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  • richardmitnick 8:38 am on June 12, 2019 Permalink | Reply
    Tags: , , CERN LHCb, , , , , ,   

    From Science Magazine: “Exotic particles called pentaquarks may be less weird than previously thought” 

    AAAS
    From Science Magazine

    Jun. 5, 2019
    Adrian Cho

    1
    The Large Hadron Collider beauty experiment has discovered three new pentaquarks. Peter Ginter/CERN

    Four years ago, when experimenters spotted pentaquarks—exotic, short-lived particles made of five quarks—some physicists thought they had glimpsed the strong nuclear force, which binds the atomic nucleus, engaging in a bizarre new trick. New observations have now expanded the zoo of pentaquarks, but suggest a tamer explanation for their structure. The findings, from the Large Hadron Collider beauty experiment (LHCb), a particle detector fed by the LHC at CERN, the European particle physics laboratory near Geneva, Switzerland, suggest pentaquarks are not bags of five quarks binding in a new way, but are more like conventional atomic nuclei.

    “I’m really excited that the new data send such a clear message,” says Tomasz Skwarnicki, an LHCb physicist at Syracuse University in New York who led the study. But, he notes, “It may not be the message some people had hoped for.”

    Pentaquarks are heavier cousins of protons and neutrons, which are also made of quarks. In ordinary matter, quarks come in two types, up and down. Atom smashers can blast four heavier types of quarks into brief existence: charm, strange, top, and bottom. Quarks cling to one another through the strong force so mightily they cannot be isolated. Instead, they are almost always found in groups of three in particles known as baryons—including the proton and neutron—or in pairs called mesons, which consist of a quark and an antimatter quark.

    But for decades, some theorists have hypothesized the existence of larger bundles of quarks. In recent years, experimenters have found evidence for four-quark particles, or tetraquarks. Then, in 2015, LHCb reported signs of two pentaquarks.

    Some theorists argue that the new particles are bags of four and five quarks, bound together through the exchange of quantum particles called gluons, adding a new wrinkle to the often intractable theory of the strong force. Others argue they’re more like an atomic nucleus. In this “molecular” picture a pentaquark is a three-quark baryon stuck to a two-quark meson the same way that protons and neutrons bind in a nucleus—by exchanging short-lived pi mesons.

    LHCb’s new pentaquarks, reported today in Physical Review Letters (PRL), bolster the molecular picture. In 2015, LHCb researchers reported a pentaquark with a mass of 4450 megaelectron volts (MeV), 4.74 times the mass of the proton. With nine times more data, they now find in that mass range two nearly overlapping but separate pentaquarks with masses of 4440 MeV and 4457 MeV. They also find a lighter pentaquark at 4312 MeV. Each contains the same set of quarks: charm, anticharm, two ups, and a down. (Previous hints of a pentaquark at 4380 MeV have faded.)

    3
    Pentaquark depiction

    5
    New Large Hadron Collider data reveal that exotic quark quintets, discovered in 2016, are composites of quark-antiquark mesons and three-quark baryons.

    The lightest pentaquark has a mass just below the sum of a particular baryon and meson that together contain the correct quark ingredients. The heavier pentaquarks have masses just below the sum of the same baryon and a related meson with extra internal energy. That suggests each pentaquark is just a baryon bound to a meson, with a tiny bit of mass taken up in binding energy. “This is a no-brainer explanation,” says Marek Karliner, a theorist at Tel Aviv University in Israel.

    The molecular picture also helps explain why the pentaquarks, although fleeting, appear to be more stable than expected, Karliner says. That’s because packaging the charm quark in the baryon and anticharm quark in the meson separates them, keeping them from annihilating each other.

    Other theorists rushed to a similar conclusion when LHCb researchers discussed their results at a conference in La Thuile, Italy, in March. For example, within a day, Li-Sheng Geng, a theorist at Beihang University in Beijing, and colleagues posted a paper, in press at PRL, that uses the molecular picture to predict the existence of four more pentaquarks that should be within LHCb’s reach.

