CP-violation search with T2K data

TL;DR

The T2K experiment investigates CP violation in neutrino oscillations, finding evidence that disfavors CP-conserving values and suggesting a preference for negative CP-violating phase values.

Contribution

This paper presents the first significant constraints on the CP-violating phase δ_CP using T2K data, including exclusion of CP-conserving values at 90% confidence level.

Findings

Excess of electron neutrino candidates observed in muon neutrino beam mode.

Best fit δ_CP value around -1.97 with uncertainties.

CP-conserving values of δ_CP are excluded at 90% CL.

Abstract

The T2K experiment is a long-baseline neutrino oscillation experiment which uses $\nu_{\mu}$ and $\bar{\nu}_{\mu}$ beams to constrain CP-violating effects in a 3-flavor PMNS neutrino mixing model. Through $\nu_{\mu}\to\nu_{e}$ and $\bar{\nu}_{\mu}\to\bar{\nu}_{e}$ appearance channels, T2K is sensitive to CP-violating effects in neutrino mixing. An excess of $\nu_{e}$ candidates in the $\nu$-beam mode is observed when compared to the CP conserving cases. T2K finds a best fit value of $\delta_{\mathrm{CP}}=-1.97_{-0.70}^{+0.97}$ using Feldman-Cousins corrected intervals and excludes CP-conserving values of $\delta_{\mathrm{CP}}$ of $0$ and $\pi$ at the 90% CL. $J_{\mathrm{CP}}=0$ is also excluded at 2$\sigma$ when using a flat prior in $\delta_{\mathrm{CP}}$, favoring negative values.