Global Variables in Heavy Ion Collisions at the LHC using the ATLAS Detector

While high $p_T$ phenomena are of primary interest at the LHC, due to the expected increased rates of hard processes, the subsequent modification of jets and heavy quark transport will depend on the energy and gluon density of the medium, as well as its dynamical behavior reflected in the global properties of the "underlying event". "Global" observables, which include the total and differential charged-particle yields ($dN/d\eta$), transverse energy ($dE_T/d\eta$) and charged particle spectra, are typically thought to be dominated by the non-perturbative soft processes observed in nucleon-nucleon interactions, with some contribution from hard processes. The interplay between the two reflects the nuclear collision geometry and beam energies, and leads to various proposed scaling laws that describe various aspects of the data. These have been studied at RHIC since the earliest Au+Au runs. With the upcoming thirty-fold increase in energy and similarly large nuclei, Pb+Pb collisions at the LHC offer a powerful lever arm to test various scaling laws associated with the energy and impact-parameter dependence of bulk particle production. The ATLAS detector at the LHC has the means to do a comprehensive study of global observables both in p+p collisions (at full LHC energy) and Pb+Pb collisions (at $\sqrt{s_{NN}}$=5520 GeV) over a wide rapidity range ($|\eta| <2.5$ for charged particles, and $|\eta|<5$ for energy measurements). The collision geometry can be determined by a variety of variables, both energy and multiplicity, in comparison to Glauber models. The present status of physics performance studies will be shown in the context of theoretical expectations for the LHC based on RHIC data.