Projected transverse momentum resummation in top-antitop pair production at LHC

Abstract

The transverse momentum distribution of the tt¯ system is of both experimental and theoretical interest. In the presence of azimuthally asymmetric divergences, pursuing resummation at high logarithmic precision is rather demanding in general. In this paper, we propose the projected transverse momentum spectrum dσtt¯/dqτ , which is derived from the classical q→T spectrum by integrating out the rejection component qτ⊥ with respect to a reference unit vector τ→, to serve as an alternative solution to remove these asymmetric divergences, in addition to the azimuthally averaged case dσtt¯/d∣q→T∣. In the context of the effective field theories, SCETII and HQET, we will demonstrate that in spite of the qτ⊥ integrations, the leading asymptotic terms of dσtt¯/dqτ still observe the factorisation pattern in terms of the hard, beam, and soft functions in the vicinity of qτ = 0 GeV. Then, with the help of the renormalisation group equation techniques, we carry out the resummation at NLL+NLO, N2LL+N2LO, and approximate N2LL′+N2LO accuracy on three observables of interest, dσtt¯/dqT,in,dσtt¯/dqT,out , and dσtt¯/dΔϕtt¯, within the domain Mtt¯ ≥ 400 GeV. The first two cases are obtained by choosing τ→ parallel and perpendicular to the top quark transverse momentum, respectively. The azimuthal de-correlation Δϕtt¯ of the tt¯ pair is evaluated through its kinematical connection to qT,out. This is the first time the azimuthal spectrum Δϕtt¯ is appraised at or beyond the N2LL level including a consistent treatment of both beam collinear and soft radiation.