Accelerator neutrino

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.

Muon neutrino beam production

The process of the muon neutrino or muon antineutrino beam production consists of the following steps:[1][2]

  • Acceleration of a primary proton beam in a particle accelerator.
  • 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 (of the order of hundreds meters) decay tunnel. Charged pions decay[3] in more than 99.98% into a muon and the corresponding neutrino according to the principle of preserving electric charge and lepton number:

π+
 →
μ+
 +
ν
μ ,   
π−
 →
μ−
 +
ν
μ

It is usually intended to have a pure beam, containing only one type of neutrino: either
ν
μ or
ν
μ . 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:


μ+
 →
e+
 +
ν
μ +
ν
e ,   
μ−
 →
e−
 +
ν
μ +
ν
e

In most of kaon decays[5] the appropriate type of neutrinos (muon neutrinos for positive kaons and muon antineutrinos for negative kaons) are produced:


K+
 →
μ+
 +
ν
μ ,   
K−
 →
μ−
 +
ν
μ ,    (63.56% of decays),

K+
 →
μ+
 +
ν
μ +
π0
 ,   
K−
 →
μ−
 +
ν
μ +
π0
 ,    (3.35% of decays),

however, decays into electron (anti)neutrinos, is also a significant fraction:


K+
 →
e+
 +
ν
e +
π0
 ,   
K−
 →
e−
 +
ν
e +
π0
 ,    (5.07% of decays).
  • Absorption of the remaining hadrons and charged leptons in a beam dump (usually a block of graphite) 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 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.

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.[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]

Neutrino beams in physics experiments

Below is the list of muon (anti)neutrino beams used in past or current physics experiments:

Notes
  1. T2K Collaboration (2011). "The T2K experiment". Nucl.Instrum.Meth. A. 659 (1): 106–135. arXiv:1106.1238. Bibcode:2011NIMPA.659..106A. doi:10.1016/j.nima.2011.06.067.
  2. KOPP, S (February 2007). "Accelerator neutrino beams". Physics Reports. 439 (3): 101–159. arXiv:physics/0609129. Bibcode:2007PhR...439..101K. doi:10.1016/j.physrep.2006.11.004.
  3. M. Tanabashi; et al. (Particle Data Group). "2019 Review of Particle Physics : Mesons" (PDF). Phys. Rev. D98: 1. doi:10.1103/PhysRevD.98.030001. (2018) and 2019 update
  4. M. Tanabashi; et al. (Particle Data Group). "2019 Review of Particle Physics : Leptons" (PDF). Phys. Rev. D98: 2. doi:10.1103/PhysRevD.98.030001. (2018) and 2019 update
  5. M. Tanabashi; et al. (Particle Data Group). "2019 Review of Particle Physics : Mesons" (PDF). Phys. Rev. D98: 24. doi:10.1103/PhysRevD.98.030001. (2018) and 2019 update
  6. Kirk T McDonald (2001). "An Off-Axis Neutrino Beam". arXiv:hep-ex/0111033. Bibcode:2001hep.ex...11033M. Cite journal requires |journal= (help)
  7. T2K Collaboration (2013). "T2K neutrino flux prediction". Phys. Rev. D87 (1): 012001. arXiv:1211.0469. Bibcode:2013PhRvD..87a2001A. doi:10.1103/PhysRevD.87.012001.
  8. Giacomelli, G (1 June 2008). "The CNGS neutrino beam". Journal of Physics: Conference Series. 116 (1): 012004. arXiv:physics/0703247. Bibcode:2008JPhCS.116a2004G. doi:10.1088/1742-6596/116/1/012004.

Further reading
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