IL NUOVO CIMENTO VOL. 107 A, N. 5 Maggio 1994

Peripheral Collisions Caused by 4.5A GeV/c Carbon and Silicon Nuclei.

TAUSEEF AHMAD (1), MUSTAFA ABDUSALAM NASR (1) (*) M. IRFAN (1) and H. KHUSHNOOD(2)

(1) Department of Physics, Aligarh Muslim University - Aligarh 202002, India (~) Department of Physics, Jamia Millia Islamia - Jamia Nagar, New Delhi, India

(ricevuto il 16 Luglio 1993; approvato il 26 Ottobre 1993)

Summary. - - In order to study some interesting characteristics of peripheral collisions caused by 4.5A GeV/c carbon and silicon ions in nuclear-emulsion targets, experimental data have been analysed; multiplicity and angular distributions of relativistic charged particles have also been investigated. Results indicate that the characteristics of peripheral collisions depend on the projectile mass. Also positive evidence regarding occurrence of two-particle correlation in these collisions has been obtained.

PACS 25.70 - Heavy-ion-induced reactions and scattering.

1. - I n t r o d u c t i o n .

Heavy-ion reactions between two larger nuclei are classically divided into three categories [1-5], namely, peripheral, quasi-central and central collisions. In quasi-central and central collisions, the projectile and the target nuclei are envisaged to approach quite closer to each other. The main difference in the two types of collisions can be understood in terms of the number of nucleons which participate in the reactions. Such events are characterized by comparatively larger multiplicities of the produced particles and the emitted target fragments. The emission of particles is quite systematic with respect to the mean direction of the incident beam. In the central collisions, however, almost all the projectile nucleons are participants and there are essentially no spectators.

Peripheral collision, which is of much significance in investigating the characteristics of heavy-ion reactions, is visualized to occur when the impact parameter, denoted by b, nearly equals the sum of the radii of the projectile and the

(*) Permanent address: Department of Physics, Faculty of Science, Khalij Atehadi University, P.O. Box 18382, Misurata, Great Socialist Peoples Libyan Arab Jamahiriya, Libya.

683

684 TAUSEEF AHMAD, MUSTAFA ABDUSALAM NASR, M. IRFAN and H. KHUSHNOOD

target nuclei, i.e. b = (R~ + R2), where R 1 and R2 are the radii of the projectile and the target nuclei, respectively. The projectile and target nuclei in the peripheral collisions are quite far apart, so a small momentum is transferred between the interacting nuclei. Consequently, in such a collision, as expected, a small number of relatively faster particles as well as slower target fragments are produced.

The main objective of the present study is to determine the general characteristics of peripheral collisions caused by 4.5A GeV/c carbon and silicon nuclei in nuclear- emulsion targets. As mentioned earlier, peripheral collisions involve a comparatively smaller number of target fragments. So we consider only those events which have Nh = 0, 1, where Nh represents the number of heavily ionizing particles in an interactibn emitted with relative velocity ~ ~< 0.7.

2. - Experimental details.

Two stacks of NIKFI-BR2 emulsion of dimensions (18.6 x 9.5 x 0.06)cm 8 and (16.9 x 9.6 x 0.06) cm 3 , with printed grid, exposed to 4.5A GeV/c carbon and silicon beams at Dubna Synchrophasotron, Russia have been used in the present investigation. Line scanning was performed to collect interactions. The tracks were picked up at a distance of 3 mm from the entrance edge of the stack and were followed backward to ensure that they did not come from the previous interactions. The primary tracks are almost flat. Hence most of the primary tracks could travel the full length of the stack provided they did not interact. A Nikon microscope, having 40 x objectives and 15 x eye-pieces, was used for line scanning; scanning was performed by three independent highly skilled scanners to increase the scanning efficiency.

Random samples comprising 873 and 1024 interactions, caused by 4.5A GeV/c carbon and silicon nuclei, respectively, having Nh /> 0, were collected. It may be mentioned that 21% and 22% of all the interactions caused by carbon and silicon nuclei, respectively, are peripheral collisions.

