SEVENTY-TWO years after James Chadwick discovered the neutron, physicists believe they have made the most precise measurement ever of the particle鈥檚 lifetime 鈥� and the new figure is ruffling feathers. It differs enough from the accepted lifetime to have an impact on our understanding of the composition of the universe.
Neutrons are made of three fundamental particles called quarks and decay into protons when a 鈥渄own鈥� quark turns into an 鈥渦p鈥� quark. The rate at which neutrons turn into protons determined how much of each was created in the first seconds after the big bang and it also dictates the amount of helium in the universe today.
To find out the precise decay rate, Anatoly Serebrov from the Petersburg Nuclear Physics Institute in Russia and colleagues trapped cold neutrons in a large metal bottle and then counted them after fixed periods of time. From these observations they estimated that it took 878.5 seconds for half the neutrons to decay, 7.2 seconds less than the half-life that is the accepted standard.
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鈥淭his result is wildly discrepant,鈥� says Scott Dewey, a member of another group that measures neutron lifetimes using a different technique at the National Institute of Standards and Technology in Gaithersburg, Maryland. Both the new measurement and the world standard have an estimated error of less than 1 second.
If the new measurement is correct, it could help explain why astronomers have found less helium in the universe than expected. Grant Mathews, a cosmologist at the University of Notre Dame in Indiana, has found that the new neutron lifetime cuts the predicted abundance of helium by about 0.15 per cent to 24.61 per cent (Physical Review D, in press). Although it鈥檚 a small shift, the new prediction is closer to the estimate of abundance that astronomers have come up with from observations of young galaxies.
The measurement also affects our estimates of the strength of the weak nuclear force, making it fit better with the predictions of the standard model of particle physics. 鈥淭here is a big probability that our result is correct,鈥� says Serebrov.
But some astronomers believe that there is enough uncertainty in the measurements of the helium abundance for them to be compatible with predictions made using the standard neutron lifetime.
Dewey adds,鈥漃eople who have been in this game for a while are very suspicious.鈥� But after carefully studying Serebrov鈥檚 four-page paper, which will be published in the journal Physics Letters B, Dewey has found no flaws in the work. 鈥淚t鈥檚 very hard to poke a hole in it.鈥�
