Flavour-Changing Decays of a 125 GeV Higgs-like Particle

TL;DR

The paper investigates flavor-changing couplings of a 125 GeV Higgs-like scalar using a bottom-up effective Lagrangian with couplings c_{ij} to quarks and leptons, and translates stringent low-energy FCNC constraints into limits on these couplings. It shows that hadronic FCNC decays are strongly suppressed, with BR(h → q_i \\bar q_j) < 10^{-3}, while leptonic LFV decays can be sizeable; BR(h → τμ) or BR(h → τe) can reach order 10%, governed in part by the ratio $\\frac{{\\mathcal B}(h o f_i \,bar f_j)}{{\\mathcal B}(h o \tau \,bar \tau)} \\\approx N_f \\\frac{|c_{ij}|^2+|c_{ji}|^2}{2 y_\tau^2}$. The results indicate that LFV Higgs decays could be within reach of the LHC and offer a valuable probe of the Higgs flavor structure, complementing flavor-physics constraints from processes like μ → eγ and μ–e conversion. Overall, the work provides a framework to interpret potential 125 GeV signals in terms of flavor-changing Higgs couplings and motivates targeted experimental searches for h flavor-violating decays, especially in the τ–μ and τ–e channels.

Abstract

The ATLAS and CMS experiments at the LHC have reported the observation of a possible excess of events corresponding to a new particle $h$ with mass $\sim 125$ GeV that might be the long-sought Higgs boson, or something else. Decyphering the nature of this possible signal will require constraining the couplings of the $h$ and measuring them as accurately as possible. Here we analyze the indirect constraints on flavour-changing $h$ decays that are provided by limits on low-energy flavour-changing interactions. We find that indirect limits in the quark sector impose such strong constraints that flavour-changing $h$ decays to quark-antiquark pairs are unlikely to be observable at the LHC. On the other hand, the upper limits on lepton-flavour-changing decays are weaker, and the experimental signatures less challenging. In particular, we find that either ${\mathcal B}(h \to τ\bar μ+ \bar μτ)$ or ${\mathcal B}(h \to τ\bar e + \bar e τ) $ could be ${\cal O}(10)%$, i.e., comparable to ${\mathcal B}(h \to τ^+ τ^-)$ and potentially observable at the LHC.

Figures (1)

• Figure 1: Left: Tree-level diagram contributing to $\Delta F=2$ amplitudes. Right: One-loop diagram contributing to anomalous magnetic moments and electric dipole moments of charged leptons ($i=j$), or radiative LFV decay modes ($i\not=j$).