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\newcommand{\zDs}{\ensuremath{z(\dstar )}} \newcommand{\Wgp}{\ensuremath{W_{\gamma p}}} \newcommand{\ptds}{\ensuremath{p_t(\dstar )}} \newcommand{\etads}{\ensuremath{\eta(\dstar )}} \newcommand{\ptj}{\ensuremath{p_t(\mbox{jet})}} \newcommand{\ptjn}[1]{\ensuremath{p_t(\mbox{jet$_{#1}$})}} \newcommand{\etaj}{\ensuremath{\eta(\mbox{jet})}} \newcommand{\detadsj}{\ensuremath{\eta(\dstar )\, \mbox{-}\, \etaj}} % 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.} \begin{titlepage} % \noindent % Date: \today \\ % Version: 0.2 \\ % Editors: Katharina Mueller, Krzysztof Nowak \\ % Referees: Jacek Turnau, Gerhard Brandt \\ %H1prelim-08-033 \noindent %%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%% %%%%%%%%%%%% For conference papers %%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%% %%%%%%%%%%%% coment the header and fill the right conference %%%%% {\it {\large version of \today}} \\[.3em] \begin{center} %%% you may want to use this line for working versions \begin{small} \begin{tabular}{llrr} {\bf H1prelim-08-033} Submitted to & & & \epsfig{file=H1logo_bw_small.epsi ,width=1.5cm} \\[.2em] \hline \multicolumn{4}{l}{{\bf XVI International Workshop on Deep-Inelastic Scattering, DIS2008}, April 7-11,~2008,~London} \\ % Abstract: & {\bf } & & \\ Parallel Session & {\bf Hadronic Final States and QCD} & & \\ \hline \multicolumn{4}{l}{\footnotesize {\it Electronic Access:https://www-h1.desy.de/publications/H1preliminary.short\_list.html %www-h1.desy.de/h1/www/publications/conf/conf\_list.html }} \\[.2em] \end{tabular} \end{small} \end{center} \vspace{2cm} \begin{center} \begin{Large} {\bf Prompt Photon Production in Photoproduction \\ at HERA} \vspace{2cm} H1 Collaboration \end{Large} \end{center} \vspace{1.0cm} \begin{abstract} A measurement of prompt photons in photoproduction in ep collisions with the H1 detector is presented. The analysis is based on the data taken in the years 2004-2007, corresponding to a total integrated luminosity of $340 \pbarnt$. The photon signal is extracted using a multivariate analysis based on calorimeter cluster different shower shape variables. Inclusive and exclusive cross sections are presented as a function of the transverse energy $E_T^\gamma$ in the range $5$ to $15 \gev$ and the pseudorapidity $\eta^\gamma$ in the range $-1.0$ to $2.4$. The momentum fractions $x_\gamma$ and $x_{proton}$ of the incident photon and proton carried by the constituents participating in the hard scattering process are measured as well. The results are compared to predictions from theoretical calculations. \end{abstract} \vspace{1.cm} %\begin{center} %To be submitted to ??? %\end{center} \end{titlepage} % THE PAPER DRAFTS HAVE NO AUTHORLIST % % FOR PAPER ISSUED FOR THE FINAL READING % COPY THE AUTHOR AND INSTITUTE LISTS % INTO YOUR AREA % % from /h1/iww/ipublications/h1auts.tex % AND UNCOMMENT THE NEXT THREE LINES % %\begin{flushleft} % \input{h1auts} %\end{flushleft} % % Please not that the author list may need re-formatting. \newpage \section{Introduction} Isolated photons with high transverse momentum in the final state are a direct probe of the dynamics of the hard subprocess in ep collisions. The measurement provides complemantary information to the study of jet production with different and generally lower corrections for hadronisation. The process gives access to parton density functions of both photon and proton. The ZEUS collaboration has reported measurements of prompt photon production \cite{ZEUS,ZEUS1,ZEUS2}. The H1 collaboration has performed a measurement of prompt photon cross section both in photoproduction \cite{H1gamma} and in Deep Inelastic Scattering \cite{H1dis}. Comparison to LO and NLO calculations showed that cross sections in DIS are significantly underestimated. In photoproduction, the inclusive cross section is slightly underestimated by the calculations but the photon plus jet cross section is described reasonably well. The present preliminary analysis extends the phase space of the previous measurement in photoproduction towards larger pseudorapidities of the photon and lower event inelasticities. The data used for the measurement have been collected by the H1 detector during the years 2004-2007 corresponding to a total integrated luminosity of $340 \pbarnt$, increased by a factor of three over previous measurement. \section{Experimental Method} Photons are reconstructed as a compact electromagnetic cluster in the liquid argon calorimeter (LAr)\cite{calo}. The transverse energy of the cluster is restricted to $5$ \gev$ 0.9$. The invariant mass of the cluster, when combined with the closest neighbouring electromagnetic cluster with an energy larger than 80 MeV must be larger than $0.3 \gev$. This last requirement rejects candidates from $\pi^0$ decays with two photons reconstructed in separated clusters. For the exclusive selection of a photon accompanied by a jet, the selection on the transverse momentum of the jet larger than $4.5 \gev$ and pseudorapidity $-1.3<\eta^{jet}< 2.4$ is performed. For both inclusive and exclusive selections the photon signal is extracted by a MVA using the shower shape variables. \subsection{Extraction of the photon signal and systematic errors} Six different shower shape variables are used to discriminate between photons and neutral mesons \cite{H1dis}. \begin{itemize} \item Hot core fraction: fraction of energy deposited in four or eight - depending on the granularity of the calorimeter - contiguous cells including the cell with the highest energy. \item Transverse radius %~\footnote{Transverse radius is defined as the square root %of the second moment of the transverse cell distribution %$$R_{T}=\sqrt{\mu_{2}}$$} of the cluster, where the transverse plane is perpendicular to the direction of the incoming particle. \item Energy fraction deposited in the hottest cell of the cluster. \item Energy fraction deposited in the first layer of the LAr. \item Transverse kurtosis %~\footnote{The transverse kurtosis $K_{T}$ is defined %as the ratio of the fourth and squared second moment of the transverse energy %distribution of cluster cells $$K_{T}=\frac{\mu_{4}}{\mu_{2}^{2}}-3$$} giving the information how strongly the energy distribution is peaked. \item Transverse symmetry, defined as the ratio of the spread of the transverse cell distributions along the two principal axes. \end{itemize} The discrimination power between the prompt photon signal (photons coming from direct events, from resolved and photons radiated off the quark) and the background (di-jet production events) can be seen in figure~\ref{estimators}. The plot shows the distribution of six different shower shape variables for the photon candidate clusters compared to the PYTHIA predictions for the signal (scaled to the measured cross section) and the background. The transverse radius and the hot core fraction exhibit the best discrimination power, as can be seen by comparing the prompt photon signal (photons coming from direct events, from resolved and photons radiated off the quark) to the background (di-jet production events) distributions. The variables are combined in a MVA producing a final discriminator in bins of transverse energy $E_T^{\gamma}$ and pseudorapidity $\eta^{\gamma}$. The signal content is obtained by fitting the discriminator distributions of pure signal and pure background samples of single particle Monte Carlo to the data. As an example for the fit figure~\ref{fit} shows the discriminator for six different energy bins for one $\eta^{\gamma}$ bin. The figures show the signal and background distributions together with the data. Various