A powerful tool to access the properties of the hot and dense medium produced at LHC is the study of jet quenching. In order to overcome the limitations of a jet study based only on charged particles, a new electromagnetic calorimeter (EMCal) has been recently proposed to be added to the original design of the ALICE detector.
The combination of such additional detector to the existing tracking and identifying detectors in ALICE will greatly improve the reconstruction of particles from low to high pT, enabling also an extensive study of "jet quenching" phenomena at LHC energies.
The design of the EMCal has been planned according to the need to integrate it with the existing ALICE magnet and detectors. It will be installed inside the solenoidal magnet, in the region between the ALICE spaceframe and the magnet coils. Due to the presence of the PHOS and HMPID detectors, EMCal will cover a region of 110° in azimuth and ±0.7 units in pseudorapidity, located in order to provide a partial back-to-back coverage with the ALICE Photon Spectrometer.
The chosen technology is based on a layered Pb-scintillator sampling calorimeter with longitudinal wavelength shifting fibre light collection. The detector is segmented into 12672 towers, approximately projective in φ and η to the interaction vertex, which will be organized in supermodules, which will be the basic structural unit of the calorimeter.
The smallest block in the calorimeter design is the individual module, which contains 2 x 2 = 4 towers, built by 77 alternating layers of Pb (1.44 mm) and polystyrene (1.76 mm), with a front face dimensions of 6cm×6cm.
The scintillation light produced in each tower is collected by an array of 36 wavelength shifting fibres (WLS), which run longitudinally through the Pb/scintillator stack. Each fibre terminates in an alumized mirror at the front face of the module and it is integrated into a group of 36 fibres joining the photosensor at the back of the module. A detailed description of the EMCal Project is reported in the Technical proposal (CERN-LHCC-2006-014, CERN-1996-032-add.3).
The EMCal is a common project shared and jointly managed by several US and EU institutes. In Italy, the only institutes involved are Catania and Frascati. Of course the schedule for the construction and the installation of the EMCal is driven by the start of LHC. The test beam of the first ALICE EMCal prototype modules was carried out at Fermi National Accelerator Laboratory in November 2005. EMCal modules of the final design, with the full electronics chain, were tested during the test beam at CERN PS and SPS in October-November 2007.