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. 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. For this sample of c events we calculated a global efficiency of 98.8 ± 0.83 and estimated a background contamination of 0.4 ± 0.23, arising from 7+7- events, and e+ e-')'and ')'')' in the sample for which no jet chamber information was available. The selection ofµ+µ- events required at least two tracks with 0.7, momentum greater < than 6 Ge V/ consistent with coming from the origin, and each identified as muons. A track was c, identified as a muon if it satisfied any of the following criteria: icos(O)I there were hits in at least two of the four layers of the barrel muon chambers, associated with • the track within A = 98 there was a track segment in the barrel hadron calorimeter, with hits in at least five of the nine • layers of the calorimeter, associated with the track within t!.¢> 70 mrad the momentum of the track was larger than 15 and the sum of the energies of all clusters • Ge V = within t!.¢> = 200 mrad was less than 3 GeV. / c The efficiencies of these criteria were 91.6 ± 0.83, 60 .3 ± 1 .33 and 92.5 ± 0.73 , respectively. The absolute time, as well as difference in time of TOF hits was used to reject the background coming from cosmic rays, with a negligible loss of signal events. After all these cuts, a sample of 585 µ+ µ events was selected, for an integrated luminosity of about 1 pb-1• The geometric acceptance of the 0.7 requirement was calculated to be 60.0 ± 1.13 ,using the KORALZ Monte Carlo < program. Using the same program, we calculated a background of 3.7 ± 0.93 from events. r+r- Tak ing/cos(O)/ into account the acceptance, efficiency and background, we obtained an overall correction factor of 1.63 ± 0.04 for the number ofµ+µ- events. The selection of r+ events required that the total electromagnetic energy observed in the event T- 12 70 be between GeV and GeV. A thrust axis was then calculated, using both the electromagnetic clusters and charged tracks, and was required to satify O. 7. The event hemispheres defined < by the plane perpendicular to the thrust axis were each required to have at least one and at most four charged tracks with 100 Me V, consistent with/cos( coming Or)/ from the origin. Events identified PT by the previous selection as µ+ µ- events were rejected, as were events identified as cosmic rays. To reject two-photon events and >beam-gas/wall interactions, we fu rther required that the vector sum of the energy vectors measured in the lead glass have 0.95 with respect to the beam axis . < 506 1 These criteria resulted in the selection of r+r- events, for an integrated luminosity of about pb-1• Using the KORALZ program we estimated the/cos(O)/ acceptance of the requirement 0.7 < as 62.0 ± 1.53. The selection efficiency within that acceptance was 76.3 ± 2.23. The background in this sample was estimated to be 6.2 ± 2.23, coming mainly for multihadron events./cos( TakingOr)/ into account the acceptance, selection efficiency and background contamination, we obtained an overall correction factor of 1 .98 ± 0.1 for the number of events. r+r- 2.3 Analysis of the Hadronic and Leptonic Line Shapes Combining our measurement of the acceptance-corrected number of hadronic and leptonic events with our measurement of the luminosity, we obtain the cross sections for these four channels, at each energy point of the scan. From the measured line shapes we can extract , in a model independent analsyis, the parameters Mz , rz, uJ:�, as well as the partial widths r., rµ, rT and rhad· The systematic uncertainties in the decay widths are highly correlated. Normalization and point to-point uncertainties are completely correlated for the four channels. Further correlations are intro duced by the ratio fe/(M r ) used in extracting the four partial widths from the measured cross · sections. We therefore perform a combined fit to the four line-shapes, based on a x2 minimization which takes into account the� fu}ll covariance matrix of the data. For the lepton channels data points have been discarded if the SM