Meet ALICE - new CERNs giant detector

Feb. 20, 2010 - PRLog -- CERN's Large Hadron Collider (LHC) is known mainly as the accelerator that will soon begin searching for the Higgs particle, and other new physics, in proton collisions at unprecedented energies — up to 14 TeV (14 trillion electron volts) at the center of mass — and with unprecedented beam intensities. But the same machine will also collide massive nuclei, specifically lead ions, to energies never achieved before in the laboratory.

For four weeks each year the LHC will switch from proton-proton collisions to lead-lead collisions, devoting itself to the study of the evolution and structure of nuclear matter at high temperature.  All the LHC's major experiments will collect data from these ion collisions, but one, ALICE, is being built specifically to study them.

"ALICE was designed to study the quark-gluon plasma, a fundamental state of very hot, very dense matter that filled the universe a few microseconds after the big bang," says Peter Jacobs of Berkeley Lab's Nuclear Science Division (NSD), a member of the Project Management Board for ALICE's EMCal, the Electromagnetic Calorimeter. "One way to recreate that state is to slam massive particles into each other at high energies. Neutron stars would be good, but in the laboratory we have to settle for heavy atomic nuclei, like gold or lead."

The principal component of ALICE is a time projection chamber (TPC), a type of detector invented by David Nygren of Berkeley Lab's Physics Division. A cylindrical device filled with gas and incorporating uniform electric and magnetic fields, a TPC is ideal for separating, tracking, and identifying thousands of charged particles in a dense environment — such as the thousands of particles produced in an energetic heavy-nuclei collision. It is the main detector in many high-energy physics experiments.

For example, the TPC-equipped STAR experiment at Brookhaven's Relativistic Heavy Ion Collider (RHIC) has studied the quark-gluon plasma by collecting data from gold-gold collisions, in which each nucleon (proton or neutron in the nucleus) has an energy of 100 GeV (100 billion electron volts). The STAR TPC, which was built at Berkeley Lab and was the largest in the world when it was turned on in 2001, has been one of the centerpieces of the RHIC experimental program.

When the RHIC program started, ALICE was already under construction, looking forward to the day when the LHC would come online with the ability to accelerate lead ions to energies well over 2 TeV per nucleon, about 30 times greater than RHIC. But in 2003, discoveries by STAR and the other RHIC experiments revealed the significance of a phenomenon that ALICE had not been designed to study efficiently.

"When particles like protons collide at high energy, pairs of their constituent quarks or gluons may slam into each other and scatter back-to-back, quickly breaking up again into a 'jet' or spray of particles such as pions and kaons," Jacobs says. "Jets are fundamental to QCD" — the theory of quantum chromodynamics that underpins the interactions of quarks and gluons — "and they have been seen in high-energy physics experiments since the early 1980s. RHIC is the first accelerator in which jets and similar 'hard' processes can be seen in collisions of heavy nuclei ... and in 2003 we observed something predicted but never before seen in jet physics."

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