Rare few-body decays of the Standard Model Higgs boson

TL;DR

This work surveys rare, exclusive few-body decays of the SM Higgs boson with $\mathcal{B} \lesssim 10^{-5}$, compiling about 70 predicted channels and current experimental limits. It employs EW/QCD perturbation theory and QCD factorization formalisms to compute rates, including ~20 newly estimated channels, and provides HL-LHC projection estimates based on existing limits and luminosity scaling. The study highlights diverse decay classes—from two- to four-body final states, to gauge-boson plus meson and two-onia final states—revealing extremely suppressed SM rates (e.g., $\mathcal{O}(10^{-14})$ to $\mathcal{O}(10^{-40})$ in some channels) and identifying channels where HL-LHC could begin to probe or constrain the SM predictions (notably some $\gamma+\rho$, $\gamma+J/\psi$, and $W^{\pm}+\rho^{\mp}$ modes). By outlining the sensitivities and backgrounds relevant to exotic BSM decays, the paper provides a practical guide for prioritizing experimental searches and theoretical refinements, and it underscores the tests these decays offer of light-quark Yukawas and QCD factorization at high energy scales.

Abstract

We present a survey of rare and exclusive few-body decays of the standard model (SM) Higgs boson, defined as those into two to four final particles with branching fractions $\mathcal{B}\lesssim 10^{-5}$. Studies of such decays can be exploited to constrain Yukawa couplings of quarks and leptons, probe flavour-changing Higgs decays, estimate backgrounds for exotic Higgs decays into beyond-SM particles, and/or confirm quantum chromodynamics factorization with small nonperturbative corrections. We collect the theoretical $\mathcal{B}$ values for about 70 unobserved Higgs rare decay channels, indicating their current experimental limits, and estimating their expected bounds in p-p collisions at the HL-LHC. Among those, we include 20 new decay channels computed for the first time for ultrarare Higgs boson decays into photons and/or neutrinos, radiative quark-flavour-changing exclusive decays, and radiative decays into leptonium states. This survey can help guide and prioritize upcoming experimental and theoretical studies of unobserved Higgs boson decays.

Figures (12)

• Figure 1: Schematic diagrams of rare and exclusive two- and three-body decays of the Higgs boson into (i) two or three gauge bosons (V = Z, W, $\gamma$) or into a gauge boson plus a neutrino pair $(\nu\overline{\nu}$) through virtual loops (grey circle) (ii) a gauge boson plus a difermion bound state (meson or leptonium), and (iii) two onium states.
• Figure 2: Representative diagrams of rare 2-, 3-, and 4-body decays of the H boson into photons and/or neutrinos, and into Z bosons plus gluons or photons.
• Figure 3: Theoretical branching fractions of the Higgs boson into rare two-, three-, or four- gauge bosons and/or neutrinos shown as a function of the Higgs boson mass (left) and as blue bars in negative log scale (right). In the left panel, the dashed lines for $m_\mathrm{H}<m_\mathrm{Z}$ show the $\rm H \to Z^*gg,\,Z^*\gamma\gamma$ decays with offshell Z bosons. In the right panel, the red vertical line indicates the minimum $\mathcal{B}$ value reachable at the HL-LHC given just by the total number of H bosons expected to be produced.
• Figure 4: Schematic diagrams of exclusive decays of the H boson into a meson plus a gauge boson in direct (left), indirect (center), and W-loop (right) processes. The solid fermion lines represent quarks, and the gray blob represents the mesonic bound state.
• Figure 5: Branching ratios (in negative log scale) of exclusive $\rm H \to \gamma +vector$-meson decays. Most recent theoretical predictions (blue bars) compared to current 95% CL experimental limits (violet) and expected conservative HL-LHC bounds (orange). The red vertical line indicates the minimum $\mathcal{B}$ value reachable at the HL-LHC given just by the total number of H bosons expected to be produced.