Rare and exclusive few-body decays of the Higgs, Z, W bosons, and the top quark

TL;DR

This paper provides a comprehensive survey of rare and exclusive few-body decays of the Higgs, Z, W bosons, and top quark, including new calculations and feasibility assessments for future collider experiments.

Contribution

It introduces first-time calculations of ultrarare Higgs decays and updates predictions for various rare decay channels, along with feasibility studies for detection at future colliders.

Findings

Computed for the first time ultrarare Higgs decays into photons and neutrinos.

Updated predictions for rare decay partial widths of Higgs, Z, and top quark.

Estimated measurement feasibility at HL-LHC and FCC colliders.

Abstract

We perform an extensive survey of rare and exclusive few-body decays -- defined as those with branching fractions $\mathcal{B} \lesssim 10^{-5}$ into two to four final particles -- of the Higgs, Z, W bosons, and the top quark. Such rare decays can probe physics beyond the Standard Model (BSM), constitute a background for exotic decays into new BSM particles, and provide precise information on quantum chromodynamics factorization with small nonperturbative corrections. We tabulate the theoretical $\mathcal{B}$ values for about 200 rare decay channels of the four heaviest elementary particles, indicating the current experimental limits in their observation. Among those, we have computed for the first time ultrarare Higgs boson decays into photons and/or neutrinos, H and Z radiative decays into leptonium states, radiative H and Z quark-flavour-changing decays, and semiexclusive top-quark decays into a quark plus a meson, while updating predictions for a few other rare H, Z, and top quark partial widths. The feasibility of measuring each of these unobserved decays is estimated for p-p collisions at the high-luminosity Large Hadron Collider (HL-LHC), and for $e^+e^-$ and p-p collisions at the future circular collider (FCC).