicists” 

    Cern New Bloc

    Cern New Particle Event

    CERN New Masthead

    From CERN

    8 Aug 2018
    Ana Lopes

    1
    A proton–proton collision event detected by LHCb earlier this year.

    A new measurement of one of a particle’s properties can sometimes throw up a value that, intriguingly, is very different from previous values. In a paper posted online and submitted to the journal Physical Review Letters, the LHCb collaboration at CERN reports precisely that for the lifetime of the so-called charmed omega. Using data from proton–proton collisions, the LHCb researchers have obtained a value for the particle’s lifetime that is nearly four times larger than previous measurements. New studies are already being planned to unravel this intriguing discrepancy, at LHCb and other experiments.

    The charmed omega belongs to a family of particles known as baryons. These particles, of which protons and neutrons are examples, comprise three smaller particles called quarks. But unlike protons, which contain three light quarks and are stable, the charmed omega contains two relatively light quarks and a heavier charm quark (the third heaviest of the six known types of quark), and eventually decays into other particles. Measurements of the lifetimes of charmed particles, and more generally of particles containing heavy quarks, are important because they test models of quantum chromodynamics – the theory that describes how quarks are stuck together by gluons.

    The lifetime of the charmed omega was measured more than a decade ago by the E687 and FOCUS collaborations at Fermilab in the US and by the WA89 collaboration at CERN. These collaborations measured the lifetime of the charmed omega by examining some dozens of charmed-omega decays in experiments in which a beam of particles strikes the nuclei in a fixed target. The average of the values measured by these experiments, which are all relatively close to one another, is 69 ± 12 femtoseconds (one femtosecond is a millionth of a billionth of a second).

    The new LHCb measurement is based on proton–proton collision data comprising about 1000 charmed-omega decays. The LHCb researchers determined the particle’s lifetime by comparing these decays with those of another particle whose lifetime is known very precisely; a similar approach was recently used by the team to determine the lifetime of a “doubly charmed” particle. The charmed-omega result – a lifetime of 268 ± 26 femtoseconds – is much larger than the average of the older values.

    However, none of these measurements contradicts the theoretical estimates of the charmed omega’s lifetime, which rely on subtle calculations based on quantum chromodynamics and include predictions ranging from 60 to 520 femtoseconds. The jury is therefore out on whether the older values or the new one will stand, but the discrepancy between the values will no doubt prompt researchers to make new measurements and revise the theoretical estimates.

    See the full article here.


    five-ways-keep-your-child-safe-school-shootings
    Please help promote STEM in your local schools.

    Stem Education Coalition

    Meet CERN in a variety of places:

    Quantum Diaries
    QuantumDiaries

    Cern Courier

    THE FOUR MAJOR PROJECT COLLABORATIONS

    ATLAS
    CERN ATLAS New

    ALICE
    CERN ALICE New

    CMS
    CERN CMS New

    LHCb
    CERN LHCb New II

    LHC

    CERN map

    CERN LHC Grand Tunnel

    CERN LHC particles

    OTHER PROJECTS AT CERN

    CERN AEGIS

    CERN ALPHA

    CERN ALPHA

    CERN AMS

    CERN ACACUSA

    CERN ASACUSA

    CERN ATRAP

    CERN ATRAP

    CERN AWAKE

    CERN AWAKE

    CERN CAST

    CERN CAST Axion Solar Telescope

    CERN CLOUD

    CERN CLOUD

    CERN COMPASS

    CERN COMPASS

    CERN DIRAC

    CERN DIRAC

    CERN ISOLDE

    CERN ISOLDE

    CERN LHCf

    CERN LHCf

    CERN NA62

    CERN NA62

    CERN NTOF

    CERN TOTEM

    CERN UA9

    Advertisements
     
  • richardmitnick 2:20 pm on March 1, 2018 Permalink | Reply
    Tags: , CERN LHCb, , How are hadrons born at the huge energies available in the LHC?, , , ,   

    From phys.org: “How are hadrons born at the huge energies available in the LHC?” 

    physdotorg
    phys.org

    March 1, 2018
    The Henryk Niewodniczanski Institute of Nuclear Physics Polish Academy of Sciences

    1
    Particles produced during one of the collisions of two protons, each with energies of 7 TeV, registered by the detectors of the LHCb experiment in 2011; view from two different sides. Credit: CERN/LHCb

    CERN LHCb chamber, LHC


    CERN/LHCb detector

    Our world consists mainly of particles built up of three quarks bound by gluons. The process of the sticking together of quarks, called hadronisation, is still poorly understood. Physicists from the Institute of Nuclear Physics Polish Academy of Sciences in Cracow, working within the LHCb Collaboration, have obtained new information about it, thanks to the analysis of unique data collected in high-energy collisions of protons in the LHC.

    When protons accelerated to the greatest energy collide with each other in the LHC, their component particles – quarks and gluons – create a puzzling intermediate state. The observation that in the collisions of such relatively simple particles as protons this intermediate state exhibits the properties of a liquid, typical for collisions of much more complex structures (heavy ions), was a big surprise. Properties of this type indicate the existence of a new state of matter: a quark-gluon plasma in which quarks and gluons behave almost as free particles. This exotic liquid cools instantly. As a result, the quarks and gluons re-connect with each other in a process called hadronisation. The effect of this is the birth of hadrons, particles that are clumps of two or three quarks. Thanks to the latest analysis of data collected at energies of seven teraelectronvolts, researchers from the Institute of Nuclear Physics Polish Academy of Sciences (IFJ PAN) in Cracow, working within the LHCb Collaboration, acquired new information on the mechanism of hadronisation in proton-proton collisions.

    “The main role in proton collisions is played by strong interaction, described by the quantum chromodynamics. The phenomena occurring during the cooling of the quark-gluon plasma are, however, so complex in terms of computation that until now it has not been possible fully understand the details of hadronisation. And yet it is a process of key significance! It is thanks to this that in the first moments after the Big Bang, the dominant majority of particles forming our everyday environment was formed from quarks and gluons,” says Assoc. Prof. Marcin Kucharczyk (IFJ PAN).

    In the LHC, hadronisation is extremely fast, and occurs in an extremely small area around the point of proton collision: its dimensions reach only femtometres, or millionths of one billionth of a metre. It is no wonder then, that direct observation of this process is currently not possible. To obtain any information about its course, physicists must reach for various indirect methods. A key role is played by the basic tool of quantum mechanics: a wave function whose properties are mapped by the characteristics of particles of a given type (it is worth noting that although it is almost 100 years since the birth of quantum mechanics, there still exists various interpretations of the wave function!).

    “The wave functions of identical particles will effectively overlap, i.e. interfere. If they are enhanced as a result of interference, we are talking about Bose-Einstein correlations, if they are suppressed – Fermi-Dirac correlations. In our analyses, we were interested in the enhancements, that is, the Bose-Einstein correlations. We were looking for them between the pi mesons flying out of the area of hadronisation in directions close to the original direction of the colliding beams of protons,” explains Ph.D. student Bartosz Malecki (IFJ PAN).