prediction amounts to less than 10 events observed for the measured luminosity. For all final states, excluding e+ e-, we use the model independent line shape parametrization described in Ref (2]. This parametrization makes use of the improved Born approximation, and treats photonic corrections in first order, with exponentiation of soft photons. To avoid introducing additional parameters in the fit, we neglect the contribution due to the interference terms, 1 - Z which is small over the energy range of the scan. The approximation used has been compared to the fu ll SM calculation with second order treatment of photonic corrections and differs by less than 1.03 over the energy range of the scan. Due to the presence of the t-channel the cross section for the e+ e- final state diverges if no angular cut is applied. For the e+e- channel, however, the effect of an angular cut depends strongly on the the resonance parameters. We therefore use in our fit a parametrization of the differential cross section which is integrated for each choice of parameters in the angular range of accepted events. This parametrization, described in Ref. (3], is based on the line shape program BHABHA (4] which uses 99 ,...... _ 1 .6 �------,..--� .D c 30 µ:µ,- b 1.2 20 0.8 10 0.4 0 0 86 88 90 92 94 96 86 88 90 92 94 96 ,.-.... 1.6 �------, ,...... _ 1 .2 ------.,--,---.,---, .D .D c c '-' b 1.2 b 0.8 0.8 0.4 0.4 0 0 �·-----�---� 86 88 90 92 94 96 86 88 90 92 94 96 v1s Figure 1: Fitted line shapes superimposed on the data points. ( GeV) the formalism developed in Ref [5]. The program accounts for '( and exchange in s and t channel, Z and all possible interference terms. Photonic corrections are treated in a first order calculation with exponentiation of soft photons. Hard photons are treated in the colinear approximation. The t channel effects are large: the peak cross section is enchanced by approximately 15% for icos(ll)I < 0.7 and the line shape sig ficantly distorted with respect to the pure s-channel process. The program requires the specification of kinematic cuts on the photon energy and the opening angle ( between the cihard photon and the final state electron or positron. our selection criteria In these cuts are imposed by the requirement that the acolinearity angle (llacol)between ( k) the electron and positron8) is smaller than 5°; this criterion corresponds approximately to the requirement of < 0.083 and < 5°. We have applied a correction factor to account for the fact that the two kinematic regions are not exactly the same. Another small correction was applied to account for thek fact that whereas8 in our event selection both the electron and positron were required to lie within the region of icos(ll)I < 0.7, in the program BHABHA that requirement is only made for the electron. These correction factors varied from -1.6% to -2.3% and from 0.5% to 1.5% respectively over the energy range of the scan. The cross section calculation of the line shape program BHABHA, with the cuts icos(ll.-)1 < 0.7, 0.083 and < 0.5° were comp ared with the results of the BABAMC program, with our event < selection cut s. Taking int o account the correction factors mentioned above, the agreement of the two programsk led to8 an estimate of the systematic error in the cross section calculation used of 2.5%. Because the exponentiation of soft photons is not performed in the program BABAMC, for the purpose of this comparison we did not use exponentiation in the line shape program; no systematic uncertainty has been assigned to the exponentiation procedure used in the BHABHA program. The line shapes resulting from the combined fit are shown in Figure la-d superimposed on the data. The parameter values for this fit are summarized in table 1 column 2. The leptonic widths we observe are consistent with lepton universality. We therefore repeat the fit constraining all leptonic widths to be equal. The result of this fit is given in table 1 column 3. With the assumption of lepton 100 hadronic data all data all data all data SM expectation only without Lu. with Lu. excluding lOOGeV electrons lOOGeV m, = 0.12 Mz[GeV/c2J 91.145 ± 0.022 91.154 ± 0.021 91.154 ± 0.021 91.144 ± 0.021 mH91.154 = (input rz[GeVJ 2.526 ± 0.04 7 2.536 ± 0.045 2.536 ± 0.045 2 .532 ± 0 .045 2.483a, = uk�[nbJ 41.2 ± 1.1 41.4 ± 1.1 41.4 ± 1.1 41.4 ± 1.1 41.4 ) r.,[MeVJ 81.2 ± 2.6 r,.,.