Analysis of final state lepton polarization-dependent observables in $H\to \ell^{+}\ell^{-} \gamma$ in the SM at loop level

TL;DR

This paper investigates how the polarization of final state leptons in the Higgs decay to a lepton pair and a photon can reveal loop-induced effects and distinguish between resonant and non-resonant contributions within the Standard Model, providing new observables for experimental tests.

Contribution

The study introduces the calculation of polarized decay rates and polarization asymmetries in $H o \, ext{leptons}^+ \, ext{leptons}^- \, ext{gamma}$, highlighting their sensitivity to loop effects and interference terms, which were previously neglected.

Findings

Polarization asymmetries are dominated by loop contributions.

Longitudinal decay rate is suppressed near $m_{\ell\ell} \approx 60$ GeV for certain lepton spins.

Ratios of decay rates between different lepton flavors can test the Standard Model and probe new physics.

Abstract

Recently, the CMS and ATLAS collaborations have announced the results for $H\rightarrow Z[\rightarrow \ell^{+}\ell^{-}]\gamma$ with $\ell=e$ or $\mu$ \cite{CMS:2022ahq,CMS:2023mku}, where $H\rightarrow Z\gamma$ is a sub-process of $H\rightarrow \ell^{+} \ell^{-} \gamma$. This semi-leptonic Higgs decay receives loop induced resonant $H\rightarrow Z[\rightarrow \ell^{+}\ell^{-}]\gamma$ as well as non-resonant contributions. % as discussed in \cite{Kachanovich:2021pvx}. To probe further features coming from these contributions to $H\rightarrow \ell^{+} \ell^{-} \gamma$, we argue that the polarization of the final state leptons is also an important parameter. We show that the contribution from the interference of resonant and non-resonant terms plays an important role when the polarization of final state lepton is taken into account, which is negligible in the case of unpolarized leptons. For this purpose, we have calculated the polarized decay rates and the longitudinal ($P_L$), normal ($P_N$) and transverse ($P_T$) polarization asymmetries. We find that these asymmetries purely come from the loop contributions and are helpful to further investigate the resonant and non-resonant nature of $H\rightarrow Z[\rightarrow \ell^{+}\ell^{-}]\gamma$ decay. We observe that for $\ell=e,\mu$, the longitudinal decay rate is highly suppressed around $m_{\ell\ell}\approx 60$GeV when the final lepton spin is $-\frac{1}{2}$, dramatically increasing the corresponding lepton polarization asymmetries. Furthermore, we analyze another observable, the ratio of decay rates $R^{\ell\ell'}_{i\pm}$, where $\ell$ and $\ell'$ refer to different final state lepton generations. Precise measurements of these observables at the HL-LHC and the planned $e^{+}e^{-}$ can provide a fertile ground to test not only the SM but also to examine the signatures of possible NP beyond the SM.