%================================================================ % LaTeX file with prefered layout for H1 paper drafts % use: dvips -D600 file-name %================================================================ \documentclass[12pt]{article} \usepackage{epsfig} \usepackage{amsmath} \usepackage{hhline} \usepackage{amssymb} \usepackage{times} \usepackage{cite} \usepackage{graphicx} \usepackage{ifpdf} \ifpdf \usepackage{epstopdf} \usepackage{pdfsync} \fi %%%%%%%%%%%% Comment the next two lines to remove the line numbering %\usepackage[]{lineno} %\linenumbers %\usepackage{hyperref} %%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%% \renewcommand{\topfraction}{1.0} \renewcommand{\bottomfraction}{1.0} \renewcommand{\textfraction}{0.0} \newlength{\dinwidth} \newlength{\dinmargin} \setlength{\dinwidth}{21.0cm} \textheight23.0cm \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 %===============================title page============================= \begin{document} % The rest \newcommand{\pom}{{I\!\!P}} \newcommand{\reg}{{I\!\!R}} \def\gsim{\,\lower.25ex\hbox{$\scriptstyle\sim$}\kern-1.30ex% \raise 0.55ex\hbox{$\scriptstyle >$}\,} \def\lsim{\,\lower.25ex\hbox{$\scriptstyle\sim$}\kern-1.30ex% \raise 0.55ex\hbox{$\scriptstyle <$}\,} \newcommand{\trm}{m_{\perp}} \newcommand{\trp}{p_{\perp}} \newcommand{\trmm}{m_{\perp}^2} \newcommand{\trpp}{p_{\perp}^2} \newcommand{\alp}{\alpha_s} \newcommand{\alps}{\alpha_s} \newcommand{\sqrts}{$\sqrt{s}$} \newcommand{\PT}{p_{\perp}} \newcommand{\JPSI}{J/\psi} \newcommand{\PO}{I\!\!P} \newcommand{\xbj}{x} \newcommand{\xpom}{x_{\PO}} \newcommand{\dgr}{^\circ} \newcommand{\gev}{\,\mbox{GeV}} \newcommand{\GeV}{\mathrm{GeV}} \newcommand{\mrad}{\mathrm{mrad}} \newcommand{\GeVS}{\rm GeV^2} \newcommand{\xp}{x_p} \newcommand{\xpi}{x_\pi} \newcommand{\xg}{x_\gamma} \newcommand{\xgj}{x_\gamma^{jet}} \newcommand{\xpj}{x_p^{jet}} \newcommand{\xpij}{x_\pi^{jet}} \renewcommand{\deg}{^\circ} \newcommand{\qsq}{\ensuremath{Q^2} } \newcommand{\et}{\ensuremath{E_t^*} } \newcommand{\rap}{\ensuremath{\eta^*} } \newcommand{\gp}{\ensuremath{\gamma^*}p } \newcommand{\der}{{\mathrm d}} % Journal macro \def\Journal#1#2#3#4{{#1} {\bf #2}, #4 (#3)} \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\PR{{\em Phys. Rev.} } \def\ZPC{{\em Z. Phys.} {\bf C}} \def\ZP{{\em Z. Phys.} } \def\EJC{{\em Eur. Phys. J.} {\bf C}} \def\EJA{{\em Eur. Phys. J.} {\bf A}} \def\CPC{\em Comp. Phys. Commun.} \begin{titlepage} \begin{flushleft} {\tt DESY 11-093 \hfill ISSN 0418-9833} \\ {\tt June 2011} \\ \end{flushleft} \vspace*{30mm} \rm \begin{center} \begin{Large} {\boldmath \bf Measurement of Photon Production in the Very Forward Direction in Deep-Inelastic Scattering at HERA } \vspace{2cm} \rm H1 Collaboration \end{Large} \end{center} \vspace{2cm} % \begin{abstract} \noindent \rm The production of photons at very small angles with respect to the proton beam direction is studied in deep-inelastic positron-proton scattering at HERA. The data are taken with the H1 detector in the years 2006 and 2007 and correspond to an integrated luminosity of $126~\mathrm{pb}^{-1}$. The analysis covers the range of negative four momentum transfer squared at the positron vertex $67.9$ as a function of its transverse momentum $p_T^{lead}$ and longitudinal momentum fraction of the incoming proton $x_L^{lead}$. In addition, the cross sections are studied as a function of the sum of the longitudinal momentum fraction $x_L^{sum}$ of all photons in the pseudorapidity range $\eta>7.9$. The cross sections are normalised to the inclusive deep-inelastic scattering cross section and compared to the predictions of models of deep-inelastic scattering and models of the hadronic interactions of high energy cosmic rays. \end{abstract} \vspace*{10mm} \it \normalsize \begin{center} Submitted to \EJC \end{center} \end{titlepage} %---------------------------------------------------------------------------------- %\input{h1autsnew.tex} %-- H1AUTS Author list by names %-- Status: Wed Jun 1 14:13:45 CEST 2011 Number of authors = 203 \noindent F.D.~Aaron$^{5,48}$, %BUCH-PD 11/06 Aaron C.~Alexa$^{5}$, %BUCH-PD 06/06 Alexa V.~Andreev$^{25}$, %LPI -PD 8/88 Andreev S.~Backovic$^{30}$, %PODG-PD 03/02 Backovic A.~Baghdasaryan$^{38}$, %YERE-PD 09/03 Baghdasaryana S.~Baghdasaryan$^{38}$, %YERE-ST 02/10 Baghdasaryans E.~Barrelet$^{29}$, %PARI-PD 11/99 Barrelet W.~Bartel$^{11}$, %DESY-PD 8/88 Bartel K.~Begzsuren$^{35}$, %ULBA-PD 04/06 Begzsuren A.~Belousov$^{25}$, %LPI -PD 8/88 Belousov P.~Belov$^{11}$, %DESY-ST 07/10 Belov J.C.~Bizot$^{27}$, %ORSA-PD 8/88 Bizot V.~Boudry$^{28}$, %ECPL-PD 1/93 Boudry I.~Bozovic-Jelisavcic$^{2}$, %BEOG-PD 03/06 Bozovicjelisavcic J.