DOCSLIB.ORG

The Discovery of the Top Quark

Total Page：16

File Type：pdf, Size：1020Kb

Download full-text PDF Read full-text
The Discovery of the Top Quark The Discovery of the Top Quark Finding the sixth quark involved the world’s most energetic collisions and a cast of thousands by Tony M. Liss and Paul L. Tipton VIOLENT COLLISION between a proton and an antiproton (center) creates a top quark (red) and an antitop (blue). These decay to other particles, typically producing a number of jets and possibly an electron or positron. n March 1995 scientists gathered quarks are the building blocks of mat- must exist since 1977, when its partner, at a hastily called meeting at Fer- ter. The lightest quarks, designated “up” the bottom, was discovered. But the top I milab—the Fermi National Accel- and “down,” make up the familiar pro- proved exasperatingly hard to find. Al- erator Laboratory in Batavia, Ill., near tons and neutrons. Along with the elec- though a fundamental particle with no Chicago—to witness a historic event. In trons, these make up the entire periodic discernible structure, the top quark MICHAEL GOODMAN back-to-back seminars, physicists from table. Heavier quarks (such as the charm, turns out to have a mass of 175 billion rival experiments within the lab an- strange, top and bottom quarks) and electron volts (GeV)—as much as an nounced the discovery of a new particle, leptons, though abundant in the early atom of gold and far greater than most the top quark. A decades-long search moments after the big bang, are now theorists had anticipated. The proton, for one of the last missing pieces in the commonly produced only in accelera- made of two ups and one down, has a Standard Model of particle physics had tors. The Standard Model describes the mass of just under 1 GeV. (The electron come to an end. interactions among these building blocks. volt is a unit of energy, related to mass The top quark is the sixth, and quite It requires that leptons and quarks each via E = mc 2.) possibly the last, quark. Along with come in pairs, often called generations. Creating a top quark thus required leptons—the electron and its relatives— Physicists had known that the top concentrating immense amounts of en- 54 Scientific American September 1997 Copyright 1997 Scientific American, Inc. The Discovery of the Top Quark ergy into a minute region of space. Phys- GeV. Meanwhile the collider at Fermilab beam energies, its collisions would be icists do this by accelerating two parti- was just coming into its own with our unlikely to create top quarks heavier cles and having them smash into each young CDF (Collider Detector at Fer- than 77 GeV. The competition was now other. Out of a few trillion collisions at milab). A brief flurry of intense compe- between CDF and a new experiment least a handful, experimenters hoped, tition between us and a group at CERN across the accelerator ring at Fermilab, would cause a top quark to be created brought the decade to a close without a called Dø (pronounced “dee zero,” af- out of energy from the impact. What we top but with the knowledge that its mass ter its location on the ring). did not know was how much energy it could be no lower than 77 GeV. In the early 1980s Leon M. Leder- would take. Although many properties By this time CERN had reached its man, then director of Fermilab, decided of the top, such as its charge and spin limit. With its comparatively lower that CDF needed some local competi- (intrinsic angular momentum), were predicted by the Standard Model, the mass was unconstrained. Although particles can be created from CONSTITUENTS OF MATTER nothing but energy, certain features, CHARGE u c such as electrical charge, cannot—these UP CHARM t TOP are “conserved.” A top quark cannot MASS +2/ be born all by itself. The easiest way to (GeV) 0.3 1.5 175 3 make a top is along with an antitop— d s b identical in mass but with opposite signs d DOWN STRANGE BOTTOM QUARKS MASS 0.5 4.5 1 for other properties, so that conserved (GeV) 0.3 – /3 quantities cancel out. In 1985, when the Fermilab collider - e- µ- τ- was first activated, the search for the ELECTRON MUON TAU MASS top had already been going on for eight 0.0005 0.106 1.7 –1 years. Early forays at the Stanford Lin- (GeV) ν ν ν ear Accelerator Center in Palo Alto, e ELECTRON µ MUON τ TAU Calif., and at DESY in Hamburg, Ger- NEUTRINO NEUTRINO NEUTRINO LEPTONS MASS 0? 0? 0 many, turned up nothing. Over the years (GeV) 0? the hunt moved on to different acceler- ators as they came into operation with ever more energetic particle beams. In TRANSMITTERS OF FORCE the early 1980s at CERN, the European laboratory for particle physics near Ge- VECTOR BOSONS PHOTON GLUON neva, beams of protons and antiprotons W + W – 0 γ g hitting one another at energies up to W Z g 315 GeV generated two new particles, MASS the W and the Z. (GeV) 80 80 91 0 0 Whereas quarks and leptons consti- tute matter, these particles transmit CHARGE +1 –1 0 0 0 force—in particular the weak force, re- FORCE WEAK WEAK WEAK ELECTRO- STRONG sponsible for some types of radioactive MAGNETIC decay. Their discovery provided further confirmation of the Standard Model, which had accurately predicted their masses. It was widely believed that the discovery of the top quark at CERN Characters of the Standard Model was imminent. Finding it would still be a difficult atter consists of two types of particles: quarks and leptons. These are feat. When protons and antiprotons hit Massociated into generations. Up and down quarks, for instance, occur one another at high energies, the actual along with electrons inside atoms; they are members of the first generation. collision is between their internal quarks Much heavier quarks such as the top and bottom are created only in acceler- and gluons. Each quark or gluon car- ators. For each quark or lepton, there is an antiquark or antilepton with oppo- ries just a modest fraction of the total site charge (not shown). energy of its host proton or antiproton, Force is transmitted by a different set of particles: the W, Z, photon and glu- yet the collision must be energetic ons. The W and Z “bosons” transmit the weak nuclear force, involved in ra- enough to generate top quarks. Such dioactive decays. For instance, an up quark may change into a down quark by collisions are rare, and the higher the emitting a W particle, which then decays into a quark or lepton pair. The pho- ton transmits the electromagnetic force, which at high energies is unified required energy—that is, the higher the with the weak force. The gluons transmit the strong force that binds up and top mass—the rarer they are. down quarks into protons and neutrons. An extra particle that is believed to By 1988 the top had not yet been ob- exist, the Higgs, has not yet been found. —T.M.L. and P.L.T. served at CERN; the experimenters con- cluded its mass must be greater than 41 MICHAEL GOODMAN The Discovery of the Top Quark Copyright 1997 Scientific American, Inc. Scientific American September 1997 55 a b W JET W JET BOTTOM/ ANTIBOTTOM JET Y OR Y T A OR T A POSITRON ABOR ABOR OR L T A OR L T A BOTTOM/ CELER ANTIBOTTOM C CELER C JET TIONAL A TIONAL A 3 METERS FERMI NA FERMI NA tion. So we acquired in-house rivals: be- the best theoretical calculations, we ex- that quarks always appear stuck togeth- ginning in 1992 the Dø collaboration pected that about one out of every 10 er with other quarks and antiquarks— began to take data. In addition to spur- billion collisions would produce a top in pairs called mesons or in triplets ring on our efforts, which it certainly quark. The rest, though interesting for a called baryons. (Protons and neutrons did, having two complementary experi- host of other projects, would be a com- are examples of baryons.) When a quark ments studying the same physics was plicated backdrop from which the top emerges from a collision, it gets “dressed healthy in another way. Despite the best would have to be extracted. up” by a cloud of other quarks and an- efforts of experimenters, spurious re- Over the course of a decade, both the tiquarks. What is observed is a jet, a di- sults can occur. Having a second exper- CDF and Dø collaborations construct- rected beam of particles that have rough- iment provides a cross-check. ed enormous, complicated instruments, ly the same direction of motion as the Both CDF and Dø are international with hundreds of thousands of chan- original quark. collaborations of more than 400 physi- nels of electronics, in order to isolate cists. There are also numerous engineers, the top’s “signature”—the trace it would A Barrage of Jets technicians and support personnel. The leave in the detectors. Whereas the CDF rival teams are independent of each oth- detector emphasizes the ability to track he W can decay into a quark and er and never collaborate on their analy- accurately the paths of individual parti- Tan antiquark from the same gener- ses. Each tries to beat the other to the cles in a magnetic field (in order to mea- ation, such as an up and an antidown. punch. But it is friendly competition, sure their momenta), the Dø device re- In this case, the quark and antiquark and we regularly share tables in the lies on an extremely precise segmented show up in a particle detector as two cafeteria and enjoy both serious scien- calorimeter, which measures the energy jets.
Load more

Recommended publications
  • The Particle World
    The Particle World ² What is our Universe made of? This talk: ² Where does it come from? ² particles as we understand them now ² Why does it behave the way it does? (the Standard Model) Particle physics tries to answer these ² prepare you for the exercise questions. Later: future of particle physics. JMF Southampton Masterclass 22–23 Mar 2004 1/26 Beginning of the 20th century: atoms have a nucleus and a surrounding cloud of electrons. The electrons are responsible for almost all behaviour of matter: ² emission of light ² electricity and magnetism ² electronics ² chemistry ² mechanical properties . technology. JMF Southampton Masterclass 22–23 Mar 2004 2/26 Nucleus at the centre of the atom: tiny Subsequently, particle physicists have yet contains almost all the mass of the discovered four more types of quark, two atom. Yet, it’s composite, made up of more pairs of heavier copies of the up protons and neutrons (or nucleons). and down: Open up a nucleon . it contains ² c or charm quark, charge +2=3 quarks. ² s or strange quark, charge ¡1=3 Normal matter can be understood with ² t or top quark, charge +2=3 just two types of quark. ² b or bottom quark, charge ¡1=3 ² + u or up quark, charge 2=3 Existed only in the early stages of the ² ¡ d or down quark, charge 1=3 universe and nowadays created in high energy physics experiments. JMF Southampton Masterclass 22–23 Mar 2004 3/26 But this is not all. The electron has a friend the electron-neutrino, ºe. Needed to ensure energy and momentum are conserved in ¯-decay: ¡ n ! p + e + º¯e Neutrino: no electric charge, (almost) no mass, hardly interacts at all.
    [Show full text]
  • Decays of the Tau Lepton*
    SLAC - 292 UC - 34D (E) DECAYS OF THE TAU LEPTON* Patricia R. Burchat Stanford Linear Accelerator Center Stanford University Stanford, California 94305 February 1986 Prepared for the Department of Energy under contract number DE-AC03-76SF00515 Printed in the United States of America. Available from the National Techni- cal Information Service, U.S. Department of Commerce, 5285 Port Royal Road, Springfield, Virginia 22161. Price: Printed Copy A07, Microfiche AOl. JC Ph.D. Dissertation. Abstract Previous measurements of the branching fractions of the tau lepton result in a discrepancy between the inclusive branching fraction and the sum of the exclusive branching fractions to final states containing one charged particle. The sum of the exclusive branching fractions is significantly smaller than the inclusive branching fraction. In this analysis, the branching fractions for all the major decay modes are measured simultaneously with the sum of the branching fractions constrained to be one. The branching fractions are measured using an unbiased sample of tau decays, with little background, selected from 207 pb-l of data accumulated with the Mark II detector at the PEP e+e- storage ring. The sample is selected using the decay products of one member of the r+~- pair produced in e+e- annihilation to identify the event and then including the opposite member of the pair in the sample. The sample is divided into subgroups according to charged and neutral particle multiplicity, and charged particle identification. The branching fractions are simultaneously measured using an unfold technique and a maximum likelihood fit. The results of this analysis indicate that the discrepancy found in previous experiments is possibly due to two sources.
    [Show full text]
  • The Positons of the Three Quarks Composing the Proton Are Illustrated
    The posi1ons of the three quarks composing the proton are illustrated by the colored spheres. The surface plot illustrates the reduc1on of the vacuum ac1on density in a plane passing through the centers of the quarks. The vector field illustrates the gradient of this reduc1on. The posi1ons in space where the vacuum ac1on is maximally expelled from the interior of the proton are also illustrated by the tube-like structures, exposing the presence of flux tubes. a key point of interest is the distance at which the flux-tube formaon occurs. The animaon indicates that the transi1on to flux-tube formaon occurs when the distance of the quarks from the center of the triangle is greater than 0.5 fm. again, the diameter of the flux tubes remains approximately constant as the quarks move to large separaons. • Three quarks indicated by red, green and blue spheres (lower leb) are localized by the gluon field. • a quark-an1quark pair created from the gluon field is illustrated by the green-an1green (magenta) quark pair on the right. These quark pairs give rise to a meson cloud around the proton. hEp://www.physics.adelaide.edu.au/theory/staff/leinweber/VisualQCD/Nobel/index.html Nucl. Phys. A750, 84 (2005) 1000000 QCD mass 100000 Higgs mass 10000 1000 100 Mass (MeV) 10 1 u d s c b t GeV HOW does the rest of the proton mass arise? HOW does the rest of the proton spin (magnetic moment,…), arise? Mass from nothing Dyson-Schwinger and Lattice QCD It is known that the dynamical chiral symmetry breaking; namely, the generation of mass from nothing, does take place in QCD.
    [Show full text]
  • First Determination of the Electric Charge of the Top Quark
    First Determination of the Electric Charge of the Top Quark PER HANSSON arXiv:hep-ex/0702004v1 1 Feb 2007 Licentiate Thesis Stockholm, Sweden 2006 Licentiate Thesis First Determination of the Electric Charge of the Top Quark Per Hansson Particle and Astroparticle Physics, Department of Physics Royal Institute of Technology, SE-106 91 Stockholm, Sweden Stockholm, Sweden 2006 Cover illustration: View of a top quark pair event with an electron and four jets in the final state. Image by DØ Collaboration. Akademisk avhandling som med tillst˚and av Kungliga Tekniska H¨ogskolan i Stock- holm framl¨agges till offentlig granskning f¨or avl¨aggande av filosofie licentiatexamen fredagen den 24 november 2006 14.00 i sal FB54, AlbaNova Universitets Center, KTH Partikel- och Astropartikelfysik, Roslagstullsbacken 21, Stockholm. Avhandlingen f¨orsvaras p˚aengelska. ISBN 91-7178-493-4 TRITA-FYS 2006:69 ISSN 0280-316X ISRN KTH/FYS/--06:69--SE c Per Hansson, Oct 2006 Printed by Universitetsservice US AB 2006 Abstract In this thesis, the first determination of the electric charge of the top quark is presented using 370 pb−1 of data recorded by the DØ detector at the Fermilab Tevatron accelerator. tt¯ events are selected with one isolated electron or muon and at least four jets out of which two are b-tagged by reconstruction of a secondary decay vertex (SVT). The method is based on the discrimination between b- and ¯b-quark jets using a jet charge algorithm applied to SVT-tagged jets. A method to calibrate the jet charge algorithm with data is developed. A constrained kinematic fit is performed to associate the W bosons to the correct b-quark jets in the event and extract the top quark electric charge.
    [Show full text]
  • Searching for a Heavy Partner to the Top Quark
    SEARCHING FOR A HEAVY PARTNER TO THE TOP QUARK JOSEPH VAN DER LIST 5e Abstract. We present a search for a heavy partner to the top quark with charge 3 , where e is the electron charge. We analyze data from Run 2 of the Large Hadron Collider at a center of mass energy of 13 TeV. This data has been previously investigated without tagging boosted top quark (top tagging) jets, with a data set corresponding to 2.2 fb−1. Here, we present the analysis at 2.3 fb−1 with top tagging. We observe no excesses above the standard model indicating detection of X5=3 , so we set lower limits on the mass of X5=3 . 1. Introduction 1.1. The Standard Model One of the greatest successes of 20th century physics was the classification of subatomic particles and forces into a framework now called the Standard Model of Particle Physics (or SM). Before the development of the SM, many particles had been discovered, but had not yet been codified into a complete framework. The Standard Model provided a unified theoretical framework which explained observed phenomena very well. Furthermore, it made many experimental predictions, such as the existence of the Higgs boson, and the confirmation of many of these has made the SM one of the most well-supported theories developed in the last century. Figure 1. A table showing the particles in the standard model of particle physics. [7] Broadly, the SM organizes subatomic particles into 3 major categories: quarks, leptons, and gauge bosons. Quarks are spin-½ particles which make up most of the mass of visible matter in the universe; nucleons (protons and neutrons) are composed of quarks.
    [Show full text]
  • Three Lectures on Meson Mixing and CKM Phenomenology
    Three Lectures on Meson Mixing and CKM phenomenology Ulrich Nierste Institut f¨ur Theoretische Teilchenphysik Universit¨at Karlsruhe Karlsruhe Institute of Technology, D-76128 Karlsruhe, Germany I give an introduction to the theory of meson-antimeson mixing, aiming at students who plan to work at a flavour physics experiment or intend to do associated theoretical studies. I derive the formulae for the time evolution of a neutral meson system and show how the mass and width differences among the neutral meson eigenstates and the CP phase in mixing are calculated in the Standard Model. Special emphasis is laid on CP violation, which is covered in detail for K−K mixing, Bd−Bd mixing and Bs−Bs mixing. I explain the constraints on the apex (ρ, η) of the unitarity triangle implied by ǫK ,∆MBd ,∆MBd /∆MBs and various mixing-induced CP asymmetries such as aCP(Bd → J/ψKshort)(t). The impact of a future measurement of CP violation in flavour-specific Bd decays is also shown. 1 First lecture: A big-brush picture 1.1 Mesons, quarks and box diagrams The neutral K, D, Bd and Bs mesons are the only hadrons which mix with their antiparticles. These meson states are flavour eigenstates and the corresponding antimesons K, D, Bd and Bs have opposite flavour quantum numbers: K sd, D cu, B bd, B bs, ∼ ∼ d ∼ s ∼ K sd, D cu, B bd, B bs, (1) ∼ ∼ d ∼ s ∼ Here for example “Bs bs” means that the Bs meson has the same flavour quantum numbers as the quark pair (b,s), i.e.∼ the beauty and strangeness quantum numbers are B = 1 and S = 1, respectively.
    [Show full text]
  • QCD at Colliders
    Particle Physics Dr Victoria Martin, Spring Semester 2012 Lecture 10: QCD at Colliders !Renormalisation in QCD !Asymptotic Freedom and Confinement in QCD !Lepton and Hadron Colliders !R = (e+e!!hadrons)/(e+e!"µ+µ!) !Measuring Jets !Fragmentation 1 From Last Lecture: QCD Summary • QCD: Quantum Chromodymanics is the quantum description of the strong force. • Gluons are the propagators of the QCD and carry colour and anti-colour, described by 8 Gell-Mann matrices, !. • For M calculate the appropriate colour factor from the ! matrices. 2 2 • The coupling constant #S is large at small q (confinement) and large at high q (asymptotic freedom). • Mesons and baryons are held together by QCD. • In high energy collisions, jets are the signatures of quark and gluon production. 2 Gluon self-Interactions and Confinement , Gluon self-interactions are believed to give e+ q rise to colour confinement , Qualitative picture: •Compare QED with QCD •In QCD “gluon self-interactions squeeze lines of force into Gluona flux tube self-Interactions” ande- Confinementq , + , What happens whenGluon try self-interactions to separate two are believedcoloured to giveobjects e.g. qqe q rise to colour confinement , Qualitativeq picture: q •Compare QED with QCD •In QCD “gluon self-interactions squeeze lines of force into a flux tube” e- q •Form a flux tube, What of happensinteracting when gluons try to separate of approximately two coloured constant objects e.g. qq energy density q q •Require infinite energy to separate coloured objects to infinity •Form a flux tube of interacting gluons of approximately constant •Coloured quarks and gluons are always confined within colourless states energy density •In this way QCD provides a plausible explanation of confinement – but not yet proven (although there has been recent progress with Lattice QCD) Prof.
    [Show full text]
  • J = Τ MASS Page 1
    Citation: P.A. Zyla et al. (Particle Data Group), Prog. Theor. Exp. Phys. 2020, 083C01 (2020) J 1 τ = 2 + + τ discovery paper was PERL 75. e e− → τ τ− cross-section threshold behavior and magnitude are consistent with pointlike spin- 1/2 Dirac particle. BRANDELIK 78 ruled out pointlike spin-0 or spin-1 particle. FELDMAN 78 ruled out J = 3/2. KIRKBY 79 also ruled out J=integer, J = 3/2. τ MASS VALUE (MeV) EVTS DOCUMENT ID TECN COMMENT 1776..86 0..12 OUR AVERAGE ± +0.10 1776.91 0.12 1171 1 ABLIKIM 14D BES3 23.3 pb 1, Eee = ± 0.13 − cm − 3.54–3.60 GeV 1776.68 0.12 0.41 682k 2 AUBERT 09AK BABR 423 fb 1, Eee =10.6 GeV ± ± − cm 1776.81+0.25 0.15 81 ANASHIN 07 KEDR 6.7 pb 1, Eee = 0.23 ± − cm − 3.54–3.78 GeV 1776.61 0.13 0.35 2 BELOUS 07 BELL 414 fb 1 Eee =10.6 GeV ± ± − cm 1775.1 1.6 1.0 13.3k 3 ABBIENDI 00A OPAL 1990–1995 LEP runs ± ± 1778.2 0.8 1.2 ANASTASSOV 97 CLEO Eee = 10.6 GeV ± ± cm . +0.18 +0.25 4 Eee . 1776 96 0.21 0.17 65 BAI 96 BES cm= 3 54–3 57 GeV − − 1776.3 2.4 1.4 11k 5 ALBRECHT 92M ARG Eee = 9.4–10.6 GeV ± ± cm +3 6 Eee 1783 4 692 BACINO 78B DLCO cm= 3.1–7.4 GeV − We do not use the following data for averages, fits, limits, etc.
    [Show full text]
  • Properties of Baryons in the Chiral Quark Model
    Properties of Baryons in the Chiral Quark Model Tommy Ohlsson Teknologie licentiatavhandling Kungliga Tekniska Hogskolan¨ Stockholm 1997 Properties of Baryons in the Chiral Quark Model Tommy Ohlsson Licentiate Dissertation Theoretical Physics Department of Physics Royal Institute of Technology Stockholm, Sweden 1997 Typeset in LATEX Akademisk avhandling f¨or teknologie licentiatexamen (TeknL) inom ¨amnesomr˚adet teoretisk fysik. Scientific thesis for the degree of Licentiate of Engineering (Lic Eng) in the subject area of Theoretical Physics. TRITA-FYS-8026 ISSN 0280-316X ISRN KTH/FYS/TEO/R--97/9--SE ISBN 91-7170-211-3 c Tommy Ohlsson 1997 Printed in Sweden by KTH H¨ogskoletryckeriet, Stockholm 1997 Properties of Baryons in the Chiral Quark Model Tommy Ohlsson Teoretisk fysik, Institutionen f¨or fysik, Kungliga Tekniska H¨ogskolan SE-100 44 Stockholm SWEDEN E-mail: [email protected] Abstract In this thesis, several properties of baryons are studied using the chiral quark model. The chiral quark model is a theory which can be used to describe low energy phenomena of baryons. In Paper 1, the chiral quark model is studied using wave functions with configuration mixing. This study is motivated by the fact that the chiral quark model cannot otherwise break the Coleman–Glashow sum-rule for the magnetic moments of the octet baryons, which is experimentally broken by about ten standard deviations. Configuration mixing with quark-diquark components is also able to reproduce the octet baryon magnetic moments very accurately. In Paper 2, the chiral quark model is used to calculate the decuplet baryon ++ magnetic moments. The values for the magnetic moments of the ∆ and Ω− are in good agreement with the experimental results.
    [Show full text]
  • Detection of a Strange Particle
    10 extraordinary papers Within days, Watson and Crick had built a identify the full set of codons was completed in forensics, and research into more-futuristic new model of DNA from metal parts. Wilkins by 1966, with Har Gobind Khorana contributing applications, such as DNA-based computing, immediately accepted that it was correct. It the sequences of bases in several codons from is well advanced. was agreed between the two groups that they his experiments with synthetic polynucleotides Paradoxically, Watson and Crick’s iconic would publish three papers simultaneously in (see go.nature.com/2hebk3k). structure has also made it possible to recog- Nature, with the King’s researchers comment- With Fred Sanger and colleagues’ publica- nize the shortcomings of the central dogma, ing on the fit of Watson and Crick’s structure tion16 of an efficient method for sequencing with the discovery of small RNAs that can reg- to the experimental data, and Franklin and DNA in 1977, the way was open for the com- ulate gene expression, and of environmental Gosling publishing Photograph 51 for the plete reading of the genetic information in factors that induce heritable epigenetic first time7,8. any species. The task was completed for the change. No doubt, the concept of the double The Cambridge pair acknowledged in their human genome by 2003, another milestone helix will continue to underpin discoveries in paper that they knew of “the general nature in the history of DNA. biology for decades to come. of the unpublished experimental results and Watson devoted most of the rest of his ideas” of the King’s workers, but it wasn’t until career to education and scientific administra- Georgina Ferry is a science writer based in The Double Helix, Watson’s explosive account tion as head of the Cold Spring Harbor Labo- Oxford, UK.
