THE CURRENT STATUS OF PROTON DECAY PHYSICS

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THE CURRENT STATUS OF PROTON DECAY PHYSICS

L. Sulak

To cite this version:

L. Sulak. THE CURRENT STATUS OF PROTON DECAY PHYSICS. Journal de Physique Collo-

ques, 1982, 43 (C3), pp.C3-205-C3-210. �10.1051/jphyscol:1982342�. �jpa-00221895�

JOURNAL DE PHYSIQUE

CoZZoque C3, s u p p Z 6 m e n t a u n o 1 2 , Tome 43, d s c e m b r e 1 9 8 2 p a g e C3-205

THE CURRENT STATUS OF PROTON DECAY P H Y S I C S

L . Sulak

U n i v e r s i t y o f M i c h i g a n , Ann A r b o r , M I 48109, U.S.A.

T h i s review condenses the c o n t r i b u t i o n s o f the rapporteurs t o t h i s session.

The developments over the l a s t year r e l e v a n t t o p r o t o n decay i n v e s t i g a t i o n s , i n c l u d i n g t h e o r e t i c a l i m p l i c a t i o n s and past, present and near f u t u r e exoerimental work are summarized.

1. T h e o r e t i c a l Aspects

We begin t h i s review by a summary o f the i m p l i c a t i o n s of r e c e n t t h e o r e t i c a l

developments f o r proton decay. Employing a = 1/137.036 and Ax = 0 . 1 6 0 1 i : ~ ~ ~ GeV i n the r e n o r m a l i z a t i o n group equations o f the minimal SU(5) Georgi-Glashow modell ( 3 generations o f fermions and

3+5

Higgs s c a l a r s ) , one obtains2

2-

+O

.004

s i n ew(mw) = 0.214-0.003

These p r e d i c t i o n s are i n e x c e l l e n t agreement w i t h experiment3

and thereby p r o v i d e s t r o n g support f o r grand u n i f i c a t i o n .

O f course, minimal SU(5) also p r e d i c t s proton decay ( p + e f o , e+w

...

^). Recently, t h e p r e d i c t e d l i f e t i m e T has s i g n i f i c a n t l y decreased f o r two reasons. 1) A decrease i n Am since r p scales as Aw,

4

and 2) an increase i n the estimated magnitude o f proton decay m a t r i x elements4, e.g. the i n c l u s i o n o f 3 quark f u s i o n (uud+e+) diagrams. Presently, one f i n d s

which i s very n e a r l y i n c o n f l i c t w i t h the bound5 i n f e r r e d from the K o l a r experiment,

I s minimal SU(5) already i n t r o u b l e ? I f a l i f e t i m e o f r p = 1031 y r were t o be e s t a b l i s h e d , minimal SU(5) would r e q u i r e Am

2

0.3 GeV. Thus, p r e c i s e determination o f AK would be needed t o t e s t the model. What happens i f p r o t o n decay

is

n o t observed a t a

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1982342

JOURNAL DE PHYSIQUE

l i f e t i m e s e n s i t i v i t y o f 1031 y r ? Bigger GUTS, such as S0(1O), SU(7) etc., o f f e r v i a b l e a1 t e r n a t i v e s . U n f o r t u n a t e l y , they do n o t p r o v i d e p r e d i c t i o n s f o r ^T p; i n s t e a d they accommodates a l a r g e range o f values.

Another p o s s i b i l i t y i s t o en arge the SU(5) model. Indeed, appending new

non-degenerate Higgs mu1 t i p l e t s t o the model6 o r i n c r e a s i n g both t h e fermion and Higgs

,

c o n t e n t v i a supersymmetry7 can s i g n i f i c a n t l y modify the ^{T P} p r e d i c t i o n . We discuss these 1 a t t e r scenarios.

Minimal SU(5) has always had a problem regarding the fermion spectrum. It p r e d i c t s

%/me = ms/md, which i s o f f by an order o f magnitude. This i s g e n e r a l l y r e c t i f i e d by i n t r o d u c i n g a complex 45 o f Higgs sca1ars.l Why n o t go a l l t h e way and i n c l u d e a 10,

15

and 50 ( o r several o f each)? Indeed, only the Higgs

-

5,

10, 15, 5

^and

-

50 c o u ~ l e t r f e r m x n b i l i n e a r s ( i n 5+10).

