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%===================================== title page ==========================================
\begin{titlepage}

\noindent
\begin{flushleft}
  {\bf H1prelim-14-013} ~~ Submitted to \\
 \rule[2mm]{\textwidth}{0.2mm}
 {\bf XXII International Workshop on Deep-Inelastic Scattering and Related Subjects, \\ DIS2014},  
  April 28 -- May 2, 2014,  Warsaw, Poland    \\
  Parallel Session ~~ {\bf Small-x, Diffraction and Vector Mesons} \\
 \rule{\textwidth}{0.2mm}
  {\em Electronic Access: www-h1.desy.de/publications/H1preliminary.short list.html} \\
\end{flushleft}

\vspace*{10mm}

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\vspace{2cm}
\begin{center}
\begin{Large}

{\bf \boldmath Exclusive photoproduction of $\rho^0$ with forward neutron at HERA}

\vspace{2cm}

H1 Collaboration

\end{Large}
\end{center}
\vspace{2cm}

\begin{abstract}
The first measurement of exclusive photoproduction of $\rho^0$ mesons
associated with leading neutrons at HERA is presented.
The data are taken with the H1 detector in the years 2006-2007
and correspond to an integrated luminosity of $1.16$ pb$^{-1}$.

$\rho^0$ mesons are reconstructed from their decays to pions in the central tracking chamber,
while leading neutrons carrying a large fraction of the incoming proton momentum, $x_L>0.35$,
are detected in the Forward Neutron Calorimeter.
The phase space of the measurement is defined by the photon virtuality $Q^2<2$ GeV$^2$,
the total energy of the photon-proton system $20 < W_{\gamma \rm p} < 100$ GeV 
and the polar angle of the leading neutron, $\theta_n < 0.75$ mrad.
The cross section of the reaction $\gamma p \to \rho^0 n Y$, where $Y$ is
a small mass system escaping undetected in the proton direction, is measured
as a function of the neutron energy, of the effective mass of the $\pi^+\pi^-$ system 
as well as of the transverse momentum squared, $p_t^2$, and of the pseudorapidity $\eta$ 
of $\rho$ meson.

The data are interpreted in terms of the double peripheral process, involving
pion exchange at the proton vertex followed by elastic photoproduction
of a $\rho$ meson on the virtual pion. In the framework of OPE approach
elastic cross section of photon-pion scattering, 
$\sigma^{\rm el}(\gamma\pi^+ \to \rho^0\pi^+)$ at average energy 
$\langle W_{\gamma\pi}\rangle = 22$ GeV is extracted.
\end{abstract}

\vspace{1cm}

%\it \normalsize
%\begin{center}
% to be submitted to {\it Eur.Phys.J.C} 
%\end{center}

\end{titlepage}

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%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
%                                                                                             %
% Fig.1 -- Generic diagrams for the reaction studied.              FNC_rho.fig0a.eps          %
%                                                                  FNC_rho.fig0b.eps          %
% Fig.2 -- Control plots.                                          h1prel.FNC_rho.fig1.eps    % 
% Fig.3 -- Rho mass distribution.                                  h1prel.FNC_rho.fig2.eps    %
% Fig.4 -- Skewing parameter n_RS (previous HERA data added)       h1prel.FNC_rho.fig3.eps    %
% Fig.5 -- dsigma/dxL (a, b)                                       h1prel.FNC_rho.fig4a.eps   %
%                                                                  h1prel.FNC_rho.fig4b.eps   %
% Fig.6 -- sigma gamma-pi vs W                                     h1prel.FNC_rho.fig5.v2.eps %
% Fig.7 -- W_gamma-p (changed: differential --> total x-section)   h1prel.FNC_rho.fig6.eps    %
% Fig.8 -- dsigma/deta                                             h1prel.FNC_rho.fig7.eps    %
% Fig.9 -- dsigma/dpt2                                             h1prel.FNC_rho.fig8a.eps   %
%                                                                                             %
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%

\section{Introduction}
%---------------------

\noindent

The aim of the analysis is to measure exclusive $\rho^0$ production on virtual pion
in the photoproduction regime at HERA and to extract for the first time experimentally 
elastic $\gamma\pi$ cross section. In the Regge framework the events of such class are 
explained by the diagram shown in Fig.~\ref{fig:FD}a which involves an exchange of two 
Regge trajectories in the process $2 \to 3$, known as {\em Double Peripheral Process} (DPP),
or Double-Regge-pole exchange reaction~\cite{DPP}. 
Events of this type are modelled by the two-step Monte Carlo generator POMPYT~\cite{Pompyt}
in which the virtual pion is produced at the proton vertex according to one of the available
pion flux parametrisations. This pion then scatters elastically on the photon from the
electron beam, thus producing vector meson ($\rho^0$ in our case).

