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Working Group 4 : Accelerators, Research and Development, detection techniques

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MEMPHYS (MEgaton Mass PHYSics)

The MEMPHYS Design Study has been realized in the context of three European FP7 Design Studies (DS): EUROnu[1], LAGUNA and LAGUNA-LBNO[2], aiming the construction of a large and deep underground neutrino observatory and a new high intensity neutrino facility in Europe. In particular, the LAGUNA DS was devoted to the investigation of seven possible sites in Europe, at different baseline from CERN, and to the comparison of three detctor technologies: Liquid Scintillator, Liquid Argon and Water Cherenkov. Two options have been selected for the subsequent LAGUNA-LBNO DS: the construction of Giant Liquid Argon detector in Pyhasalmi (Finland), at 2300km from CERN, and a Megaton Water Cherenkov detector at Fréjus, at 130km from CERN. These two configurations allow complementary studies, for example for the measurement of the CP violation phase in the lepton sector and the determination of the Neutrino Mass Hierarchy. The so-called MEMPHYS option has been studied in details, from the engeneering and from the physics point of view, as will be summarized in the following.

The site and the underground layout


Fréjus site is located in France near to the Italian-French border next to the Fréjus Highway tunnel connecting Modane (France) and Bardonecchia (Italy). At Fréjus site all the necessary infrastructures are present, in particular: the Fréjus railway tunnel, Fréjus road tunnel (one tube completed in 1982 and the second one under construction) and Laboratoire Souterraine de Modane (LSM).
The underground layouts and logistics of cavern construction has been studied in detailed by the Lombardi SA [4], already responsible of the Fréjus tunnel excavations.
MEMPHYS will be placed in French country near to Italian-French border close to the existing underground LSM with over 1700 m of overburden, as can be seen in Figure 1.

It will consist of two identical shaft shape caverns containing a total fiducial mass of 540 kt of ultra pure water. The position of the 2 MEMPHYS caverns have been fixed along an ideal straight flow corridor connecting the CERN infrastructures with the site, as shown in Figure 2.


In Figure 3, the general layout of the MEMPHYS caverns is shown. It includes: the main independent access from the town of Modane to the top and the bottom of the tanks, two vertical shafts, the tunnel directly connecting the MEMPHYS plant to the road tunnel and a series of ancillary caverns.

The Detector Design and Instrumentation


The detector layout has been defined based on the present expertise of the Super Kamiokande detector [5] and on the feasibility studies reported by Lombardi SA. The chosen layout is represented by two “dome shape” cavities, of 65 m of diameter and 100 m of height, divided in: an Inner Detector of about 283,000 m3 of volume and an annular Outer Muon Veto of about 62,360 m3 of volume. A scheme of the layout is shown in Figure 4.

A detailed design of the Optical Instrumentations has been developed as shown in Figure 5. The optical module is composed by 12” diameter PMTs, encapsulated, in order to prevent implosion, and surrounded by light concentrators, in order to increase the light collection.

Veto and Inner PMTs will be mounted on PMT’s matrices (see Figure 6) having the double function of supporting the OMs of the Inner Detector and the Outer Muon Veto and of light barrier between these two volumes. In particular, each matrix will support 16 inner PMT’s and one outer PMT.

A common electronic read out unit will take care of the High Voltage supply and signal digitization for each matrix, following the PMm2 philosophy [7]. This will allow the reduction of long cable connecting the submerged electronic to the surface one. Matrices will be supported by means of a Support Structure directly anchored to the tank walls (see again Figure ).

Physics Reach: Neutrino Beam


In order to evaluate realistic performance for the above-described baseline detector, a de- tailed simulation has been developed, mainly in the context of the EUROnu FP7 Desgin Study [8]. More details can be found also in [9,10].
Particular care has been devoted to the evaluation of the MEMPHYS sensitivity to the measurement of the CP violation phase and to the mass hierarchy (MH) determination. In the context of the EUROnu DS, two neutrino beams have been proposed and tuned for MEMPHYS: a Super Beam and a Beta Beam.
The concept of the Super Beam is based on the creation of neutrinos by impinging a high power proton beam onto a target and focusing the produced pions towards a far detector using a magnetic horn. For the MEMPHYS detector studies, the proton driver is the the Superconducting Proton Linac (SPL) producing a 4 MW beam at 5 GeV operating at a frequency of 50 Hz [11] [12].
The Beta Beam consists of electron (anti-)neutrinos obtained from the beta decay of radioactive isotopes circulating in a storage ring. The main advantage of this kind of beam is the very low contamination components. The composition of the beam depends on whether the accelerated isotope is a β+ or a β− emitter. The project envisages the use of existing infrastructures at CERN and some modifications of the actual accelerators [8].
Due to the relatively short baseline (130 km), neutrino oscillations are nearly unaffected by matter effects, therefore the experiment is well suited for the determination of the CP violation in the leptonic sector but is nearly insensitive to mass hierarchy. This is confirmed by Figure 7 which shows the allowed regions for Normal and Inverted hierarchy in terms of antineutrino versus neutrino events for different values of δCP.

As a consequence, potential studies for δCP are done under the hypothesis of known Mass Hierarchy. δCP studies have been developed both for the Super Beam and the Beta Beam. As an example, Figure 8 shows the median sensitivity to CP violation for the baseline Super Beam configuration of 20% neutrino and 80% antineutrino running. We expect a sensitivity to CP violation above 3σ (5σ) for around 58% (18%) of the true values of δCP.

Physics Reach: Atmospheric Neutrinos


The study of atmospheric neutrinos with large masses detector and good angular and energy resolutions, can help in the MH determination. This is particularly attractive for the MEMPHYS detector, which can only achieve a small sensitivity to the mass hierarchy from the study of a neutrino beam, due to the shortest baseline CERN-Fréjus. In particular, the MH can be determined by comparing the measured event rates for νe and anti-νe, with particular care to the energy range 4-8GeV [13,14]. The sensitivity for the MH determination has been studied for the three different values of θ23: 40◦, 45◦ and 50◦ and is shown as a function of the exposure in Figure 9 for the Normal Hierachy. Similar results have been obtained fot the Inverted Hierarchy.

References
[1] http://cordis.europa.eu/result/rcn/54441_en.html
[2] http://laguna.ethz.ch/
[4] http://www.lombardi.ch/en
[5] The Super-Kamiokande Collaboration, Nucl. Instrum. Meth. A501(2003)418-462.
[6] deliverable on Arxiv
[7] http://pmm2.in2p3.fr
[8] Phys. Rev. ST Accel. Beams 16 021002 (2013)
[9] JCAP 1301 (2013) 024
[10] Phys. Rev. ST Accel. Beams 16, 061001 (2013)
[11] A. Longhin, Eur.Phys.J. C71 (2011) 1745
[12] E. Baussan et al. arXiv:1212.0732 [physics.acc-ph]
[13] Lee Ka Pik, PhD Thesis, University of Tokyo, Oct. 2012.
[14] deliverable on arxiv

Last update: 6 March 2015

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