    The method used was originally developed for radioastronomy and is called HBT interferometry (from the names of its two creators: Robert Hanbury Brown and Richard Twiss). When used with reference to particles, HBT interferometry makes it possible to determine the size of the area of hadronisation and its evolution over time. It helps to provide information about, for example, whether this area is different for different numbers of emitted particles or for their different types.

    The data from the LHCb detector made it possible to study the hadronisation process in the area of so-called small angles, i.e. for hadrons produced in directions close to the direction of the initial proton beams. The analysis performed by the group from the IFJ PAN provided indications that the parameters describing the source of hadronisation in this unique region covered by LHCb experiment at LHC are different from the results obtained for larger angles.

    “The analysis that provided these interesting results will be continued in the LHCb experiment for various collision energies and different types of colliding structures. Thanks to this, it will be possible to verify some of the models describing hadronisation and, consequently, to better understand the course of the process itself,” sums up Prof. Mariusz Witek (IFJ PAN).

    The work of the team from the IFJ PAN was financed in part by the OPUS grant from the Polish National Science Centre.

    Science paper:
    Bose-Einstein correlations of same-sign charged pions in the forward region in pp collisions at s√=7TeV
    Journal of High Energy Physics, December 2017, 2017

    See the full article here .

    Please help promote STEM in your local schools.

    STEM Icon

    Stem Education Coalition

    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.

     
  • richardmitnick 4:39 pm on February 20, 2018 Permalink | Reply
    Tags: "Rare hyperon-decay anomaly under the spotlight, , , , CERN LHCb, , , ,   

    From CERN Courier: “Rare hyperon-decay anomaly under the spotlight” 


    CERN Courier

    Feb 16, 2018

    1
    The invariant mass distribution

    The LHCb collaboration has shed light on a long-standing anomaly in the very rare hyperon decay Σ+ → pµ+µ– first observed in 2005 by Fermilab’s HyperCP experiment. The HyperCP team found that the branching fraction for this process is consistent with Standard Model (SM) predictions, but that the three signal events observed exhibited an interesting feature: all muon pairs had invariant masses very close to each other, instead of following a scattered distribution.

    This suggested the existence of a new light particle, X0, with a mass of about 214 MeV/c2, which would be produced in the Σ+ decay along with the proton and would decay subsequently to two muons. Although this particle has been long sought in various other decays and at several experiments, no experiment other than HyperCP has so far been able to perform searches using the same Σ+ decay mode.

    The large rate of hyperon production in proton–proton collisions at the LHC has recently allowed the LHCb collaboration to search for the Σ+ → pµ+µ– decay. Given the modest transverse momentum of the final-state particles, the probability that such a decay is able to pass the LHCb trigger requirements is very small. Consequently, events where the trigger is activated by particles produced in the collisions other than those in the decay under study are also employed.

    This search was performed using the full Run 1 dataset, corresponding to an integrated luminosity of 3 fb–1 and about 1014 Σ+ hyperons. An excess of about 13 signal events is found with respect to the background-only expectation, with a significance of four standard deviations. The dimuon invariant- mass distribution of these events was examined and found to be consistent with the SM expectation, with no evidence of a cluster around 214  eV/c2. The signal yield was converted to a branching fraction of (2.1+1.6–1.2) × 10–8 using the known Σ+ → pπ0 decay as a normalisation channel, in excellent agreement with the SM prediction. When restricting the sample explicitly to the case of a decay with the putative X0 particle as an intermediate state, no excess was found. This sets an upper limit on the branching fraction at 9.5 × 10–9 at 90% CL, to be compared with the HyperCP result (3.1+2.4–1.9 ± 1.5) × 10–8.

    This result, together with the recent search for the rare decay KS → μ+μ– shows the potential of LHCb in performing challenging measurements with strange hadrons. As with a number of results in other areas reported recently, LHCb is demonstrating its power not only as a b-physics experiment but as a general-purpose one in the forward region. With current data, and in particular with the upgraded detector thanks to the software trigger from Run 3 onwards, LHCb will be the dominant experiment for the study of both hyperons and KS mesons, exploiting their rare decays to provide a new perspective in the quest for physics beyond the SM.

    See the full article here .

    Please help promote STEM in your local schools.

    STEM Icon

    Stem Education Coalition
    THE FOUR MAJOR PROJECT COLLABORATIONS

    ATLAS
    CERN ATLAS New

    ALICE
    CERN ALICE New

    CMS
    CERN CMS New

    LHCb
    CERN LHCb New II

    LHC

    CERN LHC Map
    CERN LHC Grand Tunnel

    CERN LHC particles

     
  • richardmitnick 11:59 am on December 5, 2017 Permalink | Reply
    Tags: , CERN LHCb, , , , , ,   

    From GIZMODO via FNAL: “Two Teams Have Simultaneously Unearthed Evidence of an Exotic New Particle” Revised to include the DZero result 

    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.

    GIZMODO bloc

    GIZMODO
    11/17/17
    Ryan F. Mandelbaum

    I can’t believe I’ve written three articles about this weird XI particle.

    1
    A tetraquark (Artwork: Fermilab)

    A few months ago, physicists observed a new subatomic particle—essentially an awkwardly-named, crazy cousin of the proton. Its mere existence has energized teams of particle physicists to dream up new ways about how matter forms, arranges itself, and exists.

    Now, a pair of new research papers using different theoretical methods have independently unearthed another, crazier particle predicted by the laws of physics. If discovered in an experiment, it would provide conclusive evidence of a whole new class of exotic particles called tetraquarks, which exist outside the established expectations of the behavior of the proton sub-parts called quarks. And this result is more than just mathematics.

    “We think this is not totally academic,” Chris Quigg, theoretical physicist from the Fermi National Accelerator Laboratory told Gizmodo. “Its discovery may well happen.”

    Bust first, some physics. Zoom all the way in and you’ll find that matter is made of atoms. Atoms, in turn, are made of protons, neutrons, and electrons. Protons and neutrons can further be divided into three quarks.

    Physicists have discovered six types of quarks, which also have names, masses, and electrical charges. Protons and neutrons are made from “up” and “down” quarks, the lightest two. But there are four rarer, heavier ones. From least to most massive, they are: “strange,” “charm,” “bottom,” and “top.” Each one has an antimatter partner—the same particle, but with the opposite electrical sign. As far as physicists have confirmed, these quarks and antiquarks can only arrange themselves in pairs or threes. They cannot exist on their own in nature.

    Scientists in the Large Hadron Collider’s LHCb collaboration recently announced spotting a new arrangement of three quarks, called the Ξcc++ or the “doubly charged, doubly charmed xi particle.”

    CERN/LHCb detector

    It had an up quark and two heavy charm quarks. But “most of these particles” with three quarks “containing two heavy quarks, charm or beauty, have not yet been found,” physicist Patrick Koppenburg from Nikhef, the Dutch National Institute for Subatomic Physics, told Gizmodo back then. “This is the first in a sense.”

    The DZero collaboration at Fermilab announced the discovery of a new particle whose quark content appears to be qualitatively different from normal.

    5
    The particle newly discovered by DZero decays into a Bs meson and pi meson. The Bs meson decays into a J/psi and a phi meson, and these in turn decay into two muons and two kaons, respectively. The dotted lines indicate promptly decaying particles.

