The Standard Model (SM) of particle physics provides a remarkably successful description of observed high-energy microscopic phenomena. The momentous discovery of the enigmatic Higgs boson, by the LHC at CERN is the culmination of the SM success. However, the SM, and particularly the Higgs mechanism, does not predict the neutrino to have mass. However, the discovery of neutrino (ν-)oscillations (the property that it may change its type between the three neutrino families) in the late 1990's, awarded with the Nobel Prize in 2015, demands that neutrinos do have a small but non-zero rest mass. This discovery was among the first empirical indications that the SM is incomplete. The implications are far reaching, leading to the inference that neutrino interactions may in fact be responsible for the non-survival of all of the antimatter created in the Big Bang by annihilation with the matter created at the same time. Based on the tiny differences in how matter and antimatter behave at the scale of elementary particles, known as charge-parity violation or (CPV), 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. The current problem is that the already observed CPV in the hadronic sector is not enough, and that the SM, also, does not predict enough to explain the observed matter/anti-matter asymmetry. Thus, the challenge which the physicists are facing is to study these differences in detail not only in the hadronic sector but in the leptonic sector as well, where the CPV has not been confirmed yet. Recent neutrino oscillation measurements indicate that the discovery of neutrino CPV becomes an important candidate to explain the observed matter dominance in the Universe. An understanding of the contribution of neutrinos in these areas requires precise measurements of the parameters governing ν-oscillations, especially the leptonic CP-violating phase, δCP. On the other hand, the measurement of the third neutrino mixing angle, θ13, with significantly larger value, with respect to what was expected before its measurment, opened the possibility of measuring with high precision the δCP with intense “super” neutrino beam experiments.
The European Spallation Source neutrino Super Beam (ESSνSB) aims to benefit from the high power of the European Spallation Source (ESS) LINAC in Lund-Sweden, to produce the world’s most intense neutrino beam enabling measurements to be made at the second oscillation maximum. Figure 1, shows the ESS site layout . The goal of ESSνSB [hyperlink: https://essnusb.eu/] project, with its extended project ESSvSB+, is to discover and measure with precision the neutrino CP-violation, through the study of the vμ → ve oscillation at the second rather than the first oscillation maximum, with unprecedented sensitivity.
ESSνSB+ is the successor of the ESSvSB, which is a European scientific collaboration, gathering scientists from 19 institutions from 11 European countries (France, Germany, Sweden, Bulgaria, Croatia, Greece, Italy, Spain, Switzerland and Turkey) and Japan. ESSvSB, the first phase of the project, was financed from January 2018 to March 2022 through the Research Infrastructure Development Programme of the European Commission (Horizon 2020). The project was initiated by the COST networking Project titled: “EuroNuNet“, which was concluded in March 2020. The ESSvSB+, the extension project, will be financed through the Horizon-Europe program from Jan. 2023 to Dec. 2025. The neutrino group of the Institut für Experimentalphysik – Hamburg University is the first and the representative of Germany in the ESSνSB/+ collaboration.
The recently published ESSvSB CDR has demonstrated that the initially foreseen physics performance of the ESSvSB has surpassed all earlier expectations by covering, figure 2 [left], after 10 years of data collection, more than 70% of the range of possible δCP values with a confidence level of more than 5σ to reject the no-CPV hypothesis, figure 2 [middle]. The expected measurement precision of the value of δCP is better than 8° for all δCP values, figure 2 [right], making it the most precise proposed experiment by a large margin.
Publications of the neutrino group in the ESSvSB collaboration:
A. Alekou et al., Eur. Phys. J. C 81, (2021) 1130.
“Updated physics performance of the ESSnuSB experiment”
A. Alekou et al., arXiv: 2206.01208 (2022).
“The European Spallation Source neutrino Super Beam Conceptual Design Report”
A. Alekou et al., arXiv:2203.08803 (2022).
https://doi.org/10.48550/arXiv.2203.08803“The European Spallation Source neutrino Super Beam“. (White Paper submitted to the Snowmass 2021)
E. Baussan et al., PoS NuFact2021 (2022) 100
“Status of the ESSnuSB Target Station"
T. Tolba for the ESSnuSB Collaboration, PoS PANIC2021 (2021) 284.
“The ESS based neutrino Super Beam Experiment (ESSnuSB)"
E. Baussan et al., JACoW IPAC2021 (2021) 466-469
“The Multi-Mega-Watt Target Station for the European Spallation Source Neutrino Super Beam”