THE case for a curiously slippery new particle in nature is building up.
杏吧原创s say the very fact that heavy elements such as gold, iodine and
uranium exist suggests that a particle called a 鈥渟terile鈥� neutrino must be
working unseen.
These heavy elements are thought to be created in the expanding debris of a
supernova explosion when a 鈥渟eed鈥� nucleus, typically iron, comes under
machine-gun bombardment by neutrons. This process, known as the rapid neutron
capture process, or 鈥渞 process鈥�, requires at least 100 neutrons for every seed
nucleus.
But scientists have suspected that not enough neutrons would have been
available to account for all the neutron-rich heavy elements in the Universe.
The problem concerns tiny particles called neutrinos, which come in three normal
types鈥攅lectron, muon and tau. Electron neutrinos play a pivotal role in
blowing off a star鈥檚 envelope in a supernova, and they interact with free
neutrons to make protons.
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The protons then mop up even more neutrons to make alpha particles. 鈥淚t鈥檚 a
double whammy which removes neutrons from circulation,鈥� says David Caldwell of
the University of California at Santa Barbara.
However, in a paper just submitted to Physical Review D, Caldwell
and his colleagues say that a 鈥渟terile鈥� neutrino may resolve the puzzle.
Experiments looking at the decay of a particle called the Z0 suggest that if
this fourth neutrino does exist, it does not interact through the weak force
like the three more familiar types. 鈥淎part from that we know nothing about it,鈥�
says Caldwell. 鈥淚f it exists, it is an extremely strange and interesting
产别补蝉迟.鈥�
The team reasons that electron neutrinos still blow the star鈥檚 envelope away.
But about 10 seconds after the blast, a two-step process gets under way. First,
muon and tau neutrinos from the blast that would normally change into the
electron type are converted into harmless sterile neutrinos.
Then, farther out, electron neutrinos are converted into muon and tau
neutrinos. 鈥淭his depletes the stock of electron neutrinos, saving free neutrons
from flipping into protons,鈥� says Caldwell.
This makes it possible for lots of heavy atoms such as gold and uranium to
form. 鈥淪uch atoms could never have been built up from light atoms without the
help of a fourth neutrino,鈥� Caldwell concludes. Other evidence for a sterile
neutrino comes from putting together observations of neutrinos from the Sun, in
the atmosphere and in lab experiments.
The new particle doesn鈥檛 fit into the Standard Model of particle physics.
However, some theories predict the existence of a 鈥渟hadow world鈥� of particles
which interact only through gravity
(New 杏吧原创, 13 February, p 5).
鈥淥ne speculative possibility is that the sterile neutrino is the partner of the
electron neutrino in the shadow world,鈥� says Caldwell.
Other physicists are cautious about the claims. 鈥淚t is possible,鈥� says Wick
Haxton of the University of Washington in Seattle. 鈥淗owever, there is a lot we
don鈥檛 know about the r process.鈥� He notes that although supernovae are the only
plausible places where the r process might go on to build up the heaviest
elements, it is still possible that the process happens in some other unknown
place in the Universe.