    The study, using the full data set acquired at the Tevatron collider from 2002 to 2011 totaling 10 inverse femtobarns, identified the Bs meson through its decay into intermediate J/psi and phi mesons, which subsequently decayed into a pair of oppositely charged muons and a pair of oppositely charged K mesons respectively. Science paper in Physical Review Letters.

    With the knowledge such a particle could exist (and with the knowledge of its properties like its mass), two teams of physicists crunched the numbers in two separate ways. One team used extrapolations of the experimental data and methods they’d previously used to predict this past summer’s particle. The other used a mathematical abstraction of the real world, using approximations that take into account just how much heavier the charm, bottom, and top are than the rest to simplify the calculations.

    In both new papers published in Physical Review Letters https://journals.aps.org/prl/abstract/10.1103/PhysRevLett.119.202002 and https://journals.aps.org/prl/abstract/10.1103/PhysRevLett.119.202001, a stable four-quark particle with two bottom quarks, an anti-up quark, and an anti-down quark fell out of the math. Furthermore, the predicted particles’ masses were not quite the same, but similar enough to raise eyebrows.

    “As you notice, the conclusions are basically identical on a qualitative level,” Marek Karliner, author of the first study from Tel Aviv University in Israel, told Gizmodo. And while lots of tetraquark candidates have been spotted, this particle’s strange identity—including the added properties and stabilization from its two heavy bottom quarks—would offer unambiguous evidence of the particle’s existence.

    “The things we’re talking about are so weird that they couldn’t be something else,” said Quigg.

    But now it’s just a manner of finding the dang things. Quigg thought a new collider such as one proposed for China might be required.

    2
    Rendering of the proposed CEPC [CEPC-SppC for Circular Electron-Positron Collider and Super Proton-Proton Collider]. Photo: IHEP [China’s Institute of High Energy Physics]

    But physicists are in agreement that the sometimes-overlooked LHCb experiment has been doing some of the year’s most exciting work—Karliner thought the experiment could soon spot the particle. “My experimental colleagues are quite firm in this statement. They say that if it’s there, they will see it.” He thought the observation could come in perhaps two to three years time, though Quigg was less optimistic.

    Such unambiguous detection of the tetraquark would confirm guesses from as far back as 1964 as to how quarks arrange themselves. And the independent confirmation from different methods have made both teams confident.

    “I think we have pretty great confidence that the doubly-b tetraquark could exist,” said Quigg. “It’s just a matter of looking hard for it.”

    See the full article here .

    Please help promote STEM in your local schools.

    STEM Icon

    Stem Education Coalition

    “We come from the future.”

    GIZMOGO pictorial

     
  • richardmitnick 6:55 pm on November 18, 2017 Permalink | Reply
    Tags: , , , CERN LHCb, , , ,   

    From Futurism: “Measurements From CERN Suggest the Possibility of a New Physics” 

    futurism-bloc

    Futurism

    November 18, 2017
    Brad Bergan

    A New Quantum Physics?

    2

    During the mid- to late-twentieth century, quantum physicists picked apart the unified theory of physics that Einstein’s theory of relativity offered. The physics of the large was governed by gravity, but only quantum physics could describe observations of the small. Since then, a theoretical tug-o-war between gravity and the other three fundamental forces has continued as physicists try to extend gravity or quantum physics to subsume the other as more fundamental.

    Recent measurements from the Large Hadron Collider show a discrepancy with Standard Model predictions that may hint at entirely new realms of the universe underlying what’s described by quantum physics.

    LHC

    CERN/LHC Map

    CERN LHC Tunnel

    CERN LHC particles

    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.

    Although repeated tests are required to confirm these anomalies, a confirmation would signify a turning point in our most fundamental description of particle physics to date.

    1
    Image credit: starsandspirals

    Quantum physicists found in a recent study [JHEP} that mesons don’t decay into kaon and muon particles often enough, according to the Standard Model predictions of frequency. The authors agree that enhancing the power [The Guardian] of the Large Hadron Collider (LHC) will reveal a new kind of particle responsible for this discrepancy. Although errors in data or theory may have caused the discrepancy, instead of a new particle, an improved LHC would prove a boon for several projects on the cutting edge of physics.

    The Standard Model

    The Standard Model is a well-established fundamental theory of quantum physics that describes three of the four fundamental forces believed to govern our physical reality. Quantum particles occur in two basic types, quarks and leptons. Quarks bind together in different combinations to build particles like protons and neutrons. We’re familiar with protons, neutrons, and electrons because they’re the building blocks of atoms.

    The “lepton family” features heavier versions of the electron — like the muon — and the quarks can coalesce into hundreds of other composite particles. Two of these, the Bottom and Kaon mesons, were culprits in this quantum mystery. The Bottom meson (B) decays to a Kaon meson (K) accompanied by a muon (mu-) and anti-muon (mu+) particle.

    The Anomaly

    They found a 2.5 sigma variance, or 1 in 80 probability, “which means that, in the absence of unexpected effects, i.e. new physics, a distribution more deviant than observed would be produced about 1.25 percent of the time,” Professor Spencer Klein, senior scientist at Lawrence Berkeley National Laboratory, told Futurism. Klein was not involved in the study.

    This means the frequency of mesons decaying into strange quarks during the LHC proton-collision tests fell a little below the expected frequency. “The tension here is that, with a 2.5 sigma [or standard deviation from the normal decay rate], either the data is off by a little bit, the theory is off by a little bit, or it’s a hint of something beyond the standard model,” Klein said. “I would say, naïvely, one of the first two is correct.”

    To Klein, this variance is inevitable considering the high volume of data run by computers for LHC operations. “With Petabyte-(1015 bytes)-sized datasets from the LHC, and with modern computers, we can make a very large number of measurements of different quantities,” Klein said. “The LHC has produced many hundreds of results. Statistically, some of them are expected to show 2.5 sigma fluctuations.” Klein noted that particle physicists usually wait for a 5-sigma fluctuation before crying wolf — corresponding to roughly a 1-in-3.5-million fluctuation in data [physics.org].

    These latest anomalous observations do not exist in a vacuum. “The interesting aspect of the two taken in combination is how aligned they are with other anomalous measurements of processes involving B mesons that had been made in previous years,” Dr. Tevong You, co-author of the study and junior research fellow in theoretical physics at Gonville and Caius College, University of Cambridge, told Futurism. “These independent measurements were less clean but more significant. Altogether, the chance of measuring these different things and having them all deviate from the Standard Model in a consistent way is closer to 1 in 16000 probability, or 4 sigma,” Tevong said.

    Extending the Standard Model

    Barring statistical or theoretical errors, Tevong suspects that the anomalies mask the presence of entirely new particles, called leptoquarks or Z prime particles. Inside bottom mesons, quantum excitations of new particles could be interfering with normal decay frequency. In the study, researchers conclude that an upgraded LHC could confirm the existence of new particles, making a major update to the Standard Model in the process.

    “It would be revolutionary for our fundamental understanding of the universe,” said Tevong. “For particle physics […] it would mean that we are peeling back another layer of Nature and continuing on a journey of discovering the most elementary building blocks. This would have implications for cosmology, since it relies on our fundamental theories for understanding the early universe,” he added. “The interplay between cosmology and particle physics has been very fruitful in the past. As for dark matter, if it emerges from the same new physics sector in which the Zprime or leptoquark is embedded, then we may also find signs of it when we explore this new sector.”

