New directions for top quark polarization in the t -channel process

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...

Section: Jhep04(2016)023mentioning

confidence: 99%

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

Aad¹,

Abbott²,

Abdallah³

et al. 2016

“…As A N FB depends on P , the measured value of the A FB asymmetry is required to constrain P for the limit computation. The quadratic variation of P and α as a function of Im g R [5,18] is taken into account when setting the limits through the -6 -JHEP04(2017)124 procedure explained in section 11. The A FB asymmetry is chosen to constrain P because its measurement is found to be independent of the value of Im g R assumed when unfolding the corresponding angular distribution; this is discussed in section 9.…”

Section: Jhep04(2017)124mentioning

confidence: 99%

“…By taking into account the analytic expressions and parameterisations introduced in refs. [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].…”

Section: Jhep04(2017)124mentioning

confidence: 99%

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

Aaboud¹,

Aad²,

Alstaty³

et al. 2017

To probe the W tb vertex structure, top-quark and W -boson polarisation observables are measured from t-channel single-top-quark events produced in proton-proton collisions at a centre-of-mass energy of 8 TeV. The dataset corresponds to an integrated luminosity of 20.2 fb −1 , recorded with the ATLAS detector at the LHC. Selected events contain one isolated electron or muon, large missing transverse momentum and exactly two jets, with one of them identified as likely to contain a b-hadron. Stringent selection requirements are applied to discriminate t-channel single-top-quark events from background. The polarisation observables are extracted from asymmetries in angular distributions measured with respect to spin quantisation axes appropriately chosen for the top quark and the W boson. The asymmetry measurements are performed at parton level by correcting the observed angular distributions for detector effects and hadronisation after subtracting the background contributions. The measured top-quark and W -boson polarisation values are in agreement with the Standard Model predictions. Limits on the imaginary part of the anomalous coupling g R are also set from model-independent measurements. The ATLAS collaboration 33 IntroductionAt hadron colliders, top quarks are predominantly produced in pairs (tt) via the flavourconserving strong interaction, but single top-quark production can occur via chargedcurrent electroweak processes involving a W tb vertex. At leading order in QCD perturbation theory, three sub-processes contribute to single top-quark production: an exchange of a virtual W boson either in the t-channel or in the s-channel, or the associated production of a top quark with an on-shell W boson (W t). The t-channel and s-channel processes do not interfere at next-to-leading-order in QCD and are thus well defined with that precision [1]. In proton-proton (pp) collisions, the t-channel exchange, depicted in figure 1, is the dominant production process of single top quarks. The exchange of a space-like W boson due to the interaction of a light quark with a b-quark produces a top quark and a forward light-quark (called the spectator quark) in the final state. Furthermore, as a consequence of the vector minus axial-vector (V-A) form of the W tb vertex in the Standard Model, the Figure 1. Leading-order Feynman diagram for t-channel production of single top quarks in pp collisions. In the depicted four-flavour scheme (2 → 3 process) the initial b-quark arises from a gluon splitting into a bb pair.produced top quarks are highly polarised, in particular along the direction of the spectatorquark momentum [2,3]. Within the Standard Model the top quark decays through the electroweak interaction into an on-shell W boson and a b-quark, with a lifetime much shorter than the time scale necessary to depolarise the spin. The information on the top-quark spin can thus be obtained from its decay products. The produced real W boson also possesses a polarisation (or helicity state), which can be extracted from angular ...

“…It is therefore expected that non-standard top-Higgs couplings will affect the polarisation states of top quark and change the angular distributions of the top quark decay products [36]. Such a polarisation asymmetry has been widely used to probe the anomalous top quark interactions at the LHC [52][53][54][55][56][57][58][59][60][61][62][63][64][65][66][67][68][69]. The precise measurement of thj channel opens a new window to probe the top quark Yukawa couplings and new physics at the LHC.…”

Section: Jhep10(2014)100mentioning

confidence: 99%

Anomalous top-Higgs couplings and top polarisation in single top and Higgs associated production at the LHC

Kobakhidze

Yue

2014

In this paper, we put constraints on anomalous CP-violating top-Higgs couplings using the currently available Higgs data and explore the prospect of measuring these couplings at 240 GeV TLEP. We find that the CP-violating phase ξ is currently limited in the range |ξ| < 0.6π at 95% C.L. and may be further constrained to |ξ| < 0.07π at TLEP. Under this consideration, we further investigate the observability of the scalar (ξ = 0), pseudoscalar (ξ = 0.5π) and mixed (ξ = 0.25π) top-Higgs interactions through the channel pp → t(→ ℓ + ν ℓ b)h → bb j. We find that it is most promising to observe pure pseudoscalar interactions with y t = y SM t , although this will be challenging due to a low signal to background ratio. We also find that the anomalous top-Higgs couplings can lead to sizeable differences in lepton forward-backward asymmetries and can be distinguished by measuring the lepton angular distributions from polarised top quarks at 14 TeV LHC.