    But the bag-of-quarks picture is not dead. Pentaquarks should occasionally form when protons are bombarded with gamma ray photons, as physicists at Thomas Jefferson National Accelerator Facility in Newport News, Virginia, are trying to do. But they have yet to spot any pentaquarks. That undermines the molecular picture because it predicts higher rates for such photoproduction than the bag-of-quarks model does, says Ahmed Ali, a theorist at DESY, the German accelerator laboratory in Hamburg. “They are already almost excluding the molecular interpretation,” he says. Others say it’s too early to draw such conclusions.

    The structure of pentaquarks isn’t necessarily an either/or proposition, notes Feng-Kun Guo, a theorist at the Chinese Academy of Sciences in Beijing. Quantum mechanics allows a tiny object to be both a particle and a wave, or to be in two places at once. Similarly, a pentaquark could have both structures simultaneously. “It’s just a question of which one is dominant,” Guo says.

    Regardless of the binding mechanism, the new pentaquarks are exciting because they suggest the existence of a whole new family of such particles, Karliner says. “It’s like a whole new periodic table.”

    See the full article here .


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  • richardmitnick 12:58 pm on April 11, 2019 Permalink | Reply
    Tags: "LHCb results add clues to pentaquark mystery", , , CERN LHCb, , , ,   

    From Symmetry: “LHCb results add clues to pentaquark mystery” 

    Symmetry Mag
    From Symmetry

    04/11/19
    Sarah Charley

    A re-examination of a particle discovered in 2015 has scientists debating its true identity.

    Syracuse professor Tomasz Skwarnicki has been a physicist for 30 years. He and his collaborators have measured rare processes and even discovered new particles. But he says their recent re-examination of a particle they discovered in 2015 was one the few analyses that made him exclaim, “Oh my gosh.”

    CERN LHCb Pentaquark mystery

    Skwarnicki has been working on the LHCb experiment at CERN for more than a decade. He uses the collisions generated by the Large Hadron Collider to search for exotic combinations of quarks.

    LHC

    CERN map


    CERN LHC Tunnel

    CERN LHC particles

    LHCb
    CERN LHCb New II

    Quarks are fundamental particles that bond together to form hadrons—the most common ones being the protons and neutrons found in the atoms that make up almost everything around us.

    For decades scientists had only ever seen hadrons containing three quarks, or a quark and an antiquark. Recent observations of particles made from four and five quarks have begun to challenge this paradigm. A question that remains about these exotic particles, however, is how the quarks are structured within them.

    This interest in how quarks bond stems from the study of a fundamental property of quarks called color charge.

    A splash of color

    Color charge is similar to the electric charge in that it induces an attractive force between particles. Just as magnets with opposite electromagnetic charges stick together, quarks with different color charges stick together. There are three possible color charges: red, blue and green. (And for antiquarks, anti-red, anti-blue and anti-green.)

    It’s not a coincidence that quarks prefer to bond into groups of two and three. In nature, scientists have found only color-neutral objects. They have found hadrons made up of a quark and an antiquark of opposite color charges (for example, red and anti-red), and hadrons made up of three quarks of different color charges (a red quark, a blue quark and a green quark, which also neutralize one another).

    For decades scientists have been looking for new kinds of quark combinations that break this mold—specifically, two matter-quarks bound together into a diquark.

    Because the force between color-charged quarks is orders of magnitude stronger than that of an electric field, most experts think that only hadrons which are completely color neutral are possible. But some theorists hold out hope that with the right combination, pockets that maintain a non-neutral color charge could momentarily exist.

    If they could find a combination of quarks that are not completely color-neutral, “it would open a new world,” says Ahmed Ali, a theoretical physicist at DESY.

    And if they could find a way to harness that charge, he says, “the implications could be far-reaching.” The last time scientists figured out how to separate the charges fundamental particles, the result was electricity.

    Scientists think it’s physically impossible to isolate a non-neutral quark cluster. But some hope that in the collisions generated by the LHC, one of these theoretical diquark combinations could momentarily manifest itself. “Finding that experimentally would be a breakthrough,” Ali says.

    Scientists have made some promising observations that show complex combinations of quarks. Since 2003, numerous experiments have observed particles that appear to be made up of four quarks. And in 2015, LHCb announced the discovery of the first particle made up of five quarks—a pentaquark.

    But this latest analysis by the LHCb collaboration raises questions about the identity of this pentaquark—and may have taken scientists back to square one in the search for a particle that could shed light on questions about color.

    Seeing triple

    The first time Skwarnicki saw LHCb’s pentaquark, it appeared as a large and broad bump that unexpectedly showed up in data from collisions that produced protons and particles called J-psi. It wasn’t especially clear, he says: “It was like looking at an image that was far away and out of focus.”

    This year, Skwarnicki and his colleagues redid the analysis using 10 times as much data, and the difference was striking. “I was the first person to see the data,” he says. “It was beautiful.”

    And startling. Rather than a single bump, Skwarnicki was suddenly looking at three: three distinct pentaquarks. “The peaks were so sharp and narrow,” he says. “Each pentaquark has the same quark content, but they are in different quantum states, which gives them different masses.”

    The new result has reignited a debate about what pentaquarks actually look like. “The key question is how the quarks organize themselves,” Ali says.

    “There is a certain latent—but not so latent—competition between the different theoretical camps.”

    The atom camp

    Theorist Marek Karliner at Tel Aviv University, and his colleague Jon Rosner at the University of Chicago, were not surprised at the appearance of three separate pentaquarks.

    “The three masses just happen to sit right where you would expect them,” Karliner says.

    That is, right where you would expect them if the pentaquark isn’t a tightly bound pentaquark, but rather a new type of atomic nucleus, formed from two well-understood, color-neutral hadrons—one made up of two quarks, and one made up of three.

    Their reasoning? Simple addition. “We expect the mass of a nucleus to be very close to the sum of its constituent parts,” he says.

    The mass of the lightest pentaquark is suspiciously close to the combined masses of a two-quark particle called a D-meson and a three-quark particle called a Sigma-C baryon. The heavier two pentaquarks could be made of the same two particles, but with their internal quarks misaligned—a configuration that slightly bumps up their energy and therefore bumps up the overall mass of the pentaquark.

    Another feature that jumped out at Karliner is the lifetime of these particles. In this nuclear interpretation of the pentaquark, the two clusters of quarks are distinct and feel only a weak pull towards each other, forming a new type of meta-stable atomic nucleus.

    “They are long-lived compared to what we normally observe in composite unstable states made out of quarks,” Karliner says. “In the nuclear picture, the long lifetime is natural and very easy to understand.”

    3
    Some scientists think a tightly bound pentaquark could have pockets of non-neutral color charge. CERN

    The clustered quark camp

    To put it simply, Ali finds the nuclear model of the pentaquark a bit disappointing. Even though a molecular pentaquark is still a new discovery, “no new no new color structures are involved,” he says. “They are formed by the recombination of known hadrons.”

    What he really wants to find is evidence of a tightly bound pentaquark, in which the five quarks are held close together by the strong force. Ali suspects that within a true tightly bound pentaquark, the quark colors could mix in a way that would allow for some non-neutral color charge.

    He remains optimistic that more complex quark combinations are possible, he says. “The theory which describes quark behavior is rich, and there are many forms and representations which could still show themselves.”

    The LHC is currently shut down for upgrades that will allow the experiments to collect even more data, letting scientists take a closer look at the pentaquark. “I anticipate that this is not the end of the story,” Ali says. “It’s the beginning. We’re entering a new hadronic world, and I suspect that more objects will be found.”