[MeVJ 82.6 ± 5.8 rn[MeV] 85.7 ± 7.1 r1+1-[MeVJ 81.9 ± 7.1 82.1 ± 2.2 83.4 rhad[GeVJ 1.854 ± 0.058 1.838 ± 0.046 1.822 ± 0.052 1.734 rinv[MeVJ 433 ± 61 453 ± 44 463 ± 48 499 x2/NDOF 4.5/8 30.8/32 31.2/34 19.3/25 Table 1: Results of the fit to the hadronic data and of the combined fit to hadronic and leptonic data. Mz has an additional uncertainty of 30 MeV/c2 deriving form the energy uncertainty of the LEP machine. In the table, Lu. stands for lepton universality. universality the line shape parameters can also be extracted without reference to the + e- e+ e e -> data. While this procedure degrades the statistical significance of the results, it removes potential sources of systematic errors in the treatment of the electron channel through the approximation given in Ref. 5]. The results of this fit is given in table 1 column 4. table 1 column 5 the SM expectation for the leptonic, hadronic and total widths are given for In our measured( value of the zo mass, with the assumption of three neutrino generations. Here we have set GeV/c2; for the strong coupling constant we have chosen a value of 100 = 0.12, consistent with the results of our study of three jet production rates in the hadronic decays of the m,It =can ffiH be =seen that the measured total width is larger than the expectation by morea, than one z0• standard deviation and that the hadronic width is larger by more than two standard deviations, whereas the leptonic width is in good agreement but somewhat smaller than the predicted value. SM The partial widths all increase appreciably with increasing top quark mass. In order to study the dependence of the SM prediction on the values assumed for the top quark mass and the strong coupling constant, we vary between 50 and 250 GeV/c2 and between 0.09 and 0.15. Figure 2 shows our measurements of the hadronic and leptonic partial widths; also indicated is the uncertainty in the SM prediction withinm, the parameter range considered here.a, the ratio of the partial widths In the dependence on the top quark mass is reduced. In particular, the allowed range of values for the ratio of the hadronic to the leptonic partial width , R�M, is where the range of R�M reflects mainly the uncertainty assigned to in second order QCD. The ratio Rz has the additional feature that some experimental systematic errors cancel; in particular it is independent of the absolute luminosity measurement . Our determinationa, of Rz, from the combined fit, is rhad Rz -- 22.43 ± o.75. r1+1- The value of Rz is in agreement with the= corrected= ratio of the number of hadronic to µ+ µ- and T+T- events, for the three energy points around the peak, (Nhad · <1+1- )/(N1+i- · 2 1.9 . . . ' ·... ·. \ ". _ _ __ _ . . 1.8 . _ ...... ,·o.....········-············�···/- _ . . · .-;jI �-�-��-c.�·· · 1 .7 . · ..0.08.. 0.084 !,+,- (GeV) Figure 2: The hadronic versus leptonic· partial widhts. The SM prediction is shown as the shaded area, for 50 250 and 0.9 1.3. � � � � 1.013 accountsm, for photon exchangea, in the s-channel. Whithin the parameter range considered f here our measurement of differs from the SM prediction by about two standard deviations. =The results of the combined fit can also be expressed as a model independent , albeit indirect, measurement of the invisibleRz width by the relation, We obtain, rinv - 453 ± 44MeV. Based on the SM value of 166.2:!: : , we obtain, rinv = rz 3r1+1- - rhad· = �;': riM2.73 ±= 0.26(ez�p):!:g:g�(th� eor). This measurement of the invisible =width Nv = excludes four generations of light neutrinos with SM cou plings by more than four standard deviations. The measurement lies below the SM prediction by about one standard deviation. This observation sould not, however, be considered independently of the deviation in discussed above. Rz 3 Jet production Rates in Hadronic decays of the z0, and �he running of as (Ref.