The Forward Physics Facility (FPF) at the High-Luminosity LHC (HL-LHC) will enable high-statistics measurements of TeV-scale neutrinos, but the intense flux of forward muons poses a major challenge for neutrino detectors in the far-forward region. We investigate the suppression of background muons using sweeper magnets with a simulation framework combining SIBYLL event generation, BDSIM beam transport, Geant4 particle tracking, and realistic magnetic field maps. Starting from a muon flux of 3.8×10^3 cm^-2 per fb^-1 without magnets, a magnet in the LHC tunnel alone achieves the target level of 2×10^3 cm^-2 per fb^-1. Additional magnets at the TI18 tunnel and FPF entrance further reduce the flux to 1.5×10^3 cm^-2 per fb^-1 in the optimized configuration. These results demonstrate that a properly optimized multi-stage sweeper magnet system can significantly reduce the forward muon background, while also highlighting the importance of realistic transport simulations and geometrical constraints in achieving further suppression.

We present a momentum measurement method based on multiple Coulomb scattering (MCS) in the FASERν emulsion detector. The measurement of charged-particle momenta is essential for studying neutrino interactions in the TeV energy range at the FASER experiment. This method exploits the sub-micron spatial resolution and long tracking length of the FASERν detector, enabling momentum determination from a few GeV up to a few TeV. The performance was evaluated using Geant4-based Monte Carlo simulations and validated with muon test beam data in the momentum range 100-300 GeV. As a first probe of the method for higher momentum muons, background muons recorded by the FASERν detector were examined, showing reconstructed momenta consistent with expectations from their angular spread.

We present methods for electromagnetic shower reconstruction and identification in the FASERnu emulsion detector using 100 GeV and 200 GeV electron test-beam data from the CERN SPS H4 beamline. The reconstruction employs a clustering-based algorithm without energy-dependent tuning to determine shower axes. A multi-level identification chain comprising track pre-selection, a cut-based selection, and a BDT classifier achieves combined background rejection rates of 99.99

The Forward Physics Facility (FPF) is a proposed extension of the HL-LHC program designed to exploit the unique scientific opportunities offered by the intense flux of high energy neutrinos, and possibly new particles, in the far-forward direction. Located in a well-shielded cavern 627 m downstream of one of the LHC interaction points, the facility will support a broad and ambitious physics program that significantly expands the discovery potential of the HL-LHC. Equipped with four complementary detectors – FLArE, FASERν2, FASER2, and FORMOSA – the FPF will enable breakthrough measurements that will advance our understanding of neutrino physics, quantum chromodynamics, and astroparticle physics, and will search for dark matter and other new particles. With this Letter of Intent, we propose the construction of the FPF cavern and the construction, integration, and installation of its experiments. We summarize the physics case, the facility design, the layout and components of the detectors, as well as the envisioned collaboration structure, cost estimate, and implementation timeline.

This Letter presents the measurement of the energy-dependent neutrino-nucleon cross section in tungsten and the differential flux of muon neutrinos and antineutrinos. The analysis is performed using proton-proton collision data at a center-of-mass energy of 13.6 TeV and corresponding to an integrated luminosity of (65.6±1.4)  fb^{-1}. Using the active electronic components of the FASER detector, 338.1±21.0 charged current muon neutrino interaction events are identified, with backgrounds from other processes subtracted. We unfold the neutrino events into a fiducial volume corresponding to the sensitive regions of the FASER detector and interpret the results in two ways: (i) we use the expected neutrino flux to measure the cross section, and (ii) we use the predicted cross section to measure the neutrino flux. Both results are presented in six bins of neutrino energy, achieving the first differential measurement in the TeV range. The observed distributions align with standard model predictions. Using this differential data, we extract the contributions of neutrinos from pion and kaon decays.

This paper presents the reconstruction and performance evaluation of the FASERν emulsion detector, which aims to measure interactions from neutrinos produced in the forward direction of proton-proton collisions at the CERN Large Hadron Collider. The detector, composed of tungsten plates interleaved with emulsion films, records charged particles with sub-micron precision. A key challenge arises from the extremely high track density environment, reaching 𝒪(10^5) tracks per cm^2. To address this, dedicated alignment techniques and track reconstruction algorithms have been developed, building on techniques from previous experiments and introducing further optimizations. The performance of the detector is studied by evaluating the single-film efficiency, position and angular resolution, and the impact parameter distribution of reconstructed vertices. The results demonstrate that an alignment precision of 0.3 micrometers and robust track and vertex reconstruction are achieved, enabling accurate neutrino measurements in the TeV energy range.

