Single top and Higgs associated production as a probe of the Htt coupling sign at the LHC

TL;DR

This work investigates whether the LHC can determine the sign of the Higgs-top coupling by studying the t-channel production of a single top quark in association with a Higgs boson, focusing on the decay H → γγ. By parameterizing deviations in the top Yukawa coupling with a scale factor C_t (keeping C_W=1) and exploiting the strong interference between diagrams, the authors show that the cross section for pp → t q H is highly sensitive to the sign of C_t, with potential observable enhancements for negative C_t at √s = 14 TeV. A parton-level analysis with realistic selection cuts suggests S/B up to ~5 and S ≈ 10 for 60 fb⁻¹ when C_t is negative, while SM expectations require much larger luminosities; allowing BR(H→γγ) to vary can further improve sensitivity. The study highlights the potential of this channel to probe the top Yukawa sign at the LHC, and calls for full detector simulations and exploration of additional Higgs decay modes to fully exploit this coupling structure.

Abstract

The LHC sensitivity to an anomalous Higgs coupling to the top quark in the Higgs-top associated production is analyzed. Thanks to the strong destructive interference in the t-channel for standard model couplings, this process can be very sensitive to both the magnitude and the sign of a nonstandard top-Higgs coupling. We analyze cross sections and the main irreducible backgrounds for the $H \to γγ$ decay channel. Sensitivities to an anomalous sign for the top Yukawa coupling are found to be large. In particular, at \sqrt{s}=14 TeV, assuming a universal rescaling in the Yukawa sector, a parton-level analysis with realistic selection cuts gives a signal-to-background ratio S/B ~ 5, for -1.5 < Y_t/Y_t^{SM} < 0. A number of events S ~ 10 (with corresponding significances ~ 3 σ) are expected for 60 fb^{-1}, to be compared with the standard-model expectation S ~ 0.3.

Figures (7)

• Figure 1: Feynman diagrams for the single-top plus Higgs associated production in the $t$-channel $q\, b \to t \,q' H\;$ at hadron colliders. Higgs radiation by the initial $b$-quark line is not shown (see text).
• Figure 2: Production cross sections for $p\, p \to t \,q\, H\;$ versus $C_t$, for $\sqrt s=8 {\rm ~and} ~14$ TeV. The inside plot is an enlargement of the positive $C_t$ region.
• Figure 3: Enhancement factors $R_i$ for the $p\, p \to t \,q\, H\;$ production cross section $\sigma$, $BR_{\gamma \gamma}$, and their product with respect to their $SM$ values, versus $C_t$, for $\sqrt s=8$ TeV. The lower plot is just an enlarged view of the positive $C_t$ range.
• Figure 4: Enhancement factors $R_i$ for the $p\, p \to t \,q\, H\;$ production cross section $\sigma$, $BR_{\gamma \gamma}$, and their product with respect to their $SM$ values, versus $C_t$, for $\sqrt s=14$ TeV. The lower plot is just an enlarged view of the positive $C_t$ range.
• Figure 5: Signal significance versus $C_t$. Different assumptions are made for the value of $R^{\gamma\gamma}_{BR}=$$BR_{\gamma \gamma}\;$/$BR^{SM}_{\gamma \gamma}$. The black solid line represents the Yukawa universal-rescaling hypothesis, where $R^{\gamma\gamma}_{BR}$ is just a function of $C_t$, with $C_b=C_{\tau}=C_t$. The remaining (colored) lines refer to the constant $R^{\gamma\gamma}_{BR}=1,2,3,4$ hypothesis.