by Despina Hatzifotiadou. Published: 18 November 2010

Applause echoed from the unusually crowded ALICE control room at 11:20 on Monday 8 November - when the first reconstructed lead-lead collision, at an unprecedentedly high energy, was displayed on the large screen. The time for studying quark gluon plasma had come.


The ALICE Experiment/CERN

ALICE's event display showing an early lead-lead collision

Only a few days earlier, on 4 November at 8 am, the Large Hadron Collider (LHC) completed its 2010 proton program and prepared to switch to heavy ions.

Finally, after 20 years of preparation - from the conception through to the commissioning of the detector, and after a year of successful data-taking from proton-proton collisions - ALICE saw its first heavy-ion collision.

The switch over to ions was fast. During the early hours of Saturday 7 November, the LHC accelerated two beams of lead nuclei - stripped of all their electrons - to the unmatched energy of 287 TeV per beam, and collided them for the first time, at 0.30 am.

While the LHC was still in the final tuning phase, and with only the silicon detectors of the Inner Tracking System switched on, ALICE immediately recorded precious ion collision data. When the LHC declared stable beams on Monday morning, ALICE was recording within minutes collision data, with all detectors operational.


The ALICE Experiment/CERN

ALICE's control room, 8 November 2010

The quark-gluon plasma, an exotic state of matter where quarks and gluons roam freely across a large volume of the size of the colliding lead nuclei - instead of being confined inside hadrons - is presumed to have existed for a few microseconds after the Big Bang.

The collisions of lead nuclei at just under the speed of light will create matter under extreme conditions of energy density and temperature - around 100,000 times hotter than the core of the sun.

Unveiling the yet unknown properties of the quark-gluon plasma is a major part of the scientific program LHC. CERN experiments at the Super Proton Synchrotron, in the late 90s, had seen hints of the fleetingly short existence of this state of matter. Experiments at the Relativistic Heavy Ion Collider (RHIC) at Brookhaven – which has been operating since 2000 - have unambiguously confirmed the formation of quark matter and provided some unexpected insights into its properties.

At the LHC, with a 14-fold increase in energy with respect to the RHIC - 2.76 TeV per nucleon pair, compared with RHIC’s 200 GeV - quark matter will live much longer before freezing into ordinary matter. This will leave us with much more time to explore, in full, its nature.

ALICE was well prepared for the lead ion collisions. During the long proton runs, the performance of the detectors was finely tuned - and in the last few weeks before the lead collisions the data acquisition system was exercised rigorously to verify its ability to cope with the sheer volume of information delivered to it, on the tens of thousands of particles produced in each collision. The event reconstruction software has also been tuned to the new conditions.


The ALICE Experiment/CERN

A laptop computer in the control room displays ALICE's online event display

As the beginning of this first lead run coincided with the quarterly ALICE Week, many preliminary results were able to be immediately discussed within the collaboration, on both detector performance and data analysis.

These first results are related to the density of produced particles, the collective behaviour of the formed medium, its space-time dimension and opacity.

The run coordinator, Chilo Garabatos, has reported that he is more than satisfied with the data taking and the detector performance.

As the number of bunches in the beam increases from one to 17, on to 69 and eventually up to 121 - the ultimate target for this year - the luminosity increases as well. The trigger is being tightened, so that higher multiplicity events from central collisions are recorded.

At the time of writing, about 8 million collisions have been recorded, from a total of 8 fills over 9 days of heavy ion running.

The heavy ion programme this year will last for four weeks - continuing until the 6 December, when the LHC will cease operations for a two-and-a half-month long technical stop.

We are all looking forward to interesting results during the weeks and months to come.

On the 17 November results were presented in the LHCC Open Session, made public on the web, and the first two papers were submitted for publication. they can be found on the preprint server at the following links : http://xxx.lanl.gov/abs/1011.3914 http://xxx.lanl.gov/abs/1011.3916

recent: 
yes