System Scan For Freeze-out Properties In Relativistic Heavy-ion Collisions By Using A Multi-phase Transport Model

Both LHC-ALICE and RHIC-STAR have announced a series of upgrade plans.In the plan,they have put forward the scientific research goal of conducting more in-depth research on small system physics based on system scanning;For example,to better understand the system dependence of physical quantities such as collective flow.Therefore,we want to systematically scan the physics of collision systems at the freeze-out stage.In this work,we employ a multiphase transport(AMPT)model to study the bulk properties at freeze-out stage for 10B+10B,12C+12 C,16O+16O,20Ne+20 Ne,40Ca+40 Ca,96Zr+96 Zr and 197Au+197 Au collisions at RHIC energy(?)=200,20 and 7.7 GeV.We present the AMPT prediction of pT and dN/dy spec-tra of identified particles including ?±,k±,p,and p in different symmetric collision systems.The results in 197Au+197 Au collisions are comparable with previous STAR data.The system(Npart)dependence of(dN/dy)can be described by a simple function,log(dN/dy)=p+q*log((Npart))where parameter q actually reflects the(NPart)depen-dence of(dN/dy).It will get larger value in more larger system size.The transverse momentum pT spectra of charged particles(?±,K±,p,and p)at the kinetic freeze-out stage,based on a blast-wave model,are also discussed.In 0.2-1.5 GeV pT range,pT spectrum could be fitted well.From fitting results,the kinetic freeze-out temperature Tkin decreases with the increasing of collision-system size and the average radial flow velocity<?T>presents an upward trend of system dependence.Besides,we use a statistical thermal model to extract parameters at the chemical freeze-out stage which agree with those from other thermal model calculations.It is found that there is a competitive relationship between the kinetic freeze-out parameter Tki,and radial expansion velocity ?T,which is also used in experiment to study freeze-out properties.We find that the chemical freeze-out strangeness potential ?s remains constant in all collision systems and fireball radius R is dominated by<NPart>which can be well fit by a function of a<NPart>b with coefficient b approximately equals to 1/3 for all collision systems.In addition,we calculate that nuclear modification factors for different collision systems with respect to the 10B+10B system and find that they present gradual suppres-sion in higher pT range from small system to larger system.In summary,we believe that in systems smaller than gold-gold collision system,the fluid dynamics assumption still holds.At the same energy,the system dependence of the physical quantities in the freeze-out stage is similar to the dependence of centrality in gold-gold collision.