Bounding the top Yukawa coupling with Higgs-associated single-top production

After the discovery of the 125 GeV scalar boson with gauge properties similar to the Standard Model (SM) Higgs, the search for beyond the SM interactions will focus on studying the discovered particles’ coupling properties more precisely and shedding light on the relation of fermion masses with the electroweak vacuum. The large mass of the top quark and the SM-predicted order one top Yukawa coupling is a natural candidate for beyond the SM physics, though experimentally challenging to constrain. In this paper, we argue that investigating angular correlations in $pp\to tHj$ production provides an excellent handle to constrain the top Yukawa coupling $y_t$ via direct measurements, even when we focus on rare exclusive final states. We perform a hadron-level analysis and show that we may expect to constrain $y_t\gtrsim 0.5y_t^{\mathrm{SM}}$ at 95%–99% confidence level at the high luminosity LHC using semileptonic top decays and $H\to \gamma \gamma$ alone, by employing a two-channel measurement approach.

Figure 1

Feynman diagrams for tree-level production of $\text{single}\text{-top}+\text{Higgs}$. The diagram on the left (right) is proportional to $C_{WWH}$ ($C_{ttH}$).

Figure 2

Parton-level leading-order distribution of the reconstructed top quark rapidity $y_t$ (left) and the rapidity distance between the top quark and the Higgs boson $|\mathrm{\Delta }y(t,H)|$ (right). Plots have been obtained using the cuts in Eq. (3) and have been normalized to unity. In the lower insets the ratio between the $c_t=0.5$ and the SM distributions is shown.

Figure 3

Rapidity distance between the top quark and the Higgs boson in the presence of the cuts in Eq. (3). On the left (right) panel, the requirement of having a $b$ jet softer (harder) than the light jet is enforced.

Figure 4

Lego-plot separation and rapidity difference of the reconstructed Higgs boson and $b$-tagged jet. We show the expected distribution for a target luminosity of $3/\mathrm{ab}$ after the selection criteria detailed in the text have been applied. To get an idea of the involved statistical uncertainty of such a measurement with an SM-consistent outcome, we include toy data and the 95% Bayesian confidence level error bars around the central values. We use these distributions and MC-sampled toy measurements to compute a confidence level interval for the top quark Yukawa coupling (see text); the $c_t=0.5$ sample includes a modified $h\to \gamma \gamma$ branching ratio. Note that the signal hypotheses overlap. The background does not contain the modifications due to $c_t<1$ for illustration purposes.

Figure 5

Same as Fig. 4 for a measurement performed in Channel 2 as defined in the text.