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W Boson mass and width

W Boson mass and width. measurements at CDF. Emily Nurse. University College London. Imperial College London seminar, July 13th 2007. Overview. Standard Model Precision Measurements Motivation for W mass and width measurements The Tevatron and CDF W and Z production

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W Boson mass and width

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  1. W Boson mass and width measurements at CDF Emily Nurse University College London Imperial College London seminar, July 13th 2007 Emily Nurse

  2. Overview • Standard Model Precision Measurements • Motivation for W mass and width measurements • The Tevatron and CDF • W and Z production • W and Z reconstruction at CDF • Analysis Strategy and Measurement steps • Results and implications Emily Nurse

  3. Standard Model The Standard Model (SM) describes the Universe’s fundamental building blocks and their interactions. Comparisons of predictions with experimental data have successfully tested the theory to a high precision but some questions remain un-answered. What’s the origin of particle mass? (SM Higgs?) Is the SM the full story?(SUSY?, extra-dimensions?, …??) Emily Nurse

  4. Discovering new physics Precision Measurements of SM Parameters Direct Discovery of New Particles 80 180 280 Reconstructed Mass(GeV) Emily Nurse

  5. Testing the SM - W and Z bosons • The W and Z bosons were predicted by Glashow, Salam and Weinberg’s electroweak theory in the 1960s • discovered by the UA1/UA2 experiments in 1983, with masses (MW and MZ) consistent with the tree level predictions. • Current SM calculations make very accurate predictions of MW and MZand the widths (W and Z) including higher order radiative corrections (i.e. through remormalisation of SM parameters). • LEP experiments measure MZ=91187.6  2.1 MeV (0.002%) and Z=2495.2  2.3 MeV (0.09%). • LEP2 and Tevatron experiments measure MW=80403  29 MeV (0.04%) and W=2141  41 MeV (1.9%). prior to these results Emily Nurse

  6. Testing the SM - W mass GF is found from muon lifetime measurements and can be predicted in terms ofMW e g e+ W+ + g  (tree level) Write g in terms of  and cosw=MW / MZ and rearrange: known to 0.015% MW known to 0.036% known to 0.0009% MZ known to 0.002% rW: radiative corrections dominated by tb and Higgs loops  we can constrain MH by precisely measuringMWand Mt rW could also have contributions from new particle loops Emily Nurse

  7. Testing the SM - W width Within the SMWis predicted by summing leptonic and hadronic partial widths: W0 =(We) is precisely predicted in terms of MWand GF : PDG: J. Phys. G 33, 1 W= 2091 2 MeV predominantly from MW (Note: Most higher order corrections are absorbed in the experimental values of MWand GF.) • Measuring W tests this accurate SM prediction (deviations of which suggest non-SM decay modes). • W is an input to the MW measurement: MW~W / 7. Emily Nurse

  8. The Tevatron The Tevatron currently has ~2.5 fb-1 on tape (6-8fb-1 expected by the end of Run II). The Tevatron is a W/Z factory (as well as many other things!) : (Wl) ~ 2700 pb (currently ~7 million created, ~0.9 million to analyse). (Zll) ~ 250 pb (currently 0.7 million created, ~40 thousand to analyse). But : precision measurements are hard! We need a “precision level” calibration of our detector to keep systematics low. These analyses are based on 200/350pb-1 of CDF data. Emily Nurse

  9. W and Z production at the Tevatron LEADING ORDER The large masses (~100 GeV ) of W and Z bosons gives their decay products large pT. The electron and muon channels are used to measure W properties, due to their clean experimental signature. Similar for Z production (decays into two charged leptons) W events: Charged lepton is detected and momentum directly measured. Neutrino cannot be detected! Transverse momentum (pT) is inferred by a vector sum of the total “transverse energy (Esin)” in the detector. The “missing ET (ETmiss)” is found by constraining the sum to zero interpreted as the neutrino pT. Z events: Both charged leptons are detected and their momenta measured. Emily Nurse

  10. W and Z production at the Tevatron HIGHER ORDER CORRECTIONS Initial state gluon radiation from incoming quarks gives the W a boost in the transverse direction  W pT The recoiling gluons form hadrons that are detected in the calorimeter  Hadronic recoil Final state  radiation affects the kinematics of the charged lepton Goes into ETmiss = pT measurement! Emily Nurse

