TOP QUARK IN ELECTROWEAK PRODUCTION CHANNELS AT ATLAS

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1 MASS MEASUREMENTS OF THE TOP QUARK IN ELECTROWEAK PRODUCTION CHANNELS AT ATLAS PROEFSCHRIFT ter verkrijging van de graad van doctor aan de Universiteit Twente, op gezag van de rector magnificus, prof. dr. W.H.M. Zijm, in het openbaar te verdedigen op woensdag 31 mei 2006 om uur door Marcello Barisonzi geboren op 20 augustus 1975 te Pavia, Italië

2 Dit proefschrift is goedgekeurd door: prof. dr. ing. B. van Eijk (promotor) dr. ir. J.C. Vermeulen (assistent-promotor)

3 The slovenliness of our language makes it easier for us to have foolish thoughts. GEORGE ORWELL

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5 Contents Introduction 5 1 The Top Quark Properties of the top quark Top pair production Single top production Single top production in the s-channel gluon fusion Associated production Top quark decay Top detection and top mass determination Top pair events Single top events Performance requirements for top physics Vertex identification and -tagging Jet energy calibration identification identification Muon momentum measurement The ATLAS Experiment The Combined Test Beam setup The Inner Detector The Pixel Detector The Semi Conductor Tracker The Transition Radiation Tracker Tracking performance Particle identification performance Calorimetry The Electromagnetic Calorimeter The Tile Calorimeter

6 2.3.3 The Hadronic End-Cap Calorimeter The Muon Spectrometer Trigger chambers Precision chambers Combined test results Trigger and Data Acquisition The LHC beam structure Data Acquisition and Trigger Architecture Trigger Menu The Level 1 Trigger Calorimeter Trigger Muon Trigger Central Trigger Processor Timing Trigger and Control High-Level Trigger Level 2 trigger algorithms Event Filter algorithms Trigger Efficiencies Electron selection performance Muon selection performance The MROD The MDT Readout chain Hedgehog boards Mezzanine cards CSM The MROD The TIM The MROD crate controller The MROD board The GOLA card The ALTERA FPGA The ZBT buffer The SHARC DSP The S-link The MROD I/O server MROD-In functionality FPGA procedure MROD-In SHARC procedure MROD-Out software

7 4.5 Testing the MROD The cosmic ray test-stand at NIKHEF The H8 Test-beam Performance Single Top: Event Generation, Selection and Reconstruction The ATHENA framework Monte Carlo Generators TopReX MC@NLO Comparison TOPREX/NLO AlpGen The Top Data Sample AOD Definitions Electrons Jet algorithms tagged jets Selection criteria Reconstruction of the top mass Systematics Conclusions A Distributions of selection variables for signal and background processes 151 Bibliography 159 Summary 167 Samenvatting 169 3

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9 & & 9 " Introduction The Standard Model represents the most complete model of physical phenomena at the sub-nuclear scale. The model describes elementary particles interacting with the strong and the electroweak fields; the description of the fields is based on the gauge symmetry groups for the strong interaction and for the electroweak interaction. In the electroweak sector, the model postulates the existence of left handed fermion doublets and for leptons and quarks respectively, which transform under [1]. The gauge bosons for the electroweak interactions!#"%$! are massless, and such are leptons and quarks. However, in the real world we observe massive leptons and quarks and linear combinations of the electroweak gauge bosons:! ' $!)(+*,.-0/21 43!,5768-0/ :9! ' <; :A!>=@?! CED B! ' F $!G,H5768-0/41 3! (I*,J-0/ where -/ CED is the electroweak mixing angle. Since the bosons are massive while the photon (! ) is not, the electroweak symmetry is broken. The Standard Model postulates the existence of a scalar field, the Higgs field, which couples to the electroweak bosons, thus breaking the symmetry. In the Higgs mechanism, the masses of bosons and fermions emerge as the coupling strengths of the interaction with the Higgs field. The parameters of the electroweak model have been measured with great precision in past experiments. Table 1 shows some of the measured observables; in this table, the parameter with the worst relative precision is the mass of the top quark. A very precise measurement of the top quark is required to reduce the theoretical uncertainties in the electroweak sector; in particular, since the top quark is the heaviest of all fundamental fermions and therefore couples strongly to the Higgs field, it should play a major role in the understanding of the Higgs mechanism. 5

