Observation of a non-elastic cosmic ray neutrino interaction

Volume 19, number 1

OBSERVATION

PHYSICS LETTERS

OF

A NON-ELASTIC

COSMIC

15 September 1965

RAY

NEUTRINO

INTERACTION

C. V. ACHAR, M. G. K. MENON, V . S . NARASIMHAM, P. V. RAMANA MURTHY and B. V. SREEKANTAN Tata Institute of Fundamental Research, Bombay, India K. HINOTANI and S. MIYAKE Osaka City University, Osaka, Japan D. R. CREED, J. L. OSBORNE, J . B . M . P A T T I S O N and A. W. W O L F E N D A L E University of Durham, Durham, U.K. Received 18 August 1965

In a r e c e n t c o m m u n i c a t i o n to this j o u r n a l [1] we have d e s c r i b e d b r i e f l y our e x p e r i m e n t a l a r r a n g e m e n t f o r the d e t e c t i o n of m u o n s p r o d u c e d in the i n t e r a c t i o n of " n a t u r a l " ( c o s m i c ray) neut r i n o s deep u n d e r g r o u n d ; and d i s c u s s e d t h r e e e v e n t s , r e c o r d e d in 50 days of o b s e r v a t i o n , s o m e at l e a s t of which w e r e due to n a t u r a l n e u t r i n o int e r a c U o n s . The n e u t r i n o t e l e s c o p e s have been in o p e r a t i o n s i n c e M a r c h , 1965 at a depth of 7600 ft below g r o u n d in the K o l a r Gold Mines in South India. Since we c o m m u n i c a t e d our e a r l i e r p a p e r , eight f u r t h e r e v e n t s have been o b s e r v e d , m a k i n g a tota l of e l e v e n e v e n t s so f a r . The f o u r t h e v e n t is c h a r a c t e r i z e d by f e a t u r e s which m a k e it a c l e a r c a s e of a n o n - e l a s t i c n a t u r a l n e u t r i n o i n t e r action. In this note we w i s h to d e s c r i b e and d i s c u s s this p a r t i c u l a r e v e n t in s o m e detail. (The o t h e r e v e n t s will be r e p o r t e d in a l a t e r m o r e det a i l e d publication.) F o r i n f o r m a t i o n c o n c e r n i n g the e x p e r i m e n t a l s e t - u p in the m i n e s , o p e r a t i o n a l f e a t u r e s and c l a s s i f i c a t i o n of e v e n t s into v a r i o u s types, the e a r l i e r p a p e r [1] m ay be r e f e r r e d to. A s k e t c h of the f o u r t h event, b a s e d on the f l a s h - t u b e a r r a y photograph, i s shown in fig. 1. In t a bl e 1 we g i v e r e l e v a n t d e t a i l s c o n c e r n i n g the event, u s i n g the n o m e n c l a t u r e of the e a r l i e r paper. Table 1 Event Type of no. coincidence 4

78

TEL. 2 N1 +S 1

Projected zenith angle

Date

Time

Track Tat 99.2 ° + 0.3 ° 3.7.65 12.30 T r a c k ' b t 96.2 ° + 0 . 8 °

~ "~

ZENITH

3"7"¢¢5 ~

NORTH

,=,oNr

pLASTIC SCWI1LLATORS

I[ ! ! i

+

SOUTH

CENTRE PLASTIC LINE J 2.Scrfl SCINTILLATORS "1 CK L| ~D

II .......

~ORIGIN -~-

IrpSI

i SCALE

I~OTTOM

Fig. 1. A sketch of the event based on the flash-tube array photograph showing the relative position of plastic scintillators and lead layers with respect to the a r rays of flash-tubes. The black dots show the discharged tubes; (only the bottom .~ section of the telescope is shown in the figure). A~ indicated in the figure, the meeting point of the two tracks could be either in air close to the rock wall on the southern side or ifiside the rock up to a thickness of one meter.

Volume 19, number 1

PHYSICS LETTERS

The m a i n f e a t u r e s of the event that can be deduced f r o m the f l a s h - t u b e photograph and the o s c i l l o s c o p i c d i s p l a y s y s t e m a r e the following: (a) C o n v e r g e n c e of the two t r a c k s : The two t r a c k s ' a ' and ' b ' shown i n fig. 1 converge to a point * below the h o r i z o n t a l plane on the south side; (the p r o j e c t e d z e n i t h angle between the t r a c k s is 3° ~: 1°). The point of i n t e r a c t i o n t h e r e f o r e a p p e a r s to be i n the s o u t h e r n rock wall. B e c a u s e of the u n c e r t a i n t i e s that exist in the det e r m i n a t i o n of the z e n i t h and a z i m u t h a l angles, the exact location of the point at which the two t r a c k s c o n v e r g e c a n n o t be d e t e r m i n e d a c c u r a t e l y . Within the e r r o r s , the point could be c l o s e to the s u r f a c e of the r o c k wall, or up to a t h i c k n e s s of one m e t e r i n s i d e the rock; in the case of the latt e r p o s s i b i l i t y , the a c t u a l d i s t a n c e t r a v e r s e d by the p a r t i c l e s through r o c k would be up to 1.7 m e ters. (b) A z i m u t h a l a n g l e s : F r o m the s c i n t i l l a t o r and f l a s h - t u b e data we c a n s e t l i m i t s on the a z i m u t h a l angles of the two t r a c k s ; the m a x i m u m d i f f e r e n c e in the a z i m u t h a l a n g l e s of Ta~ and ' b v is 21 °. (c) T r a c k Ta': The p r o j e c t e d zenith angle is 99 o. The p a r t i c l e p r o d u c i n g this t r a c k p e n e t r a t e d the 5 c m lead p l a c e d i n two l a y e r s between the t h r e e f l a s h - t u b e a r r a y s . T h e r e is no evidence of c a s c a s e m u l t i p l i c a t i o n o r of l a r g e angle s c a t t e r i n g . This t r a c k i s t h e r e f o r e definitely not due to a n e l e c t r o n and is p r e s u m a b l y due to a muon o r a pion. (d) T r a c k 'b~: The p r o j e c t e d zenith angle i s 96 °. The p a r t i c l e p r o d u c i n g this t r a c k c l e a r l y p e n e t r a t e d a m i n i m u m of one l a y e r of lead, 2.5 c m thick, at a p r o j e c t e d zenith angle of 96 ° , (i.e., it t r a v e r s e d > 5 r a d i a t i o n lengths), without any evidence of c a s c a d e m u l t i p l i c a t i o n or of l a r g e angle s c a t t e r i n g . T r a c k t b' is not s e e n in the f l a s h - t u b e a r r a y t o w a r d s the south, and the pulse amplitude r e c o r d e d by the s c i n t i l l a t o r on the s o u t h e r n side is s m a l l e r than that r e c o r d e d on the n o r t h e r n side. The m o s t likely explanation i s that the a z i m u t h a l angle was such that the p a r ticle m i s s e d the s o u t h e r n f l a s h - t u b e a r r a y and a s s o c i a t e d s c i n t i l l a t o r . By a detailed g e o m e t r i c a l r e c o n s t r u c t i o n of the l i m i t i n g p o s s i b i l i t i e s for the t r a c k c o n f i g u r a t i o n s , a s defined by the s c i n t i l l a t o r and f l a s h - t u b e data, it can be shown that t r a c k Tb' * At the depth of operation, single tracks are recorded in one of our telescopes at the rate of one in 2 × 106 seconds; the probability of observing two unrelated tracks in the same picture can be shown to be ~<10-10 . Since we have so far seen only 11 events, the probability that the event discussed here is due to two unrelated tracks is vantshingly small.

15 September 1965

also a l m o s t c e r t a i n l y t r a v e l l e d a m i n i m u m of 5 cm of lead, provided the d i r e c t i o n of t r a v e l i s taken to be f r o m south to north. It i s m o s t u n l i k e l y t h e r e fore that p a r t i c l e ' b ' was an e l e c t r o n ; and, again, it was p r e s u m a b l y a muon or a pion. (e) P o s s i b i l i t y that the i n t e r a c t i o n o c c u r r e d in the n o r t h e r n wall: T h e r e is the l e s s likely p o s s i b i l i t y that the i n t e r a c t i o n took place i n the n o r t h e r n rock wall and one of the two p a r t i c l e s div e r g i n g into the telescope was s c a t t e r e d in the rock itself t h e r e b y giving r i s e to the c o n f i g u r a Lion o b s e r v e d . In this case no g e o m e t r i c a l r e c o n s t r u c t i o n can be c a r r i e d out to define the point of interaction. If the i n t e r a c t i o n did o c c u r i n the n o r t h e r n rock wall, it could be asked whether t r a c k ' b ' might be an e l e c t r o n which was s c a t t e r e d i n the rock wall. We c o n s i d e r this e x t r e m e l y u n l i k e l y s i n c e for this the e l e c t r o n would have to undergo both a l a r g e angle s c a t t e r i n g in the rock wall and t r a v e r s e eight o r m o r e r a d i a t i o n lengths in the rock plus lead without any evidence of m u l t i p l i cation. F r o m what has been said above, it is c l e a r that the two t r a c k s a r e ~ m o s t c e r t a i n l y due to pions or m u o n s ; and this is t r u e whether the i n t e r a c t i o n o c c u r r e d in the s o u t h e r n o r n o r t h e r n rock wall. The event in q u e s t i o n follows as a non-elastic interaction. The m o s t s i g n i f i c a n t f e a t u r e of the event is the fact that the zenith a n g l e s of the two t r a c k s a r e v e r y l a r g e , i.e., 96 ° and 99 ° . We can now p r o c e e d to c o n s i d e r the n a t u r e of the p r i m a r y p a r t i c l e that could have given r i s e to such a none l a s t i c i n t e r a c t i o n at such a l a r g e zenith angle and at a depth of 7 600 ft below ground. T h e r e a r e the following t h r e e p o s s i b i l i t i e s : (A) A p h o t o n u c l e a r i n t e r a c t i o n of a n a t m o s p h e r i c muon t r a v e l l i n g at the r e q u i r e d zenith angle: the muon and a c r e a t e d pion could then c o n s t i t u t e the two p a r t i c l e s Ta' and ' b ' of the event. (]3) An i n t e r a c t i o n c a u s e d by a pion t r a v e l l i n g at the r e q u i r e d zenith angle; the pion, in t u r n , was produced in a p h o t o n u c l e a r p r o c e s s by an atm o s p h e r i c muon n e a r the level of o b s e r v a t i o n ; f o r this, the pion would have had to suffer a l a r g e angle s c a t t e r i n g to the o b s e r v e d - a l m o s t h o r i zontal - d i r e c t i o n f r o m the d i r e c t i o n of m a x i m u m i n t e n s i t y of m u o n s , which is the v e r t i c a l . The p a r t i c l e s ' a ' and Vb' would then both be pions. (C) An i n t e r a c t i o n c a u s e d by a n a t u r a l n e u trino. The f i r s t two p o s s i b i l i t i e s can be r u l e d out on the following grounds: (i) the v e r y low v e r t i c a l

79

Volume 19, number 1

PHYSICS LETTERS

i n t e n s i t y of a t m o s p h e r i c m u o n s at this depth *; (if) the steep a n g u l a r d i s t r i b u t i o n of a t m o s p h e r i c muons, as a r e s u l t of which the i n t e n s i t y at the l a r g e zenith angle i n q u e s t i o n is quite uegligible; (iit) the s m a l l c r o s s s e c t i o n for the p r o d u c t i o n of ptons by m u o n s in p h o t o n u c l e a r p r o c e s s e s ; (iv) the s m a l l p r o b a b i l i t y for the r e q u i r e d l a r g e angle s c a t t e r i n g of the pion. We t h e r e f o r e c o n s i d e r it e x t r e m e l y I m p r o b a b l e that the event i s due to a n a t m o s p h e r i c muon o r the pion component in e q u i l i b r i u m with it. We c o n s i d e r that the event is a c l e a r e x a m p l e of the n o n - e l a s t i c i n t e r a c t i o n of a n a t u r a l n e u t r i n o t r a v e l l i n g a p p r o x i m a t e l y in the h o r i z o n t a l d i r e c t i o n ; i n a l l p r o b a b i l i t y the n e u t r i n o t r a v e U e d u p w a r d s f r o m below the h o r i z o n t a l . The two m o s t l i k e l y e x p l a n a t i o n s of the event would he : (i) P r o d u c t i o n and decay of an i n t e r m e d i a t e boson: u u + Z --* Z' +/~ + W and s u b s e q u e n t leptonic (or' n o n - l e p t o n i c ) decay of W. The two obs e r v e d t r a c k s would then m o s t p r o b a b l y be due to m u o n s .

15 September 1965

(if) P r o d u c t i o n of a p t o n in an i n e l a s t i c i n t e r action: vu + N ~ / ~ + N ~ + # + . . . . The two t r a c k s would then be due to a muon and a pion. If the point of i n t e r a c t i o n was well inside the rock - say a few m e t e r s i n s i d e - then the s e c o n d a r i e s would n e c e s s a r i l y have to be muons to be able to p e n e t r a t e the rock and p a s s through the telescope. In the p r e s e n t event the u n c e r t a i n t i e s in the e s t i m a t e d point of o r i g i n and in the a m o u n t of m a t t e r t r a v e r s e d by the two p a r t i c l e s a r e too l a r g e to decide with c e r t a i n t y b e t w e e n the two a l t e r n a t i v e s cited above. The a c c u m u l a t i o n of m o r e events should c l e a r l y enable a d e c i s i o n to be made. R~fe~'enc~

1. C.V.Achar et al., Physics Letters 18 (1965) 196 * From our own observations in this experiment, we deduce that the intensity of atmospheric mup~s in the vertical direction at this depth is ~ 10-~v particlos/cm 2 sec .sr.

ERRATA

°"re ~ , ~ l a r i z a U o n effect in 209pb P. Sailing, ,~,, studied in the 09TI --, 209Pb decay, P h y s i c s

Letters 17 (1965) p. 141. The s e n t e n c e on page 141, r i g h t c o l u m n 6 l i n e s f r o m above should r e a d : I n u s i n g the f o r m u l a we have adopted the value -60.2 fm 2 for the r a d i a l m a t r i x e l e m e n t of 209pb.

80

B. B. Goodman, The e l e c t r o d y n a m i c s of flux p e n e t r a t i o n in s u p e r c o n d u c t o r s , P h y s i c s L e t t e r s

18 (1965) p. 8. E q u a t i o n (5) should r e a d :

= % ~ h

4~2

,=1 ~

exp(-r/XL) r

dl i ,

(5)