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  • richardmitnick 4:10 pm on May 21, 2017 Permalink | Reply
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    From FNAL: Gas microstrip chambers on silicon for DZero in 1995 

    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.

    5.21.17

    1
    Science is beautiful. These are gas microstrip chambers on silicon for DZero in 1995. #Fermilabs50th

    See the full article here .

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    collaborate at Fermilab on experiments at the frontiers of discovery.

     
  • richardmitnick 4:00 pm on June 29, 2016 Permalink | Reply
    Tags: , , , , FNAL DZero, , , Tetraquarks? For real?   

    From Symmetry: “LHCb discovers family of tetraquarks” 

    Symmetry Mag

    Symmetry

    06/29/16
    Sarah Charley

    1
    LHCb. Courtesy of CERN

    Researchers found four new particles made of the same four building blocks.

    It’s quadruplets! Syracuse University researchers on the LHCb experiment confirmed the existence of a new four-quark particle and serendipitously discovered three of its siblings.

    Quarks are the solid scaffolding inside composite particles like protons and neutrons. Normally quarks come in pairs of two or three, but in 2014 LHCb researchers confirmed the existence four-quark particles and, one year later, five-quark particles.

    The particles in this new family were named based on their respective masses, denoted in mega-electronvolts: X(4140), X(4274), X(4500) and X(4700). Each particle contains two charm quarks and two strange quarks arranged in a unique way, making them the first four-quark particles composed entirely of heavy quarks. Researchers also measured each particle’s quantum numbers, which describe their subatomic properties. Theorists will use these new measurements to enhance their understanding of the formation of particles and the fundamental structures of matter.

    “What we have discovered is a unique system,” says Tomasz Skwarnicki, a physics professor at Syracuse University. “We have four exotic particles of the same type; it’s the first time we have seen this and this discovery is already helping us distinguish between the theoretical models.”

    Evidence of the lightest particle in this family of four and a hint of another were first seen by the CDF experiment at the US Department of Energy’s Fermi National Accelerator Lab in 2009.

    FNAL/Tevatron CDF detector
    FNAL/Tevatron machine
    FNAL/Tevatron map
    CDF; Tevatron; Tevtron map

    However, other experiments were unable to confirm this observation until 2012, when the CMS experiment at CERN reported seeing the same particle-like bumps with a much greater statistical certainty.

    CERN/CMS Detector
    CERN/CMS Detector

    Later, the D0 collaboration at Fermilab also reported another observation of this particle.

    FNAL/Tevatron DZero detector
    D0/FNAL

    “It was a long road to get here,” says University of Iowa physicist Kai Yi, who works on both the CDF and CMS experiments. “This has been a collective effort by many complementary experiments. I’m very happy that LHCb has now reconfirmed this particle’s existence and measured its quantum numbers.”

    The US contribution to the LHCb experiment is funded by the National Science Foundation.

    LHCb researcher Thomas Britton performed this analysis as his PhD thesis at Syracuse University.

    “When I first saw the structures jumping out of the data, little did I know this analysis would be such an aporetic saga,” Britton says. “We looked at every known particle and process to make sure these four structures couldn’t be explained by any pre-existing physics. It was like baking a six-dimensional cake with 98 ingredients and no recipe—just a picture of a cake.”

    Even though the four new particles all contain the same quark composition, they each have a unique internal structure, mass and their own sets of quantum numbers. These characteristics are determined by the internal spatial configurations of the quarks.

    “The quarks inside these particles behave like electrons inside atoms,” Skwarnicki says. “They can be ‘excited’ and jump into higher energy orbitals. The energy configuration of the quarks gives each particle its unique mass and identity.”

    According to theoretical predictions, the quarks inside could be tightly bound (like three quarks packed inside a single proton) or loosely bound (like two atoms forming a molecule.) By closely examining each particle’s quantum numbers, scientists were able to narrow down the possible structures.

    “The molecular explanation does not fit with the data,” Skwarnicki says. “But I personally would not conclude that these are definitely tightly bound states of four quarks. It could be possible that these are not even particles. The result could show the complex interplays of known particle pairs flippantly changing their identities.”

    Theorists are currently working on models to explain these new results—be it a family of four new particles or bizarre ripple effects from known particles. Either way, this study will help shape our understanding of the subatomic universe.

    “The huge amount of data generated by the LHC is enabling a resurgence in searches for exotic particles and rare physical phenomena,” Britton says. “There’s so many possible things for us to find and I’m happy to be a part of it.”

    See the full article here .

