LETTERE AL NUOVO CIMENTO VOL. 10, N. 10 6 Luglio 1974

M u h i h a d r o n P r o d u c t i o n T h r o u g h the P h o t o n - P h o t o n I n t erac t i on .

L . PAOLUZI

I s t i t u t o di F i s i c a dell ' U n i v e r s i t ~ - R o m a

I s t i t u t o N a z i o n a l e di .Fisica N u c l e a t e - Se z i one di R o m a

F. CERADINI , ]V[. L. FERRER and R. SANTONICO

I s t i t u t o di F i s i c a dell ' U n i v e r s i t 5 - R o m a

Cw. BARBIELLINI, S. ORITO and T. TSURU

L a b o r a t o r i N a z i o n a l i d i .Frascat i del C N E N - F r a s c a t i ( R o m a )

(ricevuto il 26 Marzo 1974)

The reaction e+e--~e+e-X, where X is a hadronic system, provides a powerful method of studying the coupling of the C = + 1 mesonic states with two photons. In particular, if the ha(ironic system is a resonance R of mass M and spin J , its decay width in two gammas Fa~wy can be obtained by measuring the cross-section ~ of the process c+c--+e+e-R that, in the equivalent-photon approximation (1-3), is given by the equation

8 ( 2 J + 1) (1) ~ _ M 3 ~2 rR__~y ~/(E) .

In this relationship a is the fine-structure constant and the function ] (E) takes into account the vir tual-photon energy spectrum at the energy E of the colliding beams.

In this paper we study the interaction yy~mul t i had rons through the process

e+e --~ e+e - mult ihadrons.

The experiment was performed with Adone, the Frascati e +, e- storage ring. The experi- mental set-up is shown schematically in Fig. 1. I t consists of two tagging systems (4)

(t) N. A. ROMERO, A. JACCARINI, P. KESSLER and J . 1)ARISI: Compt. Rend., 296 B, 153 (1969); Left. Nuovo Cimento, 4, 933 (1970); 1, 936 (1971). (2) S . J . BRODSKY, T. KINOSHItTA bad t t . TERAZAWA: Phys. Rev. Left . , 25, 972 (1970); Phys. Rev. D, 4, 1532 (1971). (~) M. GrtECO: Naovo Cimento, 4, 689 (1971); A. BR&MO~ and M. GRECO: Left. Nuovo Cimento, 2, 522 (1971). (4) G. BARBIELLIN'I and S. ORITO.* F r a s c a t i LNF-71/17 (1971); Proceedings o/ the Firs t Conference on Meson Resonances and Related Electromagnetic Phenomena (Bologna, 1971).

435

~ L. PAOLUZI, F. CERADINI , M. L. F E R R E R , n . SANTONICO, G. B A R B I E L L I N I , ETC.

T+ and T_ used to detect the outgoing forward emitted e + and e- and of two wide-angle telescopes, forming the (~ main apparatus ~) (5), used here to detect the particles produced in the photon-photon collisions. T+(_ I consists of two plastic scintillators whose gee-

Fig. 1.

metrical acceptance is approximately 50% for the scattered e �9 with momentum in the interval (0.2--0.85)E. The tagging technique adopted(4) utilizes the machine bending magnets as momentum analysers, correlating the momentum Pe of the detected e +(-) with the impact point in T+(_). A typical momentum resolution is Ape ~ 60 MeV.

The relevant part of the wide-angle detector (5), as shown in Fig. 2, consists essen- tially of two counters and optical chamber telescopes covering a solid angle of about 0.25 • 4~r. In addition six scintillation counters, three above (Ai) and three below the vacuum chamber (Bi), increase the solid angle covered to about 0.5•

Finally two shower detectors (6) S are used to veto the real photons from beam- beam and beam-gas brcmsstrahlung.

Runs were carried out at beam energies E : 1.3, 1.4 and 1.5 GeV for a total integrated luminosity fLdt= 350nb -1. The chambers were triggered by a master coincidence requiring:

1) a 5-fold coincidence among the counters of the main apparatus, which implied a minimum penetration of the equivalent of 10 g c m -2 of iron in one telescope and 22 g e m -2 in the other;

2) coincident pulses in both tagging systems, T+ and T_;

3) t ime coincidence with the instant of beam-beam collision.

The candidate events for the process e+e--+c+e-mult ihadrons arc required, in addition, to fulfil the following criteria:

4) at least two tracks, one in each telescope, converging within -4-3 cm at a common origin in the e+e - interaction region, must be found in the thin-foil (( kine- matical ~) spark chambers C~;

5) for two-track events the noncoplanarity angle A~, defined as the angle between two planes each containing the beam and one of the tracks, must be A~o ~ 8 ~

(a) F . CERADINI, M. COXgVERSI, S. D'A~OELO, L. PAOLUZI, R . SATqTO~ICO a n d t~. VISENTIN: Phys. Lell., (7 B, 8O (1973). 4') H.C. DEm~E, M. PREGnR, S. TAZZXR~ and G. VIG]qOLA: Fraseati report LNF-72/75 (1972).

MULTIHADRON PRODUCTION THROUGH THE PHOTON-PHOTON INTER&CTION 4 3 7

Only two events passed the criteria 1)-5), one with M = 800 MeV and the other with M = 1400 MeV, where M is the two-interact ing-photon equivalent mass, which is measured by the tagging system with a typical accuracy A M = 4- 90 McV.

f l a s h - t u b e chamber plastic sc in t i l l a tors

c= I s, 1~2 I% I% F%

/

1111

,4;

i :

8;

% I c, IPb IPb

spnrk chambers (~bsorbers

Fig. 2.

