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  • richardmitnick 3:12 pm on August 27, 2014 Permalink | Reply
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    From Quanta: “Quark Quartet Fuels Quantum Feud” 

    Quanta Magazine
    Quanta Magazine

    August 27, 2014
    Natalie Wolchover

    In August 2003, an experiment at the KEKB particle accelerator in Japan found hints of an unexpected particle: A composite of elementary building blocks called quarks, it contained not two quarks like mesons or three like the protons and neutrons that constitute all visible matter, but four — a number that theoretical physicists had come to think the laws of nature did not permit. This candidate “tetraquark” disintegrated so quickly that it seemed a stretch to call it a particle at all. But as similar formations appeared in experiments around the world, they incited a fierce debate among experts about the correct picture of matter at the quantum scale.

    KEKB
    View of the KEKB accelerator beamlines, acceleration cavities, and steering magnets, at the intersection with one of the injection beamlines

    Most believed tetraquarks were a new kind of miniature molecule — essentially, two orbiting mesons, each made of one regular quark and one antimatter quark, or antiquark — while a smaller contingent saw them as stand-alone particles in which the two quarks and two antiquarks overlapped in the same small volume of space.

    “We hate each other,” said Antonio Polosa, a theorist at Sapienza University of Rome who takes the latter stance, chuckling about the rival factions. “We really hate each other.”

    Most critics find the molecular model hard to swallow. “It’s kind of a crystal glass in a nuclear explosion.”

    All parties remained uncertain that tetraquarks were real — until one turned up in data from the Large Hadron Collider, the 17-mile, proton-smashing ring near Geneva.

    CERN LHC Map
    CERN LHC Grand Tunnel
    LHC at CERN

    Detailed measurements reported in June in Physical Review Letters confirm that the particle, which was first detected in 2007 at the accelerator in Japan and designated Z(4430), is unambiguously a tetraquark. Now, the discovery is forcing physicists to extend their simple picture of quark interactions, or finally replace it with a more nuanced understanding.

    And, to mixed reviews, the properties of Z(4430) clearly favor the underdog “diquark model” and the hypothesis that tetraquarks are genuine particles. The existence of such states would suggest a menagerie of exotic “hadrons,” or particles made of quarks, including groupings of more than four. It would also attest to subtle quantum interactions that may shape the cores of hypothetical “quark stars,” the piping hot quark soup thought to have saturated the infant universe, and, closer to home, the proton and neutron building blocks of ordinary matter.

    proton
    The quark structure of the proton. (The color assignment of individual quarks is not important, only that all three colors are present.)

    neutron
    The quark structure of the neutron. (The color assignment of individual quarks is not important, only that all three colors are present.)

    “Other interpretations are really not very tenable, to be honest, with respect to this particle,” said Polosa, one of the originators of the diquark model.

    But advocates of the rival molecular model disagree. To their minds, tetraquarks tell a novel but more conservative story of mesons engaging in chemistry below the ordinary atomic scale, without challenging the dogma that two- and three-quark particles are the only hadrons that exist. Z(4430), a tetraquark that looks unlike any combination of two types of mesons mingling as a molecule, certainly “makes it more difficult,” said Marek Karliner, a particle physicist at Tel Aviv University in Israel. But, he said, the diquark model has troubles of its own.

    With each model able to account for some of the 20-or-so candidate tetraquarks that seem perplexing from the other perspective, a third view has recently gained ground: the belief that any simple model falls short. “These models might all have some aspect of the truth,” said Eric Braaten, a theoretical physicist at Ohio State University, but like the proverbial blind men encountering different parts of an elephant, “they can’t describe the elephant globally.”

    The diquark and molecular models are both attempts to salvage a cartoonish picture of hadrons that worked perfectly, if inexplicably, for the half-century before tetraquarks came along. This “quark model” treats the proton as if it is made of three bloated quarks, each contributing a third to its total mass. However, experiments have shown that the quarks in a proton (and other hadrons) are actually much lighter than their sum, each one mere thousandths of the proton’s mass. The rest of its mass derives from the energy involved in gluing the three quarks together, a stickiness known as the strong force that is conveyed by particles called gluons. “The thing you call the ‘quark’ might have quark-antiquark pairs and glue and all the rest built into it,” explained Thomas Cohen, a physics professor at the University of Maryland.

    The exact structure of hadrons is hidden in the folds of a 40-year-old theory of the strong force called quantum chromodynamics (QCD), an easy-to-write-down but infinitely self-referential and thus unsolvable set of equations. No one understands why QCD’s boundless complexity seems equivalent to the quark model, or in other words, why the dynamic confluence of quarks and gluons known as a proton “somehow behaves as if it’s a simple composite of three particles,” Braaten said. Up to now, all hadrons feigned such simplicity. Tetraquarks, which the renowned theorists Edward Witten and Sidney Coleman mistakenly argued in the 1970s were inconsistent with a simplified analogue of QCD, have turned out to be the first manifestations of the theory that aren’t also captured by the quark model.

