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The new accelerator LHC at CERN will provide heavy ion collisions at the highest energies ever achieved making it possible to explore the QCD phase transition of the nuclear matter to a new state of matter, the Quark Gluon Plasma (QGP), made of deconfined quarks and gluons. This study will provide information on one of the aspects of fundamental physics which remains to be solved: the confinement phenomenon which lies at the root of the strong interaction. QGP is also thought to have existed ten millionths of a second after the Big Bang. Therefore the study of the QGP will also impact our understanding of the primary instants of the Universe and the related astrophysical models.

The most promising tool to analyze the properties of the QGP is the comparison of the yield of an hard probe in a heavy ion collision, compared to that in a proton-proton collision. RHIC experiments at BNL have provided conclusive evidence that the energetic partons produced in the interaction loose a large fraction of their energy in the medium. At LHC the hard scattering cross section is an order of magnitude larger than at RHIC while the medium density created in the interaction is expected to increase by a factor 4-10. This fact opens a completely new experimental domain where a large number of high energy partons can be used as probes of the surrounding low momentum quark gluon plasma. The most promising way to study the QGP properties at LHC is to reconstruct all particles in the jet. Recovering a large fraction of the jet energy will reduce the sensitivity to the specific patterns of fragmentation, avoiding the strong biases of the leading-particle analysis as performed up to now at RHIC. The resulting jet sample will give a much more detailed and complete view of partonic energy loss and the medium-induced modification of jet fragmentation (the so called “jet-quenching”). Potentially the most detailed investigation of jet quenching utilizes the coincidence of a jet recoiling back to back with a direct photon. The colorless photon only weakly interacts with the medium, providing a direct measurement of the recoiling jet energy.

In order to study jet-quenching through the full jet reconstruction as well as through the direct photon measurement, a large acceptance Electromagnetic Calorimeter (EMCal) for the ALICE detector is under study. This detector is expected to greatly improve the jet rate and reconstruction allowing an extensive study of g-g, g-jet, and jet-jet events at the LHC.

Jet-reconstruction techniques in heavy ion collisions are complex and therefore the development of suitable algorithms has to be carefully studied. Thus an important activity in this project will be to explore different methods for jet finding, for jet energy reconstruction and for rejection of the uncorrelated background. These studies can be performed only by developing new Monte Carlo simulations based on the expected underlying physical processes. Therefore the optimization of detector response will require a continuous interplay between experiment and theory.

The objective of the proposed Joint Research Activities is threefold:

1.      Develop concepts and techniques for the full jet reconstruction in heavy ion collisions.

2.      Stimulating synergetic co-operation between experimentalists and theorists for the analysis and interpretation of the new data and search for signatures of QGP production and saturation.

3.      Create links between the corresponding experimental, theoretical and organizational infrastructures in Europe.

In conclusion, unprecedented possibilities for the characterization of the QGP properties are possible at LHC due to a kinematical reach never probed before. The proposed JRA will allow an extensive study of jets in heavy ion collisions by a synergy of technological and conceptual innovations and theoretical developments. Thus this initiative will represent a significant step forward towards an extensive study of jets in heavy ion collisions.

The full jet reconstruction in heavy ion collisions at ALICE, achievable through the implementation of the Electromagnetic Calorimeter, faces several experimental challenges and conceptual innovations:

The realization of a Fast Trigger on high p_T jets.

Comprehensive study of jet quenching requires an unbiased sample of high p_T jets, obtained by means of a “jet patch” trigger on total energy summed over finite phase space area. The fast trigger provided by the electromagnetic calorimeter will allow to increase the statistics by an order of magnitude of Pb-Pb collisions and more for lighter systems, and to extend the kinematically accessible range for jet study by about 70 GeV. The increased reach is crucial for mapping the energy evolution of the jet quenching.

A flexible trigger system is needed for optimizing jet triggers, and specific trigger hardware is being developed for this purpose. The challenge for this trigger is to perform a trigger selection optimized for different running modes in order to achieve uniform trigger efficiency as a function of centrality and colliding system and the optimization of jet trigger patch size versus trigger efficiency and background. Moreover, detailed studies on the effect of jet quenching on the trigger efficiency have to be performed. Preliminary studies have in fact shown strong and model-dependent reductions in efficiency, thus pointing out the importance of the jet quenching modeling.

The jet energy measurement based on the combination of high resolution charged particle tracking and of electromagnetic calorimeter

This method is alternative to the one applied in elementary collisions based on the total energy measurement by electromagnetic and hadronic calorimeters, and it allows a more targeted rejection of low-energy hadrons from soft background. This point is crucial for heavy ion collisions where background fluctuations are large and it is challenging achieve good energy resolutions. The electromagnetic calorimeter will allow to measure a significantly larger fraction of the jet energy compared with charged-particle tracking alone. This will result in a minimization of jet energy bias and geometric bias providing essential improvement for the jet quenching study.

Detailed studies vs. the cone radius, the track p_T cut, and different jet reconstruction algorithms have to be performed in order to optimize the jet energy resolutions and the jet energy bias.

The development of dedicated algorithms for jet finding and reconstruction for heavy ion collision.

Good procedures for identifying and reconstruct jets are central to the experimental analysis and comparisons with theory. Various kinds of jet-finders have been proposed, among them cone-type, sequential clustering, k_t-finder; however they have limited applicability in high track density of heavy ion collisions. In fact, due to higher multiplicity compared to e⁺e^- or p-p collisions, in conventional jet-finding algorithms the background energy will be swept up into the jet-cone, and strongly distorts the reconstructed jet energy. For the first time in heavy ion collisions new algorithms have to be developed in order to properly subtract background and optimize in order to reduce the running time and to achieve high efficiency. Thus an important activity in this project will be to explore different methods for jet finding, for jet energy reconstruction and for rejection of the uncorrelated background.

The development of methods of detection and identification of direct photons.

The identification of direct photon is challenging due to the background created by the decay photons. The PHOS detector presently in ALICE is a finely segmented lead-Tungstate calorimeter, targeted at precision measurement of direct photons and correlations. The EMCal will allow to extend ALICE photon measurements to higher p_T due to its larger acceptance (about one order of magnitude larger). Detailed studies on the feasibility of identifying direct photons in p-p and heavy ion collisions by analysis of topological characteristics of the shower and isolation criteria have to be performed.

The work broken down into tasks, the contribution to each task, the expertise and the task responsibilities of the participating Institutions are detailed in the following table:

The technologies and the conceptual innovations proposed in this research activity can be immediately employed in the ALICE experiment; however the new solutions that will be investigated for the trigger on high p_T jets and for the jet finding and reconstruction will surely benefit other present and future heavy ion experiments (like CBM at FAIR).

In addition, the proposed activity will allow to focus in Europe the development of techniques of jet finding and reconstruction in heavy ion collisions that, up to now, have been mainly used and only partially developed in the USA.

The participation of European groups that are leading in the developments of electromagnetic calorimeter detectors, fast trigger and readout, algorithms for jet profile and reconstruction, theoretical predictions and computational methods for jet quenching, ensures that Europe will have the best infrastructure for the study of the jet quenching in the world.