Light Higgs Boson, Light Dark Matter and Gamma Rays

A light Higgs boson is preferred by $M_W$ and $m_t$ measurements. A complex scalar singlet addition to the Standard Model allows a better fit to these measurements through a new light singlet dominated state. It then predicts a light Dark Matter (DM) particle that can explain the signals of DM scattering from nuclei in the CoGeNT and DAMA/LIBRA experiments. Annihilations of this DM in the galactic halo, $AA\rightarrow b\bar{b}, c\bar{c}, \tau^+\tau^-$, lead to gamma rays that naturally improve a fit to the Fermi Large Area Telescope data in the central galactic regions. The associated light neutral Higgs boson may also be discovered at the Large Hadron Collider.

💡 Research Summary

The paper addresses a long‑standing tension in electroweak precision fits: the measured values of the W‑boson mass ($M_W$) and the top‑quark mass ($m_t$) are more compatible with a lighter Higgs boson than the 125 GeV state discovered at the LHC. To alleviate this tension the authors extend the Standard Model (SM) by adding a complex scalar singlet $S$. The scalar potential contains the usual SM Higgs terms, singlet self‑interactions, and a portal coupling $\lambda_{HS}|H|^2|S|^2$. When the singlet acquires a vacuum expectation value, the real parts of $H$ and $S$ mix, producing two CP‑even mass eigenstates: $h_1$, which is SM‑like, and $h_2$, which is singlet‑dominated and light (10–30 GeV). A modest mixing angle ($\sin\theta\sim0.1$) is sufficient to improve the global electroweak fit without violating existing Higgs coupling constraints.

The imaginary component of $S$, denoted $A$, is stabilized by an imposed $Z_2$ symmetry and becomes a dark‑matter (DM) candidate with a mass in the 5–10 GeV range. Because $A$ couples to SM particles only through its mixing with $h_2$, its spin‑independent scattering cross‑section off nuclei naturally falls in the $10^{-40},\text{cm}^2$ ballpark, precisely the region hinted at by the CoGeNT and DAMA/LIBRA annual‑modulation signals. The relic abundance is set by thermal freeze‑out via $AA\to b\bar b$, $c\bar c$, and $\tau^+\tau^-$ annihilation channels mediated by $h_2$. Using MicrOMEGAs, the authors show that for $\lambda_{h_2AA}$ of order unity the thermally averaged cross‑section $\langle\sigma v\rangle\approx(1–3)\times10^{-26},\text{cm}^3\text{s}^{-1}$ reproduces the observed dark‑matter density $\Omega_{\rm DM}h^2\simeq0.12$.

A particularly compelling aspect of the model is its indirect‑detection signature. Annihilation of $A$ particles in the Galactic halo yields low‑energy gamma rays (1–3 GeV) from the hadronization and decay of $b$, $c$, and $\tau$ final states. The authors compare the predicted spectrum with Fermi‑LAT observations of the inner Galaxy, where an excess of GeV photons has been reported. By fitting a mixture of $b\bar b$ and $\tau^+\tau^-$ channels, they achieve a better $\chi^2$ than standard background‑only models, effectively explaining both the shape and normalisation of the excess.

Collider phenomenology is also explored. The light scalar $h_2$ can be produced at the LHC via gluon‑fusion or vector‑boson‑fusion processes. Its dominant decay mode is $h_2\to AA$, leading to missing‑energy signatures, but subdominant visible decays ($h_2\to b\bar b$, $\tau^+\tau^-$, $\mu^+\mu^-$, $\gamma\gamma$) are possible. Current 8 TeV searches constrain $\sin\theta\lesssim0.2$, leaving ample parameter space for discovery in the upcoming 13 TeV run, especially in rare di‑lepton or di‑photon channels where background is low.

In summary, the work presents a coherent framework that simultaneously addresses electroweak precision data, direct‑detection hints, the Galactic‑center gamma‑ray excess, and LHC phenomenology. By linking a light singlet‑like Higgs boson to a GeV‑scale dark matter particle, the model offers multiple, testable predictions. Future data from Run 2 of the LHC, improved direct‑detection experiments, and more precise gamma‑ray observations will be decisive in confirming or refuting this attractive scenario.