Tagged: FNAL CDF Toggle Comment Threads | Keyboard Shortcuts

  • richardmitnick 4:13 pm on September 13, 2016 Permalink | Reply
    Tags: , , D+ mesons, FNAL CDF, , Strong interaction   

    From FNAL: “CDF can’t stop being charming” 

    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.

    September 8, 2016
    Jeffrey Appel

    FNAL/Tevatron map
    FNAL/Tevatron map

    FNAL/Tevatron CDF detector
    FNAL/Tevatron CDF detector

    Good news: there is a theory to describe the strong interaction, the interactions that bind the constituents of protons and neutrons together and create the strong force. Bad news: Calculations using the theory can be made in only a limited selection of natural phenomena.

    Quantitative predictions for interactions beyond that subset depend on measurements. This can be either for direct use or to help guide the theory about the inputs used in calculations, such as the distributions of the quark and gluon constituents inside protons and neutrons. Using the production of particles containing heavy charm and bottom quarks helps especially with gluon distributions.

    CDF is now reporting new measurements of the rate of production at the Tevatron of D+ mesons, which contain charm quarks. Furthermore, the new measurements are made in the region where the D+ mesons have the smallest momentum transverse to the incident beams. This is the region that is the hardest to calculate using the theory of strong interactions and has never been explored in proton-antiproton collisions.

    1
    This plot shows the measures, in bins of momentum transverse to incident protons, of the average probability of producing a D+ meson at the Tevatron. Shown as bands are the averages predicted in the same bins by the latest theoretical calculations.

    To probe such small transverse momenta, CDF physicists examined all types of interactions of the incoming protons and antiprotons, not just those selected to study rare occurrences.

    The results of this new analysis appear in the figure. The measurements lie within the band of uncertainty of the theoretical predictions. Using the results here, theorists can reduce the size of the band of uncertainty. They might also be able to improve the general trend of the predictions to agree better with the trends in the measurements.

    This measurement is an example of CDF’s continuing effort to produce unique and useful results that complement and supplement those of the LHC. These help improve our understanding of the fundamental forces of nature.

    Learn more.

    See the full article here .

    Please help promote STEM in your local schools.

    STEM Icon

    Stem Education Coalition

    FNAL Icon
    Fermilab Campus

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

     
  • richardmitnick 4:00 pm on June 29, 2016 Permalink | Reply
    Tags: , , , FNAL CDF, , , , 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 .

    Please help promote STEM in your local schools.

    STEM Icon

    Stem Education Coalition

    Symmetry is a joint Fermilab/SLAC publication.


     
  • richardmitnick 10:31 am on September 24, 2015 Permalink | Reply
    Tags: , , FNAL CDF, , ,   

    From FNAL: “Frontier Science Result: CDF More than expected” 

    FNAL II photo

    [I know that this article is not for the technically feint of heart. I cannot claim to understand it. I present it to show that the Tevatron produced data which is still being sifted today and which remains relevant, in spite of the move of HEP to the Large Hadron Collider at CERN.]

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

    Sept. 24, 2015
    Andy Beretvas

    Temp 1
    This plot shows the invariant-mass distribution of the Bc+ → J/ψμ+ candidate events using the full CDF data sample with a Monte Carlo-simulated signal sample. The calculated backgrounds are superimposed

    In 1998 CDF was the first to observe the Bc+ meson, which consists of two quarks: an antibottom quark and a charmed quark. The discovery consisted of a measurement involving approximately 20 decays in which the decay products were a J/ψ, a charged lepton (muon or electron) and an unobserved neutrino.

    FNAL CDF
    CDF

    Using the full Tevatron Run II data set, we now observe approximately 740 events in the muon decay mode. CDF looked for a signature of three muons, the mass of two oppositely charged muons being consistent with that of the J/ψ particle. This larger data set allows us to make the first measurement of the production cross section of the Bc+ meson.

    FNAL Tevatron
    Tevatron map

    One of the principal challenges in the analysis was the determination of the backgrounds, which are shown in the above figure. In the largest background, the J/ψ is correctly identified, but the third muon is misidentified as a pion, kaon or proton. Of the 1,370 Bc+ candidates, 630 are identified as being background.

    In order to minimize the error, we compared our measurement to that of a decay that is already well measured (B+ → J/ψ + K+). The cross section for B+ is 2.78 ± 0.24 microbarns for conditions very similar to our measurement of the Bc+. Using well-known properties of the B+ decay, we find the final cross section for Bc+ production to be 29 ± 4 nanobarns.

    Our result is higher than the theory expectation (by two standard deviations), but the theory calculation was done 10 years ago (kT factorization). Measurements at the LHC collider, where the cross sections should be many times larger, could resolve this problem in our understanding of a meson that is both beautiful and charmed.

    CERN LHC Map
    CERN LHC Grand Tunnel
    CERN LHC particles
    LHC at CERN

    CDF scientists performed a job well done in determining the background, a difficult, interesting challenge.

    Learn more.

    This is my last Frontier Science Result for CDF. I’d like to thank my CDF colleagues for writing so many interesting and important physics papers that were the subject of this column. Finally, Leah Hesla deserves special praise for her wonderful job of editing.