10 A A ; The top quark can be produced at hadron colliders either by QCD processes in pairs or electroweak processes where a single top emerges. The Large Hadron Collider (LHC) currently being built at CERN 3 will produce, due to its high design luminosity of cm s, up to 80 million pairs as well as 30 million single top quarks per year in 14 TeV CMS proton-proton collisions. This will allow to reduce the statistical error on the top mass measurement. Observable Measurement Precision / (GeV) ± %,5 6-0/ ± % (GeV) ± % (GeV) ± % (GeV) 178.0± % Table 1: Precision measurements of electroweak observables. Table taken from [2]. The aim of the study presented in this thesis is to simulate the single top production process and subsequent top decay in the ATLAS detector, which is one of the two multipurpose experiments at the LHC. The single top channel is statistically less significant compared to production; however, the less complex signature allows for a better understanding of the systematic effects, resulting in a precision of the mass measurement that is competitive with that in the pair production channel. Chapter 1 of this thesis outlines the current knowledge on top quark physics and briefly describes the recipe for a detailed analysis; Chapter 2 illustrates the structure of the ATLAS detector and discusses the effect of detector performance on the analysis. Chapter 3 is devoted to the description of the trigger and data acquisition of the detector, focusing on the trigger efficiencies for top physics channels. Chapter 4 discusses in detail the readout electronics and data acquisition chain for the Monitored Drift Tube (MDT) chambers. NIKHEF is involved in the development and production of MDT chambers and the associated read-out electronics. Chapter 5 describes the tools used to generate Monte Carlo data for top physics processes, the software simulation of the ATLAS detector and the algorithms used to reconstruct physical objects from raw simulated data. This chapter also discusses the analysis of simulated data and gives an outlook towards real data taking in

11 9 9 Chapter 1 The Top Quark During the past thirty years, physicists have been building comprehensive and accurate models to describe the universe at sub-nuclear level. The result is an elegant set of theories collectively known as the Standard Model (usually abbreviated as SM). These theories provide powerful tools; their predictive power has been confirmed in many experiments worldwide. The inspiring principle of the Standard Model is gauge symmetry. This principle states that the description of all physical phenomena does not vary if the Lagrangian equations describing the phenomena are subjected to a special set of local transformations the local gauge transformations. In analogy with classical mechanics, local gauge invariance in the interaction Lagrangian is associated with conserved currents and boson fields, which act as the medium of the interaction. A great success for gauge theories (and thus for the Standard Model) was the prediction of the properties of the intermediate vector bosons and CED, which mediate the weak interaction, and, which were discovered at CERN in 1983 [3, 4, 5, 6]. Despite its glorious career, the Standard Model is far from writing the final word in the book of physics: there are several puzzles that still need to be solved and may prove to be a limit to the validity of the Standard Model. The most obvious puzzle is the existence of mass: the electroweak gauge symmetry applies only if the intermediate vector bosons are massless; this of course is contradicted by experimental measurements, which assign to and CED a mass of and # GeV respectively [1]. In order to reconcile gauge symmetry and the existence of mass, a spontaneous symmetry breaking mechanism known as the Higgs mechanism is introduced in the Standard Model. The Higgs mechanism postulates the existence of at least one scalar, electrically neutral boson field the Higgs field which breaks the electroweak symmetry and interacts with particles. The coupling strength of the interaction of 7

12 ' 9 " 9 / / 9 CHAPTER 1. THE TOP QUARK a particle with the Higgs field is proportional to the mass of the particle itself. The carrier of the Higgs field the Higgs boson still needs to be found before the Higgs mechanism can be validated; however, the task of finding this elusive particle is rendered more difficult by the fact that the Higgs theory does not predict a value for, the mass of the Higgs boson. Despite this fact, it is still possible to constrain the allowed values of : since the Higgs field is responsible for generating the mass of the electroweak bosons, by measuring precisely the masses of the and CED one can derive an expectation value for. The Higgs mass can be constrained by analyzing the one-loop corrections to the mass of (Figure 1.1) These corrections are proportional to : " / A " " ' F A 1 where contains the leading logarithmic contributions from the light fermion loops, contains the A dependence from top/bottom loops and contains the non-leading terms where plays a role [7]. Thus, a precise measurement of / and the other physical parameters and have a smaller impact can give us, through the evaluation of, an estimate on and thereby provide a test of the Standard Model. Figure 1.1: One-loop Higgs correction to the W boson propagator. Although the mass of the has been measured with a precision of a few tens of MeV by several experiments over the past twenty years, an experimental measurement of the mass of the top quark with the same degree of precision is not possible, since the measurement of the invariant mass of the decay products of the top quark involves non-perturbative corrections of the order J 200 MeV [7]. However, at hadronic colliders the experimental systematic uncertainties dominate over the theoretical ones, and a mass resolution of the order of 1 GeV is sufficient to obtain a good estimate of the Higgs mass. 8

13 A 1.1. PROPERTIES OF THE TOP QUARK The top quark was discovered at the Tevatron 1 collider in 1995 [8], and its mass was measured by two experiments, CDF and DØ. The average top mass in Run I was =178.0±4.3 GeV (Figure 1.2), while preliminary results for Run II indicate a lower estimate of 172.2±2.9 GeV [9]. The limiting factors in the top mass measurements are uncertainties in the jet energy scale and limited statistics [7]. Hopefully, the statistics will improve once the LHC accelerator starts functioning, pushing top quark physics from the discovery phase to the precision measurement phase. LHC will become a true top factory, producing up to 80 million pairs per year at design luminosity, allowing us to refine the top mass measurement to a precision of 1 GeV [7]. Although statistically important, pair production is not the only process by, which top quarks can be produced at the LHC; there is also the socalled single top production, where a single top is present among the final state particles. This process may play an important role in the measurement of : as only one top is present in the final state, the assignment of decay particles is simplified whereas in pair production one has to find the correct assignment of decay products for both the and the. Hence the systematic error in the measurement of the top mass, due to the incorrect assignment of decay products, may be reduced, resulting in a more precise measurement. Another interesting property of electroweak single top production is the cross-section, which is directly proportional to the element of the Cabibbo-Kobayashi-Maskawa (CKM) matrix. A precise measurement of the cross-section for this class of processes may confirm the unitarity of the CKM matrix, or give an indirect evidence for the existence of a fourth (quark) generation, mixing with the " H doublet. 1.1 Properties of the top quark The top quark was postulated in the Standard Model to complement the bottom quark in the third generation doublet; the model predicts an uplike quark, with spin= A ;, weak isospin= A ; and charge=1 3. Results from - physics at the electron-positron collider LEP for example, the precision measurement of the C partial width constrain the top mass to a value around 170 GeV [11]. 1 The Tevatron is a collider located at the Fermi National Laboratories, USA. During Run I ( ) the Tevatron had a CMS energy of 1.8 TeV. Since 2001 (Run II) Tevatron has a CMS energy of 1.96 TeV. 9