systematic uncertainties were considered. The total systematic error per bin is obtained by adding all different systematic errors in quadrature. The dominant errors are listed below. \begin{itemize} \item Description of the shower shape variables in the simulation. Systematic shift of shower shape variables between 3.5\% and 8\% (depending on the variable and pseudorapidity region) motivated by previous study \cite{H1dis} leads to errors up to 30\% on the cross sections. \item The uncertainty of the description of the trigger correction together with the steep slope of the trigger efficiency at low transverse energies leads to a systematic error up to 20\% in the lowest $E_{T}^{\gamma}$ bin. \item The model dependence was studied by comparing acceptance corrections obtained with HERWIG and PYTHIA, leading to a systematic error of up to 10\% in the most forward $\eta^{\gamma}$ bin. \item Description of the photon conversion rate in the detector. A variation of the conversion rate by factors 0.5 and 1.5 leads to a systematic error of 15\% in the most forward $\eta^{\gamma}$ bin. \item From the difference between the single particle simulations and the PYTHIA simulation including the simulation of full hadronic final state systematic error of 5\% (15\% in the most forward $\eta^{\gamma}$ bin) is assigned. \end{itemize} \section{Monte Carlo simulation and calculations} Monte Carlo (MC) simulations are used in this analysis for the determination of the detector acceptance and for the extraction of the photon signal. The acceptance correction is determined from events generated with PYTHIA 6.2\cite{PYTHIA} event generator. The simulated signal contains contributions from direct and resolved production of prompt photons as well as photons radiated from the quark in di-jet events. The background sample contains genuine di-jet events without high energetic photons from the quark. For the extraction of the photon signal using shower-shape-based MVA high statistics single particle Monte Carlo samples are used. They consist of single photons (signal part) and single hadrons (mainly $\pi^0$ and $\eta$ - background part). All generated events are passed through a full GEANT \cite{geant} simulation of the H1 detector and through the same reconstruction and analysis program as used for the data. All the results are compared to two sets of calculations corrected to hadron level and multiple interactions: \begin{itemize} \item The FGH (Fontannaz-Guillet-Heinrich) \cite{heinrich,heinrich2} calculation includes NLO corrections to the direct and resolved photon production as well as contribution from quark-to-photon fragmentation and the direct box diagram $\gamma g \rightarrow \gamma g$ which is calculated to order $\alpha_{s}^{2}$. The error is evaluated by varying the fragmentation and renormalisation scales together by a factor 0.5 and 2.0 leading to overall error of at most $10\%$. \item The ZL (Zotov-Lipatov) ~\cite{zotov} calculation is based on the $k_{T}$ factorisation approach and uses the unintegrated quark and gluon densities of the proton and the photon according to the Kimber-Martin-Ryskin prescription. Direct and resolved contributions are taken into account. Estimation of the error has been performed by multiplying the $p_{T}^{2}$ scale by a factor $4$ and $1/4$ leading to an error of at most $10\%$. \end{itemize} \section{Results} Differential cross sections for the production of prompt photons in photoproduction are measured for $Q^2<1 \gev^2$, $0.1 0.9$. Figure~\ref{xsec_incl} shows the single differential inclusive cross sections as a function of transverse energy and the pseudorapidity of the photon. Both calculations are slightly lower than the data most significantly at low $E_{T}^{\gamma}$. Exclusive (photon plus jet) single differential cross sections measurement as a