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Precise Measurement of the e + e   +   () Cross Section with BaBar and the Muon g-2

Precise Measurement of the e + e   +   () Cross Section with BaBar and the Muon g-2. Michel Davier ( LAL – Orsay, BaBar Collaboration). the muon magnetic anomaly e + e  and (revisited)  spectral functions the BaBar ISR (Initial State Radiation) analysis

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Precise Measurement of the e + e   +   () Cross Section with BaBar and the Muon g-2

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  1. Precise Measurement of the e+e+() Cross Section with BaBar and the Muon g-2 Michel Davier (LAL – Orsay, BaBar Collaboration) • the muon magnetic anomaly • e+e and (revisited)  spectral functions • the BaBar ISR (Initial State Radiation) analysis • test of the method: e+e+() • results on e+ep+p() • combination of all ee data • discussion and perspectives Oxford 27/10/2009

  2. Lepton Magnetic Anomaly: from Dirac to QED Dirac (1928) ge=2 ae=0 anomaly discovered: Kusch-Foley (1948) ae= (1.19  0.05) 103 and explained by O() QED contribution: Schwinger (1948) ae = /2 = 1.16 103 first triumph of QED  ae sensitive to quantum fluctuations of fields Oxford 27/10/2009

  3. More Quantum Fluctuations + ? a new physics ? typical contributions: QED up to O(4), 5 in progress (Kinoshita et al.) Hadronsvacuum polarizationlight-by-light (models) Electroweak new physics at high mass scale  a much more sensitive to high scales Oxford 27/10/2009

  4. Hadronic Vacuum Polarization and Muon (g–2) Dominant uncertainty from lowest-order HVP piece Cannot be calculated from QCD (low mass scale), but one can use experimental data on e+ehadrons cross section Bouchiat-Michel (1961) Brodsky-de Rafael (1968) Im[ ]  | hadrons |2  had    Dispersion relation Oxford 27/10/2009

  5. The E-821 Direct a Measurement at BNL Storage ring technique pioneered at CERN (Farley-Picasso…) a obtained from a ratio of frequencies result updated with new value for /p (+0.9 1010) (see next review in RPP2009 (Hoecker-Marciano) aexp = (11 659 208.9  5.4  3.3) 1010 ( 6.3) (0.54 ppm) precessionrotation Oxford 27/10/2009

  6. The Role of Data through CVC – SU(2) W: I=1 &V,A CVC: I=1 &V : I=0,1 &V  e+   hadrons W e– hadrons Hadronic physics factorizes (spectral Functions) branching fractionsmass spectrum kinematic factor (PS) Oxford 27/10/2009

  7. SU(2) Breaking • Corrections for SU(2) breaking applied to  data for dominant  – + contrib.: • Electroweak radiative corrections: • dominant contribution from short distance correction SEW • subleading corrections (small) • long distance radiative correction GEM(s) • Charged/neutral mass splitting: • m–  m0leads to phase space (cross sec.) and width (FF) corrections •  - mixing (EM    – + decay) corrected using FF model • m–  m0 *** and –  0 *** • Electromagnetic decays:      ***,    ,    ,   l+l – • Quark mass difference mu  md (negligible) Marciano-Sirlin’ 88 Braaten-Li’ 90 Cirigliano-Ecker-Neufeld’ 02 Lopez Castro et al.’ 06 Alemany-Davier-Höcker’ 97, Czyż-Kühn’ 01 Flores-Baez-Lopez Castro’ 08 Davier et al.’09 Oxford 27/10/2009

  8. Situation at ICHEP’06 / 08 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 e+e  BNL .0 Knecht-Nyffeler (2002), Melnikov-Vainhstein (2003) Davier-Marciano(2004) Kinoshita-Nio (2006) Observed Difference with BNL using e+e: Estimate using  data consistent with E-821 Oxford 27/10/2009