Secondary charged particles produced in an interaction are placed in the usual categories, i.e., shower, grey and black according to the ionization produced by them; their multiplicities in an interaction are denoted by Ns, N~ and Nb, respectively. Grey and black tracks together are referred to as the heavily ionizing particles or heavy tracks in an interaction and their number is denoted by Nh (= Ng + Nb). Other details regarding measurements and behaviour of these parameters are given in our earlier publications [6, 7].

3. - E x p e r i m e n t a l r e s u l t s .

Multiplicity distribution of relativistic charged particles produced in peripheral collisions initiated by 4.5A GeV/c carbon and silicon nuclei in emulsion are shown in fig. 1. It is observed that the distribution tends to become relatively broader with increasing projectile mass.

The values of the mean multiplicity of relativistic charged particles, their dispersion, D, defined as [(N~>- (N~>2] 1/2, and the ratio of D/(Ns> are presented in table I for carbon- and silicon-emulsion interactions. It may be of interest to mention that the mean multiplicity and dispersion of the relativistic charged particles grow rapidly with increasing projectile mass. However, the value of

PERIPHERAL COLLISIONS CAUSED BY 4 . 5 A GeV/c CARBON AND SILICON NUCLEI 685

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Fig. 1. - Multiplicity distributions of shower particles produced in peripheral collisions. - - - c a r b o n , - silicon.

Fig. 2. - v-distributions of relativistic charged particles emitted in peripheral collisions of carbon ( - - - ) and silicon ( ) nuclei.

the ratio D/(Nsl, within the limit of statistical error , slowly increases with increasing projectile mass.

Angular characteristics of charged shower particles produced in nucleus-nucleus collisions are investigated in terms of the rapidity variable, Y, defined as

(1) y 1 In E+P~E 2 E - P,

where E and Pl, are total energy and longitudinal momentum of a shower particle, respectively. At relativistic energies this equation reduces to

(2) v = - In tg 0/2,

where 0 is the emission angle of a shower track with respect to the mean direction of the incident particles in the laboratory frame. Figure 2 shows the pseudorapidity distributions of relativistic charged shower particles produced in peripheral collision caused by carbon and silicon nuclei. F rom the figure, it is observed that the peaks of the ~-distributions shift towards higher values of ~ with increasing projectile mass. Fur thermore , the excess of particles arising due to increase in the mass number tends to appear only in the central region of the rapidity space.

TABLE I. - Values of mean multiplicity of shower particles and its dispersion.

Projectile ( N~) D D / ( Ns)

C 2.18 -+ 0.18 1.74 + 0.13 0.80 -+ 0.09 Si 3.04 -+ 0.20 3.00 -+ 0.14 0.98 -+ 0.08

686 TAUSEEF AHMAD, MUSTAFA ABDUSALAM NASR, M. IRFAN and H. KHUSHNOOD

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Fig. 3. - Variation of (~> with Ns. © carbon, • silicon.

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Variation of (~> with Ns for peripheral collision is shown in fig. 3. From the figure, it may be noticed that <7> decreases slowly with increasing values of Ns for both types of interactions. Incidentally, this behaviour is inconsistent with the predictions of the tube-type models.

An attempt is made to examine the formation of clusters in terms of rapidity gap length distributions. To this purpose, the charged particles in each event are arranged in decreasing order of their pseudorapidity values. The particles having maximum and minimum pseudorapidity values are not considered, as they are visualized to constitute the leading and target particles, and the cluster characteristics are investigated by considering only the non-diffracticve component of the cross-section.

Rapidity gap distributions between adjacent charged shower particles for peripheral collisions produced in carbon- and silicon-emulsion interactions at

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Fig. 4. - Two-particle rapidity gap distributions for peripheral collisions. - - - silicon.

Fig. 5. - R-distributions for peripheral collisions in carbon ( - - - ) and silicon ( interactions.

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PERIPHERAL COLLISIONS CAUSED BY 4.5A GeV/c CARBON AND SILICON NUCLEI

TABLE II. - Values of constants A, B, C and D for two-particle correlations.

687

Projectile A B C D X 2/d.f.