    The Power to Know

    So far, scientists at the LHC have only observed ghosts and anomalies hinting at particles that exist at higher energy levels. To prove their existence, physicists “need to confirm the indirect signs […], and that means being patient while the LHCb experiment gathers more data on B decays to make a more precise measurement,” Tevong said.

    CERN/LHCb

    “We will also get an independent confirmation by another experiment, Belle II, that should be coming online in the next few years. After all that, if the measurement of B decays still disagrees with the predictions of the Standard Model, then we can be confident that something beyond the Standard Model must be responsible, and that would point towards leptoquarks or Zprime particles as the explanation,” he added.

    To establish their existence, physicists would then aim to produce the particles in colliders the same way Bottom mesons or Higgs bosons are produced, and watch them decay. “We need to be able to see a leptoquark or Zprime pop out of LHC collisions,” Tevong said. “The fact that we haven’t seen any such exotic particles at the LHC (so far) means that they may be too heavy, and more energy will be required to produce them. That is what we estimated in our paper: the feasibility of directly discovering leptoquarks or Zprime particles at future colliders with higher energy.”

    Quantum Leap for the LHC

    Seeking out new particles in the LHC isn’t a waiting game. The likelihood of observing new phenomena is directly proportional to how many new particles pop up in collisions. “The more the particle appears the higher the chances of spotting it amongst many other background events taking place during those collisions,” Tevong explained. For the purposes of finding new particles, he likens it to searching for a needle in a haystack; it’s easier to find a needle if the haystack is filled with them, as opposed to one. “The rate of production depends on the particle’s mass and couplings: heavier particles require more energy to produce,” he said.

    This is why Tevong and co-authors B.C. Allanach and Ben Gripaios recommend either extending the LHC loop’s length, thus reducing the amount of magnetic power needed to accelerate particles, or replacing the current magnets with stronger ones.

    According to Tevong, the CERN laboratory is slated to keep running the LHC in present configuration until mid-2030s. Afterwards, they might upgrade the LHC’s magnets, roughly doubling its strength. In addition to souped-up magnets, the tunnel could see an enlargement from present 27 to 100 km (17 to 62 miles). “The combined effect […] would give about seven times more energy than the LHC,” Tevong said. “The timescale for completion would be at least in the 2040s, though it is still too early to make any meaningful projections.”

    If the leptoquark or Z prime anomalies are confirmed, the Standard Model has to change, Tevong reiterates. “It is very likely that it has to change at energy scales directly accessible to the next generation of colliders, which would guarantee us answers,” he added. While noting that there’s no telling if dark matter has anything to do with the physics behind Zprimes or leptoquarks, the best we can do is seek “as many anomalous measurements as possible, whether at colliders, smaller particle physics experiments, dark matter searches, or cosmological and astrophysical observations,” he said. “Then the dream is that we may be able to form connections between various anomalies that can be linked by a single, elegant theory.”

    See the full article here .

    Please help promote STEM in your local schools.

    STEM Icon

    Stem Education Coalition

    Futurism covers the breakthrough technologies and scientific discoveries that will shape humanity’s future. Our mission is to empower our readers and drive the development of these transformative technologies towards maximizing human potential.

     
  • richardmitnick 10:58 am on July 6, 2017 Permalink | Reply
    Tags: , , CERN LHCb, , Observation of an exceptionally charming particle, ,   

    From LHCb at CERN: ” Observation of an exceptionally charming particle” 

    Cern New Bloc

    Cern New Particle Event

    CERN New Masthead

    LHCb at CERN

    Today, at the EPS Conference on High Energy Physics, EPS-HEP 2017, in Venice, Italy, the LHCb collaboration presented the first observation of a doubly charmed particle. This particle, called the Ξcc++, is a baryon (particle composed of three quarks) containing two charm quarks and one up quark, resulting in an overall doubly positive charge. It is a doubly charm counterpart of the well-known lower mass Ξ0 baryon, which is composed of two strange quarks and an up quark.

    1

    2

    The Ξcc++ baryon is identified via its decay into a Λc+ baryon and three lighter mesons K-, π+ and π+. The image above shows an example of a Feynman diagram contributing to this decay. The Λc+ baryon decays in turn into a proton p, a K- and a π+ meson. The image shows the Λc+K-π+π+ invariant mass spectrum obtained with 1.7 fb-1 of data collected by LHCb in 2016 at the LHC centre-of-mass energy of 13 TeV. The mass is measured to be about 3621 MeV/c2 which is almost four times heavier than the most familiar baryon, the proton, a property that arises from its doubly charmed-quark content. The signal candidates are consistent with particles that traveled a significant distance before decaying: even selecting only those Ξcc++ particles that survived more than approximately five times the expected decay time resolution, the signal remains highly significant. This state is therefore incompatible with a strongly decaying particle, but is consistent with a longer-lived decay involving weak interactions as would be expected for this particle.

    3
    The new particle, awkwardly known as Xi-cc++ (pronounced ka-sigh-see-see-plus-plus)

    The existence of doubly charmed baryons was already known to be a possibility in the 1970s, after the discovery of the charm quark. In the early 2000s the observation of a similar particle was reported by the SELEX collaboration. This observation was not confirmed by subsequent experiments and the measured properties of this particle are not compatible with those of the Ξcc++ baryon discovered by LHCb. The discovery of the Ξcc++ performed by LHCb has been made possible by the high production rate of heavy quarks at the LHC and thanks to the unique capabilities of the experiment, which can identify the decay products with excellent efficiency and purity. The image shows an artist view of this new particle. This animation shows how the signal accumulated in the Λc+K-π+π+ invariant-mass spectrum throughout 2016.

    4

    This discovery opens a new field of particle physics research. An entire family of doubly charmed baryons related to the Ξcc++ is predicted, and will be searched for with added enthusiasm. Furthermore, other hadrons containing different configurations of two heavy quarks, for example two beauty quarks or a beauty and charm quark, are waiting to be discovered. Measurements of the properties of all these particles will allow for precise tests of QCD, the theory of strong interactions, in a unique environment. LHCb is very well equipped to face this very exciting challenge.

    More information can be found in the LHCb EPS-HEP presentation, in the LHCb publication and soon in a forthcoming CERN seminar. Read also the CERN Press Release in English and French as well as the CERN Courier article in near future.

    click here to get direct access to all LHCb published papers

    See the full article here.

    Please help promote STEM in your local schools.