function of transverse energy and pseudorapidity of the photon (figure~\ref{xsec_excl}), transverse momentum and pseudorapidity of jet (figure~\ref{xsec_exclJet}) and momentum fractions $x_{\gamma}^{obs}$ and $x_{proton}^{obs}$ (figure~\ref{xsec_exclX}) show better agreement with the calculations. The largest discrepancy arises for the highest bin in $x_\gamma^{obs}$, where both calculations give different prediction, while data does not favour any of them. \section{Conclusions} Results on both inclusive and exclusive prompt photon production in photoproduction as measured by the H1 collaboration have been presented. The measurement has been compared to two calculations, one using a collinear approach and the other based on $k_t$ factorisation. The predictions for the inclusive sample are slightly lower than the data, most significantly at low $E_{T}^{\gamma}$ but there is reasonable agreement for the photon plus jet sample except at high $x_{\gamma}^{obs}$ \newpage \begin{thebibliography}{99} \bibitem{ZEUS} J. Breitweg {\it et al.} %``Measurement of inclusive prompt photon photoproduction at HERA'' Physics Letters B 472 (2000) 1-2, 175-188 [arXiv:hep-ex/9910045] \bibitem{ZEUS1} S. Chekanov {\it et al.} [ZEUS Collaboration], %``Study of the effective transverse momentum of partons in the proton using prompt photons in photoproduction at HERA'' Phys.Lett. B511 (2001) 19-32 [arXiv:hep-ex/0104001] \bibitem{ZEUS2} S. Chekanov {\it et al.} [ZEUS Collaboration], %``Measurement of prompt photons with associated jets in photoproduction at HERA'' Eur.Phys.J. C49 (2007) 511-522 [arXiv:hep-ex/0608028] \bibitem{H1gamma} A. Aktas {\it et al.} [H1 Collaboration], % ``Measurement of prompt photon cross sections in photoproduction at HERA'' Eur. Phys. J.{\bf C38} (2005) 437 [arXiv:hep-ex/0407018]. \bibitem{H1dis} F.D.Aaron {\it et al.} [H1 Collaboration], %`` Measurement of isolated photon production in deep-inelastic scattering at HERA'' 0711.4578 [hep-ex]. \bibitem{calo} B. Andrieu {\it et al.} [H1 Calorimeter Group Collaboration], Nucl. Instrum. Meth. A{\bf 336} (1993) 460. \bibitem{tmva} J. Friedman, T. Hastie and R. Tibshirani, “The Elements of Statistical Learning”, Springer Series in Statistics, 2001 \bibitem{cip} J.~Becker {\it et al.}, %``A vertex trigger based on cylindrical multiwire proportional chambers,'' Nucl.\ Instrum.\ Meth.\ A {\bf 586} (2008) 190 [arXiv:physics/0701002]. %%CITATION = NUIMA,A586,190;%% \bibitem{PYTHIA} T. Sj\"ostrand {\it et al.}, PYTHIA 6.2 Physics and Manual [hep-ph/0108264]. \bibitem{geant} GEANT 3, R.Brun {\it et al.}, CERN\_DD/EE/84-1. \bibitem{H1det} I. Abt {\it et al.} [H1 Collaboration] Nucl. Instr. and Meth. {\bf A 386} (1997) 310, ibid, 348. \bibitem{jetalgo} Stephen D. Ellis, Davison E. Soper, Phys.Rev. {\bf D48} (1993) 3160 [hep-ph/9305266]. \bibitem{heinrich} M.~Fontannaz, J.~P.~Guillet and G.~Heinrich, %``Isolated prompt photon photoproduction at NLO,'' Eur.\ Phys.\ J.\ C {\bf 21} (2001) 303 [arXiv:hep-ph/0105121]. \bibitem{heinrich2} M.~Fontannaz and G.~Heinrich, %``Isolated photon + jet photoproduction as a tool to constrain the gluon %distribution in the proton and the photon,'' Eur.\ Phys.\ J.\ C {\bf 34}, 191 (2004) [arXiv:hep-ph/0312009]. \bibitem{zotov} A.~V.~Lipatov and N.~P.~Zotov, %``Prompt photon photoproduction at HERA in the k(T)-factorization %approach,'' Phys.\ Rev.\ D {\bf 72} (2005) 054002 [arXiv:hep-ph/0506044]. %%CITATION = PHRVA,D72,054002;%% \end{thebibliography} \newpage \begin{figure}[hhh] \center \epsfig{file=H1prelim-08-033.fig1.eps,width=15cm} \setlength{\unitlength}{1cm} \caption{Shower shape variables used to discriminate between photons and neutral mesons: transverse kurtosis, transverse symmetry, transverse radius, energy fraction in the first layer, energy fraction in the hottest cell and energy fraction in the hot core of cluster. The measured data points are shown together with the radiated, resolved and direct signal Monte Carlo and remaining background, as estimated by PYTHIA. The MC prediction is scaled to the measured cross section.