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Feasibility Study for employing the uniquely powerful ESS linear accelerator to generate an intense neutrino beam for leptonic CP violation discovery and measurement.

Periodic Reporting for period 3 - ESSnuSB (Feasibility Study for employing the uniquely powerful ESS linear accelerator to generate an intense neutrino beam for leptonic CP violation discovery and measurement.)

Reporting period: 2021-01-01 to 2022-03-31

In the seconds and minutes after the Big Bang, the universe was composed of equal quantities of matter and antimatter. But at some instant in the early lifetime of the universe, processes occurred that favoured the survival of matter as opposed to antimatter and today’s universe is composed only of matter. We do not know yet what these processes were. When matter and antimatter come into contact, they annihilate each other and the mass is converted into energy - as expressed by Einstein’s famous equation E = mc2 - in the form of radiation. Intuitively that is what ought to have happened after the Big Bang, leaving a universe composed only of radiation. Clearly that did not happen. Only one part in ten billion survived this cataclysmic annihilation event as matter, and yet that relatively minute quantity that did survive was sufficient to create the whole vast universe in which we live today. There was a - yet to be understood - asymmetry, or in other words there was a violation of charge-parity (CP) symmetry, that allowed that to happen. But what were these processes and what was the physical reason for the charge-parity violation after Big Bang?
Addressed issue:
A deeper knowledge of the neutrino and its surprising properties can help us to solve a mystery that researchers have not yet understood - that today's universe is only composed of matter and no antimatter. The European Spallation Source (ESS) neutrino Super Beam project – ESSνSB – can change this situation dramatically, surpassing in power current neutrino sources based on accelerators, which are unlikely to be sufficiently intense to allow experiments to be conducted that will unravel this mystery.
Why is it important for society?
Scientific importance:
The ultimate goal of ESSνSB project, once put into operation, is the search for a mechanism beyond the Standard Model (SM) of particle physics that could explain the matter/antimatter asymmetry in the Universe. This mechanism would have far-reaching implications for cosmology, particle physics and, more generally, philosophy, answering questions such as the origin and evolution of the Universe, its characteristics, and what is our part in it?
Economic importance:
To plan and construct a modern research infrastructure, or scientific installation, like that of the ESSnuSB project, is a very large enterprise. About 70 years (20 years to construct from now and 50 years’ operations) will be required from start of the planning of the project till end of operations, several billion € will be required to cover the investment costs and several hundred physicists will be needed to carry the project through.

What are the overall objectives?
The objectives of the ESSνSB Design Study are to demonstrate the feasibility of using the European Spallation Source proton linac (linear accelerator) to deliver a 5 MW H- beam to produce and detect the world's most intense neutrino beam concurrently with the 5 MW proton beam that will be used for the production of spallation neutrons. In order to achieve this, ESSνSB high-level objectives are:
- to specify and design the necessary upgrades to the current ESS linear accelerator in order to raise the average beam power from 5 MW to 10 MW by inserting, between the proton pulses for spallation neutron production, additional H- ion pulses for neutrino production;
- to design an intermediate proton accumulator reducing the proton pulse duration in order to decrease the physics background and also to comply with the requirements of the hadron collector that is needed for the generation of a well-focused neutrino beam;
- to update, adapt and optimise the existing design of the pion production target and collector, the pion to neutrino and muon decay tunnel and the Water Cherenkov far detector, already studied in detail in the FP6/FP7 projects CARE-BENE, EUROν, LAGUNA and LAGUNA-LBNO, to the specific requirements of ESSνSB;
- to study the design of a near detector to be used for both the monitoring of the neutrino beam flux and for the measurements of the neutrino cross-sections of interest, with the aim of minimising the systematic uncertainties of the neutrino oscillation measurements;
- to carry out an investigation of the geological and logistical challenges for the construction of the far detector at the currently preferred site, which is located at a distance from ESS corresponding to the second neutrino oscillation maximum, and also to consider possible alternative sites;
- to promote the ESSνSB project proposal to its stakeholders, including scientists.
During the duration of the project height postdocs have been engaged to work on the objectives of the six Work Packages (WP). All deliverables (21) and milestones (18) have been achieved on time. One main activity has been to define and update common design parameters for the different components of the experimental equipment in order to ensure that the activities in all WPs are consistent with each other. A baseline scenario and additional options in terms of the duration and the frequency of the H- pulses have been defined for the accelerator operation. Studies have been performed of the implications for the accumulator and target station operation of the accelerator baseline scenario and of the additional options. Optimisation of the parameters for the target station components (hadronic collector, decay tunnel, the target itself) have successfully been performed and showed that there is still room for increasing the neutrino beam intensity and thus the physics performance of the overall project. A study of the near detector has been launched using computer-based simulation tools to design particle detectors that are optimal for the monitoring of the neutrino beam as well as for measuring the neutrino cross-sections. New methods have been used for the analysis of the far detector data increasing significantly the neutrino detection efficiency.
During the last period of the project, the optimisation of the experimental design of the Target Station and new methods to analyse the far detector data showed that a fraction of the CP violation parameter, above 70% (60% expected previously), a fraction that no other facility can reliably cover. At the same time, it has been proved that a high precision in the measurement of the CP violating phase-angle can be obtained with an uncertainty lower than 8 degrees, again, better than any expectation.
Doubling the accelerator power from 5 MW to 10 MW will definitely be a technological achievement beyond the state of the art. Already with the equipment of the experiment as currently planned, a uniquely intense muon beam will also be produced, concurrently with the high intensity neutrino beam. The further development and use of this muon beam will provide the project with a physics potential that goes far beyond the current scope of the project. The prospection and build-up of the underground large volume neutrino far detector will have a high socio-economic impact on the region hosting it, since the far detector site will become an important international scientific centre.
Layout of the European Spallation Source