A Measurement of b-quark Fragmentation Fractions in pp Collisions at fi = 1.8 TeV

A Measurement of b-quark Fragmentation Fractions in pp Collisions at fi = 1.8 TeV Wendy Jane Taylor -4 t.hesis submitted in conformity with the requirements for the degree of Doctor of Philosophy. Graduate
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A Measurement of b-quark Fragmentation Fractions in pp Collisions at fi = 1.8 TeV Wendy Jane Taylor -4 t.hesis submitted in conformity with the requirements for the degree of Doctor of Philosophy. Graduate Department of Physics, University of by Wendy Jane Taylor, 1999 National Library I*I of Canada Acquisitions and Bibliographic Services Bibliothèque nationale du Canada Acquisitions et services bibliographiques 395 Wellington Street 395, nie Wellington Ottawa ON KI A ON4 Ottawa ON KI A ON4 Canada Canada Your fikr Vorre referença Our file Notre ref&mcd The author has granted a nonexclusive licence allowing the National Library of Canada to reproduce, loan, distribute or sell copies of ths thesis in microfom, paper or electronic formats. The author retains ownership of the copyright in this thesis. Neither the thesis nor substantial extracts fkom it may be pruited or othemise reproduced without the author's permission. L'auteur a accordé une Licence non exclusive permettant à la Bibliothèque nationale du Canada de reproduire, prêter, distribuer ou vendre des copies de cette thèse sous la forme de microfiche/film, de reproduction sur papier ou sur format électronique. L'auteur conserve la propriété du droit d'auteur qui protège cette thèse. Ni la thèse ni des extraits substantiels de celle-ci ne doivent être imprimés ou autrement reproduits sans son autorisation. Abstract A Measurement of b-quark Fragmentation Fractions in pp Collisions at fi = 1.8 TeV Wendy Jane Taylor Doctor of Philosophy in Physics University of Toronto Toronto, Canada 1999 Fragmentation is the process by which quarks and gluons organize thernselves into hadrons. The fragmentation properties of the bottom quark cannot be predicted from fundamental principies and hence must be determined empirically. We investigate one such proper-. narnely the flavour dependence of the fragmentation process for bottom quarks produced in 1.8-TeV proton-antiproton collisions. This flavour dependence is investigated by determining the B-hadron production ratios. l e use a sample of pp data corresponding to an integrated Luminosity of 110 pb-' and reconstruct the foilowing semileptonic B-hadron decays into eiectrons and charmed hadrons: Bt -+ e+v,g~. Bo + et&d'-.y, Bo + ef v,d-.y, BI -+ e'v,d;s and -a A, + e+u,i\;s. With these data. we measure the ratios of fragmentation fractions fdlfu = (88 * 21)%, fs/(fu + fd) = (21.3 f Tl)% and fbargm/(fu + fd) = (12.0 d~ 4.2)%. lssuming that these four hadrons saturate production of weakly-decaying B = 1' we determine hadrons. that is, that f, + fd + f, + fbar,, These results represent the first measurement of al1 four b-quark fragmentation fractions in a single experiment. These fractions are in agreement both with previous phenomenological interpretations and with other experimental measurernents. Acknowledgements 1 have enjoyed the assistance and support of many people throughout the duration of my research career. First and foremost, 1 must thank my supervisor, Professor Pekka Sinervo for giving me the opportunity to pursue this degree. It was a pleasure to collaborate with Pekka and his confidence in me helped me through more than a few rough spots. Pekka's encouragement and patience were important ingredients to rny success. 11y CDF colleague and office mate, Dr. Bjorn Hinrichsen, has been a good friend and confidant. Our frequent venting sessions were essential for maintaining my sanity through the tough times. 1 will certainly miss them. 1 would also Iike to thank my other GDF colleagues at Toronto, in particular, Dr. -4ndreas Warburton: Professor William Trischuk, Dr. Szymon Gadomski and Dr. -\ndreas Holscher. The- were great resources and always took the time to offer advice and eschange knodedge. Dr. Cortney Sampson also deserves great thanks. His willingness to help and interminable patience were most appreciated. He also was an exceptional drinking companion. I have several CDF colleagues who have been helpful. Dr. Barry Wicklund, Dr. Karen Byrum and Dr. Jonathan Lewis have al1 contributed to this work. Dr. Jeff Tseng always managed to squeeze in a few moments to help with crucial aspects of the analysis. 1 am indebted to Professor Joe Kroll for helping to guide me along the path that 1 now tread. Financial support for this work came in part from the National Science and Engineering Research Council, the University of Toronto, the Department of Physics and my supervisor: Professor Pekka Sinervo. 1 would like to thank al1 parties for their generous contributions. 