[6]) We have measured the relative rates of multi-jet final states in hadronic decays of the zo. For this study we used the jet finding alorithm introduced by the JADE [6] collaboration. each hadronic event , the squares of the scaled pair masses, In is calculated for each pair particles i and j in the event. Starting with the pair resulting in the lowest scaled mass, particles are combined pairwise into pseudo-particles with four-momentum + The procedure is repeated until i for (pseudo-)particle pairs in the event. The clusters Y i 2': Ycut all of particles and pseudo-particles remaining in the event is then identified with the "jets" . Figure(P; P; 3a). shows the relative production rates of 2-, 3-, 4-, 5- jet events measured by OPAL, as a function of the parameter Ycut . An important feature of the algorithm is the close agreement between that the number of jets reconstructed at the hadron level and at the parton level over a wide range of center-of-mass energies, as has been determined through studies with QCD Monte Carlo programs. Figure 3b shows the OPAL measurement of the relative 3-jet rate in comparison with measurements of in e+e- experiments at lower center-of-mass energies, for a value of = 0.08. Ra, Ycut Ra 102 JO ,...... ,_...,_,_, .,,.::-cO::P �AL'.""""..,....�. -J"'AD=E ....,...... ,....,_...... , o Mark II + TASSO • Tnstan (AMY, VENUS. TOPAZ) 25 �' i ' - --\ !f:.:i.""'--I -- - 20 f . --�a.s=const. 1'f ...... -.... --- ······ QCD µ2= E�; A;i? ,.. 250 McV 2== � QCD µ 0.0017E;m ; Am-""1 07 MeV IS ·1:.::.:.:.:. -20 40 60 80 100 [GeV] Ecm Figure 3: shows the measured relative production rates of 2-,3-,4- and 5-jet events, as a fu nction of a the jet resolution parameter b shows the measured 3 jet rate, for 0.08 in e+ e- collisions = at different center-of-mass energies. Ycut ; Ycut A clear scaling violation is observed: the 3-jet rate decreases as the center-of-mass energy increases. In the context of second order QCD this scaling violation can be attributed entirely to the running of the strong coupling constant The prediction of the second order QCD calculation, formulated so o,. that all energy dependence is contained in a running is also shown in Figure 3b, for two choices o., of the renormalization scale µ. The results of a calculation to orders in perturbation theory must all be independent of the choice of the scale µ; the values or in the second order QCD calculation are seen to be rather insensitive to the value of µ. The QCD prediction describes the data well. The hypothesis that is constant with energy would lead to aRa constant rate for 3-jet events; the data o, exclude this hypothesis at the 5.7 standard deviation level. Search for the Standard Model Neutral Higgs Boson 4 (Ref.[7]) In the Standard Model of the electroweak interaction, spontaneous symmetry breaking is invoked in order to generate masses for the and bosons, while keeping the theory renormal izable. In the minimal model this is realized, through the Higgs mechanism, with the introduction of a complex scalar field and implies the presencew± of a physicalzo neutral Higgs scalar particle The minimal model predicts the couplings of the Higgs boson, but not its mass. Theoretical lower bounds on the Higgs mass can nevertheless be obtained, using vacuum stability arguments, as a fu( H0nction). of the (unknown) top quark mass. This lower bound is approximately 8 GeV/c2 for < 80 GeV/c2; it vanishes, however, as the top quark mass approaches 80 GeV/c2. For 80 GeV/c2 the lower bound changes rapidly towards large values of the Higgs mass. Previous experimentalmt lower bounds cover the mass range up to approximately 5 Ge V/ c2• These limits suffer,mt howe> ver, from significant uncertainty due to the difficulty in estimating resonant enhancements to the coupling of the Higgs boson to strongly-bound states. LEP I provides an excellent instrument for discovering, or unambiguosly ruling out, a SM Higgs with mass below about 40-50 GeV/c2• Indeed the production mechanism, through the process e+ e- _, _, is well