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Recent Results from the UA2 Experiment U.E.R ÇIL'ISO H 2'J LAL 87-53 Nov. 1987 Recent Results from the UA2 Experiment Jean-Paul REPELLIN Invited talk presented at the International Conference on Physics in Collision VII Tsukuba, Japan, August 25-27,1987 U.E.R raâ^ Institut National de Physique Nucléaire de et l'Université Peris-Sud de Physique des Particules Bâtiment 200 - 91105 ORSAY Cedex LAL 87-53 Nov. 1987 RECENT RESULTS FROM THE UA2 EXPERIMENT Presenied by J.P. REPELLIN Laboratoire de l'Accélérateur Linéaire, Bât 200,91405 Orsay - France Abstract Recent results from the UA2 experiment at the CERN pp collider are presented. The full statistics is used to measure the properties of the Intermediate Vector Bosons and to test the Standard Mode: Searchet for exotic processes are also described. 1. INTRODUCTION This review presents results obtained by the UA2 experiment at the CERN pp collider. The ex­ periment took data from 1981 until 1985 and the total integrated luminosity amounts to about 900 nb_1. A selection bas been made among the results obtained in the UA2 experiment In section 2, we first recall the main caracteristics of the detection of electrons and jets in the UA2 detector. Tests of the standard model are then presented. Results using the full statistics are given on the measurement of the masses of the Inlermediaie Vector Bosons (TVB) and on their production properties. The third section collects a few results on hadron physics. It describes a search for the decay of the IVB into quark-antiquark pairs. A study of four-jet events is presenied and a preliminary measurement of the direct photo* production cross section is given. In section 4, we report on searches for various exotic processes. 2. TEST OF THE STANDARD MODEL The W and die Z are observed in the UA2 detector by their decays W -> ev <0 Z ->eV <2> The apparatus is instrumented1) to delect electrons in the central and forward backward regions. The central region covers the full azimuth and polar angle between 40° and 140°, corresponding lo about 2 units of pseudorapidily. The forward (backward) region covers 90 % of the azimuth and polar angle between 20° and 37.5° (142.5° to 160°). Both regions are equipped with electromagnetic calorimeters preceeded by an active preshower detector. The resolution on the photon or electron energy is - 2 - °E/E ~ ^ % AI E (GeV) and the energy scale is known with an accuracy of 1.5 %. These properties have been maintained for the whole operation of the apparatus and a fraction of the calorimeters have been recalibrated with an electron beam at the end of the experiment In the central region the electromagnetic calorimeter ( 17 radiation length) is followed by a hadronic calorimeter (4 absorption lengths). The calorimeters arc segmented into 240 cells built in a tower structure pointing to the center of the interaction region. The response or the calorimeters was also studied with hadron beams. The energy resolution for single pions varies from 32 % at 1 Gc V to 11 % at 70 GeV and is about 12 % Tor 40 GeV hadron-jets. Jets of particles arc defined in the analysis of the calorimeters as clusters of adjacent cells with an energy exceeding a threshold set at 0.4 Gc V. Electrons are defined as jets which satisfy further requirements2'3). The energy distribution in the calorimeter cells which belong to the cluster must be compatible with that of a single electromagnetic shower. A charge particle track which paints to the cluster must be reconstructed, and aligned with a hit observed in the preshowcr counter. The efficiency of the electron signature for isolated electrons is 75 % and the rejection against hadron jets is about 104. 2.1 Measurements of the W boson mass A sample of 251 events is selected4) to study the decay mode W-» e v by requiring the presence in the event of an electron candidate, and of a missing transverse momentum (identified as the possible emission of a neutrino). The following cuts are applied ; pj > 20 GeV/c and ituf > 50 GeV (ml is the mass calculated from the transverse momenta of the electron and the neutrino). The background contamination to this sample is estimated. It comprises 11.2 hadronic QCD contribution ; 5.0 W~»tv,T->evv decays and 9.3 Z->e+e~ decays where one lepton is undetected. An estimate of the W mass, mw, is obtained from a comparison of the mF? distribution with that expected from W decays. A Monte-Carlo program is used which generates the dN/drru, distribution for different values of mw. The W longitudinal momentum distribution is obtained from the quark (antiquark) 5 structure