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Tord Ekelof: ESS Super Beam for Neutrino CP Violation Discovery

March 14, 2013

The European Neutron Spallation Source will be built in Lund using a 2.5 5 MW linac to produce 10^23 protons on target per year, i.e. two orders of magnitude more than other accelerators. This requires three years of design and four years of construction. The first beams would be in 2019. The finance volume is of 1800 MEuro.

A large value of theta_13 has consequences. The observation of the CP violation signal requires the interference term in the neutrino oscillation probability to be of observable magnitude. Most of the so far existing proposals were made under the assumption of a very small theta_13, for which the S(N ratio was the main limiting factor. For large theta_13 the situation is one where an accurate control of systematic errors becomes the limiting factor.
Having a broad neutrino energy distribution allows for an oscillation curve shape analysis rather than just a counting experiment analysis. This implies that the neutrino beam should not be tilted with respcect to the proton beam.

The MEMPHYS detector, studied so far in the context of the FP7 Laguna project, is a Hyperkamiokande-like detector to study neutrino from accelerators, but also supernovae and all other known sources of neutrinos.

The requirements for the ESS super-beam experiment are demanding: a volume of 440000 m^2 is needed to obtain a sufficiently large volume. A location in scandinavia is possible, but a medium baseline of 200-500 km is best for an energy of 300 MeV. Several locations in Sweden were considered. The Garpenberg mine is at 540km, at a depth of 1232m.

The proton pulse length of the ESS linac has to be compressed by a factor of 2000, to 1.5 microseconds. This compression can be achieved using a compressor ring in which the whole pulse is accumulated by multi-turn injection and then ejected in one turn. This ring should have a 450m circumference.

Each pulse from the ESS Linac will contain about 10^15 protons. During the 3ms injection, the negative charge will repel the successively arriving protons. The only chance is then to inject H- ions and use a laser to strip off the electrons at the moment when they enter the stored beam. This implies that the linac must accelerate also H- ions. This must be simultaneous with proton acceleration.

For the simultaneous acceleration of 14 proton pulses and 14 H- pulses per second, the average power of the linac will have to be doubled from 5MW to 10MW. The cost of this doubling is a dominant cost for the modification project of the linac.

The compressor ring will contain an enormous amount of protons stored at low energy, so the space charge will lead to a defocusing of the beam. This is at the limit of being unmanageable. A way to alleviate this problem is to divide the charge on four compressor rings, reducing the defocusing effect by a factor 4. This trick was used for the CERN PS already.

For a target one design is a packed bed of titanium spheres cooled with cold helium gas. Questions have been raised whether the beam would generate vibrations, under investigation.

The optimal baseline of the superbeam for CP violation discovery has been studied. 500km is maximizing the 5-sigma discovery probability.

The Euro SB project proposes to study the use of the ESS proton linac to build a physics competitive and cost effective neutrino Super Beam facility. The stron synergy between the use of the ESS linac for spallation neutron production and the use for neutrino beam production presents a unique possibility in Europe to play a significant role in neutrino physics, in particular in the search for CP violation.


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