1 would like to thank Dr. Paul Padley for inspiring me to pursue a graduate degree and for convincing me that 1 kvas good enough to do so. Paul continues to have a hand in securing a successful future for me and 1 am indebted to him for that. 1 must also thank Dr. Mark Sutherland for the wonderful education he provided during my course work at Toronto. Mark's time and effort went a long way to ensuriag that 1 was well prepared for my graduate research work. Finally, I would like to thank Dr. James Brown for his unwavering confidence, support and love always. Contents 1 Introduction ThestandardModel Quarks and Hadrons Heavy Quarks Heavy Quark Production Heavy Quark Fragmentation b-quark Fragmentation Fractions Semileptonic B-Hadron Decay Dissertation Overview Experimental Apparatus The Fermilab Tevatron pp Collider The Collider Detector at Ferrnilab Tracking Detectors Calorirneters The Trigger Systems Electron Identification Electron Properties Fiducial Requirements ElectromagneticEnergy Lateral Shower Profile Hadronic Energy Fraction... 41 3.1.5 High- Pr Charged Track Strip and Wire Chamber Profiles Trigger Criteria Level Level Non-B Decay EIectrons Photon Conversions Wk and Z Boson Decays Cham Decay Electrons Electron Candidate Sample Charmed Hadron Identification Common Selection Criteria.... a t Track Quality Criteria Prirnary Vertex Selection Charmed-Hadron Decay Vertex Track Impact Parameter B-Hadron Mass Cut Charmed-Hadron Signals D Meson Reconstruction D'(2010)- Meson Reconstruction D- Meson Reconstruction D; Meson Reconstruction A; Baryon Reconstruction Summary Efficiency Calculations 73 1 Sample Cross Contamination Monte Car10 Calculation Parameters Reconstruction Efficiencies Tracking Efficiency vii 5.3.2 L2 Trigger Efficiency Vertex Constraint Efficiency Acceptance Dependence on Fragmentation A, Polarization SystematicUncertainties Results Fragmentation Fraction Fitting Program Structure of the Fit Systematic Uncertainties... 9'7 6.2 Determination of fd/fu... 9'7 6.3 Measurement of f, /( f, + fd) Measurement of fbnrym/( fu + fd) Absolute Fragmentation Fraction Values Conclusions Future Prospects A The CDF Collaboration... Vlll List of Tables 1.1 The three forces governing the interactions of quarks and leptons and their mediating particles B-hadron semileptonic decay channels Level 3 inclusive electron trigger requirements Summary of electron identification criteria Summary of charmed-hadron signal fits Branching fractions for semileptonic B+-meson decays Branching fractions for semileptonic Bo-meson decays Branching fractions for semileptonic Bg-meson decays Acceptances and reconstruction efficiencies for directly-produced charmed hadrons in semileptonic B-hadron decays Acceptances and reconstruction efficiencies for directly-produced charmed hadrons in semileptonic B-hadron decays Acceptances and reconstruction efficiencies for D mesons in semilep- tonic B-meson decays Acceptances and reconstruction efficiencies for D mesons in semilep- tonic 3-meson clecays Accept ances and reconstruction efficiencies for D- mesons from semilep- tonic B-meson decays cceptances and reconstruction efficiencies for charmed mesons from semileptonic B-meson decays 5.10 Acceptance dependence on Peterson fragmentation parameter ~b for -0 semileptonic -4, decays Peterson fragmentation parameter systematic uncertainties for semilep- tonic B-hadron decays Reconstruction efficiency dependence on A, production polarization times weak decay asymmetry P Fractional systematic uncertainties associated wit h event reconstruction Results from the MINUIT least-squares fit to the free parameters Comparison of the fit results for the semileptonic branching fractions to the measured values to which they are constrained List of Figures 1.1 Feynman diagrams of b-quark production Feynman diagrams of ~(a:) bquark production The fragmentation of a heavy quark Q into a meson H(Qq) Feynman diagram of semileptonic B meson decay The Ferrnilab Tevatron pp Collider Isometric view of the CDF detector 27 Schematic of the CDF detector.... '27... Isornetric view of an SVX barre1 '29... Schematic of three axial-layer drift cells Schematic of one end plate of the CTC Schematic of a CEM module Schematic of a CES module Ehod/ Eem distribution for electron candidates Photon conversion radius distribution Transverse momentum distribution of candidate electrons prior to W' and Z boson removal Distribution of the invariant mass of the candidate electron and the high EM fraction cluster Background-subtracted distribution of the invariant mass of the can- didate electron and the high EM fraction cluster Missing transverse energy distribution in candidate electron events &-significance distribution in candidate electron events... 