First observation of neutrino interaction candidates at the Large Hadron Collider
Neutrinos are mysterious particles in the Standard Model of particle physics, having odd features, such as the unknown origin of their masses and large mixing across generations. They then may be a key to uncover the existence of new physics. Several neutrino experiments at the GeV energy scale, e.g. MicroBooNE, DUNE, and Hyper-K, are ongoing or in preparation. However, the higher energy scale around 1 TeV (1000 GeV) has been left so far untouched. Therefore, we launched the FASERnu project which deploys the Large Hadron Collider (LHC) as a neutrino source and aims to study all three neutrino flavors at the high energy frontier of man-made neutrinos. FASERnu would possibly test new physics effects at an unexplored kinematical regime.
The FASER collaboration recently reported the first observation of neutrino interaction candidates from the LHC in the data taken in 2018. LHC neutrinos are produced by decays of mesons created by proton-proton collisions. Although the number of neutrinos created at the LHC is very large, those neutrinos hardly leave their footprint in the detector. To date, no neutrino has ever been detected at any colliders, including the LHC. In 2018, we installed a pilot neutrino detector with a mass of 30 kg in the LHC tunnel to attempt a first detection of such high-energy neutrinos. We encountered a large background from proton-proton collisions, of the order of 20 million particles recorded in the detector, while the expected number of neutrino interactions was about 10. By developing a suitable event reconstruction algorithm to resolve the huge pile-up of particle tracks, we extracted interaction vertices from the data. Finally, we performed a multi-variate analysis to discriminate a possible neutrino signal from the neutral hadron background and reported the first observation of neutrino interaction candidates from the LHC. The result was published in Physical Review D on 24 November 2021.
Our result opens a new era of particle physics research, bringing together collider and neutrino physics, which have typically been very distinct domains. The FASER collaboration will continue data taking during the Run 3 of LHC operation (2022-2024) also to search for unknown new particles, such as dark photons and axions.
Article information, DOI: 10.1103/PhysRevD.104.091101, https://journals.aps.org/prd/abstract/10.1103/PhysRevD.104.L091101
Acknowledgements: We warmly thank CERN for their support, and funding agencies; Heising-Simons Foundation Grant Nos. 2018-1135, 2019-1179, 2020-1840, Simons Foundation Grant No. 623683, JSPS KAKENHI Grant Nos, JP19H01909, JP20H01919, JP20K04004, JP20K23373, the Mitsubishi Foundation, the joint research program of the Institute of Materials and Systems for Sustainability at Nagoya University Kyushu University QR program R.2.
The FASERnu project in Run 3 has received funding from the European Research Council (ERC) under the European Union’s Horizon 2020 research and innovation programme (Grant agreement No. 101002690).