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Observation of $C P$ violation in charm decays

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Abstract

A search for charge-parity ($C P$) violation in $D^0 \to K^- K^+$ and $D^0 \to \pi^- \pi^+$ decays is reported, using $pp$ collision data corresponding to an integrated luminosity of 6 $\mathrm{fb}^{-1}$ collected at a center-of-mass energy of 13 TeV with the LHCb detector. The flavor of the charm meson is inferred from the charge of the pion in $D^*(2010)^+ \to D^0 \pi^+$ decays or from the charge of the muon in $\overline{B} \to D^0 \mu^-\bar{\nu}_\mu X$ decays. The difference between the $C P$ asymmetries in $D^0 \to K^- K^+$ and $D^0 \to \pi^- \pi^+$ decays is measured to be $\Delta A_{C P} = [ -18.2 \pm 3.2 (\rm stat.) \pm 0.9 (\rm syst.) ] \times 10^{-4}$ for $\pi$-tagged and $\Delta A_{C P} = [ -9 \pm 8 (\rm stat.) \pm 5 (\rm syst.) ] \times 10^{-4} $ for $\mu$-tagged $D^0$ mesons. Combining these with previous LHCb results leads to $$\Delta A_{C P} = ( -15.4 \pm 2.9) \times 10^{-4},$$ where the uncertainty includes both statistical and systematic contributions. The measured value differs from zero by more than five standard deviations. This is the first observation of $C P$ violation in the decay of charm hadrons.

Figures and captions

Mass distributions of selected (top) $\pi^\pm$-tagged and (bottom) $\mu^\pm$-tagged candidates for (left) $ K ^- K ^+ $ and (right) $\pi ^- \pi ^+ $ final states of the $ D ^0$ -meson decays, with fit projections overlaid.

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Raw asymmetries of the tagging pion for the $\pi$-tagged $ D ^0 \rightarrow K ^- K ^+ $ sample, with the polarity of the magnet (top) up and (bottom) down. The plots on the left include only candidates with $|p_y / p_z| < 0.02$, i.e., close to the horizontal plane, and the fiducial requirements used to exclude the kinematic region surrounding the beam pipe, characterized by large values of the raw asymmetry, are indicated as black lines (in addition to the forementioned requirement $|p_y / p_z| < 0.02$). The plots on the right include all candidates except those excluded by the beam-pipe fiducial requirements, and the black lines indicate the fiducial requirements used to exclude regions at the boundary of the detector acceptance, which are also characterized by large values of the raw asymmetry. Distributions for the $ D ^0 \rightarrow \pi ^- \pi ^+ $ sample are very similar.

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Background-subtracted distributions of momentum ($p$), transverse momentum ($ p_{\mathrm{T}} $), azimuthal angle ($\varphi$) and pseudorapidity ($\eta$) of $ D ^{*+}$ mesons for the prompt sample: (left column) before and (right column) after the weighting procedure for $ D ^0 \rightarrow K ^- K ^+ $ and $ D ^0 \rightarrow \pi ^- \pi ^+ $ decays, as indicated in the legends. The distributions are normalized to unit area.

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Background-subtracted distributions of momentum ($p$), transverse momentum ($ p_{\mathrm{T}} $), azimuthal angle ($\varphi$) and pseudorapidity ($\eta$) of $ D ^0$ mesons for the semileptonic sample: (left column) before and (right column) after the weighting procedure for $ D ^0 \rightarrow K ^- K ^+ $ and $ D ^0 \rightarrow \pi ^- \pi ^+ $ decays, as indicated in the legends. The distributions are normalized to unit area.

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Raw asymmetries in the $\pi$-tagged sample for (left) $ D ^0 \rightarrow K ^- K ^+ $ and (right) $ D ^0 \rightarrow \pi ^- \pi ^+ $ candidates as a function of $m( D ^0 \pi ^+ )$.

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Raw asymmetries in the $\mu$-tagged sample for (left) $ D ^0 \rightarrow K ^- K ^+ $ and (right) $ D ^0 \rightarrow \pi ^- \pi ^+ $ candidates as a function of $m( D ^0 )$.

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Distributions of the signed $ D ^0$ impact parameter in the plane transverse to the beam direction, $\text{TIP}$ , in bins of reconstructed $ D ^0$ decay time, for the $\pi$-tagged $ D ^0 \rightarrow K ^- K ^+ $ sample: (top left) $\tilde{t} < 1.5$, (top right) $1.5 < \tilde{t} < 3.0$, (bottom left) $3.0 < \tilde{t} < 4.5$ and (bottom right) $\tilde{t} > 4.5$, where $\tilde{t} \equiv t / \tau( D ^0 )$. The fit results are overlaid and the contributions from prompt and secondary decays are shown, as indicated in the legends. Distributions for the $ D ^0 \rightarrow \pi ^- \pi ^+ $ sample are very similar.

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Invariant-mass distributions of (top) $ D ^0 \rightarrow K ^- K ^+ $ and (bottom) $ D ^0 \rightarrow \pi ^- \pi ^+ $ candidates in the prompt sample with fit results overlaid. These fits are used to determine the yields and raw asymmetries of (top) $ D ^0 \rightarrow K^-\pi^+\pi^0$ and (bottom) $ D ^0 \rightarrow \pi^-l^+\nu_l$ backgrounds, whose mass shapes extend to the $ D ^0$ -signal mass region. The various components included in the fit model are indicated in the legends.

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Measurements of $\Delta A_{ C P }$ in time-ordered data-taking subsamples (referred to as run blocks) for (left) prompt and (right) semileptonic samples. The uncertainties are statistical only. The horizontal red-dashed lines show the central values of the nominal results.

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Measurements of $\Delta A_{ C P }$ in bins of $ D ^0$ decay time for (left) prompt and (right) semileptonic samples. In each plot, the last bin on the right also includes a few overflow candidates. The uncertainties are statistical only. The horizontal red-dashed lines show the central values of the nominal results.

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Measurements of $\Delta A_{ C P }$ in bins of (top left) pseudorapidity, (top right) azimuthal angle, (bottom left) transverse momentum and (bottom right) $\chi^2_{\text{IP}}$ of tagging pions for the prompt sample. In each plot but that of the azimuthal angle, the last bin on the right also includes a few overflow candidates. The uncertainties are statistical only. The horizontal red-dashed lines show the central value of the nominal result.

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Measurements of $\Delta A_{ C P }$ in bins of (top left) pseudorapidity, (top right) azimuthal angle, (bottom left) transverse momentum and (bottom right) $\chi^2_{\text{IP}}$ of tagging muons for the semileptonic sample. In each plot but that of the azimuthal angle, the last bin on the right also includes a few overflow candidates. The uncertainties are statistical only. The horizontal red-dashed lines show the central value of the nominal result.

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Measurements of $\Delta A_{ C P }$ in bins of $ D ^0$ $\chi^2_{\text{IP}}$ for (left) prompt and (right) semileptonic samples. In each plot, the last bin on the right also includes a few overflow candidates. The uncertainties are statistical only. The horizontal red-dashed lines show the central values of the nominal results.

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Tables and captions

Systematic uncertainties on $\Delta A_{ C P }$ for $\pi$- and $\mu$-tagged decays (in $10^{-4}$). The total uncertainties are obtained as the sums in quadrature of the individual contributions.

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Supplementary Material [file]

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Created on 30 May 2020.