CMS-PAS-SMP-20-008

CMS-PAS-SMP-20-008
Search for the rare exclusive hadronic decay of a W boson into a pion and a photon in proton-proton collisions at 13 TeV
CMS Collaboration
July 2020

Abstract: A search is performed for the rare exclusive hadronic decay $\mathrm{W}\to\pi\gamma$ in pp collisions at $\sqrt{s} = $ 13 TeV. Data collected by the CMS experiment at the LHC during the 2016, 2017 and 2018 data-taking periods, which correspond to an integrated luminosity of 137 fb$^{-1}$, are analyzed. The analysis exploits the W boson production from $\mathrm{t\bar{t}}$ events. The leptonic decay of the W boson from one of the top quarks is used to tag the event, and the b-jets are exploited to reduce the QCD background from the hadronization of light quarks. The W boson originating from the other top quark is used to search for the $\mathrm{W}\to\pi\gamma$ decay. The rare decay is characterized by a high-transverse momentum, isolated track and a high-energy, isolated photon. The $\mathrm{W}\to\pi\gamma$ decay is not observed, and an upper limit is set to its branching fraction, corresponding to 1.51 $ \times $ 10$^{-5} $ at 95% confidence level.
Links: CDS record (PDF) ; CADI line (restricted) ; These preliminary results are superseded in this paper, PLB 819 (2021) 136409. The superseded preliminary plots can be found here.

Figures & Tables	Summary	References	CMS Publications

Figures
png pdf	Figure 1: Distribution of the pion isolation variable. In the signal MC, represented by the dashed red line, the branching fraction for $\mathrm {W}\to \pi \gamma $ is 10$^{-2}$.
png pdf	Figure 2: Distributions of the BDT score for the muon (left) and electron (right) channels.
png pdf	Figure 2-a: Distribution of the BDT score for the muon channel.
png pdf	Figure 2-b: Distribution of the BDT score for the electron channel.
png pdf	Figure 3: Distribution of $m_{\pi \gamma}$ for the sum of the lepton channels. In the MC signal, represented by the dashed red line, the branching fraction for $\mathrm {W}\to \pi \gamma $ is assumed to be 10$^{-4}$. The blue line represents the best fit to the data using the model described in Eq. (2).

Tables
png pdf	Table 1: Signal efficiencies per year and per channel.

Summary

In summary, the first search of the rare exclusive hadronic decay $\mathrm{W}\to \pi\gamma$ at the LHC has been presented. The search is based on the proton-proton data collected at a center-of-mass energy of $13 TeV$ by the CMS experiment in 2016, 2017 and 2018, corresponding to an integrated luminosity of 137 fb$^{-1}$. The measurement is performed on $\mathrm{t\bar{t}}$ events, where one of the produced W bosons decays into leptons. The rare decay is characterized by an isolated photon and an isolated track compatible with that of a pion, for which a specific pion isolation variable is defined. The data are compatible with the background only hypothesis. The upper limit on the branching fraction of the W boson to a pion and a photon is 1.51 $ \times $ 10$^{-5} $ at 95% CL. This result demonstrates the possibility of performing a search for rare hadronic decays of the W bosons at the LHC, and defines a suitable search strategy.

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