# Long Lived Light Scalars as Probe of Low Scale Seesaw Models

## TL;DR

This paper explores a light scalar particle in TeV scale seesaw models with local B-L symmetry, highlighting its potential long-lived nature and distinctive collider signatures like displaced photon jets, offering a new probe for neutrino mass models.

## Contribution

It identifies a phenomenologically allowed light scalar in TeV scale seesaw models and analyzes its collider signatures, especially displaced photon jets, as a novel test of these models.

## Key findings

- The light scalar H_3 can be long-lived with displaced vertex signals at the LHC.
- H_3 predominantly decays into two photons in left-right seesaw models.
- Production and decay modes of H_3 vary with the symmetry breaking scale.

## Abstract

We point out that in generic TeV scale seesaw models for neutrino masses with local $B-L$ symmetry breaking, there is a phenomenologically allowed range of parameters where the Higgs field responsible for $B-L$ symmetry breaking leaves a physical real scalar field with mass around GeV scale. This particle (denoted here by $H_3$) is weakly mixed with the Standard Model Higgs field ($h$) with mixing $\theta_1\lesssim m_{H_3}/m_h$, barring fine-tuned cancellation. In the specific case when the $B-L$ symmetry is embedded into the TeV scale left-right seesaw scenario, we show that the bounds on the $h-H_3$ mixing $\theta_1$ become further strengthened due to low energy flavor constraints, thus forcing the light $H_3$ to be long lived, with displaced vertex signals at the LHC. The property of left-right TeV scale seesaw models are such that they make the $H_3$ decay to two photons as the dominant mode. This is in contrast with a generic light scalar that mixes with the SM Higgs boson, which could also have leptonic and hadronic decay modes with comparable or larger strength. We discuss the production of this new scalar field at the LHC and show that it leads to testable displaced vertex signals of collimated photon jets, which is a new distinguishing feature of the left-right seesaw model. We also study a simpler version of the model where the $SU(2)_R$ breaking scale is much higher than the ${\cal O}$(TeV) $U(1)_{B-L}$ breaking scale, in which case the production and decay of $H_3$ proceed differently, but its long lifetime feature is still preserved for a large range of parameters. Thus, the search for such long-lived light scalar particles provides a new way to probe TeV scale seesaw models for neutrino masses at colliders.