IL NUOVO CIMENTO
VOL. 26 A, N. 2
21 Marzo 1975
A Study of the Hadronic Angular Distribution in (e+e -) Processes from 1.2 to 3.0 GeV ('). M. BERNAI~DINI~ D. BOLLINI, 1). L. BRUNINI~ E. FIORENTINO~ T. MASSAM, L. MONARI, F. PALMONARI, F. RIMONDI a n d A. ZICHICHI C E B N - Geneva Istituto Nazionale di .Fisica Nucleate - Sezione di .Bologna Istituto di .Fisiea dell' Universit~ - Bologna .~aboratori N a z i o n a l i del C N . E N - .Frascati (Roma)
(rieevuto il 9 Dicembre 1974)
Summary. - - The angular distribution of 2720 tracks of 1085 hadronic final states produced from (e+e-) annihilation has been studied in the 1.2 to 3.0 GeV total centre-of-mass energy range. If we parametrize the angular distribution in terms of ](0) = 1 + A cos ~0, where 0 is the angle between the hadronic track produced and the colliding-beam direction, the results show that A is less than 0.21, with 90% confidence.
W e h a v e studied t h e a n g u l a r distributions of t h e particles p r o d u c e d in t h e annihilation process (1)
e+e- --> hadrons
in the energy r a n g e { 1 . 2 - - 3 . 0 ) G e V , using t h e F r a s c a t i (e+e-) The following points, r e l e v a n t to t h e p r e s e n t paper, h a v e r e p o r t e d in published works a n d will not be r e p e a t e d here: i) t h e a p p a r a t u s (1), ii) t h e w a y in which a h a d r o n is identified in t h e reaction (1) (5).
storage ring. already been experimental final s t a t e of
(*) The results reported in this paper have been presented at the X V I I International Conference on High-Energy Physics, London, July 1974. (1) For details of the set-up see V. ALLxs-Bo~ELLI, M. BERNiRI~I~I, D. BOLLINI, P. L. BRUNINI, E. FIORENTINO, T. IV[ASSAM,L. ~/[ONARI, F. P~LI,MONARI, ]?. RIMONDI and A. ZICHICHI: P h y s . Left., 40 B, 433 (1972), and ref. (~.1~) quoted therein. (8) V. ALLES-BORELLI, M. BERNARDINI, D. BOLLINI, P. L. BRUNINI, E. FIORENTINO, 163
164
M. B]~RNARDINI, D. BOLLINI, P. L. BRUNINI, E. FIORENTINO, T. bfASSAM3 ETC.
The fast electronic trigger for t h e above annihilation process (1) required at least one charged particle on each of the two sides of the experimental set-up and was therefore of quasi-inclusive nature. More precisely~ the reaction invest i g a t e d was
(2)
e+e- -+ h~h~ ~- a n y t h i n g ,
where h ~ can be either a 7: or a K-meson, having kinetic energies E= > 130 Meu Ex > 223 MeV, respectively. A purely inclusive trigger, i.e. only one h ~ on one side of the apparatus, had too high a background r a t e to be useful. F o r this reason the above quasi-inclusive trigger (2) was adopted. I n order to s t u d y how far this trigger is from being ((inclusive ,), we have e x a m i n e d all the events to search for correlations between particle pairs~ either on t h e same side (for events with observed multiplicity greater t h a n two) or on opposite sides (for events with observed multiplicities equal to two). No sign of correlation was found. Figure I shows t h e results of the above analysis. Another check was p e r f o r m e d in the search for possible correlations. F o r example, if we assume t h a t t h e annihilation process goes via (3)
e+e- ~ R1 -[- R~, I__>2:~ ~ 2r:
where R1 and R2 are resonances decaying into a 7: pair, the angular correlation between t h e ~ pairs is related to the R1, R2 masses. As r e p o r t e d in Fig. 2, no a n o m a l y is observed in t h e density of the events along a n y particular mass value. Other correlations were studied~ i.e. angular correlations between tracks in t h e kinematic and h e a v y chambers. Again no correlation was observed; thus the quasi-inclusive trigger (2) produces no special effects. More precisely, the fact t h a t on one side we have an h ~ which produces a trigger does not give rise to a n y peculiar p h e n o m e n o n of correlation (either on the opposite side or on the same side). I t is therefore plausible to consider the angular distribution of the produced particles as being v e r y nearly t h e same as t h a t corresponding to a process of an inclusive nature. We define 04 to be
T. MASSAM, L. ~ONARI, F. PALMONARIand A. ZICItlCHI: Proeeedings o] the First EPS Con/erenee on Meson Besonanees and Related Electromagnetic Phenomena, Bologna, 1971, edited by R. H. DALITZ and A. ZlCttlCHI (Bologna, 1972). See also V. ALLEs-BoRELLI, ]~. BERNARDINI, D. BOLLINI, P . L. BRUI~INI, F~. FIORENTINO, T. ~/[ASSAM, L. !~r
F. laALbIONARt and A. ZICHICHI: Proceedings o] the International School ol Subnuclear Physics (~Ettore Majorana)); V I I I Course: Elementary Processes at High Energy, Erice, 1970, edited by A. ZICHICHI (New York, N. u 1971), p. 790; M. BERNARDINI, D. BOLLINI, P. L. BRUIWINI, E . FIORENTINO, T. I~IASSAM,L. MONARI, F. PALMONARI, F. RIMONDI and A. ZICmCHI: Phys. ~ett., 46 B, 261 (1973).
