Measurement of large transverse momentum positive particles produced at medium angles at √s = 52.5 GeV

Volume 55B, number 3

PHYSICS LETTERS

17 February 1975

MEASUREMENT OF LARGE TRANSVERSE MOMENTUM POSITIVE PARTICLES P R O D U C E D A T M E D I U M A N G L E S A T x / s = 52.5 G e V R. COTTRELL .1 , M. DELLA NEGRA, D. DRIJARD, H.G. FISCHER, G. FONTAINE, H. FREHSE, C. GHESQUIERE, P. HANKE, P.G. INNOCENTI, W. ISENBECK, E.E. KLUGE, V. KORBEL, S. KRZYWDZINSKI, A. MINTEN, A. PUTZER, K.H. SCHMIDT, H. SCHNEIDER, W.J. SCHWlLLE .2, R. STROYNOWSKI, M. SZEPTYCKA .3, H. WAHL .4 and D. WEGENER CERN, European Organization for Nuclear Research, Geneva Coll~ge de France, Paris. lnstitut far Hochenergiephysik der Universitiit, Heidelberg Institut far Experimentelle Kernphysik der Universitiit (TH) und des Kernforschungszentrums, Karlsruhe

Received 17 Januari 1975 The inclusive spectrum of positive particles has been measured at the ISR energy yes = 52.5 GeV as function of lzansverse momentum, PT for large PT values using the Split Field Magnet facility. The angular dependence of the distribution is observed and discussed in the range 9° ~ 0 * ~ 21° . An upper limit for the production of 0 meson with PT ~ 3 GeV/e is derived from the same sample of data. In recent experiments [ 1] it was observed that at very high energies the transverse momentum distribution deviates from an exponential fall.off, at least at large angles in the centre of mass system. This behaviour could be connected with the existence of an internal structure of the proton or with the decay of heavy objects produced in high energy collisions [2]. In this letter we report on first results of an experirnent performed at the Split Field Magnet (SFM) facility at the CERN ISR. The detailed description of the SFM facility is given in ref. [3]. In this experiment an attempt was made to study the large PT behaviour in the kinematical region of medium angles unexplored until now. A trigger has been designed to select positive particles with large transverse momentum in the c.m. angular range 9 ° < 0* < 21 °. No particle identification was made in the data presented here. The multiwire proportional chambers of the forward spectrometer of the SFM were used as hodoscopes for setting up this trigger. Large FT particles were selected by five-fold coincidences between the fast signals of groups of 32 vertical wires belonging to five consecutive chambers of the spectrometer. The region covered by the trigger is shown in fig. 1. In addition, the five signals had to satisfy a given

geometrical correlation pattern in order to reject low PT tracks with strong curvature in the magnetic field of 1 tesla. During a run of 20 hours in march 1974 about 3.5 million triggers were recorded at the c.m. energy V~ = 52.5 GeV. The resolution of the correlation pattern was 6.4 cm, therefore a large number of accidental triggers occurred. In order to reject those, the data were then processed off-line through a fast selection program which reconstructed the track in the inhomogenous magnetic field using the full spatial resolution of the chambers (wire spacing = 2 mm, o ~ 0.6 mm). The quintic spline method [4] including Coulomb scattering was used to parametrize the tracks. This selection yielded about 130 000 events with a triggering track with transverse momentum above 1 GeV/c; f o r p T i> 3 GeV/c we obtained 1159 events. The momentum resolution A p / p is calculated

,1 Visiting scientist from SLAC, Stanford, CA 94305, USA. ,2 Visiting scientist from Bonn University, Bonn, Federal Republic of Germany. ,a Visiting scientist from Institute of Nuclear Research, Warsaw, Poland.

