- Number 365 |
- June 18, 2012
MINOS experiment confirms Einstein, nails other neutrino measurements
The MINOS neutrino detector weighs
more than 5,000 tons.
Scientists working on the Fermilab-based MINOS experiment have reported the final results from the first phase of their neutrino experiment, based on seven years of data taking. The 140-member collaboration, which includes scientists from DOE’s Argonne, Brookhaven and Fermi national laboratories, presented its measurements of three key neutrino properties at the Neutrino 2012 conference in Kyoto, Japan.
The MINOS collaboration has achieved the world’s best measurement of the mass difference between neutrinos and antineutrinos. While earlier results indicated that neutrinos and antineutrinos might have slightly different masses, the new result agrees with theoretical predictions that neutrinos and antineutrinos should have identical masses.
MINOS scientists also announced their final result for the transformation of muon neutrinos into electron neutrinos. The MINOS experiment sends a beam of muon neutrinos 450 miles through the earth from Fermilab to a 5,000-ton neutrino detector located half a mile underground in the Soudan Underground Laboratory in northern Minnesota. The neutrinos’ trip from Fermilab to Soudan takes about 2.5 milliseconds, giving the neutrinos enough time to change their identities. Analyzing seven years worth of data, scientists found 21 electron-neutrino-like events in excess of an expected background of 79 events.
The measurement of the corresponding transformation parameter, called sin22θ13, provides an important clue to understanding the puzzle of neutrinos – how they transform from one type to another and which of the three neutrino types is the most massive. The new MINOS measurement reduces the statistical uncertainty associated with its 2011 result for this parameter. Both results are in agreement with measurements made at the Daya Bay experiment in China and the RENO experiment in Korea, obtained by studying neutrinos generated by nuclear reactors.
MINOS scientists also achieved an improved measurement of the velocity of neutrinos. Based on 8.5 times more data, the new MINOS result significantly reduces the systematic uncertainties of its earlier work, published in 2007. For their latest measurement, scientists examined in great detail the experiment's GPS timing system, improved their understanding of the delays of electronic components at every stage of the MINOS detectors and commissioned upgraded timing equipment, designed and implemented with the assistance of the National Institute of Science and Technology and the United States Naval Observatory.
Applying these improved understandings, the MINOS collaboration measured the travel time of a neutrino going from Fermilab to Soudan, Minn. and compared it with the expected travel time at the maximum speed allowed by Einstein’s theory of special relativity, the speed of light. The difference between the measured and calculated times is -15 ± 31 nanoseconds, indicating no observable difference between the neutrino speed and the speed of light.
Submitted by DOE’s Fermi National Accelerator Laboratory