C 1.78 _+ 0.75 3.34 - 0.65 0.34 -+ 0.19 0.80 _+ 0.11 0.14 Si 3.09 _+ 1.13 3.56 + 0.76 0.08 -+ 0.01 0.90 _+ 0.10 0.25

4.5A GeV/c are shown in fig. 4; a sharp and distinct peak at relatively smaller value of the rapidity gap is observed in the figure, suggesting thereby that two-particle correlation occurs in these collisions. The theoretical curve in the figure represented by the solid line is drawn by the two-channel generalization of the Snider model [8]. According to the model, the rapidity density may be expressed as

(3) dn/dr = A exp [ - Br] + C exp [ - Dr],

where A, B, C and D are constants and dn/dr represents the cluster density. The values of these constants are shown in table II. It may be mentioned that, in order to avoid confusion, the curves corresponding to eq. (3) for the peripheral carbon- emulsion interactions are not shown in the same figure. The values of the constants for carbon-emulsion interactions are presented in table II. The first and the second terms of eq. (3) correspond, respectively, to the short- and long-range correlations. The contributions of the two parts are shown in the figure by dash-dotted lines. It may be noticed from the figure that the dash-dotted line representing the contribution of the short-range correlation almost coincides with the solid curve in the region of lower values of rapidity gaps, whilst the line representing the contribution of the long-range correlation is significantly far away from the solid curve in the region of lower rapidity gap values. This indicates that the major contribu- tion to the two-particle correlation comes from the short-range correlation, while the contribution of the long-range correlation appears to be almost negligible.

Finally, the distribution of shower widths, R, defined as R = 71 - ~n, where Vl and Vn are the maximum and minimum pseudorapidities values, respectively, in each event, is plotted in fig. 5. Clear peaks in the central region of R-distributions are observed for the peripheral collisions in carbon- and silicon-emulsion interactions. It may be noted from the figure that the space occupied by the charged secondaries in peripheral carbon-emulsion interactions is relatively smaller as compared to its corresponding value for silicon-emulsion interactions. This feature can be explained in terms of the fact that shower particles, with increasing projectile mass, produced with relatively larger angles would tend to appear in the target fragmentation region.

4. - C o n c l u s i o n s .

The following important conclusions can be drawn on the basis of the results obtained in the present study.

1) The multiplicity distribution of relativistic charged particles produced in peripheral collisions depends on projectile mass.

2) The centroid of the angular distribution of the charged shower particles shifts towards higher values of V with increasing projectile mass.

688 TAUSEEF AHMAD, MUSTAFA ABDUSALAM NASR, M. IRFAN and H. KHUSHNOOD

3) Two-particle correlation occurs in both types of collisions.

4) Shower width distribution becomes wider with increasing projectile mass. This in turn would indicate that shower particles are produced at comparatively larger angles with increasing projectile mass.

R E F E R E N C E S

[1] R. KULBERG and I. OTTERLUND: Z. Phys., 259, 245 (1973). [2] D. E. GREINER, P. J. LINDSTRON, H. H. HECKMAN, BRUCE CORK and F. S. BIESER: Phys. Rev.

Left., 35, 152 (1975). [3] A. M. POSKANZER, R. G. SEXTRO, A. M. ZEBELMAN, H. H. GUTBROD, A. SANDOVAL and

R. STOCK: Phys. Rev. Lett., 35, 1701 (1975). [4] A. A. AMSDEN, J. N. GINOCCHIO, F. H. HARLOW, J. R. NIx, M. DhNOS, E. C. HALBERT and R.

K. SMITH: Phys. Rev. Lett., 38, 1055 (1977). [5] TAUSEEF AHMAD, MUSTAFA ABDUSALAM N~R and M. IRFAN: Phys. Rev. C, 47, 2974 (1993). [6] TAUSEEF AHMAD and M. IRFAN: Nuovo Cimento A, 196, 171 (1993). [7] TAUSEEF AH~I) and M. IRFAN: Phys. Rev. C, 44, 1555 (1991). [8] D. R. SNIDER: Phys. Rev. D, 11, 140 (1975).