Implications of the Upper Bound on $h\to {\mu }^{+}{\mu }^{-}$ on the Baryon Asymmetry of the Universe

The upper bounds from the ATLAS and CMS experiments on the decay rate of the Higgs boson to two muons provide the strongest constraint on an imaginary part of the muon Yukawa coupling. This bound is more than an order of magnitude stronger than bounds from $CP$-violating observables, specifically the electric dipole moment of the electron. It excludes a scenario—which had been viable prior to these measurements—that a complex muon Yukawa coupling is the dominant source of the baryon asymmetry. Even with this bound, the muon source can still contribute $\mathcal{O}(16\%)$ of the asymmetry.

Figure 1

The blue region within the circle is the allowed region for ${\mu }_{{\mu }^{+}{\mu }^{-}}\le 1.7$ [16]. The two points on the circle correspond to maximal values of $|Y_B^{(\mu )}{|}_{\max }=1.4\times {10}^{-11}$ and of $|d_e^{(\mu )}{|}_{\max }=6.5\times {10}^{-31}e\mathrm{cm}$. The dashed lines correspond to $|Y_B^{(\mu )}{|}_{\max }/2$ and $|d_e^{(\mu )}{|}_{\max }/2$.