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• W85143781 abstract "With a fully-operational high-efficient collimation system in the LHC, nuclear interactions of circulating protons with residual gas in the machine beam pipe can be a major source of beam losses in the vicinity of the collider detectors, responsible for the machine-induced backgrounds. Realistic modeling of Coulomb scattering, elastic and inelastic interactions of 7-TeV protons with nuclei in the vacuum chamber of the cold and warm sections of the LHC ring with an appropriate pressure profile is performed with the STRUCT and MARS15 codes. Multi-turn tracking of the primary beams, propagation of secondaries through the lattice, their interception by the tertiary collimators TCT as well as properties of corresponding particle distributions at the CMS and ATLAS detectors are studied in great detail and results presented in this paper. MACHINE-INDUCED BACKGROUNDS Beam loss in the vicinity of interaction points (IP) at the LHC is an outstanding source of background rates in the collider detectors, called machine-induced backgrounds (MIB) [1, 2]. As shown in [3], the relative importance of this component can be comparable to that from the ppcollisions at early operation of the LHC because MIB is mostly related to beam intensity and not luminosity, and tuning the machine will require substantial time and efforts. In this paper we consider the design LHC beam with 2808 bunches of 7-TeV protons in the scrubbed machine. At nominal operation, there are three sources of MIB for the experiments [2]: • Collimator tails: protons escaping the betatron and momentum cleaning insertions (IP7 and IP3, respectively) and being intercepted by the tertiary collimators TCT. This term, related to the inefficiency of the main collimation system, is called “tails from collimators” or “tertiary beam halo”. The TCTs are situated between the neutral beam absorber (TAN) and D2 separation dipole at about 148m on each side of IP1 and IP5. They are set to 8.3σ to fully protect the triplet magnets. For the betatron cleaning in IP7 at the rate of 8.3×10 p/s, a 10-hr beam lifetime and nominal intensity, the loss rates on the TCTs are 2.61×10 p/s and 4.28×10 p/s for Beam-2 approaching IP5 and Beam-1 approaching IP1, respectively [2]. • Inelastic beam-gas: nuclear inelastic (including quasi-elastic and diffractive) interactions of the incoming beam with the residual gas. Products of such interactions in straight sections and arcs upstream of the experiments have a good chance to be lost on limiting apertures in front of the collider detectors. The rate of beam-gas interactions is proportional to the beam intensity and residual gas pressure in the beam pipe. Detailed studies since the first papers on MIB in LHC [1, 3] have shown that relatively large-angle inelastic nuclear interactions in the 550-m regions upstream of IP1 and IP5 are mostly responsible for the beam-gas component of MIB. The total number of inelastic and quasi-elastic nuclear interactions in these regions for each of the beams coming to IP1 and IP5 is 3.07×10 p/s [2]. • Elastic beam-gas: nuclear elastic – coherent and incoherent (quasi-elastic) interactions as well as Coulomb scattering on residual gas around the ring. First two sources are studied in great detail in thorough MARS15 [4] calculations. Modeling approach and results of calculations of energy-dependent particle fluxes in the machine components, tunnel, and ATLAS and CMS experimental halls as described in [2]. Third component is described in this paper. ELASTIC BEAM-GAS INTERACTIONS As described in [5, 6], the main process of beam-gas interaction, Coulomb scattering, results in slow diffusion of protons from the beam core causing emittance growth. These particles increase their betatron amplitudes gradually during many turns and are intercepted by the main collimators before they reach other limiting apertures. Similar behavior takes place for small-angle elastic nuclear scattering. For certain beam parameters, large-angle nuclear elastic and Coulomb scattering can behave quite differently [6]. Such processes can result in a substantial increase of the betatron amplitude and, if not intercepted by the main collimators, the scattered protons can be lost in the vicinity of the experimental insertions, predominantly giving rise to the “scraping” rate on the TCTs and adding to MIB in the detectors. A differential cross section of nuclear elastic scattering can be parameterized as a sum of two exponential distributions. The slope of coherent elastic scattering has weak energy dependence and can be taken from [7]. The slope of quasi-elastic scattering is close to the slope of pp elastic scattering at high energies. An approximation [8] of experimental data on the slope of elastic pp-scattering at 2-2000 GeV/c is extrapolated to 7 TeV. Total cross sections of these processes are calculated using the ____________________________________________ *Work supported by Fermi Research Alliance, LLC, under contract No. DE-AC02-07CH11359 with the U.S. Department of Energy. mokhov@fnal.gov Proceedings of PAC09, Vancouver, BC, Canada WE6PFP027" @default.
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• W85143781 date "2009-04-01" @default.
• W85143781 modified "2023-09-25" @default.
• W85143781 title "Beam Losses and Background Loads on Collider Detectors Due to Beam-Gas Interactions in the LHC" @default.
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