Transverse Energy Measurement and Fluctuation Studies in Ultra-Relativistic Heavy Ion Collisions

Abstract

Transverse energy ($E_T$) has been measured with both of it's components, namely hadronic ($E_T^{had}$) and electromagnetic ($E_T^{em}$) at mid-rapidity, for 62.4 GeV Au+Au collisions by the STAR experiment. In the common phase space of TPC and BEMC ($0 < \eta < 1$ and full $\phi$), we have obtained $E_T^{had}$ from the TPC reconstructed tracks and $E_T^{em}$ from the BEMC tower hits after correcting for the hadronic contaminations in the calorimeter. The centrality behavior of $<dE_T/d\eta>/(0.5 N_{part})$ and the excitation function of $<dE_T/dy>/(0.5 N_{part})$ has been compared with the final state gluon saturation model (EKRT) along with similar measurements from SPS to RHIC. The most striking feature is the observation of a nearly constant value of $E_T/N_{ch} \sim 0.8$ GeV from AGS, SPS to RHIC. The $E_T$ and $N_{ch}$ production has been observed to follow a logarithmic behavior. The initial energy density estimated by the boost-invariant Bjorken hydrodynamic model, is well above the lattice QCD value for a deconfined matter of Quarks and Gluons. Taking similar colliding species i.e. Au+Au, the $\epsilon_{Bj} .\tau$ has been predicted for LHC, based on the measurements at RHIC. We have also made predictions for $(dE_T/d\eta)/(0.5 N_{part})$ and $(dN_{ch}/d\eta)/(0.5 N_{part})$ for the LHC energy. Furthermore, we have studied the event-by event fluctuations in $E_T$ and in the ratio of it's components using $\sigma/\mu$ as the fluctuation observable.