• Physics 16, 113
The first observation of neutrinos produced in a particle collider opens up a new field of study and offers ways to test the limitations of the standard model.
Neutrinos are among the most abundant particles in the Universe, but they rarely interact with matter: Trillions pass through us every second, but most of us will never have even one interacting with matter in our bodies. However, scientists can study these particles using high-intensity neutrino sources and detectors large enough to overcome the rarity of neutrino interactions. In this way, neutrinos have been observed from the Sun, from cosmic ray interactions in the atmosphere, from the interior of the Earth, from supernovae and other astrophysical objects, and from man-made sources such as nuclear reactors and particle accelerators in which a particle beam strikes a fixed target. But no one had ever detected the neutrinos produced in the colliding beams. This feat has now been achieved by the Forward Search Experiment (FASER), located at CERN’s Large Hadron Collider (LHC) in Switzerland [1].
As neutral particles, neutrinos cannot be directly observed by detectors of the type used in particle colliders. Instead, scientists study neutrinos through the particles produced when incoming neutrinos interact with matter: The properties of the incoming neutrinos can be inferred from the measured properties of their interaction products. Although these interactions are always rare, their probability increases with neutrino energy. In a particle collider, the highest energy neutrinos are more likely to be produced in a region of the collider where there are no particle detectors. Collision experiments are constructed to surround colliding beams with detectors, with only a small central region left empty to allow the beams to enter and exit. It is in this “forward” empty region, along the collision axis, that the highest energy neutrinos are most likely to be produced. Furthermore, typical collision experiments are very crowded environments, with many charged particles emerging from the collision, making it impossible to isolate neutrino events.
FASER is specifically designed and positioned to detect weakly interacting particles such as neutrinos in the anterior region of the LHC’s ATLAS experiment. It is located in a separate tunnel about 480 meters from the ATLAS interaction point (IP), the place where the beams collide, so as not to interfere with the beams’ trajectories (Fig. 1). The charged particles are deflected by the magnets that control the LHC beams and about 100 meters of rock and concrete separate FASER and ATLAS IP. Consequently, only neutral particles which interact with matter via the weak interaction, and which can therefore pass through rock and concrete unimpeded, are allowed to travel from the IP to FASER.
CERN
When neutrinos interact with matter via a charged current interaction (involving the exchange of a w boson), a lepton charged with the same flavor as the neutrino is produced. For example, a charged current interaction involving an electron neutrino always produces an electron, while an interaction involving a muon neutrino always produces a muon. The FASER Collaboration analysis focuses on the identification of muon neutrinos and antineutrinos and then identifies events where a muon or antimuon is produced within a tungsten target at the end of the experiment closest to the ATLAS IP (Fig. 2). Scintillator-based “veto” detectors, which emit light as charged particles pass through them, select for events consistent with a single muon produced in the target and reject events in which a muon or other charged particle enters from outside the detector. A tracking spectrometer, consisting of silicon microstrip within a magnetic field, is used to measure the muon’s momentum and trajectory: the analysis requires that the muon have more momentum than would be expected for non-signal events, and a trajectory consistent with an origin within the target.
The researchers analyze data collected between July and November 2022. Of the thousands of events studied, 153 pass the selection criteria and are identified as consistent with a muon or antimuon neutrino interaction. Based on simulations and statistical analyses, the team determines that nearly all of these events result from true charge-current interactions involving muon or anti-muon neutrinos. Only a handful of events are potentially “background”, defined as non-signal events that pass the selection criteria, such as events resulting from neutral hadrons interacting in the target or from muons entering from outside the detector with trajectories that avoid the activation of veto detectors. The final number of neutrino events, including statistical uncertainty and background estimation, is 153 , which has a significance of 16 standard deviations on a background-only assumption. Given that this observation is consistent with the expectations of the simulations, and that the spatial distribution and properties of these events are consistent with the fact that they are neutrino interactions, the experiment provides the first definitive detection of neutrinos from a particle collider. Soon after FASER’s 153 signal events were observed, another LHC experiment, the Scattering and Neutrino Detector, also reported eight events of great significance, providing further verification that neutrinos from particle collisions are now being observed at the LHC [2].
Observing particles in a new way is always exciting, but the main significance of this result is that it opens the door to a future program of neutrino physics measurements in collider experiments. We never know what we might see through a new experimental window like this, but physicists are already thinking about the measurements they would like to make and future experiments that could exploit the potential demonstrated by this result. A white paper on a proposed LHC research facility: the Forward Physics Facility (FPF) [3]-describes a series of experiments that would include updated versions of the detectors used in the work of the FASER collaboration. The FPF is designed to address a wide range of topics, including searches for hypothetical particles and dark matter, astrophysics, quantum chromodynamics tests, and neutrino physics.
The neutrino energy range accessible to the LHC has not been directly probed by other experiments, and unlike most artificial neutrino sources, LHC collisions produce all three neutrino types (electron, muon, and tau) in abundance. Comparison of measurements of the neutrino interaction rate and observed neutrino properties with theoretical models will improve our understanding of the underlying fundamental processes and help us search for new physics not described by current models. As one example among many, only a few tau neutrinos have been detected, while using the next iteration of FASER, the energy spectrum of thousands of tau neutrinos could be measured and compared with theoretical predictions. In some models that include a hypothetical additional Higgs particle, the observed tau neutrino spectrum would have energies lower than those predicted by the standard model. As a result, with the new field of collider neutrino physics heralded by this sample of 153 muon neutrino candidates, many thousands of high-energy neutrinos of all flavors can be observed, which can be used to search for new physics and extend our understanding of the fundamental forces of nature.
References
- H. Abreu et al. (FASER Collaboration), First direct observation of collider neutrinos with FASER at the LHC, Phys. Rev. Lett. 131031801 (2023).
- A. Albanian et al. (SND@LHC Collaboration), Observation of the muon neutrinos of the collider with the SND@LHC experiment, Phys. Rev. Lett. 131031802 (2023).
- JL Feng et al.The advanced physics facility at the high-luminosity LHC, J. Fis. G: Nucl. Part. Phys. 50030501 (2023).
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