Measurement of the W boson polarization in top quark decays with the ATLAS detector

Abstract

This paper presents measurements of the polarization of W bosons in top quark decays, derived from tt events with missing transverse momentum, one charged lepton and at least four jets, or two charged leptons and at least two jets. Data from pp collisions at a centre-of-mass energy of 7 TeV were collected with the ATLAS experiment at the LHC and correspond to an integrated luminosity of 1.04 fb −1 . The measured fractions of longitudinally, left-and right-handed polarization are F 0 = 0.67 ± 0.07, F L = 0.32 ± 0.04 and F R = 0.01 ± 0.05, in agreement with the Standard Model predictions. As the polarization of the W bosons in top quark decays is sensitive to the W tb vertex Lorentz structure and couplings, the measurements were used to set limits on anomalous contributions to the W tb couplings. Keywords: Hadron-Hadron ScatteringOpen Access, Copyright CERN, for the benefit of the ATLAS collaboration One particular test is the study of the W tb vertex Lorentz structure and couplings, which can be probed by measuring the polarization of W bosons produced in top quark decays. At the LHC, top quarks are produced mainly in pairs via the strong interaction and are predicted to decay via the electroweak interaction into a W boson and a bottom quark with a nearly 100% branching fraction. Events with tt pairs can thus be classified according to the decay of the two produced W bosons. Each boson can decay either into -1 - JHEP06(2012)088a quark-antiquark pair or into a charged lepton and a neutrino. The single-lepton and dilepton topologies, both considered in the analyses presented in this paper, have one and two isolated charged leptons in the final state. Only electrons and muons, including those from τ decays, are considered here.The W tb vertex is defined by the electroweak interaction and has a (V − A) structure where V and A are the vector and axial-vector contributions to the vertex. Since the W bosons are produced as real particles in top quark decays, their polarization can be longitudinal, left-handed or right-handed. The fractions of events with a particular polarization, F 0 , F L and F R , are referred to as helicity fractions. They are predicted in next-to-nextto-leading-order (NNLO) QCD calculations to be F 0 = 0.687 ± 0.005, F L = 0.311 ± 0.005, F R = 0.0017 ± 0.0001 [4]. These fractions can be extracted from measurements of the angular distribution of the decay products of the top quark. The angle θ * is defined as the angle between the momentum direction of the charged lepton from the decay of the W boson and the reversed momentum direction of the b-quark from the decay of the top quark, both boosted into the W boson rest frame [5]. The angular distribution is:All previous measurements of the helicity fractions, performed by the CDF and DØ Collaborations [6][7][8] at the Tevatron, are in agreement with Standard Model predictions. Information about the polarization of the W bosons can also be obtained through complementary observables, such as the angular asymmetries, A + and A − , defined as:wi...

Search for anomalous couplings in the W tb vertex from the measurement of double differential angular decay rates of single top quarks produced in the t-channel with the ATLAS detector

“…However, within the models studied so far for tensor couplings, the magnitude of g R is expected to be < 0.01 [14,15]. Measurements of the W boson polarisation fractions [16][17][18][19] are sensitive to the magnitude of combinations of anomalous couplings. In order to obtain constraints on Re [g R ] in those measurements, the anomalous couplings are taken to be purely real, V R = g L = 0 is assumed and V L is fixed to 1.…”

“…Theẑ-axis is chosen along the direction of the W boson boosted into the top-quark rest frame,ẑ ≡q = q/| q|. The angle θ * betweenq and the momentum of the lepton in the W boson rest frame, p , is the same angle used to measure the W boson helicity fractions in top-quark decays [17][18][19]. The spin of single top quarks produced in the t-channel, s t , is predicted to be polarised along the direction of the light quark (q in figure 1) in the top-quark rest frame [20], called the spectator-quark direction,p s .…”

“…Thus δ + cannot be constrained and f + 0 cannot be separated from P . Constraints on f + 1 are currently better determined from limits on F R [17,18]. The variations that the parameters f 1 and δ − induce in the model are demonstrated in figure 3 for t-channel signal events generated with Protos.…”

“…The correlation in the measurement of these coupling ratios is [17] and from CMS [18], respectively. However, these limits use the measured single top-quark production cross-section [46,70] …”

New directions for top quark polarization in thet-channel process

“…At present, the most stringent direct limits on them result from the measurement of two asymmetries [10] in tt production by the ATLAS Collaboration, ReV R ∈ ½−0.20; 0.23, Reg L ∈ ½−0.14; 0.11, and Reg R ∈ ½−0.08; 0.04 with a 95% confidence level, assuming only one nonzero anomalous coupling [11]. (The combination with single-top cross sections would improve these limits [12].)…”