No more escape for particle jets in ALICE

Particle jets are key tools for physicists to probe the quark-gluon plasma, a state of matter that existed a few moments after the Big Bang and that is reproduced in heavy-ion collisions at the LHC. The ALICE experiment is being upgraded to include a new calorimeter arm designed to extend significantly its capabilities to detect and measure jets of particles.


DCal and PHOS new support beams and support cradle are assembled and ready for installation.

The new calorimeter, called the “DCal”, is a large lead-scintillator detector with photo-diode readout placed in the opposite azimuth to the existing electromagnetic calorimeter (EMCal). This is the optimal configuration for the measurements of back-to-back jets, which originate in the interactions of ultra-high-energy quarks and gluons.

The Dcal has been built by the same international team from institutes in France, Italy and the US that built the EMCal, with additional new contributions from institutes in Japan and China. “The DCal uses the same type of detectors as the EMCal and will be operated in conjunction with the PHOS detector, another existing lead-tungstate calorimeter with smaller acceptance but very high precision,” says Tom Cormier of Wayne State University and the lead physicist on the ALICE Calorimeter project.

“With the addition of the DCal, ALICE will be able to measure with increased precision and over a wide energy range the jets carrying the information about the interactions of highly energetic elementary particles formed within the quark-gluon plasma,” says Mateusz Ploskon, an ALICE scientist at the Lawrence Berkeley National Laboratory. “Some of the basic properties of the quark-gluon plasma can be studied by measuring the jet-quenching phenomenon that is observed when particle jets interact with the medium and their energy is absorbed. These parameters can also be calculated theoretically, thus allowing scientists to work out the dynamics of the interactions.”

DCal and PHOS supermodules insertion test was performed above ground at Point 2.

The new DCal/PHOS/EMCal configuration will allow ALICE to perform new measurements. “An example is the photon-plus-jet coincidence which will be measured with high precision, providing additional precise probes of the quark-gluon plasma,” says Mateusz. “The DCal and EMCal will also measure the correlation of fully reconstructed jet pairs, allowing the study of the energy balance between recoiling jet pairs, which is another tool to probe the jet quenching phenomenon. The production of back-to-back jets in proton-proton collisions will allow ALICE to study in more detail the energy resolution and the energy scale of its jet measurements.”

The installation of the new DCAl is proceeding on schedule. The detector will be completed in autumn 2014, well in time for commissioning with the LHC at top energy and luminosity.

by Antonella Del Rosso