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Instrum. Methods} \def\NIMA{{\em Nucl. Instrum. Methods} {\bf A}} \def\NPA{{\em Nucl. Phys.} {\bf A}} \def\NPB{{\em Nucl. Phys.} {\bf B}} \def\PLB{{\em Phys. Lett.} {\bf B}} \def\PRL{\em Phys. Rev. Lett.} \def\PRC{{\em Phys. Rev.} {\bf C}} \def\PRD{{\em Phys. Rev.} {\bf D}} \def\ZPA{{\em Z. Phys.} {\bf A}} \def\ZPC{{\em Z. Phys.} {\bf C}} \def\EJC{{\em Eur. Phys. J.} {\bf C}} \def\CPC{\em Comp. Phys. Commun.} %===================================== 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} %%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%% %%%%%%%%%%%% For conference papers %%%%%%%%%% %%%%%%%%%%%% coment the header and fill the right conference %%%%%%%%%% %%%%%%%%%%%% {\it {\large version of \today}} \\[.3em] %%%%%%%%%% %%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%% \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} %-------------------------------------- %\input h1auts %\newpage %-------------------------------------- %\newpage %%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%% % % % 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