    Quantum Chromodynamics

    Quarks have one of three “color charges,” which are analogous to the primary colors red, green and blue. Just as an atom strikes a balance between positive and negative electrical charges, particles made of quarks balance colors to reach a neutral state. In the color analogy, that means combining colors to make white.

    proton
    A PROTON MADE OF THREE QUARKS

    “The embarrassing fact that tetraquarks have been discovered experimentally and weren’t predicted is an indication that we don’t understand QCD as well as we thought we did,” Braaten said.

    Now, rather than abandon the quark model altogether, proponents of the molecular and diquark models hope to extend it to encompass the new discoveries.

    In the spirit of the original model, the two proposed extensions pretend that tetraquarks are foursomes of plump quarks. But they present opposite visions for how these components are arranged. According to both QCD and the quark model, quarks have a property called “color,” and they must enter collective states that are color-neutral. The colors of three quarks cancel one another out inside a proton just as, for example, combining the primary colors red, green and blue makes white. And, as with complementary colors like blue and yellow, quarks pair with antiquarks to form colorless mesons. But how do four quarks achieve color neutrality inside tetraquarks?

    Exotic Compositions

    Tetraquarks are composed of two primary color quarks and two antiquarks, each with one of the complementary colors yellow, magenta or cyan. Scientists are debating how these four particles combine into a color-neutral state.

    quater

    In the molecular model, quark-antiquark pairs form two color-neutral mesons that become weakly linked as a molecule.

    diquark DIQUARK MODEL

    In the diquark model, the particles form quark-quark and antiquark-antiquark pairs, which are forced to combine to balance their color charges.

    In the molecular picture, each of the two quarks pairs with one of the two antiquarks, forming two color-neutral mesons that rendezvous momentarily as a meson molecule before flying apart. But in the diquark model, the quarks form a “diquark” pair, as do the antiquarks; both pairs have net color, so they must fuse as a compact, color-neutral particle for an instant before switching partners and dissociating.

    The molecular model’s key supporting examples have masses very close to the sum of two separate mesons, implying that these mesons have briefly become bound by the smallest drop of glue (changing their combined molecular mass by a hair) before breaking up. The tetraquark signal found in Japan in 2003, designated X(3872) for its unknown identity and measured mass, exhibited just this mass coincidence; it and a smattering of subsequent examples elevated the molecular model to early prominence.

    “The Occam’s razor approach says try the simplest thing,” Karliner said. “If it works, then that’s what it is.”

    But with the recent discovery of Z(4430), the razor has turned against the molecular model. The tetraquark’s mass and other meticulously measured characteristics, such as a property called “spin-parity,” do not match up with those of any two mesons. To its critics, this kills the molecule idea. As for its holdouts, “the importance of the evidence against them hasn’t sunk in yet,” said diquark proponent Richard Lebed of Arizona State University. “There’s a certain resistance, if you’ve spent a long time working on a particular picture, to evidence that it’s not right.”

    Although Lebed finds it mildly perplexing that so many tetraquark candidates have almost exactly the masses of known meson combinations, other diquark supporters call these coincidences unavoidable: With no less than 12 varieties of quarks and antiquarks, some of their four-way combined masses are bound to accidentally fall close to those of meson pairs, they say.

    And, ultimately, most critics find the molecular model hard to swallow. How could a fragile construct of two mesons hold together with so little glue in the middle of a near-light-speed collision of protons at the Large Hadron Collider? “It’s kind of a crystal glass in a nuclear explosion,” Polosa said.

    The diquark picture may be pulling ahead in the aftermath of the Z(4430) discovery, but it too remains incomplete. Recently, adherents like Polosa have been working to round out the model with new symmetry rules that would explain why certain quark combinations seem to be permitted and others not, a pattern that was much better captured by the molecular model. Meanwhile, in a new paper accepted for publication in Physical Review Letters, Lebed and colleagues attempt to explain why the colorful diquark and antidiquark pairs might form in the first place and why they quickly decay. If correct, these rules will influence other predictions about hadron physics, such as whether exotic, quark-filled stars exist. They would also deepen the understanding of familiar hadrons like protons and neutrons.

    But not everyone feels satisfied with embellishing the diquark picture until it works.

    “These models have lots of knobs in them,” Cohen said. “When it gets things right, you declare victory, and when it doesn’t quite get things right, you start turning knobs.” The dispute between the two models, he said, is about figuring out “which one is the better starting point for describing the data reasonably well” — not whether either one is true.

    Physicists like Cohen and Braaten believe the full spectrum of tetraquarks can only be predicted by better approximating the unending equations of QCD, an effort that would also elucidate other unknown features of quarks and gluons, such as their behavior milliseconds after the Big Bang. In a recent paper in Physical Review D, Braaten and colleagues suggested an approximation scheme operable on a supercomputer that they think could work for predicting tetraquarks.