  11. Detecting particles at CDF Electrons:detected in central trackers (drift chamber provides p measurement) and EM calorimeter (provides energy measurement). Muons:detected in central trackers (drift chamber provides p measurement), calorimeter (MIP signal) and muon chambers. ETmiss :Hadronic recoil found by summing the EM and HADRONIC calorimeter energy. S I L I C O N DRIFT CHAMBER S O L E N O I D EM HADRONIC MUON CHAMBERS e  TRACKERS CALORIMETERS Emily Nurse

  12. Analysis strategy: measuring MW and W • Ideal world: MW and Wwould be reconstructed from from the invariant mass of the W decay products (Breit-Wigner lineshape of propagator peaks at the mass and has an intrinsic width). • Reality:The neutrino is not detected thus the invariant mass cannot be reconstructed. Instead we reconstruct the transverse mass. W MW Breit-Wigner: -channel: central tracker e-channel: EM calorimeter inferred from missing transverse energy Emily Nurse

  13. Analysis strategy: measuring MW and W • MW/Wfound from MT MC template fits data. • Simulate MT distribution with a dedicated fast parameterised MC. • Utilise well understood data samples (Z events used extensively) to calibrate detector simulation to high precision - we need an excellent description of the lineshape! - W fit range: 90 -200 GeV - MW fit range:65 - 90 GeV W templates MW templates Emily Nurse

  14. MW vs W • The MW and W analyses are very similar - with different dominant uncertainties. • They are performed independently using 200(350)pb-1 of data for the MW (W) analyses . • As I describe the the measurement steps I will discuss the method used in the analysis for which the effect is more important: MW = W = Emily Nurse

  15. Measurement Steps Backgrounds: Generator effects: PDFs, QCD, QED corrections. Hadronic recoil measurement: Muon momentum measurement: Electron energy measurement: pT = ETmiss = -(U + pTlep) Emily Nurse

  16. Measurement Steps : 1 Backgrounds: Generator effects: PDFs, QCD, QED corrections. Hadronic recoil measurement: Muon momentum measurement: Electron energy measurement: pT = ETmiss = -(U + pTlep) Emily Nurse

  17. Generator effects : PDFs Parton Distribution Functions (PDFs) are parameterised functions that describe the momentum distribution of quarks in the (anti)proton. Different PDFs result in different acceptance and spectra: Use CTEQ6M and the CTEQ6 ensemble of 2x20 error PDFs (20 orthogonal parameters varied up and down within their errors). MW = 11 MeV, W = 17 MeV Emily Nurse

  18. Generator effects :QCD/QED corrections • Simulate QCD corrections (initial state gluon radiation) using RESBOS [Balazs et.al. PRD56, 5558]:NLO QCD + resummation + non-perturabtive. • Constrain non-perturbative parameter using our own Z data : MW = 3 MeV, W = 7 MeV • QED bremsstrahlung reduces l  pT • Simulated at NLO (one-) using Berends&Kleiss [Berends et.al. ZPhys. C27, 155] / WGRAD [Baur et.al. PRD59, 013002]. • PHOTOS[Barberio et.al. Comput. Phys. Comm., 66, 115] used to establish systematic due to neglecting NNLO (two-) terms. MW () = 12 MeV, W () = 1 MeV MW (e) = 11 MeV, W (e) = 8 MeV Emily Nurse

  19. Measurement Steps : 2 Backgrounds: Generator effects: PDFs, QCD, QED corrections. Hadronic recoil measurement: Muon momentum measurement: Electron energy measurement: pT = ETmiss = -(U + pTlep) Emily Nurse

  20. Lepton momentum calibration (pT) Momentum scale set with di-muon resonance peaks in data, using well known particle masses: J/  mm;(1S)  mm;Zmm Data MC M (GeV) Data MC M (GeV) p scale known to 0.021% MW () = 17 MeV, W () = 17 MeV 1/<pT>(GeV-1) Emily Nurse

  21. Lepton momentum resolution (pT) • fullMC =(q/pT)meas - (q/pT )gentaken from full GEANT MC. • Sample this histogram and multiply by a constant parameter:fastMC = SresfullMC • Sresfound by tuning to M in Zmm data. MW () = 3 MeV, W () = 26 MeV Emily Nurse

  22. Measurement Steps : 3 Backgrounds: Generator effects: PDFs, QCD, QED corrections. Hadronic recoil measurement: Muon momentum measurement: Electron energy measurement: pT = ETmiss = -(U + pTlep) Emily Nurse

  23. The electron’s journey through CDF energy leakage “out the back” of the EM calorimeter energy measurement in EM calorimeter energy loss in solenoid track momentum measurement in COT bremsstrahlung in silicon Emily Nurse

  24. Electron energy calibration/resolution(pTe) Calorimeter scale: Emeas = scaleEtrue Calorimeter resolution: (E) / E = 13.5% / √ET   Scale and  found in two independent ways: • Fit to Mee peak in Zee data using well known Z mass/width. • Fit to E/p in We data (since p has already been well calibrated.) Fundamentally E = p (electron mass is negligible). Photons are emitted from electron (bremsstrahlung) which reduces p. The photons usually end up in the same calorimeter tower as the electron thus E doesn’t decrease. electron energy measured in EM calorimeter electron momentum measured in central tracker Emily Nurse

  25. Electron energy calibration/resolution(pTe) E Data MC Data MC p E/p Mee (GeV) Leakage “out the back” of the EM calorimeter Bremsstrahlung in tracker E scale known to 0.034% scale: resolution: MW (e) = 30 MeV, W (e) = 17 MeV MW (e) = 11 MeV, W (e) = 31 MeV Emily Nurse

  26. Measurement Steps : 4 Backgrounds: Generator effects: PDFs, QCD, QED corrections. Hadronic recoil measurement: Muon momentum measurement: Electron energy measurement: pT = ETmiss = -(U + pTlep) Emily Nurse

  27. Hadronic Recoil: U pT = ETmiss = -(U + pTlep) • To get pT we need a good model of the total energy in W events. • U=(Ux, Uy)=towersEsin (cos,sin) • Vector sum over calorimeter towers Excluding those surrounding lepton • Recoil has 3 components: (3) Underlying energy Multiple interactions and remnants from collision. (1) QCD Gluons recoiling off the boson (2) Bremsstrahlung Photons emitted by lepton that do not end up in the excluded region Emily Nurse

  28. Hadronic Recoil: U • Accurate predictions of U is difficult (and slow) from first principles. • U simulated with ad-hoc parameterised model, tuned on Zll data. • U split into components parallel (U1) and • perpendicular (U2) to Z pT • 7 parameter model describes the response and resolution in the U1 and U2 directions as a function of the Z pT. • Systematic comes from parameter uncertainties (limited Z stats). U2 U1 MW () = 12 MeV W () = 49 MeV response resolution Data MC MW (e) = 14 MeV W (e) = 54 MeV Zee Zee Emily Nurse

  29. Measurement Steps : 5 Backgrounds: Generator effects: PDFs, QCD, QED corrections. Hadronic recoil measurement: Muon momentum measurement: Electron energy measurement: pT = ETmiss = -(U + pTlep) Emily Nurse

  30. Backgrounds x x x x K, fake high-pT track x x x x muon channel only: Need the mT distributions and the normalisations! Decay In-Flight electron and muon channel: • Kaon/pion decays “In-Flight” to a . • Kink in track gives high-pT measurement. • ETmiss from mis-measured track pT. Zll W • One lepton lost • ETmiss from missing lepton • decays to e/ • intrinsic ETmiss multijet • Jet fakes/contains a lepton • ETmiss from misconstruction Emily Nurse

  31. Backgrounds: Muon channel final cut value /NDF • Dominant background is Z (it’s easy to lose a muon leg!) - but we can estimate this background very reliably (using full MC). • Decay In-Flight (DIF) has large mT tails: problematic for the width! The handles we have on DIF are track quality 2track and impact parameter. Fractional background found from a template fit to the 2track distribution. Z provides the signal template MW () = 9 MeV, W () = 33 MeV High impact parameter cuts provide the DIF template Emily Nurse

  32. Backgrounds: Electron channel final cut value • Multijet has large mT tails: problematic for the width! Fractional multijet background found from a template fit to the ETmiss distribution. “Anti-electron” sample provides the mulitjet template MW (e) = 8 MeV, W (e) = 32 MeV Emily Nurse

