EXPERIMENTS ON NUCLEON STABILITY

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EXPERIMENTS ON NUCLEON STABILITY

E. Fiorini

To cite this version:

E. Fiorini. EXPERIMENTS ON NUCLEON STABILITY. Journal de Physique Colloques, 1984, 45 (C3), pp.C3-151-C3-171. �10.1051/jphyscol:1984329�. �jpa-00224043�

J O U R N A L D E PHYSIQUE

Colloque C3, suppl6ment au n03, T o m e 45, mars 1984 page C3-151

EXPERIMENTS ON NUCLEON STABILITY E. Fiorini

Dipartimento di Fisica deZZIUniversit& di MiZano,Btituto Nazionale di Fisica Nudeare, Sezione di MiZano, Itdy

RESUME

Diffdrentesexpdriencestendant P mettre en dvidence la dd- croissance neutronique, ainsi que quelques expdriences en cours sont passdes en revue et les r6sultats obtenus sont compards et discutds. Des exp6riences futures qui devraient amdliorer la sensibilitd actuelle sont ddcrites.

ABSTRACT

The various experiments aiming to the detection of nucleon decay and presently running will be reviewed, and the results obtained compared and discussed. Future experiments presently being designed to improve the present sensitivity will be also considered.

1. IhTRODUCTION

Experiments specially designed to search for nucleon decay have been stimulated in the last three or four years by cosmological considerations based on the nucleon-antinucleon asynnnetry in the universe1) and especially by Grand Unified Theories of electroweak and strong interactions2-j). These theories predict in fact, at least in minimal SU(5), that proton and bound neutron decay with a lifetimes ranging from loz8 to 10jl years, namely within the limits of experimental detectability. Unfortunately the predicted branching ratios for the various decay modes are strongly model-dependent and the experimentalist do not easily get from theory a clear-cut indication where to search for proton decay. I would like only to remind that standard SU(5) favours proton (neutron) decay into a neutral (negs tive) non-strange meson and a positron, while the more recent Super SYmetric theories (SUSY) favour decays into leptons and strange particles3).

A model independent way to search for nucleon decay could be the use of indirect techniques like those based on the search for stable or radiactive daughters left by nuclei where a nucleon has decayed (geological and radiochemical experiments), or for tracks left by the secondaries of nucleon decay in rocks of geological age. These methods have been discussed in detail even recently4) and are of obvious interest, but their sensitivity is at present low ( 10L5-27 years) and still far from theoretical predictions.

A much better sensitivity (by at least three orders of magnitude) can be achieved in direct experiments aiming to the real detection of the secondaries of nucleon decay. One approach is called (rather incorrectly) the calorimetric one and is based on set-ups made by a sandwich of horizonthal or vertical plates of p a s s i v e ~ ~ i a t e r i a l ( n o n n a l l y Iron), inter- leaved with planes of reasonably inexpensive and simple detectors5) (Geiger, proportional of limited streamer tubes of flash chambers). A second approach, undoubtely stipulated by the SU(5) prediction that proton decays into a positron and a neutral pion should be favoured, is based on the detection of the light emitted in a transparent medium by high

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1984329

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velocity charged secondaries produced by nucleon decay. The "source" for nucleon decay can then be a large pool of highly purified water, where the Cerenkov light emitted by the charged secondaries is collected by photomultipliers placed on the walls of the pool or even inside it.

The calorimetric approach is undoubtely more flexible and allows detection of decay modes on which the Cerenkov technique can be blind, and the performance of the detector can be determined easily before by means of test models with the same granularity. The Cerenkov approach allows conversely to construct very large detectors with a generally lower cost per unit mass. The "rings" of fired photomultipliers produced by the charged secondaries on the walls of the pool allow to obtain directly their sense of motion, while in calorimeters it can be obtained by the increase of multiple scattering or by the presence of a visible iecay at the end of the track. The position of the vertex in a Cerenkov detector can be obtained (Fig.1) from the relative time or intensity of the fired photomultiplier and is rather inaccurate (of the order of the metre), while tlie spacial resolution can be of a centimeter or less in a fine grain calorimeter.