    The advantages of such an approach might be marginal, however. In 40 years, researchers have failed to build abridgments of QCD that fit the data much better than the naive quark model. Many theorists prefer to extend this simple and effective understanding rather than enter a QCD quagmire.

    So the question remains: In future diagrams of tetraquarks, will the quarks be shown hand-in-hand with quarks or with antiquarks?

    “There’s going to be a picture that’s substantially more successful than others once all the data is known,” Lebed said. “But I would not at all be surprised if there were still a few nagging mysteries.”

    See the full article here.

    Formerly known as Simons Science News, Quanta Magazine is an editorially independent online publication launched by the Simons Foundation to enhance public understanding of science. Why Quanta? Albert Einstein called photons “quanta of light.” Our goal is to “illuminate science.” At Quanta Magazine, scientific accuracy is every bit as important as telling a good story. All of our articles are meticulously researched, reported, edited, copy-edited and fact-checked.

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  • richardmitnick 9:27 am on July 24, 2014 Permalink | Reply
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    From Fermilab: Searching for boosted tops 


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

    Thursday, July 24, 2014
    Pekka Sinervo and Andy Beretvas

    At CDF, protons of energy 1 TeV, or 1 trillion electronvolts, collided with antiprotons of equal energy.

    proton
    The quark structure of the proton. (The color assignment of individual quarks is not important, only that all three colors are present.)

    anti
    The quark structure of the antiproton.

    Fermilab CDF
    CDF

    In many of these events, we observe a phenomenon called a jet. A jet is a spray of particles all moving in the same direction and typically originating from a practically massless subatomic particle, which is why it is also expected to have a low mass. It is fascinating that in some cases these jets have masses that are a substantial fraction of their energy.

    Scientists have studied events in which a very large fraction — at least 40 percent — of the collision energy is transformed into just two such jets. Based on the internal structure of these jets, we have found that they appear mostly to come from very energetic quarks.

    There are six different flavors of quarks, with five of the six having masses that are small compared to the masses of the jets we see in these two-jet (or “dijet”) events.

    quarks
    This plot shows the mjet1 versus mjet2 distribution for the data taken in this experiment.

    If these jets originate from the lighter quarks, then we would expect to see a high occurrence of jets with low masses. The above figure plots the masses of one jet against the other, and indeed we see that most of the events in our sample have two jets where each has a mass between 40 and 60 GeV/c2, or between 50 and 70 proton masses. This amount of mass is consistent with predictions of quantum chromodynamics, the theory describing the strong interaction.

    But what if some of these massive two-jet events were really coming from the production of the super-massive top quark, which has a mass of 173.34 ± 0.76 GeV/c2? We then would expect to see a cluster of events in which both jets had masses between about 140 and 200 GeV/c2. Although there are roughly 30 such events in our data, as seen in the figure, it is only slightly more than we might have expected from the very occasional production of two very massive jets from the lighter quarks.

    top
    A collision event involving top quarks

    We can use these data to set an upper limit on the rate of top quarks being produced at these very high energies at about 40 femtobarns, or no more often than about one collision in every trillion. Our current understanding of the strong interactions is that the expected rate of top quark production corresponding to two-jet events is about 5 femtobarns.

    See the full article here.

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    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.


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  • richardmitnick 10:02 am on July 23, 2014 Permalink | Reply
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    From Caltech: “50 Years of Quarks” 

    Caltech Logo
    Caltech

    07/22/2014
    A Milestone in Physics

    Douglas Smith

    Caltech’s Murray Gell-Mann simplified the world of particle physics in 1964 by standing it on its head. He theorized that protons—subatomic particles as solid as billiard balls and as stable as the universe—were actually cobbled together from bizarre entities, dubbed “quarks,” whose properties are unlike anything seen in our world. Unlike protons, quarks cannot be separated from their fellows and studied in isolation; despite this, our understanding of the universe is built on their amply documented existence.

    proton
    he quark structure of the proton. (The color assignment of individual quarks is not important, only that all three colors are present.)

    mgm
    Nobel Laureate Murray Gell-Mann

    These days, the subatomic particle catalog has hundreds of entries. Back in the 1920s, there were only two—the massive proton, which had a charge of +1 and was found in the atom’s nucleus; and the electron, which had very little mass, a charge of –1, and orbited the nucleus. Every proton occupied one of two possible spin states in relation to the surrounding space. These spins could easily be flipped in a behavior described by a mathematical construct called the SU(2) symmetry group. “Quantum spin states do not have a familiar analog in everyday experience,” says Caltech’s Steven Frautschi, professor of theoretical physics, emeritus. “However, they can be turned into one another by 180-degree rotations in ordinary space, which is what SU(2) does.”

    In 1932, the neutron was discovered.

    neut
    The quark structure of the neutron. (The color assignment of individual quarks is not important, only that all three colors are present.)

