Particle physics can be a bit like playing Whac-a-Mole: knock one mystery down, and another one pops right up. True to form, a study that last week ended controversy about the sterile neutrino – a particle that shouldn’t exist – also stirred up new intrigue. The findings, if confirmed, could provide tantalising hints of extra dimensions.
Neutrinos are tiny particles that barely interact with matter. They come in three types or “flavours” – electron, muon and tau – that can, along with their antiparticle counterparts, flip from one flavour to another, or “oscillate”, as they travel.
One of the experiments to show this, the Liquid Scintillator Neutrino Detector (LSND) at Los Alamos National Laboratory in New Mexico, ran from 1993 to 1998 and suggested that some muon anti-neutrinos had flipped into electron anti-neutrinos after travelling about 30 metres. To reconcile this result with results of other experiments, researchers had to posit the existence of at least one other type of neutrino, with about a millionth the mass of an electron. This kind of neutrino would be “sterile”, meaning it wouldn’t interact with matter at all, except through gravity.
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This spelled trouble, because sterile neutrinos have no place in the standard model of particle physics. The neutrino hinted at by the LSND would also have interfered with the growth of galaxies, changing the distribution of matter in the universe in a way that we do not observe. “The implications were staggering,” says cosmologist Scott Dodelson, of Fermilab in Batavia, Illinois. “Cosmologically, there should not be a sterile neutrino, so to some extent, our butts were on the line.”
“Cosmologically, there should not be a sterile neutrino, so to some extent, our butts were on the line”
With so much at stake, the LSND results had to be confirmed. The researchers dismantled the LSND and used the parts to build a more sensitive version at Fermilab called MiniBoone, the first phase of a bigger project called Boone (Booster Neutrino Experiment). Between 2002 and 2005, MiniBoone shot a beam of mostly muon neutrinos, and a smidgen of electron neutrinos, through nearly half a kilometre of earth towards an electron neutrino detector (see Diagram).
Last week, the MiniBoone team announced that the muon neutrinos did not oscillate into the electron flavour. There is no need for a sterile neutrino. This is consistent with other results and the standard three-neutrino picture. “This confirms what we were saying,” says Dodelson. “It makes us believe we understand our cosmology, so that’s good.”
But MiniBoone had a surprise in store that may still pose a challenge to the standard model. The electron neutrinos arriving at the detector had a range of energies, and the team saw more at low energies than expected. In the lowest 18 per cent of the energy scale, they saw 369 electron neutrinos against a predicted 273.
Muon to electron neutrino oscillations cannot be to blame for this puzzling excess; if they were, extra electron neutrinos would have shown up at all energies. It is possible that they are a mirage. For instance, muon neutrinos can scatter off carbon atoms in the detector oil, leading to flashes of light that the detector could mistake for electron neutrinos. The team believes it has accounted for this, but is keen to double-check.
If these checks don’t resolve the discrepancy, physicists may have to turn to exotic explanations, such as sterile neutrinos that apparently exceed the speed of light by taking short cuts through hidden dimensions. But until the straightforward explanations are ruled out, physicists should keep their feet firmly on the ground, warns Bill Louis of the MiniBoone group. “We have to be really hard-nosed and not jump to conclusions.”
Neutrinos remain a favoured tool to prise open cracks in the standard model. “So far we have not struck gold,” says Dodelson. “But I suspect there’s some gold to be found.”
