Search for Dark Photons from Supersymmetric Hidden Valleys V.M. Abazov,37 B. Abbott,75 M. Abolins,65 B. S. Acharya,30 M. Adams,51 T. Adams,49 E. Aguilo,6 M. Ahsan,59 G.D. Alexeev,37 G. Alkhazov,41 A. Alton,64,* G. Alverson,63 G. A. Alves,2 L. S. Ancu,36 T. Andeen,53 M. S. Anzelc,53 M. Aoki,50 Y. Arnoud,14 M. Arov,60 M. Arthaud,18 A. Askew,49,† B. Åsman,42 O. Atramentov,49,† C. Avila,8 J. BackusMayes,82 F. Badaud,13 L. Bagby,50 B. Baldin,50 D.V. Bandurin,59 S. Banerjee,30 E. Barberis,63 A.-F. Barfuss,15 P. Bargassa,80 P. Baringer,58 J. Barreto,2 J. F. Bartlett,50 U. Bassler,18 D. Bauer,44 S. Beale,6 A. Bean,58 M. Begalli,3 M. Begel,73 C. Belanger-Champagne,42 L. Bellantoni,50 A. Bellavance,50 J. A. Benitez,65 S. B. Beri,28 G. Bernardi,17 R. Bernhard,23 I. Bertram,43 M. Besançon,18 R. Beuselinck,44 V.A. Bezzubov,40 P. C. Bhat,50 V. Bhatnagar,28 G. Blazey,52 S. Blessing,49 K. Bloom,67 A. Boehnlein,50 D. Boline,62 T. A. Bolton,59 E. E. Boos,39 G. Borissov,43 T. Bose,62 A. Brandt,78 R. Brock,65 G. Brooijmans,70 A. Bross,50 D. Brown,19 X. B. Bu,7 D. Buchholz,53 M. Buehler,81 V. Buescher,22 V. Bunichev,39 S. Burdin,43,‡ T. H. Burnett,82 C. P. Buszello,44 P. Calfayan,26 B. Calpas,15 S. Calvet,16 J. Cammin,71 M.A. Carrasco-Lizarraga,34 E. Carrera,49 W. Carvalho,3 B. C. K. Casey,50 H. Castilla-Valdez,34 S. Chakrabarti,72 D. Chakraborty,52 K.M. Chan,55 A. Chandra,48 E. Cheu,46 D. K. Cho,62 S. Choi,33 B. Choudhary,29 T. Christoudias,44 S. Cihangir,50 D. Claes,67 J. Clutter,58 M. Cooke,50 W. E. Cooper,50 M. Corcoran,80 F. Couderc,18 M.-C. Cousinou,15 S. Crépé-Renaudin,14 V. Cuplov,59 D. Cutts,77 M. Ćwiok,31 A. Das,46 G. Davies,44 K. De,78 S. J. de Jong,36 E. De La Cruz-Burelo,34 K. DeVaughan,67 F. Déliot,18 M. Demarteau,50 R. Demina,71 D. Denisov,50 S. P. Denisov,40 S. Desai,50 H. T. Diehl,50 M. Diesburg,50 A. Dominguez,67 T. Dorland,82 A. Dubey,29 L. V. Dudko,39 L. Duflot,16 D. Duggan,49 A. Duperrin,15 S. Dutt,28 A. Dyshkant,52 M. Eads,67 D. Edmunds,65 J. Ellison,48 V. D. Elvira,50 Y. Enari,77 S. Eno,61 P. Ermolov,39,‡‡ M. Escalier,15 H. Evans,54 A. Evdokimov,73 V.N. Evdokimov,40 G. Facini,63 A. V. Ferapontov,59 T. Ferbel,61,71 F. Fiedler,25 F. Filthaut,36 W. Fisher,50 H. E. Fisk,50 M. Fortner,52 H. Fox,43 S. Fu,50 S. Fuess,50 T. Gadfort,70 C. F. Galea,36 A. Garcia-Bellido,71 V. Gavrilov,38 P. Gay,13 W. Geist,19 W. Geng,15,65 C. E. Gerber,51 Y. Gershtein,49,† D. Gillberg,6 G. Ginther,50,71 B. Gómez,8 A. Goussiou,82 P. D. Grannis,72 S. Greder,19 H. Greenlee,50 Z. D. Greenwood,60 E.M. Gregores,4 G. Grenier,20 Ph. Gris,13 J.-F. Grivaz,16 A. Grohsjean,26 S. Grünendahl,50 M.W. Grünewald,31 F. Guo,72 J. Guo,72 G. Gutierrez,50 P. Gutierrez,75 A. Haas,70 N. J. Hadley,61 P. Haefner,26 S. Hagopian,49 J. Haley,68 I. Hall,65 R. E. Hall,47 L. Han,7 K. Harder,45 A. Harel,71 J.M. Hauptman,57 J. Hays,44 T. Hebbeker,21 D. Hedin,52 J. G. Hegeman,35 A. P. Heinson,48 U. Heintz,62 C. Hensel,24 I. Heredia-De La Cruz,34 K. Herner,64 G. Hesketh,63 M.D. Hildreth,55 R. Hirosky,81 T. Hoang,49 J. D. Hobbs,72 B. Hoeneisen,12 M. Hohlfeld,22 S. Hossain,75 P. Houben,35 Y. Hu,72 Z. Hubacek,10 N. Huske,17 V. Hynek,10 I. Iashvili,69 R. Illingworth,50 A. S. Ito,50 S. Jabeen,62 M. Jaffré,16 S. Jain,75 K. Jakobs,23 D. Jamin,15 C. Jarvis,61 R. Jesik,44 K. Johns,46 C. Johnson,70 M. Johnson,50 D. Johnston,67 A. Jonckheere,50 P. Jonsson,44 A. Juste,50 E. Kajfasz,15 D. Karmanov,39 P. A. Kasper,50 I. Katsanos,67 V. Kaushik,78 R. Kehoe,79 S. Kermiche,15 N. Khalatyan,50 A. Khanov,76 A. Kharchilava,69 Y. N. Kharzheev,37 D. Khatidze,70 T. J. Kim,32 M.H. Kirby,53 M. Kirsch,21 B. Klima,50 J.M. Kohli,28 J.