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.

The Gamma-Ray Astro Imager with Nuclear Emulsion (GRAINE) project conducts precision observations of sub-GeV–GeV cosmic gamma rays using a balloon-borne nuclear-emulsion telescope with high angular resolution. In GRAINE 2023, a 2.5-m^2 telescope was flown in the project's first overnight balloon flight, including observation periods for the Vela pulsar and Galactic center region. To operate the telescope under the low-pressure and low-temperature stratospheric environment, the balloon-style pressure-vessel concept was scaled up to a lightweight gondola with an internal length of 4.9 m. A new aluminum-alloy ring structure and a lightweight membranous-shell material, SHL-300MDL, were developed. While the telescope aperture was increased by a factor of 6.6 over GRAINE 2018, the pressure-vessel gondola mass was limited to 179 kg. Ground tests of the completed flight assembly demonstrated a differential pressure above 100 hPa at room temperature and at a mean temperature of -66.0^∘C. The payload was launched from Alice Springs, Australia, in April 2023 and achieved a total flight duration of approximately 27 h, including 24.3 h of level flight. Although the upper membranous shell reached approximately -60^∘C at night, the vessel internal pressure remained above the required 100 hPa throughout level flight. These results demonstrate that the developed gondola can accommodate a 2.5-m^2 emulsion gamma-ray telescope and maintain the required pressure during overnight stratospheric flight. Scientific analyses of astrophysical and atmospheric gamma rays, including dedicated analysis of the Galactic center region, are ongoing using the recovered emulsion data. This development provides a technical basis for repeated observations with future large-area GRAINE telescopes.

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 GRAINE project observes cosmic gamma-rays, using a balloon-borne emulsion-film-based telescope in the sub-GeV/GeV energy band. We reported in our previous balloon experiment in 2018, GRAINE2018, the detection of the known brightest source, Vela pulsar, with the highest angular resolution ever reported in an energy range of >80 MeV. However, the emulsion scanning system used in the experiment was designed to achieve a high-speed scanning, and it was not accurate enough to ensure the optimum spatial resolution of the emulsion film and limited the performance. Here, we report anew high-precision scanning system that can be used to greatly improve the observation result of GRAINE2018 and also be employed in future experiments. The scanning system involves a new algorithm that recognizes each silver grain on an emulsion film and is capable of measuring tracks with a positional resolution for the passing points of tracks of almost the same as the intrinsic resolution of nuclear emulsion film (similar to 70 similar to 70 nm). This resolution is approximately one order of magnitude smaller than that obtained with the high-speed scanning system. With this scanning system, an angular resolution for gamma-rays of 0.1 degrees degrees at 1 GeV is expected to be achieved. Furthermore, we successfully combine the new high-precision scanning system with the existing high-speed scanning system, enabling the high-speed and high-precision measurements. Employing these techniques, we reanalyze the gamma-ray events detected previously by only the high-speed scanning system in GRAINE2018 and obtain an about three times higher angular resolution (0.22 degrees) degrees ) in the 500-700 MeV energy range. Adopting this technique in future observations may provide new insights into the gamma-ray emission from the Galactic center region and may realize polarization measurements of high-energy cosmic gamma-rays.

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.

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.

The resetting of a nuclear emulsion is one of the most important technologies for the start of its new era and was realized in the OPERA experiment. Since the OPERA experiment was carried out several months after the production of its nuclear emulsion layers, AgBrI grains in the layers were exposed to ionizing particles before experiments, and formed latent image centers as noises on their surfaces. The technology to reset the emulsion layers was needed and achieved by the refresh treatment, which erased the latent image centers by enhancing their fading under the condition with high humidity at high temperature in the presence of 5-methyl-benzotriazole (5-MBT). While the treatment is being applied to other experiments with nuclear emulsions, its mechanism has not yet been made clear. Taking into account the fact that 5-MBT played a central role for the effect of the treatment, this study has been undertaken to reveal the mechanism of the treatment by focusing on the behavior of 5-MBT. It has been revealed that the irreversible phase transition of 5-MBT for the formation of epitaxial silver salt nanoparticles of 5-MBT anions at the corners of the AgBrI grains during the treatment enhances the detachment of a silver ion from a latent image center on a site with electric charge of +1/2, thus making the center to bear the electric charge of -1/2 and unstable against oxidation. Ideas have been proposed to make the treatment available for other experiments with nuclear emulsions.

