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TRANSVERSE ENERGY PRODUCTION
AT RHIC
Raghunath Sahoo, IoP, Bhubaneswar
(for the STAR collaboration)
OUTLINE:
Introduction & Motivation
Results
Summary
QGP-Meet’06, VECC, Kolkata.
6th Feb-2006
RAGHUNATH SAHOO, INSTITUTE OF PHYSICS,
BHUBANESWAR
INTRODUCTION
Transverse energy(ET) 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 ET 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 ET and the ratio of it’s components.
QGP-Meet’06, VECC, Kolkata.
6th Feb-2006
RAGHUNATH SAHOO, INSTITUTE OF PHYSICS,
BHUBANESWAR
Method of ET 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 (EThad) is
measured thru the TPC reconstructed tracks
(PID and momentum information).
The electromagnetic transverse energy (ETem)
is measured thru the calorimeter tower hits
after correcting for the hadronic contaminations.
QGP-Meet’06, VECC, Kolkata.
6th Feb-2006
RAGHUNATH SAHOO, INSTITUTE OF PHYSICS,
BHUBANESWAR
Results (62.4 GeV Au+Au)
Minimum-bias distribution of
hadronic transverse energy
QGP-Meet’06, VECC, Kolkata.
6th Feb-2006
Minimum-bias distribution of
electromgnetic transverse energy
RAGHUNATH SAHOO, INSTITUTE OF PHYSICS,
BHUBANESWAR
Results (62.4 GeV Au+Au)(contd..)
STAR Preliminary
Minimum-bias distribution of
total transverse energy
QGP-Meet’06, VECC, Kolkata.
6th Feb-2006
For 62.4 GeV Au+Au collisions
with top 5% central events, the
electromagnetic fraction of total
transverse energy is ~ 0.31.
RAGHUNATH SAHOO, INSTITUTE OF PHYSICS,
BHUBANESWAR
Results (62.4 GeV Au+Au) (contd..)
STAR Preliminary
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.
QGP-Meet’06, VECC, Kolkata.
6th Feb-2006
RAGHUNATH SAHOO, INSTITUTE OF PHYSICS,
BHUBANESWAR
Results (62.4 GeV Au+Au) (contd..)
STAR Preliminary
dET/d = 447.60 ± 0.54 (stat.)
for top 5% central collisions.
Bjorken energy density
Bj = (dET /dy)/0 A
A = transverse overlap area
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/fm3.
Variation of dET/d per participant pair
with collision centrality.
QGP-Meet’06, VECC, Kolkata.
6th Feb-2006
This energy density (Bj) is well above
the lattice QCD prediction of the
required energy density (~1 GeV/fm3)
for the transition to a deconfined
Quark-Gluon-Plasma to occur.
RAGHUNATH SAHOO, INSTITUTE OF PHYSICS,
BHUBANESWAR
Results (contd..)
STAR Preliminary
If the Bjorken hydrodynamic
model is a good model at
energies higher to RHIC, then
the energy density could be
predicted for LHC.
The excitation function of Bj  fitted
to a logarithmic function.
QGP-Meet’06, VECC, Kolkata.
6th Feb-2006
RAGHUNATH SAHOO, INSTITUTE OF PHYSICS,
BHUBANESWAR
Results (contd..)
STAR Preliminary
STAR Preliminary
The excitation function of dET/d and dNch/d fitted
to logarithmic functions.
At LHC energy, hard processes (high pT) will be the dominant contribution
to transverse energy production. Hence, the Bjorken hydrodynamic
scenario may not hold good.
QGP-Meet’06, VECC, Kolkata.
6th Feb-2006
RAGHUNATH SAHOO, INSTITUTE OF PHYSICS,
BHUBANESWAR
Results (contd..)
STAR Preliminary
The excitation function of dET/dy per
participant pair from AGS to RHIC.
The EKRT model (based on final state
Gluon saturation) underestimates the
final transverse energy.
QGP-Meet’06, VECC, Kolkata.
6th Feb-2006
STAR Preliminary
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.
RAGHUNATH SAHOO, INSTITUTE OF PHYSICS,
BHUBANESWAR
Results (contd..)
STAR Preliminary
STAR Preliminary
The centrality dependence of ET/Nch
Participant number dependence of
electromagnetic fraction of total energy.
Hydrodynamic flow effect is reflected
in the peripheral collisions.
No significance dependence of
electromagnetic fraction on collision
centrality.
 Tells about the particle production
mechanism.
If the expansion is isentropic, dNch/d
will remain constant, whereas dET/d
will decrease due to the performance of
longitudinal work.
QGP-Meet’06, VECC, Kolkata.
6th Feb-2006
RAGHUNATH SAHOO, INSTITUTE OF PHYSICS,
BHUBANESWAR
Results (contd..)
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.
Excitation function of ET/Nch
QGP-Meet’06, VECC, Kolkata.
6th Feb-2006
RAGHUNATH SAHOO, INSTITUTE OF PHYSICS,
BHUBANESWAR
Results (contd..) ET – Fluctuation Studies
STAR Preliminary
Transverse energy distribution for
all centrality classes.
Transverse energy distribution of top
5% central event A Gaussian
STAR Preliminary
-measure of fluctuation.
QGP-Meet’06, VECC, Kolkata.
6th 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/fm3 which is well above the lattice QCD predicted value (~1 GeV/fm3) 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.
QGP-Meet’06, VECC, Kolkata.
6th Feb-2006
RAGHUNATH SAHOO, INSTITUTE OF PHYSICS,
BHUBANESWAR