RESEARCH NEWS STORY
August 5, 2024
Chiba University
Kyushu University
Nagoya University
High Energy Accelerator Research Organization

First Measurement of Electron- and Muon-Neutrino
Interaction Rates at the Highest Energy Ever Detectedfrom an Artificial Source
Researchers obtain the first direct observation of electron and muon neutrino interactions atthe Large Hadron Collider using the FASERν detector
Understanding neutrino interactions is crucial for obtaining a complete picture of particle
physics and the universe. To date, neutrino interaction cross sections have not beenmeasured at high energy above some hundred gigaelectronvolts at particle colliders. Now,
researchers have obtained the first direct observation of electron and muon neutrino
interactions in the Teraelectronvolt range at CERN’s Large Hadron Collider, using the FASERν
detector. This study marks a significant step for particle physics research.

Image title: An electron neutrino detected by the FASERν detector at the LHC, the mostenergetic ever observed from a human source
Image caption: This study marks the first direct observation of electron and muon neutrino
interactions at a particle collider, providing crucial insights into particle physics and thefundamental nature of the universe.
Image credit: FASER Collaboration
Image license: CC BY 4.0

Neutrinos are fundamental particles in the Standard Model of particle physics, notable for
their extremely small masses and weak interactions with matter. They are important foranswering fundamental questions about the universe, including why particles have mass andwhy there is more matter than antimatter. Despite being abundant, their weak interactions
make their detection difficult, and hence they are called “ghost particles.” At any givenmoment, numerous neutrinos freely pass through the Earth and our bodies, which originate
from the Sun or cosmic rays. Understanding their rare interactions with matter is crucial for
obtaining a more complete picture of particle physics and the universe.
There are three types, or flavors, of neutrinos: electron neutrinos (νe), muon neutrinos (νμ),
and tau neutrinos (ντ). So far, most neutrinos studied by researchers have been low-energy
neutrinos. To date, neutrino interaction cross sections, which is the probability of a neutrino
interacting with a target particle, had not been measured at energies above 300
gigaelectronvolts (GeV) for electron neutrinos and between 400 GeV and six teraelectronvolts(6000 GeV) for muon neutrinos.
In a groundbreaking study, a team of researchers led by Associate Professor Akitaka Ariga
from the Graduate School of Science, Chiba University, Japan, also affiliated with theLaboratory for High Energy Physics, University of Bern, Switzerland, and Associate Professor
Tomoko Ariga from the Faculty of Arts and Science, Kyushu University, Japan, utilized theForward Search Experiment (FASER) at CERN’s Large Hadron Collider (LHC), to achieve thefirst direct observation of high energy electron and muon neutrino interactions at a particle
collider. The team included Dr. Ken Ohashi from the Laboratory for High Energy Physics at theUniversity of Bern, Dr. Hiroaki Kawahara from the Faculty of Arts and Science at Kyushu
University, and Project-specific Assistant Professor Daiki Hayakawa from the Graduate Schoolof Science, Chiba University, along with other members of the FASER collaboration. Theirfindings were published in the journal Physical Review Letters on July 11, 2024.
One of the primary objectives of FASER is the study of high-energy neutrinos produced in theLHC’s proton-proton (pp) collisions using the dedicated FASERν detector. Dr. Akitaka Ariga
explains, “With FASERν, charged particle tracks produced by neutrino interactions in thedetector can be reconstructed with sub-micron precision. This allows us to identify electron
and muon charged-current (CC) neutrino interactions and the measurement of neutrino
interaction cross-sections in the currently unexplored TeV energy range.”
The FASERν emulsion detector is made of 730 layers of interleaved tungsten plates andemulsion films, with a total target mass of 1.1 tons. The researchers analyzed a subset of theexposed detector volume, corresponding to a mass of 128.6 kg, for high-energy neutrinos