    This new particle appeared to be the proton’s close relative—even its mass was the same, to within 0.2 percent—but the neutron had no electric charge. SU(2) symmetry in ordinary space could not account for the neutron’s existence, but quantum mechanic Werner Heisenberg fixed the problem by declaring that the two particles were indeed fraternal twins . . . if you took SU(2) from another point of view. Frautschi explains: “Like rotating a physical object in ordinary space, Heisenberg extended SU(2) by rotating the symmetry group in a ‘space’ that quantum theorists made up.”

    Heisenberg gave his rotation a quantum number, now called isospin, which described the particle’s interaction with the so-called strong nuclear force. (The strong force overcomes the mutual repulsion between positively charged protons, binding them and neutrons to one another and allowing stable atomic nuclei to exist.) The mathematical treatment of isospin in Heisenberg’s theoretical space was identical to that of the proton’s spin in ordinary space, allowing neutrons to turn into protons and vice versa. In the physical world, Heisenberg’s version of SU(2) is like a slowly spinning roulette wheel after the ball has come to rest—if the white ball (a proton) could transmute itself into a black ball (a neutron) and then back again to a white ball once every revolution.

    A comprehensive theory of the strong force was published three years later by Hideki Yukawa of Osaka University. Quantum-mechanical forces need particles to carry them, and Yukawa calculated that the strong-force carriers would be much more massive than electrons but not nearly as massive as protons. Soon after, in 1937, Caltech research fellow Seth Neddermeyer (PhD ’35) and Nobel laureate physics professor Carl Anderson (BS ’27, PhD ’30) stumbled upon a likely candidate: a new particle with about 200 times the electron’s mass and about one-ninth the mass of the proton.

    Although it was widely assumed that Neddermeyer and Anderson had found the force-carrying particles that would prove Yukawa’s theory, the paper announcing the discovery merely described them as “higher mass states of ordinary electrons.” This proved to be the case—the new particles, now called muons, did not behave as Yukawa had predicted but instead behaved exactly like electrons. This offered the first inkling that otherwise identical particles came in multigenerational “families” of very different masses.

    The search for Yukawa’s strong-force carriers did not bear fruit until 1947, when particles dubbed pions finally turned up…

    pion
    The quark structure of the pion.

    …as did kaons, the massive second-generation members of the pion family. These kaons, however, were oddly long-lived, lasting a quadrillion times longer than expected. (“Long-lived” is relative, as the average kaon decayed into other particles in less than a millionth of a second.)

    Then, in 1953, Murray Gell-Mann, then at the University of Chicago, and Kazuhiko Nishijima (also at Osaka University) independently demystified the kaons’ strange longevity by proposing yet another new quantum number to explain it. This number, imaginatively called “strangeness,” permits particles possessing it to decay—but only by shedding one strangeness unit at a time. This relatively slow process created stepwise cascades of successively less-strange particles, ultimately ending in particles whose strangeness is zero.

    Unfortunately, strangeness and SU(2) did not mesh mathematically. The theorists remained at an impasse; meanwhile, the experimentalists built ever-more-powerful machines that created ever-more-massive, ever-more-exotic particles whose ever-briefer existences could only be inferred by working backward from the collections of mundane particles into which they decayed.

    The mushrooming catalog of discoveries defied all attempts at organization until 1961, when Gell-Mann—who had moved to Caltech in 1955—and Israeli physicist Yuval Ne’eman independently proposed sorting particles into mini-periodic tables organized by electric charge and strangeness number. Gell-Mann dubbed his version the “Eightfold Way,” after Buddhism’s Eightfold Path to enlightenment, because the tables tended to contain eight members each.

    The Eightfold Way brought physicists full circle, as it proved to be a rotating SU(3) symmetry group. Just as charge had driven the isospin axis in Heisenberg’s SU(2) symmetry, strangeness provided a second, perpendicular rotation. In other words, SU(2) spun only around the y axis, as it were, but SU(3) spun on both the x and y axes simultaneously. It was as if the roulette wheel had morphed into a globe spinning around the poles while the polar axis itself spun around two points on the equator. Relationships between particles could be represented as rotations in isospin, in strangeness, or in both.

    Although the Eightfold Way solved one problem, it created another. Whereas SU(2) manifests itself through doublets—the proton-neutron dichotomy—SU(3)’s hallmark is the triplet. “Nature is likely to use this fundamental representation,” says physics professor Frautschi, “but there was no sign of triplets in the data.” Triplets could be conjured into existence, however, if the rock-solid proton could be broken apart. In that case, SU(3)’s fundamental triplet could be a menu of three hypothetical entities, each with its own unique set of quantum numbers.

    If the menu choices were truly independent—much like allowing a diner to order an enchilada with all beans and no rice on the side, for example—a fundamental triplet offered enough possibilities to build every massive particle known, and then some. Intermediate-mass pions and kaons would contain two menu selections; protons, neutrons, and a slew of more massive particles would be three-item combos. “It’s all about making patterns,” Frautschi explains. “You write down sets of quantum numbers, add them up, and see what fits.”

