%====================================================== \documentclass[12pt]{article} \usepackage{epsfig} \usepackage{amsmath} \usepackage{hhline} \usepackage{amssymb} \usepackage{times} \usepackage{color} \usepackage{cite} \usepackage{multirow} %%%%%%%%%%%% Comment the next two lines to remove the line numbering \usepackage[]{lineno} %\linenumbers %%%%%%%%%%%% \renewcommand{\topfraction}{1.0} \renewcommand{\bottomfraction}{1.0} \renewcommand{\textfraction}{0.0} \newcommand{\Rp}{\mbox{$\not \hspace{-0.15cm} R_p$}} \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{\picob}{\mbox{{\rm ~pb}}} \hyphenation{deposits} %===================title page============================= % Some useful tex commands % \def\GeV{\hbox{$\;\hbox{\rm GeV}$}} \def\MeV{\hbox{$\;\hbox{\rm MeV}$}} \def\TeV{\hbox{$\;\hbox{\rm TeV}$}} \def\PLB{{\em Phys. Lett.} {\bf B}} \newcommand{\pb}{\rm pb} \newcommand{\cm}{\rm cm} \newcommand{\hdick}{\noalign{\hrule height1.4pt}} %\linenumbers \begin{document} %\pagestyle{empty} \begin{titlepage} \noindent \begin{flushleft} {\tt DESY 11-123\hfill ISSN 0418-9833} \\ {\tt July 2011} \end{flushleft} \vspace*{3cm} \begin{center} \begin{Large} {\bf Search for First Generation Leptoquarks in {\boldmath{$ep$}} Collisions at HERA} \vspace{2cm} H1 Collaboration \end{Large} \end{center} \vspace{2cm} \begin{abstract} \noindent A search for first generation scalar and vector leptoquarks produced in $ep$ collisions is performed by the H1 experiment at HERA. % The full H1 data sample is used in the analysis, corresponding to an integrated luminosity of $446$~pb$^{-1}$. % No evidence for the production of leptoquarks is observed in final states with a large transverse momentum electron or with large missing transverse momentum, and constraints on leptoquark models are derived. % For leptoquark couplings of electromagnetic strength $\lambda=0.3$, first generation leptoquarks with masses up to $800$~GeV are excluded at $95\%$ confidence level. \noindent \end{abstract} \vspace{5mm} \begin{center} Accepted by \PLB \end{center} \end{titlepage} \begin{flushleft} 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 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,}$$^{11,42}$, %DESY-ST 08/04 Panagoulias M.~Pandurovic$^{2}$, %BEOG-PD 03/11 Pandurovic Th.~Papadopoulou$^{l,}$$^{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{\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 $ ^{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 $ ^{\dagger}$ Deceased \\ \bigskip $ ^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 \\ } \end{flushleft} \newpage %%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%% \section{Introduction} \label{sec:intro} The $ep$ collisions at HERA provide a unique opportunity to search for new particles coupling directly to a lepton and a quark. % An example of such particles are leptoquarks (LQs), colour triplet bosons which appear in a variety of beyond the Standard Model (SM) theories~\cite{Pati:1974yy,Schrempp:1984nj,Dimopoulos:1979es,Nilles:1983ge}. % Particle interactions in the SM conserve lepton flavour. % If this property is extended to LQ models any such particles produced at HERA would decay exclusively into a quark and a first generation lepton, namely an electron\footnote{In this paper the term ``electron'' is used generically to refer to both electrons and positrons, if not otherwise stated.} or a neutrino. % Searches for such signatures have previously been performed at HERA~\cite{Aktas:2005pr,Adloff:1999tp,Chekanov:2003af}. % A dedicated analysis investigating the production of second and third generation leptoquarks has also been performed by the H1 Collaboration, where the final state contains a muon or the decay products of a tau lepton in combination with a hadronic system~\cite{h1lfv2011}. %%% In this paper a search for leptoquarks coupling exclusively to a quark and a first generation lepton is performed using the full $e^{\pm}p$ collision data set taken in the years 1994-2007 by the H1 experiment at HERA. % The data were recorded with an electron beam of energy $27.6$~GeV, which was longitudinally polarised up to $38$\%, and a proton beam of energy up to $920$~GeV, corresponding to a centre-of-mass energy $\sqrt{s}$ of up to $319$~GeV. % The total integrated luminosity of the analysed data is $446$~pb$^{-1}$, which represents a factor of almost four increase with respect to the previously published H1 results. % The presented results supersede those derived in previous searches for first generation leptoquarks by the H1 Collaboration~\cite{Aktas:2005pr}. %%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%% \section{Leptoquark Phenomenology and Standard Model Processes} \label{sec:theory} %%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%% \subsection{Leptoquark production at HERA} \label{sec:lqs} The phenomenology of LQs at HERA is discussed in detail elsewhere~\cite{Adloff:1999tp}. % The effective Lagrangian considered conserves lepton and baryon number, obeys the symmetries of the SM gauge groups ${\rm SU}(2)_L \times {\rm U}(1)_Y$ and ${\rm SU}(3)_C$ and includes both scalar and vector LQs. % In the framework of the phenomenological Buchm\"uller-R\"uckl-Wyler (BRW) model~\cite{BRW}, LQs are classified into $14$ types~\cite{14LQ} with respect to the quantum numbers spin $J$, weak isospin $I$ and chirality $C$ (left-handed $L$, right-handed $R$). % Scalar ($J=0$) LQs are denoted as $S_{I}^{C}$ and vector ($J=1$) LQs are denoted $V_{I}^{C}$ in the following. % LQs with identical quantum numbers except for weak hypercharge are distinguished using a tilde, for example $V_0^R$ and $\tilde{V}_0^R$. % Whereas all $14$ LQs couple to electron-quark pairs, four of the left-handed LQs, namely $S_0^L$, $S_{1}^L$, $V_0^L$ and $V_{1}^L$, may also decay to a neutrino-quark pair. % In particular, for $S_0^L$ and $V_0^L$ the branching fraction of decays into an electron-quark pair is predicted by the model to be \mbox{$\beta_e\!=\!\Gamma_{\rm eq}/(\Gamma_{\rm eq}+\Gamma_{\rm \nu q})\!=0.5$}, where \mbox{$\Gamma_{\rm eq}$} (\mbox{$\Gamma_{\rm \nu q}$}) denotes the partial width for the LQ decay to an electron (neutrino) and a quark $q$. % The branching fraction of decays into a neutrino-quark pair is then given by $\beta_{\nu} =1-\beta_{e}$. %%% Leptoquarks carry both lepton ($L$) and baryon ($B$) quantum numbers. % The fermion number \mbox{$F\!=\!L\!+\!3\,B$} is conserved. % Leptoquark processes in $ep$ collisions proceed directly via $s$-channel resonant LQ production or indirectly via $u$-channel virtual LQ exchange. % A dimensionless parameter $\lambda$ defines the coupling at the lepton-quark-LQ vertex. % For LQ masses $M_{\rm LQ}$ below $\sqrt{s}$, the $s$-channel production of $F = 2$ ($F = 0$) LQs dominates in $e^-p$ ($e^+p$) collisions. % For LQ masses above $\sqrt{s}$, both the $s$ and $u$-channel, as well as the interference with SM processes, are important such that both $e^-p$ and $e^+p$ collisions have similar sensitivity to all LQ types. %%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%% \subsection{Standard Model processes} \label{sec:sm} The search reported here considers final states where the leptoquark decays into an electron and a quark or a neutrino and a quark. % Such decays lead to topologies similar to those of deep-inelastic scattering (DIS) neutral current (NC) and charged current (CC) interactions at high negative four-momentum transfer squared $Q^2$. % The analysis is therefore performed using event selections (see section~\ref{sec:sel}) similar to those used in inclusive DIS analyses~\cite{Adloff:2003uh} and previous LQ searches~\cite{Aktas:2005pr}. %%% The SM prediction for both NC and CC DIS processes is obtained using the Monte Carlo (MC) event generator DJANGOH~\cite{django}, which is based on LEPTO~\cite{lepto} for the hard interaction and HERACLES~\cite{heracles} for leptonic single photon emission and virtual electroweak corrections. % LEPTO combines $\mathcal{O}(\alpha_s)$ matrix elements with higher order QCD effects