Limiting Fragmentation Behavior of Projectile Helium (Z ¼ 2) Fragments in Nucleus–Nucleus Interactions at 14.6 A GeV Ashwini KUMAR 1 , Gurmukh SINGH 2 , and Bhartendu K. SINGH 1 1 High Energy Physics Laboratory, Department of Physics, Banaras Hindu University, Varanasi 221005, India 2 Department of Computer and Information Sciences, State University of New York at Fredonia, NY 14063, U.S.A. (Received September 11, 2012; accepted October 15, 2012; published online November 19, 2012) An analysis of projectile helium (Z ¼ 2) fragments based on 855 minimum-bias inelastic events induced by 14.6 A GeV 28 Si beam in a nuclear emulsion is presented in order to test the hypothesis of limiting fragmentation. The projected angular distributions of projectile helium fragments have been fitted with Gaussian curves in individual helium reaction channels and also in different emulsion target events. Furthermore, average emission angle of projectile helium fragments has been studied in individual helium reaction channels with different emulsion target groups. On the basis of pseudo-rapidity distribution, an energy independent limiting fragmentation behavior of projectile helium fragments is also investigated in the fragmentation region. KEYWORDS: nucleus–nucleus interactions, H, C, N, O, Ag, Br, emulsion targets, emission angle, helium reaction channel, projected angular distribution 1. Introduction Heavy-ion collision experiments provide us a unique opportunity to study the exciting phenomena happening in nucleus–nucleus interactions at relativistic high energies. Among these phenomena, limiting fragmentation retains its own unique significance from the very beginning since it was first proposed by Benecke et al. 1) in 1969. Since then, a special attention has been paid by physics community associated with various heavy-ion experiments. According to which, in a high energy collision, in the frame where a target or a projectile is at rest, some of the outgoing particles approach limiting distributions. That is to say, the distribu- tion exhibits a negligible change at finite bombarding energy for large S (the square of center-of-mass energy) such as the pseudo-rapidity density as a function of –Y beam of the outgoing particles near beam or target rapidity for various energies appears to approach a fixed curve in the fragmentation region. This hypothesis is based on the geometrical picture of scattering as considered by Yang and coworkers. 2–4) According to their assumption, in the laboratory frame the projectile nuclei undergoes a Lorentz contraction in the form of a thin disk (or pancake) in the collision with the target nuclei which is further and further compressed with the increasing energy. However, the momentum and quantum number transfer process between the projectile and target does not appreciably change when the projectile is further and further compressed. This behavior leads the outgoing particles to retain the limiting fragmentation behavior in high energy collisions. Because of the large separation between the projectile and target fragments, limiting fragmentation admits no correlation between projectile and target fragments. 5) The hypothesis of limiting fragmentation, an asymptotic property of the nucleus–nucleus collisions at relativistic high energies, has been observed experimentally for a variety of collision processes such as hadron–hadron, 6,7) hadron–nucleus, 7,8) and nucleus–nucleus interactions 9–19) for produced charged hadrons at different energies and also for photons at forward pseudo-rapidity. 15,16,20) Apart from the multitude of pro- duced charged hadrons, the light, intermediate and heavy fragments (which are mainly the decay products of the projectile spectator and found near the beam rapidity region) are also emitted in a substantial amount in these heavy-ion collision experiments. The main sources of production of these fragments are nucleons and nucleon clusters formed in these nuclear collisions. 21) Out of these nuclear fragments, projectile helium fragments with charge Z ¼ 2, are most abundant in the projectile fragmentation region and are of great importance in studying the fragmentation process of the projectile nuclei. In this perspective, the study of limiting fragmentation phenomenon for these projectile helium fragments needs some more attention to be paid in order to understand the underlying physics of the fragmentation mechanism involved in such nuclear collisions at relativistic high energies. The projectile helium fragments with charge Z ¼ 2, have been studied earlier in nuclear emulsion experiments from the point of view of limiting fragmenta- tion by Heckman et al. 22) and by Bhanja et al. 5) at 2.1 A GeV energy. In this sequence, some other authors 23–25) further confirmed this hypothesis experimentally for projectile helium fragments in nucleus–nucleus interactions in later years. Such a quantitative study of limiting fragmentation hypothesis on the experimental basis for the projectile helium fragments in nucleus–nucleus interactions at 14.6 A GeV has not been performed so far. Recently, we have performed an extensive study to investigate some of the general characteristics of these projectile helium fragments emitted in nucleus–nucleus interactions for the same energy. 26) In the present paper, we discuss our detailed analysis from the point of view of testing the limiting fragmentation behavior in case of projectile helium frag- ments produced in nucleus–nucleus interactions at 14.6 A GeV energy, which will also throw some more light on the significant role of the fragmentation region in relativistic nuclear collisions. For this purpose, we discuss quantitatively projected angular distribution and average emission angle in individual helium reaction channels with different emulsion targets. We have also shown the energy independent behavior of limiting fragmentation phenomenon in the fragmentation region for projectile helium fragments in nucleus–nucleus interactions. Journal of the Physical Society of Japan 81 (2012) 124202 124202-1 FULL PAPERS #2012 The Physical Society of Japan http://dx.doi.org/10.1143/JPSJ.81.124202