Constraining the charm Yukawa and Higgs-quark coupling universality

We introduce four different types of data-driven analyses with different levels of robustness that constrain the size of the Higgs–charm Yukawa coupling: (i) Recasting the vector-boson associated $Vh$ analyses that search for the bottom-pair final state. We use this mode to directly and model independently constrain the Higgs-to-charm coupling, $y_c/y_c^{\mathrm{SM}}\lesssim 234$. (ii) The direct measurement of the total width, $y_c/y_c^{\mathrm{SM}}\lesssim 120–140$. (iii) The search for $h\to J/\psi \gamma$, $y_c/y_c^{\mathrm{SM}}\lesssim 220$. (iv) A global fit to the Higgs signal strengths, $y_c/y_c^{\mathrm{SM}}\lesssim 6.2$. A comparison with $t\overline{t}h$ data allows us to show that the Higgs does not couple to quarks in a universal way, as is expected in the Standard Model. Finally, we demonstrate how the experimental collaborations can further improve our direct bound by roughly an order of magnitude by charm tagging, as is already used in new-physics searches.

Figure 1

68.3% C.L. (cyan) and 95% C.L. (gray) allowed regions in the ${\mu }_c-{\mu }_b$ plane. The best-fit (SM) point is indicated by the black circle (blue rectangle). The green (orange) bands are the 68.3% C.L. bands obtained from ATLAS (CMS) data. The labels (a)–(f) refer to the criteria in Table 2. Note that region (d) is not shown because it is too broad.

Figure 2

Example diagram that modifies $Vh$ production when the charm-quark Yukawa is enhanced.

Figure 3

$Vh$ enhancement with ${\kappa }_c$ from the new production mechanism, using the preselection cuts of CMS and ATLAS.

Figure 4

68.3% C.L. (cyan) and 95% C.L. (gray) allowed regions of the recast study in the ${\kappa }_c-{\kappa }_b$ plane, with the best-fit (SM) point indicated by the black circle (blue rectangle). Shaded areas represent the regions excluded by the total width (ATLAS and CMS) and the exclusive Higgs decay of $h\to J/\psi \gamma$.

Figure 5

Summary of current constraints on the Higgs couplings to fermions, including the new bounds on the charm Yukawa.

Figure 6

Expected reach for the signal-strength measurement of $h\to b\overline{b}$ and $h\to c\overline{c}$ at the LHC run II and HL-LHC: The thick black (thin purple) curves correspond to the reach with $3000(300){\mathrm{fb}}^{-1}$. The solid (dashed) ones correspond to the 68.3% (95%) C.L. The SM expectation is ${\mu }_{b,c}=1$.