The electroweak measurements made at LEP using 1989-1993 data are presented
in preliminary form. The agreement with the Standard Model is satisfactory, and
allows a combined fit to all available data for the masses of the top quark and
standard Higgs boson. The fit yields M_t = 177 +11 -11 +18 -19 GeV/c2, where
the second error reflects the uncertainty in the Higgs mass.
  Talk given at the XXIXth Rencontre de Moriond, `QCD and High Energy Hadronic
Interactions', March 1994, Meribel FranceComment: 8 pages, 330K uufiled Postscript, preprint CERN-PPE/94-9

Jacobsen, Bob

English

arXiv

The electroweak measurements made at LEP using 1989-1993 data are presented in preliminary form. The agreement with the Standard Model is satisfactory, and allows a combined fit to all available data for the masses of the top quark and standard Higgs boson. The fit yields M_t = 177 +11 -11 +18 -19 GeV/c2, where the second error reflects the uncertainty in the Higgs mass. Talk given at the XXIXth Rencontre de Moriond, `QCD and High Energy Hadronic Interactions', March 1994, Meribel Franc

CERN Document Server

hep-ex/9407002   06 Jul 94 EUROPEAN ORGANIZATION FOR NUCLEAR RESEARCHCERN-PPE/94-97June 27, 19941 T OP  M ASS   FROM  E LECTROWEAK  M EASUREMENTS 1CERN, PPE DivisionCH-1211 Geneva 23Switzerland Bob Jacobsen The electroweak measurements made at LEP using 1989-1993 data are presentedin preliminary form. The agreement with the Standard Model is satisfactory, andallows a combined fit to all available data for the masses of the top quark andstandard Higgs boson. The fit yields  GeV/c 2 , where the seconderror reflects the uncertainty in the Higgs mass. 1 Introduction The 1993 LEP running was dedicated to a scan of the Z boson lineshape[1]. The errors onmeasurements of other electroweak[2] observables, notably the asymmetries[3] and heavyquark widths, were also improved. Table 1 lists the 1993 preliminary combined LEPvalues. This talk presented these measurements in light of what they tell us about the massof the top quark in the minimal Standard Model.[4]The combination of an approximately four-fold increase in off-peak data, and an improvedcalibration[5] of the LEP energy, allowed the combined error from the four LEPexperiments on the Z boson mass to be decreased from 7 to 4.4 MeV/c 2 , and on the widthfrom 7 to 3.8 MeV/c 2 . The mass error is now dominated by the systematic uncertainty inthe LEP energy, while the width error has approximately equal contributions fromstatistics and LEP energy uncertainty. Small additional improvements are expected whenthe measurements are final. This has resulted in significant improvements on the leptonicbranching ratio . Improvements in several of the LEP experiment’s luminosity 1 Talk presented at the XXIXth Rencontres de Moriond, “QCD and Hadronic Interactions”, Meribel, France, March 19-26, 1994Mt 177 11– 19–+11+18=Rl  2 Measurement Standard Model fit valuepull LEP line shape: (GeV) 91.1895  ±  0.044 91.192 0.6 (GeV) 2.4969  ±  0.0038 2.4967 0.1 (nb) 41.51  ±  0.12 41.44 0.620.789  ±  0.040 20.781 0.20.0170  ±  0.0016 0.0152 1.1 τ  polarization:0.150  ±  0.010 0.142 0.80.120  ±  0.012 0.142 1.8b and c quark results:0.2208  ±  0.0024 0.2158 2.00.170  ±  0.014 0.172 0.10.0960  ±  0.0043 0.0997 0.80.070  ±  0.011 0.071 0.1 charge asymmetry: from 0.2320  ±  0.0016 0.2321 0.1  and  ν N  (GeV) 80.23  ±  0.18 80.31 0.40.2256  ±  0.0047 0.2246 0.2 SLC  from 0.2294  ±  0.0010 0.2321 2.7 Table 1: Data input and results of the combined Standard Modelfit. All LEP values are preliminary. The  collider data are fromUA2[6] CDF[7][8] and D0[8]. The neutrino experiment data arefrom CDHS[9] CHARM[10] and CCFR[11]. The SLC value isfrom the SLD measurement[12] of the left-right asymmetry. Thesecond column is the measurement value and error used in thecombined fits. The third column value results from the fit to all data(fourth column of Table 3), and the fourth column is the differencebetween the measured and