K2K experiment

K2K is a neutrino experiment which directed a beam of muon neutrinos ({{Subatomic particle|muon neutrino}}) from the {{val|12|ul=GeV}} proton synchrotron at the KEK, located in Tsukuba, Ibaraki, to the Kamioka Observatory, located in Kamioka, Gifu, about 250 km away.

{{cite web

|date=13 June 2002

|title=Long Baseline neutrino oscillation experiment, from KEK to Kamioka (K2K)

|url=http://neutrino.kek.jp/intro/k2k.html

|publisher=High Energy Accelerator Research Organization

|access-date=2010-09-03

}} The muon neutrinos travelled through Earth, which allowed them to oscillate (change) into other flavours of neutrinos, namely into electron neutrinos ({{Subatomic particle|electron neutrino}}) and tau neutrinos ({{Subatomic particle|tau neutrino}}). K2K however, focused only on {{nowrap|{{Subatomic particle|muon neutrino}} → {{Subatomic particle|tau neutrino}}}} oscillations.

The proton beam from the synchrotron was directed onto an aluminium target, and the resulting collisions produced a copious amount of pions. These pions were then focused into a 200 m decay pipe, where they would decay into muons and muon neutrinos. The muons were stopped at the end of the pipe, leaving a beam of muon neutrinos. The exact composition of the beam contained over 97% muon neutrinos, with the other 3% being made of electron neutrinos ({{Subatomic particle|electron neutrino}}), electron antineutrinos ({{Subatomic particle|electron antineutrino}}) and muon antineutrinos ({{Subatomic particle|muon antineutrino}}).

After they exited the pipe, the neutrinos went through a 1-kiloton water Cherenkov neutrino detector ("near detector") located at about 300 m from the aluminium target to determine the neutrino beam characteristics. This 1-kiloton "near detector" was a scaled-down version of the 50-kiloton Super-Kamiokande "far detector" located at the Kamioka Observatory, which allowed scientists to eliminate certain systematic uncertainties that would be present if two different detector types were used.{{cite web

|date=19 June 1999

|title=K2K: Near Detector

|url=http://k2k.physics.sunysb.edu/k2k/near_detector.shtml

|publisher=[Stony Brook Super-Kamiokande/K2K group]

|access-date=2010-09-03

|archive-date=20 July 2011

|archive-url=https://web.archive.org/web/20110720053235/http://k2k.physics.sunysb.edu/k2k/near_detector.shtml

|url-status=dead

}} This dual-detector configuration allowed the comparison of the neutrino beam at the near detector with the neutrino beam at the far detector to determine if neutrinos had oscillated or not.{{cite web

|date=20 June 1999

|title=K2K: Introduction

|url=http://k2k.physics.sunysb.edu/k2k/intro.shtml

|publisher=[Stony Brook Super-Kamiokande/K2K group]

|access-date=2010-09-03

|archive-date=20 July 2011

|archive-url=https://web.archive.org/web/20110720052309/http://k2k.physics.sunysb.edu/k2k/intro.shtml

|url-status=dead

}}

The K2K collaboration consisted of roughly 130 physicists from 27 universities and research institutes from all over the world, listed below.

{{cite web

|date=20 January 2004

|title=K2K Member Institutes

|url=http://neutrino.kek.jp/member.html

|publisher=High Energy Accelerator Research Organization

|access-date=2010-09-03

}} The full list of scientists and their countries of origin is available on the [http://neutrino.kek.jp/member.html K2K website].

{{div col|colwidth=22em}}

• Boston University
• Chonnam National University
• CEA Saclay (DSM-DAPNIA)
• Dongshin University
• High Energy Accelerator Research Organization
• Hiroshima University
• Institute for Cosmic Ray Research
• Institute for Nuclear Research
• Kobe University
• Korea University
• Kyoto University
• Massachusetts Institute of Technology
• Niigata University
• Okayama University
• Sapienza University of Rome
• Seoul National University
• State University of New York at Stony Brook
• Tokyo University of Science
• Tohoku University
• Autonomous University of Barcelona/IFAE
• University of California, Irvine
• University of Geneva
• University of Hawaii
• University of Tokyo
• University of Valencia
• University of Warsaw
• University of Washington

{{div col end}}

The final K2K results found that at 99.9985% confidence (4.3 σ) there had been a disappearance of muon neutrinos. Fitting the data under the oscillation hypothesis, the best fit for the square of the mass difference between muon neutrinos and tau neutrinos was Δm² = {{val|2.8|e=-3|u=eV²}}.

{{cite journal

|author = M. H. Ahn

|collaboration=K2K Collaboration

|year = 2006

|title = Measurement of Neutrino Oscillation by the K2K Experiment

|journal = Physical Review D

|volume = 74|pages = 072003

|doi = 10.1103/PhysRevD.74.072003

|arxiv=hep-ex/0606032

|bibcode = 2006PhRvD..74g2003A

|issue = 7 |s2cid=22053653

}} This result is in good agreement with the previous Super-Kamiokande result,

{{cite journal

|author=Y. Fukuda

|collaboration=Super-K Collaboration

|date=1998

|title=Measurements of the Solar Neutrino Flux from Super-Kamiokande's First 300 Days

|journal=Physical Review Letters

|volume=81 |pages=1158–1162

|doi=10.1103/PhysRevLett.81.1158

|bibcode=1998PhRvL..81.1158F

|arxiv = hep-ex/9805021

|issue=6 |s2cid=14217731

}} and erratum {{cite journal

|author=

|date=1998

|title=Erratum: Measurements of the Solar Neutrino Flux from Super-Kamiokande's First 300 Days

|journal=Physical Review Letters

|volume=81 |pages=4279

|doi=10.1103/PhysRevLett.81.4279

|bibcode=1998PhRvL..81.4279F

|issue=19

|doi-access=free

}} and the later MINOS result.

{{cite journal

|author=D.G. Michael

|collaboration=MINOS Collaboration

|year=2006

|title=Observation of muon neutrino disappearance with the MINOS detectors in the NuMI neutrino beam

|journal=Physical Review Letters

|volume=97 |pages=191801

|doi=10.1103/PhysRevLett.97.191801

|pmid=17155614

|arxiv=hep-ex/0607088

|bibcode=2006PhRvL..97s1801M

|issue=19

|s2cid=119458915

}}

• T2K experiment – the successor of the K2K experiment

{{reflist}}

• [http://neutrino.kek.jp/ K2K official website]
• [http://neutrino.kek.jp/publications/ K2K publications]

{{Breakthrough Prize laureates}}

{{neutrino detectors}}

Category:Neutrino experiments