Revival of $H^-$ interpretation of $R_{D^{(*)}}$ anomaly and closing low mass window

TL;DR

This paper revisits the charged Higgs ($H^-$) explanation for the $R_{D^{(*)}}$ anomaly, especially in the low mass range, and discusses collider search constraints and future prospects at the HL-LHC.

Contribution

It demonstrates that a lighter $H^-$ with mass between 180 and 400 GeV can still explain the anomaly within 1$\sigma$, updating collider bounds and exploring future detection potential.

Findings

$H^-$ can explain the $R_{D^{(*)}}$ anomaly within 1$\sigma$ for masses 180-400 GeV.

Current collider searches constrain $H^-$ mass, excluding some parameter space.

Future HL-LHC searches will further probe the low mass $H^-$ region.

Abstract

Thanks to the recent careful revisit of the theoretical prediction of the $B_c$ meson lifetime, the conservative upper bound on the branching ratio (BR) of $\tau \nu$ mode is found to be $\simeq 63\%$ due to the large charm quark mass uncertainty. Although it is well known that a charged Higgs ($H^-$) interpretation of the $R_{D^{(*)}}$ anomaly is excluded by the previously proposed bounds, BR$(B_c\to\tau\nu)\le 30\%$ and $\le10\%$, $H^-$ can still explain the anomaly within $1\sigma$ if we adopt the 63$\%$ one. The scalar contribution is also favored by the polarization data $F_L^{D^*}$ measured at the Belle. Since the implied NP scale is within the reach of the Large Hadron Collider (LHC), collider searches are powerful tools to test the scenario. For instance, the $\tau\nu$ resonance search has already put the more stringent bound for $m_{H^-}\ge 400\,$GeV. In this work we revisit the further lighter mass range, $180\,$GeV$\le m_{H^-}\le 400\,$GeV which has not been covered yet. We will see that a combination of the conventional stau search and low mass flavor inclusive and bottom flavored di-jet resonance searches can place a new limit on the interpretation. We summarize the current status of the low mass region and discuss the future sensitivity in the high luminosity (HL)-LHC based on the existent collider constraints.