Due to the large mass and sizes of the detectors and the long running time, one has to place then1 as deep as possible underground in order to reduce the probability that cosmic rays muons or muon induced neutral particles can simulate nucleon decay. In conclusion one has to reach a compromise among:cost, depth, mass of the detector and its resolution.

This last parameterdependsessentially on the granularity, and therefore on the nmber of read-out channels, for calorimeters,and on the surface covered by photomultipliers in Cerenkov detectors.

Fig. 1. Determination of the vertex in a Cerenkov proton decay detector.

2. THE PRESENXLY R W I N G EXPERIEEhTS

I will review here the presently running experiments dedicated to search for nucleon decay, with the exception of Soudan I ~ ) which, due to its relative small mass, is considered by the authors themselves as a prototype for an already approved experiment of much larger mass7).

The first experiment to become operational, more thal two years ago, is the one per- formed by the collaboration of groups from the Tata Institute (Bombay) in India and from Osaka and Tokyo in Japan in the Kolar Gold Field Pllne in Southern India. The depth, also due to the unusually large density and Z/A ratio of the rock, corresponds to about 7600 hectograms of standard rock, and is the largest of all those of running or designed experi- ments. The detector is a tracking calorimeter of -6 x 4 x 3.7 m3 '1, made by 34 horizonthal

Iron plates interspaced with planes of Iron proportional tubes (16cW) in total) placed alternatively in the X and Y direction (Z being the vertical one). Their cross sectlon is 10 x 10 an2 and their lengths alternatively 6 and 4 m. In order to compare with other experiments one has to note that the thickness, including obviously the walls of the tubes, is 1.7 cm of Iron between consecutive layers, but this value if the two consecutive layers are considered in the Xi or YZ view. In other words the read-out is not bidimentional, which can cause ambiguities in the interpretation of some events. The total mass of the detector is 140 tons and the average density of about 1.57. The trigger is based on two requirements: a 5-fold coincidence among any 11 consecutive planes and the coincidence of two planes over four consecutive ones.

The present results, as from a telex at the end of October by Prof. V.S. Narasimham, refer to 2.4 years of effective running time where about 15W muons crossing the detector have bven recorded (about 1.8 per day). Even if these autliors have also reported evidence for "non contained " events, I will consider here only the contained ones, namely only those where all secondaries unambiguously stop in the detector. In the other cases it is in my opinion difficult to be sure that the track crossing the walls is indeed leaving the detector and not entering in it.

Eight contained events have been recorded with total visible energy in excess of 300 hW: one with an energy of -L 1.6 GeV, and the others with an energy around 1 GeV or lower.

Three of them are considered as candidates for nucleon decay: one in the mode e+no, another in the mode Tnir+, and the third in the mode e'n-or IJ'KO, the visible energies being about 1000, 400 and 850 MeV, respectively. A fourth contained event, found on October 10, 1983 is being analyzed as a possible condidate. As shown in Fig. 2 it is difficult,due to poor granularity, to clearly discriminate the nucleon decay candidates from the background of interactions by atmospheric neutrinos. One has to note in fact that the most intriguing background in nucleon decay experiment, at least for those placed deep underground, comes from interactions ofneutrinosproduced by the decay of pions, kaons and muons produced in the interactions of cosmic rays in the nuclei of the Earth's atmosphere. The flux of these neutrinos, which has been evaluated by various authors even recently9-Ill depend: on the geomagnetical latitude (3' in the Kolar Gold Field experiment). The percentage oi. electron neutrons is about 30% and the horizonthal intensity is about twice the vertical one.

I have evaluated that the Kolar Gold Field experiment should have found about 9 events with an incertitude of about 50% due to the errors in the evaluation of the neutrino f l u and of the neutrino cross sections at these relative low energies. This expected figure is in reasonable agreement with the number of detected events both in the hypothesis that the three candidate are indeed proton decap,and in the case that they are due to interactions by atmospheric neutrinos.

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Fig. 2 . The t h r e e nucleon decay candidates of t h e Kolar Gold Field experiment.

