Abstract
A search is performed for the rare decay W±→π±γ in proton-proton collisions at s=13TeV. Data corresponding to an integrated luminosity of 137fb−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±→π±γ 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±→π±γ 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±→π±γ decay is not observed. An upper exclusion limit is set to this branching fraction, corresponding to 1.50×10−5 at 95% confidence level, whereas the expected upper exclusion limit is 0.85−0.29+0.52×10−5.
Highlights
Rare hadronic decays of W bosons represent probes of the strong interaction at the boundary between the perturbative and nonperturbative domains of quantum chromodynamics (QCD), offering insights into factorization and meson form factors at large energy scales [1,2,3]
An observed upper limit at 95% confidence level (CL) is set on the branching fraction of the W boson to a pion and a photon using the asymptotic CLs method [38,39]: B(W± → π±γ) < 1.50 × 10−5, (5)
The total uncertainty is dominated by the statistical contributions, which account for ≈80%
Summary
Rare hadronic decays of W bosons represent probes of the strong interaction at the boundary between the perturbative and nonperturbative domains of quantum chromodynamics (QCD), offering insights into factorization and meson form factors at large energy scales [1,2,3]. Exclusive decays of the W boson into final states containing a single meson can be used to test these theoretical frameworks in a context where the energy scales are sufficiently large. At such scales, corrections that depend on the momentum transferred to the hadron in the final state can be neglected. At future colliders they could provide a new way to measure the mass of the W boson that is based solely on visible singleparticle decay products Theoretical calculations for such branching fractions (B) have large uncertainties: the expected value of B(W± → π±γ), for instance, is in the range of 10−9 − 10−7 [1,4]. The symbol τ indicates that the leptonic decays of the τ lepton originating from W → τν are considered in the computation
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