52 3.8 Transverse mass distribution for candidate electrons that fail the &- significance requirement Transverse momentum distribution of candidate electrons after the re- rnoval of Z and Wi bosons Event display diagram depicting a typical electron event Schematic diagram depicting B meson semileptonic decay Invariant mass distribution of D rneson candidates Invariant mass distribution of wrong-sign D meson candidates Mass difference distribution? AM = M(K?ins) -M (KT) for 0' (2010)- candidates Mass difference distribution, ALCI = Ad(Ii3rsrs) - M(Kn)? for wvrong- sign K+n-x: candidates Invariant mass distribution of D- meson candidates Invariant mass distribution of Do meson candidates Invariant mass distribution of Q, meson candidates Distribution of de/dx versus momentum for pions and protons Invariant mass distribution of A; baryon candidates Invariant mass distributions of D- candidates for passes and failures of the CL 0.01 cut cos 0 distribution from a Monte Carlo cakulation Cornparison of the measured fd/fu ratio to other measurements Comparison of the measured value for f. to other measurements Comparison of the measured value for fba, to other rneasurernents. 109 Chapter 1 Introduction As long ago as the fifth century B.C.? philosophers studied the essential nature of matter. a pursuit called physis by the ancient Greeks. At that time, the Greek philosopher Democritus developed the idea that al1 matter was made up of small indivisible objects called atoms. The idea of the atom persisted through the ages until the early nineteenth century when John Dalton expanded upon the work of his predecessors. He suggested that the chemical elements differed from one another because each one was made up of a specific kind of atom [l]. In 1869, Dmitry Mendeleev organized the elements according to their atomic weights and chemical properties into what is now known as the Periodic Table of Elements. He found three gaps in the table and predicted the existence of three elements with specific characteristics. A few years later, the three elements predicted by Mendeleev, gallium, scandium and germanium, were discovered.,4torns? however, were not the indivisible units of matter that Democritus tiad postulated. In 1896, Henri Becquerel discovered while studying uranium salts t hat the element uranium spontaneously emitted radiation [2, 31. One year later, J. J. Thomson showed that cathode rays observed when a current was passed through a rarified gas were actually beams of electrically-charged particles streaming frorn the negative electrode (cathode) to the positive eelctrode (anode) [4, 51. These negativelycharged particles came to be called electrons and it was originally thought that they came from the atoms in the cathode. Since atoms were known to be electrically neutral: it was then proposed that there must be a positively-charged portion of the atorn. Fourteen years later, Ernest Rutherford and his colleagues aimed a beam of doubly-charged a-particles at a sheet of gold foil and discovered that while most beam particles passed straight through the foil, some were deflected at large angles [6]. He inferred that the positively-charged portion of the gold atom \vas densely packed at the atomic centre. This atomic nucleus was believed to be made of positively-charged particles called protons. In the subsequent twenty years, it became clear that the atomic nucleus also cont ained neutral particles called neutrons. as discovered b- J. Chadwick in 1932 [Il. The discovery of the neutron posed two new problems for physicists. First, it became obvious that a new force of nature must exist to bind together the neutrons and the protons inside the nucleus. Such a force would have to be of very short range (- IO-'' m) and very strongly attractive to overcome the electromagnetic repulsion of the protons confined to the small volume of the nucleus. Second. the neutron \vas found to decay spontaneously into a proton and an electron with a half-life of fifteen minutes. This radioactive 0 decay, as it was called, had a time scale much too long to be attributable to the strong or electromagnetic force. It appeared that a third, much weaker force was ât play Studies of decay revealed another puzzle. The energy of the electrons emitted by the decay of a particular element was not sirnply the energy difference between the parent and daughter atomic masses; rather, it was a continuous spectrum, where the electrons were observed at al1 energies up to a maximum equivalent to the parentdaughter mas difference. In 1933, Wolfgang Pauli postulated that a third particle, the neutrino, was produced in,û decay [8]. Such a particle was neutral and very light, if not massless. Around the same time, Paul Dirac hypothesized the existence of a positivelycharged particle with the same characteristics as an electron except for its charge [9]. This anti-electron, or positron as it came to be called, was discovered in 1933 by Carl.Anderson in a cloud chamber esperiment [IO]. Scientists soon discovered other kinds of particles. The muon, discovered in 1936 [Il], was found to have properties much like a heaw electron? although it was originally thought to be the pion proposed by H. Yukawa the previous Far (121. Yukawa had postulated that the strong force binding neutrons and protons together inside the nucleus was mediated by a massive particle, called a 71- meson or pion. The rnass of the mediating particle would ensure that the force only acts over a finite range. The pion was eventually discovered in 1947 [13: 141; even though it was soon apparent that it was