  33. Measurement Steps Backgrounds: Generator effects: PDFs, QCD, QED corrections. Hadronic recoil measurement: Muon momentum measurement: Electron energy measurement: pT = ETmiss = -(U + pTlep) Emily Nurse

  34. Results: MWfits Data MC Data MC MT(e) (GeV) MT() (GeV) Also includes fits to pTlandpT: MW = 80413  48 (stat + syst) MeV Emily Nurse

  35. MW systematic uncertainties Emily Nurse

  36. MW : world average World’s most precise single measurement! Central value increases by 6 MeV: 80392  80398 MeV Uncertainty reduced by 15%: 29  25 MeV Emily Nurse

  37. MW : Implications Previous World Data : Including New MW : Including New Mt : Direct search from LEP II : mH > 114.4 GeV @ 95% C.L. Emily Nurse

  38. Results: Wfits W = 2032  71 (stat + syst) MeV Emily Nurse

  39. W systematic uncertainties Emily Nurse

  40. W : world average World’s most precise single measurement! Central value decreases by 44 MeV: 2139  2095 MeV Uncertainty reduced by 22%: 60  47 MeV Emily Nurse

  41. Indirect Width Measurement CDF Run II INDIRECT width : 2092 ± 42 MeV CDF Run II DIRECT width : 2032 ± 71 MeV preliminary PRL 94, 091803   BR (Wl) Rexp =   BR (Zll) Precision LEP Measurements SM Calculation NNLO Calculation W (Wl) (Z) R = X X Z (W) (Zll) Emily Nurse

  42. Projections Naïve statistical scaling, 20 MeV syst. limit 20 MeV syst limit 2.5fb-1 : ~25 MeV ~35 MeV Emily Nurse

  43. Summary • Two new measurements from CDF: • W mass : 80413 ± 48MeV (stat + syst) • W width : 2032 ± 71MeV (stat + syst) • Both are the world’s most precise single measurements!! • Getting to this point requires a “precision” level calibration of the detector. • Together with direct Higgs searches we will continue to squeeze the phase space available to the SM Higgs. • Analyses utilised 200 pb-1 and 350 pb-1 respectively, both CDF and DØ already have ~2.5 fb-1 on tape. • Working on improved mass/width measurements to further test the SM and constrain mH Emily Nurse

  44. Back-up slides… Emily Nurse

  45. W mass GF (fermi-coupling constant) can be predicted in terms of MW e  e  W width: W’ analysis excludes W’ < 788 GeV Emily Nurse

  46. Generator effects : PDFs 9 - highx valence quarks. ∆XW=0.5*√ (∑i ([∆iup-∆idown) 2 ))/1.6 Emily Nurse

  47. Generator effects :QCD corrections RESBOS:NLO QCD + resummation + non-perturbative. Collins-Soper-Sterman (CSS) resummation formalism. Sums LL terms + sub-logs Brock-Landry-Nadolsky-Yuan (BLNY) form: exp [-g1 - g2ln(Q/2Q0) - g1g3ln(100x1x2)]b2 g1 =0.210.01; g1 =0.68+0.01-0.02; g3 =-0.6+0.05-0.04; From fits to R209, E288, E605 fixed target Drell-Yan data (5< shat <18 GeV) + CDF RunI Emily Nurse

  48. Bremsstrahlung in si (Bethe-Heitler equation) Migdal suppression Conversions (Bethe-Heitler equation) Compton scattering (for low energy photons: scattering off e ~ conversions) Ionisation energy loss Energy loss in coil Leakge into HAD calorimeter Acceptance Ionisation energy loss Multiple scattering Acceptance Simulated effects Electrons Muons Emily Nurse

  49. visible EM energy fraction hadronic calorimeter electromagnetic cal. superconducting coil log10(incident electron energy) Simulating Electrons () : CAL Energetic electrons leak into the hadronic compartment Soft electrons suffer absorption in the coil Emily Nurse

  50. Hadronic Recoil: U (pT) U|| U • U split into components parallel (U||) and • perpendicular (U) to charged lepton. • Many distributions used to cross check the model in Wl data: We We Data MC Emily Nurse

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