~Bracinik$^{3}$, %BIRM-PD 01/2 Bracinik G.~Brandt$^{11}$, %DESY-PD 01/20 Brandt M.~Brinkmann$^{11}$, %DESY-PD 03/10 Brinkmann V.~Brisson$^{27}$, %ORSA-PD 8/88 Brisson D.~Britzger$^{11}$, %DESY-ST 10/09 Britzger D.~Bruncko$^{16}$, %KOSI-PD 8/88 Bruncko A.~Bunyatyan$^{13,38}$, %MPIH-PD 12/95 Bunyatyan G.~Buschhorn$^{26, \dagger}$, %MPIM-LEFT 05/11 Buschhorn L.~Bystritskaya$^{24}$, %ITEP-PD 05/99 Bystritskaya A.J.~Campbell$^{11}$, %DESY-PD 8/88 Campbella K.B.~Cantun~Avila$^{22}$, %MEX1-ST 04/06 Cantunavila F.~Ceccopieri$^{4}$, %BRUX-PD 10/09 Ceccopieri K.~Cerny$^{32}$, %PRG2-PD 09/08 Cernyk V.~Cerny$^{16,47}$, %KOSI-PD 06/04 Cernyv V.~Chekelian$^{26}$, %MPIM-PD 01/90 Chekelian J.G.~Contreras$^{22}$, %MEX1-PD 04/97 Contreras J.A.~Coughlan$^{6}$, %RAL -PD 8/88 Coughlan J.~Cvach$^{31}$, %PRAG-PD 8/88 Cvach J.B.~Dainton$^{18}$, %LIVE-PD 8/88 Dainton K.~Daum$^{37,43}$, %WUPP-PD 06/96 Daum B.~Delcourt$^{27}$, %ORSA-PD 8/88 Delcourt J.~Delvax$^{4}$, %BRUX-PD 11/10 Delvax E.A.~De~Wolf$^{4}$, %ANTW-PD 3/93 Dewolf C.~Diaconu$^{21}$, %MARS-PD 01/05 Diaconu M.~Dobre$^{12,50,51}$, %HAM2-ST 07/09 Dobre V.~Dodonov$^{13}$, %MPIH-PD 04/98 Dodonov A.~Dossanov$^{26}$, %MPIM-ST 01/07 Dossanov A.~Dubak$^{30,46}$, %PODG-PD 10/03 Dubak G.~Eckerlin$^{11}$, %DESY-PD 8/88 Eckerlin S.~Egli$^{36}$, %PSI -PD 01/10 Egli A.~Eliseev$^{25}$, %LPI -PD 01/06 Eliseev E.~Elsen$^{11}$, %DESY-PD 8/88 Elsen L.~Favart$^{4}$, %BRUX-PD 8/88 Favart A.~Fedotov$^{24}$, %ITEP-PD 8/88 Fedotov R.~Felst$^{11}$, %DESY-PD 11/0 Felst J.~Feltesse$^{10}$, %SACL-PD 03/05 Feltesse J.~Ferencei$^{16}$, %KOSI-PD 01/05 Ferencei D.-J.~Fischer$^{11}$, %DESY-ST 03/08 Fischer M.~Fleischer$^{11}$, %DESY-PD 07/0 Fleischer A.~Fomenko$^{25}$, %LPI -PD 8/88 Fomenko E.~Gabathuler$^{18}$, %LIVE-PD 10/89 Gabathulere J.~Gayler$^{11}$, %DESY-PD 8/88 Gayler S.~Ghazaryan$^{11}$, %DFLC-PD 09/09 Ghazaryan A.~Glazov$^{11}$, %DESY-PD 01/04 Glazov L.~Goerlich$^{7}$, %CRAC-PD 8/88 Goerlich N.~Gogitidze$^{25}$, %LPI -PD 8/88 Gogitidze M.~Gouzevitch$^{11,45}$, %DESY-PD 09/10 Gouzevitch C.~Grab$^{40}$, %ZUTH-PD 8/88 Grab A.~Grebenyuk$^{11}$, %DESY-ST 03/09 Grebenyuk T.~Greenshaw$^{18}$, %LIVE-PD 8/88 Greenshaw B.R.~Grell$^{11}$, %DESY-LEFT 10/10 Grell G.~Grindhammer$^{26}$, %MPIM-PD 8/88 Grindhammer S.~Habib$^{11}$, %DESY-PD 09/09 Habib D.~Haidt$^{11}$, %DESY-PD 8/88 Haidt C.~Helebrant$^{11}$, %DFLC-LEFT 01/11 Helebrant R.C.W.~Henderson$^{17}$, %LANC-PD 8/88 Henderson E.~Hennekemper$^{15}$, %HDB2-ST 11/07 Hennekemper H.~Henschel$^{39}$, %ZEUT-PD 06/99 Henschel M.~Herbst$^{15}$, %HDB2-ST 06/08 Herbst G.~Herrera$^{23}$, %MEX2-PD 07/98 Herrera M.~Hildebrandt$^{36}$, %PSI -PD 01/10 Hildebrandtm K.H.~Hiller$^{39}$, %ZEUT-PD 8/88 Hiller D.~Hoffmann$^{21}$, %MARS-PD 10/0 Hoffmann R.~Horisberger$^{36}$, %PSI -PD 01/10 Horisberger T.~Hreus$^{4,44}$, %BRUX-PD 10/08 Hreus F.~Huber$^{14}$, %HDB1-ST 09/09 Huberf M.~Jacquet$^{27}$, %ORSA-PD 09/96 Jacquet X.~Janssen$^{4}$, %ANTW-PD 02/03 Janssenx L.~J\"onsson$^{20}$, %LUND-PD 8/88 Joensson H.~Jung$^{11,4,52}$, %DESY-PD 07/00 Jungh M.~Kapichine$^{9}$, %JINR-PD 3/97 Kapichine I.R.~Kenyon$^{3}$, %BIRM-PD 8/88 Kenyon C.~Kiesling$^{26}$, %MPIM-PD 8/88 Kiesling M.~Klein$^{18}$, %LIVE-PD 8/88 Klein C.~Kleinwort$^{11}$, %DESY-PD 8/88 Kleinwort T.~Kluge$^{18}$, %LIVE-PD 05/04 Kluge R.~Kogler$^{11}$, %DESY-PD 12/10 Kogler P.~Kostka$^{39}$, %ZEUT-PD 8/88 Kostka M.~Kraemer$^{11}$, %DESY-PD 10/09 Kraemer J.~Kretzschmar$^{18}$, %LIVE-PD 01/08 Kretzschmar K.~Kr\"uger$^{15}$, %HDB2-PD 01/04 Kruegerk M.P.J.~Landon$^{19}$, %QMWC-PD 8/88 Landon W.~Lange$^{39}$, %ZEUT-PD 8/88 Lange G.~La\v{s}tovi\v{c}ka-Medin$^{30}$, %PODG-PD 06/04 Lastovickamedin P.~Laycock$^{18}$, %LIVE-PD 11/03 Laycock A.~Lebedev$^{25}$, %LPI -PD 8/88 Lebedev V.~Lendermann$^{15}$, %HDB2-PD 01/2 Lendermann S.~Levonian$^{11}$, %DESY-PD 8/88 Levonian K.~Lipka$^{11,50}$, %DESY-PD 01/03 Lipka B.~List$^{11}$, %DESY-LEFT 01/11 Listb J.~List$^{11}$, %DFLC-PD 01/05 Listj R.~Lopez-Fernandez$^{23}$, %MEX2-PD 03/2 Lopezfernandez V.~Lubimov$^{24}$, %ITEP-PD 01/95 Lubimov L.~Lytkin$^{9}$, %JINR A.~Makankine$^{9}$, %JINR-PD 11/02 Makankine E.~Malinovski$^{25}$, %LPI -PD 01/89 Malinovskie P.~Marage$^{4}$, %BRUX-LEFT 10/10 Marage H.-U.~Martyn$^{1}$, %AAC1-PD 8/88 Martyn S.J.~Maxfield$^{18}$, %LIVE-PD 8/88 Maxfield A.~Mehta$^{18}$, %LIVE-PD 8/88 Mehta A.B.~Meyer$^{11}$, %DESY-PD 01/00 Meyeran H.~Meyer$^{37}$, %WUPP-PD 8/88 Meyerhi J.~Meyer$^{11}$, %DESY-PD 8/88 Meyerj S.~Mikocki$^{7}$, %CRAC-PD 8/88 Mikocki I.~Milcewicz-Mika$^{7}$, %CRAC-ST 10/02 Milcewicz F.~Moreau$^{28}$, %ECPL-PD 01/90 Moreau A.~Morozov$^{9}$, %JINR-PD 06/99 Morozova J.V.~Morris$^{6}$, %RAL -PD 8/88 Morris M.~Mudrinic$^{2}$, %BEOG-LEFT 01/11 Mudrinic K.~M\"uller$^{41}$, %ZUER-PD 8/88 Muellerk Th.~Naumann$^{39}$, %ZEUT-PD 01/89 Naumannt P.R.~Newman$^{3}$, %BIRM-PD 10/92 Newman C.~Niebuhr$^{11}$, %DESY-PD 3/93 Niebuhr D.~Nikitin$^{9}$, %JINR-PD 06/08 Nikitin G.~Nowak$^{7}$, %CRAC-PD 8/88 Nowakg K.~Nowak$^{11}$, %DESY-PD 10/09 Nowakk J.E.~Olsson$^{11}$, %DESY-PD 8/88 Olsson D.~Ozerov$^{24}$, %ITEP-PD 08/08 Ozerov P.~Pahl$^{11}$, %DESY-ST 10/08 Pahl V.~Palichik$^{9}$, %JINR-PD 01/04 Palichik I.~Panagoulias$^{l,}$$^{11,42}$, %DESY-ST 08/04 Panagoulias M.~Pandurovic$^{2}$, %BEOG-PD 03/11 Pandurovic \break Th.~Papadopoulou$^{l,}$$^{11,42}$, %DESY-PD 06/04 Papadopoulou C.~Pascaud$^{27}$, %ORSA-PD 8/88 Pascaud G.D.~Patel$^{18}$, %LIVE-PD 8/88 Patel E.~Perez$^{10,45}$, %SACL-PD 10/07 Perez A.~Petrukhin$^{11}$, %DESY-PD 10/09 Petrukhin I.~Picuric$^{30}$, %PODG-PD 01/06 Picuric S.~Piec$^{11}$, %DESY-PD 11/09 Piec H.~Pirumov$^{14}$, %HDB1-ST 09/09 Pirumov D.~Pitzl$^{11}$, %DESY-PD 8/88 Pitzl R.~Pla\v{c}akyt\.{e}$^{12}$, %HAM2-PD 07/10 Placakyte B.~Pokorny$^{32}$, %PRG2-ST 10/09 Pokorny R.~Polifka$^{32}$, %PRG2-ST 10/06 Polifka B.~Povh$^{13}$, %MPIH-PD 8/88 Povh V.~Radescu$^{14}$, %HDB1-PD 10/06 Radescu N.~Raicevic$^{30}$, %PODG-PD 03/2 Raicevic T.~Ravdandorj$^{35}$, %ULBA-PD 06/06 Ravdandorj P.~Reimer$^{31}$, %PRAG-PD 8/88 Reimer E.~Rizvi$^{19}$, %QMWC-PD 01/05 Rizvi P.~Robmann$^{41}$, %ZUER-PD 8/88 Robmann R.~Roosen$^{4}$, %BRUX-PD 8/88 Roosen A.~Rostovtsev$^{24}$, %ITEP-PD 8/88 Rostovtsev M.~Rotaru$^{5}$, %BUCH-ST 02/07 Rotaru J.E.~Ruiz~Tabasco$^{22}$, %MEX1-PD 05/10 Ruiztabascojuliaelis S.~Rusakov$^{25}$, %LPI -PD 8/88 Rusakov D.~\v S\'alek$^{32}$, %PRG2-PD 10/10 Salek D.P.C.~Sankey$^{6}$, %RAL -PD 8/88 Sankey M.~Sauter$^{14}$, %HDB1-PD 10/09 Sauter E.~Sauvan$^{21}$, %MARS-PD 11/1 Sauvan S.~Schmitt$^{11}$, %DESY-PD 09/07 Schmittst L.~Schoeffel$^{10}$, %SACL-PD 12/98 Schoeffel A.~Sch\"oning$^{14}$, %HDB1-PD 04/09 Schoening H.-C.~Schultz-Coulon$^{15}$, %HDB2-PD 01/04 Schultzcoulon F.~Sefkow$^{11}$, %DFLC-PD 09/99 Sefkow L.N.~Shtarkov$^{25}$, %LPI -PD 8/88 Shtarkov S.~Shushkevich$^{26}$, %MPIM-ST 08/07 Shushkevich T.~Sloan$^{17}$, %LANC-PD 1/96 Sloan I.~Smiljanic$^{2}$, %BEOG-LEFT 01/11 Smiljanic Y.~Soloviev$^{25}$, %LPI -PD 8/88 Soloviev P.~Sopicki$^{7}$, %CRAC-ST 09/07 Sopicki D.~South$^{11}$, %DESY-PD 07/10 South V.~Spaskov$^{9}$, %JINR-PD 12/97 Spaskov A.~Specka$^{28}$, %ECPL-PD 3/95 Specka Z.~Staykova$^{4}$, %ANTW-PD 10/10 Staykova M.~Steder$^{11}$, %DESY-PD 09/08 Steder B.~Stella$^{33}$, %ROME-PD 8/88 Stella G.~Stoicea$^{5}$, %BUCH-PD 02/08 Stoicea U.~Straumann$^{41}$, %ZUER-PD 8/88 Straumann T.~Sykora$^{4,32}$, %ANTW-PD 01/06 Sykora P.D.~Thompson$^{3}$, %BIRM-PD 08/99 Thompsonp T.H.~Tran$^{27}$, %ORSA-PD 03/10 Tran D.~Traynor$^{19}$, %QMWC-PD 12/01 Traynor P.~Tru\"ol$^{41}$, %ZUER-PD 8/88 Truoel I.~Tsakov$^{34}$, %SOFI-PD 04/03 Tsakov B.~Tseepeldorj$^{35,49}$, %ULBA-PD 06/06 Tseepeldorj J.~Turnau$^{7}$, %CRAC-PD 8/88 Turnau K.~Urban$^{15}$, %HDB2-LEFT 07/10 Urbank A.~Valk\'arov\'a$^{32}$, %PRG2-PD 8/88 Valkarova C.~Vall\'ee$^{21}$, %MARS-PD 8/88 Vallee P.~Van~Mechelen$^{4}$, %ANTW-PD 12/98 Vanmechelen Y.~Vazdik$^{25}$, %LPI -PD 8/88 Vazdik D.~Wegener$^{8}$, %DORT-PD 8/88 Wegener E.~W\"unsch$^{11}$, %DESY-PD 8/88 Wuensch J.~\v{Z}\'a\v{c}ek$^{32}$, %PRG2-PD 8/88 Zacek J.~Z\'ale\v{s}\'ak$^{31}$, %PRAG-PD 01/05 Zalesak Z.~Zhang$^{27}$, %ORSA-PD 10/92 Zhang A.~Zhokin$^{24}$, %ITEP-PD 04/99 Zhokine H.~Zohrabyan$^{38}$, %YERE-PD 11/02 Zohrabyan and F.~Zomer$^{27}$ %ORSA-PD 8/88 Zomer %-- H1 Institutes \bigskip\noindent{\it $ ^{1}$ I. Physikalisches Institut der RWTH, Aachen, Germany \\ $ ^{2}$ Vinca Institute of Nuclear Sciences, University of Belgrade, 1100 Belgrade, Serbia \\ $ ^{3}$ School of Physics and Astronomy, University of Birmingham, Birmingham, UK$^{ b}$ \\ $ ^{4}$ Inter-University Institute for High Energies ULB-VUB, Brussels and Universiteit Antwerpen, Antwerpen, Belgium$^{ c}$ \\ $ ^{5}$ National Institute for Physics and Nuclear Engineering (NIPNE) , Bucharest, Romania$^{ m}$ \\ $ ^{6}$ Rutherford Appleton Laboratory, Chilton, Didcot, UK$^{ b}$ \\ $ ^{7}$ Institute for Nuclear Physics, Cracow, Poland$^{ d}$ \\ $ ^{8}$ Institut f\"ur Physik, TU Dortmund, Dortmund, Germany$^{ a}$ \\ $ ^{9}$ Joint Institute for Nuclear Research, Dubna, Russia \\ $ ^{10}$ CEA, DSM/Irfu, CE-Saclay, Gif-sur-Yvette, France \\ $ ^{11}$ DESY, Hamburg, Germany \\ $ ^{12}$ Institut f\"ur Experimentalphysik, Universit\"at Hamburg, Hamburg, Germany$^{ a}$ \\ $ ^{13}$ Max-Planck-Institut f\"ur Kernphysik, Heidelberg, Germany \\ $ ^{14}$ Physikalisches Institut, Universit\"at Heidelberg, Heidelberg, Germany$^{ a}$ \\ $ ^{15}$ Kirchhoff-Institut f\"ur Physik, Universit\"at Heidelberg, Heidelberg, Germany$^{ a}$ \\ $ ^{16}$ Institute of Experimental Physics, Slovak Academy of Sciences, Ko\v{s}ice, Slovak Republic$^{ f}$ \\ $ ^{17}$ Department of Physics, University of Lancaster, Lancaster, UK$^{ b}$ \\ $ ^{18}$ Department of Physics, University of Liverpool, Liverpool, UK$^{ b}$ \\ $ ^{19}$ Queen Mary and Westfield College, London, UK$^{ b}$ \\ $ ^{20}$ Physics Department, University of Lund, Lund, Sweden$^{ g}$ \\ $ ^{21}$ CPPM, Aix-Marseille Universit\'e, CNRS/IN2P3, Marseille, France \\ $ ^{22}$ Departamento de Fisica Aplicada, CINVESTAV, M\'erida, Yucat\'an, M\'exico$^{ j}$ \\ $ ^{23}$ Departamento de Fisica, CINVESTAV IPN, M\'exico City, M\'exico$^{ j}$ \\ $ ^{24}$ Institute for Theoretical and Experimental Physics, Moscow, Russia$^{ k}$ \\ $ ^{25}$ Lebedev Physical Institute, Moscow, Russia$^{ e}$ \\ $ ^{26}$ Max-Planck-Institut f\"ur Physik, M\"unchen, Germany \\ $ ^{27}$ LAL, Universit\'e Paris-Sud, CNRS/IN2P3, Orsay, France \\ $ ^{28}$ LLR, Ecole Polytechnique, CNRS/IN2P3, Palaiseau, France \\ $ ^{29}$ LPNHE, Universit\'e Pierre et Marie Curie Paris 6, Universit\'e Denis Diderot Paris 7, CNRS/IN2P3, Paris, France \\ $ ^{30}$ Faculty of Science, University of Montenegro, Podgorica, Montenegro$^{ n}$ \\ $ ^{31}$ Institute of Physics, Academy of Sciences of the Czech Republic, Praha, Czech Republic$^{ h}$ \\ $ ^{32}$ Faculty of Mathematics and Physics, Charles University, Praha, Czech Republic$^{ h}$ \\ $ ^{33}$ Dipartimento di Fisica Universit\`a di Roma Tre and INFN Roma~3, Roma, Italy \\ $ ^{34}$ Institute for Nuclear Research and Nuclear Energy, Sofia, Bulgaria$^{ e}$ \\ $ ^{35}$ Institute of Physics and Technology of the Mongolian Academy of Sciences, Ulaanbaatar, Mongolia \\ $ ^{36}$ Paul Scherrer Institut, Villigen, Switzerland \\ $ ^{37}$ Fachbereich C, Universit\"at Wuppertal, Wuppertal, Germany \\ $ ^{38}$ Yerevan Physics Institute, Yerevan, Armenia \\ $ ^{39}$ DESY, Zeuthen, Germany \\ $ ^{40}$ Institut f\"ur Teilchenphysik, ETH, Z\"urich, Switzerland$^{ i}$ \\ $ ^{41}$ Physik-Institut der Universit\"at Z\"urich, Z\"urich, Switzerland$^{ i}$ \bigskip\noindent $ ^{42}$ Also at Physics Department, National Technical University, Zografou Campus, GR-15773 Athens, Greece \\ $ ^{43}$ Also at Rechenzentrum, Universit\"at Wuppertal, Wuppertal, Germany \\ $ ^{44}$ Also at University of P.J. \v{S}af\'{a}rik, Ko\v{s}ice, Slovak Republic \\ $ ^{45}$ Also at CERN, Geneva, Switzerland \\ $ ^{46}$ Also at Max-Planck-Institut f\"ur Physik, M\"unchen, Germany \\ $ ^{47}$ Also at Comenius University, Bratislava, Slovak Republic \\ $ ^{48}$ Also at Faculty of Physics, University of Bucharest, Bucharest, Romania \\ $ ^{49}$ Also at Ulaanbaatar University, Ulaanbaatar, Mongolia \\ $ ^{50}$ Supported by the Initiative and Networking Fund of the Helmholtz Association (HGF) under the contract VH-NG-401. \\ $ ^{51}$ Absent on leave from NIPNE-HH, Bucharest, Romania \\ $ ^{52}$ On leave of absence at CERN, Geneva, Switzerland \smallskip\noindent $ ^{\dagger}$ Deceased \\ \bigskip\noindent $ ^a$ Supported by the Bundesministerium f\"ur Bildung und Forschung, FRG, under contract numbers 05H09GUF, 05H09VHC, 05H09VHF, 05H16PEA \\ $ ^b$ Supported by the UK Science and Technology Facilities Council, and formerly by the UK Particle Physics and Astronomy Research Council \\ $ ^c$ Supported by FNRS-FWO-Vlaanderen, IISN-IIKW and IWT and by Interuniversity Attraction Poles Programme, Belgian Science Policy \\ $ ^d$ Partially Supported by Polish Ministry of Science and Higher Education, grant DPN/N168/DESY/2009 \\ $ ^e$ Supported by the Deutsche Forschungsgemeinschaft \\ $ ^f$ Supported by VEGA SR grant no. 2/7062/ 27 \\ $ ^g$ Supported by the Swedish Natural Science Research Council \\ $ ^h$ Supported by the Ministry of Education of the Czech Republic under the projects LC527, INGO-LA09042 and MSM0021620859 \\ $ ^i$ Supported by the Swiss National Science Foundation \\ $ ^j$ Supported by CONACYT, M\'exico, grant 48778-F \\ $ ^k$ Russian Foundation for Basic Research (RFBR), grant no 1329.2008.2 \\ $ ^l$ This project is co-funded by the European Social Fund (75\%) and National Resources (25\%) - (EPEAEK II) - PYTHAGORAS II \\ $ ^m$ Supported by the Romanian National Authority for Scientific Research under the contract PN 09370101 \\ $ ^n$ Partially Supported by Ministry of Science of Montenegro, no. 05-1/3-3352 \\ } \newpage %---------------------------------------------------------------------------------- \section{Introduction} \noindent %-------------------------------------------------------------------------------------- Measurements of particle production at very small angles with respect to