Two-loop QCD corrections to top quark decay

We present O(alpha_s^2) corrections to the decay t-->bW in the limit of a very large top quark mass, m_t >> m_W. We find that the O(alpha_s^2) effects decrease the top quark decay width Gamma_t by about 2%: Gamma_t = Gamma_0(1-0.8alpha_s(m_t)-1.7alpha_s^2). The complete corrections are smaller by about 24% than their estimate based on the BLM effects O(beta_0 alpha_s^2). We explain how to compute a new type of diagrams which contribute to Gamma(t --> bW) at the O(alpha_s^2) l...

We present a determination of a new class of three-loop Feynman diagrams describing heavy-to-light transitions. We apply it to find the O(alpha_s^2) corrections to the top quark decay t -> bW and to the distribution of lepton invariant mass in the semileptonic b quark decay b -> ul\nu. We also confirm the previously determined total rate of that process as well as the O(alpha^2) corrections to the muon lifetime.

In this article we discuss the calculation of single top-quark production in the t channel at two-loop order in QCD. In particular we present the decomposition of the amplitude according to its spin and colour structure and present complete results for the two-loop amplitudes in terms of master integrals. For the vertex corrections compact analytic expressions are given. The box contributions are implemented in a publicly available C program.

Corrections of $O(\alpha_s^2)$ to the decay of the top quark into a W boson and a bottom quark are calculated. The method is based on an expansion of the top quark propagator for small external momentum, q, as compared to the top quark mass, M_t. The physical point q^2 = M_t^2 is reached through Pad\'e approximations. The described method allows to take effects induced by a finite W boson mass into account. The numerical relevance of the result is discussed. Important cross-c...

The $tW$ associated production has the second-largest cross section among three single top production channels. A complete study of NNLO QCD corrections to $tW$ production is still missing in literature. We present the calculation of part of the requisite two-loop master integrals for the process $b+g \to t+W$ at NNLO QCD. It turns out that the 80 master integrals in two families, involving up to three massive internal lines, can be expressed by Goncharov polylogarithms. The ...

We present the status of on-going efforts to compute the two-loop virtual corrections to $pp\to t\bar{t} j$ in the leading colour approximation. We review the recent study of the master integrals and their differential equations and present some ideas on modern techniques that can be applied to provide complete evaluation of helicity amplitudes containing information on top quark decays in the narrow width approximation.

We report on a recent calculation of the top decay rate up to order \alpha_s^2. It is based on asymptotic expansions of the off-shell top propagator, followed by a Pad\'e approximation in order to reach the physically relevant point q^2=Mt^2.

We calculate the b-dependent self-energy of the top quark at O(g^2 \alpha_s) by using a general massive two-loop algorithm proposed in a previous article. From this we derive by unitarity the O(\alpha_s) radiative corrections to the decay width of the top quark, where all effects associated with the $b$ quark mass are included without resorting to a mass expansion. Our results agree with the analytical results available for the O(\alpha_s) correction to the top quark width.

A method of computing two-loop fermionic contributions to the width of a heavy quark is described. An analytical formula for this effect in the limit of mass of the quark much larger than the decay products is obtained for the first time. The result confirms previous numerical studies.

We present the analytic calculation of two-loop master integrals that are relevant for $tW$ production at hadron colliders. We focus on the integral families with only one massive propagator. After choosing a canonical basis, the differential equations for the master integrals can be transformed into the $d$ln form. The boundaries are determined by simple direct integrations or regularity conditions at kinematic points without physical singularities. The analytical results in...