Volume 29B, number 1 P H Y S I C S L E T T E R S 31 March 1969

P H O T O P R O D U C T I O N O F K ° - M E S O N S

M. G. ALBROW, D. ASTON, D. P. BARBER, L. BIRD, R . J . ELLISON, C. HALLIWELL, A. E. HARCKHAM, F .K . LOEBINGER, P . G . MURPHY, J . WALTERS, A . J . W Y N R O E

Department of Physics, The University, Manchester, UK

and

R. F. TEMPLEMAN Daresbury Nuclear Physics Laboratory, Daresbury, Warrington, Lancashire, UK

Received 27 January 1969

The photoproduction of K~-mesons from protons has been studied at an angle of 3½ ° and for photon energies up to 5 GeV. The cross-section for production of K~-mesons with momenta above 1.5 GeV/c rises to 14 /2b/sr/equivalent quantum at a bremsstrahlung peak energy of 5 GeV.

The photoproduction of K ° - m e s o n s has been studied in a b r e m s s t r a h l u n g beam at NINA, the 5 GeV e lec t ron synchrot ron at the Daresbury Nuclear Phys ics Labora tory . The momentum spec t ra of K ° - m e s o n s have been studied at angles of 9 ° and 3.5 ° to the b r e m s s t r a h l u n g for photon energ ies up to 5 GeV and for a va r i e ty of t a rge t ma t e r i a l s . This l e t t e r r epo r t s the spec - t r a obse rved at 3.5 °, using a liquid hydrogen ta rge t . The number of h igh-momentum KV-me - sons, produced by the reac t ion ~ + p -~ K ° + ~,+, was l e s s than p red ic ted by Dre l l and Jacob [1], but substant ial production was obse rved at lower momenta .

The b r e m s s t r a h l u n g beam was produced f rom an in terna l t a rge t of th ickness 0.1 radiat ion length; i t passed through two sweeping magnets and a co l l ima to r . Af ter emerg ing f rom a hole in the shielding wall the beam passed through the l iquid hydrogen ta rge t 60 cm in length. Down- s t r e a m f rom the t a rge t it was absorbed in a quantameter which was used to m e a s u r e the total energy of the b remss t r ah lung . Neutra l pa r t i c l e s produced at an angle of 3.5 ° passed through 20.3 cm of lead, which s e r v e d to absorb most of the e lec t rons and photons emerg ing f rom the ta rge t ; a l a rge magnet swept any low energy charged p a r t i c l e s out of this neut ra l beam. The detect ion appara tus (fig. l) was placed at a dis tance of 41 m e t r e s f rom the hydrogen ta rge t . The neut ra l beam f i r s t passed through an i ron r e g e n e r a t o r R 15 cm thick; K ~ - m e s o n s in the beam w e r e par t ia l ly conver ted to K~-mesons in this m a t e r i - al. The K~-mesons w e r e r e q u i r e d to pass through

a l a rge sc int i l la t ion counter A before decaying into charged pa r t i c l e s (the branching ra t io for decay into ~+ + ~- is 68% [2]). The d i rec t ions of

I k \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ Y R , A

~ S 3

i J s 4

B I IS5 C

Fig. 1. Layout of the detection system (not to scale). R is the iron regenerator, A, H, B and C are scintillation

counters. S 1 to S 5 are wire spark chambers.

54

Volume29B, number 1 PHYSICS L E T T E R S 31 March 1969

the charged pa r t i c l e s were de te rmined by an a r - ray of wire spark chamber planes placed between 40 and 110 cm f rom the downst ream face of the r egene ra to r . The a r r a y consis ted of two groups of th ree spark chamber p lanes (S 1 and $2) , the s ize of each being 120 cm by 70 cm, with wires at a spacing of 1.1 mm. These s ix spark cham- be r s were followed by a hodoscope of twelve sc in t i l la t ion counters H i . The charged par t i c les then entered the gap of a magnet with poles 203 cm (across the beam) by 102 cm and a gap of 71 cm. After the magnet were six fur ther spark chamber planes , ($3, $4, S 5) followed by two large sc in t i l la t ion counters (B, C) in tandem. The spark chambers were t r iggered where two of the hodo- scope counters gave pulses in coincidence with the two r e a r counters , no charged par t i c le hav- ing passed through the f ront counter . The data f rom the chambers were s tored t empora r i l y in a magnet ic core m e m o r y before being t r a n s f e r r e d to a PDP8 computer and s tored on magnet ic tape. Selected events were displayed on an osci l loscope to check the pe r fo rmance of the appara tus .