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    Symmetry is a joint Fermilab/SLAC publication.


     
  • richardmitnick 10:22 am on February 25, 2016 Permalink | Reply
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    From Symmetry: “Fermilab scientists discover new four-flavor particle” 

    Symmetry

    02/25/16
    Leah Hesla

    FNAL Four flavour particle
    FNAL

    Scientists on the DZero collaboration at the U.S. Department of Energy’s Fermilab have discovered a new particle—the latest member to be added to the exotic species of particle known as tetraquarks.

    FNAL DZero
    DZero

    Quarks are point-like particles that typically come in packages of two or three, the most familiar of which are the proton and neutron (each is made of three quarks). There are six types, or [flavours], of quark to choose from: up, down, strange, charm, bottom and top. Each of these also has an antimatter counterpart.

    Over the last 60 years, scientists have observed hundreds of combinations of quark duos and trios.

    In 2008 scientists on the [KEK]Belle experiment in Japan reported the first evidence of quarks hanging out as a foursome, forming a tetraquark.

    KEK Belle detector
    KEK Belle detector

    Since then physicists have glimpsed a handful of different tetraquark candidates, including now the recent discovery by DZero—the first observed to contain four different quark [flavours].

    DZero is one of two experiments at Fermilab’s Tevatron collider.

    FNALTevatron
    FNAL Tevatron machine
    FNAL/Tevatron

    Although the Tevatron was retired in 2011, the experiments continue to analyze billions of previously recorded events from its collisions.

    As is the case with many discoveries, the tetraquark observation came as a surprise when DZero scientists first saw hints in July 2015 of the new particle, called X(5568), named for its mass—5568 megaelectronvolts.

    “At first, we didn’t believe it was a new particle,” says DZero co-spokesperson Dmitri Denisov. “Only after we performed multiple cross-checks did we start to believe that the signal we saw could not be explained by backgrounds or known processes, but was evidence of a new particle.”

    Mesons Baryons Tetraquarks

    And the X(5568) is not just any new tetraquark. While all other observed tetraquarks contain at least two of the same flavor, X(5568) has four different flavors: up, down, strange and bottom.

    “The next question will be to understand how the four quarks are put together,” says DZero co-spokesperson Paul Grannis. “They could all be scrunched together in one tight ball, or they might be one pair of tightly bound quarks that revolves at some distance from the other pair.”

    Four-quark states are rare, and although there’s nothing in nature that forbids the formation of a tetraquark, scientists don’t understand them nearly as well as they do two- and three-quark states.

    This latest discovery comes on the heels of the first observation of a pentaquark—a five-quark particle—announced last year by the LHCb experiment at the Large Hadron Collider.

    CERN LHCb pentaquark
    Pentaquark. CERN LHCb

    Scientists will sharpen their picture of the quark quartet by making measurements of properties such as the ways X(5568) decays or how much it spins on its axis. Like investigations of the tetraquarks that came before it, the studies of the X(5568) will provide another window into the workings of the strong [interaction] that holds these particles together.

    And perhaps the emerging tetraquark species will become an established class in the future, showing themselves to be as numerous as their two- and three-quark siblings.

    “The discovery of a unique member of the tetraquark family with four different quark [flavours] will help theorists develop models that will allow for a deeper understanding of these particles,” says Fermilab Director Nigel Lockyer.

    Seventy-five institutions from 18 countries collaborated on this result from DZero.

    See the full article here .

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    Symmetry is a joint Fermilab/SLAC publication.


     
  • richardmitnick 1:27 pm on December 18, 2014 Permalink | Reply
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    From FNAL:- “Frontier Science Result: DZero Measuring the strange sea with silicon” 

    FNAL Home


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

    Thursday, Dec. 18, 2014
    Leo Bellantoni

    Our last DZero result began like so:

    FNAL DZero
    DZero

    “The parts inside of a proton are called, in a not terribly imaginative terminology, partons. The partons that we tend to think of first and foremost are quarks — two up quarks and a down quark in each proton — but there are other kinds of partons as well.”

    This time, we start in the same place — with those unimaginatively named partons. There are three types.

    The first type comprises those alluded to above: quarks. The two up quarks and one down quark that make up protons are called valence quarks. They determine the electrical charge of the proton. There are six flavors of quark, and all the different combinations of three out of the six correspond to a particle of a specific type, called a baryon. (Well, almost. Top flavored quarks decay so quickly they never form a particle.)

    The second type of parton is the gluon. Gluons hold the quarks inside the proton together and are the mediators of the strong nuclear force. Just as electromagnetic energy comes in point-like units called photons, so energy of the strong nuclear force comes in units of the gluon.

    The third type of parton is the sea quark. A gluon can split into a quark-antiquark pair that exists for a fleetingly short time (10-24 seconds or less) before reforming back into a gluon.

    Sea quarks can be of any flavor. They very often are up or down quarks, just like the valence quarks. But they can also be strange quarks, and strange quarks do not exist as valence quarks in protons. A reaction with a strange quark in the initial state lets you measure these strange sea quarks in proton collisions.

    The reaction involves the collision of a strange sea quark from one proton (or antiproton) with a gluon from an antiproton (or proton) to produce a W boson and a charm quark. The charm quark, when produced with a large momentum transverse to the direction of the initial collision, will produce a narrow spray of particles all moving in roughly the same direction. Such a particle spray is called a jet. Because the charm quark will travel a few millimeters before decaying, the fact that there was a charm quark producing the jet can be inferred using the silicon based microstrip tracking detector at the very center of the DZero detector.

    q
    Top quark and anti top quark pair decaying into jets, visible as collimated collections of particle tracks, and other fermions in the CDF detector at Tevatron.

    FNAL CDF
    CDF

    FNALTevatron
    Tevatron map

    Silicon technology also helps identify jets produced from bottom flavored quarks. In fact, bottom quark jets are easier to find than charm quark jets. Measuring the production of bottom quark jets in events with a W boson provides important information about the nonvalence partons — specifically, gluons — of the proton.

    DZero has recently measured the production of both charm and bottom jets when a W boson is also produced. The new measurement uses more data than earlier analyses, and for the first time, we obtain information about the production (with a W) of charm and bottom jets that are produced with different momenta transverse to the collision axis. How the production varies with the transverse momentum is a valuable measurement tool to understand the various subprocesses at work. This is also the first measurement of charm-W production that relies upon the silicon microstrip tracking technology; previous measurements were based on less effective techniques.

    —Leo Bellantoni

    See the full article here.

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  • richardmitnick 9:34 am on August 28, 2014 Permalink | Reply
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    From Fermilab- “Frontier Science Result: DZero Which way did it go?” 


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

    Thursday, Aug. 28, 2014
    Leo Bellantoni

    In the DZero detector., the direction in which a particle travels — the direction of its momentum — is often easier to measure precisely than its energy or, equivalently, its amount of momentum. This is the key idea behind a recent result.

    Fermilab DZero
    DZero

    Put a flat piece of paper on your desktop. This is what particle physicists call the transverse plane. Now take a pencil and try to balance it on its point on the paper. You don’t have to succeed — just get the idea of something going perpendicularly into the paper. This depicts the proton going into the collision. The antiproton is traveling in the opposite direction, also perpendicular to the paper (excuse me: also perpendicular to the transverse plane). If the two are aligned and strike each other, then there is a collision point in the transverse plane. Pick the pencil up; there might be a little mark left from the pencil tip to show you where the collision happened.

    pro
    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.

    Now let us draw some arrows coming out from that little mark. These are the directions of the particles coming out of the collision. By drawing them on the paper, you draw only two dimensions of the motion — a particle might come out with a motion that is partly upwards and to the left, but on the paper you can only draw it to the left. This is what particle physicists call projecting onto the transverse plane.

    plane

    In the above figure you see a pair of possibilities for the projection onto the transverse plane of two particles from a collision. These two particles are the decay products of a very heavy particle created in the collision. The two particles on the left came from some heavy particle that was clearly headed south according to the compass. The two particles on the right came from a heavy particle that was clearly going nowhere.

    The method of the recent DZero result is a bit more complicated than this, but you have enough to get the basic idea. When a proton and an antiproton collide and produce a Z boson, the Z boson might decay into a pair of muons. The direction of the muons tells us how the Z was moving in the transverse plane. What scientists actually do is take the angle that you see in the figure, which is called the acoplanarity, and apply a certain correction to it to obtain an angle that theorists can predict and experimentalists can measure. More to the point, this angle, called Φ* and invented in part by DZero collaborators, is experimentally a more precise measure of the motion of the Z (as projected onto the transverse plane) than that obtained by measuring just the momentum of the muons in the transverse plane directly.

    The theoretical prediction relies on being able to calculate the effects of the strong nuclear force accurately, and this is a notoriously difficult thing to do. So in this case, comparing the data to the prediction is more a test of our ability to apply the theory than a test of the theory itself.

    DZero finds that the data and the theoretical predictions are in good but not perfect agreement. At Φ* near 30 degrees, the DZero measurement is higher than the prediction, but the prediction is not very definite. Now it is the theorists’ turn to try to improve our ability to apply the theory.

    See the full article here.

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  • richardmitnick 11:34 am on August 21, 2014 Permalink | Reply
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    From Fermilab- “Frontier Science Result: CDF Testing the Higgs boson’s spin and parity with CDF 


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

    Thursday, Aug. 21, 2014
    Tom Junk and Andy Beretvas

    Since the discovery of the Higgs boson at CERN and the three-standard-deviation excess seen by the CDF and DZero experiments here at Fermilab, the experimental community has been focusing on measuring the properties of this new particle. It’s important to determine whether there is just one new particle or if two or more are contributing to the observed data. The answer at the Tevatron could be different from that at the LHC, as the mixture of production mechanisms and the decays contributing to the most sensitive searches are different.

    Fermilab CDF
    CDF

    Fermilab DZero
    DZero

    Fermilab Tevatron
    Tevatron map

    CERN LHC Map
    LHC map

    The Standard Model’s predictions are that the Higgs boson has all the properties of a heavy piece of the vacuum — there’s no electric charge, or any other kind of charge for that matter. It should have no intrinsic spin, and it should look the same when reflected in a mirror. In short, the only non-zero properties it is expected to have are its mass and its interaction rates with other particles.

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

    But what if the particle is not the Standard Model Higgs boson but rather an exotic impostor? A Higgs-like particle could have spin, like a graviton. Or, like a pion,

    pion
    The quark structure of the pion.

    it could be a pseudoscalar — that is, its wave function could change sign (from positive to negative or vice versa) when viewed in a mirror. A group of theorists recently suggested that collisions at the Tevatron producing such Higgs impostors in association with the vector bosons W or Z would have very different measurable properties from those predicted for Standard Model Higgs events. The exotic Higgs bosons would have more kinetic energy on average than their Standard Model twins, and the recoiling vector bosons would also be going faster.

    graphs
    Best-fit signal strengths μ for two cases. Left: the Standard Model versus graviton-like boson (spin 2, positive parity) versus the Standard Model Higgs boson (spin 0, positive parity). Right: the pseudoscalar boson (spin 0, negative parity) versus the Standard Model Higgs boson.

    CDF has adapted its sophisticated searches for the Standard Model Higgs boson in the WH→lνbb, the ZH→llbb, and the VH→METbb modes to search for exotic Higgs bosons — the pseudoscalar boson, labeled 0- in the above figure, and a graviton-like spin-2 boson, labeled 2+. The exotic particles were assumed to be produced either in place of the Standard Model Higgs boson or in addition to it. The figure shows the best-fit signal strength parameters μ, which are scaled to the Standard Model Higgs boson signal strength, for both the 0- and 2+ searches.

    In neither combined search is there any hint of the presence of an exotic Higgs boson, and we observe consistency with the presence of the Standard Model Higgs boson.

    See the full article here.

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  • richardmitnick 1:30 pm on August 14, 2014 Permalink | Reply
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    From Fermilab- “Frontier Science Result: DZero Antimatter and interference” 


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

    Thursday, Aug. 14, 2014
    Leo Bellantoni

    Q: When is 3 plus 4 not 7?

    A: When the 3 and the 4 are in different directions.

    Walk 3 miles due east, say, then 4 miles due north. You are not 7 miles from your starting point; you are 5 miles from your starting point.

    paths
    A particle can take a number of different decay paths to arrive at the same result

    To be fair, telling someone to go “3 miles due east” isn’t the same as saying to go “3 miles.” It also includes the direction “due east.” The reason your final distance from the starting point is less than 7 miles is because you have a 90-degree angle between the two legs of your journey. On the other hand, were the two directions the same, then indeed you would be 7 miles from your starting point.

    A similar thing can happen in the decay of subatomic particles. In the process of calculating the rate of the decay, we compute a thing called the amplitude, which has both a distance and a direction to it. The distance and direction are in an abstract mathematical space rather than on the surface of the Earth, but otherwise it is similar. If there are two ways or paths in which the decay can happen, we add the two amplitudes and take into account the angle between them.

    For example, the Upsilon, Υ, can turn into a photon, which turns into a muon pair; that is one path. The Υ can also turn into a Z boson, which turns into a muon pair; that is another path. To add the two paths correctly, one has to allow for the angle between them in this abstract space. This effect is called interference.