from the LHC pp collisions. After applying strict criteria, selecting events with electrons ormuons with an energy above 200 GeV, four electron neutrino interaction candidate eventsand eight muon neutrino interaction candidate events were observed. These interactions hadhigh statistical significance (5.2σ for electron neutrinos and 5.7σ for muon neutrinos),meaning they are highly unlikely to be random background fluctuations and therefore
represent actual neutrinos.
The neutrinos detected in the study had energies in the teraelectronvolts range, the highest
ever detected from an artificial source. This study marks the first measurement of neutrinointeraction cross-sections in the unexplored energy range of 560–1740 GeV for electronneutrinos and 520–1760 GeV for muon neutrinos. Additionally, the measured interaction
cross-sections were consistent with Standard Model predictions.
Highlighting the significance of the study, Dr. Akitaka Ariga says, “These results demonstrate
the capability of studying flavor-tagged neutrino interactions at TeV energies with the FASERν
emulsion-based detector at the LHC. This marks the first ever physics result on neutrinos froma particle collider, a breakthrough in particle physics that could revolutionize the strategy oflarge-scale experimental research in the field.”
About Associate Professor Akitaka Ariga
Akitaka Ariga is currently an Associate Professor at the Graduate School of Science, Chiba
University, Japan. He obtained his MS and Ph.D. degrees from Nagoya University in 2004 and2008, respectively. He is also the leader of the FASER group at the University of Bern’sLaboratory for High Energy Physics (LHEP). He has about 300 publications with over 13000
citations. He also received the 2023 Chiba University Advanced Academic Award for
Pioneering the Frontier of High Energy Neutrino Using Colliders. His research interests include
neutrinos, particle physics, nuclear physics, cosmic rays, and astrophysics.
Funding:
This work was supported in part by Heising-Simons Foundation Grant Nos. 2018-1135, 20191179, and 2020-1840, Simons Foundation Grant No. 623683, ERC Consolidator Grant No.
101002690, US National Science Foundation Grant Nos. PHY-2111427, PHY-2110929, andPHY-2110648, JSPS KAKENHI Grant Nos. 19H01909, 22H01233, 20K23373, 23H00103,20H01919, and 21H00082, the joint research program of the Institute of Materials andSystems for Sustainability, BMBF Grant No. 05H20PDRC1, DFG EXC 2121 Quantum Universe
Grant No. 390833306, Royal Society Grant No. URF\R1\201519, UK Science and Technology
Funding Councils Grant No. ST/ T505870/1, the National Natural Science Foundation of China,
Tsinghua University Initiative Scientific Research Program, and the Swiss National ScienceFoundation.
Reference:
Title of original paper: First Measurement of the νe and νμ Interaction Cross Sections at theLHC with FASER’s Emulsion DetectorAuthors: Roshan Mammen Abrahama, John Andersb, Claire Antel c, Akitaka Ariga d,e, Tomoko
Ariga ∗∗,f, Jeremy Atkinson d, Florian U. Bernlochner g, Tobias Boeckh g, Jamie Boyd b, Lydia
Brenner h, Angela Burger b, Franck Cadouxc, Roberto Cardella c, David W. Casper a, Charlotte
Cavanagh i, Xin Chen j, Andrea Coccaro k, Stephane Debieux ´ c, Monica D’Onofrio i, AnshDesai l, Sergey Dmitrievsky m, Sinead Eley i, Yannick Favrec, Deion Fellers l, Jonathan L. Feng a,
Carlo Alberto Fenoglio c, Didier Ferrere c, Max Fieg a, Wissal Filali j, Haruhi Fujimori e, Ali
Garabaglu n, Stephen Gibson o, Sergio Gonzalez-Sevilla c, Yuri Gornushkin m, Carl Gwilliam i,
Daiki Hayakawa e, Shih-Chieh Hsu n, Zhen Hu j, Giuseppe Iacobucci c, Tomohiro Inada b, Luca
Iodice c, Sune Jakobsen b, Hans Joos b,p, Enrique Kajomovitz q, Takumi Kanai e, Hiroaki
Kawahara f, Alex Keyken o, Felix Kling r, Daniela Kock ¨ l, Pantelis Kontaxakis c, Umut Kose s,Rafaella Kotitsa b, Susanne Kuehn b, Thanushan Kugathasan c, Helena Lefebvre o, Lorne
Levinson t, Ke Li n, Jinfeng Liu j, Margaret S. Lutz b, Jack MacDonald u, Chiara Magliocca c,Fulvio Martinelli c, Lawson McCoy a, Josh McFayden v, Andrea Pizarro Medina c, Matteo