-P. Konrath,23 A.V. Kozelov,40 J. Kraus,65 T. Kuhl,25 A. Kumar,69 A. Kupco,11 T. Kurča,20 V. A. Kuzmin,39 J. Kvita,9 F. Lacroix,13 D. Lam,55 S. Lammers,54 G. Landsberg,77 P. Lebrun,20 W.M. Lee,50 A. Leflat,39 J. Lellouch,17 J. Li,78,‡‡ L. Li,48 Q. Z. Li,50 S.M. Lietti,5 J. K. Lim,32 D. Lincoln,50 J. Linnemann,65 V.V. Lipaev,40 R. Lipton,50 Y. Liu,7 Z. Liu,6 A. Lobodenko,41 M. Lokajicek,11 P. Love,43 H. J. Lubatti,82 R. Luna-Garcia,34,xA.L. Lyon,50 A.K.A. Maciel,2 D. Mackin,80 P. Mättig,27 A. Magerkurth,64 P. K. Mal,82 H. B. Malbouisson,3 S. Malik,67 V. L. Malyshev,37 Y. Maravin,59 B. Martin,14 R. McCarthy,72 C. L. McGivern,58 M.M. Meijer,36 A. Melnitchouk,66 L. Mendoza,8 D. Menezes,52 P. G. Mercadante,5 M. Merkin,39 K.W. Merritt,50 A. Meyer,21 J. Meyer,24 J. Mitrevski,70 R.K. Mommsen,45 N.K. Mondal,30 R.W. Moore,6 T. Moulik,58 G. S. Muanza,15 M. Mulhearn,70 O. Mundal,22 L. Mundim,3 E. Nagy,15 M. Naimuddin,50 M. Narain,77 H. A. Neal,64 J. P. Negret,8 P. Neustroev,41 H. Nilsen,23 H. Nogima,3 S. F. Novaes,5 T. Nunnemann,26 G. Obrant,41 C. Ochando,16 D. Onoprienko,59 J. Orduna,34 N. Oshima,50 N. Osman,44 J. Osta,55 R. Otec,10 G. J. Otero y Garzón,1 M. Owen,45 M. Padilla,48 P. Padley,80 M. Pangilinan,77 N. Parashar,56 S.-J. Park,24 S. K. Park,32 J. Parsons,70 R. Partridge,77 N. Parua,54 A. Patwa,73 G. Pawloski,80 B. Penning,23 M. Perfilov,39 K. Peters,45 Y. Peters,45 P. Pétroff,16 R. Piegaia,1 J. Piper,65 M.-A. Pleier,22 P. L.M. Podesta-Lerma,34,k V.M. Podstavkov,50 Y. Pogorelov,55 M.-E. Pol,2 P. Polozov,38 A.V. Popov,40 C. Potter,6 W. L. Prado da Silva,3 S. Protopopescu,73 J. Qian,64 A. Quadt,24 B. Quinn,66 A. Rakitine,43 M. S. Rangel,16 K. Ranjan,29 P. N. Ratoff,43 P. Renkel,79 P. Rich,45 M. Rijssenbeek,72 I. Ripp-Baudot,19 F. Rizatdinova,76 S. Robinson,44 R. F. Rodrigues,3 M. Rominsky,75 C. Royon,18 P. Rubinov,50 R. Ruchti,55 G. Safronov,38 G. Sajot,14 A. Sánchez-Hernández,34 M. P. Sanders,17 B. Sanghi,50 G. Savage,50 L. Sawyer,60 T. Scanlon,44 D. Schaile,26 R.D. Schamberger,72 Y. Scheglov,41 PRL 103, 081802 (2009) P HY S I CA L R EV I EW LE T T E R S week ending 21 AUGUST 2009 0031-9007=09=103(8)=081802(7) 081802-1 � 2009 The American Physical Society H. Schellman,53 T. Schliephake,27 S. Schlobohm,82 C. Schwanenberger,45 R. Schwienhorst,65 J. Sekaric,49 H. Severini,75 E. Shabalina,24 M. Shamim,59 V. Shary,18 A.A. Shchukin,40 R. K. Shivpuri,29 V. Siccardi,19 V. Simak,10 V. Sirotenko,50 P. Skubic,75 P. Slattery,71 D. Smirnov,55 G. R. Snow,67 J. Snow,74 S. Snyder,73 S. Söldner-Rembold,45 L. Sonnenschein,21 A. Sopczak,43 M. Sosebee,78 K. Soustruznik,9 B. Spurlock,78 J. Stark,14 V. Stolin,38 D.A. Stoyanova,40 J. Strandberg,64 S. Strandberg,42 M.A. Strang,69 E. Strauss,72 M. Strauss,75 R. Ströhmer,26 D. Strom,53 L. Stutte,50 S. Sumowidagdo,49 P. Svoisky,36 M. Takahashi,45 A. Tanasijczuk,1 W. Taylor,6 B. Tiller,26 F. Tissandier,13 M. Titov,18 V. V. Tokmenin,37 I. Torchiani,23 D. Tsybychev,72 B. Tuchming,18 C. Tully,68 P.M. Tuts,70 R. Unalan,65 L. Uvarov,41 S. Uvarov,41 S. Uzunyan,52 B. Vachon,6 P. J. van den Berg,35 R. Van Kooten,54 W.M. van Leeuwen,35 N. Varelas,51 E.W. Varnes,46 I. A. Vasilyev,40 P. Verdier,20 L. S. Vertogradov,37 M. Verzocchi,50 D. Vilanova,18 P. Vint,44 P. Vokac,10 M. Voutilainen,67,{ R. Wagner,68 H.D. Wahl,49 M.H. L. S. Wang,71 J. Warchol,55 G. Watts,82 M. Wayne,55 G. Weber,25 M. Weber,50,** L. Welty-Rieger,54 A. Wenger,23,†† M. Wetstein,61 A. White,78 D. Wicke,25 M.R. J. Williams,43 G.W. Wilson,58 S. J. Wimpenny,48 M.Wobisch,60 D. R. Wood,63 T. R. Wyatt,45 Y. Xie,77 C. Xu,64 S. Yacoob,53 R. Yamada,50 W.