.4 Now on leave to the State University of New York, Stoney Brook, Ill. NY 11790, USA. 341

Volume 55B, number 3

PHYSICS LETTERS Fe/~.c¢opa f

17 February 1975 7-e/escop~ 2

Fig. 1. Plan view of the Split Field Magnet facility showing lay-out of mttltiwire proportional chambers. The shaded area corresponds to the geometrical region covered by the trigger. to be o f the order o f 1.5%. The resulting data were corrected for: (a) geometrical acceptance using a Monte Carlo method which takes into account the distribution of the interaction vertices as derived from the Schottky scan [5]; (b) absorption in the beam tube in the chamber system. The correction for decays is found negligible in the range o f m o m e n t a considerd in this experiment

(P~b I> 5 GeV/c);

(c) trigger losses due to chamber time resolution; (d) trigger losses due to electronic dead time; (e) inefficiency o f the off-line track finding method. The absolute cross section was obtained using the information o f scintillation m o n i t o r counters calibrated with the Van der Meer method [6]. The whole PT range was covered in one geometrical set-up o f the apparatus and the relative normalization are independent of the over-all normalization uncertainty. The contamination, due to beam-gas interactions was measured

Table 1 Inclusive invariant cross sections for positive particles E(d3o/dp3)mbc3/GeV2 at .~'s= 52.5 GeV.

PT(GeV/c) x = 0.25 1.3 1.5 1.7 1.9 2.1 2.3 2.5 2.7 2.9 3.1 3.3 3.5 3.7 3.9 4.1 4.3 4.5 4.7

342

(1.17±0.10)X (3.98±0.37)X (1.36±0.24)X (7.52±1.35)X (4.34±0.49)X (2.22±0.41)X (1.53±0.13)X

x = 0..30 10 -I 10 -2 10 -2 10 -3 10 -3 10 -3 10 -3

(4,84±0.34)X I0-2 (2,11±0.16)X 10 -2. (8,73±0.97)XI0-3 (3.20±0.53)X I0-3 (1.77±0.14)X 10 -3 (I .lO±O.07)X I0-3 (6.0520.41 )X 10 -4 (3.34±0.30)X 10-4

x = 0.35

(1.85±0.19)X 10 -2 (7.81±0.66)X10 -3 (3.38±0.34)X I0-3 (1.77±0.21)X 10 -3 (7.56+1.19)X 10 --4 (3.83+0.27)X 10-4 (2.30±0.17)X 10 --4 (I .47±0.14)X I0 --4 (6.76±2.42)X lO-S

x = 0.40

(6.15+0.71)X10 -3 (3.27±0.32)X 10-3 (1.37±0.16)X 10 -3 (8.2021.06)X 10-4"4 (2.77±0.54)x 10-44 (1.66±0.15)X 10 -4 (8.26±0.91)x 10-s (4.78±0.67)x 10-s (2.57+0.49)x 10 -s (6.91±4.30)x 10 -6

x = 0.45

x = 0.50

(1.39+0.15)X 10 -3 (6.16+0.88)x 10 --4 (2.27+0.45)x 10-4 (1.52±0.31)x 10 -4-4 (5.94+0.80)X 10 -s (3.70+0.61)X lO-S (1.58±0.36)X 10-s (1.35±0.37)X lO-S

(4.33±0.68)x (2.58±0.47)x (I.04±0.25)x (3.10±1.22)× (2.22+0.46)x (1.27+0.36)x

I0-4 10 --4 10-4 I0 -s 10-s 10 -5

(3.38±1.78)x 10 -6

(3.55+1.23)X 10 -6 (5.58±3.63)x 10 -6

(2.97±1.66)x 10 -6 (3.36 ±2.70)x 10 -6

Volume 55B, number 3

PHYSICS L E T T E R S

p p~positive

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17 F e b r u a r y 1975

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by the time o f flight method to be less than 1%. Reconstructing the vertices o f a sample of events, the background faking large PT tracks was found to be negligible. The over-all normalization uncertainty is estimated to be about 10% which is mainly due to the error o f the calibration o f monitor counters. The errors given are statistical only.