The second experiment t o s t a r t data taking was hVSEX ( f o r Nucleon Decay Experiment) c a r r i e d out by t h e Frascati-lililano-Torino-CERh' Collaboration i n a laboratory placed i n t h e middle of the blont Blanc Tunnel between France and I t a l y , a t a depth of about 5000 hecto- gram equivalent of standard rock, second only t o t h e Kolar Gold Field Laboratory. The d e t e c t o r c o n s i s t s i n a cube of 3.5 m s i d e , made by 136 horizonthal Iron p l a t e s of 1 cni thickness, interleaved with planes of limited streamer tubes of 1 x 1 cm2 cross section and 3.5 m lenght. These tubes a r e made with p l a s t i c p r o f i l e s , varnished i n s i d e with graphite a c t i n g a s cathode, while t h e axode i s a wire of 1CO vm diameter. Since the w a l l s of the tubes a r e transparent t o t h e electromagnetic pulses, those generated by t h e l i m i t e d streamers produced i n t h e tube by t h e crossing p a r t i c l e , a r e read out (Fig.3) by means of X and Y s t r i p s placed e x t e r n a l l y t o t h e tubes along and orthogonally the wires. The read out i s therefore bidirnentional and the thickness between consecutive detector planes i n each view i s indeed one centimeter of Iron. I t has been found t h a t bidimentionality of read o u t , namely t h e p o s s i b i l i t y t o c o r r e l a t e t h e XZ an2 YL views of each detected streamer, is very helpful i n reducing the ambiguities of an event. The d e t e c t o r , which has a t o t a l mass of about 150 tons and an average density of 3.5 contains about 42,000 tubes and 82,000 read out channels 12).

A s p e c i a l f e a t u r e of t h i s experiment i s t h a t a t e s t model of the d e t e c t o r , with about a t e n t h of i t s mass, has been exposed a t CERN t o beams of pions, e l e c t r o n s and muons of momenta ranging from 150 t o 2000 bleV/c, i n order t o simulate the charged secondaries of nucleon decay o r neutron-antineutron o s c i l l a t i o l ; ~ and t o an unfocused neutrino bean1 from

Y-STRIPS (8 m v i d t h )

x 9 uun2 R-TUBES SERIAL

DATA OUT

X-STRIPS

( 5 m w i d t h )

Fig. 3. Bidimentional read-out in NUSEX.

10 Gel; protons in order to simulate the interactions by atmospheric neutrinos13). The detector is operating in the Mont Blanc Laboratory since S m e r 1982. Basic trigger require- ments are that four contiguous planes, or two contiguous planes plus a group of three or more contiguous planes are fired. Since April 1983 a special trigger has been added in order to detect magnetic monopoles with ionization as low as lom2 the minimum and velocity as low as .fie speed of light.

At the end of October, after 1.2 years of effective running time, NUSEX had detected 11705 crossing muons (about 25 per day) and 154 bundles of parallel muons, of which 19, 1, 2 and 1 with 3, 4, 5 and 6 muons, respectively. The total number of contained events is 15, in good agreement with predictions where the geomagnetic cut off is taken into account (in my rough calculation I have obtained a figure of 14). These events are shown in Fig.4. It be seen that 11 of these events can be easily interpreted as due to interactions of atmo- spheric neutrinos: two are electronic elastic, one electronic inelastic, six muonic elastic, one muonic inelastic and one muonic elastic or inelastic. Their energy distribution is in good agreement with the theoretical predictions, and there is no evidence for a large up down asymmetry. The three prong event number 1 can hardly fit the neutrino interaction hypothesis, while it would fit very well the hypothesis of proton decay into a muon and ?

neutral kaons. The number of neutrino events expected in this topology from the results of the CERN run with artificially generated neutrinos is less than 0.1 at the 90% confidence level.

Interpretation of event 6 is very difficult since it is hard to be explained as an interaction from an atmospheric neutrino. In the alternative hypothesis of a proton decay into two secondaries one of them would interact and no kinematical constraint can be applied to test this possibility. The event cannot even be used as candidate for n-fi oscillation.