-C. Yang,45 T. Yasuda,50 Y. A. Yatsunenko,37 Z. Ye,50 H. Yin,7 K. Yip,73 H.D. Yoo,77 S.W. Youn,53 J. Yu,78 C. Zeitnitz,27 S. Zelitch,81 T. Zhao,82 B. Zhou,64 J. Zhu,72 M. Zielinski,71 D. Zieminska,54 L. Zivkovic,70 V. Zutshi,52 and E.G. Zverev39 (D0 Collaboration) 1Universidad de Buenos Aires, Buenos Aires, Argentina 2LAFEX, Centro Brasileiro de Pesquisas Fı́sicas, Rio de Janeiro, Brazil 3Universidade do Estado do Rio de Janeiro, Rio de Janeiro, Brazil 4Universidade Federal do ABC, Santo André, Brazil 5Instituto de Fı́sica Teórica, Universidade Estadual Paulista, São Paulo, Brazil 6University of Alberta, Edmonton, Alberta, Canada; Simon Fraser University, Burnaby, British Columbia, Canada; York University, Toronto, Ontario, Canada and McGill University, Montreal, Quebec, Canada 7University of Science and Technology of China, Hefei, People’s Republic of China 8Universidad de los Andes, Bogotá, Colombia 9Center for Particle Physics, Charles University, Faculty of Mathematics and Physics, Prague, Czech Republic 10Czech Technical University in Prague, Prague, Czech Republic 11Center for Particle Physics, Institute of Physics, Academy of Sciences of the Czech Republic, Prague, Czech Republic 12Universidad San Francisco de Quito, Quito, Ecuador 13LPC, Université Blaise Pascal, CNRS/IN2P3, Clermont, France 14LPSC, Université Joseph Fourier Grenoble 1, CNRS/IN2P3, Institut National Polytechnique de Grenoble, Grenoble, France 15CPPM, Aix-Marseille Université, CNRS/IN2P3, Marseille, France 16LAL, Université Paris-Sud, IN2P3/CNRS, Orsay, France 17LPNHE, IN2P3/CNRS, Universités Paris VI and VII, Paris, France 18CEA, Irfu, SPP, Saclay, France 19IPHC, Université de Strasbourg, CNRS/IN2P3, Strasbourg, France 20IPNL, Université Lyon 1, CNRS/IN2P3, Villeurbanne, France and Université de Lyon, Lyon, France 21III. Physikalisches Institut A, RWTH Aachen University, Aachen, Germany 22Physikalisches Institut, Universität Bonn, Bonn, Germany 23Physikalisches Institut, Universität Freiburg, Freiburg, Germany 24II. Physikalisches Institut, Georg-August-Universität Göttingen, Göttingen, Germany 25Institut für Physik, Universität Mainz, Mainz, Germany 26Ludwig-Maximilians-Universität München, München, Germany 27Fachbereich Physik, University of Wuppertal, Wuppertal, Germany 28Panjab University, Chandigarh, India 29Delhi University, Delhi, India 30Tata Institute of Fundamental Research, Mumbai, India 31University College Dublin, Dublin, Ireland 32Korea Detector Laboratory, Korea University, Seoul, Korea 33SungKyunKwan University, Suwon, Korea 34CINVESTAV, Mexico City, Mexico 35FOM-Institute NIKHEF and University of Amsterdam/NIKHEF, Amsterdam, The Netherlands 36Radboud University Nijmegen/NIKHEF, Nijmegen, The Netherlands 37Joint Institute for Nuclear Research, Dubna, Russia PRL 103, 081802 (2009) P HY S I CA L R EV I EW LE T T E R S week ending 21 AUGUST 2009 081802-2 38Institute for Theoretical and Experimental Physics, Moscow, Russia 39Moscow State