Measurement of B ( t → Wb ) / B ( t → Wq ) at √ s = 1 . 96 TeV DØ

We present the measurement of R = B(t →Wb)/B(t →Wq) in pp̄ collisions at √s = 1.96 TeV, using 230 pb−1 of data collected by the DØ experiment at the Fermilab Tevatron Collider. We fit simultaneously R and the number (Ntt̄ ) of selected top quark pairs (t t̄ ), to the number of identified b-quark jets in events with one electron or one muon, three or more jets, and high transverse energy imbalance. To improve sensitivity, kinematical properties of events with no identified b-quark jets are included in the fit. We measure R = 1.03+0.19 −0.17 (stat+ syst), in good agreement with the standard model. We set lower limits of R > 0.61 and |Vtb|> 0.78 at 95% confidence level. © 2006 Elsevier B.V. All rights reserved. PACS: 12.15.Hh; 14.65.Ha Within the standard model (SM), the top quark decays 99.8% of the time to a W boson and a b quark, with the ratio R = B(t →Wb)/B(t →Wq) (here q refers to d, s, or b quarks) expressible in terms of the Cabbibo–Kobayashi–Maskawa (CKM) matrix elements [1] R = |Vtb|2 |Vtb|2+|Vts |2+|Vtd |2 . The unitarity of the CKM matrix and experimental constraints on its elements [2] yield the SM prediction 0.9980 < R < 0.9984 at the 90% C.L. Nevertheless, a fourth generation of quarks or non-SM * Corresponding author. E-mail address: [email protected] (C. Clément). 1 On leave from IEP SAS Kosice, Slovakia. 2 Visitor from Purdue University Calumet, Hammond, IN, USA. 3 Visitor from Helsinki Institute of Physics, Helsinki, Finland. processes in the production or decay of the top quark could lead to significant deviations from the SM. So far, measurements of R by the CDF Collaboration [3,4] have not established a deviation of R from unity. In the present analysis, we assume that the top quark decays into a W boson, but that the associated quark can be d , s, or b. Lepton + jets final states arise in t t̄ when one W boson decays leptonically and the other into a qq̄ ′ pair. About 6% of the signal arises from t t̄ events in which both W bosons decay leptonically, but one charged lepton is not reconstructed, while additional jets are produced by initial or final state radiation. In this Letter, we report the measurement of R in the lepton (electron or muon) + jets channel ( + jets). The lepton can come either from a direct W decay or from W → τ → e/μ. We use b-jet identification (b-tagging) techniques, exploiting the long life620 DØ Collaboration / Physics Letters B 639 (2006) 616–622 time of B hadrons, to separate t t̄ events from the background processes. The data were collected by the DØ experiment from August 2002 through March 2004, and correspond to an integrated luminosity of 230 pb−1. The DØ detector incorporates a tracking system, calorimeters, and a muon spectrometer [5]. The tracking system is made up of a silicon micro-strip tracker (SMT) and a central fiber tracker (CFT), located inside a 2 T superconducting solenoid. The tracking system provides efficient charged particle detection in the pseudorapidity region |η|< 3.4 The SMT strip pitch of 50–80 μm allows a precise determination of the primary interaction vertex (PV) and an accurate measurement of the impact parameter of a track relative to the PV.5 These are key components of the lifetime-based b-tagging algorithms. The PV is required to be within the fiducial region of the SMT and to contain at least three tracks. The calorimeter consists of a barrel section covering |η|< 1.1, and two end-caps extending the coverage to |η| ≈ 4.2. The muon spectrometer surrounds the calorimeter and consists of three layers of drift chambers and several layers of scintillators [6]. A 1.8 T iron toroidal magnet is located outside the innermost layer of the muon system. The luminosity is calculated from the rate of pp̄ inelastic collisions, detected by two arrays of scintillation counters mounted close to the beam-pipe on the front surfaces of the calorimeter end-caps. We select data in the electron and muon decay channels by requiring an isolated electron with pT > 20 GeV and |η|< 1.1, or an isolated muon with pT > 20 GeV and |η| < 2.0. The lepton isolation criteria are based on calorimeter and tracking information. More details on lepton identification and trigger requirements are available in Ref. [7]. In both channels, we require the missing transverse energy (/ ET ) to exceed 20 GeV and not be collinear with the direction of the lepton projected on the transverse plane. The candidate events must be accompanied by jets with pT > 15 GeV and rapidity |y|< 2.5 (footnote 4). Jets are defined using a cone algorithm with radius R= 0.5 [8]. We use a secondary vertex tagging (SVT) algorithm to reconstruct displaced vertices produced by the decay of B hadrons