Signal background interference effects in heavy scalar production and decay to a top-anti-top pair

Abstract

Abstract:We analyse the production of a top quark pair through a heavy scalar at the LHC. We first review the main features of the signal as well as the interference with the top-anti-top background at leading order in QCD. We then study higher order QCD effects. While the background and the signal can be obtained at NNLO and NLO in QCD respectively, that is not the case for their interference, which is currently only approximately known at NLO. In order to improve the accuracy of the prediction for the interference term, we consider the effects of extra QCD radiation, i.e. the 2 → 3 (loop-induced) processes and obtain an estimate of the NLO corrections. As a result, we find that the contribution of the interference is important both at the total cross-section level and, most importantly, for the line-shape of the heavy scalar. In particular for resonances with widths larger than a couple of percent of the resonance mass, the interference term distorts the invariant mass distribution and generically leads to a non-trivial peak-dip structure. We study this process in a simplified model involving an additional scalar or pseudoscalar resonance as well as in the Two-Higgs-Doublet-Model for a set of representative benchmarks. We present the constraints on simplified models featuring an extra scalar as set by the LHC searches for top-anti-top resonances, and the implications of the 750 GeV diphoton excess recently reported by CMS and ATLAS for the top pair production assuming a scalar or a pseudoscalar resonance.

Scalar production and decay to top quarks including interference effects at NLO in QCD in an EFT approach

“…[23][24][25][26]. It appears to receive similar QCD corrections, as indicated by estimates in an effective field theory (EFT) approach in the soft gluon approximation [27], and in a simplified K-factor derived from the geometric mean of the background and signal K-factors [28].…”

“…It has been noticed in an earlier work [22], and discussed in a series of recent works [25,26,28,[48][49][50][51][52], that the production of a heavy scalar resonance leads to large interference with the SM tt background. This large interference could be further augmented by a nontrivial relative phase between the signal and the SM background amplitude, possibly leading to more complicated structures.…”

“…More recently in ref. [28], predictions based on K-factors inferred from the pure signal and background components have also been provided. In this work, we aim to follow a more solid approach to the NLO computation of the interference, based on the EFT framework, which in the unresolved case provides accurate results (i.e.…”

“…We note here that with our setup we can also include the exact real emission amplitudes in a fashion similar to what has been done for double Higgs production in [29,30], but we refrain from doing so for simplicity. A discussion of the real emission amplitudes in the interference for this process can be found in [28].…”

“…Finally, as a comparison, we follow the approach in ref. [28] and rescale the LO lineshape by a K-factor inferred by using √ K S K B , where K S and K B are the (bin by bin) K-factors for the SM and the signal. As shown in the plot, the resulting lineshape is a reasonable approximation in the absence of a full NLO computation for the interference which we provide in this work, but it tends to overestimate the size of the interference at and below the resonance.…”