understood; furthermore it represents a sizeable branching ratio at the peak, from "' 10-2 zo z0• H0 for a very light Higgs, to "' 5 10-5 for a Higgs of 50 GeV/c2 mass. x We report here on a search for a Higgs boson with mass above 3 Ge V/ c2; tozo the extent that the sensitivity of the search technique presented here depends on the character of the Higgs decay prod ucts, the results of the search cannot be unambiguosly interpreted for Higgs masses below 3 GeV/c2 where the Higgs decay branching ratios are subject to considerable uncertainty. The results presented here are based on approximately 825pb-1 of integrated luminosity. We consider the channels: 103 102 IZ OPAL OPAL 10 10 0 0 '- " � � " " N '- " c .> � . 10 t � I , , I 10 0 , I 10 20 30 40 50 0 10 15 20 (GeV/c) t Energyt1t1 in Sackword-cone# (GeV) t p, Figure 4: shows the distribution after all other cuts; b shows the energytt in backward .hemisphere a +t+tt after all other cuts. The dots are the data, the histogram the Monte Carlo expectation for a Higgs of mass 12 GeV/c2• PT and Both these channels have distinctive experimental signatures. The first, involving the 'invisible decay' of the results in 'mono-jet like' events characterized by an asymmetric topology and missing momentum; it has a large branching ratio (203) and provides the most sensitive channel. For this search, the resultsz0•, of an on-line analysis were used to reject events having more than 4 Ge V energy deposited in electromagnetic calorimeter clusters, if there was more than 1 GeV of energy deposited in the hemisphere opposite the most energetic cluster. this online fu analysis, clusters were required to have at least two contiguous lead glass blocks with significant energy deposition, to protect against pedestal fluctuations and noisy channels. To ensure adequate containment, events were rejected if the thrust axis of the event pointed close to the beam line (icos(BThr)I 0.8). Lepton pairs were rejected by imposing an acoplanarity angle requirement cos(A.P) < -0.85) on events containing only two tracks. Events were also rejected that had a significant ( 2 Ge V) >energy deposition in the forward detector calorimeters. The veto on energy deposited in the forward detector calorimeters implies a 3 Ge V/ c cut-off for two-photon events. We require> that candidate events have 6 GeV/c, and that the sum of charged track :;:: momenta and energy deposited in the electromagnetic calorimeter in a backwardPT cone of full angle goo centered about the thrust axis of the event be lessPT than 0.5 Ge V. Figures 4b-c show the distributions of each of these quantities, after all cuts, except that applied on the distribution shown, have been applied. The large excess of events in the data below 3 Ge V/ c is due to the residual sample of untagged two-photon events. Also shown in each case is the expected signal from a Higgs of 12 GeV/c2 mass. No events survive in the data, whereas the efficiencyPT to the signal is always above 503 in the mass range of interest. To search forevents in the channel involving the leptonic decay of the we capitalized on the presence of two isolated high-momentum tracks in the event. The two highest-momentum oppositely charged tracks in the event (major tracks) were each required to have zo,10 GeV/c, and to have p an opening angle of at least Next, the total ECAL energy (less the energy associated with the goo. major tracks themselves) in the two cones of full angle 30° centered on the> major tracks (isolation cones) was required to be less than 5 GeV. Also the scalar momentum sum of any additional tracks inside the isolation cones had to be less than 5 Ge / c. Finally, the event had to have at least 4 V additional tracks outside the isolation cones. Again, no events in the data pass the requirements, 104 whereas the efficiency to the signal ranges from 363 at = 3 Ge V/ c2 to greater than 603 for 7 GeV/c2. > As a result of this search the mass region 3 GeV/c2 :::;mH l9.3 GeV/c2, is excluded at 953 :::; mHconfidence level. The Higgs search has since been extended to include allmHo the data accumulated in the 