{\noindent \it ISR studies at Belle\\}

Until now most of the Belle analyses using radiative return 
focused on studies of the charmonium and charmo\-ni\-um-like 
states. They can be subdivided into final states with open and
hidden charm.\\   

{\noindent \it Final states with open charm\\}

Belle performed a systematic study of various exclusive channels of
$e^+e^-$ annihilation into charmed mesons and baryons using ISR, 
often based on the so called partial reconstruction to increase 
the detection efficiency.

In Ref.~\cite{Abe:2006fj} they measured the cross sections of the processes
$e^+e^- \to D^{*\pm}D^{*\mp}$  and $e^+e^- \to D^+D^{*-} + c.c.\,$.
The shape of the former is complicated and has several local maxima
and minima. The first two maxima are close to the $\psi(4040)$ and
$\psi(4160)$ states. The latter shows significant excess of events 
near the $\psi(4040)$.

The cross sections of the processes  $e^+e^- \to D^+D^-$ and
 $e^+e^- \to D^0\bar{D}^0$ show a signal of the $\psi(3770)$, 
as well as hints of the $\psi(4040)$, $\psi(4160)$ and 
$\psi(4415)$~\cite{Pakhlova:2008zza}. There is
also an enhancement  near 3.9~GeV, which qualitatively agrees with 
the prediction of the coupled channel model \cite{Eichten:1979ms}. 

The cross section of the process $e^+e^- \to D^0D^-\pi^+$ has a
prominent peak at the energy corresponding to the
$\psi(4415)$~\cite{Pakhlova:2007fq}. From a study of the resonant 
substructure in
the decay $\psi(4415) \to D^0D^-\pi^+$ they conclude that it is
dominated by the intermediate $D\bar{D}^*_2(2460)$ mechanism.

In contrast to expectations of some hybrid models predicting
$Y(4260) \to D^{(*)}{\bar D^{(*)}}\pi$ decays, no clear structures were 
observed in the cross section of the process 
$e^+e^- \to D^0D^{*-}\pi^+$~\cite{Pakhlova:2009jv}. There is only 
some evidence ($\sim 3.1\,\sigma$) for the $\psi(4415)$. 

Finally, they measure the cross section of the reaction
$e^+e^- \to \Lambda^+_c\Lambda^-_c$ and observe a significant peak near 
threshold that they dub $X(4630)$~\cite{Pakhlova:2008vn}. 
Assuming that the peak 
is a resonance, they find that its mass and width are compatible
within errors with those of the $Y(4660)$ 
state found by Belle in the $\psi(2S)\pi^+\pi^-$ final state 
via ISR~\cite{:2007ea}. However, interpretations other than 
$X(4630) \equiv Y(4660)$ cannot be excluded. For example, peaks 
at the baryon-antibaryon threshold are observed in various 
processes.
%~\cite{Abe:2002ds}. 
According to other assumptions, 
the $X(4630)$ is
a $\psi(5S)$~\cite{Badalian:2008dv} or $\psi(6S)$~\cite{Li:2009zu} 
charmonium state, 
or, for example, a threshold effect which is due to the $\psi(3D)$, 
slightly below the $\Lambda^+_c\Lambda^-_c$ 
threshold~\cite{vanBeveren:2008rt}. 
Figure~\ref{fig:allcc} shows all cross sections mentioned above, with
the vertical lines showing positions of both well established states
like 
%$\psi(3770)$, 
$\psi(4040)$, $\psi(4160)$
and $\psi(4415)$, and new charmonium-like 
states $Y(4008)$, $Y(4260)$, $Y(4360)$ and $Y(4660)$ discussed below.

\begin{center}
\begin{figure}
\begin{center}
\includegraphics[height=0.43\textheight]{excl_six.eps}
\caption{Cross sections of various exclusive processes measured by
Belle: a)  $e^+e^- \to D^{*\pm}D^{*\mp}$, b) $e^+e^- \to D^+D^{*-} + c.c.$,
c) $e^+e^- \to D\bar{D}$,
d) $e^+e^- \to D^0D^-\pi^+ + c.c.$, e)  $e^+e^- \to D^0D^{*-}\pi^+ + c.c.$, and
f) $e^+e^- \to \Lambda^+_c\Lambda^-_c$. The dashed lines show the
position of the $\psi$ states, while the dotted lines correspond
to the $Y(4008),~Y(4260),~Y(4360)$, and $Y(4660)$ states.}
\label{fig:allcc}
\end{center}
\end{figure}
\end{center}

Summing the measured cross sections and taking into account not yet
observed final states on base of isospin symmetry they find that 
%until about 4.3~GeV 
the sum of exclusive cross sections almost saturates the 
total inclusive cross section measured by BES~\cite{Bai:2001ct}.\\

{\noindent \it Final states with hidden charm\\}

Studying the $J/\psi\pi^+\pi^-$ final state, Belle confirmed the
$Y(4260)$ discovered by BaBar and in addition observed a new structure
dubbed $Y(4008)$~\cite{:2007sj}, see Fig.~\ref{fig:bel42}.
They also observe the reaction $e^+e^- \to J/\psi K^+K^-$
and find first evidence for the reaction  
$e^+e^- \to J/\psi K^0_SK^0_S$~\cite{:2007bt}.


