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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 $10^{-4}$.

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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. In this analysis $\alpha$ and $\beta$ are both muons.

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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 filled histograms are the result of the fit to the data.

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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 23 March 2024.