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\begin{document}

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\def\NPB{{\em Nucl. Phys.}   {\bf B}}
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\begin{titlepage}
\noindent
% {\bf H-UM \& JS: version of \today} \\[.3em] 
Submitted to the 30th International Conference on 
High-Energy Physics ICHEP2000, \\ 
Osaka, Japan, July 2000

\vspace{4cm}


\begin{center}
\begin{Large}

\noindent


\begin{center}
\begin{Large}

{\bf Diffractive \boldmath$\psi(2S)$ Photoproduction at HERA}

\vspace{1.5cm}

\end{Large}

 

\end{center}
\vspace{2cm}

H1 Collaboration

\end{Large}
\end{center}

\vspace{2cm}

\begin{abstract}
\noindent The elastic photoproduction of $\psi(2S)$ mesons is analyzed 
using data collected between 1996 and 1999 
with the H1 detector at the HERA collider. The data sample corresponds
to an integrated 
luminosity of about $\sim38\:\mbox{pb}^{-1}$. 
The $\psi(2S)$ mesons are identified through their leptonic decay.
The $t$-dependence of the elastic $\psi(2S)$ production
differential cross section
%in photoproduction
is compared to that of $J/\psi$ production, and
the slope, parameterized as $e^{-b|t|}$, is measured
to be $b=4.5\pm 1.2 ^{~~+1.4}_{~~-0.7}~ \GeV^{-2}$.



\end{abstract}

\vspace{1.5cm}

\begin{flushleft}
{\bf Abstract: 987, 985  } \\
{\bf Parallel sessions: PA-02, 07} \\
{\bf Plenary sessions: PL-12, 03} \\ 
\vspace*{0.2cm}
%{\bf Electronic Access: http://www-h1.desy.de/publications/ }
\end{flushleft}

\end{titlepage}


\pagestyle{plain}
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%

\section*{Introduction}
The cross section of elastic $J/\psi$ meson photoproduction, 
$\gamma p\rightarrow J/\psi\,p$,  at HERA energies has been
measured to rise steeply with increasing energy \cite{h1jpsi2}. 
This is interpreted as a signature for a short distance 
process \cite{ryskin, fks} due to the small
transverse size of the $c\overline{c}$ system. 
The ``size'' of the $c\overline{c}$ system is related also to the dependence of
the cross section on $t$, the square of the 4-momentum transfer at 
the proton vertex. Smaller systems are expected to 
have a less peripheral $t$-dependence.
The $t$-slope, that is the value of $b$ in a parametrisation of the differential
production cross section $\frac{{\rm d}\sigma}{{\rm d}t}\propto e^{-b|t|}$,
has indeed been measured to be much smaller for 
the $J/\psi$ \cite{h1jpsi2} than
for elastic $\rho$ meson production \cite{rhoh1zeus}.

\noindent 
The $c{\bar c}$ radial excitation
$\psi(2S)$ has a larger radius than the ground state 
$J/\psi$ and its radial wave function has a node. 
In a QCD calculation this leads one to expect two distinct features for
elastic $\psi(2S)$ photoproduction \cite{Nemchik}. 
Firstly, the cross section should be suppressed compared to that of 
the $J/\psi$. 
This has been measured \cite{h1psip}\footnote{see however \cite{hoyer}.}.
Secondly, the $t$-slope should be roughly equal to or 
smaller than that of the $J/\psi$, 
even though the two $c$-quarks are on average 
further apart than in the ground state. 

\noindent 
Here we present a first measurement of the $t$-dependence of $\psi(2S)$
photoproduction.

