Hints for Leptonic $CP$ Violation or New Physics?

One of the major open questions in the neutrino sector is the issue of leptonic $CP$ violation. Current global oscillation data show a mild preference for a large, potentially maximal value for the Dirac $CP$ phase in the neutrino mixing matrix. In this Letter, we point out that new physics in the form of neutral-current-like nonstandard interactions with real couplings would likely yield a similar conclusion even if $CP$ in the neutrino sector were conserved. Therefore, the claim for a discovery of leptonic $CP$ violation will require a robust ability to test new physics scenarios.

Figure 1

Birate plots. The full line curve corresponds to the SM for all Dirac $CP$ phase values. The dashed and dotted curves correspond to a fixed NSI magnitude $|ϵ|=0.3$ and for all NSI phase $\varphi$ values shown but for different values of the standard $CP$ phase. The cross for the SM point ${\delta }_{CP}=-\pi /2$ shows the statistical uncertainty. The left (right) panel corresponds to our implementation of the T2K (NOvA) experiment. All parameters not shown in the plot were fixed to their best fit values; see the text for details.

Figure 2

Results assuming ${\delta }_{CP}^{\text{True}}=\pi$. In the left panel, the Dirac $CP$ phase best fit distributions for SM and NSI interactions are shown. The NSI magnitude was fixed to the value $|ϵ|=0.3$ for the different NSI phase $\varphi$ values showed in the plot. In the right panel appears the minimum ${\chi }^2$ distributions from the fit to the SM Dirac $CP$ phase for each of the SM and NSI cases showed in the left panel. All not shown parameters were marginalized over; see the text for details of the analysis.