Volume 41 B, number 2 PHYSICS LETTERS 18 September 1972

CHARGED PARTICLE PRODUCTION RATIOS AT THE CERN ISR

FOR A TRANSVERSE MOMENTUM OF 0.4 GeV/c

A. BERTIN, P. CAPILUPPI, M. D’AGOSTINO-BRUNO, R.J. ELLIS*, G. GIACOMELLI**, A.M. ROSS1 and G. VANNINI

Istituto di Fisica dell’Universitb di Bologna, Italy; Istituto Nazionale di Fisica Nucleare, Sezione di Bologna, Italy

A. BUSSIBRE CERN, Geneva, Switzerland, and CEN-Saclay, DPhN/HE, France

R.T. POE CERN, Geneva, SwitzPrland, and University of California, Riverside, Cali&, USA

Received 31 July 1972

Experimental results are presented on the production ratios of charged particles in inclusive reactions at ISR energies. The data have been taken at a fixed transverse momentum of 0.4 GeV/c. At ISR energies and for 0.1 <x< 0.3 the production of all charged particles seems to be energy independent, suggesting that limiting distribu- tions have been reached.

We report further experimental results on charged

particle production in inclusive reactions at the CERN Intersecting Storage Rings (ISR). The reactions studied are :

p+p+n’tX, p+p+K’+X, ptp+p+tx.

Data were taken at c.m. energies of 2 X 11.8,2 X 15.3, 2 X 22.5, and 2 X 26.5 GeV, corresponding to labora- tory momenta of approximately 270,500, 1100, and 1500 GeV/c, respectively. The produced particles were detected by a magnetic spectrometer, using Cerenkov counters and time of flight [ 1,2].

The experimental apparatus, described elsewhere [ 11, was modified in order to incorporate the following (fig. 1):

(i) A third gas threshold Cerenkov counter (C, in fig. l), placed between the second magnet and the first plastic scintillator of the spectrometer. This was either operated in parallel with one of the other Cerenkov counters in order to achieve a higher rejection of pions

* Royal Society European Exchange Fellow. ** and University of Padova.

while detecting kaons and antiprotons, or set at a pres- sure below the pion knee so as to determine the elec- tron contamination.

(ii) One more plastic scintillator (Se) placed after S,. This was introduced to obtain a shorter decay path (27 m) in order to reduce the decay correction factor for kaons. The data presented here were mainly ob- tained by this short spectrometer, which accepted a momentum bite off 11%.

(iii) A three-element hodoscope (HI ,2,3), instead of

the last plastic scintillator, to increase the counting rate in the long spectrometer. Each of these elements accepted a momentum bite of +4%.

We present here a systematic set of ratios of the invariant cross-section E(d3 a/d3p) for particles to an- tiparticles, and for particles to pions of the same sign, as a function of the variable x = pI/pmax, where pl and

P max are the c.m. longitudinal and maximum kine- matically allowed momentum, respectively, of the produced particles. Particle-to-antiparticle ratios are also presented as a function of the variable

Y max -Y =y,, - 3 In NE + PI)/@ - PI)) j

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Volume 4 1 B, number 2 PHYSICS LETTERS 18 September 1972

- 10 METERS

=4 ‘h_ Ss

4 PLAN VIEW

Tl, ‘6 -1

Fig. 1. Layout of the spectrometer: B, -B, are bending magnets; S, - S, and H, -H, are scintillation counters; C, -C, are gas threshold Cerenkov counters.

where E andy are the c.m. total energy and the rapid- ity of the observed particles, respectively;y,, is the c.m. rapidity of the incoming protons.

The data cover the angular and momentum ranges 80-300 mrad, and 1.3-5.0 GeV/c. We have chosen to work at a fixed value of the c.m. transverse momentum

pt of the observed particles, i.e. pt = 0.4 GeV/c, which is close to the average value of the transverse momenta of the produced particles and is well in the middle of our measurable range.

The raw data have to be corrected for: i) beam-gas

background*, ii) electron (positron) contamination, iii) multiple Coulomb scattering, iv) nuclear interactions

of the produced particles in the various materials of the spectrometer, and v)decay of the unstable parti- cles.

The statistical error on each point ranges from 2% to 5% for pions, 3% to 10% for protons, and 10% to 25% for the other particles. For the absolute values, the largest systematic uncertainty arises from the luminos- ity determination (probably 10-l 5%). However, this last uncertainty is minimized for each of the particles’ ratios since each point corresponds to measurements performed during the same ISR run.

Fig. 2 shows the measured ratios of the invariant

* The beam-gas background was measured with beam 2 only in the ISR. At our angles it was always less than 20% of the beam-beam rate.

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Fig. 2. The ratios of invariant cross-sections E(d30/d3p) for (a) m+/m-, (b) K+/K-, and (c)p/p plotted versus x for pt = 0.4 GeV/c. The hatched bands represent interpolation through the 24 GeV/c data of Allaby et al. [6] and Miick et al. [7]. The points at x = 0 come from the Saclay-Strasbourg [3] (SS) and

BrUish-Scandinavian CBS) Collaborations.