14 F & CHAPTER 1. THE TOP QUARK Figure 1.2: Combined measurement of the top quark mass at the Tevatron during Run I. The Run I average includes measurements from lepton-plusjets and di-leptonic decay channels for both CDF and DØ, plus the CDF jets-only channel [10]. Several properties of the top quark were studied during the first run at the Tevatron: N the production cross-section; N kinematics of the pair; N the top mass; N the nature of the electroweak coupling by determining the helicity of the produced in the decay of the top quark; N spin correlations in production; N electroweak production of top quarks; N decays of the top quark; N exotic decays; N decay channels involving Flavour Changing Neutral Currents. Results at the Tevatron were hampered by low statistics. The studies will proceed, with evident benefits from increased luminosities, both for Tevatron Run II and at the LHC. 10

15 1.1. PROPERTIES OF THE TOP QUARK Among the Tevatron studies, the most interesting concerns the mass of the top quark. Such a large mass is quite extraordinary in the quark hierarchy: the top quark is roughly 35 times more massive than the b quark. In the Higgs model, the top quark (like any other fermion) acquires mass via the Yukawa coupling to the Higgs field: ' B where is the strength of the Yukawa coupling between the top quark and the Higgs field, and is the vacuum expectation value that is the energy associated to the Higgs field in the vacuum. The Yukawa coupling factor for the top quark is close to unity; this suggests that the top quark may play an important role in models describing the fermionic masses. The Higgs model, however, cannot explain why the Yukawa coupling of the top quark is so large compared to other quarks; Technicolor models, instead, provide both a dynamical breaking of the electroweak symmetry and a theory for fermion masses. One of these models, the Topcolor-assisted Technicolor (usually referred to as TC2), predicts the existence of a new interaction that couples only with the third quark family [12] and generates the large top mass. This large mass provides a large phase space in the top decay, which A results in a lifetime of 4 10 seconds, an order of magnitude shorter A than the characteristic QCD hadronisation time of seconds. This has some interesting experimental and theoretical aspects: N toponium resonances will not be formed because of the short lifetime of the top quark [13]; N hadronisation will not degrade the mass measurement of top quark; however, this effect is spoiled by the fact that the majority of the decay products hadronise; N since the time constant for spin-flip via gluon emission is longer than the lifetime of the top quark, decay products retain the spin information of the original top quark [7]; in particular, because of constructive interference between decays of longitudinal and lefthanded s, charged leptons in the top decay chain are 100% polarized with respect to the top quark [14]; N pair production at threshold is not affected by non-perturbative QCD contributions, thus the threshold shape can be predicted by perturbative QCD [15]. 11

16 D ; D ; CHAPTER 1. THE TOP QUARK Top quark pair production (Section 1.2) has a larger cross-section than electroweak single top production (Section 1.3). 1.2 Top pair production The cross-section for QCD " ' ; production is expressed as: A " A " A " " I A H (1.1) where? " indicate the contribution of quark flavours and gluons to the cross-section. This equation descends from the following assumption: the QCD scattering process can be expressed by independent factorised terms. Consider the scattering of two protons: they are complex objects, made of several components called partons. The fractional momentum carried by the partons inside the protons is described by probability density functions called Parton Distribution Functions PDFs, indicated by " A in the equation. However, when we consider the scattering process, we assume that the scattering partons are independent from the protons that contained them; then, we have factorised the amplitude of the scattering of the two partons from the PDFs. We choose an energy scale, the factorisation scale that separates the statistical description of the parton from the quantum description of the scattering process. In the factorization scheme, the partonic cross-section for production depends only on the square of the partonic center of mass energy ', the top mass A and the running strong coupling constant I. The coupling constant is evaluated at the renormalisation scale, which describes the energy scale of the hard scattering process. Although the cross-section should be independent from the factorisation and renormalisation scales, the calculation of the scattering matrix element up to finite order introduces an unphysical dependence. At Leading Order (LO) the cross-section is usually evaluated with ' ', and has an uncertainty of about 50%. The scale-dependence of the crosssection can be reduced if we perform Next-to-Leading Order (NLO) calculations of the same cross-section: the expected cross-section at the LHC energy scale increases by 30%, the factorisation scale dependence reduces to 12% [7]. NLO calculations, however, are not safe from troubles: truncating the calculation of the cross-section to some fixed order of gives reliable results only if the physics processes included in the calculation happen at 12

