Cosmological Constraint on Vector Mediator of Neutrino-Electron Interaction in light of XENON1T Excess

TL;DR

This paper examines constraints on a light vector mediator for neutrino-electron interactions, showing that cosmological data from the CMB strongly limits the parameter space relevant to the XENON1T excess, excluding mediators heavier than about 1 eV.

Contribution

It introduces an independent cosmological constraint on light mediators from the effective number of neutrinos, complementing stellar cooling bounds.

Findings

Neff constraints exclude mediators heavier than ~1 eV.

XENON1T favored parameter space is incompatible with Neff limits.

Cosmological data provides robust bounds on light mediator models.

Abstract

Recently, the XENON1T collaboration reported an excess in the electron recoil energy spectrum. One of the simplest new physics interpretation is a new neutrino-electron interaction mediated by a light vector particle. However, for the parameter region favored by this excess, the constraints from the stellar cooling are severe. Still, there are astrophysical uncertainties on those constraints. In this paper, we discuss the constraint on the light mediator from the effective number of neutrino Neff in the CMB era, which provides an independent constraint. We show that Neff is significantly enhanced and exceeds the current constraint in the parameter region favored for the XENON1T excess. As a result, the interpretation by a light mediator heavier than about 1 eV is excluded by the Neff constraint.