University, Moscow, Russia 40Institute for High Energy Physics, Protvino, Russia 41Petersburg Nuclear Physics Institute, St. Petersburg, Russia 42Stockholm University, Stockholm, Sweden, and Uppsala University, Uppsala, Sweden 43Lancaster University, Lancaster, United Kingdom 44Imperial College, London, United Kingdom 45University of Manchester, Manchester, United Kingdom 46University of Arizona, Tucson, Arizona 85721, USA 47California State University, Fresno, California 93740, USA 48University of California, Riverside, California 92521, USA 49Florida State University, Tallahassee, Florida 32306, USA 50Fermi National Accelerator Laboratory, Batavia, Illinois 60510, USA 51University of Illinois at Chicago, Chicago, Illinois 60607, USA 52Northern Illinois University, DeKalb, Illinois 60115, USA 53Northwestern University, Evanston, Illinois 60208, USA 54Indiana University, Bloomington, Indiana 47405, USA 55University of Notre Dame, Notre Dame, Indiana 46556, USA 56Purdue University Calumet, Hammond, Indiana 46323, USA 57Iowa State University, Ames, Iowa 50011, USA 58University of Kansas, Lawrence, Kansas 66045, USA 59Kansas State University, Manhattan, Kansas 66506, USA 60Louisiana Tech University, Ruston, Louisiana 71272, USA 61University of Maryland, College Park, Maryland 20742, USA 62Boston University, Boston, Massachusetts 02215, USA 63Northeastern University, Boston, Massachusetts 02115, USA 64University of Michigan, Ann Arbor, Michigan 48109, USA 65Michigan State University, East Lansing, Michigan 48824, USA 66University of Mississippi, University, Mississippi 38677, USA 67University of Nebraska, Lincoln, Nebraska 68588, USA 68Princeton University, Princeton, New Jersey 08544, USA 69State University of New York, Buffalo, New York 14260, USA 70Columbia University, New York, New York 10027, USA 71University of Rochester, Rochester, New York 14627, USA 72State University of New York, Stony Brook, New York 11794, USA 73Brookhaven National Laboratory, Upton, New York 11973, USA 74Langston University, Langston, Oklahoma 73050, USA 75University of Oklahoma, Norman, Oklahoma 73019, USA 76Oklahoma State University, Stillwater, Oklahoma 74078, USA 77Brown University, Providence, Rhode Island 02912, USA 78University of Texas, Arlington, Texas 76019, USA 79Southern Methodist University, Dallas, Texas 75275, USA 80Rice University, Houston, Texas 77005, USA 81University of Virginia, Charlottesville, Virginia 22901, USA 82University of Washington, Seattle, Washington 98195, USA (Received 11 May 2009; published 17 August 2009) We search for a new light gauge boson, a dark photon, with the D0 experiment. In the model we consider, supersymmetric partners are pair produced and cascade to the lightest neutralinos that can decay into the hidden sector state plus either a photon or a dark photon. The dark photon decays through its mixing with a photon into fermion pairs. We therefore investigate a previously unexplored final state that contains a photon, two spatially close leptons, and large missing transverse energy. We do not observe any evidence for dark photons and set a limit on their production. DOI: 10.1103/PhysRevLett.103.081802 PACS numbers: 14.80.Ly, 12.60.Jv, 13.85.Qk, 95.35.