Diffractive dissociation of the proton into a system $Y$ (Fig.~\ref{fig:FD}b) is
an evident and major background to DPP. Using H1 detector capabilities in the forward region
one can suppress such background to some level, but not completely.
In particular, it contains an essentially irreducible part, 
originating from events where $Y=N^*$ decaying into $n+\pi^+$
and leading to the identical final state as the process of interest.
This background is modelled by the DIFFVM generator~\cite{DiffVM} and has to be subtracted
from the data. 
%Although the final state is the same, such a background still differs
%kinematically from DPP events, since there is no second `rapidity gap' between neutron
%and pion in diffractive proton dissociation.  
    
%%%%%%%%%%%%% Fig. 0 %%%%%%%%%%%%%%

\begin{figure}[hhh]
\center
 \setlength{\unitlength}{1cm}
 \begin{picture}(15,7)(0,0)
   \put(0.0,1.0){\epsfig{file=H1prelim-14-013.fig1a.eps,height=4.8cm}}
   \put(9.0,0.3){\epsfig{file=H1prelim-14-013.fig1b.eps,height=5.3cm}}
   \put(4.0,0.8){\small \boldmath $x_L\!\simeq\!E_{\rm n}/E_{\rm p}$}
   \put(3.9,2.1){\small \boldmath $(1-x_L)$}
   \put(3.5,0.2){\normalsize \sf (a)}
   \put(11.2,0.2){\normalsize \sf (b)}
 \end{picture} 
\caption{Generic diagrams for the two processes, contributing to exclusive 
         photoproduction of $\rho$ mesons associated with leading neutrons at HERA. 
         (a) Double peripheral process, involving $\pi$-exchange at the proton vertex,
         which is considered as a {\em signal} in present analysis.
         (b) Diffractive scattering in which neutron may be produced as a part of the 
         proton dissociation system, $M_Y$, and which is treated as {\em background}
         to the events of class (a).}
\label{fig:FD} 
\end{figure}

\section{Analysis Outline and Main Results}
%------------------------------------------

\noindent
The analysis is based on $\sim 6600$ events, containing only two charged pions from $\rho^0$ decay
and a leading neutron with energy $E_n>120$ GeV, and nothing else above noise level in the detector. 
The sample corresponds to an integrated luminosity of $1.16$ pb$^{-1}$, collected 
by a special minimum bias track trigger in the years 2006-2007.
The photon virtuality is limited to $Q^2<2$ GeV$^2$ and according to MC its average value is $0.05$  GeV$^2$.
Hence we use WWA~\cite{WWA} formula to calculate photon flux from the electron and to convert
$ep$ cross sections to $\gamma p$ ones.


After all selections the remaining background fraction in this sample is estimated as $0.36\pm 0.06$.
Control plots illustrating the data description by the Monte Carlo models 
using this signal to background ratio are shown in Fig.~\ref{fig:CP}. 
Since both POMPYT and DIFFVM cannot provide reliable absolute cross section prediction 
for such final state, only a shape comparison is possible. 

The effective mass distribution for two charged pions with $p_t>200$ MeV each 
and within the central detector range $20^o<\theta<160^o$ is shown in Fig.~\ref{fig:mass}. 
The distribution is corrected for the mass dependent detector efficiency.
A fit is performed in the range $M_{\pi\pi}>0.4$ GeV
using the Ross-Stodolsky parametrisation~\cite{RS} for the $\rho^0$ meson shape 
and adding the contributions for the reflection from $\omega \rightarrow \pi^+\pi^-\pi^0$ 
and for the non-resonant background.
The total contamination from the last two terms inside the analysis region $0.6<M_{\pi\pi}<1.1$ GeV 
was found at a negligible level, $(1.1 \pm 0.7)\%$. The fitted values of the mass, $M_{\rho}$, 
and the width, $\Gamma_{\rho}$, are in agreement with the nominal PDG values.
The cross section is then calculated for the full range 
$2m_{\pi}<M_{\pi\pi}<M_{\rho}+5\Gamma_{\rho}$ using the resonant part only, represented by the
relativistic Breit-Wigner function $BW_{\rho}(M_{\pi\pi})$.

