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\multicolumn{4}{l}{{\bf
                International Europhysics
                Conference on High Energy Physics},
                July~12,~2001,~Budapest} \\
 {\bf EPS 2001:} 
                 & Abstract:        & {\bf 806}    &\\
                 & Parallel Session & {\bf - }     & {\bf Soft
Interactions, Hadronic Structure and Diffraction}\\[.7em]
\multicolumn{4}{l}{{\bf
               XX International Symposium on Lepton and Photon Interactions}, 
               July~23,~2001,~Rome} \\ 
{\bf LP 2001:}  
                 & Abstract:        & {\bf 499} &\\
                 & Plenary Session  & {\bf 8c}  &\\ \hline
 & \multicolumn{3}{r}{\footnotesize {\it
    www-h1.desy.de/h1/www/publications/conf/conf\_list.html}} \\[.2em]
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\begin{center}
  \Large
  {\bf Proton Dissociative $\mathrm{J/\psi}$ Production at High
${|t|}$ \\ at HERA}

  \vspace*{1cm}
    {\Large H1 Collaboration} 
\end{center}
%================================abstract===================
\begin{abstract}

\noindent
A measurement of the diffractive production of ${J/\psi}$ 
mesons at large four-momentum transfers squared
($t$) is presented in photoproduction at HERA. The ${J/\psi}$ 
meson is identified via its decay into two unlike-sign 
muons. The cross section $\sigma(\gamma p \rightarrow J/\psi X)$ is
measured as a function of $|t|$ and as a function of the
photon-proton centre-of-mass energy $W_{\gamma p}$ 
in two $|t|$ intervals. The results are compared with perturbative QCD 
calculations.
\end{abstract}

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\section{Introduction} \label{sect:intro}


The large \({|t|}\) region (\(|t| \ge 1.0~\mathrm{GeV^{2}}\)), where $t$ 
is the four-momentum transfer squared at the proton vertex,
represents the region relevant to the so-called
hard BFKL pomeron. This is described by an effective ladder of 
Reggeized gluons within the framework of perturbative QCD. 
Predictions for the diffractive production
of the \({J/\psi}\) meson, through the exchange of the
hard BFKL pomeron, have been completed in leading order, where the
mass of the \({J/\psi}\) and the large \({|t|}\) values both provide relevant scales, and demonstrate
that the total cross section for this process, although small, is 
not expected to be restrictively small at the energies accessible to
the HERA collider \cite{forshaw}. The differential cross section 
\({d \sigma/d|t|}\) is expected to have a more gradual
\({|t|}\)-dependence in the high \({|t|}\) region \cite{bartels} than 
at low \({|t|}\) where the differential cross section can be described
by the exponential parameterisation of Regge theory. The large $|t|$ values
result predominantly in the dissociation of the proton giving
rise to a proton remnant final state X.

The pQCD predictions are based upon a `low-energy' approximation to the
full perturbative BFKL expansion. The incoming photon at HERA is
assumed to fluctuate into a quark-antiquark pair some long time before 
interacting with the proton. The wavefunction of the outgoing meson is
not known but is assumed to be a delta function which partitions the 
momenta of the incoming quarks in the photon equally. This choice of 
wavefunction to describe the produced meson is expected to be 
particularly appropriate for heavier mesons. The predictions 
depend upon the `scaling variable' \({\tau = -t/ (M_{V}
+ Q^{2})}\), where \({Q^{2}}\) is the photon virtuality and \({M_{V}}\)
is the mass of the produced meson. The only parameter in the 
leading-order model is $\alps$, which is related to the QCD coupling 
constant \({\alpha_{s}}\) according to \(\bar{\alpha}_{s} = (N \alpha_{s}
)/\pi\) for a colour group SU(N) and is treated as a free
parameter. The event generator HITVM is used
to generate proton dissociative, diffractive ${J/\psi}$ events at 
large $|t|>1.0~\mathrm{GeV^{2}}$ according to the pQCD calculation
\cite{hitvm}.

The proton-dissociative production of the \({J/\psi}\) meson,
${\gamma p \rightarrow J/\psi X}$, is studied here
in the kinematic region of photoproduction.  In photoproduction at HERA
the incoming lepton scatters through a small angle emitting an
`almost-real' photon. The total cross section is presented as a 
function of the centre-of-mass energy of the initial
photon-proton system (\({W_{\gamma p}}\)) in two separate regions of 
\({|t|}\). The differential cross section
\({d\sigma/d|t|}\) is presented as a function of 
\({|t|}\) for the \({|t|}\)-range \(1.0<|t|<21.0~\mathrm{GeV^{2}}\). 

  

\section{Event Selection.} \label{sect:ident}

Proton dissociative, diffractive ${J/\psi}$ events are
selected from data collected by the H1 detector at HERA during 1999. 
During the first part of 1999 HERA collided beams of 27.6\,GeV
electrons with 920\,GeV protons. The electron beam was
replaced by a positron beam of the same energy during the year. 

The analysis is based on a total integrated luminosity of 
\(19.1~\mathrm{pb^{-1}}\). The measurement is performed in the 
kinematic region of photoproduction in the photon-proton 
centre-of-mass energy range \(50 < W_{\gamma p} < 160~\mathrm{GeV}\). 
The $J/\psi$ mesons are identified via their decays to two unlike-sign muons.

To restrict the analysis to small photon virtualities, events are
rejected in which a scattered lepton with an energy deposit 
\(E>8.0 ~\mathrm{GeV}\) is detected in the main part of the
detector. The kinematic acceptance is thereby restricted to 
\(Q^{2} < 1.0 ~\mathrm{GeV^{2}}\). 

The polar angle \({\theta}\) is measured with respect to the incoming
proton, or forward,
direction. Events in which only two tracks are reconstructed with transverse 
momenta greater than 0.8 GeV  in the polar angular 
range \({30^{\circ} <\theta<160^{\circ}}\) are selected. Both
tracks originate from a vertex within 
\(z=\pm 40~\mathrm{cm}\) of the nominal interaction point. 
 Both tracks are identified as minimum ionizing particles in the 
calorimeter and/or by their hits in the instrumented iron yoke. Two different
triggers which use a combination of tracking, muon and z-vertex 
requirements are used to collect the events. A detailed description
of the H1 detector can be found in \cite{H1_det}.


Proton dissociative events are selected by requiring either hits
in the forward muon detector, activity in the proton
remnant tagger or energy deposits in the forward part of the LAr 
calorimeter (\({4^\circ < \theta < 10^\circ}\)). Proton dissociative
events in which narrower final states are produced can be
detected by this method if interactions of the final state
particles with material in the forward region leads to secondary
scattering in the forward detectors. 
The acceptance of these detectors restricts the 
proton remnant mass to (\(M_{X}>1.6~\mathrm{GeV}\)).

The proton remnant is restricted to the range \({\theta \le
20^{\circ}}\) by requiring an energy deposit \(E \le
1.0~\mathrm{GeV}\) in the main calorimeter in the region 
\({20^{\circ} < \theta < 30^\circ}\). This restricts the selection to 
proton remnant masses \(M_{X}<30~\mathrm{GeV}\). The
expected contribution (from Monte Carlo studies) of events produced
with elasticities \({z<0.9}\) is restricted to about 1\% by
this method. The inelastic contribution is thus suppressed. 
The momentum transfer squared \({|t|}\) at the
proton vertex can then be reconstructed using the approximation 
\({|t|=p_{t,J/\psi}^{2}}\) (where \({p_{t,J/\psi}}\) is the
transverse momentum of the ${J/\psi}$ meson). 
The invariant mass distribution of the events selected in the regions
\(1.0<|t|<5.0~\mathrm{GeV^{2}}\) and \(|t|>5.0~\mathrm{GeV^{2}}\) are
shown in figure ~\ref{fig:massprel}. 