+d Hidden valley models [1] introduce a new hidden sector, which is very weakly coupled to the standard model (SM) particles, and therefore can easily escape detection. An important subset of hidden valley models also contain supersymmetry (SUSY), a fundamental symmetry between fermions and bosons postulating the existence of SUSY partners. At colliders, in the case of R-parity conservation [2], superpartners are produced in pairs and decay to the PRL 103, 081802 (2009) P HY S I CA L R EV I EW LE T T E R S week ending 21 AUGUST 2009 081802-3 http://dx.doi.org/10.1103/PhysRevLett.103.081802 SM particles and the lightest superpartner (LSP). The LSP is a stable, weakly interacting particle, and cannot be detected in collider detectors. Recently, these models were called upon to explain the results of several cosmic ray detection experiments [3,4]. Taken together with other experiments, including new results from Fermi/LAT [5], there is evidence of an excess of high energy positrons and no excessive production of antiprotons or photons. The excess can be attributed [6] to the dark matter particles annihilating into pairs of new light gauge bosons, dark photons, which are force carriers in the hidden sector. The dark photon mass cannot be much larger than 1 GeV to give rise to Sommerfeld enhancement [7] of the dark matter annihilation cross section, and not to decay into neutral pions and/or baryons. The masses of the hidden sector states are also around 1 GeV, with mass splitting around MeV, thus providing a possible explanation of the DAMA experiment [8] signal through ‘‘inelastic dark mat- ter’’ scenarios. Dark photons decay through mixing with photons into SM fermions with branching fractions that can be calculated from the measurements [9] of R ¼ �ðeþe� ! hadronsÞ=�ðeþe� ! �þ��Þ, and strongly depend on the dark photon mass. For dark photon masses (m�D ) below the dimuon threshold of ’ 200 MeV, only decays into electrons are possible. For m�D ’ 0:5 GeV the decay rates into electrons and muons are approximately 40% each. The lowest value of the leptonic branching (3.7%) occurs if the dark photon mass is accidentally equal to that of the � meson. In this Letter we will follow the phenomenological scenario developed in [10]. A diagram of a possible pro- cess at the Fermilab Tevatron Collider is shown in Fig. 1. Gauginos are pair produced and decay into SM particles and the lightest neutral gaugino (neutralino, ~�0 1), which in turn decays with comparable branching ratios into either a hidden sector dark neutralino ~X (which is the LSP), and a photon, or into dark neutralino and a dark photon (�D). Hadronic dark photon decays are overwhelmed by SM jet backgrounds. Thus, we only consider dark photon decays into isolated electron or muon pairs. Both dark neutralinos escape detection and result in large missing transverse energy (E6 T). The branching fraction of the neutralino into the dark photon, B ¼ Brð~�0 1 ! �D ~XÞ, is a free pa- rameter of the model. If it is small, the decays into a photon dominate, and signature is the same as of SUSY with gauge-mediated breaking [11] with the neutralino as next-to-lightest superpartner (NLSP). Larger values of B give rise to events where one of the two neutralinos decays into a dark photon, resulting in a final state with one photon, two spatially close (and therefore not satisfying traditional isolation requirements) leptons and large E6 T . This Letter describes a search for this, so far unexplored, final state in p �p collisions at a center of mass energy of 