Bounds on effective Majorana neutrino masses at HERA

TL;DR

This paper investigates bounds on effective Majorana neutrino masses using HERA collider data, providing new direct limits especially for tau-related sectors, and discusses potential improvements with future collider upgrades.

Contribution

It presents the first direct limits on effective Majorana neutrino masses involving tau leptons from HERA data and analyzes potential improvements with collider upgrades.

Findings

New direct limits on <m_{mu mu}> and tau sector masses.

Potential to improve bounds by a factor of 40 with upgrades.

Current bounds are still weaker than indirect limits.

Abstract

The lepton-number violating process e p \to nu_e l l' X mediated by Majorana neutrinos is studied for the HERA collider for (l l') = (e tau), (mu tau), (mu mu) and (tau tau). Only the muonic decay of the tau is considered. The direct limit on the effective muon Majorana mass, <m_{mu mu}> is improved significantly to 4.0 times 10^3 GeV and for the first time direct limits on the analogous effective masses connected with the tau sector are given, namely 4.2 times 10^3 GeV for <m_{e tau}>, 4.4 times 10^3 GeV for <m_{mu tau}> and 2.0 times 10^4 GeV for <m_{tau tau}>. We find that a more general analysis for an upgraded HERA could improve this values by a factor of up to 40, yet still being orders of magnitude worse than indirect limits.