Each submul t i p l e t o f the decomposition under SU(3)cxSU(2)~xU(l) o f t h e Higgs SU(5) m u l t i p l e t s has a d i s t i n c t mass mi. Only some m i ' s are constrained. C o n s t r a i n t s from KO-KO and DO-DO m i x i n g are n o t very stringent.6 F o r degenerate values o f the mi, the p r e d i c t i o n s o f minimal SU15) are unchanged. One can s h i f t r p r a t h e r s i g n i f i c a n t l y withou much m o d i f i c a t i o n o f sin2ew(mW) by a j u d i c i o u s choice o f mi. E.g., assuming no mass r a t i i s >10 and maximizing t h e lifetiin: p r e d i c t i o n s , one f i n d s T~ = 3x1031 y r (an increase by 2 orders of magnitude) w h i l e sin2ew(mw) = 0.215. Since the p o s s i b i l i t y o f a l a r g e r SU(5) Higgs s e c t o r i s n o t u n r e a l i s t i c , t h i s u n c e r t a i n t y i n the T~ p r e d i c t i o n provides m o t i v a t i o f o r pushing the experiments t o the

-

^{1 0 3 ~} y e a r s e n s i t ~ v i t y .

Imposing a supersymmetry on the minimal S U ( 5 ) model, where each fermion (boson) i s given a boson ( f e r m i o n ) partner, a l s o changes the predictions.7 F o r bE = 0.16 GeV and N 1 i g h t Higgs isodoublets8, supersymmetric SU(5) w i t h supersymmetry breaking a t =inw p r e d i c t mb/@ = 2.8 and

4 0.259 5x1030 y r

.

The p r e d i c t i o n f o r sin26,(mW) i s l a r g e r than the experimental value p a r t i c u l a r l y f o r N H = ~ ) . However, by e n l a r g i n g the Higgs content f u r t h e r s 9 one can g e t sin26,(mW) = 0.21-0.22 and T P = 1031 y r .

The values o f T g i v e n above correspond t o tree-1 eve1 gauge-boson-medi ated decays.

Loop e f f e c t s i n some supersymmetric t h e o r i e s (depending on supersymmetry breaking) can g i v e r i s e t o dimension 5 o p e r a t o r s l o which may l e a d t o new dominant decay modes such as p

+ V,K+ and n-+ vTKo. The-signal f o r such decays i s l e s s c l e a r than p + e+no i n the new detectors. ~ o m e s t a k e l l gives a lower bound ^T p - > 3x1031 y r f o r decays r e s u l t i n g i n a f i n a s t a t e p+. T h i s probably t r a n s l a t e s i n t o a l i m i t o f about 3x1030 y r f o r these modes.

Other theor'etical developments have relevance f o r proton decay experiments since the suggest a l t e r n a t i v e signatures t h a t should be searched f o r . Using ^T 2 3 ~ ~ 3 0

yr,

C h e t y r k i n e t a1 .I2 have obtained an e f f e c t i v e bound for t h e f r e e neugron nn o s c i l l a t i o n p e r i o d Tn- 5 2/6m

,

2x107 sec. T h i s i s b e t t e r than present r e a c t o r experimental bounds.

As the laVge p r o t o n decay experiments become o p e r a t i o n a l , they may push T into 108-109 se as a byproduct.

An enlarged Higgs s e c t o r which i n c l u d e s the 50 w i t h r a t h e r l i g h t , doubly-charged and c o l o r - s e x t e t s c a l a r s may lead-to observable r a t e s f o r double baryon decay A 5 = AL = -2:

pp + a+a+, pn + 2'7 o r nn + vv (E = e,p).13 The s i g n a t u r e f o r the f i r s t r e a c t i o n , back-to-back charged l e t o n s w i t h = 1 GeV each, i s q u i t e spectacular.