1.96 TeV recorded by the D0 detector [12] at the Fermilab Tevatron Collider. As is described below, our search is optimized for low dark photon masses, m�D < 2:5 GeV. We consider prompt dark photon decays. Although the experimental analysis is sensitive to macroscopic lifetimes, the expectation is that the neutralino decays to a photon are negligible for small couplings between photon and a dark photon [10], making this channel unfavorable for searches for long-lived dark photons. Another theoretical scenario is the case where the neutralino decays into a hidden state ~Y with somewhat higher mass than the dark neutralino. The ~Y may cascade down to the dark neutralino through other hidden states which may be long-lived and can result in the emission of highly collimated low energy SM particles, some of which could be leptons. Most of the energy of the ~Y will stay in the hidden sector and therefore the high E6 T should not be substantially reduced. This analysis is also sensitive to another possible scenario, proposed in [13], in which a light axion that decays into muon pairs takes the place of the dark photon in the decays described above. Data for this analysis correspond to an integrated lumi- nosity of 4:1 fb�1 after application of data quality and trigger requirements. Events must satisfy a set of high transverse energy (ET), single electromagnetic (EM) clus- ter triggers which are fully efficient for photons with ET > 30 GeV. EM clusters are selected from calorimeter clusters, built using the simple cone algorithm of radius R ¼ ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffið��Þ2 þ ð��Þ2p ¼ 0:4 [14], by requiring that the fraction of the energy deposited in the EM section of the calorime- ter, EMfrac, is above 95% and the calorimeter isolation variable I ¼ ½Etotð0:4Þ � EEMð0:2Þ�=EEMð0:2Þ is less than 0.2, where Etotð0:4Þ is the total energy in a cone of radius R ¼ 0:4, corrected for the underlying event con- tribution, and EEMð0:2Þ is the EM energy in a cone of radius R ¼ 0:2, which is taken to be the EM cluster energy. Photon candidates are selected from central calorimeter (j�j< 1:1) EM clusters which have (i) EMfrac > 97%, FIG. 1 (color online). One of the diagrams giving rise to the events with a photon, dark photon (�D), and large missing energy due to escaping dark neutralinos ( ~X) at the Fermilab Tevatron Collider. PRL 103, 081802 (2009) P HY S I CA L R EV I EW LE T T E R S week ending 21 AUGUST 2009 081802-4 (ii) I < 0:07, (iii) a shower shape consistent with that of a photon, and (iv) the scalar sum of the transverse momenta (pT) of all tracks originating from the primary vertex in an annulus 0:05 30 GeV and E6 T > 20 GeV (E6 T is com- puted using all calorimeter cells and corrected for EM and jet energy scales). Dark photon candidates are formed by selecting pairs of oppositely charged spatially close (R< 0:2) tracks that originate from the same point (j�zj< 2 cm) along the beam line. The leading (trailing) track pT is required to exceed 10 (5) GeV, which is highly efficient for the signal and suppresses the multijet back- ground. We then require the scalar sum of pT of all tracks excluding the pair in a cone of radius 0.4 centered on the pair momentum direction to be less than 2 GeV. To further reduce the multijet background we require that each track must have its azimuthal angle not aligned with a