View source for Accelerator neutrino

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An '''accelerator neutrino''' is a human-generated [[neutrino]] or [[Neutrino#Antineutrinos|antineutrino]] obtained using [[particle accelerator]]s, in which [[particle beam|beam]] of [[proton]]s is accelerated and collided with a fixed target, producing [[meson]]s (mainly [[pion]]s) which then [[particle decay|decay]] into [[neutrino]]s. 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 [[Neutrino#Flavor, mass, and their mixing|flavour]], energy and angular distribution. Accelerator neutrinos are used to study neutrino interactions and [[neutrino oscillation]]s 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 [[Neutrino#Sources|sources]].

== Muon neutrino beam production ==

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The process of the [[muon]] [[neutrino]] or muon antineutrino beam production consists of the following steps:<ref>
{{Cite journal | author = T2K Collaboration | title = The T2K experiment | journal = Nucl. Instrum. Methods A | volume = 659 | pages = 106–135 | year = 2011 | issue = 1 | doi = 10.1016/j.nima.2011.06.067 | arxiv = 1106.1238| bibcode = 2011NIMPA.659..106A | s2cid = 55962579 }}
</ref><ref>
{{cite journal |last1=KOPP |first1=S |title=Accelerator neutrino beams |journal=Physics Reports |date=February 2007 |volume=439 |issue=3 |pages=101–159 |doi=10.1016/j.physrep.2006.11.004 |arxiv=physics/0609129|bibcode=2007PhR...439..101K |s2cid=13894304 }}</ref>
* Acceleration of a primary [[proton]] beam in a [[particle accelerator]].
* Proton beam collision with a fixed target. In such a collision secondary particles, mainly [[pion]]s and [[kaon]]s, are produced.
* Focusing, by a set of [[magnetic horn]]s, the secondary particles with a selected [[Electric charge|charge]]: positive to produce the muon neutrino beam, negative to produce the muon anti-neutrino beam.
* [[Particle decay|Decay]] of the secondary particles in flight in a long (of the order of hundreds meters) decay tunnel. Charged pions decay<ref>
{{Cite journal
 | author = M. Tanabashi
 | collaboration=[[Particle Data Group]]
 | title = 2019 Review of Particle Physics : Mesons
 | url = http://pdg.lbl.gov/2019/tables/rpp2019-sum-mesons.pdf
 | journal = Phys. Rev.
 | volume = D98
 | page = 1
 | quote = (2018) and 2019 update
 | doi = 10.1103/PhysRevD.98.030001
| doi-access = free
 }}</ref> in more than 99.98% into a muon and the corresponding neutrino according to the principle of preserving [[electric charge]] and [[lepton number]]:
: {{SubatomicParticle|Pion+}} → {{SubatomicParticle|Muon+}} + {{SubatomicParticle|Muon neutrino}} , &nbsp;&nbsp; {{SubatomicParticle|Pion-}} → {{SubatomicParticle|Muon-}} + {{SubatomicParticle|Muon Antineutrino}}
It is usually intended to have a pure beam, containing only one type of neutrino: either {{SubatomicParticle|Muon neutrino}} or {{SubatomicParticle|Muon Antineutrino}} . Thus, the length of the decay tunnel is optimised to maximise the number of [[pion]] decays and simultaneously minimise the number of [[muon]] decays,<ref>
{{Cite journal
 | author = M. Tanabashi
 | collaboration=[[Particle Data Group]]
 | title = 2019 Review of Particle Physics : Leptons
 | url = http://pdg.lbl.gov/2019/tables/rpp2019-sum-leptons.pdf
 | journal = Phys. Rev.
 | volume = D98
 | page = 2
 | quote = (2018) and 2019 update
 | doi = 10.1103/PhysRevD.98.030001
| doi-access = free
 }}</ref> in which undesirable types of neutrinos are produced:
: {{SubatomicParticle|Muon+}} → {{SubatomicParticle|Positron}} + {{SubatomicParticle|Muon Antineutrino}} + {{SubatomicParticle|Electron neutrino}} , &nbsp;&nbsp; {{SubatomicParticle|Muon-}} → {{SubatomicParticle|Electron}} + {{SubatomicParticle|Muon neutrino}} + {{SubatomicParticle|Electron Antineutrino}}
In most of [[kaon]] decays<ref>
{{Cite journal
 | author = M. Tanabashi
 | collaboration=[[Particle Data Group]]
 | title = 2019 Review of Particle Physics : Mesons
 | url = http://pdg.lbl.gov/2019/tables/rpp2019-sum-mesons.pdf
 | journal = Phys. Rev.
 | volume = D98
 | page = 24
 | quote = (2018) and 2019 update
 | doi = 10.1103/PhysRevD.98.030001
| doi-access = free
 }}</ref> the appropriate type of neutrinos (muon neutrinos for positive kaons and muon antineutrinos for negative kaons) are produced:
: {{SubatomicParticle|Kaon+}} → {{SubatomicParticle|Muon+}} + {{SubatomicParticle|Muon neutrino}} , &nbsp;&nbsp; {{SubatomicParticle|Kaon-}} → {{SubatomicParticle|Muon-}} + {{SubatomicParticle|Muon Antineutrino}} , &nbsp;&nbsp; (63.56% of decays),
: {{SubatomicParticle|Kaon+}} → {{SubatomicParticle|Muon+}} + {{SubatomicParticle|Muon neutrino}} + {{SubatomicParticle|Pion0}} , &nbsp;&nbsp; {{SubatomicParticle|Kaon-}} → {{SubatomicParticle|Muon-}} + {{SubatomicParticle|Muon Antineutrino}} + {{SubatomicParticle|Pion0}} , &nbsp;&nbsp; (3.35% of decays),
however, decays into electron (anti)neutrinos, is also a significant fraction:
: {{SubatomicParticle|Kaon+}} → {{SubatomicParticle|Positron}} + {{SubatomicParticle|Electron neutrino}} + {{SubatomicParticle|Pion0}} , &nbsp;&nbsp; {{SubatomicParticle|Kaon-}} → {{SubatomicParticle|Electron}} + {{SubatomicParticle|Electron Antineutrino}} + {{SubatomicParticle|Pion0}} , &nbsp;&nbsp; (5.07% of decays).
* Absorption of the remaining [[hadron]]s and charged [[lepton]]s 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 field|electric]] and [[magnetic field|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.<ref>{{Cite journal | author = Kirk T McDonald | title = An Off-Axis Neutrino Beam | year = 2001 | arxiv = hep-ex/0111033 | bibcode = 2001hep.ex...11033M }}</ref> 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]]<ref name="t2kflux">{{Cite journal | author = T2K Collaboration | title = T2K neutrino flux prediction | journal = Phys. Rev. | volume = D87 | pages = 012001 | year = 2013 | issue = 1 | doi = 10.1103/PhysRevD.87.012001 | arxiv = 1211.0469| bibcode = 2013PhRvD..87a2001A | s2cid = 55114627 }}</ref>

