A review of the present status of the W boson mass and width measurements at LEP is presented. The analysis of the data collected in 1998 by the 4 LEP collaborations, corresponding to about 700 pb /sup -1/, allows to reach an experimental uncertainty on the W boson mass measured at LEPII approaching the precision of the indirect results from the high precision electroweak measurements at LEPI. (7 refs)

Cossutti, F

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Measurement of the W boson mass and width in e+e- collisions at LEP

F. Cossutti

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Nuclear Physics A

Measurement of the W boson mass and width in e+e− collisions at LEP

DELPHI Collaboration DELPHI 99-144 TALK 315 July, 1999Measurement of the W boson mass and width in e+e−collisions at LEPF. CossuttiCERN, CH-1211 Geneva 23, SwitzerlandAbstractA review of the present status of the W boson mass and width measurements at LEP ispresented. The analysis of the data collected in 1998 by the 4 LEP collaborations, corre-sponding to about 700 pb−1, allows to reach an experimental uncertainty on the W bosonmass measured at LEPII approaching the precision of the indirect results from the highprecision electroweak measurements at LEPI.Talk given at the XVth Particles and Nuclei International Conference - PANIC9910-16 june 1999 - Uppsala, Sweden1 IntroductionThe measurement of the W boson mass mW constitutes a fundamental test of the StandardModel of the electroweak interactions. The comparison of the direct mW measurement withthe indirect evaluation from the other electroweak precision measurements allows a stringentconsistency check of the Standard Model and plays a key role in constraining the Higgs bosonmass and the possible presence of new physics.2 W mass measurement at LEPIIAt LEPII the W pair production in e+e− collision is exploited to perform this measurement.Two different methods have been used depending on the centre of mass energy of the collision.2.1 Measurement at the WW production thresholdAt the WW production kinematical threshold (√s ' 161 GeV/c2) the cross section of thisprocess is highly sensitive to mW. Exploiting the WW cross section measurement performedon the statistics collected at this energy by the LEP collaborations in 1996 (' 42 pb−1) [1] thefollowing result is obtained:mW = 80.400± 0.221 GeV/c2where the error is completely dominated by the limited experimental statistics.This method cannot be used at higher energies because the sensitivity of the cross section tomW drops rapidly becoming almost negligible.2.2 Measurement using direct reconstruction of WW decay productsThe LEP collaborations have collected and analysed data at 3 centre of mass energies beyondthe threshold one, ' 40 pb−1 at 172 GeV/c2, ' 220 pb−1 at 183 GeV/c2 and ' 690 pb−1 at 189GeV/c2 (see [3, 5, 6, 7] and references therein).At these energies the method used for the W mass measurement is the direct reconstructionof the W invariant mass spectrum from the final state measured particles. According to theway the Ws decay into fermions (a quark pair or a charged lepton - neutrino pair) 3 final statechannels can be distinguished (in parenthesis the Standard Model branching ratios are given):fully hadronic (qqqq, 45.9%), semileptonic (qqlν, 43.7%) and fully leptonic (lν lν, 10.4%). Dueto the loss of kinematical information in the fully leptonic channel because of the 2 unmeasuredneutrinos, only the first 2 channels can be used in this approach.Experimentally the final states are made by either 4 hadronic jets (qqqq) or by 2 hadronicjets and an energetic charged lepton (qqlν). The reconstruction of the event is improved using akinematic constrained fit imposing the global energy and momentum conservation ∑i Ei =√sand ∑i ~pi = ~0 (in the semileptonic events the neutrino momentum is assumed to be equal to themissing momentum).In the fully hadronic channel the 4 jets can be combined in 3 different way to build theW invariant masses. ALEPH, L3 and OPAL solve the ambiguity choosing the best pairingaccording to given quality criteria, while DELPHI uses a more elaborate technique (ideogram)1where the information from all the pairings is combined in a global event by event probabilityand convoluted with the theoretical mW distribution.The further constraint that the 2 reconstructed invariant masses are equal is always appliedin the semileptonic channel; OPAL and L3 use it also on qqqq events, while ALEPH rescalesthe reconstructed masses to the beam energy. In the DELPHI ideogram technique the fullinformation from all possible mass value combinations is exploited to build the event by eventprobability.mW is finally extracted from a likelihood fit: ALEPH, L3 and OPAL fit the data directly withthe simulation prediction for the invariant mass as a function of mW, while DELPHI uses eventby event likelihoods exploiting approximated analytical parametrisations of the reconstructeddistributions and correcting the result with a calibration curve evaluated on the simulated data.In the results presented