Search for Majorana neutrinos in $B^- \to π^+μ^-μ^-$ decays

Abstract

A search for heavy Majorana neutrinos produced in the $B^- \to π^+μ^-μ^-$ decay mode is performed using 3 fb$^{-1}$ of integrated luminosity collected with the LHCb detector in $pp$ collisions at center-of-mass energies of 7 TeV and 8 TeV at the LHC. Neutrinos with masses in the range 250-5000 MeV and lifetimes from zero to 1000 ps are probed. In the absence of a signal, upper limits are set on the branching fraction ${\cal{B}}(B^- \to π^+μ^-μ^-)$ as functions of neutrino mass and lifetime. These limits are on the order of $10^{-9}$ for short neutrino lifetimes of 1 ps or less. Limits are also set on the coupling between the muon and a possible fourth-generation neutrino.

Figures (6)

• Figure 1: Feynman diagram for $\mathrm{B}\xspace\xspace^-\xspace\xspace\rightarrow\xspace\pi^+\mu^-\mu^-$ decay via a Majorana neutrino labelled $N$.
• Figure 2: Invariant mass distributions with fits overlaid of candidate mass spectra for (a) ${\mathrm{J}\xspace / \uppsi\xspace }\xspace K^-$, (b) $\uppi\xspace\xspace^+\xspace\upmu\xspace^-\xspace\upmu\xspace^-\xspace$ ($\cal{S}$) , and (c) $\uppi\xspace\xspace^+\xspace\upmu\xspace^-\xspace\upmu\xspace^-\xspace$ ($\cal{L}$). Backgrounds are (green) shaded; they peak under the signal in (b) and (c). The dotted lines show the combinatorial backgrounds only. The solid line shows the sum of both backgrounds.
• Figure 3: Invariant $\uppi\xspace\xspace^+\xspace \upmu\xspace^-\xspace$ mass distribution for $\uppi\xspace\xspace^+\xspace\upmu\xspace^-\xspace\upmu\xspace^-\xspace$ candidates with masses restricted to $\pm2\sigma$ of $\mathrm{B}\xspace\xspace^-\xspace\xspace$ mass for the (a) $\cal{S}$ and (b) $\cal{L}$ selections. The shaded regions indicate the estimated peaking backgrounds. Backgrounds that peak under the signal in (a) and (b) are (green) shaded. The dotted lines show the combinatorial backgrounds only. The solid line the sum of both backgrounds. (In (a) there are two combinations per event.)
• Figure 4: Upper limit on ${\cal{B}}(\mathrm{B}\xspace\xspace^-\xspace\xspace \rightarrow\xspace \uppi\xspace\xspace^+\xspace \upmu\xspace^-\xspace \upmu\xspace^-\xspace)$ at $95\%$ C.L. as a function of $m_N$ in 5 MeV intervals for $\cal{S}$ selected events.
• Figure 5: Upper limits on ${\cal{B}}(\mathrm{B}\xspace\xspace^-\xspace\xspace \rightarrow\xspace \uppi\xspace\xspace^+\xspace \upmu\xspace^-\xspace \upmu\xspace^-\xspace)$ at 95% C.L. as a function of $m_N$, in 5 MeV intervals, for specific values of $\tau_N$.