1989 run. The missing energy channel analysis was sligthly modified so as to enhance the acceptance to high mass H0, above the H0 --> bb threshold, at the price of sacrifying acceptance in the low mass region. When combined with the analysis described here, this results in an excluded mass region of 3 GeV/c2 25 GeV / c2 at the 95 3 confidence level. � :::;H" m Mass Limits for top and b' Quarks 5 Ref. 8] The top quark must exist, in the context of the Standard Model, as the weak isodoublet partner of the b quark; so far , however, the top quark has eluded direct detection due, presumably, to( its( large) mass. The production of a heavy top quark ( ;::: 20 Ge V/ c2) in hadronic z0 decays would signifi cantly alter the observed event shape distributions. particular, top quark production would lead In to hadronic events having large acoplanaritym, and sphericity, or correspondingly a small value of the thrust variable. The production of a new b' quark would also have a similar effect on these distri butions. This observation holds true also in the exotic scenario of top or b' quark decay through a light charged Higgs or Technipion as well as for the possible flavour changing neutral current decays b' --> 1b or b' --> gb. We have compared the Monte Carlo expectation for the distributions of these three variables reconstructed using our electromagnetic calorimeter only, and using the jet chamber only, with the data. This study is sensitive to both the quality of the event simulation, since different particle spectra are measured in the two cases, and the simulation of the detector response. The study was performed on a set of 2185 hadronic zo decays selected according to criteria similar to those described in section 2. all cases we find good agreement with the distributions expected for 5 flavour production, and In thus no compelling evidence for the production of a sixth flavour. order to derive mass limits In on top quark , we choose to use the acoplanarity distribution as measured using the electromagnetic calorimeter. The acoplanarity distribution can also be used to set mass limits on a hypothetical fourth generation b' quark. Figure 5 shows the acoplanarity distribution for the data, superimposed on the expected distribution for 5 flavourproduction and for the additional production of a 35 Ge VJ c2 top, or b', quark. By combining the results of our null search with those of previous lower energy e+e- experiments, we conclude at the 953 confidence level that the top quark is heavier than 44.5 GeV/c2 and that the b' quark, if it exists, must be heavier than 41.4 GeV/c2• This limit stands for all combinations of the decay modes considered here. 6 Search for Standard Model Charged Sequential Heavy Leptons Re 9] Charged sequential heavy leptons with mass less than /2, if they exist, must be pair produced in zo decays. For a sequential heavy lepton we expect: Br(L- --> (1/9) for ;:,, each( f.[ ) e, µor and Br(L- --> 2/3 this search we allowmz for the possibility of a heavy r, In but assume that it is stable. Thus z0 --> L+ L- events typically contain at1-V leastivL )two energetic neutrinosI = in the finalstate and, in qifvLapproximately) ;:,, 463 of the cases, at least one energetic electron or muon.VL, To identify such events, we have searched both for events with large missing energy and and acoplanar events containing energetic isolated leptons. The results presented here are based on an integrated luminosity of approximately 750 pb-1. P, , For the missing energy search, we require that the visible energy be in the range 5 GeV < E.;, < 55 GeV and that the transverse momentum satisfy 12 GeV/c. We further require that there be > less than 5 GeV energy deposited in the forward detector calorimeter, and at least two tracks in the P, 105 event. The efficiency of these cuts to decays is about 503. After these cuts 315 events L+ L- remain in the data; we expect 204 ± 13 events form decays, and 80 ± 15 from hadronic r+r- decays. zo --+ To reduce the contamination fromtau pair decays,zo we --+ require that the thrust of the event be less