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

Abstract

Abstract:The electroweak production and subsequent decay of single top quarks is determined by the properties of the Wtb vertex. This vertex can be described by the complex parameters of an effective Lagrangian. An analysis of angular distributions of the decay products of single top quarks produced in the t-channel constrains these parameters simultaneously. The analysis described in this paper uses 4.6 fb −1 of proton-proton collision data at √ s = 7 TeV collected with the ATLAS detector at the LHC. Two parameters are measured simultaneously in this analysis. The fraction f 1 of decays containing transversely polarised W bosons is measured to be 0.37 ± 0.07 (stat.⊕syst.). The phase δ − between amplitudes for transversely and longitudinally polarised W bosons recoiling against lefthanded b-quarks is measured to be −0.014π ± 0.036π (stat.⊕syst.). The correlation in the measurement of these parameters is 0.15. These values result in two-dimensional limits at the 95% confidence level on the ratio of the complex coupling parameters g R and A Parameter dependence of the folding and background models 23The ATLAS collaboration 29 IntroductionThe top quark is the heaviest known fundamental particle, making the measurement of its production and decay kinematics a unique probe of physical processes beyond the Standard Model (SM). The top quark was first observed in 1995 at the Tevatron [1, 2] through its dominant production channel, top-quark pair (tt) production via the flavour-conserving strong interaction. An alternative process produces single top quarks through the weak interaction, first observed at the Tevatron in 2009 [3, 4]. The t-channel exchange of a virtual W boson is the dominant process contributing to single top-quark production (see figure 1). The production cross-section is calculated for Figure 1. Representative Feynman diagrams for t-channel single top-quark production and decay. Here q represents a u ord quark, and q represents a d orū quark, respectively. The initial b-quark arises from (a) a sea b-quark in the 2 → 2 process, or (b) a gluon splitting into a bb pair in the 2 → 3 process.proton-proton (pp) collisions at √ s = 7 TeV using a next-to-leading-order (NLO) QCD prediction with resummed next-to-next-to-leading logarithmic (NNLL) accuracy, referred to as approximate next-to-next-to-leading order (NNLO). With a top-quark mass of 172.5 GeV and MSTW2008NNLO [5] parton distribution function (PDF) sets, the cross-section for the t-channel process is calculated to be 64.6+2.6 −1.7 pb [6]. The uncertainties correspond to the sum in quadrature of the error obtained from the MSTW PDF set at the 90% confidence level (C.L.) and the factorisation and renormalisation scale uncertainties.New physics resulting in corrections to the Wtb vertex would affect t-channel single top-quark production and decay, and can be probed through processes which affect variables sensitive to the angular distributions of the final-state particles from the top-quark decay. Within the effective field theory framework, the Lagrang...

Probing the W tb vertex structure in t-channel single-top-quark production and decay in pp collisions at s = 8 $$ \sqrt{s}=8 $$ TeV with the ATLAS detector

“…[4,5,18] for the W tb coupling dependence of S 2 , α and P , it is possible to determine the allowed region for Im g R from the measured values of A N FB and A FB . The limit setting is based on the computation of the χ 2 test statistic using the covariance matrix associated with the A N The measured interval of allowed values slightly improves on the limits set at 7 TeV by the ATLAS Collaboration from the measurement of double-differential angular decay rates [6].…”

“…If there are two real solutions, the solution giving the smallest magnitude of the longitudinal neutrino momentum is taken. If there are complex solutions, the magnitude of the measured missing transverse momentum is rescaled in order to obtain a physical solution [6]. The top-quark candidate is reconstructed by combining the four-momenta of the reconstructed W boson and the b-tagged jet.…”

“…They also include the effects due to hadronisation and parton showering. 6 Partons are defined from the matrix-element hard process and immediate decays. FB .…”

“…The imaginary part of g R (Im g R ) can be probed with the best precision in the t-channel production of single top quarks through the measurement of polarisation observables [5]. Limits on Im g R have been set at the LHC by the ATLAS Collaboration at a centre-of-mass energy of 7 TeV from the analysis of the double-differential angular decay rates of the produced t-channel single-top-quark events [6]. Searches for anomalous -2 -…”

Pinning down electroweak dipole operators of the top quark

“…This is reflected by precise measurements of W helicity fractions and top quark spin correlations [14][15][16][17] which can be translated into bounds on anomalous couplings. All other electroweak interactions of the top quark with the Z , γ , and the Higgs boson are much less explored.…”