Observation of $C\!P$ violation in charm decays (1903.08726v2)

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.

• The paper presents the first observation of direct CP violation in charm decays, with a combined ΔACP of −15.4×10⁻⁴ exceeding five standard deviations from zero.
• The study employs advanced π-tagging and μ-tagging methods to measure raw asymmetries in D0 and D̄0 decays while correcting for detection and production biases.
• The observation aligns with Standard Model predictions and opens new avenues to explore potential contributions from physics beyond the Standard Model.

Observation of CP Violation in Charm Decays

The paper presents the first observation of Charge-Parity (CP) violation in charm hadron decays, a fundamental aspect of particle physics. Conducted by the LHCb collaboration at CERN, this paper analyzes data collected from proton-proton collisions to measure CP asymmetries in the decays of neutral D mesons ($D^0$) to $K^+K^-$ and $\pi^+\pi^-$ final states.

Key Findings

The primary result of the investigation is the measurement of the difference in time-integrated CP asymmetries between $D^0$ decays to $K^+K^-$ and $\pi^+\pi^-$ final states. The observed asymmetry difference is:

• For $\pi$-tagged decays: $$\Delta A_{CP} = [-18.2 \pm 3.2 \, \text{(stat.)} \pm 0.9 \, \text{(syst.)}] \times 10^{-4}$$
• For $\mu$-tagged decays: $$\Delta A_{CP} = [-9 \pm 8 \, \text{(stat.)} \pm 5 \, \text{(syst.)}] \times 10^{-4}$$

The combined result from this data and prior measurements is $\Delta A_{CP} = [-15.4 \pm 2.9] \times 10^{-4}$, showing a deviation from zero exceeding five standard deviations. These results mark the first evidence of direct CP violation in the decay of charm mesons.

Methodology

The experiment utilized $\pi$-tagging and $\mu$-tagging methods to identify the flavor of the $D^0$ mesons produced in proton-proton collisions at a center-of-mass energy of 13 TeV. The analysis involved measuring the raw asymmetries from the decay rates of $D^0$ and $\bar{D^0}$ mesons, while systematically correcting for potential biases due to detection and production asymmetries. Advanced statistical methods and simulation models were essential to estimate systematic uncertainties and ensure the robustness of the results.

Implications

This observation of CP violation in charm decays is a critical finding that complements the established CP violation in the kaon and B-meson sectors. The magnitude of CP violation in charm is consistent with the upper limits of Standard Model (SM) predictions, which anticipate tiny asymmetries on the order of $10^{-4}$ to $10^{-3}$. This result provides a new domain to test the SM and explore potential contributions from physics beyond the Standard Model.

Future Directions

Theoretical interpretation of this CP violation demands further investigation, particularly concerning contributions from direct CP violation and its interplay with mixing effects. Future experimental studies will aim to refine these measurements and reduce uncertainties. Additionally, further theoretical developments in quantum chromodynamics are necessary to enhance predictions of CP violation amplitudes in charm decays.

In the broader landscape of particle physics, this result from the charm sector enhances our understanding of CP symmetry and its violations, which are fundamental to explaining the matter-antimatter asymmetry in the universe. As experimental techniques continue to advance at facilities like the LHC, researchers anticipate uncovering new phenomena that may challenge or extend the prevailing theoretical models.