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RECENT RESULTS from the OPAL EXPERIMENT at LEP the OPAL 95 RECENT RESULTS FROM THE OPAL EXPERIMENT AT LEP The OPAL Collaboration, presented by: Marcello Mannelli CERN Geneva, Switzerland Abstract the of 1989, during the first operating period of LEP, the OPAL experiment collected in In fall excess of 1.3 pb-1 integrated luminosity, distributed over 11 center-of-mass energies between 88.28 and 95.04 GeV. This data set allows for constraints to be placed on the Standard Model SM), and possible physics beyond the SM, through accurate measurements of the resonance line-shape zo parameters, as well as through direct searches for new particle production. Studies of the properties( of hadronic decays can be used to test QCD. zo Here we report on a measurement of the mass of the boson, its total width, and its partial zo widths into hadrons and leptons. From the measured total and partial widths we extract a model independent value for the invisible width. The implications of these measurements in the context of the Standard Model are discussed. Next we report on a study of jet production rates in hadronic decays of the and their interpretation in terms of the running of the strong coupling constant zo, Finally we report on litnits placed on new particle production through direct searches. a,. The results presented here are essentially those presented at the New and Exotic Phenomena session of the XXV'th Rencontres de Moriond. Results presented as prelitninary at that time have been updated to final results. Where of interest, the approximate integrated luminosity represented by a given data sample is provided; it should be understood that this represents a luminosity accumulated over the various points of the energy scan, and is only to be treated as indicative. Aknowledgements It is a pleasure to thank the LEP division for the efficient operation of the machine and their continuing cooperation with our experimental group. 96 1 The OPAL detector OPAL is a general purpose e+e- detector. The tracking of charged particles is done by a central detector, consisting of a vertex chamber and large volume jet chamber complemented by Z-chamhers on its outer perimeter. The central detector is enclosed by a magnet coil which provides axial an field for momentum measurement. The magnet coil is surrounded by an array of TOF scintillation counters. The central detector assembly is in turn surrounded by a lead glass calorimeter, with a solid angle coverage of 983 of 4,,.. This is enclosed by an instrumented magnet return yoke, serving as a hadron calorimeter, and by four layers of muon chambers. Small angle ("forward" ) detectors are placed in the forward and backward direction, and are used to measure luminosity. 2 Measurement of the line shape parameters and partial widths zo (Ref.[1]) 2.1 The Luminosity Measurement The integrated luminosity of the colliding beams was determined by measuring small angle Bhabha scattering; in the angular region of interest, this process is insensitive to effects due to the boson. zo The measurement used the forward detector, consisting of two identical hybrid element s placed around the beam pipe at either end of the tracking chambers. Each element of the forwarddetector consists of a cylindrical lead-scintillator sandwich calorimeter divided into 16 azimuthal sectors and segmented longitudinally in a presampler of 4 radiation lengths followedby a main calorimeter of 22 radiation lengths. For well contained 45 Ge V electrons the energy resolution of the calorimeter was measured to be 1.3 Ge V, with 84 of the energy deposited in the % main calorimeter. Light-sharing between adj acent segments and inner and outer readouts of the main calorimeter was used to determine position of the showers centroids. The polar angle resolution varied between 1 and 10 mrad, being best near the inner edge of the calorimeter, while the resolution in azimuth varied between 3.5 and 35 mrad, being best at the sector boundaries. The acceptance of the calorimeter extends from 39 to 150 mrad, and is essentially complete in azimuth. A set of proportional tube chambers placed between the presampler and main sections of the calorimeter provides another measure of the position of the shower centroid, with a resolution of 1.35 mrad in both polar and azimuthal angles. The acceptance of the tube chambers extends from 50 to 135 mrad. The relative luminosity for each energy point of the scan was measured using a sample of events selected with the main calorimeters only. Events were selected in which the average energy of the largest cluster seen in each main calorimeter exceeded 703 of the beam energy. This requirement defined a polar angle acceptance extending from 47 to 142 mrad from the nominal beam axis. Because the energy cut was imposed on the average of the energies in the two calorimeters, the acceptance was largely independent of beam displacements and the size of the beam intersection region. To reduce background due to off-momentum beam particles, the difference, 6¢, in the azimuthal angles of the largest cluster in each calorimeter was required to he in the range 160° 200°. A total < 6.