} \label{estimators} \end{figure} %%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%% \newpage \begin{figure}[hhh] \center \epsfig{file=H1prelim-08-033.fig2.eps,width=15cm} \setlength{\unitlength}{1cm} \caption{Examples of the MVA discriminator distributions: $\eta^{\gamma}$ bin $0.23 <\eta^{\gamma}< 0.96$ and six transverse energy intervals. The fitted distributions of the single particle signal and background are also shown.} \label{fit} \end{figure} %%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%% inclusive xsections \newpage \begin{figure}[hhh] \center \epsfig{file=H1prelim-08-033.fig3.eps,width=15cm} \setlength{\unitlength}{1cm} \caption{Inclusive prompt photon single differential cross sections as function of $E_T^\gamma$ (left) and $\eta^\gamma$ (right). The cross sections are measured in visible range defined by $5 < E_T^\gamma < 15 \gev$, $Q^2< 1 \gev^2$, $0.1 < y_h< 0.7$ and $z=E_{T}^{\gamma} / E_{T}^{photon-jet} > 0.9$. The inner errors in the figures represent the statistical errors, while the outer are the statistical and the systematical errors added in quadrature. The measured cross section is compared to the Fontannaz-Guillet-Heinrich (continuous red) and Zotov-Lipatov (dashed blue) calculations.} \label{xsec_incl} \end{figure} %%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%% exclusive xsections 1 \newpage \begin{figure}[hhh] \center \epsfig{file=H1prelim-08-033.fig4.eps,width=15cm} \setlength{\unitlength}{1cm} \caption{Exclusive prompt photon single differential cross sections as function of $E_T^\gamma$ (left) and $\eta^\gamma$ (right). The cross sections are measured in range defined in caption of figure~\ref{xsec_incl}. In addition jet with transverse momentum $p_T^{jet}>4.5 \gev$ and pseudorapidity $-1.3<\eta^{jet}<2.4$ is required. The inner errors in the figures represent the statistical errors, while the outer are the statistical and the systematical errors added in quadrature. The measured cross section is compared to the Fontannaz-Guillet-Heinrich (continuous red) and Zotov-Lipatov (dashed blue) calculations.} \label{xsec_excl} \end{figure} %%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%% exclusive xsections 3 \newpage \begin{figure}[hhh] \center \epsfig{file=H1prelim-08-033.fig5.eps,width=15cm} \setlength{\unitlength}{1cm} \caption{Exclusive prompt photon single differential cross sections as function of $p_{T}^{jet}$ (left) and $\eta^{jet}$ (right). The cross sections are measured in range defined in caption of figure~\ref{xsec_excl}. The inner errors in the figures represent the statistical errors, while the outer are the statistical and the systematical errors added in quadrature. The measured cross section is compared to the Fontannaz-Guillet-Heinrich (continuous red) and Zotov-Lipatov (dashed blue) calculations.} \label{xsec_exclJet} \end{figure} %%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%% exclusive xsections 2 \newpage \begin{figure}[hhh] \center \epsfig{file=H1prelim-08-033.fig6.eps,width=15cm} \setlength{\unitlength}{1cm} \caption{Exclusive prompt photon single differential cross sections as function of $x_\gamma^{obs}$ (left) and $x_{p}^{obs}$ (right). The cross sections are measured in range defined in caption of figure~\ref{xsec_excl}. The inner errors in the figures represent the statistical errors, while the outer are the statistical and the systematical errors added in quadrature. The measured cross section is compared to the Fontannaz-Guillet-Heinrich (continuous red) and Zotov-Lipatov (dashed blue) calculations.} \label{xsec_exclX} \end{figure} \end{document}