Hints of unexpected differences between neutrinos and antineutrinos from MINOS
New results from the MINOS experiment at Fermilab hint at unexpected differences between neutrinos and antineutrinos. This new result was one of the highlights of Neutrino 2010 conference in Athens, a bi-annual event which brings together the world's leading scientists in the area of neutrino physics.
MINOS is a long baseline neutrino oscillation experiment. A beam of muon neutrinos is produced from protons extracted from the Main Injector at Fermilab (just outside Chicago). These are then detected 735 km away in the main 5400 ton MINOS Far Detector half a mile underground in the Soudan mine. Most of the neutrinos pass straight through the Earth but a small fraction interact in the far detector. On their journey from Fermilab to Soudan, some of the muon neutrinos oscillate into tau neutrinos. By studying these oscillations MINOS is able to constrain the masses of the neutrinos and the fundamental parameters governing their mixing.
Over the course of the last year MINOS has been running with antineutrinos rather than neutrinos; a feature which is unique to the experiment. It is expected that the masses and mixing parameters governing the oscillations of antineutrinos will be the same as those for neutrinos. However, the analysis of the new antineutrino data suggest that the mass difference parameter, called Δm2 (the differences of the squares of the masses), is smaller by approximately 40 percent for neutrinos than for antineutrino. The probability of the observed difference being due to chance is approximately five percent. Prof Mark Thomson, leader of the Cambridge MINOS group explained, `if this difference is real then it would have profound implications for our understanding of particle physics. However, at this stage the most likely explanation is that the observed difference is a statistical fluctuation. Nevertheless, this is first precise measurement of the oscillations of antineutrinos, and it should be remembered that in recent years results from neutrino experiments have yielded surprises. MINOS is proposing to run with antineutrinos in the coming year and the analysis of this new data will allow us to establish whether this tantalising observation is real or not.`
As reported on the BBC: