You will be presented with the full text of a science research paper. Please write an abstract for this paper. Your response should include the abstract and no additional text.

Paper text:
one of the early goals of heavy - ion collisions at relativistic heavy ion collider ( rhic ) is to establish the existence of a new state of matter which is called quark - gluon plasma ( qgp ) [ 1 ] . according to lattice quantum chromodynamics calculations ( qcd ) , the theory of strong interactions ,
qgp occurs when a sufficiently high temperature and high energy density ( @xmath4 gev/@xmath5 ) is reached .
the partonic system is transformed into hadronic matter as the system cools to lower temperatures .
the phase diagram of qcd is in general characterized by two quantities , the temperature ( t ) and the baryon chemical potential ( @xmath6 ) or the ( net ) baryon density ( @xmath7 ) .
the phase diagram should contain information about the phase boundary that separates the qgp and hadronic phases [ 2 ] .
lattice qcd finds a rapid , but smooth crossover transition from hadron gas to qgp at vanishing baryon chemical potential and large temperature t , while various models predict a strong , first - order phase transition at large @xmath6 .
if this is the case , then there should be a critical point at intermediate values in the ( t , @xmath6 ) plane where the transition changes from a smooth crossover to a first order [ 3 ] .
the bes program at rhic is carried out using several center - of - mass energies of colliding nuclei , to explore the above aspects of this qcd phase diagram .
+ the constituents of the hot and dense medium produced during a heavy - ion collision interact with each other by inelastic and elastic collisions and it evolves into a state of free particles .
this process of hadron decoupling is called freeze - out .
two kinds of freeze - out are found : chemical freeze - out ( t@xmath8 ) when inelastic collisions cease and the particle yields become fixed ; thermal ( kinetic ) freeze - out ( t@xmath9 ) when elastic collisions cease and particle transverse momenta ( @xmath1 ) spectra get fixed .
+ we present a study of the centrality dependence of hadronic freeze - out parameters in au+au collisions at mid - rapidity for @xmath10 7.7 , 11.5 , and 39 gev measured by the star experiment . to extract chemical freeze - out parameters we use a statistical thermal model ( thermus ) [ 5 ] where we fit experimental particle ratios using a grand canonical ensemble ( gce ) approach with the inclusion of a strangeness saturation factor ( @xmath11 ) and also with strangeness canonical ensemble ( sce ) where strangeness quantum number is conserved exactly . in this study
we have used mid - rapidity particle ratios that include the pions ( @xmath12 , @xmath13 ) , kaons ( @xmath14 , @xmath15 ) , protons ( @xmath16 , @xmath17 ) , @xmath18 , lambdas ( @xmath19 , @xmath20 ) and cascades ( @xmath21 , @xmath22 ) [ 6 , 7 ] .
the chemical freeze - out parameters extracted are t@xmath8 , @xmath6 , @xmath23 and @xmath11 .
the kinetic freeze - out parameters are determined from the blast - wave model ( bw ) [ 8 ] fits to the @xmath1 spectra of @xmath24 , @xmath25 and @xmath16 [ 6 ] .
the main kinetic freeze - out parameters extracted are t@xmath9 and average flow velocity ( @xmath26 ) .
[ cols="^,^ " , ]
the new measurement at bes energies 7.7 , 11.5 , and 39 gev at rhic extends the @xmath27 range from 20 to 400 mev in the qcd phase diagram .
kinetic freeze - out parameters are obtained using the measured particle spectra and a bw model .
for all the beam energies studied , the central collisions are characterized by a lower t@xmath9 and larger @xmath26 while the peripheral collisions are found to have a higher t@xmath9 and smaller @xmath26 .
chemical freeze - out parameters are obtained using the measured particle ratios and a thermus model .
we have observed a centrality dependence of the chemical freeze - out parameters at the lower energies . we have observed different behavior of chemical freeze - out parameters ( t@xmath8 , @xmath27 ) for peripheral collisions in gce and sce .
00 j. adams et al .
( star collaboration ) , nucl .
a757 , 102 ( 2005 ) .
p braun - munzinger et al .
arxiv:1101.3167 , 2011 ; b.mohanty , nucl . phys . a 830 , 899c ( 2009 ) .
s. gupta et al . , science 332 , 1525 ( 2011 ) ; e. s. bowman and j. i. kapusta , phys .
c 79 , 015202 ( 2009 ) .
b. i. abelev et al .
( star collaboration ) , phys .
c 79 , 034909 ( 2009 ) .
j. cleymans et al . ,
computer physics communications , 180 , 84 ( 2009 ) .
l. kumar ( star collaboration ) , arxiv:1201.4203 ( 2012 ) , j. phys .
38 , 124145 ( 2011 ) .
x. zhu ( star collaboration ) , acta phys .
supp . 5 ( 2012 ) 213 - 218 .
e. schnedermann et al . , phys .
c 48 , 2462 ( 1993 ) .
b. i. abelev et al .
( star collaboration ) , phys .
c 81 , 024911 ( 2010 ) .
m. m. aggarwal et al .
( star collaboration ) , phys .
c 83 , 024901 ( 2011 ) .
the star experiment at rhic has a unique capability of measuring identified hadrons over a wide range of pseudorapidity ( @xmath0 ) , transverse momentum ( @xmath1 ) , and azimuthal angle ( @xmath2 ) acceptance . 
 the data collected ( @xmath3 = 7.7 , 11.5 , and 39 gev ) in its beam energy scan ( bes ) program provide a chance to investigate the final hadronic state freeze - out conditions of ultrarelativistic au+au collisions . 
 the particle ratios are used to compare to a statistical model calculation using both grand canonical and strangeness canonical ensembles to extract the chemical freeze - out parameters . 
 the @xmath1 distributions are fitted to calculations using a blast - wave model to obtain the kinetic freeze - out parameters . 
 we discuss the centrality dependence of the extracted chemical and kinetic freeze - out parameters at these lower energies .