Image courtesy of scq.ubc.ca.

Although scientists cannot travel back in time to study the primordial particle “soup” of the very early universe, they can do the next best thing–recreate it. In addition to accelerating and colliding protons, the Large Hadron Collider at CERN will collide high-energy lead ions one month each year primarily for ALICE (A Large Ion Collider Experiment). ALICE scientists hope to use these collisions to recreate inside the detector tiny drops of primordial matter, the so called quark-gluon plasma, that presumably existed a few micro-seconds after the Big Bang.

Before the universe cooled enough for quarks to be bound inside protons and neutrons as they are today, quarks (elementary particles that constitute matter together with electrons) and gluons (particles that carry the strong force that holds nuclear matter together) moved around freely in a kind of plasma state. ALICE collaborators will study this primordial plasma to gain a better understanding of the early universe and the strong force.

It takes temperatures above 2×10¹² Kelvin (about 100,000 times hotter than the sun’s core) to melt protons and neutrons into quark-gluon plasma. The lead ions at the LHC are expected to collide at about 30 times higher energy than the current highest-energy heavy ion collisions at the Relativistic Heavy Ion Collider (RHIC) at Brookhaven National Laboratory, and thus reach the necessary temperatures. ALICE will record its data in the form of electronic signals delivered by the detectors. The scientists will combine these signals to reconstruct the final state of the collisions and analyze the properties of the hot plasma that generated this observed state.

A schematic of the ALICE detector.  Legend: EMCAL=electromagnetic calorimeter, ITS=inner tracking system, HMPID=high momentum particle identification detector, PHOS=photon spectrometer, PMD=photon multiplicity detector, TOF=time of flight detector, TPC=time projection chamber, TRD=transition radiation detector Image courtesy of ALICE.

ALICE is expected to produce around 100 Terabytes of data each day – the equivalent of about 20,000 DVDs.

“Processing this data will require extensive computing power – roughly 10,000 CPUs running continuously,” said Ron Soltz, an ALICE collaborator and researcher at Lawrence Livermore National Laboratory. “To pull together that much computing, the data need to be distributed to the different scientific centers around the world, and that’s where the grid comes in.” The Worldwide LHC Grid enables the raw detector data to be distributed among the 1,000 ALICE collaborators in 31 countries.

In preparation for the outpouring of ALICE data, expected to begin soon after LHC resumes operations in 2009, collaborators have been writing data analysis software and purchasing the necessary computers.

The U.S. will handle just under 10% of ALICE’s computing, Soltz said, and hardware outlays are currently being drafted at four U.S. institutions: the NERSC computing facility (affiliated with Lawrence Berkeley National Laboratory), Lawrence Livermore National Laboratory, the Ohio State Supercomputing Center, and the Texas Learning and Computation Center (affiliated with the University of Houston).

—Amelia Williamson, for iSGTW