Alexey Boyarsky

Long-range muons produced in proton–proton collisions at the ATLAS interaction point constitute the primary background for neutrino interaction searches at the SND@LHC experiment. This work presents a comprehensive characterization of the muon flux throughout LHC Run-3, benchmarking Monte Carlo simulations against experimental measurements. Measured and simulated muon rates agree within 10–15% across all Run-3 configurations. Following the substantial background increase in 2024 as a result of a beam optics change, the reversion to nominal optics in 2025 did not restore the 2022–2023 levels due to the unprecedented adoption of horizontal crossing in ATLAS. As indicated by simulation results, horizontal crossing enhanced the contribution of high-angle muons originating from diffractive proton losses in the LHC Dispersion Suppressor region. Their identification enabled the design of mitigation strategies that were experimentally validated. The simulation framework was also applied to the future high-luminosity LHC configuration, resulting in a considerable muon rate rise, driven by both the planned luminosity increase and the enlarged magnet aperture. Nevertheless, the upgrade from emulsion films to silicon vertex detectors will preserve the efficiency of the experiment even in such a high-rate environment.

This paper presents the design, construction, and simulation-based validation of the ColdBox, a combined neutron shielding and insulating enclosure for the Scattering and Neutrino Detector at the LHC (SND@LHC). The emulsion films in the detector's target region require protection from the intense neutron radiation background and a stable environment of 15 ± 1 °C and 50–55 % relative humidity for long-term stability. The ColdBox meets these requirements through a dual-layer structure: an external 5 cm plexiglass wall to moderate fast neutrons, and an internal 4 cm layer of borated polyethylene (with 35 % boron content) to capture thermal neutrons. The mechanical design, based on a robust aluminum frame, accommodates the constraints of the TI18 tunnel. FLUKA simulations were used to optimize the shielding configuration, showing a significant reduction in the neutron flux, with a simulated ratio of shielded to unshielded thermal neutron fluence of 2.3 × 10 ^-3 . This result is consistent with initial measurements from BatMon detectors. The design also provides a sealed volume for a cooling system to maintain the required temperature and humidity, ensuring the necessary conditions for the emulsion films' integrity.

The SND@LHC experiment was built for observing neutrinos arising from LHC pp collisions. The detector consists of two sections: a target instrumented with SciFi modules and a hadronic calorimeter/muon detector. Energetic νN collisions in the target produce hadronic showers. Reconstruction of the shower total energy requires an estimate of the fractions deposited in both the target and the calorimeter. In order to calibrate the SND@LHC response, a replica of the detector was exposed to hadron beams with 100 to 300 GeV in the CERN SPS H8 test beam line in Summer 2023. This report describes the methods developed to tag the presence of a shower, to locate the shower origin in the target, and to combine the target SciFi and the calorimeter signals so to measure the shower total energy.

During 2022/2023 the optimal inefficiency of the Veto system of the SND@LHC detector was measured to be (7.8 ± 2.8) × 10^-8. To reduce this inefficiency, a third Veto plane was installed during the 2023-2024 Year End Technical Stop. In addition, the Veto system was lowered to cover the target fully, thereby increasing acceptance. This paper describes how the inefficiency of the Veto system was reduced from (7.8 ± 2.8) × 10^-8 with an acceptance of about 64% of the target area in 2022-2023 to (4.9 ± 1.9) × 10^-9 on the full area in 2024.

We report the observation of neutrino interactions without final state muons at the LHC, with a significance of 6.4 σ . A dataset of proton-proton collisions at s = 13.6 TeV collected by SND@LHC in 2022 and 2023 is used, corresponding to an integrated luminosity of 68.6 fb − 1 . Neutrino interactions without a reconstructed muon are selected, resulting in an event sample consisting mainly of neutral-current and electron neutrino charged-current interactions in the detector. After selection cuts, 9 neutrino interaction candidate events are observed with an estimated background of 0.32 events.

The 3D distribution of matter at small scales encodes valuable information about the nature of dark matter and other fundamental physics. A prominent probe of such scales outside galaxies is the Lyman-alpha forest, which studies absorption features in the spectra of high-redshift quasars caused by neutral hydrogen. The measured quantity is the power spectrum of the absorbed flux, which indirectly traces the underlying matter distribution. However, the connection between the measured flux power spectrum and the underlying 3D dark matter power spectrum is highly nontrivial. The flux power spectrum (i) represents a one-dimensional projection of the density field; (ii) traces only neutral hydrogen, subject to thermodynamic pressure; and (iii) is a nonlinear function of local matter density. Additionally, thermal broadening and redshift-space distortions-determined not only by the hydrogen distribution but also by its thermal state and local velocity field-further complicate interpretation. To robustly constrain dark matter properties using the Lyman-alpha forest, these systematics must be carefully modeled and controlled. In this paper, we present a simple phenomenological recipe for mapping the 3D matter power spectrum to the flux power spectrum. We first motivate our approach in the linear regime, then extend it to later times and into the nonlinear regime. We validate our model against a broad suite of warm and cold dark matter simulations, demonstrating that our recipe yields consistent and accurate estimates across a wide parameter space.

The proposed upgrade of the SND@LHC experiment for the High Luminosity phase of the LHC (HL-LHC) will strongly benefit from the presence of a magnetized region, allowing for muon momentum and charge measurement. In this paper we describe an iron core magnet system that is partly integrated with the calorimeter and that is designed to respect the strict constraints from the available space in the experimental cavern, power consumption, and field requirements. Semi-analytical tools are introduced to explore the parameter space, in order to define the primary design options. A full 3-D analysis is then performed in order to validate the optimal choice, and to propose a conceptual design, including sizing of the components, detector performances and stray fields. Several technical options are also discussed, anticipating the design phase.