In conclusion the only good candidate for proton decay in this experiment would be event number one, which fits very well the hop decay mode. Being a single event the colla- boration does not think it can be used alone as evidence for nucleon decay. In this case the corresponding lifetime would be

-

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Fig. 4 . The contained events in NUSEX.

Event 4

Event 5

Event 6

Fig. 4. The contained event in NUSEX.

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Fig. 4. The contained events in iuUSD(.

E v e n t 10

E v e n t

Fig. 4. The contained events in NUSEX.

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Event 13

E v e n t 14

Fig. 4. The contained events i n NUSEX.

A t h i r d experiment1*) i s being c a r r i e d out i n t h e Morton S a l t Mine (Ohio-USA) a t a depth of 1570 h. w.e. by a collaboration i n i t i a l l y of t h e I r v i n e , Michigan and Brookhaven groups (IFfB) presently including a l s o p h y c i s i s t s from Caltech, Cleveland, Hawaii and Uni- v e r s i t y College (London). The detector (Fig.5) i s a very large pool of p u r i f i e d water

(22.8 x 17.8 x 16.9 m3) with a t o t a l volume of about 7000 tons, viewed by 2048 photomulti- p l i e r s of 5" diameter i m e r s e d i n the watcr near t h e walls. The d e t e c t o r has been t e s t e d and c a l i b r a t e d with Laser beans of 337 MI wavelength, with Light Emitting Diodes (550+50 m wavelength) and with atmospheric muons. I t has been found t h a t the d e t e c t o r response t o the l i g h t emitted i n i t corresponds t o about 0.25 photoelectrons per MeV of v i s i b l e energy l o s t i n i t .

The p o s i t i o n of t h e vertex of t h e decay o r i n t e r a c t i o n i s determined by t h e r e l a t i v e i n t e n s i t y of t h e f i r e d p h o t o m l t i p l i e r s within about a metre. For t h i s reason, and t o reduce t h e background of neutrons produced by muoninteractionsin t h e surrounding rocks, an external c r u s t of 2 m thickness from the walls i s eliminated i n t h e analysis of t h e events, thus leading t o a f i d u c i a l mass of 3300 tons.

Due t o i t s r e l a t i v e l y shallow p o s i t i o n and l a r g e mass, t h i s d e t e c t o r i s exposed t o a considerable f l u x of cosmic ray muons (about t h r e e per second). The corresponding very

IMB

DETECTOR

'I

MAX.

DIMENSIONS

Fig. 5. The Cerenkov set-up of 1E.B.

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large rate is reduced electronically by restricting the number of lit phototubes between 30 and 3 0 , by imposing geometrical constraints to the position and structure of the event, and further by visual inspection and scanning. The final detection efficiency has been eva- luated to be 75 % for neutrino interactions above 3 0 MeV and 90% for proton decay in the e+no channel. For other decay modes, like K O p it can be obviously much lower since charged particles with 6 < 0.75 cannot be recorded.

At the end of October the analysis for the search for protcn decay into the e + n ~ and u+nD channels had totalled 0.55 years of nmning time and found no candidate, while for other decay modes and for neutrino interactions has totalled 0.56 years. 109 contained events have been found (in my rough calculation they should have found 180 neutrino inte- ractions, but, due to uncertainties in neutrino fluxes and cross sections, I believe there is no real disagreement). The distribution in vertex and direction, reported in Fig. 6,

TOP VlEW

NORTH

...

.

SOUTH

SOUTH VlEW

T O P

e... o.....

...-...a*...

.

BOTTOM

Fig. 6. Angular distribution of the contained events in the DlB experiment.

shows a reassuring isotropicity, and the energy distribution is as expected (Fig.7). The proportion of contained events with p-e signature is 0.4 i 0.2 in agreement with expecta- tions based on the electronic -muonic composition of the atmospheric neutrino fluxes, and on the p-e detection efficiency of the set-up (2. 50%).

Only three contained events show a back-to-back structure with angles between the two Cerenkov cones exceeding 110". This low percentage is expected for interactions of atmo- spheric neutrinos since in inelastic reactions with a back-ward going secondary, this latter particle is often too slow to give enough Cerenkov light to be detected. The event appears therefore as an elastic one.

Fig. 7. I!nerby distribution of the contained events in the IhfB experiment.

No evidence has been found in this experiment for any decay mode of the nucleon, as it will be discussed later on.

UNIOKAATIE (for Kamioka Nucleon Decay) is a ,Cerenkov experiment being carried out1 5, since July 1983 by a KEK-Tokyo-Nijgara-Tsukuba Collaboration in the Kamioka Mine in Japan at a depth of about 2700 h.w.e.. The detector (rig.8) is a tank of 3000 cubic metres of water (of which 880 are fiducial). Inside the pool, at about one metre distance from the walls,the light is collected by 1 0very large beautiful photomultipliers of 20" diameter, especially constructed for this experiment, which cover 2.20% of the surface, an order of magnitude more than in the IFlB experiment. As a consequence the detection efficiency for light emitted by charged particles in the detector is of about 0.37 FleV per photoelectron.

The photomultipliers are divided into 12 groups and the trigger requirement is that at least 100 photoelectrons in total and at least l b photoelectrons in at least 2 groups are recorded. Due to the large size of the phototubesthe time resolution is relatively poor

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Fig. 8. The Aamiokande detector.

(about 100 ns). w e decay is recorded if it occurs within 0.5 to 9.2 ps; if however it occurs in the first 600 PS, the light emitted by the electron is added to the light produced by the other charged particles. The counting rate is considerable (40,030 count per day), and is reduced by requiring the total pulse height to be less than 4503 photoelectroes (which unfortunately eliminates possible signals from n-rr annihilation), that the maximum pulse per phototube be less than 200 p.e. and that the ratio between the pulse from a phototube and the total pulse be less than 3 0 L In addition cuts are put on the electric noise and on the geometric position of the event.

On September 21, 1983 16) this experiment had totalled 0.155 years of effective running time, and 21 contained events had been found (I expect 22 neutrino interactions). Four of this events indicate the presence of more than one visible secondary (this ratio being larger than in IEiB due to better light collection). The angular distribution of these contained events is shown in Fig.9 and shows the expected isotropy. The event shown in Fig.10 is compatible with nucleon decay in the following channels: a) unO with P,, =255+30 W / c , k$.,=524i50 MeV and EtOt=897+80 MeV; b) u n o with I.Cy=277i40 MeV; c) ep- with Mnon-=

=552t50 MeV. The first mode is obviously the most likely one.

A third Cerenkov detector is operated since more than six months by the Hanard-Purdue- -Wisconsin Collaboration in the Silver King Mine (Utah-USA) at a depth of 1500 h.w.e.. The set-up (Fig.11) consists in a wooden tank containing 705 tons of water (about 500 are consi dered fiducial). 705 photomultipliers of 5" diameter are imersed in the water to form a lattice with one metre distance between planes. Light collection is improved by a mirror placed on the walls, which bringsthe sensitivity for the detection of the energy spent in the detector to about one MeV per photoelectrons. An anticoincidence shield made with

Fig. 9. Angular distribution of contained events in Kamiokande.

Fig. 10. The proton decay candidate in Kamiokande.

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Fig. 11. The Cerenkov set-up of tW.

proportional counters has been i n s t a l l e d around the pool, b u t i s not yet i n operation. On November 21, 1983 160 days of e f f e c t i v e running time had been collected and about h a l f of them analyzed, but no d a t a on nucleon decay o r neutrino i n t e r a c t i o n s were y e t

3. COhIPARISOL' OF THE EXPERIMENTAL RESULTS

I have t r i e d t o compare t h e r e s u l t s obtained s o f a r on nucleon s t a b i l i t y i n Table I . This comparison has t o be taken with much c a r e s i n c e proton decay candidatesare very few and t h e corresponding l i f e t i m e s have low s t a t i s t i c a l significance. I would l i k e f i r s t t o note t h a t the Kolar Gold Field present three candidates: I have therefore put i n brackets t h e corresponding h a l f l i f e t i m e s . For hJJSES I have reported t h e 90% confidence l e v e l l i m i t s f o r a11 channels, with exception of t h e KOP+ one, where I have quoted t h e value of l i f e t i m e corresponding t o one event. The same procedure has been adopted f o r the ~ + n candidate of Kamiokande.

