Bigi, M.

Chierici, R.

Cossutti, F.

Duperrin, A.

Kjaer, N.J.

Moller, R.

Mulders, J.

Ouarou, A.

Palacios, J.

Parkes, C.

Renton, P.

Simard, L.

Smadja, G.

Timmermans, J.

Todorova, S.

Tonazzo, A.

Turluer, M.L.

Bigi, M

Chierici, R

Cossutti, F

Duperrin, A

Kjaer, N J

Møller, R

Mulders, M

Ouraou, A

Palacios, J

Parkes, C

Renton, P B

Simard, L C

Smadja, G

Timmermans, J

Todorova-Nová, S

Tonazzo, A

Turluer, M L

CERN Document Server

OPEN-99-41215/06/1999HEP'99 # 6 367 DELPHI 99-64 CONF 251Submitted to Pa 6 15 June 1999Pl 6Direct measurement of mWPreliminaryDELPHI CollaborationPaper submitted to the HEP'99 ConferenceTampere, Finland, July 15-211Measurement of the mass of the W boson usingdirect reconstructionM.Bigi1, R.Chierici1, F.Cossutti2, A Duperrin3, N.J.Kjaer4, R.Moller5,M.Mulders4, A.Ouraou6, J.Palacios7, C.Parkes2, P.Renton7, L.Simard6,G.Smadja2, J.Timmermans4, S.Todorova8, A.Tonazzo2, M.L.Turluer6,AbstractFrom data corresponding to an integrated luminosity of 157:6 pb 1taken during the189GeV run in 1998, DELPHI has measured the W mass from direct reconstructionof WW ! `qq and WW ! qqqq events. Combining these channels, a value ofmW= 80:331  0:090(stat)  0:035(syst)  0:034(fsi)  0:017(LEP )GeV=c2isobtained, where fsi denotes nal state interaction. The combination with previousDELPHI measurement gives the value of mW= 80:3160:076(stat)0:047(syst)0:017(LEP )GeV=c2.1Dip. di Fisica Sperimentale, Universita di Torino and INFN, via P.Giuria 1, IT-10125 Turin, Italy2CERN, CH-1211 Geneva 23, Switzerland3Universite Claude Bernard de Lyon, IPNL, IN2P3-CNRS, F-69622 Villeurbanne Cedex, France4NIKHEF, Postbus 41882, NL-1009 DB Amsterdam, The Netherlands5Niels Bohr Institute, Blegdamsvej 17, DK-2100 Copenhagen 0, Denmark6DAPNIA/Service de Physique des Particules, CEA-Saclay, FR-91191 Gif-sur-Yvette Cedex, France7Department of Physics, University of Oxford, Keble Road, Oxford OX1 3RH, UK8Inst. de Recherches Subatomiques, IN2P3-CNRS/ULP-BP20, FR-67037 Strasbourg Cedex 05, France1 IntroductionDuring the 189GeV run in 1998 DELPHI collected a sample of e+e collisions correspond-ing to an integrated luminosity of 157:6 pb 1. At this energy a high precision measurementofmWcan be made by reconstructing the W mass spectrum using constrained tting tech-niques. This direct measurement ofmWprovides an important test of the Standard Modelby comparison with the indirect measurement from precise electroweak results at lowerenergies [1] and helps constrain the mass of the Higgs boson.This paper describes the analysis using events in which one W decays into leptons andthe other into quarks, WW! lqq (\semileptonic" events), and events in which both Wsdecay hadronically, WW! qqqq (\hadronic" events). These analyses are detailed in [2]The paper is organised as follows. In section 2, the DELPHI detector setup and theevent generators are briey reviewed. In section 3, the measurement of the W mass in thesemileptonic channel is presented, while section 4 describes this measurement in the fullyhadronic channel. In section 5, the combined result is presented, as well as combinationswith previous DELPHI results.2 Apparatus and SimulationsDetailed descriptions of the DELPHI apparatus and its performance can be found in [3].The response of the detector to various physics processes was modelled using the simu-lation program DELSIM [4], which incorporates the resolution, granularity and eciencyof the detector components. The event generator EXCALIBUR [5] was used for the simu-lation of all four-fermion nal states (signal and background), while the background frome+e ! qq() was generated with the PYTHIA [6] event generator. For the generation ofthe EXCALIBUR events, the fragmentation was performed using JETSET 7.4 [7] tunedto the DELPHI LEP1 data [8], and the initial state radiation (ISR) was generated usingthe QEDPS program [9].For the signal part, a sample of 270,000 events