Searching for Signs of the Second Higgs Doublet

TL;DR

This paper develops a strategy to search for the second Higgs doublet in 2HDMs using LHC signals, focusing on specific decay channels and parameter space to optimize discovery prospects of new scalars.

Contribution

It introduces a comprehensive approach to interpret LHC Higgs data within 2HDMs, identifying key decay channels and parameter regions for discovering additional scalars.

Findings

Decays H→hh and A→Zh are promising for discovery when kinematically accessible.

Heavier scalars decaying to γγ and τ+τ− are crucial when light Higgs couplings are SM-like.

Parameter fits guide future searches for the second Higgs doublet.

Abstract

The search for evidence of extended electroweak symmetry breaking has entered a new phase with the discovery of a Standard Model (SM)-like Higgs at the LHC. The measurement of Higgs couplings and direct searches for additional scalars provide complementary avenues for the discovery of new degrees of freedom. This complementarity is particularly sharp in two Higgs doublet models (2HDMs) where the couplings of the SM-like Higgs may be directly related to the LHC signals of additional scalars. In this work we develop a strategy for searching for the second Higgs doublet given the LHC signals of the recently discovered SM-like Higgs. We focus on a motivated parameter space of flavor- and CP-conserving 2HDMs in which the couplings of all scalars to SM states are controlled by two parameters. We construct fits in this parameter space to the signals of the SM-like Higgs and translate these fits into signal expectations for future measurements of both the SM-like Higgs and additional scalars, identifying the most promising search channels for discovery or exclusion of new physics. When kinematically accessible, decays of the heavy neutral scalar Higgs to two light Higgs scalars, $H \to hh$, and decays of the pseudoscalar Higgs to a light Higgs scalar and $Z$ boson, $A \to Zh$, provide promising avenues for discovery even when the couplings of the light Higgs are within a few percent of SM predictions. When the couplings of the light Higgs are exceptionally close to those of the SM, decays of heavier neutral scalars to $\gamma \gamma$ and $\tau^+ \tau^-$ become particularly important for discovery.