17 ; A ; 1.2. TOP PAIR PRODUCTION roughly the same energy scale. When two or more very different energy scales " ; are involved in the calculation, the effect of logarithmic terms of the type 6 ; and 6 ; H have to be included in the computation [7]. Inclusion of these logarithms in the calculation of the cross-section is called resummation. There are several classes of logarithms that need to be resummed in heavy quark production processes. The type of resummation needed depends on the measured variable: N small- logarithms; these logarithms appear in the cross-section calculations when the center of mass energy B of the colliding partons is several orders of magnitude larger than the energy scale of the hard scattering; the extrapolation of PDFs between the two energy scales results in large logarithms 6 ; N bremsstrahlung logarithms; these are connected to the emission of soft collinear gluons by scattered particles and affect the spectrum of the top quark; N threshold logarithms of the type 6 F ; these appear when the final state particles carry a large fraction of the center of mass energy; N transverse momentum logarithms that occur in the distribution of transverse momentum of systems with high mass that are produced with a vanishing in the LO process. Resummation of logarithms is performed by introducing a conjugate space to the phase space and performing a transformation of the crosssection equation in the conjugate space. Resummations performed on the cross-section [7] show that Nextto-Leading Logarithm (NLL) corrections applied to NLO diagrams further reduce factorisation scale dependence by 6% (Table 1.1). It is important to note that resummations do not only affect the absolute value of the crosssection, but the kinematical properties of the process as well. For example, transverse momentum logarithms are associated with the emission of soft gluons in the initial state; a comparison of the spectrum for low- pairs between NLO predictions and Monte Carlo shower algorithms which reproduce soft and collinear gluon radiation can point out whether resummation is needed or not. Top pair production at hadron colliders proceeds via the QCD processes and (Figure 1.3). The two processes have a different relative importance for Tevatron and LHC: when we consider 13

18 CHAPTER 1. THE TOP QUARK Factorisation scale ( ' ) NLO NLO+NLL Table 1.1: Resummation correction to the total residual factorisation scale dependence [7]. cross-section (pb) and production at threshold, the colliding A partons need to have a minimum fractional momentum of ' in order to produce a pair. Substituting in this equation the center of mass energies of the two colliders, one obtains for Tevatron and for LHC; collisions at the LHC occur in a region where colliding partons carry a small fraction of the momentum of the incoming particles. Small- regions in Parton Distribution Functions (see below) are mainly populated by gluons, hence at the LHC production occurs mainly via gluon-gluon fusion, while at the Tevatron quark/anti-quark annihilation is the most important process. (a) (b) Figure 1.3: Leading order Feynman diagrams for production via the strong interaction. Diagram. quark-antiquark annihilation, is dominant at the Tevatron, while diagrams and, gluon-gluon fusion, give the largest cross-section at the LHC. (c) 1.3 Single top production Single top production probes the weak coupling of the top quark with the down-type quarks (d,s,b); at the LHC energies the cross-section for single top production is about one third of pair production, thus providing the opportunity to obtain adequate statistics for precision measurements 14

19 1.3. SINGLE TOP PRODUCTION in the electroweak sector. The measurement of the CKM element and a precision measurement of the top quark are the main goals. Other interesting topics covered are: N single top quarks are produced with almost 100% spin polarisation; by measuring the spin polarisation of the top decay products, the V-A coupling of the vertex can be evaluated; N the production of single top quarks probes the PDF for -quarks; an accurate measurement of the cross-section of single top allows a check of the PDF; N the cross-section for single top may be enhanced by physics processes beyond the Standard Model. There are three dominant single top production mechanisms: the s- channel, the t-channel and the associated production, illustrated in Figure 1.4. At Tevatron, s-channel and t-channel have comparable cross-sections, while associated production is negligible; at the LHC the situation is reversed: because of the large gluon density at small, both associated production and -gluon fusion have larger cross-section than the s-channel process, with -gluon fusion having the largest cross-section of all three processes (Table 1.2). 15

20 CHAPTER 1. THE TOP QUARK (a) (b) (c) (d) (e) Figure 1.4: Tree-level Feynman diagram for single-top production: (a) s- channel, (b,c) t-channel and (d,e) associated with a. 16

21 B B A B 1.3. SINGLE TOP PRODUCTION Process Tevatron Run II ( ) LHC ( ) LHC ( ) =1.96 TeV =14 TeV =14 TeV (pb) (pb) (pb) Table 1.2: Single top quark production NLO cross-sections [16, 17]. The cross-sections for top and anti-top at LHC are different. t-channel top production occurs mostly via 1 1 and anti-top production via 1 1 where and are valence quarks (see Figure 1.4b). Since LHC is a proton-proton collider, the probability densities for and quarks are in a 2:1 ratio, which is reflected in the cross-section. A similar argument holds for the s-channel. Associated production, instead, is symmetric for top and anti-top because it has no valence quarks in the initial state (see Figure 1.4d,e) Single top production in the s-channel The s-channel process (fig. 1.4.a) proceeds via a virtual time-like boson that decays in a pair. A This process probes the kinematic region 1. The cross-section for this channel at the LHC is much smaller than for the t-channel process; however, the cross-section for this process is known to a better precision, since the initial state involves quarks and anti-quarks, the PDFs of which have been accurately measured. Moreover, the quark luminosity can be constrained by measuring the similar Drell-Yan process [7]. Calculations of the NLO cross-section have been performed, which show a dependence on factorisation and renormalisation scales of about 2%, and resummation effects add another 3% to the uncertainty, while the Yukawa corrections from loop diagrams involving the Higgs field are negligible. It has been shown, however, that uncertainties in the measurement of of ±5 GeV result in uncertainties in the cross-section of = 10%. 17

