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Heavy Ion Physics with the ATLAS Detector

by Peter Steinberg

Peter SteinbergThe LHC is set to be a major part of the physics of the next decade. At full energy of 14 TeV (trillion electron volts), and full beam intensity, it is intended to reveal new layers of the "cosmic onion", with potential discoveries of the Higgs, supersymmetry and maybe even extra space-time dimensions.

However, one oft-neglected fact is that the LHC has the potential not just for discovering new particles, but for creating the same "near-perfect fluid" we create at RHIC -- but hotter, denser, and longer-lived. Any machine which can effectively accelerate protons can be reconfigured to accelerate heavy ions as well, just as we do at RHIC. The only penalty you pay is the an overall factor of the nuclear charge divided by the nuclear mass (Z/A, which is 40% for gold and lead ions). Thus, the LHC heavy program is poised to provide collisions of lead ions at 5.5 TeV per nucleon-nucleon collision, a factor of over 27 times higher than that achieved at RHIC. Considering the great strides in the field made when going from CERN SPS energies (17.2 GeV) to RHIC (200 GeV), many of us at BNL and worldwide are electric with anticipation of the new phenomena we may find at these higher energies.

ATLAS detector

Overview of the ATLAS Detector layout, showing the inner detector, hermetic calorimetry, and muon system. Human figures are shown for scale.

To put this huge leap of beam energy into perspective, let's review a couple of basic features of RHIC collisions. As the nuclei, contracted along the beam axis by a factor of over 2000 (compared to 100 at RHIC), collide, it is expected that over 2000 (and maybe over 3000) charged particles will be emitted at right angles to the beam (at mid-rapidity), compared with a bit less than 700 at RHIC. At the same time, there will be substantial rates of high-energy "jets" (scattered quarks and gluons) and heavy quarks (charm, bottom, and top -- the latter weighing almost as much as a gold nucleus!). These "hard probes" have already yielded major discoveries at RHIC, as they tend to lose large amounts of energy in the fluid, leading to fascinating observed phenomena jet quenching and possibly even supersonic shock waves (Mach cones). Although the latter are detected by studying the correlation of 2 and 3 particles at a time, the LHC might produce "rings" of particles opposite to a high energy jet or photon -- a real "smoking gun".

Event display

Above: An event display of a simulated central heavy ion HIJING event in the ATLAS detector. Tracks in the inner detector as well as calorimeter cells are shown.

Although the LHC experimental runs with heavy ions will be shorter than RHIC, since they compete with proton-proton physics, the precision of the measurements made with these hard probes is expected to be similar to RHIC, since relatively more are produced at a given energy. Thus, we have every expectation that these two programs will be complementary, with new physics results and analysis ideas traveling back and forth across the ocean. As predictions vary widely as to its detailed properties, whether they will be similar to RHIC, or become weakly-coupled (as was originally hoped for RHIC physics), all of the new information coming out of the ATLAS, CMS, and ALICE experiments will be essential for the future progress of heavy ion physics in the coming decade.

Below: A photo taken in March 2007 of the ATLAS detector under construction in the experimental pit, 90 meters below ground level (Photo by Peter Steinberg)

ATLASBrookhaven is a major institution in the ATLAS experiment, with almost 1800 scientific collaborators. ATLAS is an enormous apparatus, over 60 feet high and 120 feet long, weighing 7000 tons. Already the BRAHMS and PHOBOS groups from BNL have merged and are actively collaborating in ATLAS with US and international institutions to prepare for our study of the features of the strongly-interacting matter we expect in lead-lead collisions.


If you want to know more, please see the ATLAS web pages.