Speaker
Description
The Scattering and Neutron Detector at the LHC (SND@LHC), and the Forward
Search Experiment (FASER), located on opposite sides of IP1 near the ATLAS detector, study high-energy neutrinos and dark matter produced from proton-proton
collisions. A key challenge for both experiments is the muon background from the collisions and proton losses in the LHC ring. In 2024, a reversal of the triplet polarity in IR1 caused a substantial increase in muon background in both experiments.
Although nominal triplet polarity returned in 2025, background levels remained significantly higher than in 2023. LHC FLUKA simulations transported to SND, supported by track directions observed in the data, indicated primary proton losses in the dispersion suppressor (cell 11) were the dominant source of the excess background, associated with the crossing plane change from vertical to horizontal. To mitigate these losses, orbit bumps were designed to displace the loss pattern away
from cell 11. The bumps were designed using both conventional techniques and a novel Xsuite-based matching method employing beam losses as the optimisation observable. Experimental tests done in 2025 confirmed a background reduction in SND and FASER when activating the bumps. This study demonstrates that the proposed strategy is a viable operational solution for reducing muon backgrounds in future physics operations.
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