It is very important to study the impact that backgrounds due to beam running conditions will have to the detector and any measurement. This was a hard lesson learned from past experiments (i.e. transition from HERA to HERA-II). Simulations have begun to investigate these issues as the interaction region and machine parameters are being designed. In this way, experiments can be proactive in ensuring that the machine design will not be adverse to physics in terms of background load. Two major sources of background that need to be studies are backgrounds due to protons in the beam interacting with residual gas in the beam pipe (beam-gas) and photons arising from synchrotron radiation due to the electron beam.
Presented below is the current status of the beam-gas studies and includes some useful code developed, as well as tips to continue these studies into the future. The basic workflow of the simulations studies can be summarized in the following steps: Background event generation → tracking through IR magnets and interaction with material in the IR → normalizing background rates by considering running conditions → compare to expected rates of physics measurements and see what areas of the detector will be impacted the most from the background. The steps described are detailed in individual sections below.
Background Event Generation
The first question one may ask is what is the dominant species of nuclei present in the residual beam gas. The vacuum experts at RHIC were consulted. The vacuum condition existing currently at RHIC are a good starting place for these studies. It was stated that roughly 90% of the gas is H2, with heavier gases making up the remaining content. Thus the current studies focus on p+H2 collisions. The Monte Carlo package DPMJet is used to generate these events.