1. QGP-Meet’06, VECC, Kolkata. 6 th Feb-2006 RAGHUNATH SAHOO, INSTITUTE OF PHYSICS, BHUBANESWAR TRANSVERSE ENERGY PRODUCTION AT RHIC OUTLINE: Introduction & Motivation Results Summary Raghunath Sahoo, IoP, Bhubaneswar (for the STAR collaboration)

2. QGP-Meet’06, VECC, Kolkata. 6 th Feb-2006 RAGHUNATH SAHOO, INSTITUTE OF PHYSICS, BHUBANESWAR INTRODUCTION Transverse energy(E T ) is the energy produced transverse to the beam direction. This is generated due to the initial scattering of partonic constituents of the incoming nuclei and the rescattering of the produced partons and hadrons. Transverse phase space is ideal to study the initial conditions after the collision. Motivation: =>Estimation of the Bjorken energy density of the produced fireball thru the estimation of E T on an event by event basis to varify if a condition for deconfinement do exist. =>Study of Quark-Hadron phase transitions thru fluctuation observables like E T and the ratio of it’s components.

3. QGP-Meet’06, VECC, Kolkata. 6 th Feb-2006 RAGHUNATH SAHOO, INSTITUTE OF PHYSICS, BHUBANESWAR Method of E T Estimation STAR (Solenoidal Tracker At RHIC) has got the potential to measure the hadronic and electromagnetic transverse energy separately on an event by event basis. The STAR Time Projection Chamber (TPC) and the Barrel Electromagnetic Calorimeter (BEMC) have the common phase space coverage. The hadronic transverse energy (E T had ) is measured thru the TPC reconstructed tracks (PID and momentum information). The electromagnetic transverse energy (E T em ) is measured thru the calorimeter tower hits after correcting for the hadronic contaminations.

4. QGP-Meet’06, VECC, Kolkata. 6 th Feb-2006 RAGHUNATH SAHOO, INSTITUTE OF PHYSICS, BHUBANESWAR Results (62.4 GeV Au+Au) Minimum-bias distribution of hadronic transverse energy Minimum-bias distribution of electromgnetic transverse energy

5. QGP-Meet’06, VECC, Kolkata. 6 th Feb-2006 RAGHUNATH SAHOO, INSTITUTE OF PHYSICS, BHUBANESWAR Results (62.4 GeV Au+Au)(contd..) Minimum-bias distribution of total transverse energy For 62.4 GeV Au+Au collisions with top 5% central events, the electromagnetic fraction of total transverse energy is ~ 0.31. STAR Preliminary

6. QGP-Meet’06, VECC, Kolkata. 6 th Feb-2006 RAGHUNATH SAHOO, INSTITUTE OF PHYSICS, BHUBANESWAR Results (62.4 GeV Au+Au) (contd..) STAR Preliminary For 62.4 GeV Au+Au collisions with top 5% central events, the electromagnetic fraction of hadronic transverse energy is ~ 0.47. In SPS (NA49), it was ~0.29 Hadronic fraction of total transverse energy ~ 0.68. (top 5% central events) In SPS (NA49), it was ~0.77.  Affected by baryon/meson ratio.

7. QGP-Meet’06, VECC, Kolkata. 6 th Feb-2006 RAGHUNATH SAHOO, INSTITUTE OF PHYSICS, BHUBANESWAR Results (62.4 GeV Au+Au) (contd..) Variation of dE T /d  per participant pair with collision centrality. dE T /d  = 447.60 ± 0.54 (stat.) for top 5% central collisions. Assuming the formation time of the fire ball ~ 1 fm/c and then it expands hydrodynamically the Bjorken energy density (  Bj ) is 3.450 ± 0.004 GeV/fm 3. This energy density (  Bj ) is well above the lattice QCD prediction of the required energy density (~1 GeV/fm 3 ) for the transition to a deconfined Quark-Gluon-Plasma to occur. STAR Preliminary Bjorken energy density  Bj = (dE T /dy)/  0  A A = transverse overlap area

8. QGP-Meet’06, VECC, Kolkata. 6 th Feb-2006 RAGHUNATH SAHOO, INSTITUTE OF PHYSICS, BHUBANESWAR Results (contd..) The excitation function of  Bj  fitted to a logarithmic function. If the Bjorken hydrodynamic model is a good model at energies higher to RHIC, then the energy density could be predicted for LHC. STAR Preliminary

9. QGP-Meet’06, VECC, Kolkata. 6 th Feb-2006 RAGHUNATH SAHOO, INSTITUTE OF PHYSICS, BHUBANESWAR Results (contd..) The excitation function of dE T /d  and dN ch /d  fitted to logarithmic functions. STAR Preliminary At LHC energy, hard processes (high p T ) will be the dominant contribution to transverse energy production. Hence, the Bjorken hydrodynamic scenario may not hold good.

10. QGP-Meet’06, VECC, Kolkata. 6 th Feb-2006 RAGHUNATH SAHOO, INSTITUTE OF PHYSICS, BHUBANESWAR Results (contd..) The excitation function of dE T /dy per participant pair from AGS to RHIC. The EKRT model (based on final state Gluon saturation) underestimates the final transverse energy. The variation of electromagnetic fraction of total transverse energy for a number of systems spanning from SPS to RHIC. This is influenced by baryon to meson ratio. In case of a long-lived plasma, a very high photon yield is expected. From this number nothing is conclusive about it’s formation. STAR Preliminary

11. QGP-Meet’06, VECC, Kolkata. 6 th Feb-2006 RAGHUNATH SAHOO, INSTITUTE OF PHYSICS, BHUBANESWAR Results (contd..) Participant number dependence of electromagnetic fraction of total energy. No significance dependence of electromagnetic fraction on collision centrality.  Tells about the particle production mechanism. STAR Preliminary The centrality dependence of E T /N ch Hydrodynamic flow effect is reflected in the peripheral collisions. If the expansion is isentropic, dN ch /d  will remain constant, whereas dE T /d  will decrease due to the performance of longitudinal work.

12. QGP-Meet’06, VECC, Kolkata. 6 th Feb-2006 RAGHUNATH SAHOO, INSTITUTE OF PHYSICS, BHUBANESWAR Results (contd..) Excitation function of E T /N ch Production of constant transverse energy per charge particle (~ 0.8 GeV) has been observed from AGS to RHIC.  Energy pumped into the system goes for particle production, instead of increasing energy per particle.

13. QGP-Meet’06, VECC, Kolkata. 6 th Feb-2006 RAGHUNATH SAHOO, INSTITUTE OF PHYSICS, BHUBANESWAR Results (contd..) E T – Fluctuation Studies Transverse energy distribution of top 5% central event  A Gaussian Transverse energy distribution for all centrality classes.  -measure of fluctuation. STAR Preliminary

14. QGP-Meet’06, VECC, Kolkata. 6 th Feb-2006 RAGHUNATH SAHOO, INSTITUTE OF PHYSICS, BHUBANESWAR Summary STAR experiment provides a unique way of measuring both the components of transverse energy on an event by event basis. This allows exploration of the collision dynamics and particle production. A constant transverse energy per charged particle (~0.8 GeV) has been observed from AGS-SPS to RHIC. The energy density produced in 62.4 GeV Au+Au collisions is 3.450 ± 0.004(stat) GeV/fm 3 which is well above the lattice QCD predicted value (~1 GeV/fm 3 ) for a transition to a deconfined Quark-Gluon-Plasma. Transverse energy per participant pair remains almost flat with collision centrality and it increases logarithmically with center of mass energy. The electromagnetic fraction of total transverse energy increases slowly from AGS to RHIC.