=== Monitored and tagged neutrino beams ===
{{Main|Monitored neutrino beam}}

A high level of control of neutrinos at the source can be achieved by monitoring the production of charged leptons ([[positron]]s, [[muon]]s) 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,<ref>{{cite journal |last1=Hand |first1=L. N. |title=A study of 40 to 90 GeV neutrino interactions using a tagged neutrino beam |date=31 October 1970 |url=https://www.osti.gov/biblio/4117486-study-gev-neutrino-interactions-using-tagged-neutrino-beam |access-date=28 November 2021 |publisher=Cornell Univ., Ithaca, N. Y. |osti=4117486 |language=English}}</ref> 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.<ref>{{cite arXiv |last1=Longhin |first1=A. |last2=Terranova |first2=F. |title=Enhanced NeUtrino BEams from kaon Tagging (ENUBET) |date=15 March 2022 |class=hep-ex |eprint=2203.08319}}</ref>

== Neutrino beams in physics experiments ==

Below is the list of muon (anti)neutrino beams used in past or current physics experiments:
* [[CERN Neutrinos to Gran Sasso]] (CNGS) beam<ref>{{cite journal |last1=Giacomelli |first1=G |title=The CNGS neutrino beam |journal=Journal of Physics: Conference Series |date=1 June 2008 |volume=116 |issue=1 |pages=012004 |doi=10.1088/1742-6596/116/1/012004 |arxiv=physics/0703247|bibcode=2008JPhCS.116a2004G |s2cid=2624092 }}</ref> produced by [[Super Proton Synchrotron]] at [[CERN]] used in [[OPERA experiment|OPERA]] and [[ICARUS experiment|ICARUS]] experiments.
* [[Booster Neutrino Beam]] (BNB) produced by the Booster synchrotron at [[Fermilab]] used in [[SciBooNE]], [[MiniBooNE]] and [[MicroBooNE]] experiments.
* [[NuMI|Neutrinos at the Main Injector]] (NuMI) beam produced by the Main Injector synchrotron at [[Fermilab]] used in [[MINOS]], [[MINERvA|MINERνA]] and [[NOvA|NOνA]] experiments.
* K2K neutrino beam produced by a 12 GeV proton synchrotron at [[KEK]] in [[Tsukuba, Ibaraki|Tsukuba]] used in [[K2K experiment|K2K]] experiment.
* T2K neutrino beam<ref name="t2kflux" /> produced by the Main Ring synchrotron at [[J-PARC]] in [[Tokai, Ibaraki|Tokai]] used in [[T2K experiment|T2K]] experiment.

== Notes ==
{{Reflist}}

==Further reading==
* {{cite journal |last1=Dore |first1=Ubaldo |last2=Loverre |first2=Pier |last3=Ludovici |first3=Lucio |title=History of accelerator neutrino beams |journal=The European Physical Journal H |date=2 October 2019 |volume=44 |issue=4–5 |pages=271–305 |doi=10.1140/epjh/e2019-90032-x| arxiv=1805.01373|bibcode=2019EPJH...44..271D |s2cid=73628709 }}

== External links ==
* [https://neutrinos.fnal.gov/sources/accelerator-neutrinos/ Accelerator neutrinos - Fermilab]

<!--- Categories --->
[[Category:Accelerator physics]]
[[Category:Neutrinos]]