K2K experiment

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The K2K experiment (KEK to Kamioka) was a neutrino experiment that ran from June 1999 to November 2004. It used muon neutrinos from a well-controlled and well-understood beam to verify the oscillations previously observed by Super-Kamiokande using atmospheric neutrinos. This was the first positive measurement of neutrino oscillations in which both the source and detector were fully under experimenters' control.[1][2] Previous experiments relied on neutrinos from the Sun or from cosmic sources. The experiment found oscillation parameters which were consistent with those measured by Super-Kamiokande.

Experimental design[edit]

K2K is a neutrino experiment which directed a beam of muon neutrinos (ν
) from the 12 GeV proton synchrotron at the KEK, located in Tsukuba, Ibaraki, to the Kamioka Observatory, located in Kamioka, Gifu, about 250 km away.[3] The muon neutrinos travelled through Earth, which allowed them to oscillate (change) into other flavours of neutrinos, namely into electron neutrinos (ν
) and tau neutrinos (ν
). K2K however, focused only on ν

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.[3] 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 (ν
), electron antineutrinos (ν
) and muon antineutrinos (ν

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.[5] 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.[6]


The K2K collaboration consisted of roughly 130 physicists from 27 universities and research institutes from all over the world, listed below.[7] The full list of scientists and their countries of origin is available on the K2K website.


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 = 2.8×10−3 eV2.[4] This result is in good agreement with the previous Super-Kamiokande result,[8] and the later MINOS result.[9]

See also[edit]


  1. ^ "Synthetic neutrinos appear to disappear". CERN Courier 40 (7). 18 August 2000. 
  2. ^ N. Nosengo (2006). "Neutrinos make a splash in Italy". Nature 443 (7108): 126. Bibcode:2006Natur.443..126N. doi:10.1038/443126a. PMID 16971911. 
  3. ^ a b "Long Baseline neutrino oscillation experiment, from KEK to Kamioka (K2K)". High Energy Accelerator Research Organization. 13 June 2002. Retrieved 2010-09-03. 
  4. ^ a b c M. H. Ahn et al. (K2K Collaboration) (2006). "Measurement of Neutrino Oscillation by the K2K Experiment". Physical Review D 74 (7): 072003. arXiv:hep-ex/0606032. Bibcode:2006PhRvD..74g2003A. doi:10.1103/PhysRevD.74.072003. 
  5. ^ "K2K: Near Detector". Stony Brook Super-Kamiokande/K2K group. 19 June 1999. Retrieved 2010-09-03.  External link in |publisher= (help)
  6. ^ "K2K: Introduction". Stony Brook Super-Kamiokande/K2K group. 20 June 1999. Retrieved 2010-09-03.  External link in |publisher= (help)
  7. ^ "K2K Member Institutes". High Energy Accelerator Research Organization. 20 January 2004. Retrieved 2010-09-03. 
  8. ^ Y. Fukuda et al. (Super-K Collaboration) (1998). "Measurements of the Solar Neutrino Flux from Super-Kamiokande's First 300 Days". Physical Review Letters 81 (6): 1158. arXiv:hep-ex/9805021. Bibcode:1998PhRvL..81.1158F. doi:10.1103/PhysRevLett.81.1158.  and erratum Physical Review Letters 81 (19): 4279. 1998. Bibcode:1998PhRvL..81.4279F. doi:10.1103/PhysRevLett.81.4279.  Missing or empty |title= (help)
  9. ^ D.G. Michael et al. (MINOS Collaboration) (2006). "Observation of muon neutrino disappearance with the MINOS detectors in the NuMI neutrino beam". Physical Review Letters 97 (19): 191801. arXiv:hep-ex/0607088. Bibcode:2006PhRvL..97s1801M. doi:10.1103/PhysRevLett.97.191801. PMID 17155614. 

External links[edit]