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 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.
Proton beam collision with a fixed target. In such a collision secondary particles, mainly pions and kaons, are produced.
Focusing, by a set of magnetic horns, the secondary particles with a selected charge: positive to produce the muon neutrino beam, negative to produce the muon anti-neutrino beam.
Decay of the secondary particles in flight in a long decay tunnel. Charged pions decay in more than 99.98% into a muon and the corresponding neutrino according to the principle of preserving electric charge and lepton number:
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, in which undesirable types of neutrinos are produced: In most of kaon decays the appropriate type of neutrinos are produced: however, decays into electron neutrinos, is also a significant fraction:
Absorption of the remaining hadrons and charged leptons in a beam dump and in the ground. At the same time neutrinos unimpeded travel farther, close the direction of their parent particles.
Neutrino beam kinematic properties
Neutrinos do not have an electriccharge, 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.
Off-axis beam
A method that allows to further narrow the energy distribution of the produced neutrinos is the usage of the so-called off-axis beam. 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 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
Neutrino beams in physics experiments
Below is the list of muon neutrino beams used in past or current physics experiments:
