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  • 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” 

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    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?” 


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    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 


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    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” 


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    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? 


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    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 


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    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.

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  • richardmitnick 12:46 pm on March 19, 2014 Permalink | Reply
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    From Fermilab: “International team of LHC and Tevatron scientists announces first joint result” 


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

    Wednesday, March 19, 2014
    No Writer Credit

    Scientists working on the world’s leading particle collider experiments have joined forces, combined their data and produced the first joint result from Fermilab’s Tevatron and CERN’s Large Hadron Collider (LHC), past and current holders of the record for most powerful particle collider on Earth. Scientists from the four experiments involved—ATLAS, CDF, CMS and DZero—announced their joint findings on the mass of the top quark today at the Rencontres de Moriond international physics conference in Italy.

    Fermilab Tevatron
    Fermilab Tevatron

    CERN LHC
    Inside the LHC

    CERN ATLAS New
    CERN ATLAS

    Fermilab CDF
    Fermilab CDF

    CERN CMS New
    CERN CMS

    Fermilab DZero
    Fermilab DZero

    Together the four experiments pooled their data analysis power to arrive at a new world’s best value for the mass of the top quark of 173.34 plus/minus 0.76 GeV/c2.

    Experiments at the LHC at the CERN laboratory in Geneva, Switzerland and the Tevatron collider at Fermilab near Chicago in Illinois, USA are the only ones that have ever seen top quarks—the heaviest elementary particles ever observed. The top quark’s huge mass (more than 100 times that of the proton) makes it one of the most important tools in the physicists’ quest to understand the nature of the universe.

    The new precise value of the top-quark mass will allow scientists to test further the mathematical framework that describes the quantum connections between the top quark, the Higgs particle and the carrier of the electroweak force, the W boson. Theorists will explore how the new, more precise value will change predictions regarding the stability of the Higgs field and its effects on the evolution of the universe. It will also allow scientists to look for inconsistencies in the Standard Model of particle physics – searching for hints of new physics that will lead to a better understanding of the nature of the universe.

    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 combining together of data from CERN and Fermilab to make a precision top quark mass result is a strong indication of its importance to understanding nature,” said Fermilab director Nigel Lockyer. “It’s a great example of the international collaboration in our field.”

    A total of more than six thousand scientists from more than 50 countries participate in the four experimental collaborations. The CDF and DZero experiments discovered the top quark in 1995, and the Tevatron produced about 300,000 top quark events during its 25-year lifetime, completed in 2011. Since it started collider physics operations in 2009, the LHC has produced close to 18 million events with top quarks, making it the world’s leading top quark factory.

    “Collaborative competition is the name of the game,” said CERN’s Director General Rolf Heuer. “Competition between experimental collaborations and labs spurs us on, but collaboration such as this underpins the global particle physics endeavour and is essential in advancing our knowledge of the universe we live in.”

    Each of the four collaborations previously released their individual top-quark mass measurements. Combining them together required close collaboration between the four experiments, understanding in detail each other’s techniques and uncertainties. Each experiment measured the top-quark mass using several different methods by analysing different top quark decay channels, using sophisticated analysis techniques developed and improved over more than 20 years of top quark research beginning at the Tevatron and continuing at the LHC.

    The joint measurement has been submitted to the electronic arXiv and is available at: http://arxiv.org/abs/1403.4427

    See the full article here.

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  • richardmitnick 10:31 am on March 6, 2014 Permalink | Reply
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    From Fermilab- “Frontier Science Result: DZero Angling for new physics with leptons from top quark pairs” 


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    Thursday, March 6, 2014
    Mark Williams

    The Tevatron data set, collected from proton-antiproton collisions, gives access to some unique precision tests of the Standard Model.

    sm

    chart
    In the Standard Model, pairs of top-antitop quarks are produced with a known directional bias in proton-antiproton collisions, with the top (antitop) quark following the (anti)proton direction slightly more than half of the time. By measuring the angular distributions of leptons (muons and electrons) from top quark decays, this dependence can be rigorously tested. This measurement finds good agreement between the data and Standard Model predictions (MC@NLO), as shown by the plot on the left.

    One helpful property of the collision environment is the existence of distinct proton and antiproton directions, allowing the directional preferences of matter and antimatter production to be measured. One can classify particle production as “forward” (for example, positively charged particles following the proton direction and negatively charged particles following the antiproton direction) or “backward” (vice versa). One can then create a class of “forward-backward” asymmetry observables, which can be explored in searches for new physics.

    The DZero collaboration recently published a new measurement of one such directional asymmetry, from top-antitop quark pair production. To first order, the Standard Model predicts top-antitop production to be forward-backward symmetric. However, higher-order effects introduce a small asymmetry, of a few percent, meaning that (anti)top quarks tend to be produced along the (anti)proton beam direction. Other theories beyond the Standard Model, such as so-called axigluon models, can give significant departures from the Standard Model values, generally increasing the asymmetries to larger values.

