A search for heavy Majorana neutrinos produced in the $B^ \to \pi^+\mu^\mu^$ decay mode is performed using 3 fb$^{1}$ of integrated luminosity collected with the LHCb detector in $pp$ collisions at centerofmass energies of 7 TeV and 8 TeV at the LHC. Neutrinos with masses in the range 2505000 MeV and lifetimes from zero to 1000 ps are probed. In the absence of a signal, upper limits are set on the branching fraction ${\cal{B}}(B^ \to \pi^+\mu^\mu^)$ 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 fourthgeneration neutrino.
Feynman diagram for $ B ^ \rightarrow \pi^+\mu^\mu^$ decay via a Majorana neutrino labelled $N$. 
Majora[..].pdf [23 KiB] HiDef png [100 KiB] Thumbnail [105 KiB] *.C file 

Invariant mass distributions with fits overlaid of candidate mass spectra for (a) $ { J \mskip 3mu/\mskip 2mu\psi \mskip 2mu} K^$, (b) $\pi ^+ \mu ^ \mu ^ $ ($\cal{S}$) , and (c) $\pi ^+ \mu ^ \mu ^ $ ($\cal{L}$). 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. 
hmumu[..].pdf [35 KiB] HiDef png [217 KiB] Thumbnail [121 KiB] *.C file 

Invariant $\pi ^+ \mu ^ $ mass distribution for $\pi ^+ \mu ^ \mu ^ $ candidates with masses restricted to $\pm2\sigma$ of $ B ^ $ 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.) 
A_mN_f[..].pdf [29 KiB] HiDef png [162 KiB] Thumbnail [94 KiB] *.C file 

Upper limit on ${\cal{B}}( B ^ \rightarrow \pi ^+ \mu ^ \mu ^ )$ at $95\%$ C.L. as a function of $m_N$ in 5 MeV intervals for $\cal{S}$ selected events. 
A_uppe[..].pdf [30 KiB] HiDef png [140 KiB] Thumbnail [111 KiB] *.C file 

Upper limits on ${\cal{B}}( B ^ \rightarrow \pi ^+ \mu ^ \mu ^ )$ at 95% C.L. as a function of $m_N$, in 5 MeV intervals, for specific values of $\tau_N$. 
ULmas[..].pdf [50 KiB] HiDef png [312 KiB] Thumbnail [322 KiB] *.C file 

Upper limits at 95% C.L. on $V_{\mu4}^2$ are shown as a function of $m_N$ for ${\cal{L}}$ events. 
V_mu4_[..].pdf [31 KiB] HiDef png [162 KiB] Thumbnail [104 KiB] *.C file 

Animated gif made out of all figures. 
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Created on 11 July 2020.