Transverse-energy correlations and jet shape in collisions of ultrarelativistic nuclei

Measurement of angular correlations between energy fluxes is one of the promising methods for analyzing the structure of events in ultrarelativistic heavy-ion collisions. The possibility of diagnosing the rescattering and the energy loss of hard partons in dense QCD matter is studied here on the basis of an analysis of the transverse-energy correlation function. It is shown that, if events are chosen for an analysis in a special way (that is, if at least one jet is required to have a transverse energy above some threshold) and if the procedure of background subtraction is applied in each event, the energy correlation function is sensitive to the parton energy loss and the angular spectrum of gluons emitted in a medium. The transverse-energy correlation function calculated for all events reflects the global structure of the transverse energy flux: it is independent of the azimuthal anisotropy of the energy flux for central collisions and is sensitive to the azimuthal anisotropy of the energy flux, reproducing all of its Fourier harmonics for noncentral collisions, but the coefficients of these harmonics are squared. A special correlation function in the vicinity of the maximum energy deposition in each event makes it possible to study changes in the jet shape. Within the conventional scenario of the scattering of hard jet partons on accompanying medium constituents, the correlation function is independent of the rapidity position of the jet axis and becomes much broader (symmetrically in the rapidity and azimuthal angle) than in proton-proton collisions. In the case of scattering on slow medium constituents, the broadening of the correlation function depends on the rapidity position of the jet axis and, in relation to the preceding scenario, increases sizably for jets of rather high rapidity.