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F(\T(O3 I SJ Ix LAL 86/11 DES* B6-050 F(\t(o3 i SJ ix LAL 86/11 DES* B6-050 A SEARCH FOR SINGLE PHOTONS AT FETRA by CELLO Collaboration Suimitted to Phyiia LeiteAi U.E.R Inlilal Nttnnal 4t Pkyaeat Nudtairi •I I'Univifiitt Palis-Sud tt PhyiitjM in P>rlkiihs Bitlment 200 - 9M05 ORSAY Cedex LAL 06/11 OESY 86-050 A search for single photons at PETRA by CELLO Collaboration H.-J. Behrend, J. Burger, L. Criegee, H. Fenner, J.H- Field, G. Franke, J. Fuster , Y. Holler, J. Meyer, V. Schroder, H. Sindt, U. Timm, G.G. Winter, W. Zimmermann Deutsches Elektronen-Synchrotron, DESY, Haaburg, Germany P.J. Bussey, C. Buttar, A.J. Campbell, J.B. Dainton, D. Hendry, G. HcCurrach, J.M. Scarr, 1.0. Skillicorn, K.M. Smith University of Glasgow, United Kingdoa V. Blobel, M. Feindt, M. Poppe, H. Spitzes II. Institut fur Experinentalphysik, Universitat Haaburg, Geraany W.-O. Anel, A. Bohrer, J. Engler, G. Fliigge, D.C. Fries, W. Fues ', K. Gamerdinger, P. Grosse-Wiesmann , J. Hansmeyer, Th. Menkes, G. Hopp, H. Jung, J. Knapp, H. Krù'ger, H. KUster, P. Mayer, H. Miiller, K.H. Ranitzsch, H. Schneider, J. Wolf Kernforschungszentrua Karlsruhe and Universitat KarlsrKhe, Geraany W. de Boer, G. Buschhorn, W. Christiansen, G. Grindhammer, B. Gundtrson, Ch. Kiesling, R. Kotthaus, H. Kroha, D. Ltiers, H. Oberlack, B. Sack, P. Schacht, G. Shooshtari, W. Wiedenmann Max-Planck-lnstitut fiir Physik und Astrophysik, NUnchen, Geraany A. Cordier, H. Davier, 0. Fournier, M. Gai1 Iard, .J.F. Grivaz, J. Haissinski, V. Journé, F. Le Diberder, E. Ros ', A. Spadafora, J.-J. Veillet Laboratoire de l'Accélérateur Linéaire, Orsay, France S * B. Fatah , R. George, M. Goldberg, 0. Hamon, F. Kapusta, F. Kovacs, L. Poggioli, M. Rivoal Laboratoire de Physique Nucléaire et Hautes Energies, Université de Paris, France G. d'Agostini, F. Ferrarotto, H. Gaspero, B. Stella University of Rome and INFN, Italy R. Aleksan, G. Cozzika. V. Ducros, Y. Lavagne, F. Ould Saada, J. Pamela, F. Pierre, J. Zacek ' Centre d'Etudes Nucléaires, Saclay, France G. Alexander, G. Bella, Y. Gnat, J. Grunhaus, A. Levy Tel Aviv University, Israel 1) Or leave of absence from inst. de Fisica Corpuscular, Universidad de Valencia, Spain 2) Now at SCS, Hamburg, Germany 3) No* at Stanford Linear Accelerator Center, USA 4} Nuw at Universidad -Tutônoma de Madrid, Spain 5) Now at Univ'sity oi Sebha, Physics Department, Libya 6) On leave of absence from Nuclear Center, Charles University, Prague, Czechoslovakia A search for single photons at PETRA CELLO Collaboration Abstract : A search for single photons, produced in e'e" collisions together vith particles interacting only weakly with matter, has been performed using the CELLO detector operating at the PETRA 3torage ring. From the absence of any signal, an upper limit i3 set at 15 (90V. CD on the number of light neutrino species, and lower limits on various super3ummetric particle masses are derived. For massle3s photinos, mass degenerate scalar partners of the left and right handed electrons are excluded below 37.7 Gev/c2 (90V. CD. 2 In aupersymmetric theories!)), there is no general prediction for the mass spectrum of the partners of the ordinary particles. Because of the expected R-parity conservation^!, it is commonly believed that the lightest supersummetric particle (LSP) must be stable, and cosmological arguments lead to the conjecture that it is neutral and colorless|31. Candidate LSP's are therefore the photino y, the zino z or the higgsina h, and in some models a îcalar neutrino ~> ; the gravitino G can also be considered as the LSP. The couplings of these particles are completely fixed, except for possible mixings in the gaugino sector. In Ihis letter, ve will 033ume that the J Is unmixed. The LSP is in all cases expected to interact only weakly with ordinary matter. For instance, >;e scattering has a cross-section orders of magnitude smaller than that of Compton scattering because of the large mass of the exchanged scalar electron, making the photino observable only indirectly (typically by missing momentum) if stable. As a consequence, pair production of LSP's in e'e" collisions can be detected only in association vith an initial state radiated photont4l. This tagging procedure was first suggested[5] in order to count the number of light neutrino species via the reaction e'e'-»)jvv. The cross-section for this reaction readsl6]: dole'e'-^îs) 2 a I dxdy n x(l-y2) where x=2EAJs~, y=cos8 , s=s(l-x) . Here X is any v, v, (i runs from I to N, with N the number of light neutrino species). E is the energy of the tagged photon, and 6 i3 its polar angle with respect to the electron beam