The Breit-Wigner shape is strongly distorted due to interference with non-resonant $\pi\pi$
production amplitude (dashed curve on Fig.~\ref{fig:mass}). The strength of the distortion
is $p_t$ dependent and can be characterised by the phenomenological skewing parameter, $n_{RS}$,
as suggested by Ross and Stodolsky:
\begin{equation}
     \frac{dN(M_{\pi\pi})}{dM_{\pi\pi}} \propto BW_{\rho}(M_{\pi\pi}) 
     \left( \frac{M_{\rho}}{M_{\pi\pi}}\right)^{n_{RS}} 
\label{RS_rho}
\end{equation}
%
Fig.~\ref{fig:nRS} shows $n_{RS}$ as a function of $p_t^2$
of $\rho^0$ for this measurement in comparison with previously published H1~\cite{H1_rho} 
and ZEUS~\cite{ZEUS_rho} results from elastic photoproduction of $\rho^0$. 

The total cross section for the reaction $\gamma p \to \rho^0 n (\pi^+)$ is measured as
$$
   \sigma_{\gamma p} = (280 \pm 6_{\rm stat} \pm 46_{\rm sys})~ {\rm nb}
$$ 
for the phase space $20<W_{\gamma p}<100$ GeV, $0.35<x_L<0.95$ and $\theta_n<0.75$ mrad.

In the one-pion-exchange (OPE) approximation the cross section for the leading neutron
photoproduction can be expressed as a convolution of the pion flux and a photon-pion cross section:
\begin{equation}
   \frac{d\sigma_{\gamma p\to Xn}(W^2,x_L,t)}{dx_L dt} = 
    f_{\pi^+/p}(x_L,t)\cdot\sigma_{\gamma\pi^+}((1\!-\!x_L)W^2)
\label{OPE}
\end{equation}
For small four momentum transfer squared, $t$, which is ensured in our case 
by the geometric acceptance of the Forward Neutron Calorimeter,
the $p\to n$ transition amplitude is dominated by the lightest particle in the $t$-channel,
the pion, and Eq.\ref{OPE} is a good approximation.
  
The differential cross section d$\sigma_{\gamma p}/{\rm d}x_L$ is shown in Fig.\ref{fig:dsdxl}.
Predictions from several $\pi$-flux models~\cite{pi_flux} are confronted with these data.
Again, only a shape comparison is possible, since the $\gamma\pi$ cross section is not known.
However, some models can be excluded even on the basis of this shape comparison.
The remaining parametrisations of the pion flux which are compatible with the data are used
to extract the $\gamma\pi$ cross section from this measurement. 
For the central value we use Holtmann flux~\cite{pi_flux} with coupling constant
for charged pions, $g_c/4\pi = 14.11\pm0.20$~\cite{gpiPN}.
Integrated flux values, $\Gamma_{\pi^+}=\int{f_{\pi^+/p}(x_L,t){\rm d}x_L{\rm d}t}$, 
for the region of measurement,  $0.35<x_L<0.95, ~p_{t,n}<x_L\cdot 0.69$ GeV, 
still differ significantly between different parametrisations, leading to a model
uncertainty of $\sim 25\%$: $\Gamma_{\pi^+} = 0.138^{+0.039}_{-0.027}$.

The extracted cross section for the mean value of $\langle W_{\gamma\pi}\rangle = 22$ GeV
is found to be
$$
  \sigma (\gamma\pi^+ \to \rho^0\pi^+) = (2.03 \pm 0.34_{\rm exp} \pm 0.51_{\rm model})~ \mu{\rm b}
$$
where the first error represents the total experimental uncertainty and the second one
is due to the pion flux uncertainty.
The energy dependence of the elastic photoproduction cross section 
$\sigma (\gamma\pi^+ \to \rho^0\pi^+)$  is shown on Fig.~\ref{fig:sigma_gpi}.
Taking Regge-fitted value of $\sigma (\gamma p \to \rho^0 p)= 9.5\pm 0.5~\mu{\rm b}$ 
at corresponding energy, which is an interpolation between fixed target and HERA measurements
(see e.g. Fig.~10 in \cite{ZEUS_rho})
one can obtain for the ratio of elastic $\rho^0$ photoproduction on pion and on proton
$$
   r = \frac{\sigma^{\rm el}_{\gamma\pi}}{\sigma^{\rm el}_{\gamma p}} = 0.21 \pm 0.06
$$
This can be compared to the similar ratio, but for total cross sections, estimated by
ZEUS~\cite{ZEUS_pi2p}: $r = \sigma^{\rm tot}_{\gamma\pi}/\sigma^{\rm tot}_{\gamma p} = 0.32 \pm 0.03$.
Both values are lower than na\"ively expected $r=2/3$ from AQM considerations. 

Fig.~\ref{fig:wgp} shows the energy dependence of exclusive $\rho^0$ production 
with a leading neutron, $\sigma_{\gamma p \to \rho^0n\pi^+}(W_{\gamma p})$. 
Regge motivated fit $\sigma_{\gamma p} \propto W^{\delta}$ yields a value of
$\delta = -0.33 \pm 0.06_{\rm stat} \pm 0.13_{\rm sys}$.
POMPYT MC predicts different trend, typical for Pomeron exchange only. 
This difference is also reflected in the shape of $\eta$ distribution 
of the $\rho$ meson in Fig.~\ref{fig:eta} because of the kinematic correlation 
between $W_{\gamma p}$ and $\eta_{\rho}$ in exclusive processes.

Finally, in Fig.~\ref{fig:pt2} the differential distribution d$\sigma/{\rm d}p_t^2$ 
for $\rho$ mesons is shown. It exhibits two distinctly different exponential slopes,
$b_1 = 25.0 \pm 6.1$ GeV$^{-2}$ and $b_2 = 3.28 \pm 0.31$ GeV$^{-2}$ -- a feature
known to be a characteristic for DPP reactions~\cite{DPP}. \\ 
    

\section{Summary}
%----------------

\noindent
Photoproduction cross section for exclusive $\rho^0$ production associated with 
leading neutron is measured for the first time at HERA. The elastic photon-pion
cross section, $\sigma(\gamma\pi^+ \to \rho^0\pi^+)$, at $\langle W \rangle = 22$ GeV
has been extracted in the OPE approximation.
The differential cross section d$\sigma/{\rm d}p_{t,\rho}^2$ shows the behaviour
typical for exclusive double peripheral processes.

%%\newpage

\begin{thebibliography}{99} %%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%

\bibitem{DPP}
N.F. Bali,  G.F. Chew and A. Pignotti, {\em Phys. Rev. Lett.} {\bf 19} (1967) 614; \\
G.F. Chew and A. Pignotti, {\em Phys. Rev.} {\bf 176} (1968) 2112; \\
E. L. Berger, 
``Reggeized Double-Peripheral-Model Analysis of Three-Body Final-State Processes'',
{\em Phys. Rev.} {\bf 179} (1969) 1567;\\
N.P. Zotov and V.A. Tsarev,
``Diffractive Dissociation and Drell-Hiida-Deck Model'',
(In Russian) Fiz.Elem.Chast.Atom.Yadra 9 (1978) 650. 
  
\bibitem{Pompyt}
P. Bruni and G. Ingelman, 
in Proceedings of the Europhysics Conference, Marseille, France (1993) 595.

\bibitem{DiffVM}
B.~List and A.~Mastroberardino,
``DIFFVM - A Monte Carlo generator for diffractive processes in ep scattering'', 
Proc. of the Workshop on Monte Carlo Generators for HERA Physics, 
eds. A.T. Doyle et al., DESY-PROC-1999-02 (1999) 396.

\bibitem{WWA}
E.J.~Williams,  {\em Phys. Rev.}  {\bf 45} (1934) 729; \\
C.F.~Weizs\"acker, {\em Z. Phys.} {\bf 88} (1934) 612; \\  
S.~Frixione {\it et al.}, \PLB{\bf 319} (1993) 339.