Rubakov and C a l l any4 have argued t h a t as superheavy SU(5) magnetic monopoles moue through matter, they may c a t a l y z e proton decay. Rubakov estimates a cross-section of cr = 10-26 cm2 f o r the r e a c t i o n Wp + Mte++X. T h i s would cause a s t r i n g o f proton decays

separated s p a t i a l l y by 10-100 cm and temporally by microseconds i f such a monopole t r a v e r s e d one o f t h e b i g p r o t o n decay detectors. These r e s u l t s are independent o f t h e u n i f i c a t i o n mass. However, the t h e o r e t i c a l a n a l y s i s i s p r e l i m i n a r y ; much work needs t o b done regarding p o s s i b l e suppression mechanisms. Nevertheless, a double discovery ( p r o t o n decay and magnetic monopoles) may be forthcominq i f t h e i r surmise i s v a l i d .

2. Experimentally, Where Do We Stand Today?

L i m i t s on .rp have been s e t from the r a t e s o f stopping and decaying muons i n d e t e c t o r s deep underground. I t i s assumed t h a t the muons come from nucleon decay i n the d e t e c t o r o r t h e surrounding rock v i a the decay o f i o n secondaries. F o r decay modes favored by SU(5), t h e quoted l i m i t s from Learned e t a1 .19 and Cherry e t a1 . I 6 are

r p > 1030 y r s

and ^T~ > 1.5~1030 yrs, resp.

For o t h e r decay modes, e.g. the SUSY-favoured p + K+v,, K+ ⁺ pv,,, the l i m i t s would be comparable. These l i m i t s are p o s s i b l y o p t i m i s t i c since no account i s taken o f p i o n absorption i n the parent nucleus. A s i m i l a r l i m i t has been s e t f o r confined events i n a 35 t o n c a l o r i m e t e r i n the Soudan I experiment.17

The Tata I n s t i t u t e l 0 s a k a l T o k y o group5,18 has operated a 140 t o n Fe c a l o r i m e t e r f o r 460 days a t 7600 mwe depth i n the K o l a r Gold F i e l d s . I t c o n s i s t s o f 1.2

cm

t h i c k Fe p l a t e s separated by orthogonal l a y e r s o f p r o p o r t i o n a l tubes, 10x10 cm i n cross-section.

F o r each event recorded t h e r e are t y p i c a l l y 10-15 h i t s , so t h a t any one t r a c k may be d e f i n e d by only 2 o r 3 h i t s i n each o f the two views. Track and vertex r e s o l u t i o n i s t h e r e f o r e poor and even a prong d i s t r i b u t i o n i s d i f f i c u l t t o consider r e l i a b l e . Energy estimates are based on pulse h e i g h t per h i t , ranqe, and t r a c k lenqth. The authors c l a i m an energy r e s o l u t i o n o f AEIE = 20%, b u t i t i s p o s s i b l y a f a c t o r two worse as estimated by sampling frequency and the EGS Monte C a r l o program.

Three confined nucleon decay candidates i n 60 t o n years ( f i d u c i a l ) are claimed, b u t s i n c e these events cannot be d e f i n i t i v e l y e s t a b l i s h e d as nucleon decays, the experiment s e t s a lower l i m i t i n the r e g i o n o f r p > 3 x 1 0 3 ~ yrs. The experimenters observe a t o t a l o f 19 i n t e r a c t i o n s ( o r decays) a t a r a t e o f (1&2.5)/100 ton-yr; t h i s i s c o n s i s t e n t w i t h n e u t r i n o background estimates. The g r e a t achievement o f t h i s group has been t o operate the f i r s t experiment w i t h a l a r g e dedicated d e t e c t o r deep underground, and t o i n s p i r e o t h e r groups t o embark on s i m i l a r p r o j e c t s .

The

NU SEX^^

c o l l a b o r a t i o n has ooerated a 150 t o n c a l o r i m e t e r ( 1 cm Fe p l a t e s and r e s i s t i v e streamer tubes o f 1 cm2 cross-section) f o r -30 days under Mont Blanc. A t o t a l o f one observed i n t e r a c t i o n sets a l i m i t o f r p 2 3 x 1 0 ~ ~ y r .