not the mediator of the strong force. Pions, like pro tons, are strongly-interacting particles, however, and are t hus ciassi- fied as hadrons. Electrons and muons do not experience the strong interaction and are thus classified as leptons. In the early 1950s: several other hadrons were discovered, - including the ;Io: A+ io il+, 3': il-, and eventually the X and the = particles. There appeared to be an unreasonable number of elementary particles, so in 1961: Mur- ray Gell-Mann [15, 161 and Yurval Xe'eman [17] independently developed a scheme of ordering the hadrons into families. Gell-Mann used his theory to predict the exis- tence of an as-yet unknown particle, the R- [18]' which was discovered only a year later [19]. This classification scheme was further extended by Gell-Mann [20] and, independentlx G. Zweig [21: 221, resulting in the hypothesis that al1 hadrons were composed of objects called quarks . Three types of quarks were required to esplain the hadrons known at that time.l 1.1 The Standard Model The Standard Model of particle physics is the theoretical structure t hat originated h the work of Gell-Mann and Ne7eman, with rnany physicists having contributed to its subsequent development [23, 24, 251. In the Standard Model, matter is made up of sis types of quarks and six types of leptons. Quarks corne in different flaveurs, such as up (u) or dom (d), each carrying a fractional charge. Along with the leptons, they can be classified into generations, as shown below, where the particles in a given 'Three more quarks have been discovered since that tirne, for a total of six types of quarks. generation are placed in the same column: Charge Each quark and lepton ha, an associated antiparticle. The order of the generations represents the progression of masses of the quarks, ivhere the u, d and s quarks are light ( 1 GeV/c2), the c and b quarks are heavy (1.5 and 5 GeV/c2, respectively) and the t quark, discovered recently by two exper- iments at the Fermilab Tevatron [26, 27, 28: 291, is surprisingly heavy, with a mass 180 times that of the proton: or about 175 GeV/c2. The charged leptons are also ordered according to increasing mass: but currently only upper mass limits exist for the neutrinos2. The relative mass scales are not yet understood, except that the three generations appear to be distinguished from one another by the energÿ level at which they esist. The Universe is currently dominated by particles in the first generation, but particles in the second and third generations can be produced at high energies, such as those associated ivith the early moments of the Big Bang. In the Standard Model, the quarks and leptons interact with one another via three forces3, shown in Table 1.1 with their mediating particle. The electromagnetic force binds electrons and atomic nuclei together. The strong force binds the protons and neutrons together within atoms (and the quarks together within hadrons). The weak force is responsible for radioactive decays. We now consider the electromagnetic and weak forces as two manifestations of the same force, which we cal1 the electroweak 'We note that there is no specid relationship between the leptons and the quarks in each generations escept for the reiative mass scaies at which they exist. 3The fourth known force, gravity, has not yet been successfdly incorporated into the Standard Modei. However, its action is so weak that it is not relevant on a subatomic scale. Force M ediators Range Electromagnetic Y OC UTeak Wi? Z0 bosons l8 m Strong eight gluons - IO-'' rn Table 1.1: The three forces governing the interactions of quarks and ieptons and their rnediating particles. force. The electroweak coupling of the W' boson to leptons of al1 three generations occurs with the same coupling strengt h. In contrast. the strengths of the couplings to quarks appear to be related to the magnitudes of the quark masses. Quark transitions within the same generation are favoured, but transitions across generations also occur. A 3 x 3 matrix of constants known as the Cabibbo-Kobayashi-Maskawa matrix [30,31]: describes the electroweak couplings of the quarks to the IVi boson. The element PL6; for esample: describes the strength of the electroweak coupling of b quarks to u quarks. 1.2 Quarks and Hadrons Free quarks have never been observed in the laboratory; rather, quarks have only been identified bound inside hadrons, such as mesons (a qq state) or baryons (a qqq state). The theory of Quantum Chromodynamics (QCD), the element of the Standard Model that describes the strong force, provides an explanation for this phenornenon of confinement. QCD is a renormalizable gauge theory similar to Quantum Electrodynamics (QED), the gauge theory describing electromagnetic interactions. QCD states that each quark flavour cornes in three different colours. Thus, the strong force binding quarks together is mediated via the exchange of eight types of massless vector bosons (called gluons) that carry a property called colour charge. This con
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