22 CHAPTER 1. THE TOP QUARK gluon fusion The t-channel process is also known as -gluon fusion since in this process a -quark from a gluon splitting interacts with a space-like to produce the top quark. The -gluon fusion process is the channel with the largest cross-section for single top production at the LHC, about 23 times larger than the cross-section of the s-channel. At the Next-to-Leading-Order level, the process is composed by two diagrams, shown in Figure 1.4(b,c). Both diagrams depict a -quark interacting with a boson emitted by the colliding parton to produce a top quark; diagram (b) depicts a -quark from the quark sea inside the proton, while diagram (c) is a NLO correction to diagram (b) and is relevant if instead we consider the -quark in the initial state as the product of a splitting gluon, where the splitting pair has a non-vanishing transverse momentum. When the pair is collinear with the emitting gluon, diagram (c) becomes a non-perturbative process that can be included in the -quark PDF; the NLO corrections in this kinematical region have to be subtracted from the computation to avoid double counting of such a diagram. The two diagrams (b) and (c) have the same experimental signature a forward scattered light quark, a and a - quark since the additional quark from diagram (c) has for 75% of the events 20 GeV, and thus it is hardly observable [11] Associated production In this production channel, the single top quark is created together with a real boson. Two Feynman diagrams depicted in Figure 1.4(d,e) contribute to this channel. However, the t-channel diagram (e) has a smaller contribution since this diagram describes the splitting of a gluon into a pair and is mass-suppressed; thus the initial state is affected by the low gluon density at high- values. The s-channel diagram (d) dominates the associated production; the ; scaling of this process, combined with the small -quark density, result in a negligible cross-section at the Tevatron, while at the LHC it contributes to about 20% for the total single top production. All of the NLO diagrams have been computed for this production channel [17]. Gluons in the initial state splitting into a collinear pair have been included in the -quark PDF, similar to NLO corrections to the t- channel. It must be remarked that one of the corrections corresponds to the strong production process, followed by top quark decay. This diagram represents a background for the associated production channel, and should be subtracted from the cross-section computation [7]. 18

23 & D 9 D D D 1.4. TOP QUARK DECAY 1.4 Top quark decay In the Standard Model, the top quark predominantly decays into a b quark and a with a branching ratio of The Aleph and Opal experiments at LEP conducted searches for Flavour Changing Neutral Currents C (FCNC) decays, which resulted in upper limits for and of respectively 0.17 and [18]. Other SM-allowed decays to down-like quarks are very difficult to disentangle from the QCD background, as opposed to a -tagged jet. Non-SM decays, however, may provide suitable experimental signatures. The extension of the SM Higgs sector could induce new channels for the top decay. In the so-called two-higgs double models (2HDM), the Higgs sector is composed of two neutral scalars ( " ), a neutral pseudoscalar ( ) and two charged scalars ( ) [11]. In this hypothesis, the top quark could decay into a charged Higgs:. Both CDF and DØ have performed indirect searches in Run I data. No evidence has been found, but searches will continue for Run II. A direct measurement of this 9 channel may be performed searching for the signature, while a heavier charged Higgs decaying to quarks will suffer from QCD jet background. The 2HDM model usually postulates that the charged Higgs should couple preferentially with third-generation quarks due to their large mass. When this assumption is relaxed, new decay channels of the top quark involving Flavour Changing Neutral Currents may emerge: at tree level and at one-loop indicates either, C or. However, the signatures of these channels are very hard to disentangle from the QCD background. 1.5 Top detection and top mass determination Top quarks in the SM decay almost exclusively into. Because of fermion universality in electroweak interactions, the boson decays 1/3 of the time into a lepton/neutrino pair and 2/3 of the time into a pair Top pair events In events two real bosons are present, the signatures of the events are classified according to the decay channel of the bosons: all jets channel Here both s decay into a quark/anti-quark pair. The event has at least six high- jets, two of which have to be -tagged. 19