The i n t e r a c t i o n s o f atmospheric n e u t r i n o s c o n s t i t u t e the main background t o any p r o t o n decay signal. Per k i l o t o n - y e a r exposure, one expects 150 n e u t r i n o events, o f which 50 have a v i s i b l e energy o f 0.W0.3 GeV. An equal number o f nucleon decays would

correspond t o T = 1 . 2 ~ 1031 y r . Before one can c l a i m any signal

,

the n e u t r i n o background must be understood. There are several consistency t e s t s t o be performed by observing the n e u t r i n o s :

( i The v, and v e f l u x e s are c l o s e l y r e l a t e d t o the measured sea-level muon f l u x e s and a r e known t o 30% o r b e t t e r . The observed f l u x e s should agree w i t h the expected n e u t r i n o r a t e and spectrum shape.

( i i ) The muonless t o muonic events should be i n the r a t i o o f 1:2.

( i i i ) The z e n i t h angle (8) d i s t r i b u t i o n should peak a t the h o r i z o n t a l , e.g.

N(e>60°)/N(e<600) = 1.30.

( i v ) The l a t i t u d e (A) and east-west e f f e c t s r e s u l t i n g from the magnetic c u t - o f f o f the primary spectrum should be apparent, e.g. N(h=0°)/N(A=500N) = 0.6. At A=OO, N(E)/N(W) = 0.5. U n f o r t u n a t e l y a t the l o c a t i o n o f most experiments (-50' n o r t h l a t i t u d e ) N(E)/N(W) = 0.98, so t h a t only t h e K o l a r experiment can v e r i f y the east-west asymmetry.

( v ) The n e u t r i n o event topology i s c h a r a c t e r i s t i c . This has been demonstrated by t h e NUSEX group19 w i t h an Fe c a l o r i m e t e r a t the CERN PS beam. The proton energy (10 GeV) was chosen so t h a t the n e u t r i n o spectrum was s i m i l a r t o t h a t o f cosmic rays. (Of course, o n l y v,, ' s and no v e 1 s were i n the beam.) They f i n d 58% 1-prong, 27% 2-prong, and 8% a3-prong events. So a simple d e v i a t i o n from t h i s prong d i s t r i b u t i o n c o u l d be evidence f o r a non-neutrino signal. They conclude t h a t o n l y 3% o f the n e u t r i n o events c o u l d simulate nucleon decays. The l a t t e r are defined as 2-prong events, w i t h back-to-back angle >120°, o r a3-prong events w i t h 1 secondary backward r e l a t i v e t o the r e s u l t a n t momentum o f the others.

C 3 - 2 0 8 JOURNAL DE P H Y S I Q U E

Assuming an energ.y r e s o l u t i o n o f A E E 20%, t h e i r a n a l y s i s suqgests a signal t o background r a t i o o f u n i t y f o r r = 1031 y i a r s . This i s o p t i m i s t i c i n the sense tha a 2-body nucleon decay (e.g. n + e f - ) can o f t e n appear as a s i n g l e prong due t o absorption o r s c a t t e r i n g i n the parent nucleus. F u r t h e r , t h e energy r e s o l u t i o n f o an i s o t r o p i c angular d i s t r i b u t i o n and 1 cm sampling i s more n e a r l y 30% a t best.

The NUSEX a c c e l e r a t o r studies p r o v i d e valuable i n f o r m a t i o n on r e a l n e u t r i n o event c h a r a c t e r i s t i c s i n c a l o r i m e t e r s designed f o r nucleon deca

.

They show t h a t o r SUSY decay modes.

i t should be p o s s i b l e t o demonstrate a c l e a n s i g n a l , i f r p < 10 2 years, f o r SU(5)

3

3.

-

The New Tracking Calorimeter_

U n t i l r e c e n t l y two t r a c k i n g d e t e c t o r s were o p e r a t i o n a l . I n May 1982, the Mont Blan detector19 (150 tons) j o i n e d the Soudan (30 tons) and K o l a r Gold (150 tons) detectors.

