Researchers have directly observed high-energy electron and muon neutrino interactions for the first time in the teraelectronvolt (TeV) energy range.
This landmark moment was achieved at CERN’s Large Hadron Collider (LHC) using the Forward Search Experiment (FASER).
Neutrinos are fundamental particles with extremely small masses and weak interactions with matter. These “ghost particles” have, for long, fascinated physicists. Despite their abundance— countless neutrinos pass through the Earth and our bodies every second— their detection has proven to be challenging.
“This marks the first ever physics result on neutrino from a particle collider,” said Akitaka Ariga, an associate professor at Chiba University and one of the leads on the research, describing the team’s efforts as “a breakthrough in particle physics that could revolutionize the strategy of large-scale experimental research in the field.”
A new eye on the invisible
There are three “flavors” of neutrinos- electron neutrinos (ve), muon neutrinos (νμ), and tau neutrinos (ντ). Until now, neutrino interaction cross sections had not been measured at energies over 300 gigaelectronvolts (GeV) for electron neutrinos and between 400 GeV and six teraelectronvolts (6000 GeV) for muon neutrinos.
Central to the new discovery is the FASERν detector, a specialized component of the FASER experiment at CERN. This detector comprises 730 layers of tungsten plates and emulsion films, boasting a total target mass of 1.1 tons. Additionally, the FASERν has a design that allows charged particle tracks resulting from neutrino interactions to be reconstructed with sub-micron precision.
The team analyzed a subset of the exposed detector volume, equivalent to 128.6 kg, focusing on high-energy neutrinos produced by LHC’s proton-proton collisions.
They identified four electron neutrino and eight muon neutrino interaction candidates through strict selection, all with energies above 200 GeV.
In a statement, the team highlighted the high statistical significance of these observations— 5.2σ for electron neutrinos and 5.7σ for muon neutrinos. This indicates that they are highly unlikely to be background fluctuations and, therefore, represent actual neutrinos.
“These results demonstrate the capability of studying flavor-tagged neutrino interactions at TeV energies with the FASERν emulsion-based detector at the LHC,” Dr. Ariga highlighted.
Pushing energy levels
The detected neutrinos are the highest-energy neutrinos ever observed from an artificial source, boasting energies in the teraelectronvolt range.
Specifically, the study provides the first measurements of neutrino interaction cross-sections— the probability of neutrinos interacting with target particles— in the energy ranges of 560–1740 GeV for electron neutrinos and 520–1760 GeV for muon neutrinos.
These measurements fill a crucial gap as previous studies had not extended beyond 300 GeV for electron neutrinos and between 400 GeV and 6 TeV for muon neutrinos. Additionally, these measurements were consistent with Standard Model predictions.
The ability to study neutrinos at these extreme energies could shed light on fundamental questions in physics, such as why particles have mass and why there is more matter than antimatter in the universe.
Details of the team’s research were published in Physical Review Letters on July 11, 2024.
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