    However, the numbers refused to add up. Both the two-piece kaon and a three-piece particle called the sigma came in positive, negative, and electrically neutral versions. But if the only charges available to the triplet’s members were –1, 0, and +1, no conceivable combination of choices allowed all the other quantum numbers to come out right.

    This should have been the end of the story. Robert Millikan, Caltech’s first Nobel laureate, had won his prize for showing that electric charge came only in whole-numbered units. But in 1964, Gell-Mann and George Zweig (PhD ’64) independently flew in the face of all that was known by proposing that the fundamental triplet had one member with a +2/3 charge and two members with charges of –1/3.

    Gell-Mann called the members of his triplet “quarks,” after the sentence “Three quarks for Muster Mark!” in James Joyce’s Finnegan’s Wake. Everything found in the old SU(2) symmetry group could be fashioned from +2/3 “up” quarks and –1/3 “down” quarks, both of which had a strangeness number of zero. A proton was up-up-down, for example; a neutron was down-down-up. The other –1/3 quark had a quantum of strangeness; adding these “strange” quarks to the mix took care of the particles that SU(2) couldn’t handle. Since this proposal was so heretical, Gell-Mann presented quarks as no more than an expedient accounting system, writing, “It is fun to speculate about the way quarks would behave if they were physical particles of finite mass (instead of purely mathematical entities . . . ).”

    Zweig, meanwhile, called his theoretical constructs “aces,” as they were put together into “deuces” and “treys” to make pions and protons. He was also less circumspect than Gell-Mann. “The results . . . seem somewhat miraculous,” Zweig wrote. “Perhaps the model is . . . a rather elaborate mnemonic device. [But] there is also the outside chance that the model is a closer approximation to nature than we may think, and that fractionally charged aces abound within us.” Sadly, Zweig’s paper met a very different fate than Gell-Mann’s. Since Zweig was working as a very junior postdoctoral fellow at CERN, the European Center for Particle Physics, all his manuscripts had to be reviewed by his superiors before publication. The senior staff considered Zweig’s ideas too outré, and his paper got sent to a file room instead of a journal. He returned to Caltech soon after, joining the faculty.

    Gell-Mann went on to win the Nobel Prize for Physics in 1969—although not for the quark model per se, which was still on thin ice. (The very first experiments demonstrating that protons might contain something else had been run at the Stanford Linear Accelerator the preceding year.) Instead, he was cited “for contributions and discoveries concerning the classification of elementary particles and their interactions.”

    Quarks have since been shown to be physical particles with finite masses. The up quark has been found to have about half the mass of the down, while the strange quark has been shown to be some 50 times more massive—a sure sign that it represented a second generation of quarks, just as muons had turned out to be second-generation electrons. In 1974, the other second-generation quark turned up—the “charm” quark—followed three years later by the third-generation “bottom” quark. It then took nearly two decades to find what is called the “top” quark—which, as far as we know, completes the quark family tree.

    Gell-Mann was named the Robert Andrews Millikan Professor of Theoretical Physics in 1967—a fitting irony that the man who showed that fractional electric charges are necessary holds the chair named for the man who showed that electric charge is indivisible.

    Gell-Mann’s paper introducing the quark was all of two pages long; what has been written about quarks since then would fill warehouses. This half-century of discoveries was celebrated at a conference in the 84-year-old Gell-Mann’s honor, hosted by Caltech’s theoretical high-energy physics group in December, 2013 –

    See the full article here.

    The California Institute of Technology (commonly referred to as Caltech) is a private research university located in Pasadena, California, United States. Caltech has six academic divisions with strong emphases on science and engineering. Its 124-acre (50 ha) primary campus is located approximately 11 mi (18 km) northeast of downtown Los Angeles. “The mission of the California Institute of Technology is to expand human knowledge and benefit society through research integrated with education. We investigate the most challenging, fundamental problems in science and technology in a singularly collegial, interdisciplinary atmosphere, while educating outstanding students to become creative members of society.”
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  • richardmitnick 9:26 am on April 25, 2014 Permalink | Reply
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    From Fermilab- “Frontier Science Result: CMS The shape of the jet” 


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

    Friday, April 25, 2014
    Jim Pivarski

    Despite the complexity of particle colliders and the instrumentation needed to analyze their results, the ultimate aim of most particle physics experiments is to understand something simple. At a fundamental level, most natural phenomena turn out to be simple in profound ways. By contrast, our macroscopic world is teeming with complexity: A bucket of water is by far more complex than an electron. The exact way that water sloshes, curdles in turbulent flow and pinches into droplets when it splashes would be difficult to simulate on the world’s biggest supercomputers, even though the basic interactions between individual atoms are pretty well understood.

    jet
    A jet of water sprayed through water loses energy and changes shape, as illustrated by this Jacuzzi jet. CMS scientists studied a similar phenomenon in an exotic liquid of quarks and gluons. No image credit.

