Abstract
We discuss how the lepton CP phase can be constrained by accelerator and reactor measurements in an era without dedicated experiments for CP violation search. To characterize globally the sensitivity to the CP phase \delta_{CP}, we introduce a new measure, the CP exclusion fraction, which quantifies what fraction of the \delta_{CP} space can be excluded at a given input values of \theta_{23} and \delta_{CP}. Using the measure we study the CP sensitivity which may be possessed by the accelerator experiments T2K and NOvA. We show that, if the mass hierarchy is known, T2K and NOvA alone may exclude, respectively, about 50%-60% and 40%-50% of the \delta_{CP} space at 90% CL by 10 years running, provided that a considerable fraction of beam time is devoted to the antineutrino run. The synergy between T2K and NOvA is remarkable, leading to the determination of the mass hierarchy through CP sensitivity at the same CL.
Highlights
We argue that one of the most important goals related to lepton CP violation that may be reached by the ongoing and the upcoming experiments is to exclude a significant fraction of the δCP space
It is interesting and timely to discuss the following questions: “What is the impact of running T2K in the antineutrino mode on the determination of δCP? What would be the optimal time sharing between neutrino and antineutrino beams in order that T2K can say something meaningful on δCP? How T2K and NOνA compare with each other in δCP sensitivity? Can the combination of equal-time running of T2K and NOνA say more on δCP than each one of these experiments with doubled running time? Or, rephrasing, is there a synergy between them?” In what follows we will address all these questions based on the CP exclusion fraction which quantify how much the CP phase can be constrained for a given input parameters and experimental set up
To quantify the experimental sensitivity of T2K and NOνA to δCP in a global way, we use the “CP exclusion fraction” ≡ fCPX where fCPX is defined as the fraction of δCP values which can be disfavored at a given confidence level for a given set of input parameters
Summary
In this and the following sections we discuss the results of our analyses, the sensitivities to CP phase determination or exclusion to be expected by the T2K and NOνA experiments,. In figure 1, the contours of equal CP exclusion fraction for T2K experiment are plotted in the space spanned by the true values of δCP and sin θ23. We note some characteristic features of the exclusion fraction iso-contour lines: (1) overall, the regions of higher sensitivity to CP are centered around δCP ±π/2, and (2) in the 5 + 0 years running option the CP sensitive region is restricted mostly to two regions centered at (δCP π/2, low sin θ23) and (δCP −π/2, high sin θ23), whereas in 3 + 2 and 2 + 3 years running options (center and right panels) the dependence on sin θ23 is weakened, at around δCP π/2 and δCP −π/2 for the inverted and the normal hierarchies, respectively.
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