Implications of a Modified Higgs to Diphoton Decay Width

TL;DR

The paper investigates whether the Higgs diphoton width can be enhanced beyond the Standard Model expectation by introducing new colorless charged states that couple to the Higgs, without altering production or WW/ZZ decays. Using low-energy theorems and mass-matrix determinants, it analyzes how spins 0, 1/2, and 1 new states can constructively interfere with the W loop, focusing on light masses (few hundred GeV) and sizable Higgs couplings, while discussing vacuum stability and perturbativity. It provides concrete scenarios with a light W′, charged scalars, and vector-like leptons, including mass-mixing effects that boost γγ and the associated Zγ channel, which offers a complementary probe of the new particles. The work emphasizes that simultaneous γγ and Zγ measurements can reveal the weak isospin and electric charges of the new states, and that such new physics could be accessible at the LHC through electroweak production channels if realized in nature.

Abstract

Motivated by recent results from Higgs searches at the Large Hadron Collider, we consider possibilities to enhance the diphoton decay width of the Higgs boson over the Standard Model expectation, without modifying either its production rate or the partial widths in the WW and ZZ channels. Studying effects of new charged scalars, fermions and vector bosons, we find that significant variations in the diphoton width may be possible if the new particles have light masses of the order of a few hundred GeV and sizeable couplings to the Higgs boson. Such couplings could arise naturally if there is large mass mixing between two charged particles that is induced by the Higgs vacuum expectation value. In addition, there is generically also a shift in the Z + Gamma partial width, which in the case of new vector bosons tends to be of similar magnitude as the shift in the diphoton partial width, but smaller in other cases. Therefore simultaneous measurements in these two channels could reveal properties of new charged particles at the electroweak scale.

Figures (8)

• Figure 1: SM contributions to Higgs decays in the $\gamma\gamma$ and $Z\gamma$ channel.
• Figure 2: Contours of constant diphoton partial width, normalized to the SM value, shown as a function of $c_{W'}$, the $W'$ coupling strength to the Higgs as defined in Eq. (\ref{["mW'"]}), and the new $W'$ boson mass.
• Figure 3: Contours of constant diphoton partial width, normalized to the SM value, shown as a function of $c_S=g_{hSS}/v$ and the new scalar mass.
• Figure 4: Left panel: Diphoton partial width normalized to the SM as a function of the mixing parameter between the two charged scalars. The solid (dashed) line in the $R_{\gamma\gamma}$ plots includes both (only the lightest) mass eigenstates. They are almost on top of each other since the contribution from the heavy mass eigenstate is tiny. Middle panel: Mass of the lightest (solid, red line) and heaviest (dashed, blue line) scalar mass eigenstates as a function of the mixing parameter. Right panel: Effective couplings of the lightest (solid, red line) and heaviest (dashed, blue line) scalar mass eigenstates as a function of the mixing parameter.
• Figure 5: Contours of constant diphoton partial width, normalized to the SM value, shown as a function of $c_f=g_{h\bar{f}f} \Lambda/v$ and the new vector-like lepton mass