24 & CHAPTER 1. THE TOP QUARK Despite having the highest branching ratio (44%), this decay channel suffers heavily from QCD background and ambiguities in the assignment of jets to the originating s. lepton+jets channel In this decay channel one decays into a lepton neutrino pair, the other into a quark/anti-quark pair. One isolated lepton, four jets (two with -tagging) and missing energy characterise the event. The branching ratio is about 30%. di-lepton channel In this decay channel, both s decay into a lepton neutrino pair. For practical purposes, only " are considered, since decays are difficult to distinguish from the QCD background. The events have two high- leptons, two jets (at least one of which is -tagged) and missing energy due to the neutrinos. This signature is quite clear, being affected mainly by electroweak background. The only drawback of this decay channel is its low branching ratio (5%). Top quark pairs are produced near threshold and have low kinetic energy, thus present little or no boost in the beam direction. Since the decay products of the top quark have a much smaller mass than the top quark itself, they typically carry large transverse momentum and cross the central region of the detector ( ); the low boost from the decaying top accounts for good angular separation of the decay products. If di-lepton or lepton+jets channels are considered, a large missing energy / is part of the signature. Experimental cuts on and / alone are sufficient to strongly reduce the QCD background, which has an exponentially-falling spectrum and small / [11]. Tagging one or more -jets (either by secondary vertex or soft muon tagging) further reduces background from QCD. In addition to the above cuts, further selections can be performed according to the topological features of top production and its decay channels: for example, semi- and fully leptonic decays give rise to one or two high- isolated leptons; topological variables such as (the scalar sum of the of all observed objects), sphericity ( ) and aplanarity ( ) can be employed to discriminate against QCD background. Top mass determination at CDF The selection cuts used at CDF in the lepton+jets sample require [19]: 20 N one isolated lepton with 20 GeV; N missing energy / 20 GeV;

25 F ' ' A 1.5. TOP DETECTION AND TOP MASS DETERMINATION N at least three jets with 15 GeV and 2 and one jet with 8 GeV and 2, with at least 1 -tagged jet; or 2 and no -tagging re- N at least four jets with 15 GeV and quirement. The selected events are subjected to a kinematical fit with the con- / and ' ; of the possible 24 combinations 12 if -tagging is included the one with the lowest is chosen; the reconstructed top masses are histogrammed and fitted with signal and background templates, where the signal templates vary according to the mass. The mass that provides the best likelihood in the fit determines the final result. straints Top mass determination at DØ Top mass measurements in the lepton+jets channel at DØ employ both a reconstruction technique similar to the one described above and a likelihood method [20]. This method examines the kinematical features of each event, and compares them with a template based on sample events generated at the tree-level by the simulation package VECBOS [21], both for production (signal) and 1 (background), and convoluted with a transfer function that models fragmentation and detector effects. The probability for each event to be a background or a signal event is then used to compute the likelihood for the top quark to have a given mass. Top mass determination at ATLAS Selection cuts for ATLAS require [22]: N one isolated lepton with 20 GeV and 2.5; N missing energy / 20 GeV; N at least four jets with 40 GeV and 2.5, of which at least 2 are -tagged jets. Two of the non-tagged jets are used to reconstruct the, with the constraint / 20 GeV; the reconstructed is combined with one of the two -jets to form the top quark. Of all possible combinations, 21

26 CHAPTER 1. THE TOP QUARK either the one, which gives the highest to the reconstructed top, or the one with the highest angular separation between the -jet and the other jets, is assumed to represent the top quark [22]. The efficiency of this reconstruction is estimated to be 5%, with a top mass resolution of 11.9 GeV Single top events -gluon fusion and s-channel production were studied at the Tevatron during Run I, while associated production has a negligible cross-section at the Tevatron (Table 1.2). CDF and DØ Since single top experimental signatures show a lower jet multiplicity compared to pair production, stringent cuts are required to isolate single top events from QCD background; moreover, each of the three single top production processes is a background process for the other two, the cuts need to be tailored to each of the three channels. CDF and DØ performed an inclusive single top analysis, searching for single top in the 1 sample, asking for the invariant mass of the lepton, / and highest- jet to lie between 140 and 210 GeV. This was followed by a likelihood fit of the total transverse energy. This technique gave a lower limit for inclusive single-top cross-section of 14 pb. CDF also performed two separate searches for s- and t-channel production, which resulted in lower limits of respectively, 18 and 13 pb. DØ used neural network analyses giving lower limits of 17 pb for s-channel and 22 pb for t- channel production. All experimental results are therefore consistent with the theoretical value of the cross-section of less than 1 pb (Table 1.2). ATLAS The ATLAS reconstruction technique entails a set of pre-selection cuts to reduce QCD background; this set includes [22]: N at least one isolated lepton with 20 GeV; N at least two jets with 30 GeV; N at least 1 -tagged jet with 50 GeV. The net effect of these cuts is to select leptonic decay products of the single top. In addition to the pre-selection cuts, each channel requires its own specific set of selection cuts. 22

27 1.5. TOP DETECTION AND TOP MASS DETERMINATION s-channel selection cuts The signature used for the selection of s-channel single top events is shown in Figure 1.5. The signature includes two -jets from both -quarks, one lepton and a neutrino from the decay. The selection cuts require: N exactly two jets within 2.5; this cut reduces background, which has a higher jet multiplicity; N the two jets need to be -tagged and have 75 GeV; this cut reduces both +jets and -gluon fusion, where the second -jet is either missing or has low ; N 175 GeV and total invariant mass greater than 200 GeV; these cuts reduce background, which tend to have smaller total transverse energy and smaller invariant masses; N reconstructed top mass between 150 and 200 GeV. Lepton b-jet from top decay Beam Axis b-jet from hard scattering Neutrino Figure 1.5: Experimental signature of single top production in the s- channel. 23

