Search for heavy mass particles in the sea level cosmic radiation

Volume 29B, number 4

PHYSICS LETTERS

12 May 1969

SEARCH FOR HEAVY MASS PARTICLES THE SEA LEVEL COSMIC RADIATION

IN

F. ASHTON, H . J . EDWARDS and G. N. K E L L Y Department of Physics, University of Durham, England Received 1 April 1969

The results of a search for low velocity (fl< 0.9) heavy mass particles in the sea level cosmic radiation are described. Two particles of charge e and rest mass (1.9 ± 0.5) GeV/c 2 and (2.5 • 0.5) GeV/c 2 have been detected at a low flux level. The most likely interpretation of these events is that they are deuterons.

T h e concept of the e x i s t e n c e of h e a v y m a s s q u a r k s as the c o n s t i t u e n t s of b a r y o n s and m e sons has m e t with c o n s i d e r a b l e s u c c e s s in the t h e o r y of e l e m e n t a r y p a r t i c l e s (see the r e c e n t r e v i e w by M o r p u r g o [ 1]). With one exception, e x p e r i m e n t s to s e a r c h for r e l a t i v i s t i c f r a c t i o n a l c h a r g e p a r t i c l e s [2] and heavy m a s s p a r t i c l e s i r r e s p e c t i v e of t h e i r c h a r g e [3-6] in c o s m i c r a y s have so f a r y i e l d e d n e g a t i v e r e s u l t s . The e x c e p tion is the e x p e r i m e n t of Dardo et al. [7] which was p e r f o r m e d at a depth of 70 m.w.e, u n d e r ground and f r o m which p r e l i m i n a r y r e s u l t s s u g g e s t the o c c u r r e n c e of r e l a t i v i s t i c p a r t i c l e s of m a s s in the r a n g e 10-15 nucleon m a s s e s with c h a r g e e at a r a t e of about 10-7 c m - 2 s e c - l s r -1. The e x p e r i m e n t d e s c r i b e d h e r e was d e s i g n e d to s e a r c h f o r low v e l o c i t y (/3 < 0.9) heavy m a s s p a r t i c l e s i r r e s p e c t i v e of t h e i r c h a r g e with a d e t e c t o r of much l a r g e r a p e r t u r e than had p r e v i o u s l y been used. A s c a l e d i a g r a m of the a p p a r a t u s is shown in fig. 1. The p r i n c i p l e u s e d was to s e l e c t low v e locity p a r t i c l e s (with the w a t e r ~ e r e n k o v c o u n t e r s ) which have a m i n i m u m r e s i d u a l r a n g e f r o m the point at which the v e l o c i t y s e l e c t i o n is m a d e [8]; in effect c r e a t i n g a s y s t e m c a p a b l e of m a s s d i s c r i m i n a t i o n . The e l e c t r o n i c s e l e c t i o n s y s t e m was A CII B CI C D E F and the r e s p e c t i v e d i s c r i m i n a t o r l e v e l s u s e d w e r e as follows: (A, 0.15E); (CI, 0.32E); (C, 0.15E); (D, 0.15E); (E, 0.15E) and (F, 0.10E) w h e r e E is the m o s t p r o b a b l e p u l s e height in a given c o u n t e r p r o duced by r e l a t i v i s t i c muons when s e l e c t e d by an A B C D E F c o i n c i d e n c e . With this a r r a n g e m e n t p a r t i c l e s of c h a r g e e m u s t have a m a s s > > 1.3 G e V / c 2 to t r i g g e r the s e l e c t i o n s y s t e m .

Th e a p p a r a t u s was o p e r a t e d f o r I 040 hr and the n u m b e r of events satisfying the s e l e c t i o n s y s t e m was 2 200. Of t h e s e , 1 810 w e r e angled t r a c k s t r a v e r s i n g the C to F r e g i o n of the t e l e scope and m i s s i n g A to CI; 359 w e r e weak e l e c t r o n - p h o t o n s h o w e r s and the r e m a i n i n g 31 w e r e m o r e c o m p l i c a t e d events r e q u i r i n g f u r t h e r a n a l y s i s . Only two e v e n t s showed a single t r a c k t r a v e r s i n g the whole t e l e s c o p e and the m a s s e s of t h e s e p a r t i c l e s have been d e t e r m i n e d f r o m t h e i r m e a s u r e d v e l o c i t i e s , d e r i v e d f r o m the s c i n t i l l a t o r and C e r e n k o v p u l se heights, and t h e i r known r e l a t i v e r e s i d u a l r a n g e s . In d e t e r m i n i n g the v e l o c i t y e s t i m a t e s f r o m the C e r e n k o v and s c i n t i l lation counter p u l s e heights, account was taken

A

r

c.

I

cl

]

C

r

u

~4

[

,

Steel ~

I F5 I

n

Steel

~

F~

--I ' I

o

E

t

2ocmT Front elevation

.

0--'-~¢ m Side elevation

Fig. 1. Apparatus: F1-F 7, Fa-F e flash tubes; A-F plastic scintillators; CI, CII water Cerenkov eounters. 249

Volume 29B, number 4

P H YS I C S LETTERS

12May 1969 -5

10

of t h e n o n - u n i f o r m i t y of r e s p o n s e of t h e c o u n ters. U s i n g t h e u s u a l e x p r e s s i o n f o r t h e r a t e of e n e r g y l o s s [9],

M UOnS

Io-6

dE dx -

z22CmeC2 /32

| [ 4 m 2 c 4 / 3 4 "~) t In \ ( l ~ ) ] I - 2fl2

=

= z 2 A (fl) it c a n b e shown that t h e r a n g e of a p a r t i c l e of r e s t m a s s Mc 2, c h a r g e z and v e l o c i t y fl i s g i v e n by R = M c 2 f ( f l ) / z 2 , w h e r e

1¢f~ Prot o n s

t

f(fl) = offl ~(1- 2) 3 A(/~)I"dfl •

ld a i

F o r a p a r t i c l e t r a v e r s i n g t h e t e l e s c o p e , its r e s i d u a l r a n g e can b e w r i t t e n a s R = R i + Ro, w h e r e R i i s t h e a m o u n t of a b s o r b e r b e t w e e n t h e p o i n t at w h i c h i t s v e l o c i t y fli is d e t e r m i n e d and t h e b o t t o m of t h e t e l e s c o p e , and R o i s t h e r e s i d u a l r a n g e of t h e p a r t i c l e on r e a c h i n g the b o t t o m of t h e t e l e s c o p e . In t h i s c a s e R i + R o = = M c 2 f ( f i i ) / z 2 and R i i s m e a s u r e d in e q u i v a l e n t n u m b e r of g / c m 2 of w a t e r . H e n c e a plot o f R i a g a i n s t f(fli) i s a s t r a i g h t l i n e of s l o p e M c 2 / z 2 and i n t e r c e p t - R o. T o a p p l y t h i s m e t h o d of m a s s d e t e r m i n a t i o n a v a l u e of z m u s t f i r s t b e a s s u m e d to e n a b l e an e s t i m a t e of fl to b e m a d e at e a c h c o u n t e r and the a b o v e p r o c e d u r e f o l l o w e d t h r o u g h to d e t e r m i n e Mc 2. With t h i s a p p r o a c h both h e a v y m a s s e v e n t s w e r e found to b e c o n s i s t e n t with c h a r g e e and not w i t h c h a r g e ~ e , 2e, -~e, etc. F r a c t i o n a l c h a r g e s a r e r u l e d out in both c a s e s by t h e l a r g e i n c o n sistency between the velocity estimates derived f r o m t h e s c i n t i l l a t o r s and t h o s e f r o m t h e C e r e n k o v c o u n t e r s u n d e r the a s s u m p t i o n of s u c h c h a r g e s . A l e a s t s q u a r e s fit to e a c h e v e n t g i v e s r e s t m a s s e s of (1.9 ± 0.5) G e V / c 2 and (2.5 ± 0.5) G e V / c 2 r e s p e c t i v e l y , s u g g e s t i n g that both particles are deuterons. A s s u m i n g t h a t t h e two p a r t i c l e s a r e d e u t e r o n s , t h e m o m e n t a with w h i c h t h e y e n t e r e d the d e t e c t o r w e r e (2.33 ± 0.04) G e V / c and (2.57 ± 0.08) G e V / c (the d i s c r i m i n a t o r s e t t i n g s u s e d on t h e c o u n t e r s p l a c e m o m e n t u m l i m i t s of 2.28 - 2.61 G e V / c on t h e m o m e n t u m r a n g e of d e u t e r o n s a c c e p t e d by t h e a p p a r a t u s ) . An a n g u l a r d i s t r i b u t i o n of t h e f o r m c o s 8 0 w o u l d b e e x pected for which the acceptance aperture is 1.25 × 103 c m 2 s t e r a d i a n , g i v i n g an i n t e n s i t y of d e t e c t e d d e u t e r o n s of (1.3 +1.7~× 10_12 c m _ 2 s e c _ 1 st_ 1 M e V / c _ l . -0.8" 250

L I

E u t~

c

0.1

1 Momentum

10

1 O0

(GeV/c)

Fig. 2. 1 - measured intensity of heavy mass particles; 2 - intensity corrected for loss due to inelastic i n t e r actions in the detector assuming both heavy mass p a r ticles to be deuterons. The measured sea level spectra of muons (12, 14), protons (11) and pions (15) are shown for comparison. A s s u m i n g t h e d e u t e r o n i n e l a s t i c c r o s s s e c t i o n to be 2awhere ~ is the nucleon absorption cross s e c t i o n on a n u c l e u s , t h e i n t e n s i t y of i n c i d e n t d e u t e r o n s in the v e r t i c a l d i r e c t i o n at s e a l e v e l i s +6.8 × 1 0 - 1 0 c m -2 s e c -1 st -1 M e V / c -1 at a (4.8_3.3) m e a n m o m e n t u m of 2.45 G e V / c . In d e r i v i n g t h i s r e s u l t v a l u e s of g f o r t h e d i f f e r e n t m a t e r i a l s in

Volume 29B, n u m b e r 4

PHYSICS

the telescope over the relevant momentum range w e r e t a k e n f r o m t h e w o r k of C h e n e t al. [10] a n d a c c o u n t w a s t a k e n of t h e v e l o c i t y s e l e c t i o n e f f i c i e n c y of t h e C e r e n k o v c o u n t e r s a s a f u n c t i o n of i n c i d e n t d e u t e r o n m o m e n t u m . The intensities are compared with the measu r e d s e a l e v e l s p e c t r a of m u o n s , p r o t o n s a n d p i o n s i n fig. 2. T h e

d/p r a t i o

i s (3.4 +4.8._2.3 ) % at

a m o m e n t u m of 2.45 G e V / c . No o t h e r m e a s u r e m e n t s of t h e d e u t e r o n i n t e n s i t y a t s e a l e v e l h a v e b e e n m a d e b u t a c c o r d i n g t o B a d a l i a n [12] t h e d/p r a t i o a t 3 200 m e t r e s a b o v e s e a l e v e l i s (8.6 + 1 . 0 ) % a t a m e a n m o m e n t u m of 1.3 GeV/c; w e w o u l d n o t e x p e c t a b i g v a r i a t i o n of t h i s r a t i o w i t h d e p t h i n t h e a t m o s p h e r e b u t a f a l l off w i t h increasing momentum would be expected. T h e p r e s e n t r e s u l t s c a n b e u s e d to s e t u p p e r l i m i t s on t h e i n t e n s i t y of q u a r k s in t h e s e a l e v e l radiation. The velocity range over which quarks w o u l d h a v e b e e n d e t e c t e d in t h e p r e s e n t e x p e r i m e n t a s a f u n c t i o n of t h e q u a r k m a s s i s s h o w n in f i g . 3 f o r c h a r g e s e, ] e a n d ~e. A s s u m i n g t h a t both particles detected are deuterons the upper l i m i t t o t h e i n t e n s i t y of q u a r k s a t t h e 90% c o n f i d e n c e l e v e l i s 4.9 × 1 0 - 1 0 c m - 2 s e c -1 s t -1 in t h e v e l o c i t y b a n d s s h o w n in fig. 3. No c o r r e c t i o n h a s b e e n a p p l i e d f o r i n t e r a c t i o n s i n t h e t e l e s c o p e in d e r i v i n g t h i s l i m i t . H o w e v e r , if t h e q u a r k - n u cleon cross section is the same as the nucleonnucleon cross section and the most pessimistic assumption made that all interacting quarks are lost then the limit should be raised by a factor 17, t h e t o t a l m a t e r i a l i n t h e t e l e s c o p e c o r r e 1.o .

~_ 0.8

.

.

.

.

~

.

............

.

-~/

"2" 0.6 ~o.z 0.2 O0

,

01

,

,

,

,,,,i

,

,

,

,

I Mass (GeVJc2)

,,,,I

.

10

.

"50

Fig. 3. Velocity acceptance l i m i t s as a function of m a s s for single and fractional c h a r g e s . The lower limit is d e t e r m i n e d by the m i n i m u m velocity to p r o duce a coincidence and the upper limit by the velocity cut off due to the ~ e r e n k o v counters. F o r ~e p a r t i c l e s the upper l i m i t is set by the s c i n t i l l a t o r d i s c r i m i n a t i o n levels.

LETTERS

12 May 1969

s p o n d i n g to 2.82 n u c l e o n i n t e r a c t i o n l e n g t h s . Our limits are not necessarily inconsistent w i t h t h e o b s e r v a t i o n s of D a r d o et al. [7]. T h e s e workers attributed the particles which arrived, d e l a y e d w i t h r e s p e c t t o m u o n s , a t a d e p t h of 70 m . w . e , u n d e r g r o u n d a s p a r t i c l e s of r e s t m a s s 10-15 nucleon masses which have an interaction l e n g t h of 2 to 3 p i o n i n t e r a c t i o n l e n g t h s a n d l o s e o n l y a b o u t 5% of t h e i r e n e r g y i n a n u c l e o n c o l l i sion. The long interaction length and low inelast i c i t y m e a n s t h a t a n e g l i g i b l e f r a c t i o n of s u c h p a r t i c l e s p r o d u c e d in h i g h e n e r g y n u c l e o n - n u cleon collisions would be nonrelativistic at sea level and therefore not detectable in the present work. T h e a u t h o r s a r e g r a t e f u l to P r o f e s s o r G . D . R o c h e s t e r , P r o f e s s o r A. W. W o l f e n d a l e , D r . G. H. S t a f f o r d a n d D r . J . J . T h r e s h e r f o r t h e i r h e l p a t v a r i o u s s t a g e s of t h e w o r k . T h e y a l s o wish to thank the Science Research Council for t h e a w a r d of a g r a n t a n d t h e p r o v i s i o n of R e search Studentships.

References 1. G. Morpurgo, Proc. 14th Intern. Conf. on High e n ergy physics (CERN, 1968) p. 225. 2. F. Ashton, R.B. Coats, G.N. Kelly, D. A. Simpson, N. I. Smith and T. Takahashi, J. Phys. A1 (1968) 569. 3. L.W. J o n e s , D. E. Lyon J r . , P. V. Ramana Murthy, G. De M e e s t e r , R.W. Hartung, S. Mikamo, D.D. Reeder, A. Subramanian, B. Cork, B. Dayton, A. Benvenuti, E. Marquit, P. D. Kearney, A. E. B u s sian, F. Mills, C. R a d m e r and W. R. Winter, Phys. Rev. 164 (1967) 1584. 4. J. Bjornboe, G. Damgard, K. Hansen, B.K. C h a t t e r j e e , P. G r i e d e r , A. Korning, E. Lillethun and B. P e t e r s , Nuovo Cimento 53B (1968) 241. 5. H. Kasha and R. Stefanski, Phys. Rev. 172 (1968) 1297. 6. P. Franzini and S. Shulman, Phys. Rev. L e t t e r s 21 (1968) 1013. 7. M. Dardo, P. Penengo and K. Sitte, Nuovo Cimento 58A (1968) 59. 8. F. Ashton, Proc. Intern. Conf. on Cosmic rays (London, 1965) p. 1108. 9. B. Rossi, High energy particles (Prentice Hall, 1952). 10. F. F. Chen, C. P. Leavitt and A. M. Shapiro, Phys. Rev. 99 (1955) 857. 11. G. Brooke and A.W.Wolfendale, Proc. Phys. Soc. 83 (1964) 843. 12. G. V. Badalian, Soviet Physics 8 (1959) 209. 13. M. Gardener, D.G. Jones, F. E. Taylor and A.W. Wolfendale, Proc. Phys. Soc. 80 (1962) 697. 14. P.J. Hayman and A. W. Wolfendale, Proe. Phys. Soc. 80 (1962) 710. 15. G. Brooke, M. A. Meyer and A. W. Wolfendale, Proc. Phys. Soc. 83 (1964) 871. 251