Observable gravitational waves and $\Delta N_{\rm eff}$ with global lepton number symmetry and dark matter

TL;DR

This paper explores a dark matter model with a global lepton number symmetry that predicts gravitational wave signals from domain wall annihilation and potential observable effects on the cosmic microwave background, linking particle physics and cosmology.

Contribution

It introduces a novel scenario where spontaneous symmetry breaking in a global lepton number symmetry leads to detectable gravitational waves and dark matter decay signatures, connecting high-energy physics with cosmological observations.

Findings

Gravitational waves from domain wall annihilation could be observed in near-future experiments.

Dark matter decay via higher-dimensional operators affects indirect detection signals.

Light majorons contribute to $\Delta N_{ m eff}$, testable by future CMB measurements.

Abstract

We study the possibility of testing a dark matter (DM) scenario embedded in a global lepton number symmetry $U(1)_L$ via gravitational waves (GW) and cosmic microwave background (CMB) observations. The spontaneous breaking of $U(1)_L$ symmetry generates the seesaw scale as well as DM mass dynamically. The (pseudo) Nambu-Goldstone boson, known as majoron, acquires non-zero mass due to soft symmetry breaking terms of quadratic type in the scalar potential, which eventually breaks $U(1)_L$ to its $Z_2$ subgroup. The spontaneous symmetry breaking, which effectively breaks $Z_2$, leads to the formation of domain walls (DW), posing a threat to successful cosmology, if allowed to dominate. As gravity does not respect any global symmetries, we consider higher dimensional operators suppressed by the scale of quantum gravity (QG) namely, $\Lambda_{\rm QG}$ which introduces the required bias leading to DW annihilation and emission of stochastic gravitational waves (GW) observable at near future experiments. The same operators also lead to decay of DM bringing interesting indirect detection aspects. While DM is produced non-thermally via scalar portal interactions, light majoron can give rise to additional $\Delta N_{\rm eff}$ within reach of future CMB experiments.