%\section*{Old Introduction}
%Elastic photoproduction of the  $J/\psi$ meson, 
%$\gamma p\rightarrow J/\psi\,p$, at HERA energies shows signatures of a 
%hard process calculable in pQCD. The interaction of the proton takes 
%place with the $c$ and $\overline{c}$ quarks via exchange of a colorless 
%system of (at least) two gluons.
%The transition of the photon to the quark pair happens long before and the 
%formation of the meson much later than this hard interaction.
%\\
%This picture is different from the ``soft'' approach where proton and 
%$J/\psi$  meson are viewed as being diffracted from each other as hadrons.
%The latter view would imply that the diffractive slope, i.e.~the cross section 
%dependence on the negative square of the 4-momentum transfer at 
%the proton vertex $t$, is steeper for the $\psi(2S)$ state than 
%for the $J/\psi$, because the $\psi(2S)$ has a larger size.
%Parameterizing the dependence on $t$ as $e^{-b|t|}$ one would expect 
%$b_{\psi(2S)}>b_{J/\psi}$.
%\\
%A QCD based description \cite{codi} is
%provided by a model in which the $c\overline{c}$ pair is considered as a
%color dipole interacting with the proton. In this picture 
%the $t$-dependence of
%$J/\psi$ and $\psi(2S)$ is expected to be roughly equal 
%or even $b_{\psi(2S)}\lesim b_{J/\psi}$, contrary to intuition
%\cite{Nemchik}.
%\\
%Here an analysis of elastic photoproduction of $\psi(2S)$ mesons using the H1 
%detector is reported. The dependence of the diffractive slope of the
%$\psi(2S)$
%is extracted and compared to that of the 
%$J/\psi$ meson. This comparison may shed further light on the nature of the 
%diffractive process. 

\section*{Data Analysis} 
The analysis presented here is based on data with an integrated luminosity of  
$\int{\cal L}\:\mbox{dt}\sim38\:\mbox{pb}^{-1}$.
The data were taken in the period 1996 -- 1999. In 1996 and 1997 
HERA was operated with positrons 
of $27.5\,\gev$ and protons of $820\,\gev$, while in 1999 the proton energy 
was $920\,\gev$, and electrons and positrons were used.  
The analysis is carried out in the photoproduction limit, 
i.e.~$\qsq \lesssim 1\GeV^2$, and covers a range of photon proton center of 
mass energies of $40<\wgpw<150 \GeV$. 

\noindent 
For the analysis the direct leptonic decays 
$\psi(2S)\rightarrow \mu^+\mu^-, e^+e^-$ are used.
Events in which $\psi(2S)$ decays into a $J/\psi$ and two charged pions 
with subsequent decay of the $J/\psi$ into two leptons have also been
analyzed. As the transverse momentum of the
cascade decay pions is low, the experimental uncertainty in the cascade decay 
channel is  large in comparison to the direct leptonic decay of the $\psi(2S)$. Therefore
the cascade decay events were only used for cross check purposes.


\noindent 
The data selection resembles closely the procedure used for detecting 
the $J/\psi$ described in detail e.g. in \cite{h1jpsi2}. 
Exactly two tracks each with a transverse momentum greater 
than $800 \MeV$ are required in the central tracking chambers 
in a polar angular range of 
$20^\circ < \theta < 160^\circ$\footnote{The coordinate 
system of H1 defines the positive $z$ axis to be in the direction of the
incident proton beam. The polar angle $\theta$ is then defined with
respect to
the positive $z$ axis.}.
Both tracks have to fit to the event vertex and 
satisfy the lepton identification requirements.

\noindent 
Electrons are identified using the electromagnetic part of 
the LAr calorimeter and energy loss in the tracking chamber.
In addition, the backward
electromagnetic calorimeter Spacal 
is used at large polar angles.
Muons are identified as minimum ionizing particles in the calorimeter or 
in the instrumented iron return yoke \cite{H1,Spacal}. 
Events are rejected if a scattered beam lepton candidate with more than 
$8\GeV$ energy deposit is detected in the calorimeter. The accepted 
event sample thus fulfills $Q^2 \lesssim 1 \GeV^2$ with an
average $\langle Q^2\rangle \sim 0.05$ GeV$^2$.


\noindent 
In order to separate the elastic process from that with dissociation
of the proton to a small mass system, the forward part of the
calorimeter 
($\theta< 10^\circ$) and detectors covering polar angles close to the proton beam 
($0.06^{\degree}<\theta<0.26^{\degree}$ and
$3^{\degree}<\theta<17^{\degree}$) are used.
With these detectors proton
remnants with masses $M \gtrsim 1.6 \GeV$ can be detected.
Events without any signal in the forward counters
are called the ``forward untagged'' sample. 

