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

TL;DR

This paper reports a search for the rare decay of the W boson into a pion and a photon at 13 TeV, using CMS data, and sets an upper limit on its branching fraction due to non-observation.

Contribution

A novel search strategy exploiting top quark pair events and leptonic W tagging was developed to improve sensitivity to the rare W to pion gamma decay.

Findings

No evidence of W→πγ decay was observed.

An upper limit on the branching fraction was set at 1.50×10⁻⁵.

Expected upper limit was approximately 0.85×10⁻⁵.

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 $^{+0.52}_{-0.29}$ $\times$ 10$^{-5}$.