It’s a first in the world of physics: the FASER and SND@LHC experiments at CERN have seen first ever confirmed evidence of a neutrino produced in a particle collision at a collider. Both experiments specialise in weakly interacting particles to look for as yet unknown physics phenomena, and scientists hope that this new result will ultimately lead to a better understanding of neutrinos themselves, and with that to a range of open questions in particle physics.
The FASER and SND@LHC collaborations reported evidence of collider neutrino sightings at a scientific conference in March. „This is a really big deal,” says Anna Sfyrla, Associate professor at the University of Geneva and one of the collaborators of the FASER experiment. “It’s not only that we have directly detected neutrinos produced by a particle collider for the first time – and in significant numbers! –, but it’s also the energy range of the neutrinos that makes this result very exciting.” Neutrinos have been detected before, but they are either of much lower energy or, in the case of cosmic neutrinos registered by the IceCube detector at the South Pole, for example, of much higher energy. “FASER covers this very significant gap and will allow us to study the properties of the neutrinos. After all, they are the most abundant particles in the Universe!”
FASER – short for Forward Search Experiment – is a particle detector installed in a small side tunnel near the ATLAS detector at CERN. Proposed in 2017, approved in 2019 and built and commissioned in only five years since its proposal, FASER started taking data in 2022. It searches for a special range of particles produced in collisions at CERN's Large Hadron Collider (LHC): those that the big LHC detectors cannot see. This is either because there are literally no detectors to register them as they are produced so close to the beam, or because they don’t interact much with other particles and thus fly out of the collision zone undeterred to decay some way downstream. FASER sits some 500 metres away from ATLAS and is relatively small as collider detectors go, measuring only seven metres in length and 20 centimetres in diameter. The collaboration consists of 85 members from 22 institutions in nine countries. In Switzerland, the Universities of Geneva and Bern have made major contributions in the design, construction and operation of the experiment.
One FASER subdetector – developed with expertise from the University of Bern – specialises in the detection of neutrinos. The neutrinos enter FASER’s emulsion detector which interleaves 770 emulsion plates with just as many plates of material-heavy tungsten. Neutrinos interact with the tungsten, producing muons which can then be detected with the FASER detector and its spectrometer. Accelerators like the LHC produce abundant neutrinos and antineutrinos of all kinds. FASER reported 153 neutrino events in LHC collision data recorded between July and November 2022.
The other experiment that saw collision neutrinos, Scattering and Neutrino Detector at the LHC or simply SND@LHC, is complementary to FASER. Ettore Zaffaroni from EPFL presented their findings at the same conference. “From a total of ten billion events, we have used a very accurate analysis procedure to identify eight muon neutrino candidate events,“ he explains. “This is a very exciting result,“ adds Martina Ferrillo from the University of Zurich who leads the analysis of the SND@LHC neutrino collider observation in Switzerland. “We have measured precisely the muon rate and from that we have derived an expected background of 0.2 events. With eight candidate events, we can claim an observation at the level of 5 sigma.”
SND@LHC is also located near the ATLAS detector. It was designed to study neutrinos that fly out of the LHC collisions at a small angle and is sensitive to very different neutrino production mechanisms with respect to FASER. Specialising in spotting all neutrino flavours, the physicists study charmed-hadron production in a very forward region, which can in turn provide information necessary to study high-energy neutrinos in cosmic rays and insights for future collider experiments. Additionally, they can investigate the presence of new physics in neutrino interactions. Like FASER, SND@LHC has an emulsion detector interleaved with tungsten plates, and both collaborations are currently waiting for the information from these. Once in, the scientists hope to be able to distinguish between different kinds of particle decays. However, the SND@LHC detector can also use real-time information thanks to a scintillating-fiber tracker built at the EPFL.
The collaboration is particularly proud of the fast turnaround: “We’ve gone from approval to data taking in a bit more than a year,” Zaffaroni says. Both the University of Zurich and EPFL are members of the SND@LHC collaboration. They are responsible for the development and construction of the electronic detectors, and play a leading role in the data analysis.
Neutrinos, first postulated in 1930 and discovered in the 50s, are uncharged particles. They are very light and come in three kinds, or “flavours”, that are directly linked to the family of leptons: electron neutrinos, muon neutrinos and tau neutrinos. For a long time, scientists had suspected neutrinos might not have any mass at all. This is actually how they show up in the Standard Model of particle physics, the theory that describes all elementary particles and the forces that act between them. But about twenty years ago, scientists found proof that neutrinos can change from one flavour to the other – a process called oscillation. This in turn meant that they cannot be massless – but what their exact masses are and which of the flavours is the heaviest one remains a mystery.
“As more data comes in, including the data from the dedicated emulsion neutrino detector, we should be able to distinguish different types of neutrinos and study their properties,” says Sfyrla, who is responsible for the trigger (a pre-selection and call-out system for incoming particles) and the data acquisition of the FASER experiment. They should provide new insights into the interactions of the ghostly particles at high energies, and both FASER and SND@LHC might shed new light on physics beyond the Standard Model. They will also help scientists who study high-energy neutrinos from astrophysical sources. Because the way neutrinos are produced at the LHC is the same as for the very-high-energy neutrinos produced in cosmic-ray collisions with the atmosphere, the measurements by FASER and SND@LHC can be used to precisely estimate the background caused by this type of neutrinos.
Author: Barbara Warmbein
Swiss Institute of Particle Physics (CHIPP)
c/o Prof. Dr. Ben Kilminster
Department of Physics