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Precision Measurement of the e + e   +   () Cross Section with the ISR Method

Precision Measurement of the e + e   +   () Cross Section with the ISR Method. Michel Davier ( LAL – Orsay) (on behalf of the BaBar Collaboration). physics goals analysis steps e + e   +   () e + e  p + p  () discussion.

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Precision Measurement of the e + e   +   () Cross Section with the ISR Method

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  1. Precision Measurement of the e+e+() Cross Section with the ISR Method Michel Davier (LAL – Orsay) (on behalf of the BaBar Collaboration) • physics goals • analysis steps • e+e+() • e+ep+p() • discussion Tau08 22-25/9/2008

  2. Hadronic Vacuum Polarization and Muon (g–2) Contributions to the Standard Model (SM) Prediction: Dominant uncertainty from lowest order hadronic piece. Cannot be calculated from QCD (“first principles”) – but:we can use experiment Im[ ]  | hadrons |2 had  ”Dispersion relation“  had   ... Tau08 22-25/9/2008

  3. Goals of the Analysis • Measure R=   pt (also RKK) with high accuracy for vacuum polarization calculations, using the ISR method •  channel contributes 73% of ahad • Dominant uncertainty also from  • Also important to increase precision on (MZ2) (EW tests, ILC) • Present systematic precision of ee experiments CMD-2 0.8% SND 1.5% in agreement KLOE (ISR from 1.02 GeV) 1.3% some deviation in shape • Big advantage of ISR: all mass spectrum covered at once, from threshold to 4-5 GeV, with same detector and analysis • Interesting to compare to spectral functions from  decays discrepancy /e+e evaluations (3.0  1.1)%  aim for a measurement with <1% accuracy Tau08 22-25/9/2008

  4. Situation at ICHEP-Tau06 Hadronic HO – ( 9.8 ± 0.1) 10–10 Hadronic LBL + (12.0 ± 3.5) 10–10 Electroweak (15.4 ± 0.2) 10–10 QED (11 658 471.9 ± 0.1) 10–10 inclu-ding: .0 Knecht-Nyffeler,Phys.Rev.Lett. 88 (2002) 071802 Melnikov-Vainshtein, hep-ph/0312226 Davier-Marciano, Ann. Rev. Nucl. Part. Sc. (2004) Kinoshita-Nio (2006) BNL E821 (2004): aexp = (11 659 208.0  6.3) 1010 Observed Difference with BNL using e+e: Tau08 22-25/9/2008

  5. The Relevant Processes ISR FSR ISR + add. ISR ISR + add. FSR Tau08 22-25/9/2008

  6. The Measurement • ISR photon at large angle in EMC • 1 (for efficiency) or 2 (for physics) tracks of good quality • identification of the charged particles • separate pp/KK/mm event samples • kinematic fit (not using ISR photon energy) including 1 additional photon • obtain all efficiencies (trigger, filter, tracking, ID, fit) from same data • measure ratio of ppg(g) to mmg(g) cross sections to cancel ee luminosity additional ISR vacuum polarization ISR photon efficiency • still need to correct for |FSR|2 contribution in mmg(g) and additional FSR, both calculated in QED, but also checked in data (ISR-FSR interference, additional detected photons) otherwise 3-4% syst error Tau08 22-25/9/2008

  7. PEP-II is an asymmetric e+ecollider operating at CM energy of (4S). Integrated luminosity = 531 fb-1 BaBar / PEP II BaBar EMC: • 6580 CsI(Tl) crystals, resolution ~1-2 % high E. BaBar IFR: • resistive plate chambers • BaBar DIRC • particle ID up to 4-5 GeV/c • BaBar SVT and DCH • precision tracking Tau08 22-25/9/2008

  8. Analysis Steps 230.8 fb-1 (U(4S) on-peak & off peak) • Geometrical acceptance • Triggers (L1 hardware, L3 software), background-filter efficiencies • Tracking efficiency • Particle ID matrix (ID and mis-ID efficiencies)   K • Kinematic fitting reduce non 2-body backgrounds 2 cut efficiency additional radiation (ISR and FSR) secondary interactions • Unfolding of mass spectra • Consistency checks for  (QED test, ISR luminosity) and  • Unblinding R • Results on  cross section and calculation of dispersion integral Tau08 22-25/9/2008

  9. MC Generators • Acceptance and efficiencies determined initially from simulation, with data/MC corrections applied • Large simulated samples, typically 10  data, using AfkQed generator • AfkQed: lowest-order QED with additional radiation: ISR with structure function method,  assumed collinear to the beams and with limited energy FSR using PHOTOS • Phokhara 4.0: exact second-order QED matrix element, no more radiation • Studies comparing Phokhara and AfkQed at 4-vector level with fast simulation Tau08 22-25/9/2008

  10. Particle-related Efficiency Measurements tag particle (track, ID) γISR candidate (p, , φ) • benefit from pair production for particle ID • kinematically constrained events • efficiency automatically averaged over running periods • measurement in the same environment as for physics, in fact same events! • applied to particle ID with p/K/m samples, tracking, study of secondary interactions… • assumes that efficiencies of the 2 particles are uncorrelated • in practice not true  this is where 95% of the work goes! • study of 2-particle overlap in the detector (trigger,tracking, EMC, IFR) required • a large effort to reach per mil accuracies (hence the duration of the analysis) Tau08 22-25/9/2008

  11. Data/MC Tracking Correction to ppg,mmg cross sections • single track efficiency • correlated loss probability f0 • probability to produce more than 2 tracks f3 and similarly for pp mmm mpp Tau08 22-25/9/2008

  12. Particle Identification * isolated muons Mmm > 2.5 GeV  efficiency maps (p,v1,v2) impurity (1.10.1) 10-3 * correlated efficiencies/close tracks  maps (dv1,dv2) • Particle identification required to separate XX final processes • Define 5 ID classes using cuts and PID selectors (complete and orthogonal set) • Electrons rejected at track definition level (Ecal, dE/dx) • All ID efficiencies measured xI Barrel ZIFR fIFR XIFR • a tighter  ID (h) is used for tagging in efficiency measurements and to further reject background in low cross section regions. YIFR Forward Endcap Backward Endcap Tau08 22-25/9/2008

  13. Data/MC PID corrections to mm and pp cross sections mmg Two running periods with different IFR performance ppg Tau08 22-25/9/2008

  14. PID separation and Global Test All ‘xx’  solve for all xx(0) and compare with no-ID spectrum and estimated syst. error • N(o)ii hist: predicted from PID dots: measured (no ID) mpp(GeV) Tau08 22-25/9/2008

  15. Kinematic Fitting • kinematic fits to X X ISR add • ISR photon defined as highest energy • Add. ISR fit: add assumed along beams • Add. ‘FSR’ if add detected • Each event recorded on 2D plot • Typical regions defined • Loose 2 cut (outside BG region in plot) for  and  in central  region • Tight 2 cut (ln(2+1)<3) for  in  tail region ppg(g) Tau08 22-25/9/2008

  16. Backgrounds • background larger with loose 2 cut used in 0.5-1.0 GeV mass range • q q and multi-hadronic ISR background from MC samples + normalization from data using signals from 0ISR (qq), and  and  (0) • global test in background-rich region near cut boundary BG fractions in 10-3 at mpp values Fitted BG/predicted = 0.9680.037 pp multi-hadrons mpp (GeV) Tau08 22-25/9/2008

  17. Additional ISR mmgg Angular distribution of add. ISR /beams! Energy cut-off for add. ISR in AfkQed ppgg Tau08 22-25/9/2008

  18. Additional FSR Angle between add  and closest track FSR mmgg ISR Large-angle add.ISR in data  AfkQed ppgg Evidence for FSR data  AfkQed Tau08 22-25/9/2008

  19. 2 cut Efficiency Correction loose c2 • depends on simulation of ISR (FSR), resolution effects (mostly ISR  direction) for mm and pp • c2 cut efficiency can be well measured in mm data because of low background • main correction from lack of angular distribution for additional ISR in AfkQed • common correction: 1% for loose c2, 7% for tight c2 • additional loss for pp because of interactions studied with sample of interacting events agreement data/MC mmgg 1.00  0.16 loose c2 1.07  0.11 tight c2 syst error 0.5-1.5 10-3 Tau08 22-25/9/2008

  20. Checking Known Distributions Cosq* in XX CM /g mm flat at threshold 1+cos2q* bm1 pp sin2q*   P>1 GeV track requirement  loss at cos*1 Tau08 22-25/9/2008

  21. QED Test with mmg sample • absolute comparison of mm mass spectra in data and in simulation • simulation corrected for data/MC efficiencies • AfkQed corrected for incorrect NLO using Phokhara • results for different running periods consistent: (7.9 7.5) 10-3 • full statistics J/y excluded (0.2 – 5.0 GeV) BaBar ee luminosity ISR  efficiency 5.2 syst. trig/track/PID 4.0 Tau08 22-25/9/2008

  22. Unfolding Mass Spectrum • measured mass spectrum distorted by resolution effects and FSR (mpp vs. s’) • unfolding uses mass-transfer matrix from simulation • 2 MeV bins in 0.5-1.0 GeV mass range, 10 MeV bins outside • most salient effect in - interference region (little effect on ampp) Tau08 22-25/9/2008

  23. Systematic Uncertainties • mm ISR lumi • fitted w.r.t. • LO formula pp increased to 20. for preliminary results, pending investigations Tau08 22-25/9/2008

