The Opportunity Offered by the ESSnuSB Project to Exploit the Larger Leptonic CP Violation Signal at the Second Oscillation Maximum and the Requirements of This Project on the ESS Accelerator Complex

Abstract

The European Spallation Source (ESS), currently under construction in Lund, Sweden, is a research center that will provide, by 2023, the world's most powerful neutron source. The average power of the proton linac will be 5 MW. Pulsing this linac at higher frequency will make it possible to raise the average total beam power to 10 MW to produce, in parallel with the spallation neutron production, a very intense neutrino Super Beam of about 0.4 GeV mean neutrino energy. This will allow searching for leptonic CP violation at the second oscillation maximum where the sensitivity is about 3 times higher than at the first. The ESS neutrino Super Beam, ESSnuSB operated with a 2.0 GeV linac proton beam, together with a large underground Water Cherenkov detector located at 540 km from Lund, will make it possible to discover leptonic CP violation at 5 significance level in 56% (65% for an upgrade to 2.5 GeV beam energy) of the leptonic CP-violating phase range after 10 years of data taking, assuming a 5% systematic error in the neutrino flux and 10% in the neutrino cross section. The paper presents the outstanding physics reach possible for CP violation with ESSnuSB obtainable under these assumptions for the systematic errors. It also describes the upgrade of the ESS accelerator complex required for ESSnuSB.

Current unknowns in the three-neutrino framework

“…A wise attitude is to wait for further data from all the running experiments which, in the the next few years, can reveal if these two hints at ∼ 2σ level will fluctuate down, or will consistently grow and confirm the preference for NO at a cumulative level > 3σ. On a longer time frame, discovery-level tests of the mass spectrum ordering will be provided by next-generation projects [41], not only with large-volume atmospheric neutrinos [186,187,188,189] but also with medium-baseline reactors such as JUNO [190] and new long-baseline accelerator facilities such as T2HK [191], DUNE [192] and ESSnuSB [193]. Finally, for discrete hypotheses like NO versus IO, the statistical interpretation of ∆χ 2 in terms of N σ remains effectively applicable, but must be taken with a grain of salt [194].…”

Neutrino Super Beam for lepton CP violation discovery based on the European Spallation Source

“…The baseline is then focused on the second oscillation as shown in Figure 2. The ratio between the number of electron neutrinos for δ CP = 3π/2 and δ CP = π/2 reaches 4.8 and appears to be higher by a factor of at least 2.5 compared to experiments operating at the first oscillation maximum [9].…”

A comparative study between ESSnuSB and T2HK in determining the leptonic CP phase

“…Therefore, the 3 Note that although the ESSnuSB flux peaks around 2.5 GeV, the chargedcurrent cross-section is almost negligible at this energy. Therefore, the effective energy distribution, where the number of events is maximal, appears in the region where the flux × cross-section peaks, which is around 0.35 GeV [5].…”

“…The ESSnuSB [4,5] and the T2HK [6] are two future proposed neutrino long-baseline experiments, which are specifically designed to determine δ CP with significantly large confidence level. Since both these experiments have baseline lengths less than 1000 km, their main goal will be solely focused on measuring δ CP .…”