IhIB has found no candidates f o r nucleon decay i n any channel and I have quoted t h e i r 90% confidence l e v e l l i m i t s . There i s an obvious disagreement between them and tiolar Gold Field experiment f o r t h e e+no channel. There i s on the contrary, a t l e a s t i n my opinion, no disagreement between them and the K O p + candidate found by NUSEX, since tiley have a much l a r g e r mass, but worse detection e f f i c i e n c y f o r t h i s channel, where low beta p a r t i c l e a r e emitted. There seems t o be some disagreement between them and the u+q proton decay candi- d a t e of Kamiokande, but I believe t h i s should be taken with some care. I f t h i s event was not found i n t h e i r d e t e c t o r t h i s could be due (regardless i t t o be a nucleon decay or not) t o t h e i r limited e f f i c i e n c y f o r detection of t h e low momentum muon kinematically compatible with nucleon decay. Gne can a l s o note t h a t t h e f i d u c i a l mass i s by no means t h e only

TABLE I: Conyarison of the results of the various experiments (in units of 10" years)

Channel E.G.F. _NUSEX IMI3 KAMIOKANDE

relevant parameter of an experiment, a s it i s sometimes taken f o r granted: l i m i t s obtained by NUSEX on some decay modes a r e equivalent o r b e t t e r than those of IMB, where detection e f f i c i e n c y of slow p a r t i c l e i s poor.

4 . FUTLRE NUCLEOX DECAY EXPERIlrEhTS

A F r e n c h - G e m Collaboration of groups from Aachen, Orsay, Palaiseau, Saclay and Nuppertal i s i n s t a l l i n g a nucleon decay experiment i s a new laboratory i n the Frejus tunnel between France and I t a l y a t a depth of about 40CO h.w.e.. The detector w i l l have a mass of about 3000 tons and s i z e s of 6 x 6 x 12 m 3 , and w i l l be made by v e r t i c a l Iron p l a t e s of 3 mm thickness and interspaced with planes of f l a s h tubes1') placed a l t e r n a t i v e l y along the X and Z d i r e c t i o n s l g ) (Fig.12). Therefore, t h e read out w i l l not be bidimentional and the

Fig. 12. Scheme of t h e Frejus experiment.

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thickness between two consecutive planes i n t h e same view w i l l be of 6 mn. The t r i g g e r w i l l be provided by planes of Aluminum G.M. tubes placed every 5 f l a s h chamber planes. At present 2 0 0 tons of t h e detector were b u i l t , 120 mounted and 80 a r e i n operation, and t h e r e s t of t h e d e t e c t o r i s being mounted a t a r a t e of 80 tons per week. The e n t i r e d e t e c t o r should be operational a t t h e end of 1984.

Only another new nucleon decay experiment has been approved s o f a r : the d e t e c t o r of t h e ANL-Minnesota-Oxford-Rutherford-Tuft Collaboration t o be i n s t a l l e d i n t h e Soudan mine (Minnesota-USA) a t a depth of about 2000 h.w.e. 7). I t w i l l c o n s i s t of a 1200tons tracking calorimeter made with Iron and d r i f t chambers (Fig.13). Two options a r e being discussed:

a ) 5 nnn thick Iron s h e e t s interspaced with 10 mn t h i c k , SM3 llpn long d r i f t chambers and

Fig. 13. The "planar" option of Soudnn 11.

b) an honeycomb s t r u c t u r e (Fig.14) with a system of a x i a l d r i f t tubes i n a 2 nun thick Iron sheet. The experiment should be i n operation a t t h e end of 1985.