\section{Data Analysis}

The event generator HITVM is used to generate proton dissociative, 
diffractive ${J/\psi}$ events at large $|t|>1.0~\mathrm{
GeV^{2}}$ according to the pQCD calculation with an input value
$\alps=0.2$ \cite{hitvm}. The generated events were passed through a 
full simulation of the H1 detector response. Good agreement 
between the data and the HITVM Monte Carlo simulation is obtained. 
In addition, the distributions of the HITVM Monte Carlo 
sample were compared in detail with the distributions of the 
event generator DIFFVM \cite{diffvm} in the region of overlap 
\(1.0<|t|<4.0~\mathrm{GeV^{2}}\). A good agreement of
the simulated momentum and angular distributions of the decay muons
and the detector
acceptances and efficiencies for triggering, track
reconstruction, selection and muon identification between
HITVM and DIFFVM was found. The resulting average acceptance 
exhibits a flat dependence in \({|t|}\). 

A correction factor of 4\% is subtracted and an error of ${\pm 4\%}$ 
is attributed to the measured cross sections to allow for the maximum
8\% of the selected sample which is expected to originate through the 
decay of the \({\psi(2S)}\), or \({\psi'}\), meson. The maximum  
contribution is estimated using the published ratio 
\({\sigma_{\psi'}/\sigma_{J/\psi} =  0.15 \pm 0.027_{stat} \pm
0.022_{sys}}\), measured by the H1 collaboration in the same
${Q^{2}}$ region \cite{prime}, and the expected branching ratio of 
\({55 \pm 5\%}\) for the decay of the \({\psi'}\) into the
\({J/\psi}\) with the production of other particles. The published 
$|t|$-dependency of the \({\psi'}\) cross section is compatible, 
within errors, to that of the \({J/\psi}\) therefore a $|t|$-dependent 
correction is not made.

The contribution to the sample through events in which an additional
initial state photon is emitted by the incoming lepton is found to be
neglible. The contribution of events with an additional low ${p_{t}}$
initial photon was investigated using the H1 luminosity system. The
contribution (6\%) can be almost completely attributed to Bethe-Heitler 
${ep\rightarrow ep\gamma}$ events overlapping in the same
bunch crossings. Potential migration effects of events in \(|t|\) 
were also investigated using events generated with and without radiation using
the Monte Carlo event generator DIFFVM and were found to be negligible.


Uncertainties in the detector acceptance and efficiency 
calculations contribute to systematic uncertainties in the 
cross-section measurements and are estimated based on dedicated
studies and the results of earlier analyses of
elastic $J/\psi$ production in diffraction at smaller values 
of $|t|$ \cite{dschmidt,merkel}. The most significant systematic uncertainties
arise from the trigger efficiency (6\%), lepton 
identification efficiency (5\%) and track reconstruction efficiency
(4\%) as well as
from the determination of the geometrical acceptance. To determine the
systematic uncertainty in the geometric acceptance the decay angular 
distribution for $J/\psi$ decaying directly into two muons was varied
from the prediction of s-channel helicity conservation to a flat
dependence and the observed deviation added to the systematic
uncertainty. The attributed uncertainty in the forward detector 
response for the classification of proton-dissociative events is of decreasing
magnitude and falls to 2\% with increasing \({|t|}\). All errors are added in quadrature. 





\section{The $W_{\gamma p}$ Dependence of the Total Cross Section.}

The differential cross section ${\sigma(\gamma p \rightarrow J/\psi X)}$ 
is shown as a function of ${W_{\gamma p}}$ for $1.0<|t|<5.0~\mathrm{GeV^{2}}$
and $|t|>5.0~\mathrm{GeV^{2}}$ in photoproduction in figure
\ref{fig:wfit}. The inner error bars comprise the statistical uncertainty 
in the data, and the outer error bars include all the various 
systematic contributions, including the error arising from the Monte
Carlo statistical accuracy, added to the data statistical error in quadrature. 
The centre-of-mass energy is reconstructed in photoproduction from the 
event final state using the approximation \({W_{\gamma p}^2 = ys}\), 
where $y$ represents the usual scaling variable and $s$ the centre-of-mass
energy squared of the incoming lepton-proton system. The cross sections are fitted using a function of the form ${N (W/W_0)^{\delta}}$ in both $|t|$-ranges. A central value of 
$W_{0}=90~\mathrm{GeV}$ is chosen for the fit and the statistical
and uncorrelated systematic errors are included. In the low $|t|$-range 
the best extracted value of ${\delta}$ is ${\delta=0.92 \pm 0.30}$,
and in the high $|t|$-range ${\delta=1.15 \pm 0.55}$. A similar rise
in the cross section is obtained, within the larger uncertainties at
large ${|t|}$, as previously measured for proton-elastic and
proton-dissociative ${J/\psi}$ production at smaller values of
$|t|$ \cite{merkel} and the rise is steeper than the expected
effective ${\delta \sim 0.22}$ for soft pomeron
exchange in this centre-of-mass energy range. No evidence for an
increasingly steeper 
${W_{\gamma p}}$ dependence of the cross section, however, with 
increasing ${|t|}$ can be concluded. 

The same measured cross sections in both ${|t|}$-ranges are compared to the
pQCD predictions in figure \ref{fig:wabs}. The magnitude of the cross
section is very sensitive to the input value of ${\bar{\alpha}_{s}}$.
The predictions for an input value of
${\bar{\alpha}_{s}=0.221}$ give a reasonable simultaneous description, within
experimental errors, of both the magnitude and the shape of the 
measured cross sections in both ${|t|}$-ranges. This is similar to the
expected value of $3 \alpha_{s}/\pi = 0.18$. A small increase or
decrease in the input value of ${\bar{\alpha}_{s}}$ would result in a
simultaneous shift of the two predicted distributions upwards or
downwards respectively. Larger values of ${\bar{\alpha}_{s}}$ lead to
 a steeper rise of the cross section. 






\section{The ${|t|}$ Dependence of the Cross Section.}


The photon-proton differential cross section ${d \sigma/d|t|}$
is shown as a function of ${|t|}$ for the kinematic
range \(50< W_{\gamma p} < 160~\mathrm{GeV}\), \(|t|>1.0~\mathrm{GeV^{2}}\) 
and photoproduction \(Q^{2}<1.0~\mathrm{GeV^2}\) in figure \ref{fig:tabs}. The
inner error bars once again comprise the statistical uncertainty in
the data and the outer error bars include also all the systematic
contributions added in quadrature. The measurement is in good agreement
with previous measurements of the lepton-proton differential cross
section \cite{west1} and the photon-proton differential cross section of
the ZEUS collaboration \cite{zeus}. The shape of 
the predicted differential cross section is not sensitive to the 
input value of ${\bar{\alpha}_{s}}$. The pQCD prediction for
${\bar{\alpha}_{s}=0.221}$ is compared to the measurement and describes the ${|t|}$-dependence well within experimental errors, in particular for larger $|t|>3.0~\mathrm{GeV^{2}}$.  


\section{Summary}

The differential cross-section $d\sigma/d|t|$ as a function of the momentum
transfer squared $|t|$ at the proton vertex and the total cross section
$\sigma(\gamma p \rightarrow J/\psi X)$ as a function of $W_{\gamma p}$
in two intervals of $|t|$ ($1<|t|<5$\,GeV$^2$ and $|t|>5$\,GeV$^2$)
have been measured for $J/\psi$ production with proton dissociation in
the photon-proton centre-of-mass energy range of $50<W_{\gamma
p}<160$\,GeV. The examined $|t|$-range spans the region up to
$|t|=21.0~\mathrm{GeV^{2}}$. The result shows a similar hard scale
behaviour as measured in previous analyses at lower values of ${|t|}$.
Comparisons with predictions from perturbative QCD calculations show
good agreement with the data.


\section{Acknowledgements}

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, our funding agencies for 
financial support, the DESY technical staff for continual assistance 
and the DESY directorate for the hospitality which they extend to the non-DESY 
members of the collaboration.




\begin{thebibliography}{99}
\bibitem{forshaw} J.R.~Forshaw, M.~Ryskin, Zeit. Phys. C68  (1995) 137-148.

\bibitem{bartels} J.~Bartels, H.~Lotter, J.R.~Forshaw and M.~W\"{u}sthoff,
Phys. Lett. B375 (1996) 301-305.

