Multiparticle production in proton-nucleus collisions at tevation energy

Volume 187, number 1,2

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19 March 1987

MULTIPARTICLE PRODUCTION IN PROTON-NUCLEUS COLLISIONS AT TEVATRON ENERGY

P.L. JAIN, K. SENGUPTA and G. S I N G H Htgh Energy Expertmental Laboratory, Department of Physws, State Untverstty of New York at Buffalo, Buffalo, NY 14260, USA

Received 22 July 1986

For 800 GeV proton mteracttons with nuclear emulsions, the multiplicityof shower particles,their correlations and the pseudorapidity d~stnbutions are presented.

Recently, there has been intense experimental and theoretical effort to study the various aspects of highenergy collisions with nuclei. In such collisions one can investigate the spacetime development of the formation of produced hadrons. Thus, their better understanding would lead to a deeper insight into strong-interaction processes. Unfortunately, the experimental data are not yet detailed enough to differentiate between various hadron production models. Prior to the present investigation, the highest proton beam energy for fixed targets at Fermi National Accelerator Laboratory (FNAL) was ~ 400 GeV. With the latest development at FNAL, the energy of the proton beam at the Tevatron has been boosted to 800 GeV. Here, we present results obtained from interactions of 800 GeV protons in nuclear emulsion. We discuss (i) the multiplicity of the shower particles produced, (ii) their correlation, and (iii) their rapidity distribution. These results are compared to those from lower energy (200-400 GeV) proton beams [ 1-3 ] at FNAL. A small stack of two dozens of Ilford G-5 nuclear emulsion was exposed at FNAL to a 800 GeV proton beam with a flux density of ~ 1 × 102 particles/cm 2. The emulsion was scanned by the along-the-track scanning method for about 700 primary interactions with a mean-free path, 2 = ( 3 4 . 7 5 + 2 . 3 9 ) cm. The results are based on a sample of about 450 inelastic and non-single diffractive events. The average charge

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multiplicity is ( n ) = 16.39 + 0.78. Nuclear emulsion is composed of hydrogen (H), light (CNO) and heavy (AgBr) elements. The interactions with different targets were separated as described before [4]. For proton-nucleon interactions, we used 101 white stars, i.e., events with Nh~< 1 (where Nh denotes the number of heavy tracks having p ~<0.3). Comparing our earlier data at 200, 300 and 400 GeV [ 1-3] with the present 800 GeV ones, we find a logarithmic dependence of ( n ) on the squared CMS energy s, ( n ) = a l l n S + b l , where a z = 2.26 + 0.63 and b~ = 0.14 + 0.07. The dispersion of the multiplicity distribution, D = ( ( n 2 ) _ ( n ) 2) 1/2 is a linear function of ( n ) , i.e., D = a 2 ( n ) + b 2 , with a2 = 0.487 + 0.045 and b2 = - 1.096 + 0.655. We find that the relationship between D and ( n ) is similar for p - p [ 5 ] and p-nucleus interactions. In neither case is it Poisson-like (where Doc ( n ) 1/2), except at large rapidities (cf. below), where nuclear transparency occurs (r/> 5). By assuming the validity of Feynman scaling, Koba, Nielsen and Olesen [ 6 ] predicted an asymptotic ( K N O ) scaling of multiplicity distributions in hadronic collisions. We extended the KNO scaling to nucleon-nucleus [2], and then to nucleus-nucleus [3] interactions at high energies (and finally to helium fragments produced in high-energy heavy-ion collisions [ 7 ]). Recently, at x/~ = 540 GeV, a violation of KNO scaling has been reported [ 8 ] in inelastic, non-single diffractive p - p collisions. An increased 175

Volume 187, number 1,2

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{

/x v

001 0

04

08

12 16 n/

20

24

28

Fig. 1. Plot of (n)(a,,la,neO versus n / ( n ) at 800 GeV/c for p-nucleon interactions (white stars, Nh=0, 1 ) ( × ) and for p-nucleus mteracttons (Nh > 1 ) ( © ).

probability is observed for values of the scaling variable z = n~( n ) > 3. Furthermore, in collisions with heavy targets ( A = 107 and 196) at 100-320 GeWc an A dependent violation of KNO scaling has been observed [9] on the basis of high statistics. In fig. 1, (n)(O'n/amel) is plotted against n / ( n ) for inelastic and non-diffractive white star (p-nucleon) events as well as for p-nucleus (Nh> 1 ) events in emulsion with ( A ) ~ 70. Our data sample is limited to values of the scaling variable z < 3 . Within the statistical errors, the experimental points lie on the universal curve given by Slattery [ 10 ]: g(z=n/(n) ) = (3.97z+ 33.7z 3 - 6.64z 5 +0.332z 7) × e x p ( - 3.04z).