    STEM Icon

    Stem Education Coalition

    Meet CERN in a variety of places:

    Quantum Diaries
    QuantumDiaries

    Cern Courier

    THE FOUR MAJOR PROJECT COLLABORATIONS

    ATLAS
    CERN ATLAS New

    ALICE
    CERN ALICE New

    CMS
    CERN CMS New

    LHCb
    CERN LHCb New II

    LHC

    CERN LHC Map
    CERN LHC Grand Tunnel

    CERN LHC particles

    Quantum Diaries

     
  • richardmitnick 8:48 am on June 12, 2017 Permalink | Reply
    Tags: , CERN LHCb, , , , Physicists review three experiments that hint at a phenomenon beyond the Standard Model of particle physics, ,   

    From phys.org: “Physicists review three experiments that hint at a phenomenon beyond the Standard Model of particle physics” 

    physdotorg
    phys.org

    June 8, 2017

    1
    Event display recorded by the BaBaR detector showing the decays of two B mesons into various subatomic particles, including a muon and a neutrino. Credit: SLAC NATIONAL ACCELERATOR LABORATORY

    To anyone but a physicist, it sounds like something out of “Star Trek.” But lepton universality is a real thing.

    It has to do with the Standard Model of particle physics, which describes and predicts the behavior of all known particles and forces, except gravity. Among them are charged leptons: electrons, muons and taus.

    A fundamental assumption of the Standard Model is that the interactions of these elementary particles are the same despite their different masses and lifetimes. That’s lepton universality. Precision tests comparing processes involving electrons and muons have not revealed any definite violation of this assumption, but recent studies of the higher-mass tau lepton have produced observations that challenge the theory.

    A new review of results from three experiments points to the strong possibility that lepton universality—and perhaps ultimately the Standard Model itself—may have to be revised. The findings by a team of international physicists, including UC Santa Barbara postdoctoral scholar Manuel Franco Sevilla, appear in the journal Nature.

    “As part of my doctoral thesis at Stanford, which was based on earlier work carried out at UCSB by professors Jeff Richman and Michael Mazur, we saw the first significant observation of something beyond the Standard Model at the BaBaR experiment conducted at the SLAC National Accelerator Laboratory,” Franco Sevilla said.

    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.

    SLAC BABAR

    This was significant but not definitive, he added, noting that similar results were seen in more recent experiments conducted in Japan (Belle) and in Switzerland (LHCb). According to Franco Sevilla, the three experiments, taken together, demonstrate a stronger result that challenges lepton universality at the level of four standard deviations, which indicates a 99.95 percent certainty.

    BaBaR, which stands for B-Bbar (anti-B) detector, and Belle were carried out in B factories. These particle colliders are designed to produce and detect B mesons—unstable particles that result when powerful particle beams collide—so their properties and behavior can be measured with high precision in a clean environment. The LHCb (Large Hadron Collider b) provided a higher-energy environment that more readily produced B mesons and hundreds of other particles, making identification more difficult.

    KEK Belle SuperKEKB accelerator

    CERN/LHCb

    Nonetheless, the three experiments, which measured the relative ratios of B meson decays, posted remarkably similar results. The rates for some decays involving the heavy lepton tau, relative to those involving the light leptons—electrons or muons—were higher than the Standard Model predictions.

    “The tau lepton is key because the electron and the muon have been well measured,” Franco Sevilla explained. “Taus are much harder because they decay very quickly. Now that physicists are able to better study taus, we’re seeing that perhaps lepton universality is not satisfied as the Standard Model claims.”

    While intriguing, the results are not considered sufficient to establish a violation of lepton universality. To overturn this long-held physics precept would require a significance of at least five standard deviations. However, Franco Sevilla noted, the fact that all three experiments observed a higher-than-expected tau decay rate while operating in different environments is noteworthy.

    A confirmation of these results would point to new particles or interactions and could have profound implications for the understanding of particle physics. “We’re not sure what confirmation of these results will mean in the long term,” Franco Sevilla said. “First, we need to make sure that they’re true and then we’ll need ancillary experiments to determine the meaning.”

    See the full article here .

    Please help promote STEM in your local schools.

    STEM Icon

    Stem Education Coalition

    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.

     
  • richardmitnick 9:29 pm on June 7, 2017 Permalink | Reply
    Tags: , , , Belle, CERN LHCb, , , , , , Vera Lüth,   

    From SLAC: Women in STEM – “Q&A: SLAC’s Vera Lüth Discusses the Search for New Physics” 


    SLAC Lab

    June 7, 2017
    Manuel Gnida

    4
    Vera Lüth, professor emerita of experimental particle physics at SLAC. (Dawn Harmer/SLAC National Accelerator Laboratory)

    Data from BABAR, Belle and LHCb experiments hint at phenomena beyond the Standard Model of particle physics.

    SLAC BABAR

    1
    An electron-positron annihilation producing a pair of B mesons as recorded by the BABAR detector at the PEP-II storage rings. Among the reconstructed curved particle tracks is a muon (bottom left). The direction of the associated anti-neutrino (dashed arrow) is identified as missing momentum. Both particles originate from the same B-meson decay. (SLAC National Accelerator Laboratory)

    KEK Belle detector, at the High Energy Accelerator Research Organisation (KEK) in Tsukuba, Ibaraki Prefecture, Japan

    CERN LHCb chamber, LHC

    The Standard Model of particle physics describes the properties and interactions of the constituents of matter.

    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 development of this theory began in the early 1960s, and in 2012 the last piece of the puzzle was solved by the discovery of the Higgs boson at the Large Hadron Collider (LHC) at CERN in Switzerland.

    CERN CMS Higgs Event

    CERN ATLAS Higgs Event

    LHC

    CERN/LHC Map

    CERN LHC Tunnel

    CERN LHC particles

    Experiments have confirmed time and again the Standard Model’s very accurate predictions.

    Yet, researchers have reasons to believe that physics beyond the Standard Model exists and should be found. For instance, the Standard Model does not explain why matter dominates over antimatter in the universe. It also does not provide clues about the nature of dark matter – the invisible substance that is five times more prevalent than the regular matter we observe.

    In this Q&A, particle physicist Vera Lüth discusses scientific results that potentially hint at physics beyond the Standard Model. The professor emerita of experimental particle physics at the Department of Energy’s SLAC National Accelerator Laboratory is co-author of a review article published today in Nature that summarizes the findings of three experiments: BABAR at SLAC, Belle in Japan and LHCb at CERN.

    What are the hints of new physics that you describe in your article?

    The hints originate from studies of an elementary particle, known as the B meson – an unstable particle produced in the collision of powerful particle beams. More precisely, these studies looked at decays of the B meson that involve leptons – electrically charged elementary particles and their associated neutrinos. There are three charged leptons: the electron, a critical component of atoms discovered in 1897; the muon, first observed in cosmic rays in 1937; and the much heavier tau, discovered at the SPEAR electron-positron (e+e-) storage ring at SLAC in 1975 by Martin Perl.

    Due to their very different masses, the three leptons also have very different lifetimes. The electron is stable, whereas the muon and tau decay in a matter of microseconds and a fraction of a picosecond, respectively. A fundamental assumption of the Standard Model is that the interactions of the three charged leptons are the same if their different masses and lifetimes are taken into account.

    Over many years, different experiments have tested this assumption – referred to as “lepton universality” – and to date no definite violation of this rule has been observed. We now have indications that the rates for B meson decays involving tau leptons are larger than expected compared to the measured rates of decays involving electrons or muons, taking into account the differences in mass. This observation would violate lepton universality, a fundamental assumption of the Standard Model.

    What does a violation of the Standard Model actually mean?

    It means that there is evidence for phenomena that we cannot explain in the context of the Standard Model. If such a phenomenon is firmly established, the Standard Model needs to be extended – by introducing new fundamental particles and also new interactions related to these particles.