    Some of the differences between matter and antimatter are due to this effect; the angle between two amplitudes for two paths in the decay of a particle and the corresponding angle for the antiparticle can be different. As a result the sum of the two paths for the particle and the antiparticle are different.

    The particle containing a charm quark and a down antiquark is called a D+ meson. It can decay to a kaon, K-, and a pair of pions, π+, through several different paths; two are shown in the figure. In the Standard Model though, all the amplitudes have the same direction, and they add up directly — there is no interference. Accordingly, the expectation is that the D+ meson and its antiparticle, the D-, should decay at the same rate. On the other hand, if there is new physics involved, the amplitudes might be in different directions. Then there could be interference and there could be a difference between decays of the particle and the antiparticle.

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

    The DZero collaboration has measured the difference (called the asymmetry) between the rate of this decay for the D+ and the D-. The result is that the asymmetry is -0.17 ± 0.17 percent. In other words, the asymmetry is indeed zero, allowing for the uncertainty in the measurement. The previous measurement, by the CLEO experiment, had an uncertainty of ± 0.98 percent, and so the DZero result is almost six times more precise. Even with this improved measurement, the Standard Model prediction that 3 + 4 = 7 holds up.

    Fermilab DZero
    DZero at Fermilab

    See the full article here.

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  • richardmitnick 11:22 am on August 7, 2014 Permalink | Reply
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    From Fermilab- “Frontier Science Result: CDF Sifting through the noise to identify single top quarks” 


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

    Thursday, Aug. 7, 2014
    Andy Beretvas

    CDF and DZero discovered the top quark in 1995 from proton-antiproton collisions that produced the top quark together with its antimatter partner, the antitop quark. It took 14 more years for the same collaborations to announce the observation of a top or antitop quark produced without the other. Why the long delay? The main reason has to do with the features of events with only one of the massive quarks. Single-top events are remarkably similar to other more common types of events, and the lack of a clear separation between the single-top signal and other backgrounds made it very difficult to tease it out.

    Fermilab CDF
    Fermilab CDF

    Fermilab DZero
    Fermilab DZero

    Fermilab Tevatron
    Fermilab Tevatron, Home of CDF and DZero

    tq
    A collision event involving top quarks

    In this single-top quark analysis at CDF, we selected events with the following characteristics: a lepton (either an electron or a muon) with high transverse energy; a large imbalance of transverse momentum, which indicates the presence of a neutrino; and two or three jets, at least one of which must originate from a bottom quark. The two main backgrounds are a W boson plus a bottom or charm jet and a W boson plus an up, down or strange jet. Other backgrounds include a Z boson plus jets, top-antitop pairs, two-boson events and events in which a jet has been misidentified as an electron or a muon.

    After applying event selection requirements, we found that the sum of the backgrounds was still about 15 times larger than the signal. Thus, we needed a thorough understanding of the backgrounds to bring the signal to the fore. The solution was to use a set of artificial neural networks. Using a few variables that describe the properties of the events, such as the reconstructed top quark mass and the transverse mass of the reconstructed W boson, we trained the neural networks to separate the signal from the backgrounds. This process is similar to how neurons in our brain work when we engage in pattern recognition. After applying the artificial neural networks in our analysis, we measured a single-top quark cross section of 3.04 +0.57/-0.53 picobarns.

    There are three processes for the production of single top quarks: s-channel, t-channel and associated Wt production. An important feature of this measurement is that we use a next-to-leading-order event generator for the first time to properly include the Wt contribution. In addition to the combined single-top cross section, we also extract separate values for the s-channel process and the sum of the t-channel + Wt processes, as shown in the figure above. The measurements are fully compatible with predictions of the Standard Model.

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

    See the full article here.

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  • richardmitnick 11:47 am on July 31, 2014 Permalink | Reply
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    From Fermilab- “Frontier Science Result: DZero Which Higgs? 


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

    Thursday, July 31, 2014
    Leo Bellantoni

    With all the discussion about “the Higgs,” it is worth remembering that what Peter Higgs (and Robert Brout, François Englert, Gerald Guralnik, Carl Hagen and Tom Kibble) gave us was not at first a particle. Originally, it was a trick.

    Specifically, it was a mathematical trick to solve a particular physics problem — the problem of how to retain a lovely property called gauge invariance and still allow massive particles in the theory. The trick is to add mathematical expressions to the theory that have what are called doublets.

