Search for $CP$ violation in the phase space of $D^0\rightarrow\pi^+\pi^-\pi^+\pi^-$ decays

TL;DR

This paper reports the first application of the energy test to search for $CP$ violation in four-body $D^0$ decays, finding results consistent with $CP$ symmetry within current sensitivity limits.

Contribution

It introduces the energy test method to four-body decays and extends the search for $CP$ violation to include $P$-odd asymmetries, providing the most sensitive test to date.

Findings

No significant $CP$ violation observed; data consistent with symmetry.

Achieved the world's best sensitivity to $CP$ violation in this decay.

Marginal evidence for $CP$ non-conservation at 2.7 sigma.

Abstract

A search for time-integrated $CP$ violation in the Cabibbo-suppressed decay $\mbox{$D^0\rightarrow\pi^+\pi^-\pi^+\pi^-$}$ is performed using an unbinned, model-independent technique known as the energy test. This is the first application of the energy test in four-body decays. The search is performed for $P$-even $CP$ asymmetries and, for the first time, is extended to probe the $P$-odd case. Using proton-proton collision data corresponding to an integrated luminosity of 3.0 fb$^{-1}$ collected by the LHCb detector at centre-of-mass energies of $\sqrt{s}=$7 TeV and 8 TeV, the world's best sensitivity to $CP$ violation in this decay is obtained. The data are found to be consistent with the hypothesis of $CP$ symmetry with a $p$-value of $(4.6\pm0.5)\%$ in the $P$-even case, and marginally consistent with a $p$-value of $(0.6\pm0.2)\%$ in the $P$-odd case, corresponding to a significance for $CP$ non-conservation of 2.7 standard deviations.