First observation of CP violation in the decays of Bs mesons (1304.6173v2)

Abstract: Using pp collision data, corresponding to an integrated luminosity of $1.0 \mathrm{fb}^{-1}$, collected by LHCb in 2011 at a center-of-mass energy of 7 TeV, we report the measurement of direct CP violation in $B^0_s \to K^-\pi^+$ decays, $A_{C/!P}(B^0_s \rightarrow K^- \pi^+)=0.27 \pm 0.04\,\mathrm{(stat)} \pm 0.01\,\mathrm{(syst)}$, with significance exceeding five standard deviations. This is the first observation of CP violation in the decays of $B^0_s$ mesons. Furthermore, we provide an improved determination of direct CP violation in $B^0 \to K^+\pi^-$ decays, $A_{C/!P}(B^0\rightarrow K^+ \pi^-)=-0.080 \pm 0.007\,\mathrm{(stat)} \pm 0.003\,\mathrm{(syst)}$, which is the most precise measurement of this quantity to date.

Analysis of CP Violation in $B^0_s$ Meson Decays

This paper presents the first observation of CP violation in $B^0_s \to K^-\pi^+$ decays and provides an improved measurement of CP violation in $B^0 \to K^+\pi^-$ decays. Utilizing data collected in 2011 with the LHCb detector at the Large Hadron Collider (LHC), which corresponds to an integrated luminosity of 1.0 fb$^{-1}$ at a center-of-mass energy of 7 TeV, this paper marks a significant step forward in flavor physics and CP violation research.

Key Results

The primary result from this investigation is the observation of direct CP violation in the $B^0_s \to K^-\pi^+$ decay channel, marked by the CP asymmetry parameter $$A_{C\!P}(B^0_s \to K^-\pi^+)=0.27 \pm 0.04\,\mathrm{(stat)} \pm 0.01\,\mathrm{(syst)}$$. Notably, the reported significance exceeds five standard deviations, establishing this as a statistically robust observation of CP violation in $B^0_s$ decays for the first time. Complementary to this, the paper presents a refined measurement for $B^0 \to K^+\pi^-$ decays with $$A_{C\!P}(B^0 \to K^+\pi^-)=-0.080 \pm 0.007\,\mathrm{(stat)} \pm 0.003\,\mathrm{(syst)}$$, which stands as the most precise determination of this value up to the publication date.

Methodology

The analysis follows a methodical approach employing the LHCb detector, a spectrometer optimized for studying hadrons containing $b$ or $c$ quarks. The methodology involves reconstructing the invariant mass spectra for the $K^+\pi^-$ and $K^-\pi^+$ final states and fitting these spectra to derive raw asymmetries. Subsequent corrections are applied to account for instrumental asymmetries in detection, as well as production asymmetries from $B^0_{(s)} - \overline{B}^0_{(s)}$ mixing effects. Leveraging $D^{*+} \to D^0(K^-\pi^+)\pi^+$ control samples, instrumental asymmetries are precisely calibrated and corrected for, ensuring the reliability of the CP asymmetry results.

Implications

The observed CP violation in $B^0_s$ decays provides vital empirical data for testing the Standard Model's (SM) CKM framework. While previous evidence of CP violation in $B^0$ mesons has been well-documented, the $B^0_s$ results furnish a complementary rigorous test of theoretical predictions. The significance of this observation surpasses existing evidence for non-SM physics, offering compelling opportunities to explore potential new sources of CP violation beyond the SM, which might help address the baryon asymmetry present in the universe.

Future Directions

These findings encourage continued exploration of CP violation phenomena in $B$ meson systems, particularly through increased luminosity runs at the LHC. Future experiments will likely focus on reducing systematic uncertainties and exploring additional decay channels that might further elucidate or challenge the SM's explanations for CP violation.

Overall, this paper substantially contributes to our understanding of CP violation, reinforcing the LHCb's role as a crucial instrument in precision measurements of flavor physics. The methodologies and results outline an essential framework for ongoing and future studies in the field.