The Nucleon Decay Experiment in The
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467 , THE NUCLEON DECAY EXPERI MENT IN THE FREJUS TUNNEL (ORSAY - PALAISEAU - SACLAY - WUPPERTAL) J, Ernwein DPhPE, CEN-Saclay, 91191, Gif sur Yvette Cedex, France ABSTRACT After a brief survey of recent experimental limits on the nucleon life time and a presentation of five new experiments dedicated to nucleon decay, this paper describes the 1,5 kiloton fine grain flash chamber detector which will be installed in an underground laboratory in the Frejus tunnel near Modane, France, RESUME Apres une revue des limites experimentales recentes sur la vie moyenne du nucleon, cinq nouvelles experiences specialement con�ues pour cette mesure sont presentees. En particulier le detecteur grain fin de 1,5 kilotonnes qui doit etre installe dans le laboratoire souterraina de Modane est decrit en detail. 468 I. INTRODUCTION During the last few years the question of nucleon stability has received wide-spread attention from experimenters, as it appeared that observing nucleon decays would be a unique way to test Grand Unified Theories (GUTS) of weak, electromagnetic, and strong interactions [!]. The unification mass scale of 14 16 10 - 10 GeV is otherwise out of reach of conventional accelerator experiments. 30 32 The nucleon life time predicted by GUTS lies in the range 10 - 10 years, which is remarkably close to the present experimental limits. The most common decay modes involve anti-leptons and pions in the decay products, as (B-L) is exactly conserved in SU(5) . Recently, theoretical work based on "supersymnietric GUTS" [2] has pre dicted similar nucleon life times, but with neutrinos being present in the main decay modes. This would make it harder, but quite possible, to detect nucleon decays. Two kinds of detectors are in operation or under construction - In the water Cerenkov detectors, the Cerenkov light emitted by the charged relativistic particl s among the decay products of the nucleons in the � water is detected by arrays of photomultiplier tubes. The two body decay n + e+TI and the decay p + e+TI0 in which one ends up with electrons and positrons would suit these detectors particularly well. - In the fine grain calorimeters, plates of he,avy material provide the nucleons who se decay products are detected in various types of ionization counters which are interleaved with the plates. These calorimeters are sensitive, in prin ciple, to most decay modes and provide good pattern recognition capabilities . I. Recent experimental limits on the nucleon life ti.me l. the a l The present best limits on nucleon life ti.me are presented in t b e A detailed summary of published nucleon decay limits can be found in reference [3] . I. The experiments in table I have � 100 tons of fiducial mass and are well shielded from cosmic µ's. 469 Tab le - Recent experimental limits on the nucleon lifetime I Shielding Fiducial Meters Lifetime METHOD Mass ( limit tons of Water years ( ) Equivalent ( ) ) 1954-1979 Measure µ flux 30 .Reines coworkers in 100 3200 10 W ref§ . [4] Liquid Scintillator < 'V 1981 Measure stopping 30 Homestake Gold Mine µ' s in 150 4200 ( 1.5-3) 10 ref. [5] water Cerenkov Iron Calorimeter 30 1981 with proportional 3 10 Kolar Gold Field counters 100 7000 ref. [6, 7] Detect charged 'VSee below particles ( ) The group using the calorimeter in Lhe Kolar Gold Field has recently th added a 4 event [7] to the 3 nucleon decay candidates [6] . The value of the nucleon life time obtained, if one assumes that these events are indeed nucleon 30 decays, is 8xlo years. It remains true however that these events are poorly, if at all, contained in the detector. It seems difficult, with this detector, to overcome the inherent limitations in fiducial volume and granularity. I.2. New dedicated experiments Many experiments have been planned in order to increase the experimental 32 lower limit of the nucleon lifetime and, should the lifetime be below 10 years, study the decay modes. 'V Detectors have been designed to satisfy the following requirements : - They must be very massive. The number N of expected events per year and per kiloton of detector is N = 60 where £ is the efficiency for detecting a decay mode of branching ratio BR, and 32 the nucleon lifetime in years. For example if 10 years, and £.BR � 0.2, T ( T then 1 event per year would be recorded in a 1 kiloton detector. = ) 'V 470 A massive detector will also make it possible to define a passive peri pheral veto region which is essential for being able to see contained events and reject background due to neutrals associated with muon or neutrino interac tions in the surrounding rock. - Good space and energy resolution below 1 GeV along with the determina tion of the charge and nature of the decay product particles are important to get a good signature of the nucleon decay. Therefore the calorimeters need to have a fine grain. - In order to reduce the background associated with atmospheric muons, detectors need to be located deep underground. Attenuat ion of the total muon 4 flux is dependent on the depth and ranges from 10 to 107 for a cover of 1500 to 6500 meters equivalent of water (MWE) . One known background which cannot be eliminated by going deep under ground is produced by the atmospheric neutrino interactions in the 1 GeV region which simulate nucleon decay. It has been estimated that after accurate energy measurements and pattern recognition, this background alone will prevent reliable detection of nucleon decay if the lifetime is larger ttan a few 1032 years. Many proposals have flourished with much enthuE:iasm because of the fun damental issue involved and great optimism with respect to available funds ; it seems that by now we are left with a handful of detectors likely to contribute to the field in the next few years. Table II lists these experiments. For completeness I should mention that more proposals have been made to build 1 kiloton detectors [13, 14] . Furthermore, discussions are being held on the feasability of a 12.000 ton calorimetric detec tor [15] and a deep ocean water Cerenkov of 106 tons [16] . Table - New dedicated nucleon decay experiments which are beginning operation or are under construction. II *) DEPTH (MWE) INSTITUTIONS LOCATION TOTAL/FIDUC. µflux per min SIZE DETECTOR OPERATION MASS (Tons) deteetor 1700 2000 5 inch PM's Irvine Morton Salt Mine a) in 3 Michigan 6800/2200 22 17 x 18 m on outer surface NOW Brookhaven [8] Ohio, USA 180 of cavity filled x with water � Harvard b) 700 5 inch PM's ..., Silver King Mine 1800 diameter 11 m "' Purdue Utah, USA 800/150 height 7.5 m immersed in NOW I" Wisconsin [9] water tank (") 60 "' I" iii"' KEK a) 2700 diameter 16 m >1000 20 inch PM's � Tokyo Kamioka, Japan 3400/850 height 16 m on outer surface end 82? Tsukuba [10] 6 of water tank "NUS EX" CERN c) 5000 134 layers of Frasca ti Mont Blanc Tunnel 150/110 (3. 5m) 3 1 cm i2on plates NOW Milano Italy - France 4.10-3 lxl cm limited Torino [11] streamer tubes (") )> 1480 layers of ..... "OPSW" 0 4500 3 mm iron plates 500 Tons I" Or say d) H Frejus Tunnel 3 and flash chambers end '83 :;:: Palaiseau 1500/1000 6x 6x 20 m (/)"' Modane, France (5x5 mm2 cells) ..., Sac lay "' 1 I" Wuppertal (12] Tr iggered by 1500 Tons Geiger Tubes end '84 *) In order to contain events and to record tracks entering the detector, an outer layer has to be considered as a "veto region". The depth of this layer depends on the density of the detector, the vertex resolution, the decay modes considered and particular experimental features such as the use of active veto counters. I have used the following veto depths : a) 3 m, b) 2 m, c) 30 cm, d) 50 cm. ... .... ..... 472 II. THE OPSW DETECTOR IN THEFREJUS TUNNEL II.l. The laboratory The detector will be located in a gallery specially excavated near the mid point of the 13 long highway tunnel linking Modane, France to the region of Torino , Italy. km 'SWITZERLAN� Genev� / ' ' � ' Mont BJ-�•, c;.. , ITALY Fig. 1 - Geography The average thickness of the rock above the laboratory is 1680 m and corresponds to 4500 meters of water equivalent. The atmospheric muon flux in the laboratory is 106 times lower than at sea level . The angular distribution of the muons has �been calculated by taking into account the mountain profile and corresponds to a total rate of 7.6 muons per day in a horizontal area of 1 m2 This calculation has been checked by measuring the number of muons crossing four large scintillators placed horizontally in the existing gallery : the measured muon rate is 6.2 1.2 muons/day.m2 • _:!:. This rate corresponds to about one muon crossing the 1.5 kiloton detector every minute. Most of them will satisfy the trigger conditions and they will provide a convenient way to continuously check the performance of the detector without appreciable trigger efficiency loss due to the deadtime of the flash chambers (a few seconds) . II.2. Description of the detector The detector is a very fine grain calorimeter designed to visualize tracks coming from nucleon decay products, measure their range, and identify them if po ssible. The dimensions of the detector are 6x6x20 m3 (fig. 2a) . The structure of the detector is shown in figures 2b and 3. A module consists of four biplanes of flash chamber elements made of polypropylene with the cell direction alternatively horizontal and vertical. 473 6m Fig. 2a - Dimensions of the de tector Fig.