    See the full article here .


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  • richardmitnick 8:49 am on March 27, 2019 Permalink | Reply
    Tags: "Observation of new pentaquarks.", , , CERN LHCb, , , ,   

    From CERN LHCb: “Observation of new pentaquarks.” 

    Cern New Bloc

    Cern New Particle Event

    From CERN LHCb

    26 March 2019

    5
    LHCb is an experiment set up to explore what happened after the Big Bang that allowed matter to survive and build the Universe we inhabit today

    Today at the Rencontres de Moriond QCD conference, the LHCb collaboration announced the discovery of a new narrow pentaquark particle, Pc(4312)+, decaying to a J//ψ and a proton, with a statistical significance of 7.3 standard deviations. In addition, the Pc(4450)+ pentaquark structure previously reported by LHCb is also confirmed, but a more complex structure consisting of two narrow overlapping peaks, Pc(4440)+ and Pc(4457)+, is now emerging, with the two-peak structure having a statistical significance of 5.4 standard deviations compared to a single-peak hypothesis.

    1
    2

    In the conventional quark model, strongly interacting particles known as the hadrons are formed either from quark-antiquark pairs (mesons) or three quarks (baryons). Particles which cannot be classified within this scheme are referred to as exotic hadrons. In his fundamental 1964 paper [Physical Letters], in which he proposed the quark model, Murray Gell-Mann mentioned the possibility of adding a quark-antiquark pair to a minimal meson or baryon quark configuration. It took 50 years, however, for physicists to obtain unambiguous experimental evidence of the existence of these exotic hadrons. In April 2014 the LHCb collaboration published measurements that demonstrated that the Z(4430)+ particle, first observed by the Belle collaboration, is composed of four quarks (ccdu).

    Belle II KEK High Energy Accelerator Research Organization Tsukuba, Japan

    Then, a major turning point in exotic baryon spectroscopy was achieved at the Large Hadron Collider in July 2015 when, from an analysis of Run 1 data, the LHCb collaboration reported significant pentaquark structures in the J/ψp mass distribution in Λb0→ J/ψpK- decays.

    Various interpretations of these structures have been proposed, including tightly bound pentaquark states and loosely bound molecular baryon-meson state. These two possibilities are illustrated in the above figure . The color of the central part of each quark is related to the strong interaction color charge, while the external part shows its electric charge. The above image illustrates how the quarks could be tightly bound; the below image shows a loosely bound meson-baryon molecule, in which a meson and a baryon are connected by a residual strong force similar to the one that binds protons and neutrons together within nuclei.

    The analysis presented today used the combined data set collected by the LHCb collaboration in Run 1 (with pp collision energies of 7 and 8TeV, and corresponding to a total integrated luminosity of 3 fb-1) plus Run 2 (6 fb-1 at 13TeV). From this sample, 2.5×105 Λb0→ J/ψpK- decays were selected, nine times more than in the previous Run 1 analysis. The combined data set was analysed in the same way as in the earlier 2015 paper and the parameters of the previously reported Pc(4450)+ and Pc(4380)+ structures were found to be consistent with the original results. However, analysis of the much larger data sample reveals additional peaking structures in the J/ψp invariant mass spectrum which were not visible in the data sample used before. A narrow peak is observed near 4312MeV with a width comparable to the mass resolution. The structure at 4450MeV is now resolved into two narrow peaks, at 4440 and 4457MeV. The images below show the contribution of these pentaquark states to the J/ψp invariant mass spectra.

