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

TL;DR

The paper tackles the degeneracies in Higgs Yukawa couplings by proposing and evaluating th production in association with a Higgs that decays to $b\bar b$, exploiting interference between Higgs couplings to $W$ bosons and the top. It shows that nonstandard couplings can dramatically enhance the cross section and that a perturbative unitarity cutoff remains safely above LHC scales, justifying the effective-field-theory treatment. Through parton-level studies of 3b and 4b final states, it demonstrates that 8 TeV data already constrain the sign of the top Yukawa and that 14 TeV with moderate luminosity can decisively lift the degeneracy, with combined analyses in the $(c_V,c_F)$ plane providing strong exclusions. The work motivates concrete experimental analyses and highlights the potential for complementary information beyond the traditional $t\bar t h$ channel to precisely determine Higgs couplings.

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 $h \to γγ$, 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 $Λ\gtrsim 10\,\mathrm{TeV}$, 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.

Figures (7)

• Figure 1: Feynman diagrams contributing to the partonic process $Wb \rightarrow th$.
• Figure 2: Partonic cross sections for the process $Wb\to th$ as a function of the center of mass energy $\sqrt{s}$. The parameter $c_{V}$ is set to $1$. The hard scattering cross section is defined by a cut $|\eta|<2$: the large enhancement obtained for $c_{F} = - c_{V}$ with respect to the SM case is evident. The forward cross section, defined by a cut $|\eta|>3$, is also shown (dashed curves).
• Figure 3: Feynman diagrams for the processes $p p \rightarrow thj$ and $p p \rightarrow thjb$.
• Figure 4: Cross section for $pp\rightarrow thj$ at $14$ TeV normalized to the SM one, as a function of $c_F$ for three choices of $c_{V}$. Solid, dashed and dotted lines correspond to $c_V=1, \, 0.5$ and $1.5$ respectively.
• Figure 5: Histograms of normalized $p p \rightarrow thj$ cross section as a function of the center of mass energy of the hard scattering process $Wb\to th$. The left panel is for $8$ TeV, the right one for $14$ TeV.