using the colour dipole model as implemented in ARIADNE~\cite{ariadne}. % The JETSET program~\cite{jetset} is used to simulate the hadronisation process. % Additional SM background contributions from photoproduction processes are simulated using the PYTHIA~\cite{Sjostrand:2000wi} event generator, with the GRV-G~LO~\cite{grv} parameterisation of the photon parton density functions (PDFs). % All SM expectations are based on the CTEQ6m~\cite{Pumplin:2002vw} proton parton density function parameterisation, which includes only $12$\% of the H1 data analysed in this paper, in addition to $30$~pb$^{-1}$ of ZEUS data. At high Bjorken $x$, the CTEQ6m parameterisations are dominated by data from fixed target experiments due to the limited amount of HERA data included. %%% Generated events are passed through a GEANT~\cite{Brun:1987ma} based simulation of the H1 apparatus, which takes into account the running conditions of the data taking. % Simulated events are reconstructed and analysed using the same program chain as is used for the data. %%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%% \section{Experimental Method} \label{sec:exp} %%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%% \subsection{Data sets and lepton polarisation} \label{sec:datapol} The full H1 data sample is made up of $164$~pb$^{-1}$ recorded in $e^{-}p$ collisions and $282$~pb$^{-1}$ in $e^{+}p$ collisions, of which $35$~pb$^{-1}$ were recorded at $\sqrt{s} = 301$~GeV. % Data collected from 2003 onwards were taken with a longitudinally polarised lepton beam. % As leptoquarks are chiral particles, these data are analysed in separate polarisation samples, formed by combining all data periods with similar lepton beam polarisation $P_{e} = (N_{R} - N_{L})/(N_{R} + N_{L})$, where $N_{R}$ ($N_{L}$) is the number of right- (left-) handed leptons in the beam. % The average polarisation and luminosity of all data sets are detailed in table~\ref{tab:alldatasets}. \begin{table}[h] \renewcommand{\arraystretch}{1.2} \begin{center} \begin{tabular}{|c||c|c|c|} \hline Collisions & $\sqrt{s}$~~[GeV] & $P_{e}$~~[\%] & $\mathcal{L}$~~[pb$^{-1}$]\\ \hline \hline $e^{+}p$ & $301$ & $0$ & $35$ \\ $e^{-}p$ & $319$ & $0$ & $15$ \\ $e^{+}p$ & $319$ & $0$ & $67$ \\ \hline $e^{+}p$ & $319$ & $+32$ & $98$ \\ $e^{+}p$ & $319$ & $-38$ & $82$ \\ $e^{-}p$ & $319$ & $+37$ & $46$ \\ $e^{-}p$ & $319$ & $-26$ & $103$ \\ \hline \end{tabular} \end{center} \caption{Centre-of-mass energy $\sqrt{s}$, average lepton beam polarisation $P_{e}$ and integrated luminosity $\mathcal{L}$ of the analysed H1 data sets.} \label{tab:alldatasets} \end{table} %%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%% \subsection{H1 detector} \label{sec:det} A detailed description of the H1 experiment can be found elsewhere~\cite{Abt:h1}. % Only the detector components relevant to this analysis are briefly described here. % A right-handed Cartesian coordinate system is used with the origin at the nominal primary $ep$ interaction vertex. % The proton beam direction defines the positive $z$ axis (forward direction). % The polar angle $\theta$ and the transverse momenta $P_T$ of all particles are defined with respect to this axis. % The azimuthal angle $\phi$ defines the particle direction in the transverse plane. %%% The Liquid Argon (LAr) calorimeter~\cite{Andrieu:1993kh} covers the polar angle range \mbox{$4^\circ < \theta < 154^\circ$} with full azimuthal acceptance. % The energies of electromagnetic showers are measured in the LAr calorimeter with a precision of \mbox{$\sigma (E)/E \simeq 11\%/ \sqrt{E/\mbox{GeV}} \oplus 1\%$} and hadronic energy deposits with \mbox{$\sigma (E)/E \simeq 50\%/\sqrt{E/\mbox{GeV}} \oplus 2\%$}, as determined in test beam measurements~\cite{Andrieu:1993tz,Andrieu:1994yn}. % A lead-scintillating fibre calorimeter\footnote{This device was installed in 1995, replacing a lead-scintillator sandwich calorimeter~\cite{Abt:h1}.