\bibitem{RS}
M. Ross and L. Stodolsky, {\em Phys. Rev.} {\bf 149} (1966) 1172.

\bibitem{H1_rho}
S.~Aid {\it et al.}  [H1 Collaboration],
``Elastic Photoproduction of $\rho^0$ Mesons at HERA'',
\NPB{\bf 463} (1996) 3
[hep-ex/9601004].
%
\bibitem{ZEUS_rho}
J.~Breitweg {\it et al.}  [ZEUS Collaboration], 
``Elastic and Proton-Dissociative $\rho^0$ Photoproduction at HERA'',
\EJC{\bf 2} (1998) 247. 

\bibitem{pi_flux}
 (a) M. Bishari, ``Pion exchange and inclusive spectra'', \PLB{\bf 38} (1972) 510; \\
 (b) H. Holtmann {\it et al.}, \NPA{\bf 596} (1996) 631;
     M.Przybycien, A.Szczurek and G.Ingelman, 
     ``Properties of HERA events from DIS on pions in the proton'', 
     \ZPC{\bf 74} (1997) 509; \\ 
 (c) B.Kopeliovich, B.Povh and I.Potashnikova,
     ``Deep inelastic electroproduction of neutrons in the proton fragmentation region'',
     \ZPC{\bf 73} (1996) 125; \\ 
 (d) W. Melnitchouk, J. Speth and A.W.Thomas, 
     ``Dynamics of light anti-quarks in the proton'', \PRD{\bf 59} (1999) 014033; \\
 (e) L. Frankfurt, L. Mankiewicz and M. Strikman,  \ZPA{\bf 334} (1989) 343; \\    
 (f) N.N. Nikolaev, W.Sch\"afer, A. Szczurek and J. Speth, 
     \PRD{\bf 60} (1999) 014004.

\bibitem{gpiPN} 
T.E.O.~Ericson, B.~Loiseau and A.W.~Thomas,
``Determination of the pion-nucleon coupling constant and scattering lengths'',
\PRC{\bf 66} (2002) 014005. 

\bibitem{ZEUS_pi2p}
S.~Chekanov {\it et al.}  [ZEUS Collaboration],
``Leading Neutron Production in ep collisions at HERA'', \NPB{\bf 637} (2002) 3.
 
\end{thebibliography}  %%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%


\newpage  %%%%%%%%%%%%% Fig. 1 %%%%%%%%%%%%%%

\begin{figure}[ppp]
\center
 \epsfig{file=H1prelim-14-013.fig2.eps,width=\textwidth}
 \setlength{\unitlength}{1cm}
\caption{Distributions of $p_t^2$ and $\eta$ of $\rho^0$, $x_L$ of neutron and $W_{\gamma p}$ 
         for data and Monte Carlo simulations normalised to the data.
         Data points are shown with statistical errors only. 
         The shaded green band indicates the uncertainty in the estimated background fraction.}
\label{fig:CP} 
\end{figure}

\newpage  %%%%%%%%%%%%% Fig. 2 %%%%%%%%%%%%%%

\begin{figure}[ppp]
\center
 \epsfig{file=H1prelim-14-013.fig3.eps,width=\textwidth}
 \setlength{\unitlength}{1cm}
\caption{Mass distribution of the $\pi^+\pi^-$ system for elastic
         $\rho^0$ production with $p_t^2<1.0$ GeV$^2$.
         The data points are corrected for the detector efficiency.
         The curves represent different components contributing 
         to the measured distribution and the BW resonant part 
         extracted from the fit to the data. 
%  of the fit using Ross-Stodolsky parametrisation of the $\rho$ mass shape.
         Analysis region $0.6<M_{\pi^+\pi^-}<1.1$ GeV is indicated
         by vertical arrows.} 
\label{fig:mass}
\end{figure}

