MEMPHYSMEgatonneMassPHYSics

L. Agostinoa, M. Buizza-Avanzinia, D.Duchesneaub, J.Dumarchezc, M.Dracose, P.Gorodetzkya, M.Marafinia,M.Mezzettof, L.Moscad, A.Tonazzoa, T.Patzaka,∗, N.Vassilopoulose

a AstroParticule et Cosmologie (APC), CNRS, Univ. Paris 7, CEA, Obs. de Parisb Laboratoire d’Annecy-le-vieux de Physique des Particules (LAPP), IN2P3

c Laboratoire LPNHE, CNRS, IN2P3, Univ. Paris 6, Univ. Paris 7d Laboratoire souterain de Modane (LSM), CNRS/IN2P3

e IPHC, Universite de Strasbourg, CNRS/IN2P3, F-67037 Strasbourgf Istituto Nazionale di Fisica Nucleare (INFN), sezione Padova

Abstract

The MEMPHYS experiment based on a Water Cherenkov detector is one option currently under study in the Eu-ropean FP7 program. The experiment is proposed at a distance of 130 km from CERN under the Frejus mountain. Itis dedicated to nucleon decay, neutrinos from supernovæ, solar and atmospheric neutrinos, as well as neutrinos froma future Super-Beam from CERN to measure the CP violating phase δ. A small-scale prototype, MEMPHYNO, hasbeen constructed as a test bench for new photo detection, data acquisition and electronics solutions.

Keywords:Neutrino physics, neutrino oscillations, nucleon decay, neutrino astrophysics, Water Cherenkov Detectors

1. Introduction

Neutrino physics is one of the most dynamic and ex-citing fields of research in fundamental particle physicsand astrophysics. The next generation neutrino detectorwill address two aspects: fundamental properties of theneutrino like mass hierarchy, mixing angles and the CPphase as well as low energy neutrino astronomy withsolar, atmospheric and supernova neutrinos. Such anew detector naturally allows for major improvementsin the search for nucleon decay. A next generationneutrino observatory needs a megaton scale detectorwhich in turn has to be installed in a new, internationalunderground laboratory, capable of hosting such a hugedetector.The recent confirmation of a non-zero mixing angleθ13[1], allows for a series of very exciting measure-ments for neutrino mass hierarchy and CP violation

∗Corresponding author: T. Patzak, patzak@apc.univ-paris7.fr

using CERN neutrino beams. In Europe two designstudies have been performed in the past: LAGUNA,Large Apparatus studying Grand Unification and Neu-trino Astrophysics (2008 - 2011) and EUROnu (2008- 2012). The third study, LAGUNA-LBNO, LargeApparatus studying Grand Unification and NeutrinoAstrophysics and Long Baseline Neutrino Oscillations,is approved from 2011 to 2014.

2. MEMPHYS

The MEMPHYS (MEgaton MAss PHYSics) projectis discussed here with particular interest for deploymentin an extended Modane Laboratory (LSM: LaboratoireSouterrain de Modane France) at the Frejus site. Thedistance of 130 km from CERN is optimal to study thefirst oscillation maximum with a low energy neutrinobeam [2]. Due to the short distance to CERN this exper-iment has an excellent reach for leptonic CP violation

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search. The experiment is based on one of the most un-derstood techniques for neutrino detection: Cherenkovlight emission in water by charged particles resultingfrom neutrino interactions. Two beam concepts areconsidered for the MEMPHYS experiment: The CERNto Frejus Super-beam which will use the 4 MW versionof the Superconducting Proton Linac (SPL) at CERN[3] and the Beta Beam, in which the neutrino beams areproduced from the decay of beta emitting ions storedin a storage ring[4]. At beam energies below 1 GeVthe water Cherenkov technique is well adapted to thephysics scope of LAGUNA. Each tank of MEMPHYSis about 10 times SuperKamiokande (SK) and thereforeonly a mild extrapolation from an existing detector isnecessary.The project aims at a fiducial mass around half amegaton obtained with 2 cylindrical detector modulesof 65 meters in diameter and 103 meters in height.A schematic view is shown in Figure 1. It takes intoaccount the need to have a veto volume, 1.5 m thick,plus a minimal distance of about 2 meters betweenphotodetectors and interaction vertices, leaving a suf-ficient space for ring development and to protect fromγ from the PMTs natural radioactivity. The fiducialvolume is 500 kilotons. However, a number of technicalaspects are under investigation. One of the challengesis the large number of photomultipliers required. Thebaseline design of MEMPHYS uses about 200,000 12”PMTs.

Figure 1: Possible layout for the future neutrino observatory atthe Frejus tunnel. The MEMPHYS detector is made of twoindependent tanks 60 m apart from each other. Each tank is65 m in diameter and 103 m in height.