} (SpaCal)~\cite{SpaCal} covering the backward region $153^\circ < \theta < 178^\circ$ completes the measurement of charged and neutral particles. % The central \mbox{($20^\circ < \theta < 160^\circ$)} and forward \mbox{($7^\circ < \theta < 25^\circ$)} inner tracking detectors are used to measure charged particle trajectories and to reconstruct the interaction vertex. % The measured trajectories fitted to the interaction vertex are referred to as tracks in the following. % The LAr calorimeter and inner tracking detectors are enclosed in a superconducting magnetic coil with a field strength of $1.16$~T. % From the curvature of charged particle trajectories in the magnetic field, the central tracking system provides transverse momentum measurements with a resolution of \mbox{$\sigma_{P_T}/P_T = 0.002 P_T / \mbox{GeV}$} $\oplus~0.015$. % The return yoke of the magnetic coil is the outermost part of the detector and is equipped with streamer tubes forming the central muon detector \mbox{($4^\circ < \theta < 171^\circ$)}. % In the very forward region of the detector \mbox{($3^\circ < \theta < 17^\circ$)} a set of drift chambers detects muons and measures their momenta using an iron toroidal magnet. % The luminosity is determined from the rate of the Bethe-Heitler process $ep \rightarrow ep \gamma$, measured using a photon detector located close to the beam pipe at $z=-103~{\rm m}$, in the backward direction. %%% The LAr calorimeter provides the main trigger in this analysis~\cite{Adloff:2003uh}. % In order to remove events induced by cosmic rays and other non-$ep$ background, the event vertex is required to be reconstructed within $\pm 35$~cm in $z$ of the average nominal interaction point. % In addition, topological filters and timing vetoes are applied. %%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%% \subsection{Particle identification and event reconstruction} \label{sec:partid} The scattered electron is identified as a compact and isolated cluster of energy in the electromagnetic part of the LAr calorimeter with an associated track in the inner tracking detectors. % The hadronic final state is reconstructed using a particle flow algorithm to combine tracks and calorimeter deposits not associated to the scattered electron~\cite{h1lt,roman}. % The missing transverse momentum $P_{T}^{\rm miss}$, which may indicate the presence of neutrinos in the final state, is derived from all reconstructed particles in the event. %%% The kinematic quantities in NC events are determined using the electron method~\cite{eandhadronmethod}, which uses information exclusively from the scattered electron. % In CC events, the kinematic quantities are determined exclusively from the hadronic final state~\cite{eandhadronmethod}. % The leptoquark mass $M_{\rm LQ}~=~\sqrt{Q^2/y}$ is reconstructed using the measured kinematics of the scattered electron~(had-ronic final state) in the analysis of NC (CC) topologies, where $y$ is the inelasticity. %%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%% \section{Data Analysis} \label{sec:analysis} %%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%% \subsection{DIS event selections} \label{sec:sel} Neutral current events are selected by requiring a scattered electron with energy $E_{e} > 11$~GeV and $Q^{2} > 133$~GeV$^2$. % Additionally, a kinematic cut on the inelasticity $0.1~<~y<~0.9$ is employed to remove regions of poor reconstruction, poor resolution, large QED radiative effects and background from photoproduction processes~\cite{Aktas:2005pr}. % Background from neutral hadrons or photons misidentified as leptons is suppressed by requiring a charged track to be associated to the lepton candidate. % Energy-momentum conservation requires that $\Sigma_{i} (E^{i} - P^{i}_{z}) = 2E^{0}_{e}$, where the sum runs over all reconstructed particles, $P_{z}$ is the momentum along the proton beam axis and $E^{0}_{e}$ is the electron beam energy. % Applying the condition $\Sigma_{i} (E^{i} - P^{i}_{z}) > 35$~GeV further suppresses the contamination from photoproduction background in which the scattered lepton is undetected