An accelerator neutrino is a human-generated neutrino or antineutrino obtained using particle accelerators, in which beam of protons is accelerated and collided with a fixed target, producing mesons (mainly pions) which then decay into neutrinos. Depending on the energy of the accelerated protons and whether mesons decay in flight or at rest it is possible to generate neutrinos of a different flavour, energy and angular distribution. Accelerator neutrinos are used to study neutrino interactions and neutrino oscillations taking advantage of high intensity of neutrino beams, as well as a possibility to control and understand their type and kinematic properties to a much greater extent than for neutrinos from other sources.
The process of the muon neutrino or muon antineutrino beam production consists of the following steps:[1] [2]
→ +, → + It is usually intended to have a pure beam, containing only one type of neutrino: either or . Thus, the length of the decay tunnel is optimised to maximise the number of pion decays and simultaneously minimise the number of muon decays,[4] in which undesirable types of neutrinos are produced:
→ + +, → + + In most of kaon decays[5] the appropriate type of neutrinos (muon neutrinos for positive kaons and muon antineutrinos for negative kaons) are produced:
→ +, → +, (63.56% of decays),
→ + +, → + +, (3.35% of decays),however, decays into electron (anti)neutrinos, is also a significant fraction:
→ + +, → + +, (5.07% of decays).
Neutrinos do not have an electric charge, so they cannot be focused or accelerated using electric and magnetic fields, and thus it is not possible to create a parallel, mono-energetic beam of neutrinos, as is done for charged particles beams in accelerators. To some extent, it is possible to control the direction and energy of neutrinos by properly selecting energy of the primary proton beam and focusing secondary pions and kaons, because the neutrinos take over part of their kinetic energy and move in a direction close to the parent particles.
A method that allows to further narrow the energy distribution of the produced neutrinos is the usage of the so-called off-axis beam.[6] The accelerator neutrino beam is a wide beam that has no clear boundaries, because the neutrinos in it do not move in parallel, but have a certain angular distribution. However, the farther from the axis (centre) of the beam, the smaller is the number of neutrinos, but also the distribution of energy changes. The energy spectrum becomes narrower and its maximum shifts towards lower energies. The off-axis angle, and thus the neutrino energy spectrum, can be optimised to maximize neutrino oscillation probability or to select the energy range in which the desired type of neutrino interaction is dominant.
The first experiment in which the off-axis neutrino beam was used was the T2K experiment[7]
See main article: Monitored neutrino beam.
A high level of control of neutrinos at the source can be achieved by monitoring the production of charged leptons (positrons, muons) in the decay tunnel of the neutrino beam. Facilities that employ this method are called monitored neutrino beams. If the lepton rate is sufficiently small, modern particle detectors can time-tag the charged lepton produced in the decay tunnel and associate this lepton to the neutrino observed in the neutrino detector. This idea, which dates back to the 1960s,[8] has been developed in the framework of the tagged neutrino beam concept but it has not been demonstrated, yet. Monitored neutrino beams produce neutrinos in a narrow energy range and, therefore, can employ the off-axis technique to predict the neutrino energy by measuring the interaction vertex, that is the distance of the neutrino interaction from the nominal beam axis. An energy resolution in the 10-20% range has been demonstrated in 2021 by the ENUBET Collaboration.[9]
Below is the list of muon (anti)neutrino beams used in past or current physics experiments: