After being down for more than a year, the CERN-based Large Hadron Collider (LHC) is back in action, banging together protons at the speed of light.
The world’s biggest particle accelerator had to undergo extensive repairs after an electrical short circuit damaged it in September last year.
Yesterday, the first new collision occurred at a low energy of 900 volts (GeV) and each beam supplied 450 (GeV).
The collision occurred just three days after physicists began circulating the first beam of protons inside the LHC’s 17-mile ring.
Outcry
When the LHC began its work last year, there were international outcries over fears a Doomsday scenario would occur as physicists attempted to create the Higgs boson, the last piece in the puzzle, as it were, that will satisfy the Standard Model.
The Standard Model is a theory that explains three of the four forces that determine the behaviour of matter and energy in the universe, or the nature of reality, really. If the Higgs boson (or God particle, as some like to call it) pops into existence, then we have proof that massless elementary particles cause mass.
In other words, this would provide proof of dark matter, the other 96pc of the universe that has been niggling scientists for quite some time (and would explain a hell of a lot about why the universe apparently weighs so little yet takes up so much space).
Beams crossing
Yesterday, with just one bunch of particles circulating in each direction, the beams can be made to cross in up to two places in the ring. From early in the afternoon, the beams were made to cross at points 1 and 5, home to the ATLAS and CMS detectors, both of which were on the lookout for collisions. Later, beams crossed at points 2 and 8, ALICE and LHCb.
“It’s a great achievement to have come this far in so short a time,” said CERN director general Rolf Heuer. “But we need to keep a sense of perspective – there’s still much to do before we can start the LHC physics programme.”
Beams were first tuned to produce collisions in the ATLAS detector, which recorded its first candidate for collisions at 2.22pm yesterday afternoon. Later, the beams were optimised for CMS. In the evening, ALICE had the first optimisation, followed by LHC.
A new era in physics
“This is great news, the start of a fantastic era of physics and hopefully discoveries after 20 years’ work by the international community to build a machine and detectors of unprecedented complexity and performance,” said ATLAS spokesperson Fabiola Gianotti.
“The events so far mark the start of the second half of this incredible voyage of discovery of the secrets of nature,” said CMS spokesperson Tejinder Virdee.
“It was standing room only in the ALICE control room and cheers erupted with the first collisions” said ALICE spokesperson Jurgen Schukraft. “This is simply tremendous.”
“The tracks we’re seeing are beautiful,” said LHC spokesperson Andrei Golutvin, “we’re all ready for serious data taking in a few days time.”
Three days since restart
These developments come just three days after the LHC restart, demonstrating the excellent performance of the beam control system. Since the start-up, the operators have been circulating beams around the ring alternately in one direction and then the other at the injection energy of 450 GeV.
The beam lifetime has gradually been increased to 10 hours, and today beams have been circulating simultaneously in both directions, still at the injection energy.
Next on the schedule is an intense commissioning phase aimed at increasing the beam intensity and accelerating the beams. All being well, by Christmas, the LHC should reach 1.2 TeV per beam, and have provided good quantities of collision data for the experiments’ calibrations.
By John Kennedy