    [Show full text]
  • Associated Higgs-Bottom Quark Production: Reconciling the 4FS and the 5FS Approach
    Associated Higgs-bottom quark production: reconciling the 4FS and the 5FS approach R. Harlander, M. Kr¨amer, M. Schumacher 6. April 2011 — v0.52 1 Two approaches The cross section for associated Higgs-bottom quark production, pp → b¯bH + X, can be calculated in two different schemes. As the mass of the bottom quark is large compared to the QCD scale, mb ≫ ΛQCD, bottom quark production is a perturbative process and can be calculated order by order. Thus, in a four-flavour scheme (4FS), where one does not consider b quarks as partons in the proton, the lowest-order QCD production pro- cesses are gluon-gluon fusion and quark-antiquark annihilation, gg → b¯bH and qq¯ → b¯bH, respectively. However, the inclusive cross section for gg → b¯bH develops logarithms of the form ln(µF/mb), which arise from the splitting of gluons into nearly collinear b¯b pairs. The large scale µF ≈ MH /4 corresponds to the upper limit of the collinear region up to which factorization is valid [1, 2, 3]. For MH ≫ 4mb the logarithms become large and spoil the convergence of the perturbative series. The ln(µF/mb) terms can be summed to all orders in perturbation theory by introducing bottom parton densities. This defines the so-called five-flavour scheme (5FS). The use of bottom distribution functions is based on the approximation that the outgoing b quarks are at small transverse momentum. In this scheme, the LO process for the inclusive b¯bH cross section is bottom fusion, b¯b → H. If all orders in perturbation theory were taken into account, the four- and five-flavour schemes would be identical, but the way of ordering the perturbative expansion is different.
    [Show full text]
  • Identification of Boosted Higgs Bosons Decaying Into B-Quark
    Eur. Phys. J. C (2019) 79:836 https://doi.org/10.1140/epjc/s10052-019-7335-x Regular Article - Experimental Physics Identification of boosted Higgs bosons decaying into b-quark pairs with the ATLAS detector at 13 TeV ATLAS Collaboration CERN, 1211 Geneva 23, Switzerland Received: 27 June 2019 / Accepted: 23 September 2019 © CERN for the benefit of the ATLAS collaboration 2019 Abstract This paper describes a study of techniques for and angular distribution of the jet constituents consistent with identifying Higgs bosons at high transverse momenta decay- a two-body decay and containing two b-hadrons. The tech- ing into bottom-quark pairs, H → bb¯, for proton–proton niques described in this paper to identify Higgs bosons decay- collision data collected by the ATLAS detector√ at the Large ing into bottom-quark pairs have been used successfully in Hadron Collider at a centre-of-mass energy s = 13 TeV. several analyses [8–10] of 13 TeV proton–proton collision These decays are reconstructed from calorimeter jets found data recorded by ATLAS. with the anti-kt R = 1.0 jet algorithm. To tag Higgs bosons, In order to identify, or tag, boosted Higgs bosons it is a combination of requirements is used: b-tagging of R = 0.2 paramount to understand the details of b-hadron identifica- track-jets matched to the large-R calorimeter jet, and require- tion and the internal structure of jets, or jet substructure, in ments on the jet mass and other jet substructure variables. The such an environment [11]. The approach to tagging√ presented Higgs boson tagging efficiency and corresponding multijet in this paper is built on studies from LHC runs at s = 7 and and hadronic top-quark background rejections are evaluated 8 TeV, including extensive studies of jet reconstruction and using Monte Carlo simulation.
    [Show full text]