direction. From (1), it is evident that an experiment aiming at the study of such a reaction needs good photon defection at low x and at high lyl. In addition, to ensure that only weakly interacting particles are produced in association with the tagged photon, the veto capability of the detector must be extended to the s largest possible solid angle. However, in e'e" storage rings, it is unavoidable that a hole be left for the beam pipe. This causes the reaction e*e'-»e*e";j to be the major experimental background, when both electrons are scattered at small polar angles. If B., is the minimum veto angle, this background is kinemoticolly totally eliminated as soon as Xi>2B,vith x, =xjl-yj ; but in practice, due to the dominance of the virtual Compton scattering configuration where only one electron is deflected, this background remains negligible down to Xj=6,.. For the results reported here, the minimum veto angle is 50 mrad. Thic means that, for a typical CM energy of 44 GeV, no yt cut will be needed for single photons with ly|<.83 beyond that implied by a trigger threshold of-Z.5 GeV. The data entering the present analysis were collected in 1984-85 with the CELLO detector [71 operating at the PETRA e'e" storage ring. At fixed CM energies of 38.28, 43.45, 43.60, 44.20 and 46.57 GeV, integrated luminosities of 8.9, 1.4, 17.0, 9.2 and I.I pb*1 respectively were obtained i for a cross-section behaving like (I), this is equivalent to 37.6 pb"1 at J<3>" = 42.6 GeV. Charged particles are detected down to |cos8l=.98 by a system of cylindrical proportional and drift chambers, completed by two planes of proportional chambers placed perpendicularly to the beam in the forward and backward directions (end cap chambers). Electrons and photons are detected and measured in the 20 radiation length (Xa) deep barrel '|co3BI<.86) and end c jp (.92f|eos8l<.99) lead-liquid argon calorimeters. The energy resolution is iE/E=5'/.*l0y..',jEGeV for electromagnetic showers. The fine lateral segmentation of the seven longitudinal samplings provides a resolution of~60mrad on the shower direction, measured in the calorimeter alone without vertex constraint, both in ajimuth and polar angle. This angular resolution is only weakly energy dependent in the domain of interest for thi3 analysis. The acceptance gap between the barrel and end cap calorimeters vas filled m I9S4 by a crude lead-scintillatar sandwich orrou read by 4 wavelength shifters, the 30-called 'hole tagger'. With only two samplings after 4 and 3>!0, the energy resolution is rather poor, but efficient veto capability i3 ensured in this angular range. A rough position measurement is also provided by the eight-fold azimuihal segmentation of this device. This upgrade of the detector was crucial for this analysis, 3ince it allowed the elimination of the large potential background coming from th^ reaction e"e""*SX!J • witn one photon remaining in3ide the beam pipe, another one going into the hole tagger region, and only one detected. Finally, a lead glass arratj closes the acceptance from 130 mrad (lower limit of the angular coverage of the end cap calorimeter) down to 50 mrad. High energy muons can be identified o.i/r 927. of the solid angle in the large drift chambers which surround the detector beyond an 80 cm thick iron absorber. Single photon events were triggered by a total energy deposit in excess of ai^ adjustable threshold in any of the 16 modules of the barrel calorimeter. This threshold (position at 507. of the maximum efficiency) was set ai -3 GeV for the first half of the data (3et A), and decreased to ~2 GeV afterwards (set B). This improvement was made possible by a better rejection of the out of time signals (cosmic3 and electronic noi3e) at the trigger level. This rejection is achieved by sampling the 1.5 ^is-long liquid argon signal at a set of times fixed with respect to the beam crossing (twice in A, four times in e). In the subsequent analysis the known pulse shape is adjusted to this set of measurements, from which one derives the time of initiation of the signal with a resolution of 25 ns for shower3 above ~4 GeV, degrading slowly to 45 ns for 2 GeV showers. A timing cut at '. 2.5 0 is then applied. The trigger efficiency for single photons was determined using events from QED reactions which lead to a single electromagnetic shower in the barrel calorimeter, and which were triggered independently by a large energy deposit in an end cap calorimeter module (QED-I sample). The result is shown in Fig. I for sets A and B separately. The 103s induced by the timing cut wa3 calculated in the 3ame vau.
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