%================================================================ % LaTeX file with preferred layout for the contributed papers to % the ICHEP Conference 98 in Vancouver % process with: latex hep98.tex % dvips -D600 hep98 %================================================================ \documentclass[12pt]{article} \usepackage{epsfig} \usepackage{amsmath} \usepackage{hhline} \usepackage{amssymb} \usepackage{times} \renewcommand{\topfraction}{1.0} \renewcommand{\bottomfraction}{1.0} \renewcommand{\textfraction}{0.0} \newlength{\dinwidth} \newlength{\dinmargin} \setlength{\dinwidth}{21.0cm} \textheight23.5cm \textwidth16.0cm \setlength{\dinmargin}{\dinwidth} \setlength{\unitlength}{1mm} \addtolength{\dinmargin}{-\textwidth} \setlength{\dinmargin}{0.5\dinmargin} \oddsidemargin -1.0in \addtolength{\oddsidemargin}{\dinmargin} \setlength{\evensidemargin}{\oddsidemargin} \setlength{\marginparwidth}{0.9\dinmargin} \marginparsep 8pt \marginparpush 5pt \topmargin -42pt \headheight 12pt \headsep 30pt \footskip 24pt \parskip 3mm plus 2mm minus 2mm \newcommand{\cms}{center of mass} \newcommand{\jpsi}{$J/\psi$} \newcommand{\jpsiw}{J/\psi} \newcommand{\eppsi}{$ep\rightarrow e\,\jpsiw\,p$} \newcommand{\psiprime}{$\psi'$} \newcommand{\cs}{cross section} \newcommand{\gpcs}{$\gamma p$ cross section} \newcommand{\wgp}{$W_{\gamma p}$} \newcommand{\wgpw}{W_{\gamma p}} \newcommand{\ceme}{\,\mbox{cm}} \newcommand{\gpcm}{$\gamma p$ \ceme} \newcommand{\qsqr}{$Q^2$} \newcommand{\degree}{\rm \circ} \newcommand{\GeV}{\,\mbox{GeV}} \newcommand{\MeV}{\,\mbox{MeV}} \newcommand{\GeVt}{\,\mbox{\GeV$^2$}} \newcommand{\gevt}{\,\mbox{\GeV$^2$}} \newcommand{\gev}{\,\mbox{GeV}} \newcommand{\gmt}{\,\mbox{\GeV$^{-2}$}} \newcommand{\lesim}{\mbox{$\;\raisebox{-1mm}{$\stackrel{\scriptstyle<}{\scriptstyle\sim}$}\;$}} %===============================title page============================= % Some useful tex commands % \newcommand{\TeV}{\rm TeV} \newcommand{\pb}{\rm pb} \newcommand{\cm}{\rm cm} \newcommand{\hdick}{\noalign{\hrule height1.4pt}} \begin{document} \newcommand{\qsq}{\ensuremath{Q^2} } \newcommand{\gevsq}{\ensuremath{\mathrm{GeV}^2} } \newcommand{\et}{\ensuremath{E_t^*} } \newcommand{\rap}{\ensuremath{\eta^*} } \newcommand{\gp}{\ensuremath{\gamma^*}p } \newcommand{\dsiget}{\ensuremath{{\rm d}\sigma_{ep}/{\rm d}E_t^*} } \newcommand{\dsigrap}{\ensuremath{{\rm d}\sigma_{ep}/{\rm d}\eta^*} } %\newcommand{\jpsi}{\ensuremath{J/\mit\psi} } % Journal macro \def\Journal#1#2#3#4{{#1} {\bf #2} (#3) #4} \def\NCA{\em Nuovo Cimento} \def\NIM{\em Nucl. Instrum. Methods} \def\NIMA{{\em Nucl. Instrum. Methods} {\bf A}} \def\NPB{{\em Nucl. Phys.} {\bf B}} \def\PLB{{\em Phys. Lett.} {\bf B}} \def\PRL{\em Phys. Rev. Lett.} \def\PRD{{\em Phys. Rev.} {\bf D}} \def\ZPC{{\em Z. Phys.} {\bf C}} \def\EJC{{\em Eur. Phys. J.} {\bf C}} \def\CPC{\em Comp. Phys. Commun.} \pagestyle{empty} \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