\begin{figure}
\begin{center}
%%%\includegraphics[width=0.35\textwidth]{radret_belle_y4260.eps}
\includegraphics[width=0.25\textwidth,angle=-90]{radret_belle_ppll.eps}
\caption{The $J/\psi\pi^+\pi^-$ invariant mass distribution.}
\label{fig:bel42}
\end{center}
\end{figure}



Studying the $\psi(2S)\pi^+\pi^-$ final state, Belle confirmed the
$Y(4360)$ discovered by BaBar and in addition observed a new structure
dubbed $Y(4660)$~\cite{:2007ea}, see Fig.~\ref{fig:bel43}.


\begin{figure}
\begin{center}
\includegraphics[width=0.25\textwidth,angle=-90]{radret_belle_y4360.eps}
\caption{The $\psi(2S)\pi^+\pi^-$ invariant mass distribution.} 
\label{fig:bel43}
\end{center}
\end{figure}




It is worth noting that the resonance interpretation of various
enhancements discussed above is not unambiguous and can be strongly affected
by close thresholds of different final states and rescattering
effects.
 
Various ISR studies performed at the Belle detector in the charmonium
region are summarised in Table~\ref{tab:belisr}.\\


\begin{table}
\begin{center}
\caption{Summary of ISR studies in the $c\bar{c}$ region at Belle.}
\label{tab:belisr}
\begin{tabular}{lll}
\hline
Final state & $\int{L~{\rm d}t}$, fb$^{-1}$ & Ref. \\
\hline
$D^{*+}D^{*-}$ & 547.8 &  \cite{Abe:2006fj} \\
\hline
$D^{\pm}D^{*\mp}$ & 547.8 &  \cite{Abe:2006fj} \\
\hline
$D^0\bar{D}^0,~D^+D^-$ & 673 & \cite{Pakhlova:2008zza} \\
\hline
$D^0D^-\pi^+$ & 673 &  \cite{Pakhlova:2007fq} \\
\hline
$D^0D^{*-}\pi^+$ & 695 &  \cite{Pakhlova:2009jv} \\
\hline 
$\Lambda^+_c\Lambda^-_c$ & 695 &  \cite{Pakhlova:2008vn} \\
\hline
$J/\psi\pi^+\pi^-$ & 548 &  \cite{:2007sj} \\
\hline
$\psi(2S)\pi^+\pi^-$ & 673 &  \cite{:2007ea} \\
\hline
$J/\psi K^+K^-$ & 673 & \cite{:2007bt} \\
\hline
\end{tabular}
\end{center}
\end{table}


{\noindent \it ISR studies of  light quark states\\}

In one case the ISR method was used to study the light quark 
states~\cite{:2008ska}. In this analysis the cross sections of the
reactions $e^+e^- \to \phi\pi^+\pi^-$ and  $e^+e^- \to \phi f_0(980)$
are measured from threshold to 3~GeV, using a data sample of 673~fb$^{-1}$,
see Fig.~\ref{fig:belphi} (a, b).
In the $\phi\pi^+\pi^-$ mode the authors observe and measure for
the first time the parameters of the $\phi(1680)$; they also observe
and measure the parameters of the $\phi(2170)$. Also selected in this
analysis is the $\phi f_0(980)$ final state, which shows a clear
signal of the $\phi(2170)$. For Monte Carlo simulation they use a
version of PHOKHARA in which the produced resonance decays into  
$\phi\pi^+\pi^-$ or  $\phi f_0(980)$ with the subsequent decays
$\phi \to K^+K^-$ and $f_0(980) \to \pi^+\pi^-$. The $\pi^+\pi^-$ 
system is in the $S$-wave, the $\pi^+\pi^-$ system
and the $\phi$ are also in a relative 
$S$-wave.  The $\pi^+\pi^-$ mass distribution is generated according
to phase space. They assign 0.1\% as  the systematic uncertainty
of the ISR photon radiator.

In all the ISR studies the Monte Carlo simulation is performed as
follows. First, the kinematics of the initial state radiation is
generated using the PHOKHARA v5.0 package for simulation of the
process $e^+e^- \to V\gamma_{\rm ISR}(\gamma_{\rm ISR})$ \cite{Czyz:2005as}.
Then a $q\bar{q}$ generator is used to generate $V$ decays.

\begin{figure}
\begin{center}
\begin{tabular}{cc}
\begin{minipage}{4.2cm}
\includegraphics[width=1.\textwidth]{radret_belle_fig6a.eps}
\end{minipage} &
\begin{minipage}{4.2cm}
\includegraphics[width=1.\textwidth]{radret_belle_fig6b.eps}
\end{minipage}
\end{tabular}
\caption{Cross sections of the processes 
$e^+e^- \to  \phi\pi^+\pi^-$ (a) and $e^+e^- \to \phi f_0(980)$ (b).}
\label{fig:belphi}
\end{center}
\end{figure}



