Measurement of the CP-violating phase $\beta$ in $B^0\rightarrow J/\psi \pi^+\pi^-$ decays and limits on penguin effects

TL;DR

This paper measures CP violation in B0 decays to J/psi and pion pairs, providing constraints on penguin effects and the CP-violating phase shift, using LHCb data to improve understanding of CP violation and penguin contributions.

Contribution

First measurement of time-dependent CP violation in B0→J/psi π+π− decays for each resonant final state, setting limits on penguin effects and phase shifts in CP violation.

Findings

Measured 2β^eff as (41.7±9.6_{-6.3}^{+2.8})° in B0→J/psi ρ0(770) decays.

Limited the shift in CP-violating phase φ_s to within [-1.05°, +1.18°] at 95% CL.

Provided constraints on penguin contributions affecting CP violation measurements.

Abstract

Time-dependent CP violation is measured in the $B^0\rightarrow J/\psi\pi^+\pi^-$ channel for each $\pi^+\pi^-$ resonant final state using data collected with an integrated luminosity of 3.0 fb$^{-1}$ in $pp$ collisions using the LHCb detector. The final state with the largest rate, $J/\psi\rho^0(770)$, is used to measure the CP-violating angle $2\beta^{\rm eff}$ to be $(41.7\pm 9.6_{-6.3}^{+2.8})^{\circ}$. This result can be used to limit the size of penguin amplitude contributions to CP violation measurements in, for example, $B_s^0\rightarrow J/\psi\phi$ decays. Assuming approximate SU(3) flavour symmetry and neglecting higher order diagrams, the shift in the CP-violating phase $\phi_s$ is limited to be within the interval [$-1.05^\circ$, +$1.18^\circ$] at 95% confidence level. Changes to the limit due to SU(3) symmetry breaking effects are also discussed.