Model-independent determination of the strong phase difference between $D^0$ and $\bar{D}^0 \to\pi^+\pi^-\pi^+\pi^-$ amplitudes

TL;DR

This paper measures the strong phase difference between $D^0$ and $ar{D}^0$ decays to four pions in a model-independent way, using CLEO-c data, to improve the determination of the CP-violating phase gamma in B decays.

Contribution

It provides the first model-independent measurement of the strong phase difference in $D o ourpions$ decays, crucial for precise gamma determination in B physics.

Findings

Strong phase difference measured in decay phase space bins.

CP-even fraction of $D o ourpions$ decay determined as 0.769.

Estimated gamma uncertainty of about 10 degrees at LHCb.

Abstract

For the first time, the strong phase difference between $D^0$ and $\bar{D}^0\to\pi^+\pi^-\pi^+\pi^-$ amplitudes is determined in bins of the decay phase space. The measurement uses $818\,\mathrm{pb}^{-1}$ of $e^+e^-$ collision data that is taken at the $\psi(3770)$ resonance and collected by the CLEO-c experiment. The measurement is important for the determination of the $C P$-violating phase $\gamma$ in $B^{\pm}\to D K^{\pm}$ (and similar) decays , where the $D$ meson (which represents a superposition of $D^0$ and $\bar{D}^0$) subsequently decays to $\pi^+\pi^-\pi^+\pi^-$. To obtain optimal sensitivity to $\gamma$, the phase space of the $D \to \pi^+\pi^-\pi^+\pi^-$ decay is divided into bins based on a recent amplitude model of the decay. Although an amplitude model is used to define the bins, the measurements obtained are model-independent. The $CP$-even fraction of the $D \to \pi^+\pi^-\pi^+\pi^-$ decay is determined to be $F_{+}^{4\pi} = 0.769 \pm 0.021 \pm 0.010$, where the uncertainties are statistical and systematic, respectively. Using simulated $B^{\pm}\to D K^{\pm}, D \to \pi^+\pi^-\pi^+\pi^-$ decays, it is estimated that by the end of the current LHC run, the LHCb experiment could determine $\gamma$ from this decay mode with an uncertainty of $(\pm10\pm7)^\circ$, where the first uncertainty is statistical based on estimated LHCb event yields, and the second is due to the uncertainties on the parameters determined in this paper.