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Search for heavy neutral leptons in $W^+ \to \mu^+ \mu^{\pm} jet $ decays

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Abstract

A search is performed for heavy neutrinos in the decay of a $W$ boson into two muons and a jet. The data set corresponds to an integrated luminosity of approximately $3.0 \text{ fb}^{-1}$ of proton-proton collision data at centre-of-mass energies of 7 and $8 \text{ TeV}$ collected with the LHCb experiment. Both same-sign and opposite-sign muons in the final state are considered. Data are found to be consistent with the expected background. Upper limits on the coupling of a heavy neutrino with the Standard Model neutrino are set at $95\%$ confidence level in the heavy-neutrino mass range from 5 to $50 \text{ GeV}/c^2$. These are of the order of $10^{-3}$ for lepton-number-conserving decays and of the order of $10^{-4}$ for lepton-number-violating heavy-neutrino decays.

Figures and captions

Properties of a heavy neutrino as a function of its mass [21,22]: (left) the branching fractions to final states with a muon and (right) the lifetime, assuming a coupling of $10^{-5}$ (current limit).

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Feynman diagram for the production of a heavy neutrino via mixing with a neutrino from the decay of a $ W $ boson and semileptonic decay of the heavy neutrino into a lepton and two quarks. The subscripts $\alpha$ and $\beta$ indicate the lepton flavour.

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The (left) positive and (right) negative muon transverse momentum spectra for the 2012 data set integrated over pseudorapidity for the normalisation channel. The fit is overlaid.

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Distributions of the invariant mass $m(\mu _N\text{ jet} )$ for (left) same-sign and (right) opposite-sign muons. The signal component corresponds to a $15\text{ Ge V /}c^2 $ neutrino.

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Expected (dashed line) and observed (solid line) upper limit on $\mathcal{B} (N\rightarrow \mu \text{ jet} )\left| V_{\mu N}\right|^2$ at 95% C.L. for (left) the same-sign muons sample and (right) the opposite-sign muons sample. The light and dark green bands show the $1\sigma$ and $2\sigma$ uncertainties, respectively, on the expected upper limits.

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Observed upper limit on the mixing parameter $\left| V_{\mu N}\right|^2$ between a heavy neutrino and a muon neutrino in the mass range $5-50 \text{ Ge V /}c^2 $ for same-sign and opposite-sign muons in the final states with and without lifetime correction.

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Animated gif made out of all figures.

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Tables and captions

Requirements on IP and BDT classifiers defining the signal and control regions.

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Efficiency ratios, for different heavy-neutrino masses, between normalisation and signal channels. The first uncertainty is statistical, the second is systematic.

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Extrapolated background yields in the signal region for same-sign and opposite-sign muon channels. The uncertainty is statistical.

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Supplementary Material [file]

Supplementary materials for LHCb-PAPER-2020-022

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Created on 27 February 2021.