DOE OSTI2011
One source of experimental background in the LHC is showers induced by particles hitting the upstream collimators or particles that have been scattered on the residual gas. We estimate the flux and distribution of particles entering the ATLAS and CMS detectors through FLUKA simulations starting either in the tertiary collimators or with inelastic beam-gas interactions. Comparisons to MARS15 results are also presented. Our results can be used as a source term for further simulations of the machine-induced background in the experimental detectors. To ensure optimal performance of the LHC experimental detectors, it is important to understand the background, which can come fromseveral sources. In this article we discuss machine-induced background, caused either by nearby beam losses or interactions between beam particles and the residual gas inside the vacuum pipe. Beam losses outside the experimental interaction regions (IRs) are unavoidable during collider operation. The halo is continuously repopulated and has to be cleaned by the collimation system, so that the losses in the cold magnets are kept at a safe level. The collimation system is located in two dedicated insertions (IR3 and IR7) but a small leakage of secondary and tertiary halo is expected to escape. Some particles make it to the experimental IRs, where they are intercepted by tertiary collimators (TCTs) that are installed in order to protect the inner triplet magnets. Some parts of the induced high-energy shower can escape and propagate into the detectors. Another source of background is beam-gas interactions. Beam protons can scatter elastically or inelastically on residual gas molecules. If an inelastic interaction occurs close to the detector, it causes a shower that could reach the detector. Elastic interactions can scatter protons directly onto the TCTs without passing IR7, which has to be treated separately from the beam-halo losses discussed above. Machine-induced background can also originate from a cross-talk between different IPs. In this article we focus on beam-halo losses and inelastic beam-gas interactions two LHC experiments: ATLAS in IR1 and CMS in IR5. We compare also to previous results. We simulate the machine-induced showers propagating through the interaction region up to an interface plane between the machine and the detector, which is defined to be at 22.6m from the interaction point (IP) along the beam direction. The coordinates and momenta of the particles crossing this plane are recorded and can be used as a source term for further simulations of the detector itself. The interface plane extends to 30 m radially, although 99% of the total energy was found within a 2.5 m radius in the simulations.
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