[Paper Review] XSEN: a $ν$N Cross Section Measurement using High Energy Neutrinos from pp collisions at the LHC

N. Beni, S. Buontempo+21 more|arXiv (Cornell University)|Oct 24, 2019

Particle physics theoretical and experimental studies4 citations

TL;DR

XSEN proposes a high-precision measurement of the neutrino-nucleon cross section at TeV-scale energies using nuclear emulsion detectors placed in the LHC's forward region (7.5 < η < 9.5), exploiting neutrinos from proton-proton collisions at IP1 and IP5. The experiment aims to collect ~100 tau neutrino interactions and measure cross sections in two energy bins (0.7 and 1.2 TeV), probing new physics beyond the Standard Model in the third generation via high-energy neutrino interactions.

ABSTRACT

XSEN (Cross Section of Energetic Neutrinos) is a small experiment designed to study, for the first time, neutrino-nucleon interactions (including the tau flavour) in the 0.5-1 TeV neutrino energy range. The detector will be installed in the decommissioned TI18 tunnel and uses nuclear emulsions. Its simplicity allows construction and installation before the LHC Run 3, 2021-2023; with 150/fb in Run3, the experiment can record up to two thousand neutrino interactions, and up to a hundred tau neutrino events. The XSEN detector intercepts the intense neutrino flux, generated by the LHC beams colliding in IP1, at large pseudo-rapidities, where neutrino energies can exceed the TeV. Since the neutrino-N interaction cross section grows almost linearly with energy, the detector can be light and still collect a considerable sample of neutrino interactions. In our proposal, the detector weighs less than 3 tons. It is lying slightly above the ideal prolongation of the LHC beam from the straight section; this configuration, off the beam axis, although very close to it, enhances the contribution of neutrinos from c and b decays, and consequently of tau neutrinos. The detector fits in the TI18 tunnel without modifications. We plan for a demonstrator experiment in 2021 with a small detector of about 0.5 tons; with 25/fb, nearly a hundred interactions of neutrinos of about 1 TeV can be recorded. The aim of this pilot run is a good in-situ characterisation of the machine-generated backgrounds, an experimental verification of the systematic uncertainties and efficiencies, and a tuning of the emulsion analysis infrastructure and efficiency. This Letter provides an overview of the experiment motivations, location, design constraints, technology choice, and operation.

Motivation & Objective

• To measure the neutrino-nucleon cross section at energies above 500 GeV, extending beyond existing accelerator data.
• To probe potential new physics in the third generation via high-energy tau neutrino interactions, motivated by anomalies in W and B decays.
• To characterize machine-induced backgrounds and validate detector performance using a pilot run with a 0.5-ton emulsion detector.
• To achieve independent measurements in two energy bins (0.7 and 1.2 TeV) using a 1.5–3 ton detector in LHC Run3.
• To enable precise, in-situ calibration of neutrino flux and background levels through emulsion-based tracking and angular reconstruction.

Proposed method

• Deploy nuclear emulsion detectors with lead layers in the forward region (7.5 < η < 9.5) at the TI18 cavern near ATLAS IP1.
• Use emulsions to record charged particle tracks with sub-millimeter angular resolution, distinguishing IP-originated particles from local backgrounds.
• Install emulsion stacks with neutron shielding (borated polyethylene or boron carbide foam) to prevent spurious blackening from neutron activation.
• Perform in-situ background characterization using emulsions and radiation monitors at candidate sites (e.g., VN, N, F, VF), with validation via Fluka simulations.
• Implement a two-phase operation: Phase 1 (2019–2021) for pilot run and infrastructure tuning; Phase 2 (2022–LS3) for full data collection with expanded mass.
• Use survey targets and precise brick placement to maintain known distance from the beam line, enabling accurate cross-section reconstruction.

Experimental results

Research questions

• RQ1 What is the energy-dependent νN cross section for neutrinos above 500 GeV, particularly for tau neutrinos?
• RQ2 Can high-energy neutrino interactions at the LHC reveal deviations from the Standard Model in the third generation?
• RQ3 How do machine-induced backgrounds (muons, hadrons, neutrons) affect neutrino detection in forward LHC regions?
• RQ4 What is the achievable sensitivity of emulsion detectors to rare tau neutrino interactions in the 0.7–1.2 TeV range?
• RQ5 Can a passive, low-intrusion detector system be successfully deployed and operated in the LHC tunnel without disrupting beam operations?

Key findings

• The experiment is designed to collect approximately 100 tau neutrino interactions over Run3, enabling direct measurement of νN cross sections at TeV energies.
• Background levels from local sources were measured at ~10^6 /cm² for hadrons and ~10^5 /cm² for muons, with a dominant 10^7 /cm² isotropic background from random tracks.
• Muon tracks from the IP were identified via sharp angular peaks at ~20 mrad in θx and ~0 mrad in θy, consistent with expected fluence and emulsion response.
• The pilot run with a 0.5-ton detector is expected to validate background models and optimize emulsion scanning efficiency prior to full deployment.
• The detector installation and emulsion exchange can be completed in less than eight hours with minimal disruption, ensuring compatibility with LHC schedule.
• Access via PM15 shaft and passage under the cryogenic line (80 cm wide, 34 cm high) has been confirmed safe and feasible for transporting detector components.