28 CHAPTER 1. THE TOP QUARK -gluon fusion selection cuts The signature used for the selection of -gluon single top events includes one -jet, one forward-emitted light jet, one lepton and a neutrino from the decay. The selection cuts require: N exactly two jets with 30 GeV; this cut reduces the background; N one of the two jets with 2.5 and 50 GeV; this cut selects the forward light quark, which is the trademark of the -gluon fusion process; N the other jet needs to be -tagged and have 50 GeV; this cut reduces background; N 200 GeV and total invariant mass greater than 300 GeV; these cuts reduce background; N reconstructed top mass between 150 and 200 GeV. The final state may also contain a second -jet, which is very soft and emitted at high pseudorapidity (Figures 1.4.c and 1.6). This second -jet therefore will escape detection at ATLAS, thus it is ignored by the selection cuts. Associated production selection cuts The final state of the associated production channel contains one -jet, a lepton and a neutrino from the decay of one of the bosons, and two light jets from the decay of the other boson (Figure 1.7). The selection cuts require: N exactly three jets with 50 GeV; this cut reduces the background; back- N one of the three jets needs to be -tagged; this cut reduces ground, and further reduces background; back- N total invariant mass less than 300 GeV; this cuts reduces the ground; N the invariant mass of the two non-b jets between 65 and 95 GeV; this cut enhances the probability that a real boson is present in the final state. The efficiencies of the selection cuts are summarised in Table

29 ; 1.6. PERFORMANCE REQUIREMENTS FOR TOP PHYSICS Lepton Leading b-jet Beam Axis Soft b-jet Forward light jet Neutrino Figure 1.6: Experimental signature of single top production in the fusion channel. -gluon 1.6 Performance requirements for top physics Studying electroweak top production processes at hadron colliders involves the correct identification of the decay chain of the top quark, which may contain the following ingredients : N isolated leptons; N hadronic jets; Efficiencies (%) Pre-selection Selection Events/30 fb 26800± ± ±269 Table 1.3: Efficiencies of the selection cuts for -gluon fusion ( ), s- channel production ( ) and associated production ( ) at ATLAS. The pre-selection efficiencies for the two most important background processes, and are given for comparison [22]. 25

30 CHAPTER 1. THE TOP QUARK Lepton b-jet Beam Axis Light jet Light jet Neutrino Figure 1.7: Experimental signature of single top production in the associated production channel. N one or more -tagged jets; N missing energy from the leptonic decay of the. To each item on this list correspond requirements on the performance of the detector: a good detector performance results in a higher efficiency for event analyses and lower systematic error on measurements of the parameters involved in the physical process under study. These requirements include: N correct identification of electrons, photons and pions; N calibrated calorimeter for precise jet energy measurements; N efficient track reconstruction for high- N tagging of -jets; leptons; 26 N detector coverage up to high pseudorapidity for accurate missing transverse energy measurements.

31 A ; A ; 1.6. PERFORMANCE REQUIREMENTS FOR TOP PHYSICS In the past years, detector designs have been optimised in order to fulfil these requirements. The typical experiment is composed of several concentric subdetectors, each performing a specific task, and all or part of the detector is immersed in one or more magnetic fields in order to evaluate charged particle momenta. The innermost layer of a detector is instrumented with silicon trackers. These detectors fulfill the task of tracking the path of charged particles close to the interaction point. The reconstructed tracks can be extrapolated back to the interaction point to evaluate the impact parameters or, if the extrapolation leads to a different point, reveal a secondary vertex. Secondary vertices are a signal for long-lived unstable particles, such as the $ -mesons. Outside the silicon trackers, the calorimetry system forms the next layer. The calorimetry system usually is subdivided into two parts with different characterstics: the electromagnetic calorimeter is closest to the interaction point, with the hadron calorimeter surrounding it. The relatively narrow and short electromagnetic showers are contained in the electromagnetic calorimeter, while hadron showers extend into the hadron calorimeter. The calorimeters succesfuly contain all types of particles but two: neutrinos which can be detected by measuring an imbalance of energy in the calorimeters and muons. The amount of energy deposited by muons in the calorimeters is small and almost independent of their energy. For this reason and for optimal muon identification the typical detector is provided with a Muon Spectrometer, forming the outermost layer. The spectrometer uses gaseous detectors, such as drift chambers, to track muons and measure their momenta. ATLAS is a general-purpose detector designed for the LHC, thus is required to cope with the harsh environment created by the LHC accelerator. ATLAS must operate efficiently in all luminosity regimes, from an initial low luminosity period of cm s up to the nominal LHC luminosity of 10 3 cm s. High luminosity poses a double threat to AT- LAS. For each bunch crossing, about 20 proton-proton interactions take place. Mostly, these interactions are soft elastic collisions minimum bias events that constitute a background for physics processes of interest. Moreover, high luminosity implies high levels of radiation in the detector. The detector elements need to be radiation tolerant in order to keep an acceptable level of performance during the years of operation. In the next sections I will outline the measurement techniques used at ATLAS and the performance they can provide, while the description of the detector and the evaluation of the performance in real-life tests will be laid out in Chapter 2. 27