Volume 41B. number 2 PHYSICS LETTERS 18 September 1972

,I , ;*A$ 0 0.5 IO 15 20 25 3.0 40

y,,* -y Fig. 3. The ratios of invariant cross-sections E(d30/d3p) for ,‘/lr-, K+/K-, and p/p plotted versus y_ -y for pt = 0.4

GeV/c. The notation is the same as for fig. 2.

cross-sections for particles to antiparticles (n+/n-,

K+/K-, p/F) as a function of x. These data need to be corrected only for points (i), (ii) (for pions only), and (iv). The total uncertainty is about 5% for nijn-, and ranges from 10% to 30% for K+/K- and p/p. Also show are other ISR data [3-51. The hatched bands re- presents 24 GeV/c data [6,7]. Fig. 3 shows the same ratios as a function of the (urn, -v) variable.

Fig. 4 shows the measured ratios of the invariant cross-sections for particles to pions of the same sign (p/n-, K-/n-, p/n+, and K+/n+). The total error is about 5- 10% for p/n+, and ranges from 10% to 30% for the other ratios.

The following observations may be made on these experimental results:

i) Within experimental errors, we do not observe any energy variation of the particle ratios among the four ISR energies. Other groups observe some ener- gy variation at x=0, which is outside our range.

ii) All the particle-to-antiparticle ratios decrease with decreasing values of x. The decrease is slow for n’/n- and K+/K-, very fast for p/y. The ratios decrease towards a value between 1 and 2 at x = 0

iii) The particle-to-pion ratios tend to a value around 0.1-0.2 at x = 0.

iv) The ratios where protons are involved exhibit a strong x-dependence, reflecting the leading-particle na- ture of the proton.

v) The comparison of the resuits of this experi-

01 02 03 04

Fig. 4. The ratios of invariant Goss-sections for (a)$n-, (b) K-/z-, (c) K+/n’, and (d) p/n+plotted versus x for pt = 0.4

GeV/c. The notation is the same as for fig. 2.

ment with the corresponding data measured at x=0 [3,4] shows that the ratios (in particular the n+/n-, p/p, and p/n’) level off to an essentially constant value

for x < 0.07. vi) Depending on the variable used (x or v), the

differences between the ratios measured at the ISR and those obtained at 24 GeV/c do not appear to be quite the same, thus indicating the different role played by these variables. In the plot against x the differences are very large when the ji are involved, large for the K- and moderate for the other particles. In the plots versus the rapidity these effects are smaller.

vii) We also observe that the separation between ISR and 24 GeV/c data decrease with increasing values of x. The same trend persists in the rapidity graph, where the differences between ISR and 24 GeVlc data

decrease with decreasing values of y,, -y. From these observations and taking into account

the results of previous work on absolute cross-sections [ 1,2], one infers that scaling holds for the production of I?, K’, p’ in inclusive reactions within the investi- gated range at ISR energies: one may also say that

Volume 4 1 B, number 2 PHYSICS LETTERS

limiting distributions have been reached at these ener- References

18 September 1972

gies. From the comparison with the low-energy data, the 111

present results indicate that the limiting distributions are reached at smaller energies for pions and at higher energies for kaons, protons, and antiprotons. This fact suggests some kind of threshold effect due to mass, 121 and possibly some dependence on the strangeness quan-

tum number. Also the leading particle nature of the proton plays a role. The limiting distributions are reach- ed first in the fragmentation region (small yrnax -y)

131

and later in the central or pionization region (large 141

[51 We are grateful to the ISR staff for the successful

operation of the ISR. We acknowledge the friendly collaboration of C. Maroni and L.R. Ratner. We thank

G. Busacchi, 0. Ferrari and M. Mignardi for their com- petent technical assistance.

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A. Bertin, P. Capiluppi, A. Cristallini, M. D’Agostino-Bruno, R.J. Ellis, G. Giacomelli, C. Maroni, F. Mercatali, A.M. Rossi and G. Vannini, Phys. Letters 38B (1972) 260, and data presented at 4 th Int. Conf. on High-Energy Collisions, Oxford (1972). L.G. Ratner, R.J. Ellis, G. Vannini, B.A. Babcock, A.D. Krisch and J.B. Roberts, Phys. Rev. Letters 27 (1971) 68, Proc. of the Meeting of the American Physical Society, Di- vision of Particles and Fields, Rochester (1971). Saclay-Strasbourg Collaboration, paper presented at the 4 th Int. Conf. on High-Energy Collisions, Oxford (1972). British-Scandinavian Collaboration, paper presented at the 4th Int. Conf. on High-Energy Collisions, Oxford (1972). M.G. Albrow, D.P. Barber, A. Bogaerts, B. Bosnjakovic, J.B. Brooks, A.B. Clegg, F.C.Erne,C.N.P. Gee,A.D. Ka- naris, A. Lacourt, D.H. Locke, P.G. Murphy, A. Rudge, J.C. Sens and F. van der Veen, submitted to Phys. Letters. J.V. Allaby, A.N. Diddens, R.W. Dobinson, A. Klovning, J. Litt, L.S. Rochester, K. Schliipmann, A.M. Wetherell, U. Amaldi, R. Biancastelli, C. Bosio and G. Matthiae, CERN 70-12 (1970), and paper presented at the 4th Int. Conf. on High-Energy Collisions, Oxford (1972). H.J. Miick et al., M. Schatter, F. Selonke, B. Wessels, V. Blobel, A. Brandt, G. Drews, H. Fesefeldt, B. Hellwig, D. Mdnkenmeyer, P. S&ding, G.W. Brandenburg, H. Franz, P. Freund, D. Luers, W. Richter and W. Schrankel, Phys. Letters 39B (1972) 302.

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