Probing classically conformal $B-L$ model with gravitational waves

TL;DR

This paper investigates the gravitational wave signals produced by a first-order phase transition in a classically conformal $B-L$ extension of the standard model, highlighting potential detectability by future interferometers.

Contribution

It demonstrates that the conformal $B-L$ model predicts strong gravitational waves from early universe phase transitions, which can be tested by upcoming experiments.

Findings

Gravitational wave spectrum can reach $ imes 10^{-8}$ at 0.01-1 Hz.

Phase transition is typically first-order with ultra-supercooling.

Many parameter regions are testable by future interferometers.

Abstract

We study the cosmological history of the classical conformal $B-L$ gauge extension of the standard model, in which the physical scales are generated via the Coleman-Weinberg-type symmetry breaking. Especially, we consider the thermal phase transition of the U$(1)_{B-L}$ symmetry in the early universe and resulting gravitational-wave production. Due to the classical conformal invariance, the phase transition tends to be a first-order one with ultra-supercooling, which enhances the strength of the produced gravitational waves. We show that, requiring (1) U$(1)_{B-L}$ is broken after the reheating, (2) the $B-L$ gauge coupling does not blow up below the Planck scale, (3) the thermal phase transition completes in almost all the patches in the universe, the gravitational wave spectrum can be as large as $\Omega_{\rm GW} \sim 10^{-8}$ at the frequency $f \sim 0.01$-$1$Hz for some model parameters, and a vast parameter region can be tested by future interferometer experiments such as eLISA, LISA, BBO and DECIGO.