    In recent years, searches for fundamentally new phenomena have relied on high-precision measurements to detect deviations from Standard Model predictions or on searches for new particles or interactions with properties that differ from known ones.

    What exactly are the BABAR, Belle and LHCb experiments?

    They are three experiments that have challenged lepton universality.

    Belle and BABAR were two experiments specifically designed to study B mesons with unprecedented precision – particles that are five times heavier than the proton and contain a bottom or b quark. These studies were performed at e+e- storage rings that are commonly referred to as B factories and operate at colliding-beam energies just high enough to produce a pair of B mesons, and no other particle. BABAR operated at SLAC’s PEP-II from 1999 to 2008, Belle at KEKB in Japan from 1999 to 2010. The great advantage of these experiments is that the B mesons are produced pairwise, each decaying into lighter particles – on average five charged particles and a similar number of photons.

    The LHCb experiment is continuing to operate at the proton-proton collider LHC with energies that exceed the ones of B factories by more than a factor of 1,000. At this higher energy, B mesons are produced at a much larger rate than at B factories. However, at each crossing of the beams, hundreds of other particles are produced in addition to B mesons. This feature tremendously complicates the identification of B meson decays.

    To study lepton universality, all three experiments focus on B decays involving a charged lepton and an associated neutrino. A neutrino doesn’t leave a trace in the detector, but its presence is detected as missing energy and momentum in an individual B decay.

    What evidence do you have so far for a potential violation of lepton universality?

    All three experiments have identified specific B meson decays and have compared the rates of decays involving an electron or muon to those involving the higher mass tau lepton. All three experiments observe higher-than-expected decay rates for the decays with a tau. The average value of the reported results, taking into account the statistical and systematic uncertainties, exceeds the Standard Model expectation by four standard deviations.

    This enhancement is intriguing, but not considered sufficient to unambiguously establish a violation of lepton universality. To claim a discovery, particle physicists generally demand a significance of at least five standard deviations. However, the fact that this enhancement was detected by three experiments, operating in very different environments, deserves attention. Nevertheless, more data will be needed, and are expected in the not too distant future.

    What was your role in this research?

    As the technical coordinator of the BABAR collaboration during the construction of the detector, I was the liaison between the physicists and the engineering teams, supported by the BABAR project management team at SLAC. With more than 500 BABAR members from 11 countries, this was a challenging task, but with the combined expertise and dedication of the collaboration the detector was completed and ready to take data in four years.

    Once data became available, I rejoined SLAC’s Research Group C and took over its leadership from Jonathan Dorfan. As convener of the physics working group on B decays involving leptons, I coordinated various analyses by scientists from different external groups, among them SLAC postdocs and graduate students, and helped to develop the analysis tools needed for precision measurements.

    Almost 10 years ago, we started updating an earlier analysis performed under the leadership of Jeff Richman of the University of California, Santa Barbara on B decays involving tau leptons and extended it to the complete BABAR data set. This resulted in the surprisingly large decay rate. The analysis was the topic of the PhD thesis of my last graduate student, Manuel Franco Sevilla, who over the course of four years made a number of absolutely critical contributions that significantly improved the precision of this measurement, and thereby enhanced its significance.

    What keeps you excited about particle physics?

    Over the past 50 years that I have been working in particle physics, I have witnessed enormous progress in theory and experiments leading to our current understanding of matter’s constituents and their interactions at the most fundamental level. But there are still many unanswered questions, from very basic ones like “Why do particles have certain masses and not others?” to questions about the grand scale of things, such as “What is the origin of the universe, and is there more than one?”

    Lepton universality is one of the Standard Model’s fundamental assumptions. If it were violated, unexpected new physics processes must exist. This would be a major breakthrough – even more surprising than the discovery of the Higgs boson, which was predicted to exist many decades ago.

    What results do you expect in the near future?

    There is actually a lot going on in the field. LHCb researchers are collecting more data and will try to find out if the lepton universality is indeed violated. My guess is that we should know the answer by the end of this year. A confirmation will be a great event and will undoubtedly trigger intense experimental and theoretical research.

    At present we do not understand the origin of the observed enhancement. We first assumed that it could be related to a charged partner of the Higgs boson. Although the observed features did not match the expectations, an extension of the Higgs model could do so. Another possible explanation that can neither be confirmed nor excluded is the presence of so-called lepto-quarks. These open questions will remain a very exciting topic that need to be addressed by experiments and theoretical work.

    Recently, LHCb scientists have reported an interesting result indicating that certain B meson decays more often include an electron pair than a muon pair. However, the significance of this new finding is only about 2.6 standard deviations, so it’s too early to draw any conclusions. BABAR and Belle have not confirmed this observation.

    At the next-generation B factory, Super-KEKB in Japan, the new Belle II experiment is scheduled to begin its planned 10-year research program in 2018. The expected very large new data sets will open up many opportunities for searches for these and other indications of physics beyond the Standard Model.

    4
    Super-KEKB in Japan

    5
    Belle II at the SuperKEKB accelerator complex at KEK in Tsukuba, Ibaraki Prefecture, Japan

    See the full article here .

    Please help promote STEM in your local schools.

    STEM Icon

    Stem Education Coalition

    SLAC Campus
    SLAC is a multi-program laboratory exploring frontier questions in photon science, astrophysics, particle physics and accelerator research. Located in Menlo Park, California, SLAC is operated by Stanford University for the DOE’s Office of Science.
    i1

     
  • richardmitnick 5:06 pm on May 8, 2017 Permalink | Reply
    Tags: A study of J∕ψ production in jets, CERN LHCb,   

    Physics: “Viewpoint: Probing Quarkonium Production in Jets” 

    Physics LogoAbout Physics

    Physics Logo 2

    Physics

    May 8, 2017
    Adam K. Leibovich, Pittsburgh Particle Physics Astrophysics and Cosmology Center (PITT PACC), Department of Physics and Astronomy, University of Pittsburgh, 3941 O’Hara St., Pittsburgh, PA 15260, USA
    Thomas Mehen, Department of Physics, Duke University, Durham, NC 27713, USA

    A study of J∕ψ production in jets reveals the weaknesses of a widely used method of simulating high-energy particle collisions.

    1
    Figure 1: High-energy proton-proton collisions produce quarks and gluons that form a jet of hadrons that can include quarkonia like the J∕ψ.

    On November 11, 1974, two research groups, one at SLAC National Accelerator Laboratory [1] and the other at Brookhaven National Laboratory [2], announced the discovery of a new particle, a meson soon to be called the J∕ψ. What was so surprising about the particle was that it lived about a thousand times longer than expected. Now known as the “November Revolution” of particle physics, the discovery had a huge impact on the field. Theorists jumped to interpret the result, and only 8 days after the initial announcement, a study by Thomas Appelquist and David Politzer interpreted the particle as a bound state of a charm quark and an anticharm quark [3]. Since then, much progress has been made in understanding the physics of the J∕ψ and other forms of quarkonium, a hadron comprised of a heavy quark (a charm or bottom quark) and its antiquark. Today, however, the theory describing how quarkonium is produced in high-energy collisions is still incomplete. Now, the Large Hadron Collider beauty (LHCb) collaboration [4] has used a new way to study the J∕ψ, which looks at how the particle is produced within a jet—a narrow, collimated cone of hadrons created in high-energy particle collisions. The results will help researchers refine both theoretical models for quarkonium and numerical Monte Carlo methods that are crucial for the description of particle collider experiments.

