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\title{Probing the neutron skin and symmetry energy with isobar collisions at $\sqrt{s_{\rm NN}}=200$ GeV by STAR}
\author{Haojie Xu\\ (For the STAR Collaboration) \\ Huzhou University 
\\ Track category: Baryon rich matter, neutron stars, and gravitational waves}


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Neutron skin thickness $\Delta r_{\rm np}$ of nuclei and the inferred nuclear symmetry energy are of critical importance to 
the equation of state of dense nuclear matter in neutron stars and heavy-ion collisions.
The $\Delta r_{\rm np}$ has traditionally been measured by low-energy hadron-nucleus and nucleus-nucleus scatterings over decades. 
The recent PREX-II measurement using parity-violating electroweak interactions has yielded a large neutron skin thickness of Pb nucleus,
at tension with the world-wide data established in hadronic collisions. 
More recent studies indicate that the neutron skin can also be measured,
unconventionally and possibly with better precisions than traditional methods, 
by colliding isobar nuclei at relativistic energies~\cite{Li:2019kkh}.
The idea is to compare the produced hadron multiplicities ($N_{\rm ch}$)~\cite{Li:2019kkh},
the mean transverse momenta ($\langle p_{T}\rangle$)~\cite{Xu:2021kkh}, and the net charge multiplicities ($\Delta Q$)~\cite{Xu:2021qjw}
to trace back the nuclear structure differences between the isobar nuclei. 
In this talk, we will present results on the $N_{\rm ch}$, $\langle p_{T}\rangle$, and $\Delta Q$ ratios between
isobar ($^{96}_{44}$Ru+$^{96}_{44}$Ru and $^{96}_{40}$Zr+$^{96}_{40}$Zr) collisions at $\sqrt{s_{\rm NN}}=200$ GeV by STAR.
We extract the neutron skin thickness and the symmetry energy slope parameter from these data, 
and discuss our results in the context of the global data on symmetry energy and the tension with the PREX-II data. 
We also comment on the implication of our results on the baseline
for the chiral magnetic effect search in isobar collisions~\cite{Xu:2017zcn,STAR:2021mii}. 

\bibliographystyle{unsrt}

\begin{thebibliography}{999}
\bibitem{Li:2019kkh}
H.~Li, H.~j.~Xu, Y.~Zhou, X.~Wang, J.~Zhao, L.~W.~Chen and F.~Wang,
%``Probing the neutron skin with ultrarelativistic isobaric collisions,''
Phys. Rev. Lett. \textbf{125}, 222301 (2020)
%doi:10.1103/PhysRevLett.125.222301
arXiv:1910.06170 [nucl-th].


%\cite{Xu:2021qjw}
\bibitem{Xu:2021kkh}
H.~j.~Xu, W.~Zhao, H.~Li, Y.~Zhou, L.~W.~Chen and F.~Wang,
%``Probing nuclear structure with mean transverse momentum in relativistic isobar collisions,''
arXiv:2111.14812 [nucl-th].


%\cite{Xu:2021qjw}
\bibitem{Xu:2021qjw}
H.~j.~Xu, H.~Li, Y.~Zhou, X.~Wang, J.~Zhao, L.~W.~Chen and F.~Wang,
%``Measuring neutron skin by grazing isobaric collisions,''
arXiv:2105.04052 [nucl-th].


%\cite{Xu:2017zcn}
\bibitem{Xu:2017zcn}
H.~j.~Xu, X.~Wang, H.~Li, J.~Zhao, Z.~W.~Lin, C.~Shen and F.~Wang,
%``Importance of isobar density distributions on the chiral magnetic effect search,''
Phys. Rev. Lett. \textbf{121}, 022301 (2018)
%doi:10.1103/PhysRevLett.121.022301
arXiv:1710.03086 [nucl-th].


%\cite{STAR:2021mii}
\bibitem{STAR:2021mii}
M.~Abdallah \textit{et al.} (STAR),
%``Search for the Chiral Magnetic Effect with Isobar Collisions at $\sqrt{s_{NN}}$ = 200 GeV by the STAR Collaboration at RHIC,''
arXiv:2109.00131 [nucl-ex].

\end{thebibliography}
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