%\end{document}
%The fraction of elastic events in the forward untagged sample 
%has been 
%determined with the help of Monte Carlo Simulations {\sf check cite}
%and is approximately 
%{\sf check (BLAH! Was Kluges zur tag efficiency!!!) 
\noindent
Fig.~\ref{fig1} shows the mass distribution of the combined 
forward tagged and untagged samples. The insert shows the mass distribution
restricted to the $\psi(2S)$ region. The non-resonant background is dominated
by the process $\gamma\gamma \rightarrow l^+l^-$.



\begin{figure}[ht] \centering
\setlength{\unitlength}{1cm}
\begin{picture}(15.,11.1)
\put(0.,-0.5){\epsfig{file=H1prelim-00-174.fig1a.eps,width=16cm}}
\put(7.5,5.5){\epsfig{file=H1prelim-00-174.fig1b.eps,width=8cm}}
\end{picture}
\caption{Mass spectrum of the whole data sample for both 
decay channels from 1996, 1997 and
  1999 for $|t| > 0.07 \GeV^2$. The insert shows the mass distribution
  restricted to the $\psi(2S)$ region. The curve is fitted combining
  a Gaussian and an exponential for the radiation tail from the electron 
  decay channel for the vector meson resonances with a linear 
  polynomial for the background.} 
\label{fig1}
\end{figure}



 
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%


\section*{The \boldmath $t$ Dependence for Elastic $\psi(2S)$ Production}
In elastic vector meson photoproduction events the variable $t$ can be 
approximated by $ t \simeq -(\vec{p}_t)^2 $, where
$\vec{p}_t$ is the transverse momentum of the vector meson.
The $t$-dependence of $\psi(2S)$ production 
is measured averaging over the 
entire $W_{\gamma p}$ range  of $40<W_{\gamma p}<150$ GeV.
The resolution in $t$ was estimated 
with the help of the Monte Carlo generator DIFFVM \cite{diffvm} which simulates 
elastic and proton dissociative vector meson production.
For the muonic decay channel a value of typically 0.035 GeV$^2$ was
found which increases to $0.060 \GeV^2$ for $|t| \approx 1.2 \GeV^2$.
For the electron decay channel the resolution is roughly $15\%$ worse
due to bremsstrahlung.


\noindent 
Fig.~\ref{fig2} shows the ratio between the measured 
number of events in the
region of $150\MeV$ around the nominal $\psi(2S)$ mass and 
the number of events in the $J/\psi$ signal as a function of $|t|$.
The contribution from non-resonant background is determined 
from a fit to the sidebands of the data and statistically 
subtracted. Only events without a signal in the 
forward detectors are used. No correction for detector acceptance or
efficiencies is applied here as the corrections for
$J/\psi$ and $\psi(2S)$ events are very similar.
%{\sf check error bars contain what? Ist der Fehler auch richtig bestimmt?}
The slope of the  ratio as a function of $t$ is consistent with zero, 
indicating that $\psi(2S)$ and $J/\psi$ meson production 
have similar $t$-dependences.
Note that the background contributions 
from proton dissociation processes increase towards high values 
of $|t|$ for both $J/\psi$ and $\psi(2S)$ production. Although these 
contributions are similar and thus largely cancel in the ratio, the 
interpretation of fig.~\ref{fig2} should be taken with care.
\begin{figure}[ht] 
\centering
\setlength{\unitlength}{1cm}
\begin{picture}(12.,9.)
\put(-0.25,0.){\epsfig{file=H1prelim-00-174.fig2.eps,width=12cm}}
\end{picture}
\caption{Uncorrected ratio of the number of forward untagged events in the
region $150\MeV$ around the nominal $\psi(2S)$ mass subtracting the
sidebands over the number of $J/\psi$ as a function of $|t|$.
The two curves are derived from fits to the data points 
assuming a constant (dashed line) and a linear (solid line) 
behavior respectively.}
\label{fig2}
\end{figure}

\noindent 
The $t$-dependence is measured using all forward untagged 
events in a mass window of $\pm$ 150 MeV around the nominal 
mass of the $\psi(2S)$ and dividing these into bins of $t$. The data points
shown in fig.\ref{fig3} are corrected for detector acceptance and 
efficiencies. With this selection the untagged data
sample contains $39\%$ of elastic $\psi(2S)$ events, $6\%$ of events
with proton dissociation and $55\%$ of non-resonant background. 
The contribution of $\psi(3S)$ to this data sample is expected to be
approximately four
orders of magnitude smaller than the $\psi(2S)$ contribution. 