  24. BaBar results Tau08 22-25/9/2008

  25. BaBar results in  region Tau08 22-25/9/2008

  26. BaBar vs. other experiments at large mass Tau08 22-25/9/2008

  27. BaBar vs.other ee data (0.5-1.0 GeV) direct relative comparison of cross sections in the corresponding 2-MeV BaBar bins (interpolation with 2 bins) deviation from 1 of ratio w.r.t. BaBar stat + syst errors included CMD-2 SND KLOE Tau08 22-25/9/2008

  28. BaBar vs.other ee data (r-w interference region) • mass calibration of BaBar checked with ISR-produced J/y mm • expect (0.16  0.16) MeV at  peak •  mass can be determined through mass distribution fit (in progress) • Novosibirsk data precisely calibrated using resonant depolarization • comparison BaBar/CMD-2/SND in - interference region shows no evidence for a mass shift CMD-2 SND Tau08 22-25/9/2008

  29. BaBar vs. IB-corrected  data (0.5-1.0 GeV) relative comparison w.r.t. BaBar of isospin-breaking corrected t spectral functions BaBar data averaged in wider t bins and corrected for - interference Tau08 22-25/9/2008

  30. Computing ampp ALEPH-CLEO-OPAL (DEHZ 2006) (DEHZ 2003) (2008) ampp(1010) FSR correction was missing in Belle, new value 523.5  3.0  2.5 Direct comparison 0.630-0.958 GeV BaBar 369.3  0.8  2.2 CMD-2 94-95 362.1  2.4  2.2 CMD-2 98 361.5  1.7  2.9 SND 361.0  1.2  4.7 Tau08 22-25/9/2008

  31. Conclusions • BaBar analysis of pp and mm ISR processes completed • Precision goal has been achieved: 0.6% in  region (0.6-0.9 GeV) • Absolute mm cross section agrees with NLO QED within 1.2% • Preliminary results available for pp in the range 0.5-3 GeV • Structures observed in pion form factor at large masses • Comparison with results from earlier experiments • discrepancy with CMD-2 and SND mostly below  • large disagreement with KLOE • better agreement with t results, especially Belle • Contribution to am from BaBar agrees better with t results • Deviation between BNL measurement and theory prediction significantly reduced using BaBar pp data a[exp] –a[SM]=(27.5 ±8.4)  10–10  (14.0  8.4)  1010 • Wait for final results and contributions of multi-hadronic modes Tau08 22-25/9/2008

  32. Backup Slides Tau08 22-25/9/2008

  33. PID correction to mm cross section Two running periods with different IFR performance Tau08 22-25/9/2008

  34. Measurement of p-ID efficiencies • ‘’ ID is a set of negative conditions • use  sample from ISR-produced  with h tag: • 0.6<m<0.9 GeV impurity = (3.70.5) 103 • ID and mis-ID efficiencies stored in 2D maps • unlike muons, efficiency sample is not from isolated tracks • biases from tagging and correlated loss studied with MC • 10-3 level Tau08 22-25/9/2008

  35. PID correction to pp cross section Tau08 22-25/9/2008

  36. 2 cut Efficiency Correction: Interactions ppgg loose c2 • secondary interactions mostly from beam pipe (tight doca cut on tracks) • tag events with interactions using displaced vertex with a ‘bad’ track in transverse plane (Rxy) • agreement data/MC 1.00  0.16 loose c2 1.07  0.11 tight c2 syst error 0.5-1.5 10-3 Beam pipe tight c2 Tau08 22-25/9/2008

  37. Mass Calibration Effect of a 1 MeV mass scale shift (calibration with J/y : (0.16  0.16) MeV) mpp (GeV) Tau08 22-25/9/2008

  38. Present BaBar Measurements only statistical errors syst. 5-10% to obtain R in the energy range 1-2 GeV the processes +-30, +-40, K+K-, KSKL, KSKL, KSK+-0 remain to be measured Tau08 22-25/9/2008

  39. BaBar Impact on Multi-Hadronic Modes • Using ISR method the cross sections of e+e +0, 2+2, K+K+, 2K+2Kreactions have been measured from threshold to 4.5 GeV. • These are the most precise measurements to date for c.m. energies greater than 1.4 GeV. • Examples: contributions to ahad (1010) from 2+ 2(0.56 – 1.8 GeV) from all e+ e exp. 14.21  0.87exp  0.23rad from all  data 12.35  0.96exp  0.40SU(2) fromBaBar 12.95  0.64exp  0.13rad from +  0(1.055  1.8 GeV) from all e+ e exp. 2.45  0.26exp  0.03rad fromBaBar 3.31  0.13exp  0.03rad • More modes to come Davier-Eidelman- Hoecker-Zhang 2003 total 696.37.2 Tau08 22-25/9/2008

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