19 March 1987

The overall z2/DOF= 0.65/12 = 0.054. Thus, within the present statistics, we see a universal behaviour of the multiplicity distribution in p-nucleus interactions with ( A ) ~ 70. Furthermore, we show in table 1, the experimental values of the moments Cq = ( n q) / ( n ) q for q = 2-4. When KNO scaling holds these moments should not depend on energy. Apart from the C moments which give weight to high multiplicities, we also calculated 7-moments which are more sensitive to deviations about the mean. The values of the C moments can be compared to those at lower energies [ 3]. These are the same within statistical errors, for each of p-nucleon, p - C N O and p-AgBr interactions, as shown in table 1. Most of the secondary particles produced at the Tevatron energy are concentrated at very small angles with respect to the primary proton. Nuclear emulsion, which has the highest spatial resolution is very effective for measuring these angles. Pseudo-rapidity, ~/~ - I n tan(0L/2) (0L is the laboratory angle of the emitted particles with respect to the projectile direction), has been a commonly used variable in the description of kinematics of hadron-nucleus collisions. For p - p interactions, CMS pseudo-rapidity (r/c) is defined as r/c~ - l n ( T c tan 0L), where 7c is the Lorentz factor. The pseudo-rapidity range can be divided into three regions: (i) the target fragmentation region, ~/< 2, (ii) the central production region, 2~ 5. The normalized pseudo-rapidity (qc) distribution of secondary particles produced in the CMS of p-nucleon interactions is shown in fig. 2a for 200, 300 and 800 GeV protons. The distribution has broadened with energy and the central plateeau-width has increased at 800 GeV as predicted by Feynman. The height of the plateau is energy dependent. With

Table 1 Experimental values for the rauo Cq = ( n q) / ( n ) q and y-moments for p--nucleon, p - C N O and p-AgBr interactions at 800 GeV/c. Type

p-nucleon p-CNO p-AgBr

176

Moment

C2

6"3

C4

~'2

73

!.139__.0.113 !.163_+0.108 1.151 _+0.075

1.478__.0.147 1.513+0.140 1.491 _+0.098

2.157_+0.215 2.143-+0.199 2.126_+0.139

0.139_+0.014 0.163-+0.015 0.151 _+0.010

0.059_+0.006 0025_+0.002 0 037+0.002

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the 800 GeV proton beam we observe a constant rapidity density of ~ 2.7 charged particles per unit of rapidity, while for cosmic ray [ 11 ] experiments it is 2.8 charged particles per unit of rapidity. The width of the central rapidity plateau in cosmic ray [11 ] experiments and at the Tevatron energies are about 4 and 3 units of rapidity, respectively. When pseudo-rapidity is plotted for p-nucleon interactions at these three energies (200, 300 and 800 GeV/c proton) in the fragmenting proton rest frame (q') (cf. below) the distributions of the secondaries observed over the whole q'-range remain practically the same within the statistical errors. This follows from the principle of limiting fragmentation at such energies. We are now extending the principle of limiting

i 9 March 1987

fragmentation to nucleon-nucleus collisions, thus adding a new dimension. This is achieved by a detailed study of the projectile fragmentation region in proton-nucleus collisions. We boost to the rest frame of the incident projectile (i.e., q' = q - qm~,, and qm.x = In s). In fig. 2b, the distribution of the secondary particles produced from heavy (AgBr) targets is

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zbo Fig. 2. (a) Plot of q~= - In (7~ tan 0 L) for p-nucleon mteractmns at 200, 300 and 800 GeWc. (b) Pseudo-rapidity (t/') dlstnbutmn of shower part~cles m the rest frame of the incident projectde for 200, 300 and 800 G e W c incident momenta, for a heavy (AgBr) target, q' = q - ~m,~, where qma~IS the rap~dlty boost. Proton -nucleon data ~s also shown w~th the sohd curve.

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Fig. 3 The raUo R (~/) = ( nhA) / ( n.p ) versus ~/for produced partlcles at (a) 300 OeV/c proton, (b) 800 OeV/c proton with heavy (AgBr) and hght ( C N O ) targets. Plot of R(Elab) as a fuctlon of E~,b, (c) for C N O and (d) for AgBr targets in different regions of pseudo-rapidity. Solid hnes m (a) and (b) are free hand curves.

