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

    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 7:53 pm on August 18, 2015 Permalink | Reply
    Tags: , , , , Pentaquarks   

    From Don Lincoln at FNAL: “Pentaquarks” 

    CERN LHCb NewFNAL II photo

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

    FNAL Don Lincoln
    Don Lincoln

    Scientific research isn’t always simple; in fact, it’s often like rummaging around an unfamiliar room in the dark while wearing a blindfold. Under such conditions, it is inevitable that we have to make guesses about what we encounter. Sometimes those guesses turn out to be right and sometimes they don’t.

    This kind of exploratory research is especially true at the very frontier of human understanding and a recent announcement at the LHC [LHCb Collaboration] about a new form of matter called pentaquarks exemplifies this sort of investigation. The history of the search for pentaquarks involves previous observations that eventually faded under the light of more study. So what’s the deal with this recent announcement? Fermilab’s Dr. Don Lincoln tells us of the history of this interesting possible particle and gives us an idea of what we can expect in the near future.

    See the full article here.

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    Fermilab Campus

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

     
  • richardmitnick 8:21 am on July 29, 2015 Permalink | Reply
    Tags: , , , , , , Pentaquarks   

    From NOVA: “What the Heck is a Pentaquark?” 

    PBS NOVA

    NOVA

    28 Jul 2015
    FNAL Don Lincoln
    Don Lincoln

    What do you get when you combine four quarks and an antiquark?

    If you think this sounds like the opening of a particle physicists’ riddle, you aren’t too far off. Hypothetically, this particular quark combo makes a “pentaquark.” Despite decades of searching, physicists haven’t been able to actually find a pentaquark. Now, though, there’s a hint that two pentaquarks have unexpectedly come out of hiding.

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    Illustration of a possible layout of the quarks in a pentaquark particle such as those discovered at LHCb. © CERN

    If the new result holds up—a big if—the unexpected discovery would add a new species of particle to the standard model’s menagerie. But the measurements, recently announced by the team collaborating on the LHCb experiment, are truly perplexing.

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    LHCb on the LHC at CERN

    While the results were submitted for publication a couple of days ago, the first discussion in a large public conference occurred on July 23 at the 2015 meeting of the high energy physics division of the European Physical Society, where I had the opportunity to hear Sheldon Stone, who led the analysis, talk about the result. It’s certainly a topic of both excited and skeptical discussion here at the conference.

    Pentaquarks were first predicted in 1964 by Murray Gell-man and George Zweig in the separate and competing papers in which they first hypothesized the existence of quarks. (Gell-man’s name “quark” has stood the test of time, while Zweig independently proposed the now-defunct “aces.”) Physicists have looked for pentaquarks for a long time, unsuccessfully. We don’t know why there has been no evidence for their existence for so long. Maybe they don’t exist. Or maybe they do and the LHCb experiment has finally found them.

    Quarks are the building blocks of protons and neutrons and, as far as we know, they are the smallest basic units of matter. Quarks combine with other quarks according to the rules of quantum chromodynamics (QCD), which is the theory describing the behavior of the strong nuclear force, which is the strongest of the known subatomic forces. Pair a quark with an antiquark, and you’ve got a particle called a meson; three quarks make a baryon, like a proton or neutron. The new pentaquark—if it really is a pentaquark—seems to be made up of two up quarks, a down quark, and a charm quark/antiquark pair.

    The announcement is the latest chapter in a somewhat dubious story of now-you-see-now-you-don’t discovery. In 2002, scientists in Japan announced the discovery of a particle with a mass about 1.5 times that of a proton. They called it the Θ+, and argued that it was a kind of pentaquark. This announcement triggered a flurry of searches by other groups of experimenters, with some groups confirming the Θ+ and finding other particles that were claimed to be different pentaquark candidates, while other researchers found no evidence for any new particles at all. The excitement continued for three years until 2005, when the community decided that the original announcement was wrong. The death knell of the Θ+ sounded when a group of scientists at the Thomas Jefferson National Accelerator Facility (TJNAF) in Newport News, Virginia, repeated the initial Japanese measurement with far more data. The TJNAF scientists saw no evidence for the existence of the Θ+, and the community consigned it to the dustbin of history as one of many particle “discoveries” that ultimately didn’t pan out.