    One part of the quantum world has this kind of complexity, however: the strong force that binds quarks. Unlike the electromagnetic force between atoms, the particles that make up the strong force are themselves attracted via the strong force, which begets more strong force. Physicists call them gluons because they make such a sticky mess. Like the bucket of water, the strong force is notoriously difficult to calculate because some of its properties are emergent — they arise from the interplay of many interactions.

    One of these emergent properties is the fact that a lone quark flying away from a collision creates gluons, which create quarks, which create gluons, and becomes a jet of particles flying in roughly the same direction. Another is that if you get enough quarks in a small space (by colliding heavy nuclei), they undergo a phase transition into a new kind of liquid ruled by strong force interactions. Recently, scientists discovered that jets are eaten by the liquid: They are absorbed into the droplet and sometimes disappear entirely.

    To get a more complete picture of this phenomenon, scientists have used the CMS experiment to study an in-between case, jets that are partially but not completely absorbed by the strong-force liquid. Like a hose sprayed through water, this results in misshapen jets. The angles among particles that make up the jet are noticeably wider than usual, and the exact amount of broadening tells us a little more about the nature of this new state of matter.

    See the full article here.

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    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.


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  • richardmitnick 12:20 pm on April 17, 2014 Permalink | Reply
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    From Fermilab- “Frontier Science Result: DZero Most precise single measurement of the top quark mass” 


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

    Thursday, April 17, 2014
    Mark Williams

    Just four weeks ago in this column we featured the world combination of top quark mass measurements, which used as ingredients the set of best measurements from both Tevatron and LHC experiments available in February 2014. Far from indicating that we’ve reached the end of top quark analyses at DZero and CDF, this represents just a single snapshot of a continually developing program. As a perfect illustration, the DZero experiment this week released a new measurement of the top quark mass in the lepton-plus-jets channel, which is the most precise single measurement ever made and which is actually competitive with the world average itself.

    Fermilab Tevatron
    Tevatron at Fermilab

    CERN LHC New
    LHC at CERN

    The lepton-plus-jets channel is probably familiar to regular readers. The analysis reconstructs top quark-antiquark pairs in their decays into four quark jets, a charged lepton (an electron or muon) and missing momentum consistent with an undetected neutrino. This signature is distinctive enough to be distinguished from most backgrounds, but it also provides a sufficiently large sample with which to reach the required statistical precision on the top mass. Effectively, once the sample composition has been determined, the mass of the top (or antitop) quark can be extracted by appropriately adding the energies of the six decay products, which of course is much easier said than done.

    Reaching the desired precision requires pushing our understanding of the DZero detector to its limit. In particular, with four quark jets in the final state, it is crucial to understand the correspondence between the measured signals and the true jet energies. This analysis uses some advanced techniques to help achieve this goal. Since all the top quark decays involve an intermediate W boson, we can use the existing precise measurements of the W boson mass to constrain the properties of the final jets, lepton and neutrino. Doing this provides enough additional information to make an independent jet energy calibration in situ, rather than relying on the existing calibrations, which relate the measured and true jet energies.

    top
    The DZero experiment this week released the single most precise measurement of the top quark mass ever made. This plot shows the favored region for the top mass (the x-axis) and an important experimental parameter kJES (the y-axis) which is extracted simultaneously to improve the overall precision.

    Using this method, the final analysis reports a two-dimensional measurement of the top quark mass and a jet energy scale factor kJES, as shown in the figure above. This method allows every last drop of information to be squeezed from the data and significantly reduces the dominant source of systematic uncertainty on the final measurement.

    The upshot of this two-dimensional approach is a measured top quark mass of 174.98 GeV, with a total uncertainty of just 0.76 GeV, or less than half a percent. For reference, the world combination of the top mass, which does not include this new measurement, also has an uncertainty of 0.76 GeV. It is an impressive achievement that this single measurement can achieve the same precision as the recent worldwide combination and gives a taste of things to come. The era of high-precision top quark measurements is well and truly here.

    See the full article here.

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    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.


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  • richardmitnick 9:06 am on February 11, 2014 Permalink | Reply
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    From Symmetry: “Quarks in the looking glass” 

    February 10, 2014
    Kandice Carter

    A recent experiment at Jefferson Lab probed the mirror symmetry of quarks, determining that one of their intrinsic properties is non-zero—as predicted by the Standard Model.
    sm
    Standard Model of Particle Physics

    From matching wings on butterflies to the repeating six-point pattern of snowflakes, symmetries echo through nature, even down to the smallest building blocks of matter. Since the discovery of quarks, the building blocks of protons and neutrons, physicists have been exploiting those symmetries to study quarks’ intrinsic properties and to uncover what those properties can reveal about the physical laws that govern them.

    A recent experiment carried out at Jefferson Lab has provided a new determination of an intrinsic property of quarks that’s five times more precise than the previous measurement.

    jlab
    Courtesy of Jefferson Lab

    The result has also set new limits, in a way complementary to such as the Large Hadron Collider at CERN, for the energies that researchers would need to access physics beyond the Standard Model. The Standard Model is a well-tested theory that, excluding gravity, describes the subatomic particles and their interactions, and physicists believe that peering beyond the Standard Model may help resolve many unanswered questions about the origins and underlying framework of our universe. The result was published in the February 6 edition of Nature.

