Identification of extra neutral gauge bosons at the International Linear Collider

Heavy neutral gauge bosons, Z′s, are predicted by many theoretical schemes of physics beyond the Standard Model, and intensive searches for their signatures will be performed at present and future high energy colliders. It is quite possible that Z′s are heavy enough to lie beyond the discovery reach expected at the CERN Large Hadron Collider LHC, in which case only indirect signatures of Z′ exchanges may occur at future colliders, through deviations of the measured cross sections from the Standard Model predictions. We here discuss in this context the foreseeable sensitivity to Z′s of fermion-pair production cross sections at an e+e− linear collider, especially as regards the potential of distinguishing different Z′ models once such deviations are observed. Specifically, we assess the discovery and identification reaches on Z′ gauge bosons pertinent to the E6, LR, ALR and SSM classes of models, that should be attained at the planned International Linear Collider (ILC). With the high experimental accuracies expected at the ILC, the discovery and the identification reaches on the Z′ models under consideration could be increased substantially. In particular, the identification among the different models could be achieved for values of Z′ masses in the discovery (but beyond the identification) reach of the LHC. An important role in enhancing such reaches is played by the electron (and possibly the positron) longitudinally polarized beams. Also, although the purely leptonic processes are experimentally cleaner, the measurements of c- and b-quark pair production cross sections are found to carry important, and complementary, information on these searches.