\section*{ABSTRACT}
    The proposed Circular Electron Positron Collider (CEPC), with a center-of-mass energy $\sqrt{s} = 240$ GeV, will primarily serve as a Higgs factory.
    At the same time, it can offer good opportunities to search for new physics phenomena at low energy, which can be challenging to discover at hadron colliders, but well motivated by some theoretical models developed to explain, e.g., the relic abundance of dark matter.
    This paper presents sensitivity studies of chargino pair production, considering scenarios for both a Bino-like and a Higgsino-like neutralino as the lightest supersymmetric particle, using full Monte Carlo (MC) simulation.
    With the assumption of systematic uncertainties at the level of 5\%, the CEPC has the ability to discover chargino pair production up to the kinematic limit of $\sqrt{s}/2$ for both scenarios.
    The results have a minor dependence on the reconstruction model and detector geometry. 
    These results can also be considered as a reference and benchmark for similar searches at other proposed electron-positron colliders, such as the Future Circular Collider ee (FCC-ee) or the International Linear Collider (ILC), particularly given the similar nature of the facilities, detectors, center-of-mass energies, and target luminosities.
    \par\textbf{Keywords: }CEPC; chargino; Bino; Higgsino

\let\thefootnote\relax\footnotetext{* This study was supported by the National Key Programme (Grant NO.: 2018YFA0404000).}
\tableofcontents
\clearpage

 \section{Introduction}
 \label{sec:intro}
 \input{introduction.tex}

 \section{Detector, Software and Samples}
 \label{sec:samples}
 \input{samples.tex}

 \section{Search for chargino pair production at CEPC}
 \label{sec:searchew}
 \input{searchew.tex}

 \pagebreak
 \clearpage

 \section{Conclusion}
 \label{sec:conclusion}
 \input{conclusion.tex}

 \section{Acknowledgements}
 \label{sec:acknowledgements}
 \input{acknowledge.tex}

\bibliographystyle{spphys} 







\subsection{Search for chargino pair production with Bino LSP}
\label{subsec:searchcb}
\input{searchcb.tex}
\subsection{Search for chargino pair production with Higgsino LSP}
\label{subsec:searchch}
\input{searchch.tex}
\subsection{Chargino pair production search summary}
\label{subsec:searchsum}
The studies presented in this paper demonstrate that the discovery sensitivity for electroweakinos with both a Bino-like LSP and a Higgsino-like LSP can reach masses up to $\sqrt{s}/2$ at CEPC. The sensitivity can be further improved if considering additional final states (i.e. electron and tau), alternate signal scenarios such as disappearing track or chargino pair production with photon initial-state radiation (ISR), or more complicate analysis techniques (i.e. machine learning etc). 

The prospective limits at CEPC compared with LEP and LHC results are shown in Figure \ref{fig:searchsum}. 
In general, the sensitivity at CEPC is mainly constrained by the center-of-mass energy and is not strongly dependent on the mass splitting between NLSP and LSP, allowing CEPC to cover very compressed electroweakinos that are very difficult to search for at the LHC due to the reduced acceptance and reconstruction efficiency of the soft leptons.
\begin{figure}[!htp]
  \centering
    \subfigure [] {\includegraphics[width=.48\textwidth]{Figure6a.pdf}}
    \subfigure [] {\includegraphics[width=.48\textwidth]{Figure6b.pdf}}
    \caption{Observed and expected exclusion limits on SUSY simplified models for chargino-pair production with (a) Bino-like LSP and (b) Higgsino-like LSP obtained by ATLAS. The observed limits obtained by LEP are also shown for Higgsino LSP case in Figure \ref{fig:searchsum} (b), while not shown in Figure \ref{fig:searchsum} (a) since the limit of chargino mass is below 100 GeV. The prospected limits at CEPC are also shown in the dotted purple line for rough comparison. }
    \label{fig:searchsum}
  \end{figure}