Monitored neutrino beam
Monitored neutrino beams are designed to give scientists unusually precise control over how many neutrinos are produced in and around the beam. Neutrinos are produced when protons from an accelerator smash into a target and create many particles, especially pions, which decay into muons and neutrinos. Early experiments showed there are different neutrino flavors, and modern setups use magnetic focusing devices to steer the protons and the particles they produce, increasing the number of pions that will become neutrinos and selecting their charge and momentum.
After focusing, pions travel through a decay tunnel and decay into muons and neutrinos. Most pions and muons are stopped at the tunnel’s end, while neutrinos pass through the tunnel walls and reach a distant detector. In the past, scientists estimated the neutrino flux by counting pions and muons. As the need for precision grew, researchers built dedicated monitoring systems that measure how many particles are produced when protons hit solid targets and used computer simulations to predict the beam.
Monitored beams try to measure the neutrino flux directly by watching the charged leptons produced in the same decays. For example, a decay like pi+ to mu+ and nu_mu would signal the neutrino’s production by detecting the corresponding antimuon. The same idea applies to electron neutrinos from kaon decays, which would be signaled by detecting a positron. However, detecting these leptons in the tunnel is hard because lots of other particles are present.
In the 1980s, the USSR built a tagged neutrino beam, but its flux was too small for experiments. Today’s beams can detect muons but not yet with single-particle precision, and their flux measurements are not yet as precise as the best traditional methods. ENUBET is the leading project aiming to create a truly monitored neutrino beam for high-precision neutrino cross-section measurements. The concept relies on detecting the leptons in the decay tunnel, but researchers do not always try to tag both the lepton and the neutrino from the same decay. For example, in pi+ decays to mu+ nu_mu, the antimuon can be detected in the tunnel, helping to infer the neutrino production. Neutrinos are incredibly abundant but interact very weakly, so most pass through without being detected.
The idea of time-tagged beams—where the tunnel detector and the far detector time-stamp events to link a specific neutrino to a particular lepton—was proposed decades ago, but no intense time-tagged facility has been built yet.
This page was last edited on 2 February 2026, at 09:42 (CET).