photon, 0:4<���;track < 2:74. In rare cases, when there is more than one such pair in the event, we select the one with the highest trailing track pT . For a dark photon decaying into a pair of electrons, the calorimeter depositions overlap, so we require that the dark photon candidate matches an EM cluster with ET > 10 GeV, EMfrac > 97%, and I < 0:1. For the dimuon de- cay mode, we require that at least one of the tracks is matched to a reconstructed muon, and the energy deposited in the calorimeter in the annulus 0:1 20 GeV. All three have contributions from B1, although the relative fraction of multijets, single photon production, and diphoton produc- tion varies among the three control samples. Back- grounds B2 and B3, however, can only significantly con- tribute to the QCDW sample. We observe no difference between the dark photon candidate mass distributions in the three control samples. We therefore conclude that the background to dark photon production is dominated by B1, and use the average shape of the dark photon candidates mass distributions in all control samples as our background model. The dark photon candidate invariant mass distributions in the signal sample are shown in Fig. 2 separately for the electron and muon channels, together with the expected contribution from dark photons with a mass of 1.4 GeV. We see no evidence of a dark photon signal and proceed to set limits on its production. To set limits we use the standard D0 likelihood fitter [23] that incorporates a log- -1 4.1 fb∅D -µ+µ (a) (GeV) D γm 0 0.5 1 1.5 2 2.5 E ve nt s / 0 .2 G eV 0 5 10 15 20 25 30 35 -1 4.1 fb∅D -e+e (b) (GeV) D γm 0 0.5 1 1.5 2 2.5 E ve nt s / 0 .2 G eV 0 5 10 15 20 25 30 35 FIG. 2 (color online). Observed mass distributions in the sig- nal region are represented as points with error bars, the back- ground estimation is shown as filled band, and an example signal for m�D ¼ 1:4 GeV plus background is shown as the solid histogram for the dimuon channel (a) and the dielectron channel (b). PRL 103, 081802 (2009) P HY S I CA L R EV I EW LE T T E R S week ending 21 AUGUST 2009 081802-5 likelihood ratio (LLR) statistic method [24]. The value of CLs is defined as CLs ¼ CLsþb=CLb, where CLsþb and CLb are the confidence levels for the signal plus back- ground hypothesis and the background-only (null) hy- pothesis, respectively. These confidence levels are evaluated by integrating the corresponding LLR distribu- tion populated by simulating outcomes via Poisson statis- tics. Systematic uncertainties are treated as uncertainties on the expected number of signal and background events, not the outcomes of the limit calculations. This approach ensures that the uncertainties and their correlations are propagated to the outcome with their proper weights. The limit is set by simultaneously fitting dilepton invariant mass distributions in data for the muon and electron chan- nels to the signal and background predictions for each signal point, defined by the dark photon and the lightest chargino masses. For each dark photon mass the back- ground is normalized outside of the expected signal region. The systematic uncertainty on the signal reconstruction efficiency (25%) is dominated by the uncertainty to recon- struct the two spatially close tracks from the dark photon decays (20%). This efficiency varies from 70% to 95% depending on the opening angle