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First neutrino interaction candidates at the LHC

FASER Collaboration, Henso Abreu, Yoav Afik, Claire Antel, Jason Arakawa, Akitaka Ariga, Tomoko Ariga, Florian Bernlochner, Tobias Boeckh, Jamie Boyd, Lydia Brenner, Franck Cadoux, David W. Casper, Charlotte Cavanagh, Francesco Cerutti, Xin Chen, Andrea Coccaro, Monica D'Onofrio, Candan Dozen, Yannick Favre, Deion Fellers, Jonathan L. Feng, Didier Ferrere, Stephen Gibson, Sergio Gonzalez-Sevilla, Carl Gwilliam, Shih-Chieh Hsu, Zhen Hu, Giuseppe Iacobucci, Tomohiro Inada, Ahmed Ismail, Sune Jakobsen, Enrique Kajomovitz, Felix Kling, Umut Kose, Susanne Kuehn, Helena Lefebvre, Lorne Levinson, Ke Li, Jinfeng Liu, Chiara Magliocca, Josh McFayden, Sam Meehan, Dimitar Mladenov, Mitsuhiro Nakamura, Toshiyuki Nakano, Marzio Nessi, Friedemann Neuhaus, Laurie Nevay, Hidetoshi Otono, Carlo Pandini, Hao Pang, Lorenzo Paolozzi, Brian Petersen, Francesco Pietropaolo, Markus Prim, Michaela Queitsch-Maitland, Filippo Resnati, Hiroki Rokujo, Marta Sabaté-Gilarte, Jakob Salfeld-Nebgen, Osamu Sato, Paola Scampoli, Kristof Schmieden, Matthias Schott, Anna Sfyrla, Savannah Shively, John Spencer, Yosuke Takubo, Ondrej Theiner, Eric Torrence, Sebastian Trojanowski, Serhan Tufanli, Benedikt Vormwald, Di Wang, Gang Zhang

TL;DR

This paper reports the first observation of neutrino interaction candidates from collider-produced neutrinos using the FASERν pilot detector in the LHC TI18 tunnel. It details the detector design, the 2018 Run-2 data collection (12.2 fb^-1 at 13 TeV), and a comprehensive simulation framework to estimate forward-neutrino fluxes and muon-induced neutral-hadron backgrounds. A multivariate approach, including a boosted decision tree, is employed to separate neutrino-like neutral vertices from backgrounds, yielding a 2.7σ excess consistent with neutrino interactions. The results validate collider neutrino detection and outline Run-3 improvements and projected event yields that could enable TeV-scale neutrino cross-section measurements.

Abstract

FASER$ν$ at the CERN Large Hadron Collider (LHC) is designed to directly detect collider neutrinos for the first time and study their cross sections at TeV energies, where no such measurements currently exist. In 2018, a pilot detector employing emulsion films was installed in the far-forward region of ATLAS, 480 m from the interaction point, and collected 12.2 fb$^{-1}$ of proton-proton collision data at a center-of-mass energy of 13 TeV. We describe the analysis of this pilot run data and the observation of the first neutrino interaction candidates at the LHC. This milestone paves the way for high-energy neutrino measurements at current and future colliders.

First neutrino interaction candidates at the LHC

TL;DR

This paper reports the first observation of neutrino interaction candidates from collider-produced neutrinos using the FASERν pilot detector in the LHC TI18 tunnel. It details the detector design, the 2018 Run-2 data collection (12.2 fb^-1 at 13 TeV), and a comprehensive simulation framework to estimate forward-neutrino fluxes and muon-induced neutral-hadron backgrounds. A multivariate approach, including a boosted decision tree, is employed to separate neutrino-like neutral vertices from backgrounds, yielding a 2.7σ excess consistent with neutrino interactions. The results validate collider neutrino detection and outline Run-3 improvements and projected event yields that could enable TeV-scale neutrino cross-section measurements.

Abstract

FASER at the CERN Large Hadron Collider (LHC) is designed to directly detect collider neutrinos for the first time and study their cross sections at TeV energies, where no such measurements currently exist. In 2018, a pilot detector employing emulsion films was installed in the far-forward region of ATLAS, 480 m from the interaction point, and collected 12.2 fb of proton-proton collision data at a center-of-mass energy of 13 TeV. We describe the analysis of this pilot run data and the observation of the first neutrino interaction candidates at the LHC. This milestone paves the way for high-energy neutrino measurements at current and future colliders.

Paper Structure

This paper contains 5 sections, 6 figures, 2 tables.

Table of Contents

  1. Introduction
  2. The pilot run in LHC Run-2
  3. Simulation
  4. Data analysis
  5. Conclusions and outlook

Figures (6)

  • Figure 1: Structure of the pilot emulsion detector. Metallic plates (1-mm-thick lead or 0.5-mm-thick tungsten) are interleaved with 0.3-mm-thick emulsion films. Only a schematic slice of the detector is depicted.
  • Figure 2: The muon flux as a function of energy at 409 m from the IP, as estimated by FLUKA. Muons entering 1 m $\times$ 1 m around the collision axis are shown.
  • Figure 3: Event displays of two of the neutral vertices in the $y$–$z$ projection longitudinal to the beam direction (left) and in the view transverse to the beam direction (right).
  • Figure 4: Monte Carlo simulation distributions of the BDT input variables for the neutrino signal and neutral hadron background. The observed neutral vertices in the data sample are shown in black. The Monte Carlo simulation distributions are normalized to 12.2 fb$^{-1}$.
  • Figure 5: Monte Carlo simulation distributions of the BDT input variables for charged hadron interactions and muon interactions. The observed charged vertices in the data sample are shown in black. The Monte Carlo simulation distributions are normalized to the data to compare the shapes.
  • ...and 1 more figures