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shown as seen from the colliding protons at 200, 300 and 800 GeV energies. In the projectile fragmentation region, the heavy nuclei fragment in the same way, irrespective of the energy with which they have been hit. For the AgBr target the energy independent region extends over about 5 units in pseudo-rapidity. For comparison the energy independent region for p-nucleon collisions is also shown here. In fig. 3a and 3b are presented the ratios R = (nhA)/(nhp) for light ( C N O ) and heavy (AgBr) targets as a fuction oft/, for 300 and 800 GeV proton beams, respectively. R(r/) is particularly large in the target fragmentation region (~/< 2), indicating internuclear cascading in this region. For the rapidity regions 2~ 5.5 ), R (~/) falls below 1. This is further demonstrated in table 2, where ( n ) at ~/> 5.5 is given for different even multiplicities in p-nucleon, p - C N O and p-AgBr targets from the 800 GeV beam. In figs. 3c and 3d we show the ratio R = (nhEm)/(nhN) for light nuclei ( C N O ) and heavy nuclei (AgBr) respectively, as a function of the laboratory energy (E~ab) for 300, 400'and 800 GeV proton and 200 GeV pion in the three regions o f pseudorapidity, wz. (i) target fragmentation, (ii) central and (iii) projectile fragmentation regions. We find that the ratio R (E~b) in the projectile fragmentation region (r/> 5) is almost constant (R ~ 0.8) with light and heavy targets. In the central region (2 ~<~/~<5 ), R increases with the increase in the size of the target. With the same target, the increase of R with energy is very slow. In the target fragmentation region ( r / < 2 ) , there is a large increase in R values as compared to those in the central region, i.e., a factor of Table 2 Mean multlphclty of shower parttcles produced m the very forward direction (Le., ~/> 5.5) m interactions with &fferent targets and with d~fferent multlphc~ty. Target

MulUphclty n=4-10

nucleon CNO AgBr 178

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n=>16

2.056_+0280 2.355+0.423 2.231+0.619 1.489_+0.182 1.425__.0.225 2.233_+0.408 1.048_+0229 1.704_+0.232 1.815+0.145

19 March 1987

about 2.5 and 3 in C N O and AgBr targets, respectively. The relative increase o f R with energy with either targets is low. From our data, we conclude the following: (1) The relationship between D and ( n ) is similar for p - p and p-nucleus collisions. (2) Within the present statistics, we do not observe any violation of K N O scaling for values of the scaling variable z < 3. (3) The mean multiplicity o f charged secondary particles is independent o f the incident energy and is almost independent of the target size in the forward region (r/> 5.5 ). (4) The ratio R(Etab) in the projectile region is constant and is independent o f the energy and the target. In the central region it is increasing very slowly with energy and w~th target size. In the target fragmentation region, it is increasing slowly with energy but rapidly with the target size. We thank Dr. R. Stefanski and the Fermilab staff for their help m the exposure o f the emulsion stack. Our special thanks to Professor S. Ganguli o f TIFR, Bombay, India, for getting our stack developed at his institute. This reseach work was supported in part by the National Science Foundation under Grant No. NSP/PHY8544391. K.S. would like to thank the office of Research and Graduate Studies, S U N Y at Buffalo, for the partial support.

[ 1] P.L. Jam, M. Kazuno, G. Thomas and B. G~rard, Phys. Rev. Lett. 33 (1974) 660; P.L. Jam, Experiments of high energyparticle colhslons,AIP Conf. Proc. No. 12, ed. R.S. Panvmi (American Institute of Physics, New York, 1973) p. 141; P.L. Jam et al., Lett Nuovo Omento 8 (1973) 921; 9 (1974) 113; 27 (1980) 491 [ 2 ] P L. Jam, B Girard, M. Kazuno and G. Thomas, Phys. Rev. Lett. 34 (1975) 972 [ 3 ] P L. Jam and G Das, Phys. Rev. D 24 (1981 ) 1987. [4] P.L. Jam, K.L. Gomber, M.M. Aggarwaland V. Rani, Phys. Lett B 154 (1985) 252. [5] A.K. Wroblewskl, Acta Phys. Polon. B 4 (1973) 857 [6] Z. Koba, H.B. Nielsen and P. Olesen, Nucl. Phys. B 40 (1972) 317. [ 7 ] P.L. Jam and M.M. Aggarwal,Phys. Rev. C 33 (1986) 1790. [8] UA5 Collab., G.L. Alner et al., Phys. Lett. B 138 (1984) 304. [9] N.N. BlSwaset al., Phys. Rev. D 33 (1986) 3167. [ 10] P Slattery, Phys. Rev. Lett. 29 (1972) 1624 [ 11 ] P.L. Jam and Z. Ahmad, Phys. Rev. Lett. 28 (1972) 459.