Full analysis of general non - standard tbW couplings

Possible non-standard couplings which could contribute to the t → bW process are studied based on the effective-Lagrangian approach. The corresponding effective Lagrangian consists of four kinds of dimension-6 effective operators, each of which has an independent coupling constant. In this analysis, all those couplings are treated as complex numbers and constraints on them are estimated by using recent experimental data from the LHC. We point out that the resultant constraints on those couplings are still not that strong because contributions from some couplings can work oppositely with each other. PACS: 12.38.Qk, 12.60.-i, 14.65.Ha E-mail address: [email protected] E-mail address: [email protected] The top quark, the mass of which is about 173 GeV, is still the heaviest particle we can observe up to now although a new scalar indicating the Higgs boson, the last piece of the standard model, has been discovered [1, 2]. Studying this quark from various angles will be, therefore, a quite promising approach to new physics beyond the standard model [3, 4, 5]. In particular, precise analyses of the topquark couplings could play a crucial role to reveal new-physics effects that might exist behind phenomena observed in collider experiments. We will soon have more information for those studies, considering that the Large Hadron Collider (LHC) has now re-started measuring the top-quark properties more precisely with √ s = 13 TeV and a plan of luminosity upgrade [6]. In precision measurements, a sign of new-physics will appear in various observables as deviations from the standard-model predictions, unless new (non-standard) particles are directly discovered. Since those deviations in general arise through quantum loop effects of non-standard particles, the effective-Lagrangian procedure [7]–[10] is known as a useful way to describe such effects. This approach enables a model-independent analysis if we construct the effective Lagrangian using only the standard-model fields below the new-physics scale (Λ). The top-quark-decay process we focus on, t → bW , is suitable for those studies because a top quark decays quickly within the perturbation region owing to its heavy mass [11, 12]. Although many authors have already studied top-decay processes in the effectiveLagrangian framework in order to probe possible new interactions [13]–[36], the non-standard couplings included there have been treated as real numbers, or as partially complex numbers, and/or only some couplings have been treated as free parameters at once fixing the others. In addition, it has not been unusual to adopt the linear approximation in those parameters, i.e., to neglect their quadratic (and higher-power) terms. Those limited analyses could be reasonable if the authors are implicitly considering some specific models. We cannot say however that they are the most satisfactory as purely model-independent studies. Therefore, in this short article, assuming all those non-standard couplings are complex numbers and contribute to the top-decay process at the same time, we estimate current constraints on them from recent experimental data without taking the linear approximation. In our analysis, we assume that there exist no new particles at any energy less than Λ. Based on this assumption and adopting the notations of our previous work [37, 38, 39], the effective Lagrangian for t→ bW is expressed as LtbW = − 1 √ 2 g [ ψ̄b(x)γ (f 1 PL + f R 1 PR)ψt(x)W − μ (x) + ψ̄b(x) σ MW (f 2 PL + f R 2 PR)ψt(x)∂μW − ν (x) ]