A Smooth Changeover

15 November 2010

Second Heavy ion collision watched by enthousiastic physicists.


To learn more about the plasma the Universe was made of at its beginning, the LHC has taken new steps: colliding lead ions instead of protons.

Why lead instead of protons? Peter Steinberg, co-convener of the Heavy Ion group has many years of experience in heavy ion collisions at Brookhaven National Lab. He explains: “The reason for taking lead is that is 200 times bigger in volume than protons. We want to make sure that there is enough time for the particles to bounce around and interact. In the previous collider in Brookhaven, when we collided copper or gold, we produced a 'perfect liquid' of quarks and gluons. That's something quite different from the gaseous quark–gluon state theorists and experimenters were expecting.”

For the LHC, the last proton beams were extracted in the morning of November 4, giving way to the commissioning of the lead-ion beam by early afternoon. Not even three days later on November 7 at 00:30 CET, the first collisions were recorded in ATLAS.

“We have been extremely impressed about how flawless the transition from protons to ions has been from the machine point of view. They had an aggressive commissioning plan and still they managed to provide initial collisions slightly ahead of schedule” says Benedetto Gorini, ATLAS run coordinator. “From ATLAS point of view, we had been preparing for the heavy ion data taking during the last weeks by simulating as much as we could the different conditions, in particular the high occupancy expected for the Inner Detector. Our preparatory work has obviously paid off as the transition has also been extremely smooth for us and we have been able to take data with an ideal data acquisition setup since the first collisions.”

These excellent results also impressed Peter Steinberg: “If you would have asked me last year, if I would have expected the switch [to be] so fast, I would have definitely answered 'No'. But as I got to talk more and more to people from the LHC, I learned that the time of getting to control the machine for the heavy ions and the moment to have real collisions got shorter and shorter. Luckily I [had] booked my flight for Friday! Before we knew it, we already had collisions.”

Andi Salzburger, an offline software expert, explains “This is certainly the most difficult environment to run track reconstruction. The high track densities lets the number of combinatorial [track] candidates explode, and still it was pretty much exciting to see how well the tracking performs. Even during the weekend without stable beam flag declared, and thus without Pixel and limited SCT information, we could reconstruct the first heavy ion collision events with only slightly modified tracking setup. It is somewhat surprising to me each time: we know that we've prepared ourselves properly and we've tested the setup on Monte Carlo simulation and thus we know beforehand that the reconstruction should and would work.” And he concludes “ But yet, the first time you see it popping up on the online event display it only becomes really true then. And it leaves you for this small moment with your mouth open, knowing what has been achieved, but even more looking at what nature still has to reveal to us.”

More photos in this week's gallery.

Birgit Ewert

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