Abstract
Abstract Current Higgs data show an ambiguity in the value of the Yukawa couplings to quarks and leptons. Not so much because of still large uncertainties in the measurements but as the result of several almost degenerate minima in the coupling profile likelihood function. To break these degeneracies, it is important to identify and measure processes where the Higgs coupling to fermions interferes with other coupling(s). The most prominent example, the decay of h → γγ, is not sufficient to give a definitive answer. In this paper, we argue that t-channel single top production in association with a Higgs boson, with h → bb, can provide the necessary information to lift the remaining degeneracy in the top Yukawa. Within the Standard Model, the total rate is highly reduced due to an almost perfect destructive interference in the hard process, W b → th. We first show that for non-standard couplings the cross section can be reliably computed without worrying about corrections from physics beyond the cutoff scale Λ ≳ 10 TeV, and that it can be enhanced by more than one order of magnitude compared to the SM. We then study the signal pp → thj(b) with 3 and 4 b’s in the final state, and its main backgrounds at the LHC. We find the 8 TeV run dataset to be sensitive to the sign of the anomalous top Yukawa coupling, while already a moderate integrated luminosity at 14 TeV should lift the degeneracy completely.
Highlights
JHEP05(2013)022 in an ambiguity in the value of the tree-level Higgs couplings since the coupling likelihood function exhibits several and almost degenerate minima
We first show that for non-standard couplings the cross section can be reliably computed without worrying about corrections from physics beyond the cutoff scale Λ 10 TeV, and that it can be enhanced by more than one order of magnitude compared to the SM
The large enhancement of the th cross section for nonstandard Higgs couplings is associated to the growth of the scattering amplitude at high energy, which in turn implies that perturbative unitarity is lost at some UV scale Λ
Summary
The Feynman diagrams contributing to the core process W b → th are shown in figure 1. Such a process can be efficiently described by a 5-flavor scheme where b’s are in the initial state and described by a perturbative b PDF, figure 3(a) In this scheme, the non-collinearly enhanced contribution, where the spectator b (i.e. the one not struck by the W boson) is central and at high pT (see figure 3(b)), is moved to the next-to-leading order term. The striking enhancement of the hadronic cross section for cF = cV is shown, where σ(pp → thj) for an LHC energy of 14 TeV, normalized to its SM value, is displayed as a function of cF for three different choices of cV (very similar plots are obtained considering 8 TeV and/or the pp → thjb process). PDFs the contribution of the region s 1 TeV, where sis the center of mass energy of the th system, to the hadronic cross section is negligible This implies that our perturbative computations can be fully trusted. This is compatible with the different behaviors of the partonic cross section in the two cases, shown in figure 2
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