Milanesio c, Theo Moretti ´ c, Magdalena Munker c, Mitsuhiro Nakamuraw, Toshiyuki
Nakanow, Friedemann Neuhaus u, Laurie Nevay b, Motoya Nonaka e, Kazuaki Okui e, KenOhashi d, Hidetoshi Otono f, Hao Pang j, Lorenzo Paolozzi b,c, Brian Petersen b, Markus Prim g,
Michaela Queitsch-Maitland x, Hiroki Rokujo w, Elisa Ruiz-Choliz u, Andre Rubbia ´ s, Jorge
Sabater-Iglesias c, Osamu Sato w, Paola Scampoli d,y, Kristof Schmieden u, Matthias Schott u,
Anna Sfyrla c, Mansoora Shamim b, Savannah Shively a, Yosuke Takubo z, Noshin Tarannum c,Ondrej Theiner c, Eric Torrence l, Svetlana Vasina m, Benedikt Vormwald b, Di Wang j, Yuxiao
Wang j, Eli Welch a, Samuel Zahorec b,aa, Stefano Zambito c, Shunliang Zhang j
Affiliations: a Department of Physics and Astronomy, University of California, USAb CERN, Switzerlandc D´epartement de Physique Nucl´eaire et Corpusculaire, University of Geneva, Switzerlandd Albert Einstein Center for Fundamental Physics, Laboratory for High Energy Physics,
University of Bern, Switzerlande Department of Physics, Graduate School of Science, Chiba University, Japanf Kyushu University, Japang Universit¨at Bonn, Japanh Nikhef National Institute for Subatomic Physics, Netherlands
I University of Liverpool, United Kingdom
j Department of Physics, Tsinghua University, United Kingdom
k INFN Sezione di Genova, Italyl University of Oregon, Japanm Affiliated with an international laboratory covered by a cooperation agreement withCERN, Switzerlandn Department of Physics, University of Washington, USAo Royal Holloway, University of London, United Kingdom
p II. Physikalisches Institut, Universit¨at G¨ottingen, Germany
q Department of Physics and Astronomy, Technion—Israel Institute of Technology, Israelr Deutsches Elektronen-Synchrotron DESY, Germanys ETH Zurich, Switzerlandt Department of Particle Physics and Astrophysics, Weizmann Institute of Science, Israelu Institut f¨ur Physik, Universit¨at Mainz, Germanyv Department of Physics & Astronomy, University of Sussex, United Kingdom
w Nagoya University, Japanx University of Manchester, School of Physics and Astronomy, Japany Dipartimento di Fisica “Ettore Pancini”, Universit`a di Napoli Federico II, Italyz Institute of Particle and Nuclear Studies, KEK, Japan
aa Charles University, Faculty of Mathematics and Physics, Czech Republic**Corresponding author. Email: ariga@artsci.kyushu-u.ac.jp
Journal: Physical Review Letters
DOI: 10.1103/PhysRevLett.133.021802
Contact:
Akitaka Ariga
Graduate School of Science, Chiba University and Albert Einstein Center for Fundamental
Physics, University of BernEmail: ariga@chiba-u.jp

Tomoko Ariga
Faculty of Arts and Science, Kyushu University
Email: ariga@artsci.kyushu-u.ac.jp
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