The event with M = 800 MeV exhibits two tracks with Ar = 30 ~ and a pulse from one of the three counters A i. Thc cvent with M = 1400 MeV shows two tracks with A~ = 24 ~

The background simulating these events is negligible for the following reasons:

a) The events due to beam-gas interaction give signals in at most one tagging counter. These events are rejected by thc condition 2). Events due to accidental double tagging give a negligible background, as estimated from the rate of the tagging counters. No event satisfying requirements 1)-5) has been found during the runs with sepa- rated beams.

4 3 ~ L. PAOLUZI, F. CERADINI , M. L. F E R R E R , R. SANTONICO, G. B A R B I E L L I N I , ETC.

b) No background is expec ted f rom the processes ~?(->two bodies, such as ~,~(-+~+~- or r:+r:- or e+e -, because the double t agg ing at forward angle implies A ~ < 5 ~ for t w o - b o d y events (7).

Therefore , the two selected events p rov ide evidencc for the process u mul t ihadrons . The even t wi th the equ iva l en t mass of the hadron ic system M = 800 MeV is com-

pat ib le wi th in the u n c e r t a i n t y in the mass de te rmina t ion wi th the hypothes is t ha t i t is or ig inated from the process

(2) e+e - -+ e+c-~ ' .

This even t cannot be a t t r i bu t ed to any o ther known resonances, because these have e i ther a much smal ler wide-angle de tec t ion efficiency or a mass va lue far away f rom the de tec ted one. The con t r ibu t ion of the nonresonant p roduc t ion cannot be exc luded because of the unce r t a in ty in the va lue of the cross-section.

The de tec t ion efficiency for the process e+e--+e+e-~ ' has been calcula ted by a Monte Carlo s imula t ion of the expe r imen t based on the equ iva len t -pho ton approxi- mat ion . The va l id i ty of this app rox ima t ion has been checked in a prev ious experi- ment (7) in which the processes e+c--> e+e-e+c- and e+e--> e+e-[z+~z - were inves t iga ted .

I n Table I are shown the ca lcula ted expe r imen ta l efficiencies for the appara tus to de tec t the re levant decay modes of the ~'.

The to ta l efficiency of the expe r imen ta l appara tus to detec t the ~' is e = y~ B~ e~= = (0.53 • 0.03)%. The ca lcula ted va lue of the cross-section of the process e+e--+ e+e-~ ', p roper ly averaged over the different energies invo lved in the exper iment , is a = 4.8.10 -35. "/'~'-~vv cm2 (keV) -1 (3). I f the mu l t i had ron even t wi th M = (800 • 90) MeV is inter-

+15 pre ted as an ~' p roduc t ion , the yy decay wid th of the ~' is F~,_+vv = (11_ s) keV; if the even t is coming f rom a nonresonant cont r ibu t ion of yy-+ m~, the upper l imit of F~,~vv is 33 keV wi th 95% confidence level.

T A B L E I .

~)' decay channels (i) Branch ing rat io Bi Efficiency e,

x+TT- 5, 26% (1.45 • 0.07)%

nT~ -+ 2~+27~-+neutral 14% (0.89 -r 0.06) %

nn~) -+ r:+r:- + neut ra l 41% (0.06 -4- 0.01 ) %

o r

The to ta l w id th obta ined th rough the known branching rat io (s)is, respect ively ,

r~ .. , , = (0 6~ :~ /MeV

Fn,~a n < 1.8 M e V .

The va lue of F~, is in agreement wi th the recent exper imenta l de te rmina t ion (g) and wi th theore t i ca l es t imate (lo). The analysis of the even t wi th M = 1400 MeV could

( ') (~. BARBIELLINI, S. ORITO, T. TSURIY, R . VISENTIN, F. CERADIN'I, 1~1. CONVERSI, S. D'ANGELO, iVY. L. FERRER, L. PAOLUZI a n d R . SANTONICO: Phys. Rev. Lett., 32, 385 (1974). (8) PARTICLE DATA GROUP: Rev. Mod. Phys., 45, No. 2, P a r t I I (1973) for r e fe rences to t h e o r ig ina l p a p e r s . (0) D. IVY. BINNIE, L. CAMILLERI, ~k. DAIfNE, D. A. GARBIYTT, J . 1~. ~tOL$IES, W. G. JONES, J . KEY'NE, 1Vi. LEWIS, I . SIOTIS, P . N. UPADHYAY, I . F. BURTON a n d J . G. MCEWEN: Phys. Left., 39 B, 275 (1972); A. DUANE, D. 2r BINNIE, L. CAMILLERI, J . CARR, N. C. DEBENHAM, D. X. GARBUTT, W . G. JONES, J . KEXrNE, I . SIOTIS and. J . C. 1VICEWEN': Phys. Rev. Lett., 32, 425 (1974). F o r o t h e r v a l u e s see ref . (~). (10) A. BRA~tO~r a n d M. GRECO: Phys. Left., 48 B, 137 (1974).

I~IULTIHA.DRON PRODUCTION THROUGH THE P H O T O N - P H O T O N I N T E R A C T I O N 4 3 9

be carried out in a similar way. However, the interpretat ion of this event as being due to a high-mass resonance decay is made more complicated by the large number of resonances that exist in a mass interval of i 90 MeV around M = 1400 MeV.

We arc grateful to Prof. M. CONVERSI and Drs. S. D'ANGELO and R. VISENTIN for their contributions to the present work. Drs. A. BR~_MON and M. GRECO have contributed many clarifying discussions on the theoretical implications of this research. We acknow- ledge the co-operation of the machine group and especially of Dr. M. PLACIDI. Finally, we want to thank Prof. M. CONVERSI and Dr. C. SPENCER for their critical reading of the manuscript.