Lifting degeneracies in Higgs couplings using single top production in association with a Higgs boson

TL;DR

This paper proposes using single top production with a Higgs boson decaying to b-quarks at the LHC to resolve ambiguities in the top Yukawa coupling, which are not distinguishable through current Higgs decay measurements.

Contribution

It introduces a novel method to break degeneracies in Higgs couplings by analyzing interference effects in single top plus Higgs production, providing a way to determine the sign of the top Yukawa coupling.

Findings

The cross section for $t$-channel single top plus Higgs production can be significantly enhanced for non-standard couplings.

The 8 TeV LHC data can determine the sign of the top Yukawa coupling.

Moderate luminosity at 14 TeV can fully resolve the degeneracy in the top Yukawa coupling.

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 $h \to \gamma \gamma$, is not sufficient to give a definitive answer. In this Letter, we argue that $t$-channel single top production in association with a Higgs boson, with $h\to b\bar b$, 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, $W b \rightarrow t h$. 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 $\Lambda\gtrsim 10\,\mathrm{TeV}$, and that it can be enhanced by more than one order of magnitude compared to the SM. We then study the signal $ p p \rightarrow t h j (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.