between the tracks, and was validated with data using tau decays and converted photons in radiative Z ! ��� decays. We also took into account the uncertainty on the total integrated luminosity (6.1%) and the effect of varying the dark photon mass resolution by 10%. We interpret the cross section limits as limits on the lightest chargino mass as a function of the dark photon mass and the neutralino branching fraction. For B ¼ 0:5 the excluded region of chargino and dark photon masses is shown in Fig. 3. The difference between the observed and expected limits never exceeds 2 standard deviations over the whole dark photon mass range. In Fig. 4 we display the chargino mass limit as a function of B for three represen- tative dark photon masses: 0.2 GeV (only the electron channel is open), 0.782 GeV (low branching fraction into leptons due to! and mesons), and 1.5 GeV. Our previous limit on the SUSY in the diphoton final state [21] is directly applicable to the model considered in this Letter, although it does not probe the dark photon mass. The corresponding exclusion contours are shown in Figs. 3 and 4. To summarize, we search for a previously unexplored final state consisting of a photon, two spatially close lep- tons from hypothetical dark photon decays and large miss- ing energy. We find no evidence for such events, and set limits on their production in a benchmark model [10]. For dark photon masses of 0.2, 0.782, and 1.5 GeV we exclude chargino masses of 230, 142, and 200 GeV, respectively. We would like to thank Scott Thomas and David Shih for many inspiring discussions and help with the signal simulation. We thank the staffs at Fermilab and collaborat- ing institutions, and acknowledge support from the DOE and NSF (U.S.); CEA and CNRS/IN2P3 (France); FASI, Rosatom, and RFBR (Russia); CNPq, FAPERJ, FAPESP, and FUNDUNESP (Brazil); DAE and DST (India); Colciencias (Colombia); CONACyT (Mexico); KRF and KOSEF (Korea); CONICET and UBACyT (Argentina); FOM (The Netherlands); STFC and the Royal Society (United Kingdom); MSMT and GACR (Czech Republic); CRC Program, CFI, NSERC, and WestGrid Project (Canada); BMBF and DFG (Germany); SFI (Ireland); The Swedish Research Council (Sweden); CAS and CNSF (China); and the Alexander von Humboldt Foundation (Germany). *Visitor from Augustana College, Sioux Falls, SD, USA. †Visitor from Rutgers University, Piscataway, NJ, USA. Excluded region Expected exclusion Diphoton search exclusion -1 4.1 fb∅D chargino mass (GeV) 120 140 160 180 200 220 240 260 280 300 320 ( G eV ) Dγ m 0.5 1 1.5 2 2.5 FIG. 3 (color online). The excluded region of possible masses of the lightest chargino and the dark photon for B ¼ 0:5 are shown as the shaded region. The expected limit is illustrated as the dash-dotted line. The vertical black line corresponds to the exclusion from the diphoton search [21]. )X ~γ→0 1 χBr( 0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1 ch ar gi no m as s (G eV ) 140 160 180 200 220 240 260 280 = 0.2 GeV D γm = 0.782 GeV D γm = 1.5 GeV D γm diphoton search exclusion -1 4.1 fb∅D FIG. 4 (color online). The dependence of the excluded char- gino masses on the branching ratio of the neutralino into a photon are given for dark photon masses of 0.2, 0.782, and 1.5 GeV. The black contour corresponds to the exclusion from the diphoton search [21]. 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