star focus: Identified hadron measurements in STAR using the Time Projection Chamber
Highlights from the STAR paper Systematic Measurements of Identified Particle Spectra in p+p, d+Au and Au+Au Collisions from STAR submitted recently to Physical Review C.
Posted: Sep 06, 2008

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 is predicted to scale with this quantity. The first figure shows the experimental observations for minimum bias pp and for Au+Au collisions of the various centralities. A linear dependence of the mean pT on (dN_pion/dy)/S is observed for all three particle species, as shown by the lines in the figure. It is interesting to note that the slope, characterizing the rate of increase in the mean pT, is a factor 2 larger for anti-protons than for kaons which is in turn a factor 2 larger than for pions. The intercepts of the linear fits for anti-proton and kaons are the same, but are larger than that for pions.

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 -
Systematic Measurements of Identified Particle Spectra in pp, d+Au and Au+Au Collisions from STAR - arXiv:0808.2041.

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