    Being a bound state of a quark and an antiquark, quarkonium is in some ways similar to positronium, a bound state of an electron and an antielectron (positron). Like positronium, quarkonium’s different states can be classified in terms of their spin, orbital angular momentum, and total spin. For instance, the charm-anticharm bound state with the lowest energy is the paracharmonium state (known as ηc), with spin 0, and it is analogous to parapositronium, in which the electron and positron have antiparallel spins. The higher energy J∕ψ state, with spin 1, is instead analogous to orthopositronium, in which the electron and positron spins are parallel.

    However, while positronium is bound together by electromagnetic forces, quarkonium is bound together by the strong interaction, which is described by the theory of quantum chromodynamics (QCD). Quarkonium is thus an important test bed for QCD phenomenology. From an experimental point of view, the J∕ψ is particularly interesting because it is easy to produce and observe at particle accelerators as a product of high-energy collisions. From a theoretical perspective, however, the mechanisms by which the J∕ψ and other quarkonia are produced still defy our understanding. This is because QCD calculations are only possible in a perturbative regime. But quarkonium production involves quark interactions on many energy and length scales, not all of which can be tackled perturbatively. For example, the binding of all quark states, including quarkonium, cannot be calculated analytically.

    The calculations of J∕ψ production by Appelquist and Politzer [3] were done in the so-called color-singlet model (CSM). Such a model assumes that the heavy quark-antiquark pair is a color-singlet (a state with a total color charge of zero) and is in an angular momentum state whose quantum numbers match those observed for the J∕ψ. However, the CSM fell out of favor in the 1990s when it failed to reproduce experiments on J∕ψ production at the Tevatron [5]. It appeared that new production channels were needed, which led to the adoption of an effective field theory called nonrelativistic QCD (NRQCD) [6]. In the NRQCD description, hard collisions create quark-antiquark pairs that can have any quantum numbers. The pairs then evolve into final-state particles like the J∕ψ

    While NRQCD has delivered successful predictions for many years, it has also led to conclusions that are in conflict with experiments. For instance, NRQCD wrongly predicted that J∕ψ particles emerging from a collision with large transverse momentum should be transversely polarized [7]. This is possibly the result of corrections to the calculations that turn out to be larger than expected, or because of how the matrix elements are extracted from experimental data. This extraction depends on certain theoretical assumptions that might be inaccurate. Different groups have used different assumptions, leading to significantly different matrix elements for the same processes and causing confusion in the field. The new LHCb experiments may now help researchers constrain and refine the process for NRQCD matrix-element extraction.

    The LHCb measurement was motivated by theoretical work, carried out by us and our collaborators [8], which proposed to study the production of J∕ψ in jets created in high-energy particle collisions.

    CERN LHCb chamber


    CERN/LHCb

    The initial collisions produce quarks and gluons that eventually form a jet of hadrons that can include quarkonia like the J∕ψ (see Fig. 1). Ref. [8] indicated that the distribution of the J∕ψ within a jet is sensitive to the underlying production mechanisms. In particular, it showed that measurements of the ratio of the momentum carried by the J∕ψ to the momentum carried by the jet could be used to test the accuracy of different matrix element extractions.

    The LHCb collaboration has now realized this proposal, carrying out the first experimental study of prompt quarkonium production within a jet. The success of the experiment hinged on improvements in LHCb’s data-taking scheme, which the collaboration introduced in 2015. These developments allowed the team to record many more J∕ψ candidate events than previous studies, including, for instance, candidates with low transverse momentum pT. Such particles are hard to detect because they are created with low energies, to which detectors are less sensitive, and at small angles with respect to the colliding proton beams, where there is a large background from other particles. LHCb determined z (the ratio of the transverse momentum carried by the J∕ψ to the transverse momentum carried by the whole jet) both for the J∕ψ that are produced promptly and those that are produced subsequently from the decay of other particles ( b hadrons) in the jet. The ability to measure the J∕ψ down to very low pT allowed LHCb to measure the entire z distribution.

    One of the most important conclusions of this work is that PYTHIA [9]—a Monte-Carlo-based simulation program widely used to model particle collisions at high energy—does a poor job at reproducing the z distribution for prompt J∕ψ

    production. Experimentalists use PYTHIA to numerically estimate the production rates or background signals in high-energy collisions, while theorists use it to highlight possible signatures of beyond-standard-model physics. The shortcomings of PYTHIA were anticipated by theoretical work carried out by us and by our collaborators [10], which showed that PYTHIA disagreed significantly with theoretical predictions based on NRQCD. Now, the LHCb results have shown that PYTHIA also disagrees with experimental results.

    The LHCb results will have two important implications for research on quarkonium and particle physics. First, it will spur improvements in Monte Carlo simulations, which are crucial for analyzing any particle-physics measurement. PYTHIA experts, who are working to refine the description of quarkonium production in the program, will certainly take note. Second, it will trigger theoretical and experimental work aimed at improving the extraction of NRQCD matrix elements. This may lead to important improvements of the NRQCD formalism, which is essential for understanding the complex physics of heavy quarkonium states.

    This research is published in Physical Review Letters.

    Study of J/ψ
    Production in Jets

    R. Aaij et al. (LHCb Collaboration)

    Phys. Rev. Lett. 118, 192001 (2017)

    Further references
    See the full article for further references complete with links

    See the full article here .

    Please help promote STEM in your local schools.

    STEM Icon

    Stem Education Coalition

    Physicists are drowning in a flood of research papers in their own fields and coping with an even larger deluge in other areas of physics. How can an active researcher stay informed about the most important developments in physics? Physics highlights a selection of papers from the Physical Review journals. In consultation with expert scientists, the editors choose these papers for their importance and/or intrinsic interest. To highlight these papers, Physics features three kinds of articles: Viewpoints are commentaries written by active researchers, who are asked to explain the results to physicists in other subfields. Focus stories are written by professional science writers in a journalistic style and are intended to be accessible to students and non-experts. Synopses are brief editor-written summaries. Physics provides a much-needed guide to the best in physics, and we welcome your comments (physics@aps.org).

     
  • richardmitnick 11:56 am on April 20, 2017 Permalink | Reply
    Tags: , , CERN LHCb, , New LHC Results Hint At New Physics... But Are We Crying Wolf?, ,   

    From Ethan Siegel: “New LHC Results Hint At New Physics… But Are We Crying Wolf?” 

    Ethan Siegel
    Apr 20, 2017

    1
    The LHCb collaboration is far less famous than CMS or ATLAS, but the bottom-quark-containing particles they produce holds new physics hints that the other detectors cannot probe. CERN / LHCb Collaboration

    Over at the Large Hadron Collider at CERN, particles are accelerated to the greatest energies they’ve ever reached in history. In the CMS and ATLAS detectors, new fundamental particles are continuously being searched for, although only the Higgs boson has come through. But in a much lesser-known detector — LHCb — particles containing bottom quarks are produced in tremendous numbers. One class of these particles, quark-antiquark pairs where one is a bottom quark, have recently been observed to decay in a way that runs counter to the Standard Model’s predictions. Even though the evidence isn’t very good, it’s the biggest hint for new physics we’ve had from accelerators in years.