A STUDY
OF TI-rE HADI~ONIC A N G U L A R DISTRIBUTION
3oi "Events
IN (e+e -) PROCESSES
ETC.
]65
/ 2.5 ~
20.
10
20
40
60
80
100
Degrees
70- Events /
2.5 ~
60-
50-
b)
403O-
20,
10,
20
40
60
80
100
12.0 R~ Degrees
Fig. 1. - a) The distribution in opening angle a between tracks which enter the same side of t h e a p p a r a t u s , b) The distribution in acollinearity a n g l e / ~ between two t r a c k s which enter opposite sides of the a p p a r a t u s . The solid curves are obtained from a Monte Carlo calculation using events generated according to n-body phase space.
166
M. BERNARDII~I, D. BOLLINI, P. L. BRUNINI, E. FIORF, NTIN0~ T. MASS&M, ~TC.
1400 90C
800
700
600
500 ,~eV)
400
300 200
I
I
I
I
I
I
I
I
I
10~
20 ~
30 ~
40 ~
50 ~
60 o
70 ~
80 o
90~
,
CL
Fig. 2 . - The density of points for different data, using only the events with four tracks. which enter the apparatus on the same side of R 1 and R~ which would give a gacobian energy ~7a.
mass values, Rl~/?2, obtained from our :r is the angle between a pair of tracks of the beam. MR is the Common mass peak at opening angle ~ and for beam
the angle between the colliding b e a m axis and the particle produced h//
e--
e+
then, in the one-photon approximation, if there are superelementary constituents of the hadrons, and if the p h o t o n couples to these constituents, the expected angular distribution is ~(0) = 1 ~- A cos 2 0 with A = + 1 for spin-89 constituents, and A = -- I for spin-zero constituents.
~k S T U D Y
OF
THE
HADRONIC
ANGULAR
DISTRIBUTION
IN
(e+e-)
PROCESSES
167
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168
M. BERNABDINI, D. BOLLINI, P. L. BRUNINI~ E. FIORENTINO~ T. NIASSAM, F,TC.
I t should be noticed t h a t the effective value of A will be reduced b y the effect of the (~clothing process ~ of the constituents. I n order to improve statistics the d a t a have been grouped in energy intervals, r e p o r t e d in Table I, where the corresponding values of A are also reported. Figures 3a), b), c) show the observed angular distributions in the three energy intervals. The weighted m e a n of the d a t a gives A = 0.07-4-0.11~ t h a t is A < 0.21 at t h e 9 0 % confidence level. TABLE I.
Energy interval (GeV)
Weighted energy (GeV)
1.2--1.6
1.45
Integrated luminosity (nb -1)
Number of hadronie tracks
98.33
562
A
--0.24 • 0.26
1.7 --2.1
1.94
414.14
1373
-4-0.08 :J=0.18
2.4--3.0
2.75
439.76
785
-4-0.20 • 0.18
Thus in our energy range the possible effects of neither spin-89 nor spin-zero components of superelementary n a t u r e are seen to contribute to the (e+e -) annihilation into hadrons.
W e would like to acknowledge the collaboration of our technicians and scanning girls: Messrs. G. BACCHERI~qI~A. BORGHI~ L. DASIIELLO~F. FERIOLI~ M. ~ERRA~I~ V. LELLI~ F . MARTELLI~ F. MASSERA~ G. MOLINARI~ R. PILASTRISTI~ O. POLGROSSI~ V. l~usso~ G. SOLA; Mrs. L. MASELLI~ Mrs. F . ~ICOLI and Miss L. ZITELLI.
9
RIASSUNTO
]~ stata analizzata la distribuzione angolare di 2720 tracee appartenenti a 1085 eventi adroniei prodotti nell'anniehilazione (e+e-) nell'intervallo di energia totale nel eentro di massa da 1.2 a 3.0 GeV. Parametrizzando la distribuzione angolare con una funzione de1 tipo ](0) = 1-4- A cos~0, in eui 0 ~ l'angolo fra la traceia adroniea prodotta e la direzione dei fasei, i risultati mostrano ehe A ~ inferiore a 0.21 con un livello di eonfidenza del 90%.
Pe3IOM0 He noJIy~OHO.