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We have investigated how the defonnation of the nuclear structure function per nucleon, which is known as the EMC effect, reveals itself in hard scatterings off nuclei. Our conclusion is that, in high PT hadron production reaction, the EMC effect as~ 0(10%) effect in nucleon structure function in nuclei cannot make the single hard scattering scheme survive and can have small influence upon the results within multiple hard scattering scheme. We also discuss the anomalous average PT increase observed in high energy heavy ion collisions. § 1. IntroductionThe phenomena, which are associated with large PT (transverse momentum) hadron production off nuclear target, have stimulated our interest continually.l) At the earlier stage of the study in this field, much attention has been paid to the space-time structure of large PT hadron production and also to the parton distribution within a nucleus. Because we have information on the parton distribution function within various nuclei,2) we are now able to investigate how the deformed nucleon structure function in nuclei affects the large PT hadron production off nuclei. As for the other phenomena related with high PT hadron production, recent observations of the increase of average P T in CERN -SPS collider,3) ISR 4 ) andin cosmic ray experiments 5 ),6) are also noticeable in the sense that those may be one of signals 6 ),7) of the quark-gluon plasma (QGP)8) formation.When the experimental inclusive cross section of large PT hadron production off nuclear target is parametrized as it turns out that a(PT) exceeds unity for PT<2 GeV ie. 9 ), IO) This fact has been well known as the anomalous nuclear enhancement (ANE), because of its sharp contrast with the naive expectation of no shadowing effects, a (p T) = 1. There exist two explanations for this ANE. One is the multiple hard scattering (MHS) ll)_H) of a parton in the projectile hadron with partons in the target nucleus. The other is the single hard scattering of a parton where the ANE is attributed to the deformation of the parton distribution function within a nuclear target,15) However, in both of these approaches, there are ambiguities:For the integration on the momentum of the propagating parton, a cutoff parameter, PTcut, should be introduced in the MHS scheme, to discriminate the hard scattering from the soft component. In the latter approach, it was uncertain how the parton distribution would be Present address : t)

We investigate" the multiple hard scattering in the high energy nucleus· nucleus collisions, in which partons in the different nucleons in the colliding nuclei undergo hard scattering separately. It turns out that the subsequent recombination of a quark and an antiquark into a meson of larger transverse momentum is crucial. We show that this mechanism can explain the anomalous nuclear enhancement of 7[0 production at large transverse momentum in the a-a collisions at the CERN-1SR.

The energy density of the central products in the ultra-relativistic heavy ion collisions are calculated. In our estimation, the special attention is paid to the space-time extension of the emission points of the secondary hadrons which are originated in the successive nucleon-nucleon interactions within the finite size of colliding nuclei. The average collision number per produced *On leave of absence from Institute for Nuclear Study, University of Tokyo, Tanashi, Tokyo 188. iv particle, in the final state interaction. is also calculated and is used as a criterion whether the system is thermalized or not. It turns out that the attainable energy density in the central heavy ion collision is sensitive to the space-time extension of the emission points. However, if the incident energy and the mass numbers of colliding nuclei are high and large, we can get high enough energy density for the phase transition from a hadronic state to a quark-gluon plasma state.

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