CMS-PAS-FTR-21-009

CMS-PAS-FTR-21-009
Search for rare Higgs boson decays with mesons at the HL-LHC
CMS Collaboration
March 2022

Abstract: Rare decays of the Higgs boson via quarkonium mesons Q are promising laboratories to search for physics beyond the standard model (BSM). The decay channels of interest are $\gamma$Q, ZQ, or QQ. BSM physics might alter Yukawa couplings to quarks, possibly resulting in higher decay rates than predicted by the standard model. Decays of the Higgs boson into a Z boson and a J$/\psi$ meson are searched for in four-lepton final states with the CMS detector at the LHC. In addition, Higgs decays into pairs of $\Upsilon$ mesons are studied. This analysis uses these two Higgs decays to benchmark the 95% CL upper limit reach with the CMS detector at the HL-LHC.
Links: CDS record (PDF) ; CADI line (restricted) ;

Figures & Tables	Summary	References	CMS Publications

Figures
png pdf	Figure 1: Sample Feynman diagrams depicting direct (left) and indirect (middle, right) quark coupling contributions to the H $\to $ ZQ decay, where Q represents a quarkonium and Q a quark. The diagrams represent Higgs boson decays into quarkonium pairs when replacing the bottom section with the upper half in each.
png pdf	Figure 1-a: Sample Feynman diagrams depicting direct (left) and indirect (middle, right) quark coupling contributions to the H $\to $ ZQ decay, where Q represents a quarkonium and Q a quark. The diagrams represent Higgs boson decays into quarkonium pairs when replacing the bottom section with the upper half in each.
png pdf	Figure 1-b: Sample Feynman diagrams depicting direct (left) and indirect (middle, right) quark coupling contributions to the H $\to $ ZQ decay, where Q represents a quarkonium and Q a quark. The diagrams represent Higgs boson decays into quarkonium pairs when replacing the bottom section with the upper half in each.
png pdf	Figure 1-c: Sample Feynman diagrams depicting direct (left) and indirect (middle, right) quark coupling contributions to the H $\to $ ZQ decay, where Q represents a quarkonium and Q a quark. The diagrams represent Higgs boson decays into quarkonium pairs when replacing the bottom section with the upper half in each.
png pdf	Figure 2: Four-muon invariant mass distribution for Z J$/\psi $ $ \to$ 4$\mu $ candidates of a simulated candidate sample corresponding to an integrated luminosity of 3000 fb$^{-1}$. The result of the maximum likelihood fit is superimposed (solid blue line). For illustrative purposes, the plot shows the simulated Z J$/\psi $ signal (dashed red line) normalized to the observed upper limit branching fraction at 95% CL. The SM branching fraction is assumed to be $\mathcal {B}_{\rm {SM}} = $ 2.3 $\times$ 10$^{-6}$ [13-15].
png pdf	Figure 3: The four-muon invariant mass distribution for $ \Upsilon (n\text{S}) \Upsilon (m\text{S})$ candidates for a simulated sample corresponding to an integrated luminosity of 3000 fb$^{-1}$. The result of the maximum likelihood fit is superimposed (solid blue line). For illustrative purposes, the plot shows the distribution for the simulated Higgs boson signal (dashed red lines) normalized to the 95% CL upper limit branching fraction. The SM branching fraction is assumed to be $\mathcal {B}_{\rm {SM}} = $ 6.5 $\times$ 10$^{-5}$ [19].

Tables
png pdf	Table 1: Relative changes (in percent) of the acceptance for different extreme polarization scenarios with respect to the case where both decay products are longitudinally polarized. In the other scenarios, both decay products are assumed to be either unpolarized or transversely polarized.
png pdf	Table 2: Summary of the criteria for the muon, di-muon, and four-muon candidate selection.
png pdf	Table 3: Estimated upper limits on branching fractions for rare Higgs decays into mesons at 95% CL, for integrated luminosities of 3000 and 4500 fb$^{-1}$. The factors with respect to SM predictions are listed in brackets. The $\Upsilon (n\text{S})$ refers to the $n =$ 1, 2, 3 states that are combined.

Summary

Rare Higgs decays into four-lepton final states via mesons are an avenue to discover beyond the standard model (BSM) physics indirectly. In this document, the projected sensitivities of two processes are explored for the HL-LHC data taking period: the H $\to$ Z J$\psi$ channel and the H $ \to \Upsilon (n\text{S}) \Upsilon (m\text{S})$ ($n$, $m =$ 1, 2, 3) channel. For both, upper limits on branching fractions at 95% CL are found to be $\rm{\mathcal{B}(\mathrm{H}h \to \mathrm{Z}h {\mathrm{J}/\psi} )} = $ 2.9 $\times$ 10$^{-4}$ (2.7 $\times$ 10$^{-4}$) and $\mathrm{\mathcal{B}( Hh \to {\Upsilon (n\text{S})\Upsilon (m\text{S})})} = $ 1.3 $\times$ 10$^{-5}$ (8.5 $\times$ 10$^{-6}$) at an integrated luminosity of 3000 fb$^{-1}$ (4500 fb$^{-1}$). This indicates that at the HL-LHC for several of the related Higgs boson decays into $\gamma$Q, ZQ, or QQ decay rate measurements could arrive at values of several 10$^{-5}$, either as predicted by the SM or reducing the range for several BSM scenarios.

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