here there is no DELPHI result on the qqτν semileptonic channelyet. ALEPH has presented a measurement on the lν lν final state performed at 183 GeV/c2fitting simultaneously the leptons energy and the missing energy spectra, which are sensitive tothe W mass. Also semileptonic events not entering the former analyses have been treated in asimilar way. The error quoted is 2.5 times bigger than the result on the standard semileptonicanalyses [4].2.3 Error sources on the mW direct reconstruction measurementThe main uncorrelated systematic error sources are the detector resolution effects on the jetsand leptons reconstruction, that spoil the quality of the information brought by the reconstructedfinal state.Several uncertainties correlated among the 4 LEP experiments have to be taken into ac-count: the errors arising from the data-simulation residual discrepancies due to the models ofthe hadronisation of quarks; the LEP energy measurement error and the uncertainty on the the-oretical description of the ISR (both effects imply a shift of the effective centre of mass energyof the event which has to be corrected).A category of error sources specific to the qqqq final state is that due to the final stateinteraction effects that produce interferences between the final state systems coming from the 2Ws: the colour reconnection between the quark pairs from different Ws (that are not fragmen-ting independently) and the Bose-Einstein correlations among identical bosons in the final statecoming from different Ws.The present error quoted for these effects is about 50 MeV/c2, coming from preliminarystudies on phenomenological models. The present results of LEP on the difference between themasses measured in the qqqq and in the qqlν + lν lν channels, which is sensitive to final stateinteraction effects, is [2]:mqqqqW −mqqlν+lν lνW = 152± 74 MeV/c2.2.4 Results on mW from the direct reconstruction methodThe preliminary combined results of the 4 LEP collaborations on mW from the direct recon-struction method are shown in table 1 [2]. The non − qqqq channel corresponds to the combi-nation of the semileptonic analyses and the lν lν analysis of ALEPH.The preliminary combined LEP result from the direct reconstruction approach on all the2Table 1: Preliminary combined LEP results on mW from the direct reconstruction method.Channel mW Stat. Syst. FSI LEP energyqqqq 80.429 0.049 0.043 0.058 0.017non− qqqq 80.313 0.052 0.032 – 0.017channels is:mW = 80.347± 0.057 GeV/c23 Measurement of the W boson widthThe DELPHI, L3 and OPAL collaborations have presented preliminary results on the measure-ment of the W boson width ΓW. The technique used to extract it from the data is the same usedfor the mass: due to the small correlation between the measured values of the 2 observables, thefits can be performed either simultaneously (L3 and OPAL) or separately (DELPHI).The main error sources are the detector resolution and the final state interaction effects. Thesummary of the present LEP results is presented in the table 2.Table 2: Preliminary LEP results for the W boson width ΓW. In the first column the centre ofmass energies corresponding to the data sets analysed are shown.Experiment ΓW Stat. Syst.DELPHI (183) 2.48 0.40 0.10L3 (172+183+189) 2.12 0.25 –OPAL (172+183) 1.84 0.32 0.204 ConclusionsThe preliminary LEP result on the mW measurement combining the cross section method at theWW threshold with the direct reconstruction method at higher energies is shown in the figure1, together with all the other direct (from pp¯ colliders) and indirect measurements of mW. Thecombined error from the direct measurements is approaching the uncertainty from the indirectones. In figure 2 the measured values of mW and of the top quark mass mt are compared withthe expected ones from the global electroweak fits and with the Standard Model predictions asa function of the Higgs boson mass mH .No evidence for a breakdown of the Standard Model can be observed. Also the direct mWmeasurement favours a low Higgs boson mass.References[1] OPAL COll., L3 Coll., DELPHI Coll., ALEPH Coll., LEP Electroweak Working Group,SLD Heavy Flavour Group, CERN-PPE 97-154 (1997).[2] LEP Electroweak Working Group, LEPEWWG/WW/99-01 (1999).3W-Boson Mass  [GeV]mW  [GeV]χ2/DoF: 1.4 / 180 80.2 80.4 80.6pp−-colliders 80.448 ± 0.062LEP2 80.350 ± 0.056Average 80.394 ± 0.042NuTeV/CCFR 80.25 ± 0.11LEP1/SLD/νN/mt 80.364 ± 0.029Figure 1: Global summary of the mW direct andindirect measurements.80.280.380.480.580.6130 150 170 190 210mH [GeV]90 300 1000mt  [GeV]mW  [GeV]Preliminary68% CLLEP1, SLD, νN DataLEP2, pp−  DataFigure 2: mW vs mt from electroweak fits(solid), direct measurements (dotted) and theStandard Model prediction (band).[3] ALEPH Coll., ALEPH 99-017 CONF 99-012 (1999).[4] ALEPH Coll., ALEPH 99-015 CONF 99-010 (1999).[5] DELPHI Coll., DELPHI 99-51 CONF 244 (1999).[6] L3 Coll., L3 Note 2377 (1999).[7] OPAL Coll., Physics Note PN385 (1999).4

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Measurement of the W boson mass and width in $e^{+}e^{-}$ collisions at LEP