  9. Revisited Analysis using  Data: Belle + new IB Relative comparison of  and ee spectral functions ( green band) arXiv:0906-5443 MD et al. slope… KLOE CMD-2, SND Global test of spectral functions: prediction of  BR using ee data  larger disagreement with KLOE Oxford 27/10/2009

  10. Goals of the BaBar Analysis • Measure [e+ e +  ()] with high accuracy for vacuum polarization calculations, using the ISR method e+ e +   () •  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) 2005 1.3% some deviation in shape 2008 0.9% better agreement • Big advantage of ISR: all mass spectrum covered at once, from threshold to 3 GeV, with same detector and analysis • Measure simultaneously +   () and +   () • Compare to spectral functions from previous e+ e data and  decays  aim for a measurement with <1% accuracy (syst. errors at per mil level) great interest to clarify the situation as magnitude of possible discrepancy with SM is of the order of SUSY contributions with masses of a few 100 GeV Oxford 27/10/2009

  11. The Relevant Processes e+ e +   () and +   () measured simultaneously ISR FSR LO FSR negligible for pp at s(10.6 GeV)2 ISR + add. ISR ISR + add. FSR Oxford 27/10/2009

  12. 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 Oxford 27/10/2009

  13. Analysis Steps 232 fb-1 (U(4S) on-peak & off peak) • Measure ratio of ppg(g) to mmg(g) cross sections to cancel ee luminosity, additional ISR, vacuum polarization,ISR photon efficiency (otherwise 1-2% syst.) • ISR photon at large angle in EMC + 2 tracks • Geometrical acceptance (using Monte Carlo simulation) • All efficiencies measured on data (data/MC corrections) • 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, FSR), secondary interactions • Unfolding of mass spectra • Consistency checks for  (QED test, ISR luminosity) and  • Unblinding R  partial preliminary results (Tau08, Sept. 2008) • Additional studies and checks • Final results on  cross section and calculation of dispersion integral Oxford 27/10/2009

  14. 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 (LO) QED with additional radiation: ISR with structure function method,  assumed collinear to the beams and with limited energy FSR using PHOTOS • Phokhara 4.0: (almost) exact second-order QED matrix element, limited to NLO • Studies comparing Phokhara and AfkQed at 4-vector level with fast simulation • QED test with    () cross section requires reliable NLO generator •   () cross section obtained through  /  ratio, rather insensitive to detailed description of radiation in MC Oxford 27/10/2009

  15. Particle-related Efficiency Measurements tag particle (track, ID) γISR candidate (p, , φ) • benefit from pair production for tracking and 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  study of 2-particle overlap in the detector (trigger,tracking, EMC, IFR) required a large effort to reach per mil accuracies Oxford 27/10/2009

  16. PID separation and Global Test All ‘xx’  solve for all xx(0) and compare with no-ID spectrum and estimated syst. error BaBar BaBar • N(o)ii hist: predicted from PID dots: measured (no ID) mpp(GeV) Oxford 27/10/2009

  17. Kinematic Fitting • Two 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 • First analysis to measure cross section with additional photons (NLO) • Loose 2 cut (outside BG region in plot) for  and  in central  region • Tight 2 cut (ln(2+1)<3) for  in  tail region • q q and multi-hadronic ISR background from MC samples + normalization from data using signals from 0ISR (qq), and  and  (0) ppg(g) Oxford 27/10/2009

  18. 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 incomplete NLO using Phokhara • strong test (ISR probability drops out for ) BaBar (0.2  3 GeV) BaBar ee luminosity ISR  efficiency 3.4 syst. trig/track/PID 4.0 Oxford 27/10/2009

  19. Obtaining the () cross section Acceptance from MC + data/MC corrections Unfolded spectrum Effective ISR luminosity from () analysis (similar equation + QED)  mass spectrum unfolded (Malaescu arXiv:0907-3791) for detector response Additional ISR almost cancels in the procedure (() / () ratio) Correction (2.5 1.0) 103   cross section does not rely on accurate description of NLO in the MC generator ISR luminosity from  in 50-MeV energy intervals (small compared to variation of efficiency corrections) Oxford 27/10/2009

