On the Potential Use of Zero Degree Calorimeters for LHC Luminosity Monitoring

We discuss the ZDC role in commissioning proton running at LHC. The ATLAS Zero Degree Calorimeters were designed to meet the needs of the Heavy Ion Program and follow closely experience at RHIC. In ATLAS, as at RHIC, they will be used primarily to measure absolute luminosity, determine reaction plane and centrality. They also provide a trigger sensitive to peripheral events- particularly those from diffractive photoproduction which is a promising area of research at LHC. Experience at RHIC showed that the ZDC's provide a unique background-free measure of instantaneous luminosity during pp running also. When operated with a calorimeter threshold of 10% of pbeam the coincidence rate between calorimeters forward in both beam directions corresponds to ~ 0.4% x sigma_pp inelastic. This robust signal is commonly used for accelerator tuning and for vernier scans (where it reliably measures luminosity variations over at least 3 decades) at RHIC. In this note we argue that the ATLAS ZDC (and similar devices - already in construction for ALICE and likely to be built for CMS also) will complement other available tools for commissioning the LHC with proton beams. In particular the ZDC discriminates effectively against beam-residual gas backgrounds as well as other sources which are not addressed in the ion chamber design and are notoriously hard to anticipate.