inside jets. Secondary vertices are reconstructed from two or more tracks satisfying: pT > 1 GeV, 1 hits in the SMT detector, and impact parameter significance dca/δdca > 3.5 (footnote 5). Tracks identified as arising from K0 S or Λ decays or from γ conversions are not used. If the secondary vertex reconstructed within a jet has a decay-length significance Lxy/δLxy > 7, 6 the jet is defined as b-tagged. Events with exactly 1 ( 2) b-tagged jets are referred to as 1-tag (2-tag) 4 Rapidity y and pseudorapidity η are defined as functions of the parameter β and polar angle θ w.r.t. the proton beam line, as y(θ,β) ≡ 1 2 ln [(1 + β cos θ)/(1 − β cos θ)] and η(θ) ≡ y(θ,1), where β is the ratio of a particle’s momentum to its energy. 5 Impact parameter is defined as the distance of closest approach (dca) of the track to the primary vertex in the plane transverse to the beam line. Impact parameter significance is defined as dca/δdca , where δdca is the error on dca. 6 Decay length Lxy is defined as the distance from the primary to the secondary vertex in the plane transverse to the beam line. Decay length significance is defined as Lxy/δLxy , where δLxy is the uncertainty on Lxy . events. Events with no b-tagged jets are referred to as 0-tag events. A prediction for the number of background events and the fractions of t t̄ events in the 0, 1, and 2-tag samples require the probabilities for different types of jets (b-, c-, and lightquark jets) to be b-tagged. The calculation of these probabilities is presented in Ref. [13]. We fit simultaneously R and the total number of t t̄ events in the 0, 1, and 2-tag samples (Ntt̄ ) to the number of observed 1-tag and 2-tag events, and, in 0-tag events, to the shape of a discriminant variable D that exploits kinematic differences between the backgrounds and the t t̄ signal. The main background in this analysis is from the production of leptonically decaying W bosons produced in association with jets (W + jets). Most of the jets accompanying the W boson originate from u, d , and s quarks and gluons (W + light jets). Between 2% and 14% of W + jets events contain heavyflavor jets, arising from gluon splitting into bb̄ or cc̄ (Wbb̄ or Wcc̄, respectively). About 5% of the W + jets events contain a single c quark that originates from W -boson radiation from an s quark in the proton or anti-proton sea (s → Wc). A sizable background arises from strong production of two or more jets (“multijets”), with one of the jets misidentified as an isolated lepton, and accompanied by large / ET resulting from mismeasurement of jet energies. Significantly smaller contributions to the selected sample arise from Z+ jets, WW , WZ, ZZ, and single top quark production. Together, these five smaller backgrounds are expected to contribute from 1% to 7% of the selected sample, depending on the number of b-tagged jets, and are referred to below as “other” backgrounds. Normalization of the backgrounds begins with the determination of the number of multijet events in the selected sample. The multijet background is determined using control data samples and probabilities for jets to mimic isolated lepton signatures, also derived from data [7]. Subtracting this background also provides the fraction of events with a truly isolated highpT lepton (i.e., t t̄ and all backgrounds, except multijets). The contributions from single top quark, Z+ jets, and diboson production are determined from Monte Carlo simulation (MC). The remainder corresponds either to t t̄ or W + jet production. The W + jets background normalization is constrained by the untagged data, as a function of jet multiplicity, while its flavor composition is taken from MC. The signal and background processes are generated using ALPGEN [9] with mt = 175 GeV. PYTHIA [10] is used for fragmentation and decay. B hadron decays are modeled via EVTGEN [11]. A full detector simulation is performed using GEANT [12]. In an analysis based on the SM, with R ≈ 1, the t t̄ event tagging probabilities are computed assuming that each of the signal events contains two b-jets [13]. In the present analysis, the top quark can also decay into a light quark (d or s) and a W boson. The ratio R determines the fraction of t t̄ events with 0, 1, and 2 b-jets and therefore how t t̄ events are distributed among the 0, 1, and 2-tag samples. In order to derive the t t̄ event tagging probability as a function of R, we determine the tagging probability for the three following scenarios (i) t t̄ →W+bW−b̄ (to be referred to as t t → bb), (ii) t t̄ →W+bW−q̄l or its charge conjugate (referred to as t t → bql), and (iii) t t̄ →W+qlW−q̄l (referred to as t t → qlql), where ql denotes either a d or s DØ Collaboration / Physics Letters B 639 (2006) 616–622 621