    CERN

    CERN LHC New
    LHC

    The experiment probed properties of the mirror symmetry of quarks. In mirror symmetry, the characteristics of an object remain the same even if that object is flipped as though it were reflected in a mirror.

    The mirror symmetry of quarks can be probed by gauging their interactions with other particles through fundamental forces. Three of the four forces that mediate the interactions of quarks with other particles—gravity, electromagnetism and the strong force—are mirror-symmetric. However, the weak force—the fourth force—is not. That means that the intrinsic characteristics of quarks that determine how they interact through the weak force (called the weak couplings) are different from, for example, the electric charge for the electromagnetic force, the color charge for the strong force, and the mass for gravity.

    jlab
    Info-Graphic by: Jefferson Lab

    In Jefferson Lab’s Experimental Hall A, experimenters measured the breaking of the mirror symmetry of quarks through the process of deep-inelastic scattering. A 6.067 billion-electronvolt beam of electrons was sent into deuterium nuclei, the nuclei of an isotope of hydrogen that contain one neutron and one proton each (and thus an equal number of up and down quarks).

    “When it’s deep-inelastic scattering, the momentum carried by the electron goes inside the nucleon and breaks it apart,” says Xiaochao Zheng, an associate professor of physics at the University of Virginia and a spokesperson for the collaboration that conducted the experiment.

    To produce the effect of viewing the quarks through a mirror, half of the electrons sent into the deuterium were set to spin along the direction of their travel (like a right-handed screw), and the other half were set to spin in the opposite direction. Researchers identified about 170,000 million electrons that interacted with quarks in the nuclei through both the electromagnetic and the weak forces over a two-month period of running.

    “This is called an inclusive measurement, but that just means that you only measure the scattered electrons. So, we used both spectrometers, but each detecting electrons independently from the other. The challenging part is to identify the electrons as fast as they come,” Zheng says.

    The experimenters found an asymmetry, or difference, in the number of electrons that interacted with the target when they were spinning in one direction versus the other. This asymmetry is due to the weak force between the electron and quarks in the target. The weak force experienced by quarks has two components. One is analogous to electric charge and has been measured well in previous experiments. The other component, related to the spin of the quark, has been clearly isolated for the first time in the Jefferson Lab experiment.

    Specifically, the present result led to a determination of the effective electron-quark weak coupling combination 2C2u–C2d that is five times more precise than previously determined. This particular coupling describes how much of the mirror-symmetry breaking in the electron-quark interaction originates from quarks’ spin preference in the weak interaction. The new result is the first to show that this combination is non-zero, as predicted by the Standard Model.

    The last experiment to access this coupling combination was E122 at SLAC National Accelerator Laboratory. Data from that experiment were used to establish the newly theorized Standard Model more than 30 years ago.

    The good agreement between the new 2C2u–C2d result and the Standard Model also indicates that experimenters must reach higher energy limits in order to potentially find new interactions beyond the Standard Model with respect to the violation of mirror symmetry due to the spin of the quarks. The new limits, 5.8 and 4.6 trillion electronvolts, are within reach of the Large Hadron Collider at CERN, but the spin feature provided by this experiment cannot be identified cleanly in collider experiments.

    In the meantime, the researchers plan to extend this experiment in the next era of research at Jefferson Lab. In a bid to further refine the knowledge of quarks’ mirror-symmetry breaking, experimenters will use Jefferson Lab’s upgraded accelerator to nearly double the energy of the electron beam, reducing their experimental errors and improving the precision of the measurement by five to 10 times the current value. The experiment will be scheduled following completion of the upgrade in 2017.

    See the full article here.

    Symmetry is a joint Fermilab/SLAC publication.



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  • richardmitnick 9:38 am on April 19, 2013 Permalink | Reply
    Tags: , , , , , Quarks   

    From Fermilab- “Frontier Science Result- CMS The messy strong force” 


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

    Friday, April 19, 2013
    Don Lincoln

    “When scientists explain how interactions occur at colliders like the LHC, they often have to rely on approximate descriptions. For instance, in discussions of the production and decay of a Higgs boson, we often mention that its most likely decay mode is into two bottom quarks. We then draw a simple picture, with a Higgs boson decaying and two quarks flying away from the decay point. This picture is accurate to a point, but beyond that it’s far messier.

    quark
    A simple description of a particular event might be how a Higgs boson (top) decays into a bottom quark-antiquark pair (middle). However the reality is much messier, involving a complex spray of particles. Today’s analysis is a study of the details of how a couple of quarks can turn into a much more complicated collection of particles. No image credit.

