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W mass at LEP2: toward the final results

W mass at LEP2: toward the final results. Andrea Venturi INFN Pisa Acknowledgement: all the results have been provided by LEP Experiments and LEP W Working Group Disclaimer: future perspectives are due to my own personal view. W mass measurement is important because :

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W mass at LEP2: toward the final results

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  1. W mass at LEP2:toward the final results Andrea Venturi INFN Pisa Acknowledgement: all the results have been provided by LEP Experiments and LEP W Working Group Disclaimer: future perspectives are due to my own personal view A.Venturi - W mass at LEP - From Zero to Z0: Precision EW Physics

  2. W mass measurement is important because: direct vs indirect measurements: EW radiative correction test improve indirect Higgs boson mass determination optimum: DmW ~ 0.007 Dmtop W boson at LEP2 e+e- collider ECM> 2MW: WW pair production ~ 700 pb-1/exp  ~10000 WW evts Final states ~45% qqqq ~44% lnqq ~11% lnln High purities and efficiencies W mass measurement and LEP2 A.Venturi - W mass at LEP - From Zero to Z0: Precision EW Physics

  3. W mass from direct final state reconstruction WW4q WWmnqq Jets and leptons 4 momenta reconstruction: tracks+calo Neutrino 4 momentum reconstruction: Spi=0 A.Venturi - W mass at LEP - From Zero to Z0: Precision EW Physics

  4. E, p conservation: improved resolution(6-8  2-4 GeV) neutrino reconstrcution  LEP energy affects W mass Jet pairing in 4q events Matrix element, c2, ...  ~80-90% of correct pairings W mass extraction W mass dependent reweighted MC to fit estimator distributions W mass, kinematic fit errors,... BW or event likelihood fit + MC calibration W width from 2 params fit MC is needed to correct biases: ISR, resolutions, thresholds,... Systematics due to data vs MC discrepancies W mass extraction event likelihood A.Venturi - W mass at LEP - From Zero to Z0: Precision EW Physics

  5. LEP Results (no OPAL 2000) MW=80.4120.029(stat)0.031(syst) GeV sMW = 42 MeV GW=2.150 0.068(stat)0.060(sist)GeV DMW(qqqq-lnqq)= +2243 MeV optimized linear combination dominated by correlated uncertainties direct measurement sMW = 43 MeV threshold measurement included With full statistics (OPAL 2000): sMW = 43 MeV  41 MeV(direct reco) Systematic uncertainties Fragmentation and LEP energy large because correlated Final state Interactions (CR+BE) reduce the weight of 4q channel: 9% The Future (thanks to LEP W workshops) : final/revised analyses and systematics final LEP energy calibration address the FSI systematic issue Present results (Winter 2003) systematics breakdown A.Venturi - W mass at LEP - From Zero to Z0: Precision EW Physics

  6. Because of kinematic fit (Etot=ELEP) DMW/MW DELEP/ELEP energy and expts. correlated Beam energy calibration: Resonant depol. up to 60 GeV Extrapolation up to 100 GeV with NMR probes (B field measurement) NMR extrapolation checks Flux loop measurement Spectrometer (NEW) Beam energy from beam deflection Synchrotron oscillations (NEW) Beam energy from synchrotron radiation energy losses Calibration uncertainty NEW: DEbeam=10-20 MeV  DMW=10 MeV old DEbeam=20-25 MeV  DMW=17 MeV Effect on W mass: sMW = 41 MeV  39 MeV LEP energy calibration A.Venturi - W mass at LEP - From Zero to Z0: Precision EW Physics

  7. Final State Interactions in qqqq channel • Hadronically decaying W pairs short living (~0.1 fm)their decay products can interact among each other • Colour Reconnection (CR) • color singlets across W’s • Bose-Einstein correlation (BE) • coherently produced identical pions are closer in phase space (also from different W’s?) • We cannot ask ourselves which W a particle comes from • reconstructed W mass distribution is affected • not included in usual MC models  possible bias on the W mass measurement • Relevant at the end of the hadronic shower (CR) or after the hadronization (BE) • full MC calculation is impossible  predictions rely on phenomenological MC models A.Venturi - W mass at LEP - From Zero to Z0: Precision EW Physics

  8. CR models: have to survive comparison and tuning with Z peak data BE correlation: LUBOEI (JETSET) inter W corr as strong as intra W DMW=35 MeV Constrain MC models to reduce W mass uncertainty Before Summer 2002: "theory" driven sMW=40 (CR) + 25 (BE) MeV Now: data driven (particle flow) sMW = 74-105 MeV (CR)+ 35 MeV (BE: full LUBOEI effect) Future (??): data driven cones and momentum cuts search for BE corr in data FSI syst uncertainties from MC models SKI model (strings) Mass shift in MeV SKI controlled by parameter Ki % of reconnected events A.Venturi - W mass at LEP - From Zero to Z0: Precision EW Physics

  9. SKI 1s upper limit predicts the largest W mass shift no sensitivity to Ariadne and Herwig sMW = 74 (172 GeV) MeV 105 (207 GeV) MeV (since Summer 2002) correlated among experiments  strong reduction of 4q W mass weight in combination Particle Flow to constrain CR models 3 0=4 1 2 • In WW 4q events particles projected to the closest inter-jet plane and angles w.r.t. jet measured • Inter-jet angles normalized to 1 • Distribution folded and binned ratio done. A.Venturi - W mass at LEP - From Zero to Z0: Precision EW Physics

