# Searches for Majorana neutrinos in $B^-$ decays

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## Abstract

Searches for heavy Majorana neutrinos in B- decays in final states containing hadrons plus a \mu- \mu- pair have been performed using 0.41/fb of data collected with the LHCb detector in proton-proton collisions at a center-of-mass energy of 7 TeV. The D+ \mu- \mu- and D*+ \mu- \mu- final states can arise from the presence of virtual Majorana neutrinos of any mass. Other final states containing \pi+, Ds+, or D0\pi+ can be mediated by an on-shell Majorana neutrino. No signals are found and upper limits are set on Majorana neutrino production as a function of mass, and also on the B- decay branching fractions.

## Figures and captions

 (a) Diagram of neutrinoless double $\beta$ decay when two neutrons in a nucleus decay simultaneously. (b) The fundamental diagram for changing lepton number by two units. doublebeta.pdf [14 KiB] HiDef png [105 KiB] Thumbnail [13 KiB] *.C file Feynman diagrams for $B$ decays involving an intermediate heavy neutrino ($N$). (a) $B^-\rightarrow D^{(*)+}\mu^-\mu^-$, (b) $B^-\rightarrow \pi^+(D_s^+)\mu^-\mu^-$, and (c) $B^-\rightarrow D^0 \pi^+\mu^-\mu^-$. B-Majo[..].pdf [54 KiB] HiDef png [103 KiB] Thumbnail [24 KiB] *.C file Invariant mass of (a) candidate $J/\psi K^-$ decays, and (b) candidate $J/\psi K^-\pi^+\pi^-$ decays. The data are shown as the points with error bars. Both the partially reconstructed background and the combinatorial background are shown, although the combinatorial background is small and barely visible. The solid curve shows the total. In both cases the candidate $\mu^+\mu^-$ is required to be within $\pm$50 MeV of the $J/\psi$ mass, and in (b) the dimuon pair is constrained to have the $J/\psi$ mass. B2Jpsi[..].pdf [78 KiB] HiDef png [186 KiB] Thumbnail [40 KiB] *.C file Invariant mass spectrum for (a) $B^-\rightarrow D^+\mu^-\mu^-$ candidates, and (b) $B^-\rightarrow D^{*+}\mu^-\mu^-$ candidates. The solid lines show the linear fits to the data in the mass sidebands. MassFitDp.pdf [63 KiB] HiDef png [111 KiB] Thumbnail [29 KiB] *.C file Invariant mass distribution of $\pi^+\mu^-\mu^-$. The estimated backgrounds are also shown. The curve is the sum of the peaking background and the combinatoric background. MassFitPi.pdf [60 KiB] HiDef png [185 KiB] Thumbnail [45 KiB] *.C file Invariant mass distribution of $\pi^+\mu^-$ in the $\pm 2\sigma$ region of the $B^-$ mass with both peaking and combinatorial background superimposed. The peaking background at 3100 MeV is due to misidentified $B^-\rightarrow J/\psi X$ decays.There are two combinations per event. MassSi[..].pdf [60 KiB] HiDef png [171 KiB] Thumbnail [36 KiB] *.C file Majorana mass resolutions for the three $B^-$ decays as a function of Majorana mass. Resolu[..].pdf [59 KiB] HiDef png [161 KiB] Thumbnail [48 KiB] *.C file Detection efficiencies for the three $B^-$ decays as a function of Majorana mass. Charm meson decay branching fractions are not included. Overeff.pdf [60 KiB] HiDef png [178 KiB] Thumbnail [45 KiB] *.C file Upper limits at 95% CL as a function of the putative Majorana neutrino mass, (a) for ${\cal{B}}(B^- \rightarrow \pi^+\mu^- \mu^-)$ as a function of the $\pi^+\mu^-$ mass, (b) for ${\cal{B}}(B^-\rightarrow D_s^+\mu^-\mu^-$) as a function of the $D_s^+\mu^-$ mass, and (c) for ${\cal{B}}(B^-\rightarrow D^0\pi^+\mu^-\mu^-)$ as a function of the $\pi^+\mu^-$ mass. UpperAll.pdf [60 KiB] HiDef png [170 KiB] Thumbnail [39 KiB] *.C file Invariant mass spectrum for $B^-\rightarrow D_s^+\mu^-\mu^-$ candidates. The line shows the fit to the data excluding the $B^-$ mass signal region. MassFitDs.pdf [59 KiB] HiDef png [142 KiB] Thumbnail [44 KiB] *.C file Invariant mass spectrum of $D_s^+\mu^-$ from $B^-\rightarrow D_s^+\mu^-\mu^-$ events in the signal region with the background estimate superimposed (solid curve). There are two combinations per event. DsEsti[..].pdf [58 KiB] HiDef png [277 KiB] Thumbnail [61 KiB] *.C file Invariant mass distribution of $D^0\pi^+ \mu^- \mu^-$. The solid line shows a linear fit to the data in the sidebands of the $B^-$ signal region. MassFitD0.pdf [58 KiB] HiDef png [215 KiB] Thumbnail [51 KiB] *.C file Invariant mass distribution of $\pi^+\mu^-$ for $B^-\rightarrow D^0\mu^-\mu^- \pi^+$ in the signal region and with estimated background distribution superimposed. There are two combinations per event. MassFitD0N.pdf [58 KiB] HiDef png [249 KiB] Thumbnail [60 KiB] *.C file Relative efficiencies as a function of Majorana neutrino lifetime for (a) $B^-\rightarrow \pi^+\mu^-\mu^-$ for a mass of 3 GeV, (b) $B^-\rightarrow D_s^+\mu^-\mu^-$ for a mass of 3 GeV, and (c) $B^-\rightarrow D^0\pi^+\mu^-\mu^-$ for a Majorana neutrino mass of 2 GeV. Where the error bars are not visible, they are smaller than radii of the points. Lifeti[..].pdf [105 KiB] HiDef png [68 KiB] Thumbnail [18 KiB] *.C file Upper limits on $|V_{\mu4}|^2$ at 95% CL as a function of the Majorana neutrino mass from the $B^- \rightarrow \pi^+ \mu^- \mu^-$ channel. Constr[..].pdf [63 KiB] HiDef png [60 KiB] Thumbnail [36 KiB] *.C file Animated gif made out of all figures. PAPER-2011-038.gif Thumbnail

## Tables and captions

 Charm and charmonium branching fractions Table_1.pdf [53 KiB] HiDef png [76 KiB] Thumbnail [12 KiB] tex code Systematic uncertainties for $B^- \rightarrow D X \mu^- \mu^-$ modes. Table_2.pdf [57 KiB] HiDef png [101 KiB] Thumbnail [15 KiB] tex code Systematic uncertainties for $B^- \rightarrow \pi^+\mu^- \mu^-$ measurement. Table_3.pdf [47 KiB] HiDef png [57 KiB] Thumbnail [9 KiB] tex code Summary of upper limits on branching fractions. Both the limits on the overall branching fraction assuming a phase space decay, and the range of limits on the branching fraction as a function of Majorana neutrino mass ($M_N$) are given. All limits are at 95% CL. Table_4.pdf [53 KiB] HiDef png [43 KiB] Thumbnail [7 KiB] tex code

Created on 09 December 2018.Citation count from INSPIRE on 18 December 2018.