The FASER experiment at CERN has opened a new window in collider neutrino physics by detecting TeV-energy neutrinos produced in the forward direction at the LHC. Building on this success, this document outlines the scientific case and design considerations for an upgraded FASER neutrino detector to operate during LHC Run 4 and beyond. The proposed detector will significantly enhance the neutrino physics program by increasing event statistics, improving flavor identification, and enabling precision measurements of neutrino interactions at the highest man-made energies. Key objectives include measuring neutrino cross sections, probing proton structure and forward QCD dynamics, testing lepton flavor universality, and searching for beyond-the-Standard Model physics. Several detector configurations are under study, including high-granularity scintillator-based tracking calorimeters, high-precision silicon tracking layers, and advanced emulsion-based detectors for exclusive event reconstruction. These upgrades will maximize the physics potential of the HL-LHC, contribute to astroparticle physics and QCD studies, and serve as a stepping stone toward future neutrino programs at the Forward Physics Facility.

The DsTau(NA65) experiment at CERN was proposed to measure an inclusive differential cross-section of $$D_s$$ D s production with decay to tau lepton and tau neutrino in p – A interactions. The DsTau detector is based on the nuclear emulsion technique, which provides excellent spatial resolution for detecting short-lived particles like charmed hadrons. This paper presents the first results of the analysis of the pilot-run (2018 run) data and reports the accuracy of the proton interaction vertex reconstruction. High precision in vertex reconstruction enables detailed measurement of proton interactions, even in environments with high track density. The measured data has been compared with several Monte Carlo event generators in terms of multiplicity and angular distribution of charged particles. The multiplicity distribution obtained in p–W interactions is tested for KNO-G scaling and is found to be nearly consistent. The interaction length of protons in tungsten is measured to be $$93.7 \pm 2.6~\text {mm}.$$ 93.7 ± 2.6 mm . The results presented in this study can be used to validate event generators of p – A interactions.

A bstract The first FASER search for a light, long-lived particle decaying into a pair of photons is reported. The search uses LHC proton-proton collision data at $$ \sqrt{s} $$ s = 13 . 6 TeV collected in 2022 and 2023, corresponding to an integrated luminosity of 57 . 7 fb − 1 . A model with axion-like particles (ALPs) dominantly coupled to weak gauge bosons is the primary target. Signal events are characterised by high-energy deposits in the electromagnetic calorimeter and no signal in the veto scintillators. One event is observed, compared to a background expectation of 0 . 44 ± 0 . 39 events, which is entirely dominated by neutrino interactions. World-leading constraints on ALPs are obtained for masses up to 300 MeV and couplings to the Standard Model W gauge boson, g aWW , around 10 − 4 GeV − 1 , testing a previously unexplored region of parameter space. Other new particle models that lead to the same experimental signature, including ALPs coupled to gluons or photons, U(1) B gauge bosons, up-philic scalars, and a Type-I two-Higgs doublet model, are also considered for interpretation, and new constraints on previously viable parameter space are presented in this paper.

The muon puzzle – an excess of muons relative to simulation predictions in ultra-high-energy cosmic-ray air showers – has been reported by many experiments. This suggests that forward particle production in hadronic interactions is not fully understood. Some of the scenarios proposed to resolve this predict reduced production of forward neutral pions and enhanced production of forward kaons (or other particles). The FASER experiment at the LHC is located 480 m downstream of the ATLAS interaction point and is sensitive to neutrinos and muons, which are the decay products of forward charged pions and kaons. In this study, the latest measurements of electron and muon neutrino fluxes are presented using the data corresponding to 9.5 fb^-1 and 65.6 fb^-1 of proton-proton collisions with √(s)=13.6 TeV by the FASERν and the FASER electronic detector, respectively. These fluxes are compared with predictions from recent hadronic interaction models, including EPOS-LHCr, SIBYLL 2.3e, and QGSJET 3. The predictions are generally consistent with the measured fluxes from FASER, although some discrepancies appear in certain energy bins. More precise flux measurements with additional data will follow soon, enabling validation of pion, kaon, and charm meson production with finer energy binning, reduced uncertainties, and multi-differential analyses.

A comprehensive analysis of centrality dependence of event-by-event fluctuations of pseudo-rapidity multiplicity combination in terms of strongly intensive quantity has been carried out in 22Ne-emulsion interactions at 4.1 AGeV/c. The ensemble of events has been categorized into four centrality classes based on the total charges or sum of the charges of non-interacting projectile fragments (Q). Notably, significant fluctuations in pseudo-rapidity are observed across different centrality ranges. Strongly intensive quantity Δ increases with decreasing centrality while Σ increases weakly with decreasing centrality up to Q = 4. Our study reflects that change in the strongly intensive quantity Δ[R,N] and Σ[R,N] does not depend on the number of analyzed events in each class. Experimental results are compared with the analysis of MC-RAND events.

A completely new study of forward-backward multiplicity fluctuation of produced pions in nuclear emulsion detector has been carried out by the method of strongly intensive quantity ΣFB for 16O-AgBr, 28Si-AgBr and 32S-AgBr interactions at 4.5 AGeV/c. Experimental analysis has been compared using MC-RAND events and UrQMD simulated events. UrQMD model could not reproduce the experimental results. Outcome of this analysis supports the presence of dynamical fluctuations in multiparticle production at 4.5 AGeV/c. The results on strongly intensive fluctuation measures at a lower energy show much larger fluctuations in comparison to the higher energy data studied so far.