the proton beam direction (forward direction) in positron-proton collisions are important for the %theoretical understanding of proton fragmentation. These measurements %of forward particle production also provide important constraints for the modelling of the high energy air showers and thereby are very valuable for the understanding of high energy cosmic ray data. % The H1 and ZEUS experiments at the $e^{\pm}p$ collider HERA have published several analyses on the production of forward protons and neutrons which carry a large fraction of the longitudinal momentum of the incoming proton \cite{Adloff:1998yg,Chekanov:2002pf,Chekanov:2007tv,Chekanov:2008tn,Aaron:2010ze}. % These measurements probe different mechanisms related to the baryon production in forward direction, such as elastic scattering of the proton, diffractive dissociation, pion exchange and string fragmentation. In particular, these measurements test the hypothesis of limiting fragmentation~\cite{Benecke:1969sh,Chou:1994dh}, according to which, in the high-energy limit, the cross section for the inclusive production of particles in the target fragmentation region is independent of the incident projectile energy. This hypothesis implies, that in deep-inelastic scattering (DIS) forward particle production cross sections are independent of the Bjorken-$x$ and the virtuality of the exchanged photon $Q^2$. The measurement of the photon production in the forward direction can provide new input to the understanding of proton fragmentation, and is complementary to forward baryon measurements. The production of photons and $\pi^0$ mesons in the proton fragmentation region has been studied in $\bar{p}p$ and $pp$ collisions at SPS and the LHC colliders~~\cite{Pare:1989mr,Adriani:2011nf}. The analysis presented here is the first measurement of very forward photons in DIS $e^+p$ collisions at HERA. The photons are detected at very small angles below $0.75$~mrad with respect to proton beam direction. It relies on the upgraded H1 Forward Neutron Calorimeter (FNC) which includes an electromagnetic section. \section{Experimental Procedure and Data Analysis} %-------------------------------------------------------------------------------------- The data used in this analysis were collected with the H1 detector at HERA in the years 2006 and 2007 and correspond to an integrated luminosity of $126~ \rm pb^{-1}$. %For the period used in this analysis, During the period corresponding to the analysis data set HERA collided positrons and protons with energies of $E_e=27.6$~GeV and $E_p=920$~GeV, respectively, corresponding to a centre-of-mass energy of $\sqrt{s}=319$~GeV. \subsection{H1 detector} A detailed description of the H1 detector can be found elsewhere \cite{Abt:1996hi,Abt:1996xv,Appuhn:1996na,Pitzl:2000wz,spacaltest,Andrieu:1993kh}. Only the detector components relevant to this analysis are briefly described here. % The origin of the right-handed H1 coordinate system is the nominal $e^+p$ interaction point. The direction of the proton beam defines the positive $z$ axis; the polar angle $\theta$ is measured with respect to this axis. Transverse momenta are measured in the $x$--$y$ plane. The pseudorapidity is defined by $\eta = -\ln{(\tan\frac{\theta}{2})}$ and is measured in the lab frame. The polar angles $\theta<0.75$~mrad correspond to pseudorapidity range $\eta>7.9$. The interaction region is surrounded by a two-layer silicon strip detector and two large concentric drift chambers. % , operated inside a $1.16$~T solenoidal magnetic field. Charged particle momenta are measured in the angular range \break \mbox{$25\deg<\theta<155\deg$}. The tracking system is surrounded by a finely segmented Liquid Argon (LAr) calorimeter, which covers the polar angle range of \mbox{$4\deg<\theta<154\deg$} with full azimuthal acceptance. The LAr calorimeter consists of an electromagnetic section with lead absorber and a hadronic section with steel absorber. The total depth of the LAr calorimeter ranges from $4.5$ to $8$ hadronic interaction lengths. % The backward region ($153\deg<\theta<177.8\deg$) is covered by a lead/scintillating-fibre calorimeter (SpaCal). Its main purpose is the detection of the scattered positron. % The polar angle of the positron is measured with a precision of $1$~mrad. The energy resolution for positrons is $\sigma(E)/E\approx 7.1\%/\sqrt{E[\GeV]}\oplus 1\%$, as determined in test beam measurements~\cite{spacaltest}. The LAr and SpaCal calorimeters are surrounded by a superconducting solenoid which provides a uniform magnetic field of $1.16$~T along the beam direction. The luminosity is measured via the Bethe-Heitler Bremsstrahlung process $ep \rightarrow e'p \gamma$, the final state photon being detected in a tungsten/quartz-fibre sampling calorimeter at $z=-103$~m. The data sample of this analysis was collected using triggers which require the scattered positron to be measured in the SpaCal. The trigger efficiency is about $96\%$ for the analysis phase space as determined from data using independently triggered data. \begin{figure}[h] \hspace*{40mm}{\large \bf (a)} \hspace*{60mm} {\large \bf (b)} \vspace*{-2mm} \epsfig{file=d11-093f1a.eps,width=68mm} \hspace*{20mm} \epsfig{file=d11-093f1b.eps,width=58mm} \caption{(a) A schematic view of the H1 FNC. (b) Layout of $9$ vertical and $9$ horizontal readout strips of the Preshower Calorimeter. The hatched area shows the geometrical acceptance window defined by the beam-line elements. The area corresponding to $\eta>7.9$ is indicated by dashed circle. } \label{gen_view} \end{figure} %-------------------------------------------------------------------------------------- \subsection{Detection of forward neutral particles} \label{sec:fncdet} Neutral particles produced at very small polar angles can be detected in the FNC calorimeter, which is situated at a polar angle of $0\deg$, at $z=+106$~m from the interaction point. A schematic view of the H1 FNC used during the HERA-II running period is shown in figure~\ref{gen_view}a. A detailed description of the detector is given in \cite{Aaron:2010ze}. The FNC consists of the Main Calorimeter and the Preshower Calorimeter. The Main Calorimeter is a lead-scintillator sandwich calorimeter with a total length of $8.9$ nuclear interaction lengths. % The Preshower Calorimeter is a $40$~cm long lead-scintillator sandwich calorimeter. The length corresponds to about $60$ radiation lengths. The Preshower Calorimeter is composed of $24$ planes: the first $12$ planes each consist of a lead plate of $7.5$~mm thickness and a scintillator plate of $2.6$~mm thickness. The second $12$ planes each consist of a lead plate of $14$~mm thickness and a scintillator plate of $5.2$~mm thickness. % The transverse size of the scintillating plates is \mbox{$26\times26~\rm cm^2$}. Each scintillating plate has $45$ grooves with $1.2$~mm wavelength shifter fibres attached down one side. The orientation of fibres alternates from horizontal to vertical in consecutive planes. For each plane, the fibres are bundled into nine strips of five fibres each. Longitudinally, all strips are combined leading to $9$ vertical and $9$ horizontal towers which are finally connected to $18$ photomultipliers. The acceptance of the FNC is defined by the aperture of the HERA beam-line magnets and is limited to scattering angles of $\theta\lsim 0.8$~mrad with approximately $30\%$ azimuthal coverage. The geometrical acceptance window of the FNC is shown in figure~\ref{gen_view}b together with the layout of the Preshower Calorimeter readout strips. The longitudinal segmentation of the FNC allows efficient discrimination of photons from hadrons. The photon reconstruction algorithm is based on the fact that electromagnetic showers are fully contained in the Preshower Calorimeter with no energy deposits above the noise level in the Main Calorimeter. For high energy neutrons most of the energy is contained in the Main Calorimeter. However, low energy neutrons deposit large fractions of their energy in the Preshower Calorimeter. The fraction of neutrons which can be misidentified as photons is about $10\%$ for $90$~GeV neutrons decreasing to below $1\%$ for neutrons with an energy of $200$~GeV, as determined from the Monte Carlo (MC) simulation. % The energy deposits in the FNC which are contained in the Preshower Calorimeter are classified as electromagnetic clusters and are considered as photon candidates. The detection and reconstruction efficiency for photons in the measured angular range $\theta<0.75$~mrad, as estimated from MC simulation, is about $85\%$ for $100$~GeV photons increasing to $95\%$ for photons with energies of $900$~GeV. Losses are mainly due to interactions with the beampipe. %------------ % Calibration %------------- % All modules of the FNC were initially calibrated at CERN using $120$-$230$~GeV electron and $120$-$350$~GeV hadron beams. After the calorimeter was installed at DESY, the stability of calibration constants was monitored using interactions between the proton beam and residual gas in the beam pipe, as described in \cite{Aaron:2010ze}. Refined calibration constants for electromagnetic showers are determined using an iterative procedure based on the assumption that the maximum photon energy, $E_\gamma^{max}$, as measured in the Preshower Calorimeter, is expected to be equal to the proton