The ESS𝜈SB+ Project

The European Spallation Source neutrino Super Beam (ESS𝜈SB) is a long-baseline neutrino project that will measure the CP-violation in the leptonic sector at the second, rather than the first, oscillation maximum, where the sensitivity of the experiment is ∼3 times higher. The physics simulations have shown that the ESS𝜈SB, after 10 years of data taking, will be able to cover more than 70% of the CP-violating phase, δCP , parameterrange with 5 σ C.L. to reject the no-CP-violation hypothesis. The expected measurement error of δCP is smaller than 8° for all δCP values, making it the most precise proposed experiment in the field by a large margin. The extension project, ESS𝜈SB+ to be performed between 2023 and 2026, aims in measuring the neutrino-nucleus cross-section (the dominant term of the systematic uncertainty) in the energy range of 0.2 – 0.6 GeV, using a Low Energy nuSTORM (LEnuSTORM) and a Low Energy Monitored Neutrino Beam (LEMNB) facilities.


Introduction
Based on the tiny differences in how matter and antimatter behave at the scale of elementary particles, known as charge-parity violation or "CP-violation", scientists believe that there is some subtle asymmetry between matter and antimatter, and that soon after the Big Bang, after annihilation, this led to a universe dominated by matter.While CPV has been observed on a small scale in the hadronic sector [3], it has not been seen yet in the leptonic sector.The present generation of ν-oscillations experiments [4,5] promises to determine the values of the three mixing-angle and the two mass-splitting parameters of the PMNS matrix with high precision.However, neither of these experiments can reach the confidence level required to claim the CPV discovery.On the other hand, the relatively high value of θ 13 significantly modified the optimal strategy for the discovery of the leptonic CPV and the precision measurement of δ CP value, provided that the far detector is placed at the second, rather than the first, oscillation maximum.The future long-baseline detectors such as the Deep Underground Neutrino Experiment (DUNE) in the USA [6], the Tokai to Hyper-Kamiokande (T2HK) in Japan [7] and the European Spallation Source neutrino Super Beam (ESSνSB) in Europe will rely on using an intense "super" neutrino beams working at a Mega-Watt scale power for their measurements.While the design of DUNE and T2HK experiments aims for measurement at the first oscillation maximum, the ESSνSB foresees measurement at the second oscillation maximum, with a significantly higher sensitivity to CPV.

The ESSνSB project
To be able to produce an intense neutrino beam, concurrently with the intense neutron beam of the ESS, it is necessary to apply a number of upgrades to the ESS facility.The pulse frequency of the ESS LINAC (the proton driver) must be increased from 14 Hz to 28 Hz to obtain additional acceleration cycles that will be used for neutrino production, without affecting the neutron programme.Moreover, during neutrino cycles, H − ions instead of protons need to be accelerated in order to ease the filling of the accumulator ring.An accumulator ring will be built to shorten the ESS pulse to about 1.2 µs.A neutrino production target station, composed of four identical targets enveloped by four magnetic focusing devices (horns), will be built.The horns will be used to charge select and focus the pions and thus also the neutrinos resulting from their decay toward the near and far detectors.A near detector will be used to monitor the neutrino beam and to measure neutrino interaction cross-sections, especially electron neutrino cross-sections, at a short baseline from the neutrino source, while the far detector will be used to detect the oscillated neutrino beam at the long baseline distance.
This phase of the project was concluded in March 2022.The results obtained from this design study were published in a comprehensive Conceptual Design Report (CDR) [1]. Figure 1 (left panel) shows the ESSνSB CPV discovery sensitivity as a function of true δ CP values.It shows that that for the maximal values of δ CP , around ±90 • , the discovery sensitivity reaches ca. 13 σ for the default baseline of 360 km.The middle panel shows that that the ESSνSB will cover, after 11 years of data collection, more than 70% of the range of the possible δ CP values with a confidence level of more than 5 σ to reject the no-CPV hypothesis.The right panel shows that the expected precision on the measured value of δ CP will be better than 8 • for all δ CP values, making ESSνSB the most precise proposed experiment in the field by a large margin.This analysis assumed different systematic errors; 1%, 5%, 10%, and 25%, on signal and background.

The ESSνSB+ project
Due to the lack of knowledge of the complex interactions of the neutrinos with the target nuclei, i.e. the uncertainty on the neutrino-nucleus (ν-N) interaction cross-section in the water of the water-Cherenkov far detector, the CPV discovery and the precise measurement of δ CP value will be limited foremost by systematic, rather than statistical, errors.ESSνSB will be measuring at the second oscillation maximum, where the influence of the systematic errors on the δ CP measurement will be close to three times smaller as compared to DUNE and T2HK experiments.Even so, it is of a vital importance to measure the ν-N cross-section in the low energy range as precisely as possible for the precise determination of δ CP value, especially that the data on the neutrino cross-section in the neutrino energy range of ESSνSB, ca.0.1-0.6GeV, is currently very scarce [8].
The extension phase, the ESSνSB+ [2], which was recently funded by the European commission Horizon-Europe programme and started on January 2023, aims in addressing the challenging task of measuring the neutrino-nucleus cross-section using a special target station, a Low Energy nuSTORM [9] (LEnuSTORM) and an ENUBET-like [10] Low Energy Monitored Neutrino Beam (LEMNB) facilities.

