Asymmetry distributions and mass effects in dijet events at a polarized

The asymmetry distributions for several kinematic variables are considered for finding a systematic way to maximize the signal for the extraction of the polarized gluon density. The relevance of mass effects for the corresponding dijet cross section is discussed and the different approximations for including mass effects are compared. We also compare via the programs Pepsi and Mepjet two different Monte Carlo (MC) approaches for simulating the expected signal in the dijet asymmetry at a polarized HERA. The isolation and investigation of the polarized gluon parton distribution is one of the most interesting topics for deep inelastic scattering. The three main approaches are the analysis of dijet events, on which we will concentrate here, the Q 2 dependence of structure functions [1], and the production of charmed particles [2]. Up-to-date parton distributions based on all experimental input available do not yet determine the form of the polarized gluon distribution with any degree of reliability [3]. This underscores the great significance of a polarized HERA, where all three approaches described above would be feasible. In the analysis of dijet events two different processes have to be taken into account, the photon-gluon fusion (PGF) and the gluon bremsstrahlung (GB), see Fig. 1. Only the photon-gluon fusion allows to analyze the gluon parton distribution, while the gluon bremsstrahlung is a background process. In the following we will investigate to which extend we can get information about the PGF events from the polarized dijet cross section. For this purpose we discuss in the first section, after some remarks to the kine-matic cuts used, the influence of quark and remnant masses on the dijet cross section in different MC approaches. In the second section we analyze the spin asymmetry of dijet events as a function of several kinematic variables. We present a method of how to find suitable cuts in these kinematic variables so that the significance of the asymmetry signal becomes largest. This is a continuation of the discussion started in [4], where the discussion was concentrated on the kinematic variable x g , the fractional momentum carried by the gluon. The discussion is done on the level of parton jets because on this level the QCD improved parton model provides a well defined theory, where comparison of different MC approaches is possible in a quantitative way. Moreover, this will be the bases for more parameter-dependent additions like parton showering, to describe higher-order effects, …