The FASER experiment at the LHC is designed to search for light, weakly-interacting particles produced in proton-proton collisions at the ATLAS interaction point that travel in the far-forward direction. The first results from a search for dark photons decaying to an electron-positron pair, using a dataset corresponding to an integrated luminosity of 27.0 fb$^{-1}$ collected at center-of-mass energy $\sqrt{s} = 13.6$ TeV in 2022 in LHC Run 3, are presented. No events are seen in an almost background-free analysis, yielding world-leading constraints on dark photons with couplings $\epsilon \sim 2 \times 10^{-5} - 1 \times 10^{-4}$ and masses $\sim$ 17 MeV - 70 MeV. The analysis is also used to probe the parameter space of a massive gauge boson from a U(1)$_{B-L}$ model, with couplings $g_{B-L} \sim 5 \times 10^{-6} - 2 \times 10^{-5}$ and masses $\sim$ 15 MeV - 40 MeV excluded for the first time.

The Forward Search Experiment (FASER) at CERN's Large Hadron Collider (LHC) has recently directly detected the first collider neutrinos. Neutrinos play an important role in all FASER analyses, either as signal or background, and it is therefore essential to understand the neutrino event rates. In this study, we update previous simulations and present prescriptions for theoretical predictions of neutrino fluxes and cross sections, together with their associated uncertainties. With these results, we discuss the potential for possible measurements that could be carried out in the coming years with the FASER neutrino data to be collected in LHC Run 3 and Run 4.

The first results of the study of high-energy electron neutrino (νe) and muon neutrino (νμ) charged-current interactions in the FASERν emulsion-tungsten detector of the FASER experiment at the LHC are presented. A 128.8 kg subset of the FASERν volume was analyzed after exposure to 9.5 fb−1 of s=13.6 TeV pp data. Four (eight) νe (νμ) interaction candidate events are observed with a statistical significance of 5.2σ (5.7σ). This is the first direct observation of νe interactions at a particle collider and includes the highest-energy νe and νμ ever detected from an artificial source. The interaction cross section per nucleon σ/Eν is measured over an energy range of 560–1740 GeV (520–1760 GeV) for νe (νμ) to be (1.2−0.7+0.8)×10−38 cm2 GeV−1 [(0.5±0.2)×10−38 cm2 GeV−1], consistent with standard model predictions. These are the first measurements of neutrino interaction cross sections in those energy ranges. Published by the American Physical Society 2024

FASER, the ForwArd Search ExpeRiment, is an experiment dedicated to searching for light, extremely weakly-interacting particles at CERN's Large Hadron Collider (LHC). Such particles may be produced in the very forward direction of the LHC's high-energy collisions and then decay to visible particles inside the FASER detector, which is placed 480 m downstream of the ATLAS interaction point, aligned with the beam collisions axis. FASER also includes a sub-detector, FASER$\nu$, designed to detect neutrinos produced in the LHC collisions and to study their properties. In this paper, each component of the FASER detector is described in detail, as well as the installation of the experiment system and its commissioning using cosmic-rays collected in September 2021 and during the LHC pilot beam test carried out in October 2021. FASER will start taking LHC collision data in 2022, and will run throughout LHC Run 3.

