Researchers at Fermilab's MINOS experiment have announced a surprise result that could point to a fundamental difference between neutrinos and their anti particles. The findings, if confirmed by further experimental runs, may help physicists to explore some of the elemental differences between matter and anti-matter.

The MINOS experiment is designed to test the theory that neutrinos can change between types in a process known as neutrino "oscillation". When this idea was first muted in the 1950s it was controversial because it implies that neutrinos have mass, a feature that contradicts the Standard Model of particle physics. However, the theory has been supported by subsequent experiments, which have found the Sun to be producing fewer electron neutrinos than had been expected. It is also backed-up by an apparent shortfall in muon neutrinos produced by cosmic rays interacting in the Earth's atmosphere.

It would not just demolish any particular model, it would require revision of the whole way we do particle physics. David Wark, Imperial College

The MINOS experiment was set up to study neutrino oscillation by making the first high precision measurements of a controlled beam of neutrinos produced within a particle accelerator environment. Each experimental run begins at Fermilab near Chicago where a target is bombarded with energetic protons to produce a beam of neutrinos, called the NuMI beam. This is fired through the Earth towards the Soudan mine in Minnesota, some 735 km away. Deep in the mine, the neutrinos interact with the MINOS detector, which consists of a large iron calorimeter in the presence of a magnetic field. MINOS is designed to make highly precise measurements of the energy spectrum of muons, which arise from interactions with the Fermilab neutrinos.

Dips and troughs

Where troughs appear in this energy spectrum, it is an indication that a number of muon neutrinos have oscillated into the less energetic tau neutrinos, which cannot be recorded in the detector. The energy range over which this dip appears can reveal information about the difference between the masses of the two neutrino types. A dip appearing at higher energies corresponds to a larger difference between the masses of the two different types of neutrino. When it began operations in 2006, the MINOS team were initially probing for the mass difference between muon neutrinos and tau neutrinos. After recording firing 7 x 1020 protons at the Fermilab target, they arrived at a result of 2.35 x 10-3eV2, which represents the square of the difference between the mass eigenstates (Δm2) of the two different types of neutrino.

However, more recently the MINOS team has switched its attention to antineutrinos, and the Fermilab NuMI beam was altered to produce a beam of muon antineutrinos. The detector in the Soudan mine operates in the same way except muon antineutrinos produce positively-charged muons rather than negative. Neutrino models suggest that antineutrinos should also oscillate between types, where Δm2 should correspond to the same value as their neutrino counterparts.

To their surprise, however, the MINOS team has recorded a Δm2 value of 3.35 x 10-3eV2 between muon antineutrinos and tau antineutrinos, which is larger than their neutrino result by approximately 40%. The neutrino value and the antineutrino value are inconsistent at a confidence level of 90–95%, which corresponds to a statistical significance of approximately 2 sigma. "While the neutrinos and antineutrinos do behave differently on their journey through the Earth, the Standard Model predicts the effect is immeasurably small in the MINOS experiment," says Jenny Thomas, a spokesperson for the MINOS team based at University College London.

Out of the blue

Thomas says that the result has come "completely out of the blue", but she warns that the particle physics community generally expects a statistical significance of 3 or 4 sigma before they start to take serious notice of a result. "Clearly, more antineutrino running is essential to clarify whether this effect is just due to a statistical fluctuation,” she adds.

David Wark, a neutrino physicist at Imperial College, London, shares a similar view. "[The uncertainty] isn’t a concern in the sense that it doesn’t show that they did anything wrong, it just shows that there is not enough data to make a strong conclusion." Wark points out that if there is a difference in the oscillations of neutrinos and anti-neutrinos, this would have an enormous impact on both the Standard Model and local relativistic quantum field theory. "It would not just demolish any particular model, it would require revision of the whole way we do particle physics."

The MINOS team will continue to take measurements of anti neutrino mass difference, with the current run coming to an end shortly, and the next one getting underway in September. "If the effect does prove to be real, then we could be looking at a 3 sigma significance by February 2012," says Thomas.

The results were presented earlier this week at the Neutrino 2010 conference in Athens, Greece.