In USSR, where a non-dedicated 350 tons s c i n t i l l a t i o n detector i s already i n operation, two proposals have been put forward f o r g i a n t gas nucleon decay detector^^^^^'). Two proposals f o r a s c i n t i l l a t i o n d e t e c t o r of 1 k i l o t o n and another f o r 10 k i l o t o n have been presented i n t h e USA, both from t h e Pennsylvania group") and a giant underground f a c i l i t y with a capacity of 50,000 m3 i s being constructed i n the Gran Sasso Tunnel i n l t a l y z 3 ) .

New techniques a r e a l s o being considered: l a r g e (1-2 kiloton) l i q u i d argon Time Projection ~ h a m b e r s ' ~ ) a t Irvine and a t Cern and a 3000 m3 bubble chamber with 10",uty c i c l e a t C E ~ V ~ ~ ) .

5. CONCLUSIONS

Five d e t e c t o r s i n three contifients a r e taking d a t a on nucleon decay, and another i s going t o j o i n them soon. They a r e based on d i f f e r e n t techniques ana placed a t d i f f e r e n t depths, and t h e i r r e s u l t s have t h e r e f o r e t o be considered as complementary.

Six nucleon decay candidate have been reported i n a l l : f o u r by the r a t h e r imprecise, but deeply located, Kolar Gold Field experiment, and one each by NUSEX and Kaniokande with

I I I ,ANODE WIRES

CATHODE PADS

I '

1 '

'I' I

Fig. 14. The "honeycomb" option of Soudan 11.

a much better identification. The IMB results are negative, but do not contradict these last two possible nucleon decay, which seem. difficult to be explained as due to neutrino interactions.

It seems therefore that something new is being revealed in these underground experi- ments, but it is in my opinion too early to obtain from these results an indication for nucleon decay. Should tlis be the case the half lifetimes would be of a few unities in 1031 years, and the e+ro channels, favoured by minimal SU(5), seems unlikely.

The five running experimentsand the experiment in the Frejus tunnel will undoubtely clarify the present situation in the next one or two years. If nucleon decay will be proveu, new detectors with larger masses and expecially with better "granularity", like liquid Argon Tinie Projection Chamber will become essential to study the various decay modes of the nucleon. If this will not be the case the performance of larger detectors will be strongly jeopardized by the background of atmospheric neutrinos. All running eqerinlentsagree very well on the intensity and composition of these neutrinos, and I consider this agreemsnt itself a result of extreme physical importance. These experiments show also, however, that the background of atmospheric neutrinos is already giving some trouble, even if the sensi- tivity on nucleon decay is of a few lo3' years, or at the best for particular channels of

years. It seems difficult to me that, even with substantially larger masses and better granularities, it will be possible to improve this sensitivity by more than an order of magnitude.

This pessimistic attitude has,optimistic consequences on the direct neutron-anti-

2 6

neutron oscillations experiment&, kven if the limits on these oscillations obtained presently and going to be obtained in the near future are much lower than those which can be extracted by nucleon decay experiments. Even disregarding the fact that n-Ti oscillations could be depressed of even forbidden in nuclei, and that the two types of experiments are probably complementary, I have no doubt that a direct search aiming to reach a sensitivity of lo8 s or more in the neutron free oscillation time will not suffer any competition from nucleon decay experiments.

Even if nucleon decay would not have been discovered, the results from these experi- ments would still be extremely important. The excellent agreement of them on the interesting

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and up to now experimentally obscure problem of atnlospheric neutrinos opens a window on the origin and possibly on the oscillations of these particles. The "strange" events found in all experiments will have to be further investigated and understood, and it seems possibly that from this study, and more in general from the study of the penetrating component of the cosmic radiation, results can be obtained of extreme interest in elementary particle andcosmic rays physics.

REFERENCES

1) See for instance J. Ellis,"Guts, astrophysics and superunification"Proc. of the Int.

Conf. Neutrino 82, ed. A. Frenkel and L. Jenik Vol. I p.304 (1982).

2) P. Langacker,Phys. Rep. 72, 1985 (1981).

