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\title{System size dependence of particle production in p/d+Au, Ru+Ru, Zr+Zr & Au+Au collisions at $\sqrt{s_\mathrm{NN}}$ = 200 GeV with the STAR experiment}
\author{Tong Liu$^1$ and Yang Li$^{2,3}$ for the STAR collaboration}
\date{$^1$Yale University, New Haven, CT, USA\\%
      $^2$University of Science and Technology of China, Hefei, Anhui, China
      $^3$Brookhaven National Lab, Upton, NY, USA
}

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%Why isobar
%\textcolor{blue} {The hot and dense matter called Quark-Gluon plasma (QGP) can be produced in large colliding systems (e.g. Au+Au and Pb+Pb)}
While it has been confirmed that the hot and dense QCD medium called the Quark-Gluon Plasma (QGP) can be produced in large system heavy-ion collisions (e.g. Au+Au and Pb+Pb), recent studies on hard scatterings do not show significant quenching in small systems (e.g. p+Au). Therefore, medium size systems, such as Ru+Ru and Zr+Zr collisions, become crucial to study the system size dependence of QGP formation and its properties. Such collisions provide a wide dynamic range in the number of participants ($N_\mathrm{part}$) from about 10 to 165, sitting in between p+Au and central Au+Au collisions. 
Also, the $ N_\mathrm{part} $ of Ru+Ru and Zr+Zr collisions partially overlap with that of Au+Au collisions, allowing us to study how collision geometry influences QGP formation and its evolution.
%What we use to study this question
A useful tool is the transverse momentum ($p_\mathrm{T}$) distribution of charged hadrons. On the one hand, high $p_\mathrm{T}$ hadrons serve as a proxy to hard scattering processes and can be used to probe parton energy loss in the medium. On the other hand, the distribution on the low-$p_\mathrm{T}$ end, especially those of species-identified particles, can reveal the transverse expansion and freeze-out properties of the medium. 

% High transverse momentum ($p_\mathrm{T}$) partons are produced in the initial stages of high energy nucleus-nucleus collisions, and can be used as an excellent probe of both initial-stage and final-stage phys. Especially, when a Quark-Gluon Plasma (QGP) is created, the partons will lose energy to this medium via collisional and radiative processes. One common proxy to these effects is the invariant yield of high $p_\mathrm{T}$ particles, where effects of different mechanisms can be reflected via its modification relative to $p+p$ collisions. Though previous studies focus more on energy dependence of the modification, we here present results that target its sensitivity to the size of the collision system.

In this talk, we present charged hadron yields using the large isobar ($_{44}^{96}Ru$+$_{44}^{96}Ru$ and $_{40}^{96}Zr$+$_{40}^{96}Zr$) dataset collected with the STAR detector in 2018. We perform multi-differential yield measurements of low $p_\mathrm{T}$ identified particles, and centrality-differential measurement on high $p_\mathrm{T}$ particles. Combined with existing Au+Au and small system ($p$+Au/$d$+Au) measurements, we picture an overall system size dependence of QGP production and properties as well as effects of collision geometry on them.

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