\bibitem{hitvm} H.~Abramovicz, T.~Doeker, L.~Iannotti, S.~Kamanov,
A.~Levy, and L.~West, Proc. Workshop {\em `Future Physics at HERA'}, eds.
G.~Ingelman, A.~De Roeck and R.~Klanner (1996) Vol. 2 pp.679-90.

\bibitem{H1_det} H1 Collaboration, Nucl.\ Inst.\ Meth.\ A386 (1997) 310.

\bibitem{diffvm} B.~List, Diploma Thesis, Techn. Univ. Berlin,
unpublished (1993);\\
B.~List and A.~Mastroberardino, Proc. Workshop {\em `Monte Carlo
Generators for HERA Physics'} DESY, Hamburg (1999) 396.


\bibitem{prime} H1 Collaboration, C. Adloff {\em et al.}, (1998) Phys. Lett. 
B421 385.

\bibitem{dschmidt} D.~Schmidt, Ph.D. Thesis, II. Inst. Exp.,
Univ. Hamburg, unpublished (2001).

\bibitem{merkel} H1 Collaboration, S.~Aid {\em et al.}, Nucl. Phys. 
B472 (1996) 3.

\bibitem{west1} H1 Collaboration, Submitted to HEP97, Jerusalem, Israel.

\bibitem{zeus} ZEUS Collaboration, Submitted to ICHEP~2000, Osaka,
Japan. 


                                     

\end{thebibliography}

\clearpage

\begin{figure}[p]
\begin{center}
\vskip 0.5cm
\epsfig{figure=H1prelim-01-071.fig1.eps,width=14.0cm}
\vskip 0.5cm
\vskip -18.5cm
\hspace{-10.0cm} {\Large a)}
\vskip 18.5cm
\vskip -9.8cm
\hspace{-10.0cm} {\Large b)}
\vskip 7.8cm
\caption{The invariant mass distribution of muon pairs reconstructed 
in the intervals a) $1.0 < |t| < 5.0~\mathrm{GeV^{2} }$ and
b) $|t|>5.0~\mathrm{GeV^{2}}$.} 
\label{fig:massprel}
\end{center}
\end{figure}

\begin{figure}[ht]
\begin{center}
\epsfig{figure=H1prelim-01-071.fig2.col.eps,width=16.0cm}

\caption{The photon-proton differential cross-section $\sigma (\gamma
p \rightarrow J/\psi X)$ for $J/\psi$ production with proton dissociation in the kinematic range
$Q^{2}<1.0~\mathrm{GeV^{2}}$ for two different intervals of ${|t|}$.
The cross sections are fitted using a function of the form ${N
\dot (W/W_0)^{\delta}}$ where $W_{0}=90~\mathrm{GeV}$ and show a
similar rise to the cross section for ${J/\psi}$ production at
low ${|t|}$.}
\label{fig:wfit}
\end{center}
\end{figure}


\begin{figure}[ht]
\begin{center}
\epsfig{figure=H1prelim-01-071.fig3.col.eps,width=16.0cm}
\caption{The photon-proton differential cross-section 
${\sigma(\gamma p \rightarrow J/\psi X)}$ for ${J/\psi}$ 
production with proton dissociation in the kinematic range
$Q^{2}<1.0~\mathrm{GeV^{2}}$ for two different intervals of $|t|$.
The curves show the predictions in perturbative QCD for 
${\bar{\alpha}_{s}=0.221}$.}
\label{fig:wabs}
\end{center}
\end{figure}

\begin{figure}[ht]
\begin{center}
\epsfig{figure=H1prelim-01-071.fig4.col.eps,width=16.0cm}
\caption{The photon-proton differential cross-section 
${d\sigma/ d|t|}$ for ${J/\psi}$ production with 
proton dissociation in the kinematic range
$Q^{2}<1.0~\mathrm{GeV^{2}}$, $50<W_{\gamma p}<160~\mathrm{GeV}$,
$|t|>1.0~\mathrm{GeV^{2}}$. The measurement is compared to 
the prediction in perturbative QCD for 
${\bar{\alpha}_{s}=0.221}$.}
\label{fig:tabs}
\end{center}
\end{figure}





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