    Like all quarks, bottom quarks carry color (the charge of the strong nuclear force) and feel a mutual interaction. Because of the way the strong force works, as the two quarks get farther apart, the force increases, leading to an increase in the energy stored in that force. This concentrated energy eventually results in something akin to a spark, and a gluon is emitted. Since the gluon also carries color, it too experiences a force between itself and the original quarks, and so the process repeats.”

    See the full article here.

    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.


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  • richardmitnick 9:43 am on April 5, 2013 Permalink | Reply
    Tags: , , , , Quarks   

    From JLab via DOE Pulse: “Quarks’ spins dictate their location in the proton” 

    pulse

    Jefferson Lab

    “A successful measurement of the distribution of quarks that make up protons conducted at DOE’s Jefferson Lab has found that a quark’s spin can predict its general location inside the proton. Quarks with spin pointed in the up direction will congregate in the left half of the proton, while down-spinning quarks hang out on the right. The research also confirms that scientists are on track to the first-ever three-dimensional inside view of the proton.

    proton
    The quark structure of the proton. (The color assignment of individual quarks is not important, only that all three colors are present.)

    The proton lies at the heart of every atom that builds our visible universe, yet scientists are still struggling to obtain a detailed picture of how it is composed of its primary building blocks: quarks and gluons. Too small to see with ordinary microscopes, protons and their quarks and gluons are instead illuminated by particle accelerators. At Jefferson Lab, the CEBAF accelerator directs a stream of electrons into protons, and huge detectors then collect information about how the particles interact.

    cebaf

    According to Harut Avakian, a Jefferson Lab staff scientist, these observations have so far revealed important basic information on the proton’s structure, such as the number of quarks and their momentum distribution. This information comes from scattering experiments that detect only whether a quark was hit but do not measure the particles produced from interacting quarks.

    ‘If you sum the momenta of those quarks, it can be compared to the momentum of the proton. What scientists were doing these last 40 years, they were investigating the momentum distribution of quarks along the direction in which the electron looks at it – a one-dimensional picture of the proton,’ he explains.

    Now, he and his colleagues have used a new experimental method that can potentially produce a full three-dimensional view of the proton.”

    See the full article here.

    DOE Pulse highlights work being done at the Department of Energy’s national laboratories. DOE’s laboratories house world-class facilities where more than 30,000 scientists and engineers perform cutting-edge research spanning DOE’s science, energy, National security and environmental quality missions. DOE Pulse is distributed twice each month.

    DOE Banner


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  • richardmitnick 9:25 am on April 5, 2013 Permalink | Reply
    Tags: , , , , , , , Quarks   

    From Fermilab- “Frontier Science Result: CMS CMS observes particle X” 


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

    Friday, April 5, 2013
    Jim Pivarski

    “When the discovery of a new Higgs-like particle was announced last summer, it received a lot of well-deserved media attention. It is less widely known, however, that about a dozen new particles have been discovered in the past 10 years. Why all this lack of excitement? Unlike the Higgs boson, these other new particles are bound states of quarks: the same old particles in new combinations.

    But they should not be so quickly dismissed. These new particles may be made of quarks, but most of them defy conventional explanations of how quarks fit together.

    In the standard theory of quark interactions, quarks come in three types and can only bind together in ways that would result in an equal balance of these types. By analogy with the color wheel, they are called red, green and blue, and a particle like a proton is made of one red, one green and one blue quark, which mix to form white. Antiquarks are yellow, cyan and magenta. You can form a bound state from a yellow antiquark and a blue quark, for instance, since these colors are on opposite sides of the color wheel. See this Physics in a Nutshell for more.

    proton
    A proton, composed of two up quarks and one down quark. (The color assignment of individual quarks is not important, only that all three colors be present.) (Wikipedia)

    white
    This is a nice demonstration of how you get white if you combine red, blue and green. You can try this yourself at home, or watch it demonstrated in a video online.

    Up to 2003, the only combinations of quarks and antiquarks that had ever been seen were red-green-blue particles, yellow-cyan-magenta antiparticles, and quark-antiquark pairs. Since then, a growing number of bound states have been discovered that do not fit this scheme. The first of these, discovered by the Belle experiment in Japan, was the X(3872), named “X” because we do not know what it is, and “3872” for its mass, measured in units of MeV.

    The X(3872) and its companions might be the first examples of four-quark combinations, such as red-cyan-yellow-blue. These combinations are theoretically possible since they mix to form white, but they were not expected to be stable enough to be observed. The X(3872) in particular has a mass that is very close to the sum of two well-known bound states, D0 and D*0, so it might be a bound state of bound states. It has also been suggested that these new states are part-glueball hybrids.

    In a recent experiment, CMS scientists observed the X(3872) with a strong signal and measured several of its properties with higher precision than ever before. Far from the glare of the spotlight, these scientists are working to solve one of nature’s underappreciated mysteries.”

    cms
    In a large sample of proton-proton collisions resulting in muons and pions, a few thousand of them accumulate above the backgrounds with a mass of 3,872 MeV. This is the X(3872).

    See the original article here.

    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.


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