was generated with a reference W massof 80.35 GeV=c2, while two other smaller samples of 180,000 events each were generatedwith masses of 79.35 and 81.35 GeV=c2. At each of these masses, the simulated widthwas the one predicted by the standard model. The W mass and width used in this papercorrespond to a W propagator with an s-dependent width.3 Analysis of the semileptonic decay channelEvents were selected from the data sample recorded, requiring all detectors essential forthis measurement to be fully ecient. These comprise the central tracking detectorsand the electromagnetic calorimeters. The selected sample corresponds to an integratedluminosity of 152:9 pb 1.The analysis follows closely that presented in [2] for the electron and muon channels.In addition, we present here an analysis of the  qq channel.Only events which failed the electron and muon selections were used as  qq can-didates. They were required to be clusterised in 3 or more jets using LUCLUS withycut = 5 GeV=c2, and they were then forced into a 3-jet conguration, in which the taujet was chosen as the jet with the lowest charged multiplicity. As the tau mainly decays1into a nal state with 1 or 3 charged particles, only events where the tau jet containedbetween 1 and 4 charged tracks were selected. The selection, tuned on samples of signaland background Monte Carlo, was based on properties of tau jets (fraction of charged tototal energy, isolation) and of global WW events (acollinearity, acoplanarity, b-tagging,missing energy). Cuts were optimised separately for tau candidates with one and morethan one charged tracks. The composition of the tau sample after selection was estimatedto be 64%  qq, 19% eqq, 5% of qq and the remainder to come mainly from qq(),corresponding to a 35% selection eciency for  qq events.After the selection, 265 electron, 315 muon and 218 tau candidates remained in thedata, while the numbers of expected events from simulation were respectively 290.8, 335.8and 200.2 respectively.The events were reconstructed using a constrained t, imposing conservation of 4-momentum and equality of the two W masses. The distributions of the reconstructedmasses are shown in gure 1 for real and simulated data in the electron, muon and tauchannels.DELPHI01020304050607050 55 60 65 70 75 80 85 90 95 100W mass (GeV/c2)events / 2 GeV/c2DELPHI010203040506070809050 55 60 65 70 75 80 85 90 95 100W mass (GeV/c2)events / 2 GeV/c2DELPHI0102030405050 55 60 65 70 75 80 85 90 95 100W mass (GeV/c2)events / 2 GeV/c2Figure 1: The distributions of the reconstructed masses for the electron, muon and tauchannels.As in [2], the W mass was extracted from an event-by-event likelihood t using allevents within the region of tted mass 68-92 GeV=c2, followed by a bias correction esti-mated from Monte Carlo events reweighted at various masses. Any imperfection in thesimulation will cause a systematic error on the mass. The dierent sources of systematicerrors are discussed in detail in [2]. The list of the relevant ones is presented in table 1.For the lepton combination, all sources of systematic errors are taken as uncorrelatedbetween electron, muon and tau, except for the errors coming from the jet energy, thefragmentation and the ISR which are taken as fully correlated.The event-by-event likelihood analysis on the semileptonic channels gave the followingresults:mW= 80:326  0:290(stat) 0:080(syst) 0:017(LEP )GeV=c2for electron events,mW= 80:071  0:217(stat) 0:068(syst) 0:017(LEP )GeV=c2for muon events andmW= 79:892  0:324(stat) 0:074(syst) 0:017(LEP )GeV=c22Sources of systematic error eqq qq  qq lqq qqqq Combined(MeV=c2)Statistical error on calibration 30 25 27 16 13 10Lepton energy 39 41 - 23 - 8Jet energy 59 45 66 54 20 23Background level 6 - 9 3 3 2Background shape - - - - 5 3Isolation of the lepton 13 - - 4 - 1Total uncorrelated 78 66 72 61 25 27Fragmentation 10 10 10 10 20 17I.S.R. 15 15 15 15 15 15Total correlated 