    The particles recently announced by the LHCb experiment aren’t the Θ+. Instead, the new particles have a mass of about 4.5 times that of the proton. The LHCb team wasn’t actually searching for pentaquarks when they made their measurements. Instead, they were studying how a particle called the Λb baryon decays. To their surprise, they found that a fraction of the time, some of the “daughter” particles left behind by the decay seemed to be coming from an unknown parent particle. So what the heck was it?

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    The LCHb team found the potential pentaquarks while investigating how a Λb baryon decays into a J/ψ meson and a Λ* baryon, which in turn decays into a K- meson and a proton (p+). In such a complicated decay mode, it is customary to look at the three daughter particles two at a time and calculate what the mass of the parent particle could have made them. In the case of the K- meson and a proton, you’d expect to see that they preferentially came from a particle with a mass of a Λ* baryon. Since the J/ψ and the proton weren’t thought to come from the decay of a single particle, you’d expect to see no particular mass looking special—but, as seen here, the researchers saw that a fraction of the time, these two particles seemed to come from a parent with a specific mass. Could pentaquarks be the culprit? Image adapted by Don Lincoln.

    The LHCb team was unable to reconcile their measurements with any of the known or predicted particles of the Standard Model.

    3
    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.

    They seemed to need something new. After testing out lots of hypotheses, they considered the discredited pentaquarks. (Remember that pentaquarks are a prediction of the theory of QCD, they’ve just never been seen before.) One pentaquark wasn’t enough to fit their data, but two did the trick. When they included two new pentaquark particles in their calculations, the data and theory agreed.

    The two new particles have an unusual amount of quantum mechanical spin, specifically 3/2 and 5/2. (Protons, neutrons and electrons are all spin ½.) Like all particles that are bound by the strong nuclear force and decay under its rules, they live for a very short time, specifically about 10-23 seconds.

    Given the checkered history of previous pentaquark searches, physicists are naturally skeptical. So it is worth dissecting the claim. The first question is whether scientists are confident that they’ve discovered some kind of new particle. Here, the claim is on firmer ground: the two detections have significance of nine and 12 standard deviations respectively. (The usual standard in particle physics to claim the discovery of a phenomenon is five standard deviations, and larger numbers mean more certainty. Nine and 12 are very strong numbers.)

    It’s less certain whether the new particles are really pentaquarks. There are good reasons for skepticism: For one thing, the makeup of the new pentaquarks—two ups, a down, and a charm quark/antiquark pair—seems improbable. It should be easier to make a pentaquark consisting of only up and down quarks, which are lighter than charm quarks, and such a particle has never been discovered. Discovering a charm pentaquark first feels like going fishing and pulling up two sharks and no trout. A second possibility is that the new discovery is actually a sort of “molecule”: a particle called a J/ψ attached to a proton, roughly similar to how a deuteron is a proton and neutron bound together. Both have the same quark content, but only “five things in a bag” qualifies as a “real” pentaquark.

    When I caught up with Sheldon Stone during the coffee break after his talk at the conference, he speculated that the higher mass of the charm quarks could make the resulting pentaquark more stable or perhaps somehow makes this sort of pentaquark more likely to form. He cautioned, however, that this was speculation on his part and more work would be required to substantiate these ideas.

    Theoretical physicists are likewise skeptical. Frank Wilczek, professor of physics at MIT and winner of the Nobel Prize in physics for his contributions to the development of the theory of QCD was excited about the possibility of the existence of the pentaquark, but cautious about the measurement.

    So what will it take for the community to embrace this exciting development? Well, as Carl Sagan is famous for noting, extraordinary claims require extraordinary evidence. It is also true that independent confirmation is key. Accordingly, other LHC experiments will try to repeat the analysis approach reported by the LHCb collaboration in order to see if their measurement can be replicated. In addition, theorists will try to see if they can find a mechanism within QCD that will explain why pentaquarks containing charm quarks are more likely to form than ones with lighter quarks.

    Now, taking a more personal perspective, what do I think? First, Sheldon Stone made a persuasive and thorough case at his talk. I think the LHCb experiment is a world class collaboration, with some of the finest minds on the planet and ample experience in the subject matter. Further, they are well aware of the history of the pentaquark and would not lightly propose this hypothesis without adequate care. However, I am very cautious of claims of this nature, especially without confirmation from other experiments. I think the only sensible approach is to view the claim charitably, but critically. Taking a phrase from President Ronald Reagan, I “trust, but verify.” I think the next few months will be very interesting.

    See the full article here.

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    NOVA is the highest rated science series on television and the most watched documentary series on public television. It is also one of television’s most acclaimed series, having won every major television award, most of them many times over.

     
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