functions * of the proton (antiproton). The W transverse momentum. pT, is generated from the distribution of ref. 6, which agrees well with the data. The decay is described by standard (V-A) coupling with decay parameters given by the Standard Model The best fit to die experimental distribution is : mW = 80.2 ± 0.6(staL) ± 0J(syS]) ± 1.3(sys2> GeV, (3) and if die W width, r\y, is fitted as an additional free parameter, we obtain r\y < 7 GeV (90 * confidence level). The statistical uncertainty in (3) takes into account the resolution of die energy measurement, and also cell-to-cell uncertainties of the energy calibration. Systematic uncertainties of die mass measurement have two major contributions, which are quoted separately. The uncertainty (sysj) of Eqn.(3) is mainly due to possible systematic biases in die evaluation of pi, and consequently m_. The second systematic uncertainty (sys2) reflects the measurement uncertainty on the global energy scale of the calorimeter calibration. The influence on the mass fit of the (dominandy low-pj) background contribution is small. 2.2 Measurement of the Z boson mass The study of the Z -* e+c~ decay was done on a sample of events obtained by requiring the pre­ sence of 2 electromagnetic clusters, one of which at least had to satisfy loose electron criteria. A total of 39 events satisfy m^ a 76 GcV and arc attributed to the Z, with an esdmatcd background of 1,3 events. The process of internal brcmsstrahlung can produce events of the type Z -4 e+c""y ; one such event has been observed and is included in the final event sample, This event consists of a 24 GeV photon separated in space by 31° from an electron of 11 GcV. The probability of seeing at least one event having an (cev) configuration less likely than that observed in die sample of 39 Z decays is estimated to be * 0.4, Using a relativistic Brcit-Wigncr shape modified by the mass resolution, the Z mass is evaluated Tram a fit to the mass values of a sub-sample of 25 events for which both electron energies are accurately measured and for which mcc > 76 GcV. The result is : mz = 91.5 ± 1.2 (stat) ± 1.7 (syst) GeV (4) where the systematic uncertainty results mainly from the calorimeter energy scale of the central and forward calorimeters. From (3) and (4), we measure : ir^ - mw = 11.3 ± 1.3 (stat.) ± 0.5 (sys^ ± 0.8 (sys2) GeV (5) where (sysj) again results from systematic uncertainties of the P^ evaluation in (3) and (sys2> reflects the differing global energy scale uncertainties of the forward and central calorimeters. 23 Width of the W and Z bosons. Limits on the number of neutrino types A direct measurement of the width, r%, of the Z is difficult since a precise knowledge of die shape of die mass resolution is required. The average measurement error is estimated to be 3.1 GuV, which is of the same Ofder as the expected Z width. Following?), we obtain Tz = 2.7 ± 2.0 (stat) ± 1.0 (sysl) GeV < 5.6 GeV (90 % confidence level), where the quoted systematic error reflects the uncertainty of the average measurement error. As described in ref. 3), we can obtain an independent but model-dependent7) estimate of die ratio of total widths, T^fTy ^rom rz/Tw = RMP R01 RlcP*, where RexP = o?X / a^ is the experimentally determined cross-section ratio from this experiment, where R* is die ratio <T2/0W obtained theoretically, and R'^P* = rj/ r^ is the ratio of leptonic partial widdis as evaluated from the Standard Model. Wc measure : Rexp = 7.2^2 (stat), < 10.42 (95 % confidence level), (6) Both Rln and R'cPl depend on sin^w explicitly through the neutral current couplings and implicitly via the W and Z masses. However the product Rm R,cPt is constant to within 1 % over the range sin26u/ = 0.232 + 0.009. A more important uncertainty results from the use of different sets of structure functions in the theoretical calculation of Rtn. Recent estimates give values of R10 ranging between 0.285 and 0.325. Using RCXP as in (6) and the value R,h = 0.305 ± 0.020, we evaluate r#Tw - 0.82 $'H (slat) ± 0.06 (thcor), < 1,19 ± 0.08 (thcor) (95 % confidence level), (7) where the theoretical error reflects the uncertainly on R^ as quoted above. The ratio of lout) widths is sensitive to both the number of neutrino types and the mass of the top quark, mt. Assuming that the charged members of any new family and also any other unknown particle arc loo massive to contribute significantly to W or to Z decays, we obtain the results summarized in figure 1 which shows the expected variation of r^/Tyy with mt if the number of light neutrino types is respectively 3,4 or 7.
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