18 18 18 18 25 23LEP energy 17 17 17 17 17 17Colour reconnection - - - - 50 33Bose Einstein correlations - - - - 20 13Total nal state interaction - - - - 54 34Table 1: Contributions to the systematic error on the mass measurement. The errorsources have been separated into those uncorrelated and correlated between the dierentLEP experiments.for tau events. The last error1comes from the uncertainty on the beam energy.The combination of the three gives the following result:mW= 80:102  0:153(stat) 0:063(syst) 0:017(LEP )GeV=c2:4 Analysis of the hadronic decay channelRequiring only that the central tracking was fully ecient, events were selected from adata sample corresponding to an integrated luminosity of 157:4 pb 1. A total of 1601events were selected, while 1589 were expected from simulation.This analysis uses the 2D likelihood method described in [2]. After selection, a total of1601 events remained in the data (see gure 2) while 1589 were expected from simulation.Contrary to the semi-leptonic case, the constrained t used in the event reconstructionimposes only 4-momentum conservation. The resulting goodness-of-t as a function ofthe two masses is convoluted with the theoretically expected 2D distribution to obtainan event-by-event likelihood for the W mass. This is followed by a bias correction takenfrom simulated events.The calibration of the analysis depends on the accuracy of the simulation. Table 1shows the estimated systematic errors coming from possible inaccuracies in the simulation.The two-dimensional ideogram analysis on the hadronic channel gave the following1mW=mW Ebeam=Ebeam3DELPHI 189 GeV PreliminaryW mass (GeV/c2)events / GeV/c202040608010012014050 55 60 65 70 75 80 85 90 95 100Figure 2: Mass plot for the selected WW ! qqqq candidates showing only one recon-structed mass per event (that with the best 2), and using an equal-mass constraint (notused in the determination of mW, see text).result:mW= 80:467  0:110(stat) 0:035(syst) 0:054(fsi)  0:017(LEP ) ;where `fsi' denotes the possible eect from nal state interactions and `LEP' the uncer-tainty on the beam energy.5 Combination of all resultsThe masses measured in the semileptonic and hadronic decays analysis are in good agree-ment within statistics. Combining them yieldsmW= 80:331  0:090(stat) 0:035(syst) 0:034(fsi)  0:017(LEP )GeV=c2:Our previous measurements derived from our 172 GeV data [11], and from our 183 GeVdata [2] are fully compatible with this more precise value. Combining all the hadronicand semileptonic values gives the mass dierencemW(had) mW(lep) = 0:128  0:159(stat) 0:078(syst)GeV=c2And combining all these measurements of the W mass yieldsmW= 80:316  0:076(stat) 0:047(syst)  0:017(LEP )GeV=c2:4References[1] LEP electroweak working group, A Combination of Preliminary Electroweak Mea-surements and Constraints on the Standard Model, CERN-EP/99-15.[2] DELPHI Collaboration, P. Abreu et al., CERN-EP/99{79, submitted to PhysicsLetters B.[3] DELPHI Collaboration, P. Abreu et al., Nucl. Instr. Meth. A 303 (1991) 233.DELPHI Collaboration, P. Abreu et al., Nucl. Instr. Meth. A 378 (1996) 57.[4] DELPHI Collaboration, DELPHI event generation and detector simulation - UserGuide, DELPHI 89-67.[5] F. A. Berends, R. Pittau and R. Kleiss, Comp. Phys. Comm. 85 (1995) 437.[6] T. Sjostrand, Comp. Phys. Comm. 82 (1994) 74.[7] T. Sjostrand, PYTHIA 5.7/JETSET 7.4, CERN-TH.7112/93 (1993).[8] DELPHI Collaboration, P. Abreu et al., Z. Phys. C 73 (1996) 11.[9] T. Munehisa, J. Fujimoto, Y. Kurihara and Y. Shimizu, Prog. Theor. Phys. 95 (1996)375.[10] DELPHI Collaboration, P. Abreu et al., Phys. Lett. B 397 (1997) 158.DELPHI Collaboration, P. Abreu et al., W pair production cross-section and Wbranching fractions in e+e interactions at 183 GeV, CERN-EP/99-47, submitted toPhys. Lett. B[11] DELPHI Collaboration, P. Abreu et al., Phys. Lett. B 397 (1997) 158.5

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