Measurement of CP asymmetry in radiative charm decays at LHCb
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Date
2024
Authors
Chernov, Aleksei
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The Henryk Niewodniczański Institute of Nuclear Physics Polish Academy of Sciences
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Creative commons license
Abstract
This thesis presents a measurement of CP asymmetry, ACP , for radiative decays of charm mesons D0based→onV γ,theand theirproton-protonchargecollisionconjugates,datawherecollectedV atdenotesa cenvectortre-of-masmesons Kenergy¯ ∗0, ofφ √orsρ=0. 8TheTeV bstudyy theis LHCb experiment in 2012, during Run-1 of the LHC. This data sample corresponds to an integrated luminosity of 2 fb−1. Reconstructed neutral D mesons originate from strong decays of the D∗± states, D∗+ → D0π production. The presented analysis is the first study of radiative charm decays at LHCb. Large and irreducible background from decays involving π0 mesons, D0 → V π0, is suppressed using a multivariate classifier based on distinguished shapes of energy clusters produced by photons and π0 mesons in the LHCb calorimeter system. The signal decays are separated from residual background with the three-dimensional fit to D0 invariant mass M(D0), the difference between invariant masses of D∗+ and D0, ∆M = M(D∗+) − M(D0), and V -meson helicity angle cos θ, as observables. Correlations observed between M(D0) and ∆M observables for both signal and background channels, are modelled based on the corresponding simulation samples. The total D0 and D¯ 0 signal yields are measured to be: 4263 ± 193 events for D0 →0 K¯ ∗0γ decay, and 216 ± 29 events for the D0 → φγ decay; no significant signal is observed for the D → ργ decay. Nuisance asymmetries due to asymmetric charm production and asymmetric detection of positively and negatively charged hadrons, are constrained with high-statistics and high-purity reference channels. The pion-tagged D0 → K+K− and D0 → π+π− decays are chosen as reference channels for D0 → φγ and D0 → ρ0γ signal decays, respectively, whereas D0 → K−π+π0 is used as a reference channel for D0 → K¯ ∗0γ. The nuisance asymmetries are measured to be about (−1.0 ± 0.2)% in D0 → K+K− and D0 → π+π− channels, and (−1.7 ± 0.3)% in D0 → K−π+π0. They are subtracted from asymmetries measured for the signal channels in order to access ACP. The asymmetries for the signal channels are measured by performing the three-dimensional fits in M(D0) vs. ∆M vs. cos θ space, simultaneously to the separated D0 and D¯ 0 samples. The central values of the asymmetries are still blinded, while their statistical uncertainties are measured to be 5% for D0 → K¯ ∗0γ and 12% for D0 → φγ; the Run-1 analysis is not sensitive to the asymmetry in the D0 → ρ0γ decay. The presented measurement is statistically limited. The total systematic uncertainties on the signal ACP ’s are expected to be below 1%, and dominated by statistical uncertainties on the asymmetries measured in the reference channels. Unblinding is planned together with the Run-2 measurement, which is currently ongoing.
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Attribution 4.0 International