thanzo 0.95. The backgrounds fromhadronic decays come primarily from events in which the energy of a jet has been mismeasured: in these events the missing momentum vector tends to lie along the axis of one of the jets. We reduce this backgroundz0 by requiring that the missing momentum vector be well isolated from the rest of the event: we require that less than 1 GeV in each of charged track or electromagnetic energy lie in a cone of 30° half angle centered about the missing momentum vector. After this final set of cuts the efficiencyto decays is still about 403. Two events survive L+ L- in the data; we expect approximately 3 background events from conventional decays, whereas we expect 7.6 events for 44 GeV/c2, zo --+0. = mv = the second search, we require a chargedL track multiplicity of at least two,zo and that the event In contain at least one electronmL or muon, with a momentum greater than 5 GeV/c. We require that lepton be well isolated from the rest of the event, by imposing that less than 1 GeV in charged track energy, and less than 2 Ge V in electromagnetic energy lie in a cone of 30° half angle centered about the lepton's momentum vector. To reject conventional lepton pairs we consider the two hemispheres defined by the plane perpendicular to the lepton's momentum vector. We require the the transverse mass in each hemisphere be greater than 2 Ge V/ c2, or that the acolinearity angle of the total momentum vectors in either hemisphere be at least 0.25 radians. The efficiency of these cuts to decays is approximately 203. L+ L- After these cuts 84 events remain in the data. Most of these events appear to be due to either zodilepton--+ production with initial state radiation or to dilepton production by two-photon processes. Both of these backgrounds give acolinear events, since they can have a large missing momentum due to the particles which escape the main calorimeter acceptance in the forward direction. Twelve of the 84 events have more than 5 Ge V energy deposited in the forward detector calorimeter and are rejected accordingly. Radiative events in which the radiative particles escape down the beam pipe, and consequently carry little transverse momentum, are rejected by the requirement that the acoplanarity of the total momentum vectors in the two hemispheres defined above be at least 0.2 radians, and that the total transverse momentum of the event be at laest 6 Ge V/ c. Highly asymmetric decays, where one of the r's decays into an energetic electron or muon and almost all r+r- the energy of the other goes into neutrinos, may also be acoplanar and have a large transverse r momentzo --+ um. Such events are rejected by requiring that the momentum of the second most energetic track in the event be at least 2 Ge V/ c. FromMonte Carlo simulation we expect at this stage 0.8 0.8 background events from ± r+r-")', and 0.5 ± 0.5 events from two-photon processes. We observe one event; it is kinematically consistent with being a with the converting r+r-1' 1' to produce an e+ e- pair, and is rejected accordingly. Rejecting events by this criterion does not appreciably affect the acceptance heavy letpon events. After these cuts no events are left in the data. A 44 Ge V/ c2 heavy lepton, with an associated massless neutrino is expected to produce 3.8 events passing all of our requirements. As a result of this search, a charged heavy lepton is excluded at 953 confidence level for masses below 44.3 GeV/c2 with < 20 GeV/c2 and < 5 GeV/c2 For heavier than 20 mv mv - llL GeV/c2, we exclude at 953 confidenceL level the shadedL region of Figure 5, approximately defined by < 43 GeV/c2, < 30 GeV/c2 and < 0.75 mL m L mvL mvL · mL. 7 Search for Charged Supersymmetric particles: Sleptons and Gauginos (Ref.[10]) Supersymmetry (SUSY) states that all fermions and bosons have supersymmetric partners with identical couplings, but differing by half a unit of spin from the corresponding conventional particle. particles are assigned a new multiplicative quantum number; this quantum number is All 106 10'.------; OPAL 40 OPAL 1 � fop -;::-.....,c 1 0 � induainq (J� C.V/c'} Dott dLin• . 6 �u<1rt 1 for conventional fermions and bosons, and takes the value of -1 for the SUSY partners. most + In models this quantum number is conserved: this is referred to as R-parity conservation. R-parity conservation implies that the lighest supersymmetric particle (lsp) is stable. what In follows we assume that the SUSY partner of the photon, the photino is the lsp. Under this assumption the does not interact electromagnetically with conventional matter since it cannot i convert to a fermion pair) and thus escapes detection. ( i) The SUSY partners of leptons are called sleptons, they have spin 0 and are( denoted by e, p.,and the scenario envisaged here, the decays promptly into the corresponding lepton and f. In a photino. The SUSY partners of (i)left and right handed leptons need not be degenerate; in most SUSY models the l"ii are lighter than the Thei SUSY partners of the charged bosons are called charginos The chargino is expected to have both leptonic decay modes -> and hadronic decay modes TheI�. unknown wino-higgsino mixing matrix leads(W±, to some H±) -> qu"lfdi . model dependence in(x± both). the production cross sections and decay branching ratios. x+ 1+117 sleptons or charginos x+are light enough, they may be pair produced in decays. From the If above discussion we see that decays to pairs of sleptons or charginos will be characterized by final states containing pairs of leptons, or jets, which may be highly acoplanarz0 due to the energy of undected photinos. zo We have have carried out a search for or events in which the acoplanarity angle µ µ-, ,- ,-- of the leptons is larger than 20°. We have also searched for two jet events in which the acoplanarity angle of the jets is greater than 50°. e+c, + + a sample of data corresponding ot about 550 pb no dilepton events are found with an In -1 acoplanarity angle of greater than 20°. the two jet sample, 11 events are found with an acoplanarity In angle of greater than 50°; this number is in good agreement, with the Monte Carlo expectation of 12 ± 4 such events. Thus we find no compelling evidence for the pair production of sleptons or charginos in decays. Figure 6 shows the mass limits derived from this result. the case of the chargino, limits are provided as a function of the branching ratio to leptonic In over hadronicz0 ciecay modes of the chargino, assuming equal values for each leptonic channel. The cross section for the production of x± pairs was calculated according to the minimal SUSY model and the parametrization of Ref. [11]. This results in a Born cross section of about 8.5 nb for the pair production of charginos at the peak. zo References [1] A Combined Analysis of the Hadronic and Leptonic Decays of the zo 107 al OPAL OPAL cl 30 30 20 20 .� �E � 2 2E 10 10 ..... ' I 10 20 30 50 20 30 40 so m; [GeVj , m;c,fGeVI bl OPAL 0 � 30 a:: 0.8 � 'i' �0< .6 20 CD"' 0.4 ·� 0 �E "' 2 0 10 ��0 .2 50 00 10 20 30 G•V ) mµ, [GeV! MASS ( Figure 6: d show the excluded mass regions for sleptons and gauginos respectively. a - OPAL Collaboration, Phys. Lett. B240 (1990) [2] D.Bardin et al., Z Physics at LEPl, CERN 89-08, ed. G.Altarelli et al., Vol.1 (1989) 89. [3] M.Caffo , E.Remiddi and F.Semeria, Z Physics at LEPl, CERN 89-08, ed. G.Altarelli et al., Vol.1 (1989) 171. [4] Program BHABHA by M.Caffo, E.Remiddi, and F.Semeria. [5] M.Greco, Phys. Lett. Bll7 (1986) 97. [6] A Study of Jet Production Rates and a Test of QCD on the Resonance zo OPAL Collaboration, Phys. Lett. B235 (1990) 389. [7] Mass Limits for a Standard Model Higgs Boson in collisions at LEP. OPAL Collaboration, Phys. Lett. B236 (1990) 224. e+ c [8] A Search for the Top and b' Quarks in Hadronic Decays zo OPAL Collaboration, Phys. Lett. B236 (1990) 364. [9] A Direct Search for New Charged Heavy Leptons at LEP OPAL Collaboration, Phys. Lett. B240 (1990) 250. [10] A Search forAcoplanar Pairs of Leptons or Jets in Decays; Mass Limits on Supersymmetric zo Particles OPAL Collaboration, Phys. Lett. B240 (1990) 261 [11] A.Bartel, H.Fraas,and W.Majerotto, Phys. C30 (1986) 441; Z. Phys. C41 (1988) 475. Z.