Tau neutrino is the least studied lepton of the Standard Model (SM). The NA65/DsTau experiment targets to investigate D s , the parent particle of the ν τ , using the nuclear emulsion-based detector and to decrease the systematic uncertainty of ν τ flux prediction from over 50 % to 10 % for future beam dump experiments. In the experiment, the emulsion detectors are exposed to the CERN SPS 400 GeV proton beam. To provide optimal conditions for the reconstruction of interactions, the protons are required to be uniformly distributed over the detector's surface with an average density of 10 5 cm -2 and the fluctuation of less than 10%. To address this issue, we developed a new proton irradiation system called the target mover. The new target mover provided irradiation with a proton density of 1.01 × 10 5 cm -2 and the density fluctuation of 1.9 ± 0.3% in the DsTau 2021 run.

A detailed study of centrality dependence of event-by-event fluctuations of maximum pseudo-rapidity gap of the produced particles has been carried out in terms of the scaled variance ω for 22Ne-emulsion interactions at 4.1 AGeV/c and 28Si-emulsion interactions at 4.5 AGeV/c. Depending on the values of the total charges or sum of the charges of non interacting projectile fragments, event samples were classified into three different centrality classes. Presence of centrality dependence of event-by-event fluctuations of maximum pseudo-rapidity gap is reflected in the multiparticle production process from this analysis. The event-by-event fluctuations are found to increase with decreasing centrality of collisions. Experimental analysis has been compared with the results obtained from the analysis of Monte Carlo simulated (MC-RAND) events in order to extract dynamical fluctuations. The difference between the experimental and simulated values of event-by-event fluctuations establishes the existence of true dynamical correlation among the produced particles.

The well-known statistical distribution function, Weibull distribution function, has been employed to study the multiplicity distribution and multiplicity moments for the backward shower particles produced in case of total disintegrated events of [Formula: see text]O–AgBr, [Formula: see text]Ne–AgBr and [Formula: see text]Si–AgBr interactions at (4.1–4.5) AGeV/c. Multiplicity fluctuation of backward shower particles in case of total disintegrated events by the method of scaled variance and the forward–backward asymmetry parameter had also been investigated.

Experiments in preparation for search for uranium ternary fission by means of nuclear track emulsion are summarized. The study will be focused on the possible involvement of the unstable nucleus ${}^{8}$Be in the suggested scenario of the collinear tri-partition in the fission.

This paper presents a study of bin–bin correlation of the produced shower particles in the pseudo-rapidity space by the method of factorial correlator in [Formula: see text]O-AgBr and [Formula: see text]S-AgBr interactions at 4.5[Formula: see text][Formula: see text]GeV/[Formula: see text]. The correlated moments are found to increase with decreasing bin–bin separation D, following a power law. Strong bin–bin correlation is exhibited by the experimental data. Experimental data also supports the validity of log normal approximation. Experimental analysis has been compared with the results obtained from the analysis of events simulated by UrQMD model.

Status and prospects of nuclear clustering studies by dissociation of relativistic nuclei in nuclear track emulsion are presented. The unstable 8 Be and 9 B nuclei are identified in dissociation of the isotopes 9 Be, 10 B, 10 C and n C, and the Hoyle state in the cases 12 C and 16 O. On this ground searching for the Hoyle state and more complex α -particle states in the dissociation of the heavier nuclei is suggested. A detailed study of a low-density baryonic matter arising in dissociation of the heaviest nuclei is forthcoming long-term problem. An analysis of nuclear fragmentation induced by relativistic muons is proposed to examine the mechanism dissociation.

A detailed study on the multiplicity distribution of helium (He) fragments emitted in projectile fragmentation of 22 Ne, 28 Si, and 32 S projectiles on interactions with H, CNO, and AgBr targets in a nuclear emulsion track detector has been presented at an incident momentum of (4.1–4.5) AGeV/c. The percentage of events with He fragments increases with increase in projectile mass for all three targets. Studies of multiplicity moments, dispersion of multiplicity distribution, and the investigation of dynamical fluctuations of the projectile He fragments have also been carried out.

The interest in using the radiation detectors based on high resistive chromium-compensated GaAs (GaAs:Cr) in high energy physics and others applied fields has been growing steadily due to its numerous advantages over others classical materials. High radiation hardness at room temperature stands out and needs to be systematically investigated. In this paper an experimental study of the effect of 20.9 MeV electrons generated by the LINAC-200 accelerator on some properties of GaAs:Cr based sensors is presented. In parallel, Si sensors were irradiated at the same conditions, measured and analyzed in order to perform a comparative study. The target sensors were irradiated with the dose up to 1.5 MGy. The current-voltage characteristics, resistivity, charge collection efficiency and their dependences on the bias voltage and temperature were measured at different absorbed doses. An analysis of the possible microscopic mechanisms leading to the observed effects in GaAs:Cr sensors is presented in the article.