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

TL;DR

This work provides a comprehensive survey of about 200 rare and exclusive few-body decays of the four heaviest SM particles: the Higgs, Z, W bosons, and the top quark. It integrates three theoretical frameworks—Light-Cone factorization, SCET, and NRQCD—to compute partial widths and compiles current experimental limits, while also predicting numerous channels for the first time. Ultrarares Higgs decays (e.g., $\mathrm{H}\to\nu\overline{\nu}$, $\mathrm{H}\to Zgg$, $\mathrm{H}\to Z\gamma\gamma$, leptonium modes), radiative Higgs/Z decays into flavoured mesons, and semiexclusive top decays are shown to be probes of FCNCs, LFV/LFUV, Yukawa couplings, and FCNC effects beyond the SM. The study also analyzes the feasibility of observing these decays at HL-LHC and future colliders (FCC-ee/hh), illustrating that FCC facilities could discover or constrain a large fraction of the predicted channels, thereby offering new avenues to test SM predictions and search for BSM phenomena.

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).

Figures (35)

• Figure 1: Examples of schematic diagrams of rare and exclusive two- and three-body decays of the Higgs, Z, and W bosons, and of the top quark. The leftmost diagram shows a rare decay into two or three gauge bosons V = Z, W, $\gamma$ (or into a gauge boson plus two neutrinos $\nu$) through virtual loops (grey circle). The center and rightmost diagrams show, respectively, typical exclusive decays into a gauge boson (mostly a photon) plus an onium bound state, and into two onium states (dashed blobs).
• 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 decay branching fractions of the Higgs boson into rare two-, three-, or four- gauge bosons and/or neutrinos, computed in this work 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 $\rm H \to Z^*gg,\,Z^*\gamma\gamma$ decay rates with offshell Z bosons. In the right panel the red vertical lines indicate the FCC-ee (solid) and FCC-hh (dashed) reach based only on the total number of H bosons to be produced at both facilities.
• Figure 4: Schematic diagrams of exclusive decays of the H boson into a meson plus a gauge boson: 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 experimental upper limits (violet) and expected conservative HL-LHC bounds (orange). The red vertical lines indicate the FCC-ee (solid) and FCC-hh (dashed) reach based only on the total number of H bosons to be produced at both facilities.