  10. Work in progress: reduce W mass sensitivity on CR • CR is expected to affect mainly: • low momentum particles • particles away from the jet core • Measure W mass by removing: • or low momentum particles (pcut) • or far away particles (cone) • Two-fold result: • new W mass estimator less sensitive to CR biases (and to BE too). • Worse stat (and framg?) error • the difference std W mass and (pcut/cone) W mass used to “measure” CR and constrain the MC models • it can be combined with Particle Flow A.Venturi - W mass at LEP - From Zero to Z0: Precision EW Physics

  11. BE correlation: dedicated analyses and W mass • BEcorrelations between different W’s has been being investigated at LEP • 2-particles correlations in 4q events vs two “mixed” lnqq events • ALEPH, L3 and OPAL: no hint for BE corr • DELPHI evidence for BE corr • (un)consistency under investigation • W mass uncertainty: from LUBOEI: • sMW=35 MeV • it can be reduced using BEC data analyses • cones and pcuts reduce BEC effects on W mass A.Venturi - W mass at LEP - From Zero to Z0: Precision EW Physics

  12. Constraints on CR models:  sMW(CR)=50- ??? MeV very personal estimate Depends on: actual result from pcuts and cone how result is used how many experiments do it Impact on W mass(red area)  sMW = 39  39-37.5 MeV Reduced sensitivity analysis cones or pcuts CR + BE: -50% statistical error: +30% fragmentation error: +20% Depends on actual results how many experiments Impact on W mass(blue area) sMW = 39  39-36.5 MeV FSI and W mass uncertainty: past, present and future ? CR W mass shift (MeV) (189 GeV, reduced sensitivity) MeV After Summer 02 data driven Total uncertainty (BE 35-20) Before Summer 02 "theory" driven Total uncertainty reduced sensitivity CR W mass shift (MeV) (189 GeV) A.Venturi - W mass at LEP - From Zero to Z0: Precision EW Physics

  13. Detector simulation systematic uncertainty • Present uncertainties • sMW= 5-35 MeV  14 MeV (lnqq) • sMW= 5-25 MeV  10 MeV (qqqq) • uncorrelated among experiments • Work in progress • jet mass simulation • inter-jet angles • event reconstruction tuning • Uncertainties may change ! • +30% → sMW < +1.5 MeV • Detector response  biases and resolutions • accurate simulation is needed • jets and leptons • Z events to correct/validate MC • W mass syst. estimated from: • correction statistical errors • alternative simulations • data vs MC comparison at peak Z • data vs MC in radiative Z events Ejet(data)/Ejet(MC) at Z peak A.Venturi - W mass at LEP - From Zero to Z0: Precision EW Physics

  14. Radiative (ISR) Z boson production at LEP2 Z mass (or LEP energy) measurement is the closest (by still quite different) to W mass measurement Very useful test for data/MC agreement reconstruction and simulation tuning systematics evaluation Data/MC comparison, an example: radiative Z Jet 1 Jet 2 ALEPH LEP jamboree jet boost data vs MC ALEPH LEP jamboree rescaled di-jet mass A.Venturi - W mass at LEP - From Zero to Z0: Precision EW Physics

  15. YFSWW MC used by all experiments ISR: O(a3) LL exp. NL O(a) EW corrections in Double Pole Approximation Non-factorizable corrections included RacoonWW different implementations useful for tests Not available at the beginning of LEP2 ! affect WW xsect and TGC Estimated TU: with simplified analyses sMW< 5 MeV RacoonWW vs YFSWW comparison all observed differences are understood so far Present (and future ?) real analyses uncertainty:  sMW< 10 MeV QED/EW radiative corrections -- YFSWW paper ALEPH mnqq ALEPH 4q ISR: O(a3)-O(a2) NL O(a) Real photons + constrained fit NF A.Venturi - W mass at LEP - From Zero to Z0: Precision EW Physics

  16. Hadronization/fragmentation • MC models to generate hadrons • particle spectra, angular distributions and contents (baryons) are affected • Interplay with real detector • resolutions, thresholds • biases, non-linearities,… • How well data are simulated • JETSET used by all LEP experiments • internal MC parameters tuned with Z peak data by each experiments • Systematic uncertainty on W mass • Changing the MC parameters • MC tuning statistical errors • Compare MC models • JETSET vs ARIADNE vs HERWIG • improve model tunings to reduce the W mass differences  sMW=10-30 MeV  18 MeV • correlated among channels and experiments • Work in progress • real data vs MC comparison: • change tuning parameters within errors • An important effect: • baryon masses are neglected • reconstructed jet masses are biased • can explain difference between models • baryon rate to be controlled R. Stroehmer A.Venturi - W mass at LEP - From Zero to Z0: Precision EW Physics

  17. Conclusions • Final W mass results from LEP should be available by the end of this year (Winter conferences ?) • What will be different w.r.t. the present results • Full statistics (OPAL) • “best” analyses applied to the full datasets • New and improved LEP energy calibration • sEbeam: from ~20-25 to ~10-20 MeV • Improved constraints on FSI models reduction on W mass uncertanty • reduced sensitivity on FSI effects (pcut and cone) ? • Other systematics revised • more conservative detector systematics ?? • Possible improvements on W mass uncertainty • sMW = 43 → 39 MeV → 39 - 37.5 MeV → 39 - 36.5 MeV • plus W mass from WW xsect at threshold ( ~ 1 MeV) A.Venturi - W mass at LEP - From Zero to Z0: Precision EW Physics

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