The mess over neutrino mass
PARTICLE physicists scanning the cosmos for the slippery neutrino, claim once again to have established that it has a mass. Anthony Turkevich of the University of Chicago and his collaborators have published a study claiming to have uncovered "another independent line of evidence pointing towards neutrinos having mass." Predictably, this has aroused the ridicule of physicists who tend to agree with Harvard Nobel Laureate Sheldon Glashow, who refers contemptuously to the "hullabaloo over neutrinos."
The mass -- or masslessness -- of the neutrino stands out as the single most crucial question in both cosmology and high-energy physics. The accepted characterisation of this elementary particle is that it has zero electric charge and no mass. A report in Science (Vol 256, No 5058) points out that if the neutrino has any mass at all, then physicists must move beyond the accepted model of particle physics that was first confirmed in 1973. And, if the neutrino has sufficient mass, then it offers a solution to the enigma of Dark Matter, the invisible and unfathomable matter in the universe.
There have been many attempts -- and as many approaches -- to hound out this chimera of a particle, but all the findings have been shrouded in experimental uncertainties. Weighing the neutrino -- the "ethereal" particle -- has pushed experimental techniques to the limits of their resolution. Wick Haxton, a nuclear theorist at the University of Washington, observes that the history of neutrino mass experiments is chequered with claims that were later either retracted or contradicted.
If Turkevich's measurements of the ephemeral particles, which are copiously emitted during radioactive decays, nuclear reactions within the sun, and exploding stars, had been confirmed, it would have had revolutionary implications for physics.
Turkevich chemically analysed a sample of uranium salt that had been sealed against nuclear fall-out since 1956. His quarry was plutonium 238 -- an isotope that can be generated by nuclear explosions and cosmic rays, and also by the double beta decay of uranium 238. But such decays, in which two neutrons simultaneously change into two protons, emitting two electrons and two neutrinos in the process, are extremely rare.
Critics suggest that his results might be explained by measurement difficulties and the possibility that fall-out or cosmic rays had contaminated the sample. Turkevich acknowledged the uncertainties and said he would like to repeat the experiment with a large lump of uranium in Vienna -- the relic of a German war effort sealed since 1945. Clearly, the massive neutrino hunt is not yet over.