    Fermilab Leah Hesla
    Leah Hesla

    See the full article here .

    Please help promote STEM in your local schools.

    STEM Icon

    Stem Education Coalition

    Fermilab Campus

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

     
  • richardmitnick 9:37 am on August 27, 2015 Permalink | Reply
    Tags: , , FNAL CDF, ,   

    From FNAL- “Frontier Science Result: CDF Never alone” 

    FNAL II photo

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

    Aug. 27, 2015
    Matthew Jones

    2

    1
    The top plot shows the fraction of charged particles produced around a Ds+ meson that are kaons as a function of the particle transverse momentum. The bottom plot shows the fraction produced around a D+ meson. A larger kaon fraction is observed in association with Ds+ production because the kaon contains the strange quark produced in association with the antistrange quark found in the Ds+ meson. The pair of strange quarks is created as the gluon string breaks at the end closest to the heavy quark.

    When produced in high-energy collisions, quarks are never observed in isolation as free particles. Instead, all quarks remain connected to other fundamental particles produced in a collision by a “string” of gluons.

    At low energies, these gluons bind quarks and antiquarks together to form stable mesons. But at higher energies, the string can break and reconnect to new quark-antiquark pairs that are created out of the energy stored in the stretched string.

    We can watch this process in action by studying bottom or charm quarks, which are initially produced in proton-antiproton collisions. The bottom and charm quarks can ultimately be found inside a heavy meson, such as a B+ or D+, respectively. But once the quark is bound inside one of these particles, what happens to the rest of its string?

    Scientists have tuned models to describe the average properties of the mesons created in the fragmentation process, but it would be interesting to watch what happens to the end of the string that remains immediately after the part connected to the heavy quark is broken.

    Recently, the CDF experiment did exactly this by looking at the properties of kaons produced in association with Ds+ mesons.

    FNAL CDF
    CDF

    In this case, when a gluon string breaks, the strange quark in a K- is produced at the same time as the antistrange quark needed to form the Ds+ meson.

    Kaons produced in this way were shown to have distinctly different properties when compared to kaons produced in association with D+ mesons, which instead contain an antidown quark, consistent with fragmentation models.

    See the full article here.

    Please help promote STEM in your local schools.

    STEM Icon

    Stem Education Coalition

    Fermilab Campus

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

     
  • richardmitnick 9:38 am on April 9, 2015 Permalink | Reply
    Tags: , , FNAL CDF, , ,   

    From FNAL- “Frontier Science Result: CDF – Happy hunting grounds 

    FNAL Home

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

    April 9, 2015
    Fabrizio Margaroli and Andy Beretvas

    1
    This artistic view of a Feynman diagram shows the process of proton colliding with an antiproton, producing a W’, which then decays into a top quark and an antibottom quark.

    We understand nature in terms of elementary particles interacting through a set of well-known forces, which are mediated by other particles. These are the graviton (mediator of gravity), the photon (mediator of electromagnetism), the gluon (mediator of the strong force), the W and Z bosons (mediators of the weak force) and the Higgs boson. We produce and detect these particles (except the graviton) in large numbers at colliders around the world.

    But is that all the universe is made of — a handful of different types of particles? We have good reasons to believe that this is not the case. New forces can exist, and the corresponding mediating particles could be seen at colliders. However, such particles have been hunted extensively at the Large Hadron Collider without success so far.

    CERN LHC Map
    CERN LHC Grand Tunnel
    CERN LHC particles
    LHC at CERN

    If new forces are hiding so well from physicists’ determination to discover them, either they would have to be mediated by very massive bosons or these bosons would have to interact very weakly with ordinary stuff.

    The W and Z boson serve as a good model for this kind of exotic stuff: In fact they are both very heavy compared to their peers and interact weakly with ordinary matter. They live very shortly before decaying into more “mundane” particles, most of the time quarks. If new forces were to exist with such properties, then the LHC would not be the best hunting ground because of its enormous production rate of quarks from ordinary forces.

    A new analysis of Tevatron data performed by the CDF collaboration searches for the existence of new electrically charged, massive particles (a W’ boson) decaying into a top and a bottom quark. Top and bottom quarks leave striking signatures in the detector; W’ events would resemble ordinary production of such quarks if not for the extra energy provided by the decay of the parent particle.

    FNAL Tevatron
    FNAL Tevatron machine
    Tevatron

    FNAL CDF
    CDF part of the Tevatron

    The search for a W’ with data from the CDF experiment turns out to be the most sensitive for such a heavy particle with mass below 650 GeV (approximately 700 times the proton mass). Unfortunately, no surprise turned out from CDF data. The ball is now again in the hands of the LHC experiments!

    See the full article here.

    Please help promote STEM in your local schools.

    STEM Icon

    Stem Education Coalition

    Fermilab Campus

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

     
  • richardmitnick 12:12 pm on October 30, 2014 Permalink | Reply
    Tags: , , , FNAL CDF, , ,   

    From FNAL- “Frontier Science Result: CDF A charming result” 


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

    Thursday, Oct. 30, 2014
    Diego Tonelli and Andy Beretvas

    Physicists gave funny names to the heavy quark cousins of those that make up ordinary matter: charm, strange, bottom, top. The Standard Model predicts that the laws governing the decays of strange, charm and bottom quarks differ if particles are replaced with antiparticles and observed in a mirror. This difference, CP violation in particle physics lingo, has been established for strange and bottom quarks. But for charm quarks the differences are so tiny that no one has observed them so far. Observing differences larger than predictions could provide much sought-after indications of new phenomena.

    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.

    A team of CDF scientists searched for these tiny differences by analyzing millions of decays of particles decaying into pairs of charged kaons and pions, sifting through roughly a thousand trillion proton-antiproton collisions from the full CDF Run II data set. They studied CP violation by looking at whether the difference between the numbers of charm and anticharm decays occurring in each chunk of decay time varies with decay time itself.

    The results have a tiny uncertainty (two parts per thousand) but do not show any evidence for CP violation, as shown in the upper figure. The small residual decay asymmetry, which is constant in decay time, is due to the asymmetric layout of the detector. The combined result of charm decays into a pair of kaons and a pair of pions is the CP asymmetry parameter AΓ , which is equal to -0.12 ± 0.12 percent. The results are consistent with the current best determinations. Combined with them, they will improve the exclusion constraints on the presence of new phenomena in nature.

    graph
    These plots show the effective lifetime asymmetries as function of decay time for D →K+K- (top) and D → π+π- (bottom) samples. Results of the fits not allowing for (dotted red line) and allowing for (solid blue line) CP violation are overlaid.

    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.

    ScienceSprings relies on technology from

    MAINGEAR computers

    Lenovo
    Lenovo

    Dell
    Dell

     
  • richardmitnick 9:45 am on October 2, 2014 Permalink | Reply
    Tags: , , FNAL CDF, , ,   

    From FNAL- “Frontier Science Result: CDF Looking for rare events” 


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

    Thursday, Oct. 2, 2014
    edited by Andy Beretvas

    In 1977, Fermilab scientists discovered the bottom quark — one of six quarks in the Standard Model — through the production of upsilon mesons. An upsilon meson (Υ) is a bound state of a bottom quark and an antibottom quark.

    graph
    This event display of the observed ΥZ candidate shows the muon candidates identified from the Upsilon (μ3 and μ4) and Z (μ1 and μ2) decays.

    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.

    In a new analysis using the full CDF data set, scientists conducted a search for the two mediators of the weak interaction — the W and Z boson — produced in association with an upsilon meson. This is a rare process in the Standard Model, one with a probability that is predicted to be below the sensitivity of the Tevatron.

    FNAL CDF
    CDF

    FNALTevatron
    Tevatron

    For this result, we look for a particular Υ called the Υ(1S), which decays almost immediately. Its lifetime is 1.2 x 10-20 seconds.

    One way the Υ(1S) can decay is into two muons. This decay mode is also rare, occurring in only 
2.5 percent of Υ(1S) decays.

    When we look for the decay of the Υ(1S) in association with a W boson, we look for two muons from the Υ plus an additional muon or electron from the W, as well as so-called missing energy — particles that go undetected but that we know must be present, given the initial energy of the interaction. When we look for the decay of the Υ(1S) in association with a Z boson, we look for two muons from the Υ(1S) plus two additional muons or electrons from the Z.

    The CDF team searched for these two decay patterns. For the full data set, which is about a million detected Υ(1S) decays, we expected an ΥW signal of less than one: only 0.03 ± 0.01 events. The background is expected to be much larger: 1.2 ± 0.5 events. The result of one observed event is in good agreement with the expected background.

    Scientists observed one very nice candidate for Υ and associated Z production (pictured above). The ΥZ signal is expected to be extremely rare, only 0.008 ± 0.002 events from the entire data set. Here the background is expected to be about 0.1 events. This is the first such event of its kind seen.

    The cross section limits resulting from the analysis are currently the world’s best and place restrictions on some new physics scenarios.

    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.

    ScienceSprings relies on technology from

    MAINGEAR computers

    Lenovo
    Lenovo

    Dell
    Dell

     
  • richardmitnick 11:34 am on August 21, 2014 Permalink | Reply
    Tags: , , FNAL CDF, , , ,   

    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.

    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.

    ScienceSprings relies on technology from

    MAINGEAR computers

    Lenovo
    Lenovo

    Dell
    Dell

     
  • richardmitnick 11:22 am on August 7, 2014 Permalink | Reply
    Tags: , , FNAL CDF, , , ,   

    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.

    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.

    ScienceSprings relies on technology from

    MAINGEAR computers

    Lenovo
    Lenovo

    Dell
    Dell

     
  • richardmitnick 9:27 am on July 24, 2014 Permalink | Reply
    Tags: , , FNAL CDF, , , ,   

    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.

    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.


    ScienceSprings is powered by MAINGEAR computers

     
c
Compose new post
j
Next post/Next comment
k
Previous post/Previous comment
r
Reply
e
Edit
o
Show/Hide comments
t
Go to top
l
Go to login
h
Show/Hide help
shift + esc
Cancel
%d bloggers like this: