by Peter Jacobs. Published: 14 January 2011

One of the final pieces of the ALICE detector, the EMCal, is being completed during the current LHC shutdown and will be ready for operation at the start of the next LHC run. The EMCal - a large electromagnetic calorimeter - will extend ALICE's reach to study jets and other hard processes.

One of the most striking effects seen in heavy ion collisions is the phenomenon of jet quenching. Jets occur in all types of high-energy collisions, arising from the hard scattering of quarks and gluons. They appear in the detector as a collimated spray of stable particles (pions, kaons, protons, and the like). Jets were first identified at the SppS collider at CERN in the early 1980s and have played an important role in particle physics ever since.

Heavy-ion collisions bring a new aspect to the study of jets, because the hard scattering occurs in the midst of the hot fireball of Quark-Gluon Plasma (QGP). The jet consequently must plough through the plasma, interacting with it and losing energy, before emerging into vacuum and fragmenting into the stable particles seen in the detector.

This interaction with the plasma, known as jet quenching, strongly modifies the jet structure, relative to that seen in proton–proton collisions. These modifications can be calculated theoretically using perturbative Quantum Chromo-Dynamics (pQCD), and a comparison of such calculations and jet measurements in nuclear collisions provides an invaluable tool for looking into the early moments in the life of the QGP.

Jet quenching was first observed by experiments at the Relativistic Heavy Ion Collider at Brookhaven National Laboratory, in New York. One of the first measurements reported by ALICE from the inaugural heavy ion run last November was the observation of jet quenching at the LHC, based on the suppression in the production rate of very high momentum charged particles.

This is a great start to ALICE’s jet physics program. However, full utilization of jet quenching as a probe of the QGP requires not only single particle and soon-to-be-reported particle correlation measurements, but also the reconstruction of the entire jet structure on an event-wise basis. For that, ALICE requires an additional detector that is now being completed: the large-coverage Electromagnetic Calorimeter (EMCal), which will both measure the neutral portion of jets and provide a fast online jet trigger.This will enable ALICE to record all of these rare objects for offline analysis.

The EMCal is a lead-scintillator sampling calorimeter comprising almost 13,000 individual towers that are grouped into ten super-modules. The towers are read out by wavelength-shifting optical fibers in a shashlik geometry coupled to an avalanche photodiode. The complete EMCal will contain 100,000 individual scintillator tiles and 185 kilometers of optical fiber, weighing in total about 100 tons.


The installation of an EMCal super-modules, cradled within an installation fixture that orients it and connects to the rails of the support frame

The EMCal covers almost the full length of the ALICE Time Projection Chamber and central detector, and a third of its azimuth placed back-to-back with the ALICE Photon Spectrometer - a smaller, highly granular lead-tungstate calorimeter.

The super-modules are inserted into an independent support frame situated within the ALICE magnet, between the time-of-flight counters and the magnet coil. The support frame itself is a complex structure: it weighs 20 tons and must support five times its own weight, with a maximum deflection between being empty and being fully loaded of only a couple of centimeters. Installation of the eight-ton super-modules requires a system of rails with a sophisticated insertion device to bridge across to the support structure.

The transportation by crane of the 8 ton installation fixture containing an 8 ton super-module from the base of the Point 2 access shaft

Four EMCal super-modules were installed in ALICE prior to the inaugural LHC run in 2010, participating in the data taking and providing fast triggers for part of the run. During the winter 2010-11 shutdown the remaining six super-modules are being installed, to complete the EMCal in time for the second run.

The EMCal installation is the major activity in the ALICE hall during the month of January 2011, requiring careful coordination between teams of physicists, engineers, and technicians to lower these massive and delicate instruments, from the surface, 50 meters down into the pit and to install them in place within the ALICE magnet without disturbing the rest of the experiment.

Peter Jacobs, who is writing on behalf of the EMCal collaboration, is a physicist at the Lawrence Berkeley National Laboratory.