Searching for lepton number violating $\Lambda$ baryon decays mediated by GeV-scale Majorana neutrino with LHCb

TL;DR

This paper investigates lepton-number-violating decays of $\Lambda_b$ and $\Lambda_c$ baryons mediated by GeV-scale Majorana neutrinos, calculating branching ratios and setting sensitivity bounds for LHCb in the 0.25-4.5 GeV mass range.

Contribution

It provides the first detailed calculation of branching ratios for these baryon decays mediated by sterile neutrinos, including detector effects, and derives sensitivity bounds for LHCb experiments.

Findings

Branching ratios depend on sterile neutrino mass and mixing.

LHCb can set competitive bounds in the 0.25-4.5 GeV mass range.

Stronger limits are obtained for neutrino masses between 2 and 4.5 GeV.

Abstract

We consider the lepton-number-violating processes in $\Lambda_b$/$\Lambda_c$ decays mediated by on-shell GeV-scale sterile neutrino. We calculate the branching ratio for the following processes: $\Lambda_b^{0} \rightarrow \mathcal{B}^+ \mu^{-}\mu^- \pi^+$, where $\mathcal{B}^+$ is $\Lambda_c^+$ or proton, and $\Lambda_c^{+} \rightarrow \Lambda \mu^{+}\mu^+ \pi^-$, as function of the mass of the sterile neutrino $m_N$ and the heavy-light mixing coefficient of the extended Pontecorvo-Maki-Nakagawa-Sakata (PMNS) matrix $|U_{\mu N}|^2$. The effect of finite detector size is included in our calculation. After comparing the theoretical effective branching ratio with the expected experimental ability of LHCb, we give the sensitivity upper bounds on $|U_{\mu N}|^2$ on the ($|U_{\mu N}|^2$-$m_N$) plane. These channels give comparable results with the bounds given by different search strategies (e.g., at Belle, NuTeV, DELPHI and BEBC) in the mass region $m_N \simeq$ 0.25 GeV - 4.5 GeV, and the limits are stronger in the mass range 2 GeV $<m_N<$ 4.5 GeV.