beam energy in case of unlimited statistics. This calibration procedure also utilises data from HERA runs with reduced proton beam energies of $460$~GeV and $575$~GeV. The validity of this algorithm is tested with MC simulation. % The measured photon energy spectra for the three proton beam energies are displayed in figure~\ref{fig:calibration}a. The correlation between the beam energy and the maximum photon energy $E_{\gamma}^{max}$ as determined by the iterative procedure and after applying the calibration is shown in figure~\ref{fig:calibration}b. % Using this calibration procedure, the linearity of the energy response and the absolute energy scale are verified to a precision of $5\%$. \begin{figure}[t] \epsfig{file=d11-093f2a.eps,width=77mm} %\hspace*{-.5cm}\epsfig{file=desy-11-093.fig2b.eps,width=77mm} %\epsfig{file=desy-11-093.fig2c.eps,width=77mm} \epsfig{file=d11-093f2b.eps,width=78mm} \caption{ \label{fig:calibration} (a) The measured photon energy spectra for three proton beam energies. % ($460$, $575$ and $920$~GeV). %(b) %The measured photon energy spectra for $E_p=920$~GeV. %The different lines represent the development %of the photon energy distribution when gradually increasing the statistics. %(c) % The fitted upper edges of the energy distributions as a function of % the number of iterations corresponding to the gradual increase of the event statistics. % Results of the fit with a function $f(i)=E_{\gamma}^{max}+a\cdot exp(b\cdot i)$ % are shown by the dashed lines. (b) The correlation between the proton beam energy and %the upper edge of photon energy distribution $E_{\gamma}^{max}$. } \end{figure} % %Figures~\ref{fig:calibration}b shows the %photon energy spectra for $E_{p}=920$~GeV, %where the different lines represent the development of %the distribution when gradually increasing the event statistics. %It can be seen, that the upper edge of the distribution %approaches the beam energy with the increase of statistics. %Fitting these distributions, the values of the upper edges, %$E_{\gamma}^{edge}$, are determined. %In figure~\ref{fig:calibration}c the values of $E_{\gamma}^{edge}$ %are shown as a function of the number of iterations %corresponding to the gradual raise of statistics %for three proton beam energies. %Fitting the dependence on the number of iterations with a function %$f(i)=E_\gamma^{max}+a\cdot exp(b\cdot i)$, where $i$ is the number %of iterations, the upper limits of %the photon energy distributions, $E_{\gamma}^{max}$, %are determined and the calibration constants are derived. %The validity of this algorithm is tested with MC simulation. % The energy resolution of the FNC calorimeter for electromagnetic showers is $\sigma(E)/E \approx 20\%/\sqrt{E~[\GeV]} \oplus 2\%$, as determined in test beam measurements. % The spatial resolution for single electromagnetic showers and for those hadronic showers which started to develop in the Preshower Calorimeter is about $2$~mm. %-------------------------------------------------------------------------------------- \subsection{Kinematics and event selection } The kinematic variables used to describe high energy DIS interactions are the exchanged photon virtuality $Q^2$, the inelasticity $y$ and the Bjorken scaling variable $x_{Bj}$. They are defined as \begin{equation} Q^2=-q^2\; , \hspace{3em} x_{Bj}=\frac{Q^2}{2 p \cdot q}\; , \hspace{3em} y=\frac{p \cdot q}{p \cdot k}\;, \end{equation} where $p$, $k$ and $q$ are the four-momenta of the incident proton, the incident positron and the virtual photon, respectively. The selection of DIS events is based on the identification of the scattered positron as the most energetic compact calorimetric deposit in the SpaCal with an energy $E_e'>11$~GeV and a polar angle $156\deg<\theta_e'<175\deg$. The \mbox{$z$ coordinate} of the primary event vertex is required to be within $\pm 35$~cm of the nominal position of the interaction point. The hadronic final state is reconstructed using an energy flow algorithm which combines charged particles measured in the tracker with information from the SpaCal and LAr calorimeters~\cite{Peez:2003zd,Hellwig:2004yp}. % To suppress events with hard initial state radiation, as well as events originating from non-$e^+p$ interactions, the quantity $\sum{E - p_z}$, summed over all reconstructed final state particles including the positron, is required to lie between $35$~GeV and $70$~GeV. This quantity, which uses the energy and longitudinal momentum component of each final state particle, is expected to be twice the electron beam energy for fully contained events. In addition, events are restricted to the kinematic range $60.1$, where $E_p$ and $E_\gamma$ are the proton beam and forward photon energy, respectively. The data sample contains $78740$ events. In this analysis %the normalised differential cross sections are measured for the most energetic forward photon (leading photon) with pseudorapidity $\eta>7.9$ as a function of its longitudinal momentum fraction $x_L^{lead}$ and transverse momentum $p_T^{lead}$, in the range $0.17.9$, $x_L^{sum}=\sum x_L$, in the range $0.17.9$ in DIS in the kinematic range $67.9$ are presented in figure~\ref{fig:CrsecMCsum}. The data are compared with the predictions of models for inclusive DIS (LEPTO and CDM) and models of hadronic interactions (EPOS, SIBYLL and two versions of QGSJET). The ratios of MC model predictions to the measurements are shown separately. All models tested in this paper overestimate the total rate of forward photons. The LEPTO and CDM models predict about $70\%$ more photons than measured, while EPOS, SIBYLL and QGSJET overestimate the rate of photons by about $30\%$ to $50\%$. In contrast to the excess of photons in the CDM, the same model predicts a too low rate of forward neutrons as observed in previous H1 analysis of forward neutron production \cite{Aaron:2010ze}. The shapes of all measured distributions are well described by LEPTO. The CDM predicts harder $x_L$ and $p_T$ spectra. The QGSJET model predicts slightly softer spectra in $x_L$ and $p_T$. The EPOS and SIBYLL models predict harder $x_L$ spectra, but describe reasonably the shape of $p_T$ distribution. A measurement of the energy spectra of single photons produced at $\eta>8.8$ in $pp$ collisions at $7$~TeV centre-of-mass energy at the LHC has been recently reported by the LHCf Collaboration~\cite{Adriani:2011nf}. This measurement also shows significant discrepancies with the predictions from hadronic interaction models. However, a direct comparison of the H1 and LHCf results is not possible due to the different kinematic ranges of the two measurements. The measurement of forward photons allows a test of the limiting fragmentation hypothesis, according to which the production of forward photons in DIS is insensitive to $Q^2$ and $x_{Bj}$. To investigate this prediction, the ratio of the forward photon production cross section to the inclusive DIS cross section is measured as a function of $Q^2$ and $x_{Bj}$ (table~\ref{tab:table2} and figure~\ref{fig:Q2xratio}). Within the uncertainties the fraction of DIS events with forward photons is independent from $Q^2$ and $x_{Bj}$ in agreement with the limiting fragmentation hypothesis. A similar conclusion was obtained in the earlier H1 analysis of forward neutron production~\cite{Aaron:2010ze}. The LEPTO and CDM predictions also included in %In figure~\ref{fig:Q2xratio} display a significant difference in normalisation compared to data as well as %also the LEPTO and CDM predictions % are included. Apart from the normalisation difference, a slight dependence as a function of $Q^2$ and $x_{Bj}$. %-------------------------------------------------------------------------------------- \section{Summary} \noindent The production of high energy forward photons in the pseudorapidity range $\eta>7.9$ is studied for the first time at HERA in deep-inelastic positron-proton scattering in the kinematic region $6< Q^2 < 100$~GeV$^2$, $0.057.9$ in deep-inelastic scattering in the kinematic region $6< Q^2 < 100$~GeV$^2$ and $0.057.9$. For each measurement, the statistical, the total systematic, the uncorrelated systematic uncertainties, and the bin-to-bin correlated systematic uncertainties due to the FNC absolute energy scale, the impact position of the FNC and the model dependence of data correction are given. } \label{tab:table2} \end{table} %%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%% \newpage %\input{desy-11-093.f1.tex} \begin{figure}[p] \epsfig{file=d11-093f3a.eps,width=80mm} \epsfig{file=d11-093f3b.eps,width=80mm} \epsfig{file=d11-093f3c.eps,width=80mm} \epsfig{file=d11-093f3d.eps,width=80mm} \caption{ The normalised cross sections for the production of forward photons in the pseudorapidity range $\eta>7.9$ in deep-inelastic scattering in the kinematic region $6< Q^2 < 100~$GeV$^2$ and $0.057.9$ in deep-inelastic scattering in the kinematic region $6< Q^2 < 100~$GeV$^2$ and $0.057.9$ in deep-inelastic scattering in the kinematic region $6< Q^2 < 100~$GeV$^2$ and $0.057.9$ as a function of $Q^2$ and $x_{Bj}$. The inner error bars shows the quadratic sum of the statistical and the uncorrelated systematic uncertainties. Shaded band shows the correlated systematic uncertainties. The expectation from the LEPTO and CDM models are also shown. } \label{fig:Q2xratio} \end{figure} \end{document}