fit values, normalized by themeasurement error.MZΓZσh0RlAFB0 l,AτAeRb Γbb Γhad⁄≡RcΓccΓhad⁄≡AFB0 b,AFB0 c,qqsin2θWeff QFB〈 〉ppMW1 MW2 MZ2⁄ υN( )–sin2θWeff Aepp 3calorimeters have significantly reduced the systematic errors on absolute cross sectionmeasurements, leading to improved errors on the peak hadronic cross-section, .All asymmetries at the Z can be used to measure the weak coupling angle . Themeasurements of lepton and c quark asymmetries, the polarization of tau leptons, and thetotal charge asymmetry have also been improved from 1992 values. The most preciseestimate comes from the forward-backward asymmetry of b quark events (See Table 2).Combining these to a single estimate of  from LEP data alone gives 0.2322 ± 0.005with a  of 6.3/5, corresponding to a 28% confidence level. Including the SLDmeasurement of  from the left-right asymmetry increases the  to 12.8/6,giving a confidence level of 5%. The biggest difference in the combined set ofmeasurements is between the values of  from A LR  and the  τ  forward-backwardasymmetry, which both measure the electron coupling A e , and are dominated by statisticalerrors.  The Z partial widths to b and c quarks have improved errors, due primarily to the use ofnew lifetime-based techniques. These measurements are now dominated by systematicerrors, but all four experiments expect further progress in the coming year. 2 Standard Model Fits The values listed in Table 1 were used as input to a Standard Model fit, and the results aresummarized in Table 3. Assuming a Higgs mass of 300 GeV/c 2 , the top mass is fitted to be177 ± 11 GeV/c 2  using all data. The improvement since the 1993 summer conferences isprimarily due to the new Z width measurement, and the measurement is dominated by theLEP data. The  value implied by the fit is 0.124  ±  0.005  ±  0.002, quite consistentwith independent measurements at LEP and elsewhere.  Measurement Implied  value LEP 0.0170  ±  0.0016 0.2311  ±  0.00090.150  ±  0.010 0.2311  ±  0.00130.120  ±  0.012 0.2350  ±  0.00150.0960  ±  0.0043 0.2328  ±  0.00080.070  ±  0.011 0.2324  ±  0.0026 from 0.2320  ±  0.0016 0.2320  ±  0.0016 SLC 0.1637  ±  0.0075 0.2294  ±  0.0010 Table 2:  central values and errors implied by variousmeasurements. LEP numbers are averages of the four experiment’svalues.σh0sin2θWeffsin2θWeffχ2 d.o.f⁄sin2θWeff χ2 d.o.f⁄sin2θWeffsin2θWeffAFB0 l,AτAeAFB0 b,AFB0 c,sin2θWeff QFB〈 〉ALRsin2θWeffαsMZ2( )4The Rb and the forward-backward τ polarization asymmetry measurements contribute 4.0and 3.2 respectively to the  and tend to reduce the fitted top mass. The SLDmeasurement of ALR tends to increase the top mass and contributes 7.3 to the . The corresponds to a confidence level for the Standard Model hypothesis of 8.6% forthe fit to all data, which is certainly acceptable. The fit to just LEP data and LEP pluscollider and neutrino data have confidence levels of 28% and 40% respectively.Γlepton, , and Rb give approximately independent constraints on Mt. Figure 1 showsthe Γlepton vs.  plane with the Standard Model predictions and combined fit resultsoverlaid. The preferred top and Higgs masses are clearly correlated in this projection. As can be seen in Figure 2, the Standard Model value for Rb is almost independent of theHiggs mass[13]. The measured Rb value alone provides a 90% confidence level upperlimit on the top mass of 180 GeV/c2. Figure 3 is a different way of plotting the same dataand shows the effect of Rb, when combined with the Rhad and the combined measurements, is to prefer lower Higgs masses. Figure 4 shows the variation of the fit as a function of top mass for three Higgs mass assumptions. The variation with Higgsmass is not large enough to be considered statistically significant, so the entire range of 60to 1000 GeV/c2 is used to estimate the uncertainty in the top mass due to the unknownHiggs mass. The final value and error for the top mass is then  GeV/c2, where thesecond error represents the uncertainty due to