The world's most powerful particle smasher will restart
in November at just half the energy the machine was designed to
reach. But even at this level, the Large Hadron Collider has the
potential to uncover exotic new physics, such as signs of hidden
extra dimensions, physicists say.
The LHC is
a new particle accelerator at the CERN laboratory near Geneva,
Switzerland, designed to answer fundamental questions, such as what
gives elementary particles their mass, by colliding particles at
higher energies than ever achieved in a laboratory before.
But the first attempt to turn on the
LHC failed in September 2008 when a joint connecting a pair of
superconducting wires overheated, causing an explosive release of
helium used as a coolant. Scientists have been making repairs and
checking the strength of other electrical connections since then to
pave the way for a second start attempt.
Now, CERN has announced that the LHC's first data collecting
run, to begin in November, will collide protons at only half the
energy the accelerator was designed to achieve. The run will
initially smash protons together at 7 trillion electron volts (7
TeV), compared to the design goal of 14 TeV, according to a CERN
statement on 6 August. (Protons in each of the
two opposing beams will have 3.5 TeV of energy, producing
collisions at 7 TeV.)
But even 7 TeV is much higher than physicists have ever probed
in the laboratory before. The Tevatron accelerator at Fermilab in
Batavia, Illinois, is the current record holder, with collisions at
2 TeV.
New physics
No one knows exactly what energy threshold must be crossed to
catch a glimpse of new and exotic physics that is not contained in
the standard model of particle physics, which fits everything seen
so far at lower energies.
But new phenomena are widely expected somewhere between one and
a few TeV, because that is where the mathematics underlying the
standard model starts breaking down, says Greg Landsberg of Brown
University in Providence, Rhode Island. Landsberg is involved in
the CMS experiment, one of the LHC's two main detectors.
"Nature is full of surprises and something exciting and possibly
unexpected could happen at 7 TeV," Landsberg told New
Scientist. "Extra dimensions could easily open up at that
energy."
Many collisions
This first run is supposed to last until late 2010, and CERN
plans to boost the energy to 10 TeV before it is over. Landsberg is
optimistic that the LHC will be able to quickly ramp up the energy,
allowing most of the run to be carried out at 10 TeV.
However, getting collisions at energies of even a few TeV is
harder than it might seem, because protons are made up of smaller
particles called quarks and gluons. When a pair of protons
collides, it is actually a pair of these constituents that hit one
another. This usually involves only a small fraction of the total
kinetic energy of the protons – about one-tenth on average.
Only in rare, lucky instances do the collisions involve most of
the kinetic energy of the protons. That means that many collisions
at an advertised energy of 10 TeV are required to get one that
actually unleashes energy close to that amount.
Slow and steady
After the 2009-2010 run, the LHC will be shut down, with
upgrades made to allow it to go to higher energies. Measurements
have revealed that some of the electrical connections are not
robust enough to handle operation beyond 10 TeV.
The ultimate goal is still to reach 14 TeV. Landsberg believes
the LHC will reach that energy eventually, but is not sure how long
it will take. Whether it is "one year, two years or three years is
anyone's guess", he says.
But the LHC is a very complex machine, so it makes sense to be
careful while scientists gain more experience in running it, he
says: "When learning to drive, you don't take your
car at 100 miles per hour around a hairpin curve – you take it
slowly around the parking lot."
In the meantime, Fermilab has a window of opportunity to find
the first evidence for the last unseen component of the standard
model, the Higgs boson, which is thought to endow other particles
with mass, Landsberg says. But Fermilab could only beat the LHC to
finding Higgs if the particle turns out to be relatively
lightweight, allowing it to be produced reasonably often at the
energies Fermilab can probe, he says.
Images from
Rex Features.
This article originally appeared on
New Scientist.