CMS-SMP-20-008

CMS-SMP-20-008 ; CERN-EP-2020-197
Search for the rare decay of the W boson into a pion and a photon in proton-proton collisions at $\sqrt{s} = $ 13 TeV
CMS Collaboration
11 November 2020
Phys. Lett. B 819 (2021) 136409
Abstract: A search is performed for the rare decay $\mathrm{W^{\pm}}\to\pi^{\pm}\gamma$ in proton-proton collisions at $\sqrt{s} = $ 13 TeV. Data corresponding to an integrated luminosity of 137 fb$^{-1}$ were collected during 2016 to 2018. This novel analysis exploits the W boson production in top quark pair events. The leptonic decay of the W boson from one of the top quarks is used to tag the event, and the b quark jets are used to reduce the background from the hadronization of light-flavor quarks and gluons. The W boson originating from the other top quark is used to search for the $\mathrm{W^{\pm}}\to\pi^{\pm}\gamma$ decay. Such events are characterized by an isolated track and an isolated photon of large transverse momentum. The $\mathrm{W^{\pm}}\to\pi^{\pm}\gamma$ 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.
Links: e-print arXiv:2011.06028 [hep-ex] (PDF) ; CDS record ; inSPIRE record ; HepData record ; CADI line (restricted) ;

Figures & Tables	Summary	Additional Figures	References	CMS Publications

Figures
png pdf	Figure 1: Event distribution as a function of the pion-isolation variable $\Sigma {p_{\mathrm {T}}} / {p_{\mathrm {T}}} ^{\pi}$. The simulated MC distribution for the signal is given by the dashed red line and corresponds to a 1% branching fraction for the $\mathrm{W^{\pm}} \to \pi^{\pm} \gamma $ decay. The statistical uncertainties in the data are small and thus not visible. In the lower plot, the ratio between data and the background component of the MC is shown. The gray bands represent the statistical uncertainty in the MC background.
png pdf	Figure 2: Event distributions as a function of the BDT discriminant for the muon (left) and electron (right) channels. The green and orange arrows indicate the intervals of the BDT discriminant used to define a signal (SR) and a control region (CR). The signal histogram is enhanced so that the signal and background histograms are normalized to the same area. The statistical uncertainties in the data are small and thus not visible.
png pdf	Figure 2-a: Event distributions as a function of the BDT discriminant for the muon channel. The green and orange arrows indicate the intervals of the BDT discriminant used to define a signal (SR) and a control region (CR). The signal histogram is enhanced so that the signal and background histograms are normalized to the same area. The statistical uncertainties in the data are small and thus not visible.
png pdf	Figure 2-b: Event distributions as a function of the BDT discriminant for the electron channel. The green and orange arrows indicate the intervals of the BDT discriminant used to define a signal (SR) and a control region (CR). The signal histogram is enhanced so that the signal and background histograms are normalized to the same area. The statistical uncertainties in the data are small and thus not visible.
png pdf	Figure 3: Event distribution as a function of $ {m_{\pi \gamma}} $ for the combination of the lepton channels. The simulated MC distribution for the signal is given by the dashed red line and corresponds to a $10^{-4}$ branching fraction for the $\mathrm{W^{\pm}} \to \pi^{\pm} \gamma $ decay. The uncertainties in the data are statistical only. The blue line represents the best fit to the data using the model described in Eq. (1). In the lower plot, the ratio between data and the background component of the MC is shown. The gray bands represent the uncertainty (statistical + systematic) in the MC background.

Tables
png pdf	Table 1: The product of signal efficiency and acceptance per year and per lepton channel.

Summary

A first search is reported for the rare decay $\mathrm{W^{\pm}}\to\pi^{\pm}\gamma$ at the LHC. The search is based on the proton-proton collision data collected at a center-of-mass energy of 13 TeV at the CMS experiment in 2016-2018, corresponding to an integrated luminosity of 137 fb$^{-1}$. Because of the high trigger thresholds for single photons, which make an inclusive search unsuitable at CMS, the measurement is performed using top quark-antiquark pair events, where one of the produced W bosons decays into leptons. This is the first search for $\mathrm{W^{\pm}}\to\pi^{\pm}\gamma$ that adopts such a strategy. The rare decay is characterized by an isolated track, for which a specific pion-isolation variable is defined, and an isolated photon with large transverse momentum. 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 1.50 $\times 10^{-5}$ at the confidence level of 95%. This result demonstrates the feasibility of a search for such rare decays of the W bosons at the LHC and defines a suitable search strategy.

Additional Figures
png pdf	Additional Figure 1: Event distribution as a function of the number of b jets $n_{\mathrm {b}}$ (left) and the pion-isolation variable $\Sigma {p_{\mathrm {T}}} / {p_{\mathrm {T}}} ^{{\pi}}$ (right) after the multivariate selection. The simulated MC distribution for the signal is given by the dashed red line and corresponds to a 10$^{-4}$ branching fraction for the $\to {\pi ^\mathrm {{\pm}}} {\gamma}$ decay. In the lower plots, the ratio between data and the background component of the MC is shown. The gray bands represent the statistical uncertainty in the MC background. The topology with $n_{\mathrm {b}} = $ 0 mainly incorporates events with soft hadronic activity, and is strongly suppressed by the multivariate selection. This results as well in an improved agreement between simulation and collision data for low values of $\Sigma {p_{\mathrm {T}}} / {p_{\mathrm {T}}} ^{{\pi}}$, where mostly particles of low ${p_{\mathrm {T}}}$ contribute to the ${p_{\mathrm {T}}} $-sum.
png pdf	Additional Figure 1-a: Event distribution as a function of the number of b jets $n_{\mathrm {b}}$ after the multivariate selection. The simulated MC distribution for the signal is given by the dashed red line and corresponds to a 10$^{-4}$ branching fraction for the $\to {\pi ^\mathrm {{\pm}}} {\gamma}$ decay. In the lower plot, the ratio between data and the background component of the MC is shown. The gray band represents the statistical uncertainty in the MC background. The topology with $n_{\mathrm {b}} = $ 0 mainly incorporates events with soft hadronic activity, and is strongly suppressed by the multivariate selection.
png pdf	Additional Figure 1-b: Event distribution as a function of the pion-isolation variable $\Sigma {p_{\mathrm {T}}} / {p_{\mathrm {T}}} ^{{\pi}}$ after the multivariate selection. The simulated MC distribution for the signal is given by the dashed red line and corresponds to a 10$^{-4}$ branching fraction for the $\to {\pi ^\mathrm {{\pm}}} {\gamma}$ decay. In the lower plot, the ratio between data and the background component of the MC is shown. The gray band represents the statistical uncertainty in the MC background. The topology with $n_{\mathrm {b}} = $ 0 mainly incorporates events with soft hadronic activity, and is strongly suppressed by the multivariate selection. This results as well in an improved agreement between simulation and collision data for low values of $\Sigma {p_{\mathrm {T}}} / {p_{\mathrm {T}}} ^{{\pi}}$, where mostly particles of low ${p_{\mathrm {T}}}$ contribute to the ${p_{\mathrm {T}}} $-sum.

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