\noindent 
The $t$-dependence of the elastic $\psi(2S)$ events is
extracted through a fit taking the three contributions in the forward
untagged data sample into account.
A single exponential $e^{-b|t|}$ is assumed to describe the 
elastic cross section averaging over the measured $W_{\gamma p}$ range.
The background from proton dissociative
events is described by another single exponential. The contribution of this
background is determined from data using tagged events and correcting
for the difference between tagged events and untagged events
in proton dissociation using Monte Carlo simulations of events generated with
DIFFVM \cite{diffvm}.
The non-resonant
background is described by a sum of two exponential functions which
are obtained by fitting the $t$-distribution of the sidebands. A comparison of
the exponents extracted from the LPAIR generator program \cite{lpair} which
simulates the non-resonant background shows good agreement.
The only free parameters in this fit are the
elastic slope parameter $b$ and a normalization parameter.  
The first and the last $t$-bin are not included in the fit as in these bins 
the background dominates over the elastic component.

\noindent 
The fit yields 
\[ b=(4.5\pm1.2^{~~+1.4}_{~~-0.7})~\gev^{-2}.\] 
The first value reflects the statistical and the second
the systematic uncertainty. 
The systematic uncertainty is determined
by varying the fractions of  the non-resonant and the dissociative 
background contributions. To estimate the effect from uncertainties 
on detector acceptance 
and efficiencies, the Monte Carlo simulation is used with different 
$t$-dependences.
In addition, the same fit is performed including the first and the last bin 
and for the electron and muon sample separately. The systematic error contains the
differences summed in quadrature separately for the up and the down variations.


\noindent 
The result is consistent with the slope parameter of the $J/\psi$ meson
of $(4.73\pm0.25^{~~+0.30}_{~~-0.39}) \GeV^{-2}$ measured in \cite{h1jpsi2}.
\begin{figure}[ht] 
\centering
\setlength{\unitlength}{1cm}
\begin{picture}(12.,11.)
\put(-0.5,0.){\epsfig{file=H1prelim-00-174.fig3.eps,width=13cm}}
\end{picture}
\caption{The corrected number of events $1/N \cdot dN/dt$ for untagged events around the
  $\psi(2S)$ mass as a function of $|t|$ and averaged over
  $40<\wgpw<150 \GeV$. The lines correspond to
  different contributions to the combined fit of the $t$-slope.
  The fit excludes the first and last data point and 
  is described in detail in the text.}
\label{fig3}
\end{figure}


%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%

\section*{Summary}
The measurement of the $t$-dependence of the elastic
production differential cross section $\frac{\rm d\sigma}{{\rm d}t}$ of
the $\psi(2S)$ meson 
in a range of $40 < \wgpw < 150 \GeV$ and $Q^2 \lesssim 1 \GeV^2$ has been
reported. 
The exponent $b$, when an exponential fit is
made to the $t$-dependence of $\frac{\rm d\sigma}{{\rm d}t}\propto
e^{-b|t|}$, is found to be consistent with the value 
measured for the $J/\psi$ meson,as expected in QCD calculations.


%The measurement thus does not
%support models in which the difference between the
%$t$-dependences of $J/\psi$ and
%$\psi(2S)$ production is large.
%The slope agrees with
%the expectations of the color dipole model of $b_{\psi(2S)}
%\lesim b_{J/\psi}$ {\sf check ist das nicht ein Widerspruch? Hoert sich
%zumindest so an}.


\section*{Acknowledgments}
We are grateful to the HERA machine group whose outstanding efforts
have made and continue to make this experiment possible. We thank the
engineers and technicians for their work in constructing and now
maintaining the H1 detector. We thank the funding agencies for their
financial support of the experiment. We wish to thank the DESY
directorate for the support and hospitality extended to the non-DESY
members of the collaboration.


%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%


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%
%%%%%\bibitem{zpsi} ZEUS talk DIS 2000 ????? (CHECK!)

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\end{thebibliography}
 
\end{document}
