杏吧原创

TO UNIFY the four fundamental forces of nature, theoretical physicists find
it helpful to invent extra dimensions of space. To explain away the fact that we
don鈥檛 notice them, they argue that the extra dimensions are curled up so small
they鈥檙e undetectable. But now a team of Cambridge University researchers says
that one of the extra dimensions could be big enough to show up in the latest
particle accelerators, such as the one at CERN near Geneva.

鈥淭he signature might be detectable at the Large Hadron Collider when it
begins operation in 2005,鈥� says team member Ben Allanach.

The current favourite theory for unifying the forces is known as string
theory, which requires nine space dimensions. Physicists can look for extra
dimensions by searching for the peculiar particles they would bring into being.
The force fields of nature should extend into the dimensions and bounce around,
producing 鈥渆choes鈥� in the form of particles dubbed Kaluza-Klein
particles鈥攁n infinite series for each particle of nature.

But KK echoes are not easy to catch. To experience a small extra dimension, a
particle must have a large energy, and the fact that no KK echoes have been
spotted at the highest energies reachable today鈥攁bout 1000
gigaelectronvolts (GeV)鈥攕uggests that these extra dimensions cannot be
bigger than 10^鈭�18 metres.

But the Cambridge group has calculated that this is not the case. 鈥淭his is
only true if all forces extend into the extra dimensions,鈥� says Allanach. 鈥淚t
may be that only gravity notices them.鈥�

In this case, there would be KK echoes of the graviton. Since gravitons are
force-carriers of gravity, extra gravitons modify gravity. 鈥淚ncredibly, an extra
space dimension could be as big as 1 millimetre and its effect鈥攁
modification of the inverse square law over small distance鈥攕till be missed
by lab experiments,鈥� says Allanach.

But there is another way to spot the extra dimension. Allanach and his
colleagues, Andy Parker, Bryan Webber and Kosuke Odagiri, say that if the
lowest-energy KK echo of a graviton has an energy less than 2000 GeV, it will be
created in CERN鈥檚 Large Hadron Collider. Quark-quark collisions are expected to
create large numbers of particles including electrons and positrons. Some of the
electrons and positrons could be decay products of KK gravitons.

鈥淥n a plot of the number of electron-positron pairs against their energy this
would show up as a spike at the energy of the heavy graviton,鈥� says Allanach.
Such a spike could be caused by another particle, but the collision tracks would
reveal the true culprit.

鈥淭he result is exciting not only because it shows that the LHC has
an excellent chance of detecting extra dimensions, but also because they can
distinguish KK gravitons,鈥� says Joe Lykken of Fermilab near Chicago.