    2
    A decaying B-meson, as shown here, may decay more frequently to one type of lepton pair than the other, contradicting Standard Model expectations. KEK / BELLE collaboration

    KEK Belle detector, at the High Energy Accelerator Research Organisation (KEK) in Tsukuba, Ibaraki Prefecture, Japan

    There are two ways, throughout history, that we’ve made extraordinary advances in fundamental physics. One is when an unexplained, robust phenomenon pops up, and we’re compelled to rethink our conception of the Universe. The other is when multiple, competing, but heretofore indistinguishable explanations of the same set of observations are subject to a critical test, where only one explanation emerges as a valid one. Particle physics is at a crossroads right now, because even though there are fundamentally unsolved questions, the energy scales that we can probe with experiments all give results that are perfectly in line with 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.

    3
    The discovery of the Higgs Boson in the di-photon (γγ) channel at CMS. That ‘bump’ in the data is an unambiguous new particle: the Higgs.

    CERN CMS Higgs Event

    CERN/CMS Detector

    The Higgs boson, discovered earlier this decade, was created over and over at the LHC, with its decays measured in excruciating detail. If there were any hints of departures from the Standard Model — if it decayed into one type of particle more-or-less frequently than predicted — it could be an extraordinary hint of new physics. Similarly, physicists searches exhaustively for new “bumps” where there shouldn’t be any in the data: a signal of a potential new particle. Although they showed up periodically, with some mild significance, they always went away entirely with more and better data.

    4
    The observed Higgs decay channels vs. the Standard Model agreement, with the latest data from ATLAS and CMS included. The agreement is astounding, but there are outliers (which is expected) when the error-bars are larger.

    CERN ATLAS Higgs Event

    CERN/ATLAS detector

    Statistically, this is about what you’d expect. If you had a fair coin and tossed it 10 times, you might expect that you’d get 5 heads and 5 tails. Although that’s reasonable, sometimes you’ll get 6 and 4, sometimes you’ll get 8 and 2, and sometimes you’ll get 10 and 0, respectively. If you got 10 heads and 0 tails, you might begin to suspect that the coin isn’t fair, but the odds aren’t that bad: about 0.2% of the time, you’ll have all ten flips give the same result. And if you have 1000 people each flipping a coin ten times, it’s very likely (86%) that at least one of them will get the same result all ten times.

    The Standard Model makes predictions for lots of different quantities — particle production rates, scattering amplitudes, decay probabilities, branching ratios, etc. — for every single particle (both fundamental and composite) that can be created. Literally, there are hundreds of such composite particles that have been created in such numbers, and thousands of quantities like that we can measure. Since we look at all of them, we demand an extremely high level of statistical significance before we’re willing to claim a discovery. In particle physics, the odds of a fluke need to be less than one-in-three-million to get there.

    6
    The standard model calculated predictions (the four colored points) and the LHCb results (black, with error bars) for the electron/positron to muon/antimuon ratios at two different energies. LHCb Collaboration / Tommaso Dorigo

    Earlier this week, the LHCb collaboration announced their greatest departure yet observed from the Standard Model: a difference in the rate of decay of bottom-quark-containing mesons into strange-quark-containing mesons with either a muon-antimuon pair or an electron-positron pairs. In the Standard Model, the ratios should be 1.0 (once mass differences of muons and electrons are taken into account), but they observed a ratio of 0.6. That sure sounds like a big deal, and like it might be a hint of physics beyond the Standard Model!

    7
    The known particles and antiparticles of the Standard Model all have been discovered. All told, they make explicit predictions. Any violation of those predictions would be a sign of new physics, which we’re desperately seeking. E. Siegel

    The case gets even stronger when you consider that the BELLE collaboration, last decade, discovered these decays and began to notice a slight discrepancy themselves. But a closer inspection of the latest data shows that the statistical significance is only about 2.4 and 2.5 sigma, respectively, at the two energies measured. This is about a 1.5% chance of a fluke individually, or about 3.7-sigma significance (0.023% chance of a fluke) combined. Now, 3.7-sigma is a lot more exciting than 2.5-sigma, but it’s still not exciting enough. Given that there were thousands of things these experiments looked at, these results barely even register as “suggestive” of new physics, much less as compelling evidence.

    7
    The ATLAS and CMS diphoton bumps from 2015, displayed together, clearly correlating at ~750 GeV. This suggestive result was significant at more than 3-sigma, but went away entirely with more data. CERN, CMS/ATLAS collaborations; Matt Strassler

    Yet already, just on Wednesday, there were six new papers out (with more surely coming) attempting to use beyond-the-Standard-Model physics to explain this not-even-promising result.

    Why?

    Because, quite frankly, we don’t have any good ideas in place. Supersymmetry, grand unification, string theory, technicolor, and extra dimensions, among others, were the leading extensions to the Standard Model, and colliders like the LHC have yielded absolutely no evidence for any of them. Signals from direct experiments for physics beyond the Standard Model have all yielded results completely consistent with the Standard Model alone. What we’re seeing now is rightly called ambulance-chasing, but it’s even worse than that.

    8
    The Standard Model particles and their supersymmetric counterparts. Non-white-male-American scientists have been instrumental in the development of the Standard Model and its extensions. Claire David

    We know that results like this have a history of not holding up at all; we expect there to be fluctuations like this in the data, and this one isn’t even as significant as the others that have gone away with more and better data. You expect a 2-sigma discrepancy in one out of every 20 measurements you make, and these two are little better than that. Even combined, they’re hardly impressive, and the other things you’d seek to measure about this decay line up with the Standard Model perfectly. In short, the Standard Model is much more likely than not to hold up once more and better data arrives.

    9
    The string landscape might be a fascinating idea that’s full of theoretical potential, but it doesn’t predict anything that we can observe in our Universe. University of Cambridge

    What we’re seeing right now is a response from the community is what we’d expect to an alarm that’s crying “Wolf!” There might be something fantastic and impressive out there, and so, of course we have to look. But we know that, more than 99% of the time, an alarm like this is merely the result of which way the wind blew. Physicists are so bored and so out of good, testable ideas to extend the Standard Model — which is to say, the Standard Model is so maddeningly successful — that even a paltry result like this is enough to shift the theoretical direction of the field.

    A few weeks ago, famed physicist (and supersymmetry-advocate) John Ellis asked the question, Where is Particle Physics going? Unless experiments can generate new, unexpected results, the answer is likely to be “nowhere new; nowhere good” for the indefinite future.

    See the full article here .

    Please help promote STEM in your local schools.

    STEM Icon

    Stem Education Coalition

    “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

     
c
Compose new post
j
Next post/Next comment
k
Previous post/Previous comment
r
Reply
e
Edit
o
Show/Hide comments
t
Go to top
l
Go to login
h
Show/Hide help
shift + esc
Cancel
%d bloggers like this: