Search for heavy Majorana neutrinos in $e^{\pm }{e}^{\pm }$ and $e^{\pm }{\mu }^{\pm }$ final states via WW scattering in $pp$ collisions at $\sqrt{s}=13$ TeV with the ATLAS detector

Abstract

A search for heavy Majorana neutrinos in scattering of same-sign $W$ boson pairs in proton-proton collisions at $\sqrt{s}=13$ TeV at the LHC is reported. The dataset used corresponds to an integrated luminosity of 140 fb${}^{-1}$, collected with the ATLAS detector during 2015-2018. The search is performed in final states including a same-sign $ee$ or $e\mu$ pair and at least two jets with large invariant mass and a large rapidity difference. No significant excess of events with respect to the Standard Model background predictions is observed. The results are interpreted in a benchmark scenario of the Phenomenological Type-I Seesaw model. New constraints are set on the values of the $|V_{eN}{|}^2$ and $|V_{eN}{V}_{\mu N}^{\ast }|$ parameters for heavy Majorana neutrino masses between 50 GeV and 20 TeV, where $V_{\ell N}$ is the matrix element describing the mixing of the heavy Majorana neutrino mass eigenstate with the Standard Model neutrino of flavour $\ell =e,\mu$. The sensitivity to the Weinberg operator is investigated and constraints on the effective $ee$ and $e\mu$ Majorana neutrino masses are reported. The statistical combination of the $ee$ and $e\mu$ channels with the previously published $\mu \mu$ channel is performed.

arXiv comment37 pages in total, author list starting page 20, 4 figures, 2 tables, published in Phys. Lett. B. All figures including auxiliary figures are available at https://atlas.web.cern.ch/Atlas/GROUPS/PHYSICS/PAPERS/EXOT-2023-16/

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