We are developing the Gamma-Ray Astro-Imager with Nuclear Emulsion project, designed for 10 MeV–100 GeV cosmic γ -ray observations with a high angular resolution (5′/0.°08 at 1–2 GeV) and a polarization-sensitive large-aperture (∼10 m 2 ) emulsion telescope for repeated long-duration balloon flights. In 2018, a balloon-borne experiment was carried out in Australia with a 0.38 m 2 sensitive area and a flight duration of 17.4 hr, including 6.7 hr of Vela observations. Significant improvements compared with the 2015 balloon-borne experiment were achieved by a factor of 5, including both an increase in effective area × time and a reduction in the background contribution. We aimed to demonstrate the telescope’s overall performance based on detection and imaging of a known γ -ray source, the Vela pulsar. A robust detection of the Vela pulsar was achieved with a 68% containment radius of 0.°42, at a significance of 6 σ , at energies above 80 MeV. The resulting angular profile is consistent with that of a pointlike source. We achieved the current best imaging performance of the Vela pulsar using an emulsion γ -ray telescope with the highest angular resolution of any γ -ray telescope to date.

Research utilizing the sub-micron three-dimensional spatial resolution of nuclear emulsion detectors has been expanding in various fields, triggered by the development of automated emulsion scanning technology. This paper describes a new production system capable of supplying several thousand square meters of nuclear emulsion film per year. The development and introduction of improved emulsion gel, knife coating, and drying equipment have enabled the mass production of double-side coated nuclear emulsion films with thick emulsion layers suitable for full-area scanning and analysis. This system has played an essential role in numerous nuclear emulsion experiments.

The GRAINE experiment is the balloon flight experiment with emulsion chamber telescope for observing of gamma rays that have energy range of 10 MeV to 100 GeV. We launched the emulsion chamber telescope in Australia in the April, 2023 (GRAINE2023). GRAINE2023 have the larger aperture area than previous experiments, and the aperture area increase from 0.38 square meters to 2.5 square meters. The nuclear emulsion film is the kind of photographic film, and we can observe emulsion films on the microscope after the development. The scale of experiments with the nuclear emulsion film increases, the Hyper Track Selector (THS) was developed for convert track in the nuclear emulsion film into data at high speed, and have been producing success. Additionally, the HTS2 was developed for improvement of the scanning speed over THS, by lowering the magnification and widening the field of view of the captured image. On the other hand, the THS2 is concerns about the deterioration of the detection performance of developed silver. As one way, to improve the detection performance of developed silver, we changed the developer, which makes enlarge developed silver grains. So far, we have used OPERA’s development, but we made development possible to adjust the size of developed silver grains by mixing chemicals with ourselves. We study and develop of high-contrast developing for the nuclear emulsion film and report the result of developed emulsion films of GRAINE2023.

The Fermi Gamma-ray Space Telescope has surveyed the sub-GeV/GeV gamma-ray sky and provided a large amount of data. However, observation remains difficult owing to the lack of angular resolution, and new issues have arisen. We started up a precise gamma-ray observation project, Gamma-Ray Astro-Imager with Nuclear Emulsion (GRAINE), using balloon-borne emulsion gamma-ray telescopes to enable high angular resolution (0.1 degrees at 1 GeV), polarization-sensitive, and large-aperture observations (10 m$^2$) in the 0.01--100 GeV energy region. In the last balloon experiment, which was performed in 2018, we succeeded in the first detection of a celestial gamma-ray object, Vela pulsar, via the balloon-borne emulsion telescope, and the world's highest angular resolution was demonstrated. We start the scientific observation phase by enlarging the aperture area, extending the flight duration, and repeating balloon flights. The GRAINE 2023 balloon experiment was conducted in April 2023. The experiment aims at the observation of Vela pulsar, the Galactic center, etc. in the GeV energy region, and the survey of transient phenomena by the largest aperture area telescope. In preparation for GRAINE 2023, the new facility constructed at Nagoya University was used to produce nuclear emulsion, completing a 750 m$^2$ emulsion films, the largest area of all the experiments using Nagoya-made emulsion. In this presentation, we report on the emulsion detector for gamma-ray measurement and its technology, as well as the latest status of GRAINE 2023.