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\title{KLOE first results on $\eta$, $\eta'$}

\author{The KLOE Collaboration \footnote{T\lowercase{he full} KLOE \lowercase{author list can
    be found in hep-ex/0006036 } } }

\author{\lowercase{presented by}}
\author{F. Ambrosino}

\address{Universit\`a e Sezione INFN, Napoli (Italy)\\E-mail: ambrosino@na.infn.it}

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\maketitle

\abstracts{
The KLOE experiment at the DA$\Phi$NE $e^+e^-$ collider in Frascati has
collected 2.4 pb$^{-1}$ during its first period of data taking in 1999.
The sample of $\sim 8\cdot 10^6\;\phi$ decays has been used to study the
radiative decays $\phi\to\eta\gamma$, $\phi\to\eta'\gamma$.     
Preliminary results show the BR($\phi\to\eta'\gamma$) to be $\simeq 7\cdot 10^{-3}$
relative to the BR($\phi\to\eta\gamma$); this ratio can be used to extract
the pseudoscalar mixing angle $\vartheta_P$.}

\section{Introduction}
The reason for studying the radiative process $\phi\to\eta'\gamma$ is 
twofold: first, the value of this BR enlightens the gluonium
content\cite{Desheil} of the $\eta'$, and, second, the ratio
$R=\frac{\Gamma(\phi\to\eta'\gamma)}{\Gamma(\phi\to\eta\gamma)}$ is
strictly related to the pseudoscalar mixing angle\cite{Rosner,Bramon}.
Predictions for this BR range between value as low as $10^{-6}$ for large
gluonium admixture up to $10^{-4}$ in different realizations of the quark
model and of the mixing pattern\cite{Fulvia}.
The past measurements\cite{SND,CMD-2} have large
errors; KLOE\cite{bertolucci} can
significantly increase the present knowledge by large statistics and by
observing different decay chains.

\section{Analysis and results}
We study four processes, giving rise to two different final states :
  $\phi\to\eta(\eta')\gamma\to\pi^+\pi^-\pi^0(\eta)\gamma\to\pi^+\pi^-\gamma\gamma\gamma$ and
$\phi\to\eta(\eta')\gamma\to\pi^0\pi^0\pi^0(\eta)\gamma\to 7 \gamma$.
The $\phi\to\eta\gamma$ decays are the main source of background
for the rare corresponding $\phi\to\eta'\gamma$ channels, but, since they
can be selected with very low background on a simple topological basis, they constitute
also a relatively high statistics channel on which testing systematics. 
Comparison between data and Monte Carlo
show very good agreement for the $\eta$ control sample, both for $7 \gamma$
and for $\pi^+\pi^-\gamma\gamma\gamma$ final state (see fig. \ref{fig1}). Most of the
selection systematics effects (such as uncertainty on trigger efficiencies)
are in common for the $\eta$ and $\eta'$ corresponding channels and hence
almost cancel when evaluating the ratio $R$ , while errors on integrated
luminosity and $\phi$ cross section cancel exactly in the ratio.
After a first level topological selection, where photons are individuated
as prompt clusters not associated to charged tracks using the excellent
time resolution of the detector (54ps/$\sqrt{\rm{E(GeV)}}$), a kinematic
fit is performed with all measured quantities as inputs and costraining
the intermediate particle masses together with global
energy-momentum conservation. The final selection is made on $\chi^2$
probability for the hypothesis of an event to be either $\eta\gamma$-like
or $\eta'\gamma$-like, and, for the $7\gamma$ sample, cutting on the energy
of the hardest photon. After these selection we are left with $21\pm 4.6$
$\eta'$ events in the $\pi^+\pi^-\gamma\gamma\gamma$ final state and
$6^{+3.3}_{-2.2}$ in the $7 \gamma$ final state with less than one expected
background event at 90\% CL. This is the first observation of the
$\phi\to\eta'\gamma\to 7 \gamma$ decay chain.
Using the number of selected $\eta$ and $\eta'$ events, the efficiencies
evaluated by Monte Carlo and the intermediate states BR's as given by PDG
we evaluate the ratio $R=(7.1 \pm 1.6 ({\rm stat.}) \pm 0.3 ({\rm
  syst.}))\cdot 10^{-3}$ or BR($\phi\to\eta'\gamma$)=$ (8.9\pm 2 \pm
0.6)\cdot 10^{-5}$. This result has the same level of accuracy of the
current world average. Using formula given in Bramon\cite{Bramon} we can
use this value of $R$ to extract a mixing angle $\vartheta_P\simeq
-19^{\circ}$. The value of BR($\phi\to\eta'\gamma$) disfavours
models with large gluonium admixtures of the $\eta'$.
        

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\begin{figure}[t]
\begin{center}
%\figurebox{20pc}{15pc}{} % to have a box alone
\epsfxsize=17pc % will enlarge or reduce the postscript figures based on the xsize
\epsfbox{ambrosino.merged.eps} % postscript image file name
\caption{Comparison data Monte Carlo for $\eta$ control sample: a) photon
  energy spectrum in 7$\gamma$ events ; b) charged pions momentum spectrum
  in $\pi^+\pi^-\gamma\gamma\gamma$ events}\label{fig1}
\end{center}
\end{figure}

\begin{thebibliography}{99}


\bibitem{Desheil}N. G. Deshpande, G. Eilam, \Journal{\PRD}{25}{270}{1982}

\bibitem{Rosner} J. L. Rosner, \Journal{\PRD}{27}{1101}{1983}

\bibitem{Bramon} A. Bramon {\em et al}, {\em Eur. Phys. J.} C {\bf 7}, 271 (1999) 

\bibitem{Fulvia} F.~De Fazio, M.R. Pennington {\em JHEP} {\bf 07}, 051
  (2000) and refs. therein

\bibitem{SND} V. M. Aulchenko {\em et al}, {\em JETP Lett.} {\bf 69}, 97 (1999)

\bibitem{CMD-2} R. R. Akhmetshin {\em et al} \Journal{\PLB}{473}{337}{2000}

\bibitem{bertolucci}S. Bertolucci for the KLOE collaboration, hep-ex/0002030.

%\bibitem{osaka} The KLOE collaboration, contributed paper to ICHEP2000 (hep-ex/0006036).

\end{thebibliography}

\end{document}

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