For a MEMPHYS detector at the Frejus site the firstpeak of the neutrino oscillation probability occurs at a

beam energy between 0.2 − 0.4 GeV. Extensive studieson the detector optimization are ongoing. Our workuses the neutrino event generator GENIE [5] and afull simulation of the MEMPHYS detector is based onGEANT4 [6] [7]. Full event reconstruction and particleidentification are implemented based on the experienceof SK. These procedures allow the reconstruction ofthe incoming neutrino energy and the identificationof its flavor to perform appearance or disappearanceanalyses with the different types of beams. The typicalsensitivity to CP violation as function of θ13 is shownin Figure 2, [8].

13θ22true sin0.02 0.04 0.06 0.08 0.1 0.12 0.14

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100 2% sig 2% bkg 5% sig 5% bkg 10% sig 10% bkg 15% sig 15% bkg

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Figure 2: 3 sigma sensitivity of the MEMPHYS experiment tothe CP phase for different assumptions on the systematic errorfor signal and background. Normal mass hierarchy has beenassumed and 2 years of neutrino + 8 years of anti neutrinorunning in the CERN - Frejus Super beam.

The deep underground position of the MEMPHYSneutrino observatory (4800 m.w.e.) allows a very richnon-accelerator physics program. We summarize in Ta-ble 1 the results for non-accelerator physics: the dis-covery potential of MEMPHYS for proton decay (90%C.L. in 10 years), the number of events for a supernovaexplosion at 10 kpc, the signal over background ratiofor Diffuse Supernova Neutrinos (DSN) and the rate ofsolar, atmospheric and reactor neutrinos in the detectorper year. We assume an energy threshold of 5 MeV.

L. Agostino et al. / Nuclear Physics B (Proc. Suppl.) 237–238 (2013) 311–313312

TOPIC fiducial (∼ 500 kt)

Proton decay: in 10 yearse+π0 ∼ 1.2 x 1035 [y] 90% CLνK+ ∼ 2.4 x 1034 [y] 90% CL

SN ν (10 kpc):CC ∼ 2.4 x 105 (νe)ES ∼ 1.2 x 103 (e)DSN ν (52 − 131)/57 (�) (S/B 5 y)Solar ν8B ES ∼ 1.3 x 106 per yAtm. ν ∼ 4.8 x 104 per yReactor ν ∼ 7.2 x 104 (�)per y

Table 1: Summary of non-accelerator physics in MEMPHYS.The (�) stands for the case where Gd salt is added to the waterof one tank.

3. Detector development

The coverage of large area with PMTs at a “low”cost implies a readout integrated electronics circuit forgroups of PMT (matrix of 4x4). The development ofsuch electronics is the aim of a dedicated French R&Dprogram, called PMm2 [9] [10]. The circuit under de-velopment allows to integrate for each group of PMTs: ahigh-speed discriminator, the digitization of the charge(on 12 bits ADC), the digitization of time (on 12 bitsTDC), a channel-to-channel gain adjustment and a com-mon high voltage. All the electronic and acquisitiondeveloped in the PMm2 program is going to be fullytested with the MEMPHYNO prototype installed at theAPC laboratory. MEMPHYNO is a test bench for anykind of light sensor or electronics solution for next gen-eration experiments. This prototype is realized with aPolyethylene tank of 2 x 2 x 2 m3 filled with water and ahodoscope made by 4 scintillator planes (kindly donatedby the OPERA collaboration) - 2 on the top and 2 on thebottom - to trigger cosmic muons. The development ongrouped electronics and photosensors is of very high in-terest for all the three detector options of the LAGUNAproject. In particular the strong synergy with the LENAdetector has lead to a joint study within a collaborativeeffort between german and french groups [11][12].

4. Conclusions

The proposed next generation neutrino observatoryMEMPHYS has outstanding performances in neutrinophysics and neutrino astrophysics. The huge number ofsolar and atmospheric neutrino events allows precision

studies on the solar neutrino spectrum and mixing andon the atmospheric mixing parameters. Atmosphericneutrinos will be used to determine the mass hierarchywith 3 sigma confidence within 10 years of data tak-ing. The current limits on the proton lifetime can beextended by an order of magnitude, allowing to proveor disprove a number of GUT models. MEMPHYS cancover 50% of the CP phase space at the 3 sigma level in10 years of data taking.

5. Acknowledgments

We are grateful to the European Commission for thefinancial support of the project through the FP7 DesignStudies LAGUNA (Grant Agreement No. 212343,FP7-INFRA-2007-1), LAGUNA-LBNO (Grant Agree-ment No. 284518, FP7-INFRA-2011-2.1.1.) andEUROnu (Grant Agreement No. 212372). We thankthe French Centre National de Recherche Scientifiquefor the support of the project in form of the PICS(Project number 5226) and the French UnivEarthSLabEx project.

6. References

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