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.
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."
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.