Off-shell single-top-quark production in the Standard Model Effective Field Theory

Abstract

We present a fully differential and spin-dependent t-channel single-top-quark calculation at next-to-leading order (NLO) in QCD including off-shell effects by using the complex mass scheme in the Standard Model (SM) and in the Standard Model Effective Field Theory (SMEFT). We include all relevant SMEFT operators at 1/Λ 2 that contribute at NLO in QCD for a fully consistent comparison to the SM at NLO. In addition, we include chirality flipping operators that do not interfere with the SM amplitude and contribute only at 1/Λ 4 with a massless b-quark. Such higher order effects are usually captured by considering anomalous right-handed W tb and left-handed W tb tensor couplings. Despite their formal suppression in the SMEFT, they describe an important class of models for new physics. Our calculation and analysis framework is publicly available in MCFM.

Single-top-quark production in the t-channel at NNLO

“…For the study of anomalous couplings and SMEFT contributions this perturbative stability at NNLO is important, since such analyses are performed at NLO. In fact, the inclusion of off-shell effects at NLO has previously been found to be equally or more important for some angular distributions [33].…”

“…For a precision prediction of the invariant mass of the top quark it is clearly essential to move away from this approximation and, in general, more differentially such effects are of increasing importance. Calculations of off-shell effects were first performed in an effective field theory approach, valid in the region of the resonance [27][28][29][30], and subsequently computed exactly [31][32][33].…”

Precision phenomenology with MCFM

“…MCFM is now capable of computing W ± , Z, H as well as γγ, W ± H, ZH and Zγ production processes at NNLO, as well as hundreds of processes at NLO. Some processes include NLO electroweak corrections [22], while others account for contributions from BSM sources and anomalous couplings, as well as NLO corrections for the SMEFT [23]. All leptonic decay channels of Z and W ± are included as well as the Higgs boson decays into γγ, W + W − , ZZ, Zγ, τ + τ − and bb.…”

Precise predictions for single-top production: the impact of EW corrections and QCD shower on the t-channel signature

“…Within the SM the interest in single-top production is mainly motivated by the possibility of directly extracting the value of |V tb | in the CKM matrix element [1][2][3][4][5]. Moreover, in numerous BSM scenarios, single-top production provides a sensitive probe to New Physics effects [6][7][8][9][10][11][12][13][14][15][16], possibly parametrised through dimension-6 operators [14][15][16][17][18]. Indeed, involving the heaviest of the SM particles and EW interactions, this class of processes is of paramount interest for physics beyond-the-SM (BSM).…”