Directly Measuring High Energy Neutrino Cross-Sections

Particle collider experiments have confirmed that electron neutrinos and muon neutrinos have their predicted cross-sections of interaction.

There are three types, or flavors, of neutrinos: electron neutrinos (ν(e)), muon neutrinos (ν(μ)), and tau neutrinos (ν(τ)). 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 . . . utilized the Forward Search Experiment (FASER) at CERN's Large Hadron Collider (LHC), to achieve the first direct observation of high energy electron and muon neutrino interactions at a particle collider. . . . The FASERν emulsion detector is made of 730 layers of interleaved tungsten plates and emulsion films, with a total target mass of 1.1 tons. The researchers analyzed a subset of the exposed 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 or muons with an energy above 200 GeV, four electron neutrino interaction candidate events and eight muon neutrino interaction candidate events were observed. These interactions had high 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 electron neutrinos and 520-1760 GeV for muon neutrinos. Additionally, the measured interaction cross-sections were consistent with Standard Model predictions.

From Science Daily citing: Roshan Mammen Abraham, et al., "First Measurement of ν(e) and ν(μ) Interaction Cross Sections at the LHC with FASER’s Emulsion Detector." 133(2) Physical Review Letters (2024). DOI: 10.1103/PhysRevLett.133.021802

The abstract of the paper states that:

The first results of the study of high-energy electron neutrino (𝜈𝑒) and muon neutrino (𝜈𝜇) charged-current interactions in the FASER⁢𝜈 emulsion-tungsten detector of the FASER experiment at the LHC are presented. A 128.8 kg subset of the FASER⁢𝜈 volume was analyzed after exposure to 9.5  fb−1 of √𝑠=13.6  TeV 𝑝⁢𝑝 data. Four (eight) 𝜈𝑒 (𝜈𝜇) interaction candidate events are observed with a statistical significance of 5.2⁢𝜎 (5.7⁢𝜎). This is the first direct observation of 𝜈𝑒 interactions at a particle collider and includes the highest-energy 𝜈𝑒 and 𝜈𝜇 ever detected from an artificial source. The interaction cross section per nucleon 𝜎/𝐸𝜈 is measured over an energy range of 560–1740 GeV (520–1760 GeV) for 𝜈𝑒 (𝜈𝜇) to be (1.2+0.8−0.7)×10−38  cm2 GeV−1 [(0.5±0.2)×10−38  cm2 GeV−1], consistent with standard model predictions. These are the first measurements of neutrino interaction cross sections in those energy ranges.

The body text indicates that the expected number of events in the Standard Model prediction (with a plus or minus one sigma range) was 1.1-3.3 for electron neutrinos and 6.5-12.4 for muon neutrinos. The number of electron neutrino events was within two sigma of the prediction, and the number of muon neutrino was right in the middle of the predicted range.