32 CHAPTER 1. THE TOP QUARK Vertex identification and -tagging The tagging of -quarks is a crucial tool in the study of top decays. Given the high decay branching ratio for top into, the requirement of a - tagged jet is one of the most important analytical cuts to reject backgrounds, which have a high jet multiplicity but low -jet content (such as +jets). The -tagging needs to combine high rejection power with high efficiency in order not to lose signal events. The -tagging algorithm in ATLAS is based on the long lifetime of the $ -mesons: these decay away from the primary interaction vertex 2 ; by measuring the tracks of the decay products the -decay vertex can be reconstructed. For each hadronic jet found in the calorimeter, all tracks measured in the Inner Detector ((Section 2.2)) with >1 GeV and inside a cone of radius 3 around the jet axis are assigned to the jet. Each track yields two parameters: the impact parameter D and the longitudinal impact parameter D. The transverse plane perpendicular to the jet axis and passing through the primary vertex divides the detector into two hemispheres. The impact parameter D is the minimum distance between the extrapolated track and the primary interaction vertex in the transverse plane. The parameter D is signed: it is positive if the intersection of the track with the jet axis is in the same hemisphere as the jet itself and it is negative otherwise. The longitudinal impact parameter D is the coordinate of the intersection point along the jet axis, and is signed in the same way as D. Monte Carlo data has been generated to study the distribution of the impact parameters for - and -quark jets; both distributions show a peak at zero (no secondary vertex) and a small tail at negative values (given by particles wrongly assigned to the jet); however, the -jet distributions for D and D show a tail on the positive side, corresponding to a secondary vertex. The original algorithm described in the ATLAS Technical Design Review (TDR) [23] used only the impact parameter D (hence it is regarded as 2D-algorithm), while the current algorithm combines the two impact parameters (hence it is called 3D-algorithm). The D and D parameters of each track are used to compute a significance: 8 D " D ' D D D 2 The average decay length is given by. The value of is 501 µm for mesons and 460 µm for mesons [1]. In detector coordinates, the angular distance is defined as! D 28

33 1.6. PERFORMANCE REQUIREMENTS FOR TOP PHYSICS where D and D are the resolutions on the impact parameters. The distribution of the significances for -jets and -jets is shown in Figure 1.8. Each track is assigned a value given by the ratio of the significances for the two flavours, and the jet is assigned a weight given by the logarithmic sum of the ratios of the tracks matching the jet: ' 6 The weight is the likelihood measure for a given jet to originate from a -quark; by applying a cut on this weight optimised to reach 50% efficiency on -tagging it is possible to separate light jets from -jets. Though it is not possible to investigate the performance of -tagging algorithms in a test setup, it is possible to apply Monte Carlo tools to study detector effects. The likelihood distributions for light jets and -jets obtained from the Monte Carlo simulation are shown in Figure 1.8. Figure 1.8: Significance of the impact parameters D (left) and D (right) for -jets and -jets. The Monte Carlo data simulates the production process, with the Higgs boson ( =120 GeV) decaying into a pair [24]. In a recent study [25], the comparison of the performance of -tagging in a realistic ATLAS environment against the TDR results includes: N changes in the layout of the Pixel Detector (Section 2.2.1): the initial layout has only 2 pixel layers, the intermediate pixel layer will be installed at a later date; the -layer has been moved further away from the beam with respect to the TDR layout; N the pixel size in the direction is increased from 300 to 400 µm; N ganged pixels (Section 2.2.1) are present in the layout; 29

34 CHAPTER 1. THE TOP QUARK N increase of dead material in the Pixel Detector due to a redesign of detector services; N staging of the wheels of the Transition Radiation Detector (Section 2.2.3); N simulation of detector inefficiencies: on top of the standard 3% inefficiency for Pixels, SCT strips and TRT straw, the effect of 1% dead pixel modules and 2% dead pixel modules are added; N effects of misalignment between the Pixel Detector and the Semiconductor Tracker (Section 2.2.2); N addition of minimum bias pile-up events. The study used samples with,, ( =120 GeV) and ( =400 GeV) events, to evaluate the efficiency of the -tagging algorithm for -jets over a wide range of pseudorapidity and. The Inner Detector was simulated with GEANT3, while the jets were reconstructed using both ATLFAST and full detector simulation the differences were found to be marginal. The results of the study, summarised in Table 1.4, are the following: 30 N changes in the detector layout amount to a reduction in rejection power by a factor 0.5±0.2, mainly due to increase of material in the Inner Detector; N staging of the intermediate layer in the Pixel Detector amounts to a reduction in rejection power by a factor 0.7±0.1; N pile-up events at low luminosity, realistic detector inefficiencies, and misalignment between the Pixel Detector and the SCT during the detector commissioning stages amount to a factor 0.75±0.05; N changing the length of the pixels to 400 µm in the -layer amounts to a 10% decrease in rejection; N improved track fitting algorithms increase the rejection power by a factor 1.8; the improved algorithms perform well with high track multiplicities coming from pile-up events at high luminosity: the rejection power is degraded only by 10% at high luminosity;

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