Search for light pseudoscalar boson pairs produced from Higgs boson decays using the 4$\tau$ and 2$\mu$2$\tau$ final states in proton-proton collisions at $\sqrt{s}$ = 13 TeV

TL;DR

This study searches for light pseudoscalar boson pairs from Higgs decays in proton-proton collisions at 13 TeV, setting limits on their production and decay modes with no observed excess.

Contribution

First combined analysis of 4τ and 2μ2τ channels at 13 TeV to constrain light pseudoscalar bosons from Higgs decays, including interpretations in 2HD+S models.

Findings

No excess above SM expectations observed.

Upper limits on Higgs to a1a1 production range from 0.007 to 0.079.

Constraints exclude Higgs decays to a1 pairs exceeding 16% for certain masses.

Abstract

A search for a pair of light pseudoscalar bosons (a$_1$) produced in the decay of the 125 GeV Higgs boson is presented. The analysis examines decay modes where one a$_1$ decays into a pair of tau leptons and the other decays into either another pair of tau leptons or a pair of muons. The a$_1$ boson mass probed in this study ranges from 4 to 15 GeV. The data sample was recorded by the CMS experiment in proton-proton collisions at a center-of-mass energy of 13 TeV and corresponds to an integrated luminosity of 138 fb$^{-1}$. No excess above standard model (SM) expectations is observed. The study combines the 4$\tau$ and 2$\mu$2$\tau$ channels to set upper limits at 95% confidence level (CL) on the product of the Higgs boson production cross section and the branching fraction to the 4$\tau$ final state, relative to the Higgs boson production cross section predicted by the SM. In this interpretation, the a$_1$ boson is assumed to have Yukawa-like couplings to fermions, with coupling strengths proportional to the respective fermion masses. The observed (expected) upper limits range between 0.007 (0.011) and 0.079 (0.066) across the mass range considered. The results are also interpreted in the context of models with two Higgs doublets and an additional complex singlet field (2HD+S). The tightest constraints are obtained for the Type III 2HD+S model. In this case, assuming the Higgs boson production cross section equals the SM prediction, values of the branching ratio for the Higgs boson decay into a pair of a$_1$ bosons exceeding 16% are excluded at 95% CL for a$_1$ boson masses between 5 and 15 GeV and $\tan\beta$ $\gt$ 2, with the exception of scenarios in which the a$_1$ boson mixes with charm or bottom quark-antiquark bound states.