    For this publication, the top and antitop quarks are both selected in the lepton channel, in particular, in their decays to a muon or electron. Rather than try to infer the direction of the original (anti)top quark, we use the directions of the charged leptons originating from the decay products of the original top and antitop quarks. For this analysis, we looked at original (anti)top quark decaying into either a muon or electron (its charge depending on whether it decayed from a top or an antitop), an undetected neutrino and a b quark jet. This decay pattern is very distinctive, giving a sample signal purity exceeding 80 percent. Both muons and electrons leave extended signatures in the detector, meaning that their trajectories are known with excellent precision, and the correspondence between the lepton and top quark direction is well-understood.

    In fact, this paper measures two separate asymmetry parameters: a single lepton asymmetry (for example, how often the positive lepton follows the proton direction versus the antiproton direction) and a similarly defined dilepton asymmetry. The two quantities are highly correlated, but both their individual values and their ratio are precisely predicted by the Standard Model. After subtracting the contributions from various background processes and accounting for apparent asymmetries caused by detector effects, the final results are found to agree with the Standard Model predictions. Two possible axigluon models in particular are disfavored by the data. Stay tuned for more measurements of similar top quark asymmetries from DZero in the coming weeks.

    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 1:32 pm on February 24, 2014 Permalink | Reply
    Tags: , , , , Fermilab DZero, , , ,   

    From Fermilab: “Scientists complete the top quark puzzle” 


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

    Monday, Feb. 24, 2014
    Andre Salles, Fermilab Office of Communication: asalles@fnal.gov, 630-840-6733

    Scientists on the CDF and DZero experiments at the U.S. Department of Energy’s Fermi National Accelerator Laboratory have announced that they have found the final predicted way of creating a top quark, completing a picture of this particle nearly 20 years in the making.

    two
    Matteo Cremonesi, left, of the University of Oxford and the CDF collaboration and Reinhard Schweinhorst of Michigan State University and the DZero collaboration present their joint discovery at a forum at Fermilab on Friday, Feb. 21. The two collaborations have observed the production of single top quarks in the s-channel, as seen in data collected from the Tevatron. Photo: Cindy Arnold

    Fermilab DZero
    DZero

    Fermilab CDF
    CDF

    Tevatron
    Tevatron

    The two collaborations jointly announced on Friday, Feb. 21, that they had observed one of the rarest methods of producing the elementary particle — creating a single top quark through the weak nuclear force, in what is called the s-channel. For this analysis, scientists from the CDF and DZero collaborations sifted through data from more than 500 trillion proton-antiproton collisions produced by the Tevatron from 2001 to 2011. They identified about 40 particle collisions in which the weak nuclear force produced single top quarks in conjunction with single bottom quarks.

    Top quarks are the heaviest and among the most puzzling elementary particles. They weigh even more than the Higgs boson — as much as an atom of gold — and only two machines have ever produced them: Fermilab’s Tevatron and the Large Hadron Collider at CERN. There are several ways to produce them, as predicted by the theoretical framework known as the Standard Model, and the most common one was the first one discovered: a collision in which the strong nuclear force creates a pair consisting of a top quark and its antimatter cousin, the anti-top quark.

    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.

    Collisions that produce a single top quark through the weak nuclear force are rarer, and the process scientists on the Tevatron experiments have just announced is the most challenging of these to detect. This method of producing single top quarks is among the rarest interactions allowed by the laws of physics. The detection of this process was one of the ultimate goals of the Tevatron, which for 25 years was the most powerful particle collider in the world.

    “This is an important discovery that provides a valuable addition to the picture of the Standard Model universe,” said James Siegrist, DOE associate director of science for high energy physics. “It completes a portrait of one of the fundamental particles of our universe by showing us one of the rarest ways to create them.”

    Searching for single top quarks is like looking for a needle in billions of haystacks. Only one in every 50 billion Tevatron collisions produced a single s-channel top quark, and the CDF and DZero collaborations only selected a small fraction of those to separate them from background, which is why the number of observed occurrences of this particular channel is so small. However, the statistical significance of the CDF and DZero data exceeds that required to claim a discovery.

    “Kudos to the CDF and DZero collaborations for their work in discovering this process,” said Saul Gonzalez, program director for the National Science Foundation. “Researchers from around the world, including dozens of universities in the United States, contributed to this important find.”

    The CDF and DZero experiments first observed particle collisions that created single top quarks through a different process of the weak nuclear force in 2009. This observation was later confirmed by scientists using the Large Hadron Collider.

    Scientists from 27 countries collaborated on the Tevatron CDF and DZero experiments and continue to study the reams of data produced during the collider’s run, using ever more sophisticated techniques and computing methods.

    “I’m pleased that the CDF and DZero collaborations have brought their study of the top quark full circle,” said Fermilab Director Nigel Lockyer. “The legacy of the Tevatron is indelible, and this discovery makes the breadth of that research even more remarkable.”

    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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