    3
    4

    The minimal quark content of these states is duucc: four quarks and one antiquark. Since all three states are narrow and lie just below the Σc+D0 and Σc+D*0 thresholds (meaning that their mass is slightly smaller than the sum of the masses of a Σc+ and a D0 or a D*0) by amounts that correspond to plausible hadron-hadron binding energies, they provide a possible experimental evidence for the existence of bound states of a baryon and a meson, as seen in the image above. If this interpretation is correct, the decay channels open to the states would be restricted. Being just below threshold, such states would not decay by “falling apart” into a Σc+ baryon and a D0 or a D*0 meson, but could decay instead to a J/ψ meson and a proton. In the baryon-meson configuration shown in the image, it is not easy for the c and c quarks to come close enough together to form a cc bound state (i.e. a J/ψ meson). Therefore, such a baryon-meson configuration is expected to be relatively stable and would be observed as a narrow peak, following the basic rules of quantum mechanics. A description of these states as tightly bound clusters of five quarks is also plausible. A full understanding of the internal structure of the observed states will require more experimental and theoretical study.

    Read more in the Moriond presentation, in the forthcoming LHCb paper and in the CERN news.

    See the full article here.

    There is further news in the full article.


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  • richardmitnick 2:46 pm on February 26, 2019 Permalink | Reply
    Tags: "LHCb catches fast-spinning charmonium particle", , , CERN LHCb, , , ,   

    From CERN LHCb: “LHCb catches fast-spinning charmonium particle” 

    Cern New Bloc

    Cern New Particle Event

    CERN New Masthead

    From CERN LHCb

    26 February, 2019
    Ana Lopes

    The LHCb collaboration has spotted a new particle. Its mass and other properties place it squarely in the charmonium family that includes the better-known J/ψ particle, which was the first particle containing a “charm quark” to be discovered and won its discoverers a Nobel prize in physics.

    The Nobel Prize in Physics 1976

    2
    Burton Richter. Prize share: 1/2

    3
    Samuel Chao Chung Ting. Prize share: 1/2

    Future studies of the properties of this new charmonium state and its relatives will help physicists better understand the strong force that binds together quarks, among the smallest particles that we know of.

    Charmonium particles are two-quark particles (called mesons) composed of a charm quark and its antimatter counterpart, the charm antiquark. Charm quarks are the third most massive of six quark types. Just like atoms, mesons can be observed in excited states of higher energy, in which the mesons’ constituent quarks move around each other in different configurations. These different arrangements give rise to a gamut of particles with different masses and quantum properties such as spin, which can be thought of as the rotation of a system around its axis.

    Observing such excited states and measuring their properties provides a way of testing models of quantum chromodynamics (QCD), the theory that describes how quarks are stuck together into composite particles. What’s more, knowledge of the full collection of these states helps identify exotic states with more than three quarks, such as tetraquarks, that are also predicted by QCD but have only recently been discovered.

    Tetraquarks-School of Physics and Astronomy – The University of Edinburgh

    If all of the excited states are accounted for, physicists can be more confident that any remaining ones are exotic.

    To catch the new charmonium particle, the LHCb collaboration, one of the four main experiments at the Large Hadron Collider, studied the decays of charmonium states produced in proton–proton collisions into pairs of D mesons, using data recorded between 2011 and 2018; D mesons are the lightest particles containing charm quarks. The collaboration measured the range of masses of the D-meson pairs and then added up how many times they recorded each mass value within the measured range. They then looked for an excess of events, or bump, in this mass distribution, and found a new, narrow peak at a mass that corresponds to a previously unobserved charmonium state dubbed the ψ3(1D). The particle has a spin value of 3, making this the first observation of a spin-3 charmonium state. The high spin value could account for the peak’s narrow width and the fact it has taken so long to find.

    For more information, check the LHCb website.

    See the full article here.


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  • 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.


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    THE FOUR MAJOR PROJECT COLLABORATIONS

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  • 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

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    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 .

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    THE FOUR MAJOR PROJECT COLLABORATIONS

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    CERN ATLAS New

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    CMS
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  • 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.

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    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 .

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  • 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 .

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  • 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.

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    Meet CERN in a variety of places:

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    THE FOUR MAJOR PROJECT COLLABORATIONS

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