In fig. 2(a) the invariant cross section for the positive particles is plotted as function of PT at f'Lxed values of x = averaged over the bin in x of -+0.025 around its central value. For the values of x = 0.3, 0.4 and 0.5 it was possible to compare our data with the results from the CERN-Holland-Lancaster-Manchester Collaboration [7] who measured the invariant

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343

Volume 55B, number 3

PHYSICS LETTERS

p p~posdlve

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Fig. 3. Contours of constant cross section for production of positive particles in the x - PT plane. cross section o f p, K + and 7r+ at lower PT values. In the small overlap region the agreement is satisfactory. The values of the invariant cross section are given in table 1. In fig. 2(b) the invariant cross section is plotted as function of PT at fixed values of the c.m. angle 0". Again the invariant cross section was averaged over the angular interval o f tan 0* = +0.02 around its central value. Our data at 10.2 ° are in good agreement with the results o f the CERN-Bologna Collaboration [8]. At fixed angle the invariant cross section can be approximated by E d 3 o / d p 3 = A exp(--BPT ). The slope o f the distribution decreases monotieally with increasing angle from B = 4.9 +- 0.1 for 0* = 10.2 ° to B = 3 . 4 + 0 . 1 f o r 0 * = 2 0 . 8 °. In order to illustrate the behaviour of the invariant cross section we present in fig. 3 the contour lines of the constant cross section in the x - P T plane. It is interesting to note that for transverse m o m e n t a P T < 344

17 February 1975

2 GeV/c the invariant cross section for the production of all positive particles is practically independent o f x while it shows strong x-dependence at higher PT values. This may be explained by the fact that at large x-values there is a significant contribution of diffractively produced particles which can extend even to the region o f PT about 2 GeV/c. At higher PT values this contribution is probably negligible. It was recently suggested that the abundant production of leptons at large PT may be explained by the copious production of vector mesons and in particular the ¢ meson [9]. The cross section for ¢ production would be required to be 1.5 times that of the pion cross section at the same PT- In this case the positive high PT particle detected by our trigger could originate from the K+K - decay of the ¢. Therefore, we searched in our data for another large PT negative track at small angle to the triggering positive one. We found 32 pairs, none o f them compatible with the ¢ meson. We conclude that with 95% confidence level the cross section for ~ production is less than 0.4 the cross section for positive particles at PT ~> 3 GeV/c in our angular range. The experiment was greatly helped, in different phases, by contributions from Drs. G. Sharpak, B. Heck, H. Hoffmann, E.W. Kittel, D.R.O. Morrison, G. Sinapius and D. Sotiriou. We are grateful to the SFM Detector Group and to the ISR Division for their support. The Heidelberg and Karlsruhe groups were supported by a grant from the Bundesministerium for Forschung und Technologic, Federal Republic of Germany. [1] F.W. Bilsser et al., Phys. Lett. 46B (1973)471; M. Banner et al., Phys. Lott. 44B (1972) 537; B. Alper et al., Phys. Lett. 44B (1973) 521. [2] For a review see e.g.J.D. Bjorken in Prec. Int. Conf. on Elementary particles, Aix-en-Provence (1973). [3] R. Bouclier et al., Nucl. Instr. and Methods 115 (1974) 235 [41 H. Wind, Nucl. Instr. and Methods 115 (1974)431. [5] J. Borer et al., CERN[ISR-DI/RF/74-23, presented at the 19th Intern. Conf. on High energy accelerators, SLAC, 1974. [6] S. Van der Meet, CERN internal report ISR-PO/68-31 (1968). [71 M.G. Albrow et al., Nuel. Phys. B73 (1974) 40. [8] P. Capiluppi et al., Nucl. Phys. B70 (1974) 1. [9] J.W. Cronin, Processes at large transverse momentum, review talk at the SLAC Summer Institute on Particle Physics, August 1974.