Collisions of lead ions create tiny samples of matter at energy densities not seen in the universe since microseconds after the Big Bang. At these densities, ordinary matter melts into its primordial constituents—quarks and gluons that shine brightly at a temperature more than 100,000 times hotter than the center of our sun. Studying the electromagnetic radiation emitted by this plasma as it expands and cools provides insights into the nature of primordial matter.
“When these particles interact with the material from which the calorimeter is built, they undergo what’s known as an electromagnetic shower, depositing their full energy in a relatively short distance in the detector,” said Cormier, who began development of the ALICE electromagnetic calorimeters while working with a team at Lawrence Berkeley National Laboratory. “An electromagnetic calorimeter is designed to measure the energy in these showers. By measuring the energy of these particles we can determine the temperature of the quark–gluon plasma matter produced.”
Detecting particles under the universe’s most extreme conditions is a grand challenge for scientists and engineers. To address that challenge, a team of researchers from U.S. universities and national labs, known as the ALICE-USA Collaboration, designed, deployed and tested 16 large electromagnetic calorimeter super-modules, each weighing 8 tons, for Run-1. The instruments let scientists explore the theory of the strong interaction, called quantum chromodynamics, which describes how quarks and gluons produced at the Big Bang became confined inside neutrons and protons. At temperatures exceeding 2 trillion kelvins, created in nucleus–nucleus collisions at the LHC, quarks and gluons become de-confined and are free to travel outside of neutrons and protons in a state analogous to how they existed during the very earliest universe.
ORNL members of the ALICE-USA team include Kenneth Read, Soren Sorensen, Martin Poghosyan, Charles Britton and N. Dianne Bull Ezell. The ORNL upgrade for Run-2, known as the Di-Jet Calorimeter, or DCal, expands the instrument’s “acceptance,” or fraction of emitted particles it can detect. “The larger a detector’s acceptance, the larger the fraction of all particles that can be seen,” Cormier explained. Expanded acceptance allows the observation of correlated pairs of jets, or finely collimated sprays of particles that squirt out of the quark–gluon plasma, and correlated photon-jet pairs, enabling detailed studies of the transport properties of the matter.
“By measuring correlated energies of these jets with respect to the energies of corresponding photons, we can observe how the jet loses energy while traversing the QGP matter and in turn learn the density of that matter,” Cormier said.
At the start of Run-2, the LHC will provide collisions at nearly twice the energy available during Run-1. “This will potentially open a new period of discovery as new collision dynamics begin to emerge at the higher energy and our picture of the early universe comes into sharper focus,” he continued.
U.S. funding for the upgrade was provided by the Department of Energy’s Office of Nuclear Physics.
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