![]() |
|
STAR experiment has recently reported systematic measurements of identified particle spectra in pp, d+Au and Au+Au collisions. Along with reporting several interesting results for the above collision systems at different energies we have also presented in detail the particle identification procedure in STAR Time Projection Chamber and the various correction factors associated with the extraction of the yield and shape parameters for the transverse momentum spectra of produced hadrons. In this focus article we present two results from this work. For Au+Au collisions,mean pT, which characterizes the slope of the transverse momentum spectra are found to increase significantly with increasing collision centrality or decreasing impact parameter of the collision. The trends are similar at 62.4 GeV, 130 GeV, and 200 GeV, and mean pT qualitatively agree with each other at the same dNch/dy. This suggests that the kinetic freeze-out properties in Au+Au collisions are rather energy independent for the measured collision energies. In the Color Glass Condensate (gluon saturation)
picture, small x gluons overlap and recombine,
reducing the total number of gluons and
increasing their transverse energy. These gluons
hadronize into mostly soft hadrons. Thus, a lower
particle multiplicity and larger mean pT is
predicted. In the gluon saturation picture, the
only relevant scale is (dN_pion/dy)/S (S overlap
area between the colliding nuclei in the
transverse plane), and the The next figure shows the ratio of the number of
net-protons (protons - anti-proton) to half the
number of participant nucleons, i.e. the
approximate probability of each incoming nucleon
to be transported to mid-rapidity, as a function
of number of participating nucleons. The
probability is non-zero even in pp collisions at
200 GeV. Compared to pp, the probability is
larger in central heavy-ion collisions at the
same energy by a factor ~2. The probability of
baryon transport to mid-rapidity is larger in
the lower 62.4 GeV collisions, due to the smaller
beam rapidity. This data should provide
significant information on understanding baryon
production and baryon stopping.
There are several other interesting results
reported in this paper which along with the
detail discussion of techniques of particle
identification through energy loss of charged
particles in a medium, is an ideal paper for
fresh graduate students to read. Some of the
other observations in the paper includes -
(a) Spectra of heavy particles are flatter than
those of light particles in all collision
systems. This effect becomes more prominent in
more central Au+Au collisions. In central Au+Au
collisions this could be viewed as due to
large collectivity developed in heavy ion collisions.
(b) The effect of collision energy on the production rate
is significantly smaller on
strangeness production than on anti-baryon production.
(c) Within the framework of the thermal
equilibrium model the extracted chemical
freeze-out temperature is same in p+p, d+Au and
Au+Au collisions at all measured energies in RHIC
and close to the Lattice QCD predicted phase
transition temperature between hadronic matter and the
Quark-Gluon Plasma. The extracted
strangeness suppression factor is substantially
below unity in pp, d+Au, and peripheral Au+Au
collisions. The strangeness suppression factor
in medium-central to central Au+Au collisions
closer to unity. This could in the framework of
the above model suggest that the strangeness and
light flavor are nearly equilibrated, which may
suggest a fundamental change from peripheral
to central collisions.
(d) The extracted kinetic freeze-out temperature from the
blast-wave fit to the transverse momentum spectra, on the
other hand, decreases from pp and d+Au to central Au+Au
collisions.
The extracted collective flow velocity increases
significantly with increasing centrality.There
is a significant difference between the
extracted chemical and kinetic freeze-out
temperature. Experimentally this suggests the
presence of an elastic rescattering phase
between the two freeze-outs.
(e) The equilibrium model studies presented here suggest
that the collision systems chemically
decouple at a universal temperature, independent
of the vastly different initial conditions at
different centralities.
STAR is actively pursing the idea of doing a beam energy
scan program to map the QCD phase boundary
and look for the QCD critical point. So stay
tuned for more interesting results from lower energy
collisions from STAR.
Further details can be found in the following STAR paper - |