Within the ALICE experiment, a smaller collaboration with american, italian, frech and russian physicists has beed formed to build a big electromagnetic calorimeter (EMCal – ElectroMagnetic Calorimeter). This calorimeter will allow to study in details high transverse momentum particles. This study is fondamental to understand the quark gluon plasma (QGP) nature.
Indeed, in ultra-relativistic heavy ion collisions, high transverse momentum (pT) partons interact with the dense medium produced during the collision. This leads to an energy loss through gluon radiation from those partons in the medium (this phenomena is called the jet-quenching). The radiated gluons are emitted along the jet axis and therefor, the total jet energy measurement, that is unmodified, will not give information on this energy loss. On the other hand, this energy is supposed to modify the jet constituant momentum distribution. This can be quantified measuring jet fragmentation function. Thus, jet studies can be used to probe the dense medium properties in heavy ion collisions and learn about the medium initial density. This density will be, depending on its value, companible or not with the presence of a quark and gluon plasma or with an hadronique gaz.
Parton energy loss has been measured at RHIC with high pT particles suppression. This result has been understood as the consequence of an extreamly dense and hot medium, so dense that the particles loose some of their energy in thus medium, reducing implicitly their momentum. Comparing experimental mreasurements with theoretical predictions suggest that such density are compatible with a QGP. Recent theories, relative to saturation physics suggest a new concept, a color glass condensate that would not only explain the jet suppression but also be the QGP precursor. This is why today's physicists want to study high pT particles at the LHC. Indeed, with collisions at energies 30 times more energetic than at RHIC, jets production cross-sections will be quite higher, allowing event by event studies. Hard heavy ion physics will be study in depth at the LHC and in particular with the EMCal.
The EMCal is a calorimeter composed of 11 Super Modules, each with 2888 towers using a well known techonologie of Pb-scintillator sampling. It allows to enhance the photon measurement acceptance compared to the other ALICE calorimeter PHOS. It can also be used to trigger on high energetic jets in order to encrease the statistics by a factor 200. Combining EMCal information (on charged and neutral particles) and the ALICe tracking system, the jet energy resolution is improved significatively.
The EMCal allows to study :
- Inclusive jet and photon production: comparison with pQCD predictions should allow to constrain gluon distribution functions. First results on photon hadron correlations in proton-proton collisoins (pp), baseline for the analysis in the dense medium of PbPb collisions, have been presented by one member of the group at the 2012 Quark Matter conference. The pp, pPb and PbPb analysis should be published very soon.
- Gamma-jet events, triggered: a jet and a gamma emitted back to back at 180°one from the other allow to get a handle on the jet initial energy. Comparing the energy of the measured jet to the one of the photon, one access the energy loss in the hot medium. This measurement is challenged by the statistics limitations but is very much awaited, especially going to the lowest pT possible, which is where ALICE will be performant.