The Large Hadron Collider is about to enter a new phase of operation, with proton running for this year concluding successfully this morning. For the rest of the year, lead ions will be accelerated and brought into collision in the machine for the first time.
Since the end of March, when the first proton collisions occurred at a total energy of 7 TeV, the machine and experiment teams say they have achieved all they hoped for.
A major aim was to reach a luminosity – a measure of the collision rate – of 1,032 per square centimetre per second. The team did this on 13 October, with two weeks to spare - indeed, by the time proton running came to an end, the machine had reached twice this figure, allowing experiments to double the amount of data collected in the space of only a few days.
"This shows that the objective we set ourselves for this year was realistic, but tough, and it’s very gratifying to see it achieved in such fine style," said Rolf Heuer, CERN’s director general.
The change to running with lead ions should deliver insights about matter as it would have been in the first moments after the Big Bang.
One of the main objectives is to produce tiny quantities of this matter, known as quark-gluon plasma, and to study how it evolves into the kind of matter that makes up the universe today. This should shed further light on the properties of the strong interaction, which binds quarks into bigger objects such as protons and neutrons.
"Heavy-ion collisions provide a unique micro-laboratory for studying very hot, dense matter," said Jurgen Schukraft, spokesperson for the ALICE lead-ion experiment at the LHC. "At the LHC we’ll be continuing a journey that began for CERN in 1994, which is certain to provide a new window on the fundamental behaviour of matter, and in particular the role of the strong interaction."
Lead-ion collisions will be more challenging than proton-proton collisions, as the flow of data will be significantly greater. But tests have shown that the data storage system at CERN can accept data at more than three times the rate achieved for proton-proton collisions, and more than double the rate expected for heavy ions.
The LHC will run with lead ions until 6 December, before a technical stop for maintenance. It will start again with protons in February, and physics runs will continue through 2011.