# Two Higgs bosons near 125 GeV in the NMSSM: beyond the narrow width approximation

## TL;DR

This paper investigates how quantum interference between nearly degenerate Higgs bosons in the NMSSM affects di-photon production at the LHC, challenging the narrow width approximation and revealing significant cross section modifications.

## Contribution

It provides a detailed analysis of interference effects between Higgs states in the NMSSM, including CP-violating scenarios, using the full propagator matrix for accurate cross section calculations.

## Key findings

- Interference effects can modify the di-photon cross section by up to 40%.
- Full propagator matrix approach is essential for accurate predictions.
- Interfering states can be distinguished from a single resonance under certain conditions.

## Abstract

In the next-to-minimal supersymmetric (NMS) Standard Model (SM), it is possible for either one of the additional singlet-like scalar and pseudoscalar Higgs bosons to be almost degenerate in mass with the ~125 GeV SM-like Higgs state. In the real NMSSM (rNMSSM), when the mass difference between two scalar states is comparable to their individual total decay widths, the quantum mechanical interference, due to the relevant diagonal as well as off-diagonal terms in the propagator matrix, between them can become sizable. This possibility invalidates usage of the narrow width approximation (NWA) to compute the cross section for the production of a di-photon pair with a given invariant mass via resonant Higgs boson(s) in the gluon fusion process at the Large Hadron Collider (LHC). When, motivated by the baryon asymmetry of the universe, CP-violating (CPV) phases are explicitly invoked in the Higgs sector of the NMSSM, all the interaction eigenstates mix to give five CP-indefinite physical Higgs bosons. In this scenario, the interference effects due the off-diagonal terms in the Higgs mass matrix that mix the pseudoscalar-like state with the SM-like one can also become significant, when these two are sufficiently mass-degenerate. We perform a detailed analysis, in both the real and complex NMSSM, of these interference effects, when the full propagator matrix is taken into account, in the production of a photon pair with an invariant mass near 125 GeV through gluon fusion. We find that these effects can account for up to ~40% of the total cross section for certain model parameter configurations. We also investigate how such mutually interfering states contributing to the ~125 GeV signal observed at the LHC can be distinguished from a single resonance.