The beam is back. At 10.41 am local time on 5 April, a pulse of protons flew through the enormous circular track of the Large Hadron Collider near Geneva, Switzerland for the first time in two years 鈥� followed at 12.27 pm by a beam circulating in the opposite direction.
This is the first of several milestones in the LHC鈥檚 return to life after two years of extensive upgrades. The reboot will culminate in smashing protons together with a combined energy of 13 teraelectronvolts 鈥� almost double the energy of the first run.
But first, it needs a warm-up. Today鈥檚 beams circulated at a test energy of 450 GeV, just under 10 per cent of their ultimate energy.
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鈥淭he return of beams to the LHC rewards a lot of intense, hard work from many teams of people,鈥� said Paul Collier, CERN鈥檚 head of beams, in a . 鈥淚t鈥檚 very satisfying for our operators to be back in the driver鈥檚 seat, with what鈥檚 effectively a new accelerator to bring on-stream, carefully, step by step.鈥�
Unfinished business
The reboot was delayed slightly when a short-circuit was discovered in one of the LHC鈥檚 magnets a few weeks ago. A needle-thin string of metal, probably solder, was connecting bits of circuit that should have been kept separate. After studying several ways to fix the problem, the team decided to melt the offending metal by sending a current of 400 amps through it. The effect was similar to blowing a fuse.
The charred bits of metal are still in the machine, Collier says, but out of harm鈥檚 way.
The collider operators will now check all systems before increasing the beam鈥檚 energy. They expect the first collisions between two bunches of protons to happen in June, Collier says.
Physicists hope that the revamped LHC will help find some new physics beyond the standard model, the existing paradigm of particles and forces. The Higgs boson, discovered at the LHC in 2012, was the last missing piece of the standard model, but the theory does not include a host of things physicists know exist, such as dark matter and gravity.
鈥淲e have unfinished business with understanding the universe,鈥� says of the University of Liverpool, who works on the LHCb experiment. 鈥淲e want to chase the hints we鈥檝e seen in previous measurements, whose behaviour didn鈥檛 quite match our expectations, in case these hints turn into discoveries.鈥�
Article amended on 6 April 2015
When this article was first published, it misstated the percentage of planned beam energy represented by 450 GeV.