The small mass and the coarse g r a i n resp., o f the l a t t e r two d e t e c t o r s are complemented the f i n e g r a n u l a r i t y o f the Mont Blanc detector. It i s a d i g i t a l t r a c k i n g c a l o r i m e t e r w i t h 1 cm t h i c k i r o n p l a t e s and ( 1 cm)2 p l a s t i c streamer tubes between t h e plates.

These t r a c k i n q c a l o r i m e t e r s are more complex and expensive than the a1 t e r n a t i v e wat Cerenkov detectors. They a l s o s u f f e r from the d i s t u r b i n g nuclear e f f e c t s (Fermi motion and nuclear r e i n t e r a c t i o n s ) o f the h i g h Z d e t e c t o r m a t e r i a l . However, they f e a t u r e a u n i f o r m s e n s i t i v i t y t o the various p o s s i b l e decay modes. The r e c e n t enlarqement o f the p r e d i c t e d proton decay schemes by supersymqetric t h e o r i e s have f u r t h e r enhanced by t h i s feature.

I n d i g i t a l c a l o r i m e t e r s a l l t h e i n f o r m a t i o n r e l e v a n t t o event i d e n t i f i c a t i o n comes from the p a t t e r n of h i t elements. T h i s p a t t e r n determines the event v e r t e x and topoloqy p a r t i c l e i d e n t i f i c a t i o n , energy measurement, and prong d i r e c t i o n . The l a t t e r i s general a v a i l a b l e f o r electromagnetic showers. It can a l s o be determined by t h e i d e n t i f i c a t i o n the (K,n) ⁺ u ⁺ e decay sequence. The v+e d e t e c t i o n e f f i c i e n c y o f the Mont Rlanc d e t e c t i s about 30%. Due t o the h i g h ^11- capture p r o b a b i l i t y i n i r o n (38%)) o n l y (K+,T+) + +

e+ decays are observed. As a consequence, the observation o f the decay chain i d e n t i f i e s the p o s i t i v e charge o f t h e secondary.

The s e n s i t i v i t y o f t h e Mont Blanc d e t e c t o r i s r a t h e r uniform f o r a wide spectrum o f p o s s i b l e decay modes: from e+n t o K'v. From the CERN beam t e s t s , i t s g r a n u l a r i t y appears adequate t o r e j e c t v background f o r an exposure o f a few 100 ton-years. I t shou t h e r e f o r e be able t o i n v e s t i g a t e p r o t o n l i f e t i m e up t o 1031 years.

A l a r g e r d i g i t a l t r a c k i n g c a l o r i m e t e r i s now under c o n s t r u c t i o n i n the F r e j u s tunne I t should s t a r t o p e r a t i o n next year w i t h 500 tons. The eventual t o t a l mass w i l l be 1.5 Kton. An e s s e n t i a l f e a t u r e i s i t s very f i n e g r a i n : 3 m i r o n p l a t e s , i n t e r l e a v e d w i t h f l a s h chambers o f (5 mn)2 cross section. Geiger tubes p r o v i d e the t r i q g e r . I t s g r a n u l a r i t y should be adequate t o r e j e c t v background f o r an exposure o f a few Kton-year It should be able t o explore up t o a l i f e t i m e o f years. I f t h e proton decays, the d e t e c t o r should be able t o i d e n t i f y complex decay modes such as ef + ( p , w , n ) o r p + ~ i . Two more f i n e g r a i n c a l o r i m e t e r s have been proposed. The f i r s t one, a 1 Kton devic i s advocated f o r the Soudan mine. The t r a c k i n g devices would be l o n g - d r i f t p r o p o r t i o n a l chambers, sandwiched w i t h 0.5 cm i r o n p l a t e s . The s p a t i a l g r a n u l a r i t y o f t h i s d e t e c t o r would n o t be as f i n e as t h a t o f the F r e j u s detector, b u t t h e AE/AX measurement c o u l d presumably compensate f o r t h a t , and a1 so p r o v i d e t r a c k d i r e c t i o n a l i t y .

The second proposed c a l o r i m e t e r would be i n s t a l l e d i n the Gran Sasso Laboratory.

P r e s e n t l y t h e r e a r e two a l t e r n a t i v e desiqns under study, one w i t h f l a s h chambers and one w i t h p l a s t i c Geiger tubes. E i t h e r would have a t i m e - o f - f l i g h t system t o q i v e

d i r e c t i o n a l i t y .

T h i s v a r i e t y o f t r a c k i n g d e t e c t o r s and techniques, e x h i b i t i n g non-negl i q i b l e d i f f e r e n c e s i n t h e i r d e t e c t i o n features, should ensure a c a r e f u l e x p l o r a t i o n of p r o t o n l i f e t i m e i n t o the 1032 years ranqe.

4. The New Water Cerenkov D e t e c z

Two U.S. c o l l a b o r a t i o n s have j u s t begun o p e r a t i n g l a r g e water Cerenkov detectors designed s p e c i f i c a l l y t o search f o r nucleon decay reactions. These d e t e c t o r s are run bj t h e Harvard-Purdue-Wisconsin c o l l a b o r a t i o n i n the S i l v e r King Mine, Park C i t y , Utah and the Irvine-Michigan-Brookhaven c o l l a b o r a t i o n i n the Morton S a l t Mine, Cleveland, Ohio.

L. Sulak C3-209

Another water detectorz0, under c o n s t r u c t i o n a t the Kamioka metal mine i n Japan, i s scheduled t o be i n o p e r a t i o n by l a t e 1982 by a group from Tokyo, Tsukuba, and KEK.

Water Cerenkov d e t e c t o r s p r o v i d e a t low c o s t 6 x 1 0 3 ~ nucleons per k i l o t o n y i e l d i n g 60 branching r a t i o x e f f i c i e n c y events per k i l o t o n - y e a r f o r ^T~

-

1031 years. The low d e n s i t y o f the medium and the l a r g e volume o f the d e t e c t o r allows the t r a c k s from an event t o expand t o

-

4.5 meters, w i t h Cerenkov r a d i a t i o n f u r t h e r p r o j e c t i n q the i n f o r m a t i o n t o many meters distance. From t h i s extended p a t t e r n , t h e d i r e c t i o n s o f t r a c k s a r e obtained through timing. P a t t e r n r e c o g n i t i o n , t r a c k d i r e c t i o n , and energy r e s o l u t i o n combine t o i d e n t i f y events and e l i m i n a t e background. For c l a s s i c a l SU(5) modes ( i .e., p + e+ro) the decay products are electromagnetic and produce d i s t i n c t i v e back-to-back showers w i t h r e c o n s t r u c t i o n r e s o l u t i o n s on v e r t e x l o c a t i o n o f

-

^{50 cm.,} a x i s d i r e c t i o n s o f 10' t o 15' and energy r e s o l u t i o n o f 10% t o 20%. I n c o n t r a s t , SUSY modes (e.g., p + vK) are u n i d i r e c t i o n a l

,

produce few photoelectrons and have d i f f e r e n t associated backgrounds.

Although designed f o r the electromagnetic modes, t h e water Cerenkov d e t e c t o r s w i l l be capable o f r e s o l v i n g some SUSY reactions, b u t w i t h lower e f f i c i e n c y .

The IMB d e t e c t o r i s approximately cubical w i t h 21 m sides and 2048 photo-mu1 t i p 1 ie r s . F i v e - i n c h hemi s p h e r i c a l phototubes are arranged uni form1 y over the s i x i n n e r surfaces v i e w i n g the 10 k i l o t o n s o f water. The i n n e r 4 k i l o t o n s c o n s t i t u t e the f i d u c i a l volume.

I n c o n t r a s t , the HPW d e t e c t o r i s c y l i n d r i c a l w i t h a volume d i s t r i b u t i o n o f 704

p h o t o m u l t i p l i e r s i n 850 tons o f water (-l/m3) and w i t h m i r r o r s l i n i n g t h e e n t i r e i n n e r s u r f a c e t o enhance l i g h t c o l l e c t i o n . Whereas the IMB design u t i l i z e s the o u t e r 2 m o f water as an a c t i v e shield, the HPW tank w i l l be surrounded by p r o p o r t i o n a l w i r e chambers and concrete as a shield. Both exoeriments m a i n t a i n very pure water t o achieve absorption l e n g t h s f o r photons a t -420 nm wavelength o f 25 t o 30 m. The Kamioka d e t e c t o r w i l l be c y l i n d r i c a l i n shape w i t h 1000 l a r g e 20" PMTts mounted on the i n n e r surfaces viewing a 1 k i l o t o n f i d u c i a l volume (3 k i l o t o n s t o t a l

1.

The IMB and HPW groups are c u r r e n t l y analyzing cosmic muon data t o v e r i f y the r e s o l u t i o n s o f t h e i r r e s p e c t i v e d e t e c t o r s and t o t e s t and improve the r e c o n s t r u c t i o n programs. The r e s u l t s are encouraging. A t a muon r a t e o f a few Hz, the muon

t r a j e c t o r i e s , each w i t h v a r i a b l e length, produce Cerenkov cones t h a t are c l e a r l y evident.

Event c h a r a c t e r i s t i c s are i n close accord w i t h Monte Carlo p r e d i c t i o n s . A run o f the IMB device i n December 1981 w i t h the d e t e c t o r 1/3 f u l l o f water provided a data s e t o f throughgoing and stopping muons d e p o s i t i n g

-

^{1 GeV.} This r u n gave r e s u l t s e n t i r e l y c o n s i s t e n t w i t h expectations f o r photoelectron y i e l d , f o r angular d i s t r i b u t i o n w i t h r e s p e c t t o the zenith, and f o r measurement o f t h e muon l i f e t i m e .

With estimated n e u t r i n o backqroundsof -1 eventfyear mimicking proton decay i n the IMB and HPW ex eriments, a l i m i t on the iiirdF6on l i f e t i m e can be s e t a t about 2 (HPW) t o 10

5

(IMB) x 10 2 y e a r s f o r electromagnetic decay modes. N e u t r i n o i n t e r a c t i o n s w i t h back-to-back l e p t o n - p i o n t r a c k s s i m i l a r t o nucleon decay (e.q., v n ⁺ p - p r o ) l i m i t the s e n s i t i v i t y . For SUSY modes, the number o f photoelectrons detected i s an order o f magnitude l e s s and the background e l im i n a t i o n commensurate1 y rllore d i f f i c u l t ; these modes a r e c u r r e n t l y being s t u d i e d by k n t e C a r l o techniques. F u r t h e r d i f f i c u l t i e s are due t o nuclear e f f e c t s , which reduce the number o f observable nucleon decays by p i o n absorption, and Fermi motion, which degrades the back-to-back s t r u c t u r e o f two body and quasi-two body modes. With 20% o f t h e protons f r e e and w i t h an appreciable f r a c t i o n o f the Fermi motion o f the bound protons w i t h i n angular and energy r e s o l u t i o n , 40% o f proton decays appear t o s u r v i v e these d i f f i c u l t i e s .

5. Summary

Where do we stand today? The KGF group have observed n e a r l y 20 events a t a r a t e expected from n e u t r i n o background; i t i s n e i t h e r excluded nor proven t h a t a few o f these c o u l d be nucleon decays. S i m i l a r l y , t h e s i n g l e event from the NUSEX d e t e c t o r i s c o n s i s t e n t w i t h the expected n e u t r i n o r a t e . I t i s c l e a r t h a t several experiments w i t h d i f f e r e n t techniques w i l l be needed t o e s t a b l i s h a s i g n a l . The new d e t e c t o r s provide compl imentary technologies. They i n c l u d e good s p a t i a l r e s o l u t i o n and sampl i n g t o d e f i n e v e r t i c e s , r e s o l v e m u l t i - t r a c k events, and give e/p d i s c r i m i n a t i o n . Further, p a r t i c l e d i r e c t i o n i s a v a i l a b l e t o d i s c r i m i n a t e two prong decay events from s c a t t e r s , and t o d i s t i n g u i s h decays 1 i ke p + K+V, and K+ ^-t pvU from n e u t r i n o background 1 i ke

vu

+

N + N + n + p .

The n e x t few months are going t o be very a c t i v e and e x c i t i n g f o r a l l the proton decay p r o j e c t s .

JOURNAL DE PHYSIQUE

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