Search for the rare decay of the W boson into a pion and a photon in proton-proton collisions at $\sqrt{s}=$ 13 TeV

Abstract

A search is performed for the rare decay W${}^{\pm }\to {\pi }^{\pm }\gamma$ in proton-proton collisions at $\sqrt{s}=$ 13 TeV. Data corresponding to an on W integrated luminosity of 137 fb${}^{-1}$ were collected during 2016 to 2018 with the CMS detector. This analysis exploits a novel search strategy based on W boson production in top quark pair events. An inclusive search for the W${}^{\pm }\to {\pi }^{\pm }\gamma$ decay is not optimal at the LHC because of the high trigger thresholds. Instead, a trigger selection is exploited in which the W boson originating from one of the top quarks is used to tag the event in a leptonic decay. The W boson emerging from the other top quark is used to search for the W${}^{\pm }\to {\pi }^{\pm }\gamma$ signature. Such decays are characterized by an isolated track pointing to a large energy deposit, and by an isolated photon of large transverse momentum. The presence of b quark jets reduces the background from the hadronization of light-flavor quarks and gluons. The W${}^{\pm }\to {\pi }^{\pm }\gamma$ decay is not observed. An upper exclusion limit is set to this branching fraction, corresponding to 1.50 $\times$ 10${}^{-5}$ at 95% confidence level, whereas the expected upper limit exclusion limit is 0.85 ${}_{-0.29}^{+0.52}$ $\times$ 10${}^{-5}$.

Replaced with the published version. Added the journal reference. All the figures and tables can be found at http://cms-results.web.cern.ch/cms-results/public-results/publications/SMP-20-008 (CMS Public Pages)

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• First order electroweak radiative corrections to the decay of the polarised $W$ boson
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