    Now, that trick can be played more than one way. If the trick is played in the simplest way, by adding one doublet, the existence of one and only one new particle is predicted; that is the particle that we usually call “the Higgs.” But there is no particular reason to believe that the simplest way is how nature is playing with us. The next simplest play has two of these doublets and predicts both three new neutral particles and a pair of charged particles. There are many other ways in which nature might be playing its cards.

    One way to figure out what is in this hand of cards is to measure the spin and parity of the Higgs that has been found. The spin of a particle is its intrinsic angular momentum; the parity has to do with how the particle’s interactions will appear if they are viewed in a mirror.

    If there is only one doublet and there is only one Higgs boson, then the spin must be zero and the parity must be even. If there are two doublets and if we have found one of those three neutral particles, then the particle we have found must have a spin of zero but it might not have even parity. It could have an odd parity or be a mixture of even and odd parity. Because the found Higgs decays into a pair of photons, it can not have a spin of one; but a spin of two and a positive parity is possible in some theories with extra dimensions.

    Although the spin and parity are properties of the particle itself and do not depend on what the particle decays into, it is valuable to check that one obtains the same result regardless of what the particle decays into. For this reason, DZero has leveraged the comparative advantage of the Tevatron to set constraints on the spin and parity of the Higgs that has been found in the case where it decays into a pair of bottom quarks.

    Fermilab DZero
    DZero at Fermilab

    Fermilab Tevatron
    Tevatron Campus at Fermilab

    The DZero result suggests that nature is playing the simplest hand — the single-doublet scenario. Comparing odd to even parities for the spin zero case, the odd parity hypothesis is disfavored with 97.6 percent confidence. Comparing spin zero to spin two for the even parity case, the spin two hypothesis is disfavored with 99.0 percent confidence. However, this does not quite prove that there is only one doublet. It is possible to get the same result with two doublets. We shall have to see a few more cards before we know exactly what is in that hand!

    See the full article here.

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  • richardmitnick 2:55 pm on June 19, 2014 Permalink | Reply
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    From Fermilab- “Frontier Science Result: DZero Shedding light on forward muons 


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

    Thursday, June 19, 2014
    Mark Williams

    The DZero experiment, like all such general-purpose particle detectors, is built from a number of distinct subsystems, each designed to perform a specific task. One such component is the muon system, which forms the outermost layer of the detector. Its job is to identify muons from proton-antiproton collisions and measure their trajectories and timing information for use in both the trigger and the subsequent event reconstruction.

    Fermilab DZero
    Fermilb/DZero

    In fact, the muon system is itself divided into two complementary parts: three muon tracking layers and three interleaved layers of muon scintillators. The tracking layers provide precise information about the location of muons as they leave the detector, which is used to build the trajectories. The scintillators measure the times that the muons pass through, with excellent precision of less than 1 nanosecond. Together, this is all the information necessary to fully reconstruct muons in the event. A recent limited-authorship publication describes the performance of the muon scintillator counters in the forward region, demonstrating both the excellent performance and the methods used to monitor the system.

    Each layer of forward muon scintillator counters forms an overlapping (“fish-scaled”) set of aluminum-covered plastic plates, which produce a burst of visible light photons when a muon passes through. This light is then fed into photon counters and converted to an electric current. Because the detection medium is light-based, the information is available very soon after the initial proton-antiproton collision and is hence used to make a decision about whether or not to save (trigger) the event for later use. Triggering is essential to select the roughly 100 events per second that can be saved to tape, out of the several million proton-antiproton collisions in this time.

    The timing information is also essential for identifying muons from cosmic-ray sources, which must be removed from the data sample. If a cosmic muon passes directly through the detector, it can look much like a pair of muons originating from the center. However, it will hit the top of the detector around 30 nanoseconds before the bottom, while a genuine muon pair produced in the collision will arrive at the muon system concurrently. By using appropriate timing windows, cosmic-ray muons can be almost completely eliminated from the data without significant reduction of signal efficiency.

    Scientists measured the performance of the forward muon scintillators regularly using a variety of independent methods. The efficiency of the light collection components was tested on a daily basis using built-in LED sources. The scintillating plastic plates were tested with radioactive beta ray (electron) sources. The overall performance of the full system was also tested using reconstructed muons identified by the muon tracking system. All methods show consistent results: a very slow reduction in the signal sizes over time, expected due to the effects of radiation aging. This slow change was anticipated during the detector design and had no effect on the muon identification efficiency.

    Overall, the detector performed very well during its entire life, with typically over 99.9 percent of counters working during data collection, and its excellent design provided extended spatial coverage and outstanding trigger capabilities.

    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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