Unbroken B-L Symmetry

TL;DR

This paper investigates the possibility of an unbroken gauged B-L symmetry in the Standard Model, deriving bounds on the new force's parameters across a wide mass range using cosmological, astrophysical, and collider data.

Contribution

It provides a comprehensive analysis of constraints on an unbroken gauged B-L symmetry, covering all testable mass scales from eV to 10^{13} eV, including effects on Big Bang nucleosynthesis.

Findings

Strong bounds from Big Bang nucleosynthesis for 10 eV < M_{Z'} < 10 GeV.

Astrophysics and collider data constrain M_{Z'}/g' up to 10^{10} GeV.

The model contains only three parameters: g', M_{Z'}, and the kinetic mixing angle.

Abstract

The difference between baryon number B and lepton number L is the only anomaly-free global symmetry of the Standard Model, easily promoted to a local symmetry by introducing three right-handed neutrinos, which automatically make neutrinos massive. The non-observation of any (B-L)-violating processes leads us to scrutinize the case of unbroken gauged B-L; besides Dirac neutrinos, the model contains only three parameters, the gauge coupling strength g', the Stueckelberg mass $M_{Z'}$, and the kinetic mixing angle $\chi$. The new force could manifest itself at any scale, and we collect and derive bounds on g' over the entire testable range $M_{Z'}$ = 0 - $10^{13}$ eV, also of interest for the more popular case of spontaneously broken B-L or other new light forces. We show in particular that successful Big Bang nucleosynthesis provides strong bounds for masses 10 eV < $M_{Z'}$ < 10 GeV due to resonant enhancement of the rate $\bar{f} f \leftrightarrow \bar{\nu}_R \nu_R$. The strongest limits typically arise from astrophysics and colliders, probing scales $M_{Z'}/g'$ from TeV up to $10^{10}$ GeV.