快猫短视频

A DETECTOR designed to spot dark matter using over-the-counter optical fibres
could boost the search for the Universe鈥檚 mysterious missing mass. The technique
could lead to a new generation of smaller and cheaper detectors. 鈥淚t鈥檚 a great
idea,鈥� says astrophysicist John Learned of the University of Hawaii in Honolulu.
The same device could even spot elusive low-energy neutrinos.

Many cosmologists believe that WIMPs, hypothetical weakly interacting massive
particles, are the most likely candidates to form the mysterious dark matter
that makes up most of the Universe. Because they do not emit light, WIMPs are
impossible to see. But if a WIMP were to bump into an atomic nucleus in a
detector, the nucleus would recoil, causing a tiny shock wave.

These interactions are very rare, so WIMP detectors that look for recoils
must be huge to increase the chance of detection and cooled to reduce background
noise. Even so, nobody has yet spotted a WIMP.

The impacts of low-energy neutrinos also cause nuclei to recoil but with less
than a kiloelectronvolt of energy, compared to the tens of kiloelectronvolts
that WIMP collisions produce. No existing instrument can detect such low-energy
recoils. 鈥淭his mechanism of neutrino interaction is a teaser. We know it鈥檚
there, but we cannot profit from it,鈥� says Juan Collar of the Enrico Fermi
Institute in Chicago, Illinois, who has come up with an alternative.

He believes that commercially available optical fibre could be used to detect
both the low-energy nuclear recoils caused by neutrinos and the more energetic
WIMP recoils 鈥攁ll at room temperature. 鈥淚t鈥檚 trying to kill two birds with
one stone,鈥� he says.

The device relies on a type of fibre in which light can travel both through
its core and independently through its outer layer. To begin with, light is
removed from the outer layer but allowed to pass through the core. Normally the
light cannot leave the core but Collar believes that small nuclear recoils from
interactions with WIMPs or neutrinos can nudge photons into the outer region
where they can be detected. Other particles such as electrons or muons interact
with nuclei via the electromagnetic force and so wouldn鈥檛 be picked up.

Learned was impressed when Collar first published his idea on the Internet.
鈥淚 sent him an e-mail saying, 鈥榡eez, why didn鈥檛 I think of the idea 20 years
补驳辞鈥�.鈥�

Collar has built a prototype but has yet to test it. Low-energy neutrinos are
plentiful because they are formed in the Sun and in nuclear reactors on Earth.
He believes that just a kilogram of fibre would be enough to detect the
particles from a reactor so this detector would be portable. WIMPs, however, are
scarce and the detector might have to use a tonne or more of fibre. Even so,
since the detector works at room temperature it should be smaller and cheaper
than existing ones.