Events were se lec ted which had at leas t two good t racks in the spark chamber ups t r eam of the magnet and at l eas t one good t rack af ter the magnet . Fu r the r ana lys i s was n e c e s s a r y to e l iminate (i) unco r re l a t ed pa i r s of t r acks , (ii)

pa i r s , (iii) th ree-body decays of K~ or e lec t ron K~. Uncor re la ted t r acks were e l iminated by r e - qui r ing that a pai r of i n t e r sec t ing t r acks should pass sufficiently close to eleven sparks in the front six planes for the sum of the squares of the deviat ions to be l e s s than 20 mm z (i.e., one spark was allowed to be miss ing) . Elec t ron pa i r s were e l iminated by re jec t ing all t r acks making angles less than 1.4 ° with the beam di - rec t ion, and by r equ i r ing that the opening angle of the vee be g rea te r than 5.5 °. To ensure that only ~+Tr- pa i r s f rom regene ra t ed K~ decays were accepted, the p roper t i e s of coherent r e g e n - erat ion were used. A coherent ly regenera ted K~ mus t continue in exactly the same di rec t ion as the paren t K~; when it decays in the two-body mode the plane defined by the decay products mus t contain the K~ di rec t ion and there fore it mus t also contain the K~. di rect ion. An event in which the two t racks a r e coplanar with the beam direc t ion is most l ikely to be a two-body decay of a coherent ly r egenera ted K~; s ince ~+~- is the only charged two-body mode, the t r acks mus t be caused by y -mesons .

The dis t r ibut ion of the cosine of the angle 0 c between the beam direc t ion and the plane of the vee for the events obtained in this exper iment is shown in fig. 2a. Events were selected for which

6C

4C

2C

1 r , ,

(a)

i ! ! i

• 99998 ,99996 c o s e c

4 0 i ,

L_ C ' J ~ 4 6 0 5 0 0 5 4 0

MK(MeWc2)

Iodr ' (c)

I 0

5

o ,o Is .II

T( IO s¢¢)

Fig. 2a. Distribution of angles between beam direc- tion and plane of decay products. b. Invariant mass distribution, assuming the two par- ticles to be pions. c. Distribution of lifetimes in rest system.

COS 0 c >/ 0.999 99. Monte Carlo calculat ions - in which al lowance was made for all known inaccu- r ac i e s - showed that this cut e l iminated all but a few per cent of three-body decays, even though these a r e concent ra ted near the beam direct ion. The pos t -magne t spark chambers allowed a m e a s u r e m e n t of the momentum of one of the de- cay products . The invar ian t mass of the pai r of charged par t i c les was calculated f rom this one momentum and the angles of the t racks , making the assumpt ion that they were charged pions f rom the two-body decay of a beam par t ic le . The d is t r ibut ion of calculated m a s s - v a l u e s is shown in fig. 2b. Events were accepted as coherent ly r egenera ted K~-mesons if the invar i an t mass lay between 475 MeV/c 2 and 520 MeV/c 2. As a check the d is t r ibut ion of the l ives of the K~- mesons (each calculated in i t s r e s t f rame) was examined. This is shown in fig. 2c; it i s c lear ly cons is ten t with the known mean life of 0.862 × × 10 -10 sec [2].

Data were taken for b r e m s s t r a h l u n g energy max ima of 3.5, 4.0, 4.5 and 5.0 GeV. The mo- mentum spec t ra of the detected K ° ' s a re s t rong- ly d i s tor ted f rom the spec t ra at the hydrogen ta rge t because of the following m o m e n t u m - d e - pendent cor rec t ion factors: (i) decay in flight, (ii) the combined probabi l i ty of coherent r egene - ra t ion and surv iva l of the K~ (we have used the

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Volume 29B, number 1 P H Y S I C S L E T T E R S 31March 1969

da ta of B~Jhm et a l . [3] f o r r e g e n e r a t i o n ) , (iii) the p r o b a b i l i t y of decay of the K~ within a f i d u c i - a l r e g i o n , (iv) the p robab i l i t y of the decay p r o d - u c t s p a s s i n g th rough the n e c e s s a r y s p a r k c h a m - b e r s and c o u n t e r s . The p r o d u c t of t h e s e f a c t o r s , t he " a c c e p t a n c e " , was c a l c u l a t e d by a Monte C a r l o method . The a c c e p t a n c e f o r PK < 1.5 GeV/c was so low, and h e n c e the n u m b e r of e v e n t s d e - t e c t e d was so s m a l l , that no e s t i m a t e can be m a d e of the c o r r e c t e d y i e l d in th i s r eg ion . Al l s u b s e q u e n t d i s c u s s i o n wi l l be conf ined to PK > 1.5 G e V / c , a l though i t i s wor th no t ing that the c o r - r e c t e d s p e c t r a a l l a p p e a r to be r i s i n g a s PK d e - c r e a s e s , even at the l o w e s t m o m e n t a . F u r t h e r c o r r e c t i o n s , a s s u m e d to be independen t of K ° m o m e n t u m , w e r e iv) a b s o r p t i o n in the lead; da ta quo ted by S c h i v e l l [4] l ead to an e s t i m a t e of 21% fo r the p r o p o r t i o n of K ~ - m e s o n s which p e n e t r a t e s the l ead , (vi) the e f f i c i ency of the s p a r k c h a m b e r s y s t e m ; r e q u i r i n g e l e v e n s p a r k s in the f ron t s ix p l a n e s t o g e t h e r with one w e l l - de f ined t r a c k a f t e r the m a g n e t gave an o v e r a l l e f f i c i e n c y be tween 75% and 80%, (vii) the r e - q u i r e m e n t s on t r a c k a n g l e s , c o p l a n a r i t y , i n v a r i - ant m a s s and i n t e r s e c t i n g t r a c k s , w e r e e s t i - m a t e d to h a v e l o s t be tween 9% and 15% of the good even t s , (v i i i ) a 570 l o s s due to p ion decay o r s c a t t e r i n g , (ix) 29 + 670 of the even t s w e r e e s t i - m a t e d to h a v e been p r o d u c e d in the w a l l s of the t a r g e t v e s s e l .

T h e da ta w e r e c o r r e c t e d f o r a l l the e f f ec t s (i) to (ix). The y i e l d of K ~ - m e s o n s , fo r m o m e n t a be tween 1.5 G e V / c and the m a x i m u m , i s p lo t t ed in f ig. 8a as a func t ion of the m a x i m u m e n e r g y of the b r e m s s t r a h l u n g . The t h r e s h o l d fo r p r o - duct ion of K - m e s o n s with 1.5 G e V / c is at a pho- ton e n e r g y of 1.84 GeV. The da ta fo r E m a x = 3.5 GeV and E m a x = 4 GeV h a v e been combined , as h a v e t hose f o r 4.5 GeV and 5.0 GeV. The K~, s p e c t r a fo r t h e s e c o m b i n e d data (denoted "3.75 GeV" and "4.75 GeV" , r e s p e c t i v e l y ) a r e shown in f ig. 3b. S u b t r a c t i o n of the 3.75 GeV data f r o m the 4.75 GeV data g ive s the c r o s s - s ec t i on p e r photon fo r photons in the e n e r g y r a n g e 4.25 + 0.5 GeV; th is s p e c t r u m i s shown in f ig. 3c.

F o r p roduc t ion at a f ixed ang le by photons of a g iven e n e r g y , t he p r o c e s s p roduc ing K ° - m e - sons wi th the h ighes t m o m e n t u m i s ~ + p ~ K ° +

n - ÷ + ~, . At 3.5 ° , and fo r E 7 "~ 8 GeV, th is p r o c e s s g i v e s PK(3.5 °) = p~ - 0.36 G e V / c . The two-body p r o c e s s g iv ing the next h i g h e s t m o m e n t u m i s

+ p ~ K ° + Y*(1385), fo r which the K - m e s o n m o m e n t u m i s pv - 0.65 G e V / c . Thus , any even t wi th PK be tween the m a x i m u m p o s s i b l e va lue and 0.29 G e V / c be low tha t m u s t be f r o m

56

2 0

IO

i ! i i i

(o1

(PK>J'SGeVkl t (p blst/e,q~)

E t I ~1 I |

0 I 2 3 4 S Emax(GCY)

20

I0

0 5

! w u !

T ") 'd~dPK t o 4 - 7 5 GcV

( Fb/st]GeV/c/¢.qj 1 • 3.7 5 GeV

li • I~ i • ~ ! a

I 2 3 4 PK (GtV/c)

30

2 0

IO

!

dndp K ~ b/st/GWlcl r °

::x"

I 0 I S

t ; 0 a i i

2 3 4 pKtGeVld

Fig. 3. a. Yield of K~. in microbarns per steradian per equivalent quantum, as a function of bremsstrahlung maximum energy. For each value of Ema x the K- spectrum is integrated f rom 1.5 GeV/c to the maximum. b. K~ spectra for E m a x = 3.5 GeV and 4.0 GeV com- bined("3.75 GeV") and for Ema x = 4.5 GeV and 5.0 GeV combined ("4.75 GeV') . The curve labelled ~ is the contribution f rom ~ + p ~ ~b + o 0 p, ~b ---* K L + KS, cal - culated for Ema x = 5 GeV. c. K~ spectrum in microbarns per steradian per GeV/c (per photon) for photons ~ith Ey = 4.25 ± 0.5 GeV. In- set is a diagram for the process discussed by Drell

and Jacob [1].

Volume 29B, number 1 PHYSIC S

+ p ~ K ° + ~.+ (or p o s s i b l y ~ + p ~ K ° + A +~r+). Thus, by examinat ion of the high energy par t of the spec t rum, we can p lace an upper l imi t on the c r o s s - s e c t i o n for this p r o c e s s . In this way we find upper l i m i t s (cor responding to one ob- s e r v e d event in each case) of 1.6 ~ b / s r at E~ = = 3.4 GeV; 0.7 ~ b / s r at 3.8 GeV; 2.3 ~ b / s r at 4.4 GeV and 1.7 ~ b / s r at 4.7 GeV. Dre l l and Jacob [1] sugges ted that this c r o s s - s e c t i o n might have a substant ia l contr ibut ion f r o m K* exchange through the d i ag ram shown in fig. 3c. We have ca lcu la ted the c r o s s - s e c t i o n f rom this d iagram using the f i na l - s t a t e absorpt ion model exactly as did Dre l l and Jacob, except that we have used the SU(6) value for the K ' K ? coupling [5], c o r - responding to F(K *° -~ K ° + ~) = 0.23 MeV. At F- 7 = 4 GeV this g ives a predic t ion of 4.5 p b / s r for ~ + p -~ K ° + Z+, to be compared with the ob- s e r v e d upper l imi t of 0.7 p b / s r at 3.8 GeV. A poss ib le explanation of this d i sc repancy is that the K*Kv coupling may be much weaker than the SU(6) value. Brown et al. [5] have es t ima ted the effects of m a s s - s p l i t t i n g s on vec to r meson de- cays; they p red ic t substant ia l co r r ec t i ons , g iv - ing F(K *° -* K ° + ~) = 0.028 MeV or 0.0055 MeV, e i ther of which a g r e e s with our r e su l t s .

The r e m a i n d e r of the spec t rum must be due to m o r e compl ica ted p r o c e s s e s . One poss ibi l i ty i s via the product ion of qS-mesons/~ 7 K~+P~.-~ @ + p' fol lowed by the decay mode ~b -* K~ + The d i f ferent ia l c r o s s - s e c t i o n and angular d i s t r ibu- tion is known f rom the r e s u l t s of the DESY bubble chamber col labora t ion [6]. Assuming that the @ re ta ins the t r a n s v e r s e po la r iza t ion of the photon, the y ie ld of K ~ - m e s o n s f r o m this p r o c e s s can be

L E T T E R S 31 March 1969

calcula ted. It i s shown in fig. 3b for Ema x = 5 GeV as a curve label led ~b; it explains only a sma l l f rac t ion of our data (on account of the v e r y sma l l c r o s s - s e c t i o n for ~b-production; ~tot = = 0 . 4 ~ b ) .

We conclude that mos t of the K ~ - m e s o n s ob- s e r v e d at 3½ ° a r e produced in assoc ia t ion with heavy Y* s ta tes or f rom the decay of K* s ta tes . The l a rge c r o s s - s e c t i o n might be explained if a K* has a l a rge anomalous magnet ic moment .

We would l ike to thank the staff at Da re sbu ry for the i r ene rge t i c support and cooperat ion, pa r t i cu l a r ly Dr. N. R. S. Tai t , who provided the Quantamete r , and a lso the technica l staff at Manches te r Univers i ty . We a r e gra tefu l to Ivan Mauri tson for invaluable a s s i s t a n c e with the PDP8 sys tem.

References 1. S.Drell and M.Jacob, Phys. Rev. 138 (1965) B1312. 2. N. Barash-Schmidt, A. Barbaro-Gattieri, L.R.

Price, M. Roos, A.H. Rosenfeld, P. Soding, C.G. Wohl, Lawrence Radiation Laboratory report UCRL 8030 (Aug. 1968} (unpublished).

3. A.B~hm, P.Darriulat. C.Grosso, V.Kaftanov. K. Kleinknecht, H.L. Lynch, C.Rubbia, H.Ticho and K. Tittel, Phys. Letters 27B (1968) 594.

4. J .F.Schivell , E.Engels Jr . , A.Entis, J .M.Pa t e r - son, L.N. Hand and A. Sadoff, Phys. Rev. Letters 19 (1967) 1349; J. F. Schivell, Thesis, Cornell University (1968) (unpublished).

5. L.M. Brown, H. Munezek and P. Singer. Phys. Rev. Letters 21 (1968) 707.

6. Deutsches Elektronen-Synchrotron report DESY 68/8 (1968).

57