the unknown Higgs mass.LEP only LEP, collider and ν data LEP, SLC, collider and ν data (GeV)0.125 ±0.005 ±0.002 0.125 ± 0.005 ± 0.002 0.124 ± 0.005 ± 0.00211.4/9 11.5/11 19.1/12 (GeV)Table 3: Results of fits within the Standard Model hypothesis of thedata of Table 1. The central values and first errors are quoted for a fixedHiggs mass of 300 GeV/c2. The second error reflects the change incentral value when the Higgs mass is varied from 60 GeV/c2 to 1000GeV/c2. Column two includes only the LEP data (top section in Table1), column three includes the  and νN data from the center section ofTable 1, and column four also includes the SLD result from the left-rightasymmetry.  has been left free in all fits.Mt 172 14– 20–+13+18 170 12– 20–+12+18 177 11– 19–+11+18αsMZ2( )χ2 d.o.f⁄sin2θWeff 0.2323 0.0002 0.0002–+0.0001± 0.2324 0.0002 0.0002–+0.0001± 0.2320 0.0003 0.0002–+0.0001±1 MW2 MZ2⁄– 0.2251 0.0015 0.0003–+0.0003± 0.2253 0.0013 0.0002–+0.0003± 0.2243 0.0012 0.0002–+0.0003±MW 80.28 0.08 0.02–+0.01± 80.26 0.07 0.01–+0.01± 80.31 0.06 0.01–+0.01±ppαsMZ2( )χ2χ2χ2 d.o.f⁄sin2θWeffsin2θWeffsin2θWeffχ2Mt 177 11– 19–+11+18=5AcknowledgmentsThe LEP results presented here were contributed by the LEP collaborations. Theaveraging and combined fits were done by the LEP Electroweak Working Group. I wouldparticularly like to thank A. Blondel, P. Clarke, J. Harton, M. Koratzinos, M. Martinez,R. Miquel, M. Pepe-Alterelli, B. Pietrzyk, D. Schaile, and R. Tenchini for their help. Figure 1: The  vs. Γlepton plane, overlaid with the results of the fit to alldata and Standard Model predictions. The shaded band is the 1 σ limit on the topmass from CDF. The ★ indicates the Standard Model expectation withoutelectroweak radiative corrections. sin2θWeff0.2290.230.2310.2320.2330.2340.2350.2360.23783 83.5 84 84.5 856References[1] P. Clarke, presented at the XXIXth Rencontres de Moriond, March 12-19 1994[2] CDF’s evidence for a top quark mass of 174±10±13 was presented after this talk was delivered,and is not included in these results. Their mass estimate is consistent with the LEP predictionsF. Abe et.al., Fermilab-Pub-94/097-E, submitted to Phys. Rev. D.[3] B. Pietrzyk, presented at the XXIXth Rencontres de Moriond, March 12-19 19940.20.210.220.230.240.225 0.23 0.235 0.24Figure 2: The Rb vs.  plane, overlaid with current measurements andStandard Model predictions. An  value of 0.123±0.006 was used toproduce this plot; lower values would move the Rhad band up.sin2θWeffαsMZ2( )7[4] R. Miquel, CERN-PPE/94-70, presented at the 22nd INS Symposium, “Physics with High Energy Colliders”, March 8-10, 1994[5] J. Wenninger, presented at the XXIXth Rencontres de Moriond, March 12-19 1994[6] UA2 Collaboration, J.Alitti et al., Phys. Lett. B276(1992) 354[7] CDF Collaboration, F. Abe et al., Phys. Rev. D43(1991) 2070[8] CDF and D0 preliminary results, Y. Ducros, La Thuile, March 6-12, 1994Figure 3: The Rb vs.  plane. The intersection of the Standard Modelpredictions and the fit contour (compare to Figure 2) shows that the preference forlow Higgs masses is primarily due to the higher-than-Standard-Model valuemeasured for Rb.sin2θWeff0.20.210.220.230.240.225 0.23 0.235 0.248[9] CDHS Collaboration, H. Abramowicz et al., Phys. Rev. Lett. 57(1986) 298CDHS Collaboration, A. Blondel et al., Z. Phys. C45(1990) 361[10] CHARM Collaboration, J. V. Allaby et al., Phys. Lett. B177(1986) 446 CHARM Collaboration, J. V. Allaby et al., Z. Phys. C36(1987) 611[11] CCFR Collaboration, NEVIS-R 1498, Submitted to Phys. Rev. Lett.[12] SLD Collaboration, K. Abe et al., SLAC-PUB-6456, submitted to Phys. Rev. Lett.[13] A. Blondel and C. Verzegnassi, Phys. Lett. B311(1993) 346Figure 4: The variation of the  of the fit to all data as a function of the top massfor three different assumed Higgs masses. The preference for small Higgs masses isnot statistically compelling.χ2

Top Mass from Electroweak Measurements

Abstract

Similar works

Full text

Available Versions

CERN Document Server