in the backward direction and a hadron is misidentified as an electron. % The $\Sigma_{i} (E^{i} - P^{i}_{z})$ requirement also further suppresses the influence of radiative corrections arising from initial state bremsstrahlung. %%% Charged current events are selected by requiring significant missing transverse momentum, $P_{T}^{\rm miss}>12$~GeV, which is due to the undetected neutrino. % To ensure a high trigger efficiency and good kinematic resolution, the analysis is further restricted to the region $0.1 < y < 0.85$. % The main SM background is due to photoproduction events, in which the scattered electron escapes undetected in the backward direction and transverse momentum is missing due to fluctuations in the detector response or undetected particles. % This background is suppressed by exploiting the correlation between $P_{T}^{\rm miss}$ and the ratio $V_{\rm ap} / V_{\rm p}$~\cite{Adloff:1999tp} of transverse energy flow anti-parallel and parallel to the hadronic final state transverse momentum vector~\cite{ringaile}. %%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%% \subsection{Systematic uncertainties} \label{sec:sys} The experimental systematic uncertainties included in the analysis of the polarised data taken in the years 2003-2007 are described in the following. % The systematic uncertainties on the 1994-2000 data are described in the previous H1 publication~\cite{Aktas:2005pr}. %%% In the NC event samples, a systematic scale uncertainty of $1$-$3$\% is assigned to the electromagnetic energy measured in the LAr calorimeter, depending on the $z$-coordinate of the impact position of the scattered electron. % A $0.5$\% component of this uncertainty is considered as correlated. % In addition, an uncorrelated uncertainty on the polar angle measurement of the scattered lepton of $2$~mrad for $\theta_{e} > 120^{\circ}$ and $3$~mrad elsewhere is also included. %%% An uncertainty of $2$\% is assigned to the scale of the measured hadronic energy for events in the CC event samples, of which $1$\% is considered to be a correlated component. % In addition, a $10$\% correlated uncertainty is assigned to the amount of energy in the LAr calorimeter attributed to noise for events in the CC event samples. %%% Other experimental systematic uncertainties are found to have a negligible impact on the analysis. % The effect of the above systematic uncertainties on the SM expectation is determined by varying the experimental quantities by $\pm 1$ standard deviation in the MC samples and propagating these variations through the whole analysis. %%% The luminosity measurement has an average uncertainty of $3$\%. % The uncertainty on the polarisation measurement is $3.5$\% and is found to have a negligible effect on the limit calculations performed in section~\ref{sec:limits}. %%% All data sets are compared to a SM prediction based on the CTEQ6m~\cite{Pumplin:2002vw} parameterisation of the parton densities inside the proton. % The uncertainties of this parameterisation are propagated through the analysis using the full set of eigenvector PDFs, and the effect is added in quadrature to the experimental uncertainties listed above. %%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%% \section{Results} \label{sec:results} %%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%% \subsection{Mass distributions} \label{sec:massspectra} Mass spectra of the four H1 data sets taken with a longitudinally polarised lepton beam as defined in table~\ref{tab:alldatasets} are shown in figure~\ref{fig:polmassplots}, where both the NC and CC event samples are presented. % The mass spectra of the complete electron and positron H1 data sets are presented in figure~\ref{fig:fullmassplots}. % A good description of the H1 data by the SM is observed, where the expectation is dominated by DIS processes in all event samples, with small additional contributions from photoproduction. % Since no evidence for LQ production is observed in any of the NC or CC data samples, the data are used to set constraints on LQs coupling to first generation fermions. %%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%% \subsection{Statistical Method} \label{sec:method} For the limit analysis, the data are studied in bins in the $M_{LQ}-y$ plane, where the NC and CC data samples with different lepton beam charge and polarisation are kept as distinct data sets. % In total, $N_{\rm bin} = 1408$ bins are considered, divided equally between the NC and CC event samples. % For a given bin $i$, the predicted LQ signal contribution is denoted $s_i$ and the predicted number of events in the absence of a LQ signal is denoted $b_i$. % The number of events in the presence of a LQ signal is thus $s_i+b_i$ and is obtained as a function of the LQ mass and coupling by a reweighting procedure~\cite{Aktas:2005pr}. % The limits are determined from a statistical analysis which uses the method of fractional event counting, optimised for the presence of systematic uncertainties~\cite{Bock:2004xz}. % For a given leptoquark type, mass and coupling hypothesis, a test statistic $X$ is constructed as a fractional event count of all events: % \begin{equation} X \,\, = \,\, \sum_{i=1}^{N_{\rm bin}} w_i n_i\,, \label{eqn:X} \end{equation} % where the sum runs over all bins and $n_i$ is the number of events observed in bin $i$. % The weights $w_i$ are chosen such that in the presence of a LQ signal the test statistic $X$ is larger than that expected from the SM. % In particular, bins with a large and positive $s_i$ have weights close to one, whereas bins with $s_i$ close to zero have weights close to zero. % If $s_i$ is negative in a given bin due to interference effects, the corresponding bin weight is also negative. % This has the desired effect that an event deficit in such a bin still leads to a $X$ larger than the SM expectation and thus is interpreted correctly as a signal contribution. % The presence of systematic uncertainties may reduce the sensitivity of a given bin. % The weight is therefore defined in such a way as to ensure that only bins with both a large signal contribution and small systematic uncertainties enter with sizeable weights into the test statistic $X$. % This is achieved by defining the weights as solutions of the following set of linear equations~\cite{Bock:2004xz}: % \begin{equation} s_i = k_1\left[(s_i+b_i)w_i+\sum_j V^{SB}_{ij}w_j\right]+k_2 \left[b_i w_i +\sum_j V^{B}_{ij}w_j\right]\,. \label{eqn:si} \end{equation} % In this analysis, the constants $k_1$ and $k_2$ are set to one, which is the appropriate choice for testing signals with a well defined cross section prediction \cite{Bock:2004xz}. % The covariance matrices of all bins in the presence (absence) of the LQ signal, $V^{SB}_{ij}$ ($V^{B}_{ij}$), are calculated as: % \begin{equation} V^{SB}_{ij} = \sum_{k=1}^{N_{\rm sys}} \sigma^{SB}_{ki} \sigma^{SB}_{kj} \,\,\,\,\,\, \text {and} \,\,\,\,\,\, V^{B}_{ij} = \sum_{k=1}^{N_{\rm sys}} \sigma^{B}_{ki} \sigma^{B}_{kj}\,, \label{eqn:comat} \end{equation} % where $\sigma^{SB}_{ki}$ ($\sigma^{B}_{ki}$) are the one sigma shifts induced from systematic source $k$ to the number of events expected in bin $i$ in the presence (absence) of the LQ signal. % The sums run over the $N_{\rm sys}$ sources of systematic uncertainty. % In the case of negligible systematic uncertainties, equation~\ref{eqn:comat} is equivalent to the weight definition used in~\cite{Aktas:2005pr}. %%% Limits are obtained by performing a frequentist analysis of the test statistic obtained from the data, $X^{\rm data}$. % For each signal hypothesis, a large number of MC experiments, typically ${\cal O}(10^5)$, are generated by varying the expected number of events $s_i+b_i$ within the uncertainties. % Systematic uncertainties are treated as Gaussian distributions and statistical fluctuations are simulated using Poisson statistics. % For each MC experiment $e$, a test statistic $X^e$ is calculated. % A probability $p^{\rm data}$ is calculated as the fraction of MC experiments which have $X^e