\newpage  %%%%%%%%%%%%% Fig. 3 %%%%%%%%%%%%%%

\begin{figure}[ppp]
\center
 \epsfig{file=H1prelim-14-013.fig4.eps,width=\textwidth}
 \setlength{\unitlength}{1cm}
\caption{Ross-Stodolsky skewing parameter, $n_{RS}$, as a function of
         $p_t^2$ of $\pi^+\pi^-$ system. Values measured in this analysis 
         are compared to previously obtained results for elastic photoproduction
         of $\rho^0$ mesons, $\gamma p \to \rho^0 p$,  
         by H1~\cite{H1_rho} and ZEUS\cite{ZEUS_rho} Collaborations.}
\label{fig:nRS}
\end{figure}

\newpage  %%%%%%%%%%%%% Fig. 4 %%%%%%%%%%%%%%

\begin{figure}[ppp]
\center
 \epsfig{file=H1prelim-14-013.fig5a.eps,width=12.8cm}
 \epsfig{file=H1prelim-14-013.fig5b.eps,width=12.8cm}
 \setlength{\unitlength}{1cm}
\caption{Differential cross section d$\sigma_{\gamma p}/{\rm d}x_L$ compared 
         to the predictions based on different versions of pion fluxes~\cite{pi_flux}.
         The data points are shown with statistical (inner error bars) and total
         (outer error bars) uncertainties, excluding overall normalisation
         error of $5.9\%$. All predictions are normalised to the data.}
\label{fig:dsdxl}
\end{figure}

\newpage  %%%%%%%%%%%%% Fig. 8 %%%%%%%%%%%%%%

\begin{figure}[ppp]
\center
 \epsfig{file=H1prelim-14-013.fig6.eps,width=\textwidth}
 \setlength{\unitlength}{1cm}
\caption{Elastic cross section $\sigma^{\rm el}({\gamma\pi^+} \to \rho^0\pi^+)$  
         as a function of photon-pion energy, $W_{\gamma\pi}$.
         The inner error bars represent the total experimental uncertainty
         and the outer error bars are experimental and model uncertainties 
         added in quadrature, where the model error is due to
         pion flux uncertainties ($\sim 25\%$).}
\label{fig:sigma_gpi}
\end{figure}

\newpage  %%%%%%%%%%%%% Fig. 5 %%%%%%%%%%%%%%

\begin{figure}[ppp]
\center
 \epsfig{file=H1prelim-14-013.fig7.eps,width=\textwidth}
 \setlength{\unitlength}{1cm}
\caption{Cross section of the reaction $\gamma p \to \rho^0 n \pi^+$ as
         a function of $W_{\gamma p}$ compared to prediction from POMPYT MC,
         which is normalised to the data.
         The dashed curve represents the Regge motivated fit 
         $\sigma \sim W^{\delta}$
         with $\delta = -0.33 \pm 0.06_{\rm stat}\pm 0.13_{\rm sys}$.
         The data points are shown with statistical (inner error bars) and 
         total uncertainties (outer error bars) excluding overall normalisation
         error of $5.9\%$}
\label{fig:wgp}
\end{figure}

\newpage  %%%%%%%%%%%%% Fig. 6 %%%%%%%%%%%%%%

\begin{figure}[hhh]
\center
 \epsfig{file=H1prelim-14-013.fig8.eps,width=\textwidth}
 \setlength{\unitlength}{1cm}
\caption{Differential cross section d$\sigma_{\gamma p}/{\rm d}\eta$ 
         compared to prediction from POMPYT MC.
         Inner error bars represent statistical errors and the outer
         error bars are total errors excluding overall normalisation
         uncertainty of $5.9\%$.
         Different $W$ dependence in data and Monte Carlo is reflected
         in pseudorapidity distribution of $\rho$ meson.}
\label{fig:eta}
\end{figure}

\newpage  %%%%%%%%%%%%% Fig. 7 %%%%%%%%%%%%%%

\begin{figure}[hhh]
\center
 \epsfig{file=H1prelim-14-013.fig9.eps,width=\textwidth}
 \setlength{\unitlength}{1cm}
\caption{Differential cross section d$\sigma_{\gamma p}/{\rm d}p_t^2$ 
         of $\rho^0$ mesons fitted with the sum of two exponential functions.
         The values of slopes are characteristic for double peripheral 
         processes~\cite{DPP} in which an exchange of 
         two Regge trajectories is involved.}
\label{fig:pt2}
\end{figure}


\end{document}
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