Abstract
The data produced at the particle physics experiments at the Large Hadron Collider (LHC) contain not only the signals from the collisions, but also a background component from proton losses around the accelerator. Understanding, identifying and possibly mitigating this machine-induced background is essential for an efficient data taking, especially for some new physics searches. Among the sources of background are hadronic and electromagnetic showers from proton losses on nearby collimators due to beam-halo cleaning. In this article, the first dedicated LHC measurements of this type of background are presented. Controlled losses of a low-intensity beam on collimators were induced, while monitoring the backgrounds in the ATLAS detector. The results show a clear correlation between the experimental backgrounds and the setting of the tertiary collimators (TCTs). Furthermore, the results are used to show that during normal LHC physics operation the beam halo contributes to the total beam-induced background at the level of a percent or less. A second measurement, where the collimator positions are tightened during physics operation, confirms this finding by setting a limit of about 10% to the contribution from all losses on the TCTs, i.e. the sum of beam halo and elastic beam-gas scattering around the ring. Dedicated simulations of the halo-related background are presented and good agreement with data is demonstrated. These simulations provide information about features that are not experimentally accessible, like correlations between backgrounds and the distributions of proton impacts on the collimators. The results provide vital information about the dependence between background and collimator settings, which is of central importance when optimizing the LHC optics for maximum peak luminosity.
Highlights
The Large Hadron Collider (LHC) [1,2] is a 27 km synchrotron, designed to collide two counterrotating beams of, up to, 7 TeV protons or heavy ions of equivalent magnetic rigidity, inside four experimental detectors
A second measurement, where the collimator positions are tightened during physics operation, confirms this finding by setting a limit of about 10% to the contribution from all losses on the tertiary collimators (TCTs), i.e. the sum of beam halo and elastic beam-gas scattering around the ring
The main loss locations are found on the primary collimators in interaction region 7 (IR7), and the loss levels decrease on downstream collimators in IR7
Summary
The Large Hadron Collider (LHC) [1,2] is a 27 km synchrotron, designed to collide two counterrotating beams of, up to, 7 TeV protons or heavy ions of equivalent magnetic rigidity, inside four experimental detectors. Machine-induced backgrounds arise from three conceptually different sources: (i) inelastic collisions of the beam protons with residual gas in the LHC beam vacuum; (ii) beam-halo losses on limiting apertures close to the experiment; and (iii) elastic beam-gas scattering around the accelerator, out of which a fraction results in protons lost on apertures close to the experiment. Operational experience with the LHC, so far, suggests that inelastic beam-gas interactions constitute the dominant source of machine-induced backgrounds [21,22]. Each TCT assembly includes one horizontal (TCTH) and one vertical (TCTV) set of jaws Their primary role is to protect the magnets of the final focusing system but they intercept the tertiary halo and some of the showers from upstream beam-gas interactions. In general the beam-halo background is reduced if the upstream collimators are set at tight apertures such that the tertiary halo is minimized
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