<f; < of 58,124 events were selected by these criteria, with a background contamination of less than 0.13. The energy calibration of the main calorimeters could be maintained to within 0.53 over the entire period of data taking. As a consequence, the cross section for luminosity events defined by these requirements was stable with time to within 0.83. To obtain the absolute luminosity calibration a fiducial region was defined well within the tube chamber and calorimeter acceptance, extending from 58 to 124 mrad in polar angle, and excluding angular regions within 10 degrees of the horizontal and vertical planes. Events were selected by requiring that the average of the angles measured by the tube chamb ers on each side of the event lie within this fiducial region. To reject background the requirement 160° 200° was made, using < 6.<f;< the shower centroid coordinates measured by the tube chambers. Finally, the average of the energies (sum of the presampler and main) of the largest cluster in each side of the event was required to 97 be larger than 2/3 of the beam energy. For a sample which included 703 of the data taking period 17,379 events were selected by these criteria. Taking into account the systematic uncertainty in the acceptance of this selection method, this sample provides an absolute luminosity calibration with a total uncertainty of 2.23. The beam energy dependent systematic error of the integrated luminosity was estimated to be 13 and was taken into account as a point-to-point systematic error in the cross section measurements. 2.2 Hadronic and Leptonic Event Selection The event selection for hadronic decays used mainly energy clusters in the electromagnetic zo calorimeters. For this analysis, clusters in the barrel region were required to have an energy of more than 100 Me V, while clusters in the endcap region were required to have an energy of more than 200 MeV deposited over at least two adjacent lead glass blocks. The following requirements defined a multihadron candidate: at least 8 clusters • a total energy deposited in the lead glass of at least 103 of the center-of-mass energy • an total energy imbalance along the beam direction of less than 653 of the total energy de­ o posited in the lead glass The cut on the number of clusters rejected efficiently decays to e+ e- and 7+7- . The cuts on zo energy deposited in the electromagnetic calorimeters and energy imbalance of the event discarded two-photon and beam-gas/wall events, as well as cosmic rays in the end caps. Information from the TOF detector was used to reject cosmic rays in the barrel region. All events with at least 4 TOF counters firing in time were accepted. Events with fewer than 4 TOF counters firing in time, for which at least 503 of the observed energy was deposited in the barrel lead glass were rejected. All remaining events with fewer than 4 TOF counters were visually scanned; this corresponded to less than 23 of all hadronic events. Out of this sample 36 events were rejected as being due to cosmic rays or beam-wall interactions . A total of 25,801 hadronic decays remained zo after all these cuts, corresponding to a useful integrated luminosity of 1.25 pb-1 . An overall trigger and selection acceptance of 97. 7% ±0.8% was estimated for this sample of event s. The main contamination in the hadronic data sample came from7+7- events, and was estimated to be 0.333±0.023; Whereas for multihadron eventJ the full acceptance of the electromagnetic calorimeters was used, for the lepton event samples only the barrel region 0.7) was used: in that region a high < degree of redundancy is afforded in both the triggering and selection of leptonic events. The selection of e+ e- --+ e+ e- events required two(icos(O)I clusters in the barrel lead glass calorimeter, eacli having at least 503 of the beam energy, and satisfying < 0.7; the acolinearity angle between the two clusters had to be less than 5°. Tracking information from the jet chamber, available for 903 of this data set, was used to reject 29 background eventicos(O)Is, due mainly to e+e- --+ e+e-')'and e+ e- events in whicli at least one of the two highest energy clusters was an isolated photon; --+ 'l''l' this was consistent with the expectation of 26 such events in the sample. After these cuts, a sample of 908 e+ events was selected, for an integrated luminosity of about 1.3 pb-1.
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