The ESSνSB+ target station
The ESSνSB+ proposes to build an R&D target station which will contain only one of the four ESSνSB target/horn systems, and operating at 1.25 MW beam power.In addition, the pions produced in the secondary hadron beam will be extracted and deflected towards a low energy muon storage ring (LEnuSTORM), similar to the nuSTORM project [9].Therefore, the R&D target station will consist of magnetic systems that depend on using conventional dipoles to bend the beam in horizontal and/or vertical planes is considered for the design of pions extraction, deflection and focusing system (pEDS).Sets of magnetic focusing elements, e.g.quadrupoles, placed before and/or after the extraction-system will ensure keeping the transverse envelop of the extracted pion beam minimal while propagating through the beam line.Moreover, the pEDS system will be able to, efficiently, separate the pions from the non-interacted protons present in the secondary beam.Figure 2 shows a sketch for the conceptual layout of the ESSνSB+ special target station, including the concept of the pion extraction and deflection system, and the LEnuSTORM ring

The LEnuSTORM racetrack ring
This racetrack muon storage ring will allow muons to decay and give a well-defined muon and electron neutrino beam to be used for cross-section measurements and sterile neutrino searches.A transfer line will be designed at the end of, and integrated with, the pEDS to lead the secondary particles, the pions, exiting the R&D target station into the straight section of the LEnuSTORM racetrack ring, in which pions will decay through the process π + → µ + + ν µ (or its charged conjugate version for π − mode of operation) to produce additional muons.The bend at the end of the straight section will be designed to kinematically select muons to keep them in the ring, while pions will be lead into the beam dump.The circulating muons will gradually decay through the process µ + → e + + ν e + ν µ (or its charged conjugate version for π − /µ − mode).Muon decays occurring in the straight section will produce a neutrino beam containing equal amounts of muon and electron neutrinos (with additional muon neutrinos coming from the pion decay in the first straight portion of the ring during the filling).This beam will have a significant ν e (or ν e ) component which will be used to measure the electron (anti)neutrino interaction cross-section.

The Low Energy Monitored Neutrino Beam
The ESSνSB+ project will be equipped with an ENUBET-like [10] Low Energy Monitored Neutrino Beam (LEMNB) facility.The LEMNB will share the LEnuSTORM the same near detector, hence it will be placed parallel to the LEnuSTORM ring.This will help improving the precision on the muon (and possibly electron) neutrino interaction cross-section measurements.The idea behind this facility is to tag (monitor) the lepton associated with the neutrino that has been produced from the decay of e.g., pions and muons, in an instrumented decay tunnel.The walls of the decay tunnel will be instrumented by an iron-scintillator calorimeter which will be used to reconstruct the energy and direction of the charged decay products of pions and muons (muons for pion decay and electrons for muon decay) [11].The LEMNB technique doesn't prefer the short pulses, which is required by the ESSνSB and LEnuSTORM facilities.Therefore, a bypass line of the accumulator ring, will be considered in this design study.This will require a static focusing, rather than the magnetic horn focusing, of the produced hadron beam, since horn will not be able to withstand long high-current pulses.
Figure 3 shows a lay-out of the ESS site with proposed positions of the LEnuSTORM racetrack ring, the LEMNB and the dedicated target station.

The ESSνSB+ detectors
The ESSνSB near detector will be used as a far detector of LEnuSTORM.An additional detector, which will be used by LEnuSTORM and LEMNB for cross-section measurements, and as LEnuSTORM near detector for sterile neutrino searches will be designed.Moreover, a dedicated study will be conducted on the effect of doping the near and far WC detectors with gadolinium (Gd).Gd is known to have a high neutron capture cross-sections.Detecting the delayed gammas produced from the neutron captures on Gd in the WC, in coincidence with the prompt positorn signal, helps in discriminating between the electron antineutrinos and the electron neutrinos (which tend to produce protons in the WC).

Conclusion
The first phase of the ESSνSB design study has been successfully concluded and a comprehensive conceptual design report has been published.The results published in the CDR have demonstrated the unprecedented physics reach of the experiment.An extension phase has just begun with the ESSνSB+ design study program.The new project is underway to design intermediate facilities for R&D and measurement of neutrino interaction cross-section, perform civil engineering conceptual studies and explore the additional physics possibilities of the proposed infrastructure.

Figure 1 .
Figure 1.(Left) CPV discovery sensitivity of ESSνSB as a function of true δ CP at the 360 km baseline (at the Zinkgruvan mine in Sweden).(Middle) The fraction of true values of δ CP for which CPV can be discovered at 5σ C.L. at both considered baselines; the 360 km and the 540 km (this latter is at the Garpenberg mine in Sweden) (Right) The precision on the measurement of δ CP at the 360 km baseline option.(The 360 km, which covers the second and part of the first oscillation maximum, is the default baseline of the ESSνSB experiment, while the 540 km, which covers the second oscillation maximum only, is the alternative baseline)

Figure 2 .
Figure 2. A conceptual drawing of the ESSνSB+ target station facility, with the concept of the pion extraction, deflection and focusing system, and the LEnuSTORM ring.

14thFigure 3 .
Figure 3. Layout of the ESS site with the proposed LEnuSTORM ring (in red), the LEMNB (in green) and the special target station (in yellow).(Units in mm)