3) S. Ferrara,"Grand Unification, Proc. of the Int. Europhysics Conf. on High Energy Physics", ed. Rutherford Appleton Lab. p.522 (1983). also for previous references.

4) See for instance R.I. Steinberg,"Decay mode insensitive searches for nucleon decay", Proc. of the Int. Colloquium on Matter Non-Conservation, Frascati, January 17-21 1983.

5) E. Fiorini,"Nucleon Decay Experiments",Proc. of the Int. Europhysics Conf. on High Energy Physics, ed. Rutherford Appleton Lab. p.804 (1983) also for previous references.

6) J. Bertelt et al., Phys. Rev. Lett. 50, 651 (1983).

7) "Soudan 11-a 1OOO ton tracking calorimeter fornucleon decay" Elimesota-Argonne-Oxford, Am-HEP-Pr-12 (Sept

.

6) M.R. Krishnaswami et al. ,"The KGF Nucleon Decay Detector", Invited paper to the 4th Workshop on Grand Unification, Philadelphia, April 21-23, 1983, and private communication by V.S. Narasimham.

9) M.R. Erishnaswami et al., Pranama Vol. 19, 525 (1982).

10) T.K. Gaisser et al., "The flux of atmospheric cosmic ray neutrinos", Invited talk at the 4th Int. Workshop on GLIT's, Philadelphia, April 25-28, 1983.

11) A. Dar, "Atmospherical neutrinos in proton decay experiments", Invited talk at the 4th Int. Workshop on GUT'S, Philadelphia, April 25-28, 1983.

12) G. Battistoni et al., "Nucleon stability, magnetic nlonopoles and atmospheric neutrinos in the Flont Blanc experiment, Phys. Lett. (in press).

13) G. Battistoni et al., "An experimental study of the neutrino background in underground experiments on nucleon decay" Nuclear Instr. and Methods (in press).

13) R.31. Bionta et al., "A search for nucleon decay into pKO and vEO", Phys. Rev. Lett.

(in press) also for previous references.

15) #. Arikasa et al., "Kamiokande-Kamioka nucleon decay experiment", presented by I.!. iioshiba to the 1963 Int. Symposium on Lepton and Photon Interactions at High Energies, Cornell University, August 4-9,1983 and private corrununication by Prof. M. Eoshiba.

16) D. Cline, "Operation of the HPlV proton decay experiment at Park City (Utah)"- ICalt\; 83, Int. Colloquium on Matter Non Consenlation, Frascati, Jariuary 17-21,1983 (ed. E. Bellotti and S. Stipcich) p.164.

1:) Private connnunication by D. Winn.

16) 31. Conversi and L. Federici, Nucl. Instr. and Nethods

151,

19) R. Barloutaud, "Future european nucleon decay experiments",ICClrliLU 83, Int. Colloquium on Matter Xon Conservation, Frascati, January 17-21,1983 (ed. E. Bellotti and S. Stipcich) p.17 20) H.R. Gulkanian et al., "On a possible method of experimental investigation on proton

decay modes", presented to the Int. Symposium on Lepton and Photon Interactions at High Energies,CornellUniv., August 4-9, 1983.

21) V.A. Lubimov, "On the possibilities of proton decay searches up to the level of years", ITEP 158 (preprint)

.

22) Private communication from K. Lande and A. Man.

23) A. Zichichi,"The Gran Sasso Laboratory", IC@M 83, International Colloquium on Matter Non Conservation, Frascati, Jaruary 17-21, 1983 (ed. E. Bellotti and S. Stipcich) p.3.

24) See for instance D. Cline, "Search for extraterrestrial neutrinos solar neutrinos and proton decay using a very large liquid argon detector", ICBAh' 83, Int. Colloquium on Matter Non Conservation, Frascati, January 17-21, 1983 (ed. E. Bellotti and S. Stipcich) p.177.

25) G. Harigel et al., "A giant bubble chamber with long sensitivity time for nucleon decay experiments", Nucl. Instr. and Methods (in press).

2 6 ) M. Baldo Ceolin, "Neutron oscillation", this Conference.