  20. Systematic uncertainties s’ intervals (GeV) errors in 103 Dominated by particle ID (-ID, correlated ’’, -ID in ISR luminosity) Oxford 27/10/2009

  21. BaBar results (arXiv:0908.3589) e+ e  +  () bare (no VP) cross section diagonal errors stat+syst Oxford 27/10/2009

  22. BaBar results in  region 2-MeV energy intervals Oxford 27/10/2009

  23. VDM fit of the pion form factor  Running (VP) add. FSR Oxford 27/10/2009

  24. BaBar vs. other experiments at larger mass Oxford 27/10/2009

  25. BaBar vs.other ee data (0.5-1.0 GeV) direct relative comparison of cross sections with BaBar fit (stat + syst errors included) (green band) BaBar CMD-2 SND KLOE Oxford 27/10/2009

  26. BaBar vs. IB-corrected  data (0.5-1.0 GeV) relative comparison w.r.t. BaBar of isospin-breaking corrected t spectral functions IB corrections: radiative corr.,  masses, - interference,  masses/widths each  data normalized to its own BR ALEPH CLEO Belle Oxford 27/10/2009

  27. Computing ampp (1010) 0.281.8 (GeV) BABAR 514.1  3.8 previous e +e combined 503.5  3.5 *  combined 515.2  3.5 * * * * arXiv:0906-5443 MD et al. Oxford 27/10/2009

  28. Including BaBar in the e+e Combination arXiv: 0908.4300 MD-Hoecker-Malaescu-Yuan-Zhang Improved procedure and software (HVPTools) for combining cross section data with arbitrary point spacing/binning Oxford 27/10/2009

  29. Obtaining the average cross section relative weights • local weighted average performed • full covariance matrices • local 2 used for error rescaling • average dominated by BaBar and • KLOE, BaBar covering full range error scale factor integrand (dispersion integral) error Oxford 27/10/2009

  30. Other hadronic contributions from MD-Eidelman-Hoecker-Zhang (2006)  another large long-standing discrepancy in the +  20 channel ! Oxford 27/10/2009

  31. The Problematic 2 20 Contribution preliminary BaBar data: A. Petzold, EPS-HEP (2007) ee data used now (CMD2 discarded) only statistical errors old contribution 16.8  1.3 update 17.6  1.7 probably still underestimated (BaBar)  21.4  1.4 Oxford 27/10/2009

  32. BaBar Multi-hadronic Results only statistical errors syst. 5-10% Still more channels under analysis: K+K, KK with K0 Oxford 27/10/2009

  33. Where are we? • including BaBar 2 results in the e+e combination + estimate of hadronic • LBL contribution (Prades-de Rafael-Vainhstein, 2009) yields • aSM[e+e] = (11 659 183.4 4.1 2.6 0.2) 1010 • HVP LBL EW (4.9) • E-821 updated result 11 659 208.9 6.3 • deviation (ee) 25.5  8.0 • (3.2 ) • updated  analysis • +Belle +revisited IB corrections • deviation () 15.7  8.2 • (1.9 ) Oxford 27/10/2009

  34. Discussion • BaBar 2 data complete and the most accurate, but expected precision • improvement on the average not reached because of discrepancy with KLOE • however, previous /ee disagreement strongly reduced • 2.9 (2006)  2.4 ( update)  1.5 (including BaBar) • a range of values for the deviation from the SM can be obtained, depending • on the 2 data used: • BaBar 2.4 • all ee 3.2 • all ee BaBar 3.7 • all ee KLOE 2.9 •  1.9 • all approaches yield a deviation, but SM test limited by systematic effects not • accounted for in the experimental analyses (ee) and/or the corrections to  data • at the moment some evidence for a deviation (24), but not sufficient to establish • a contribution from new physics Oxford 27/10/2009

  35. Perspectives • first priority is a clarification of the BaBar/KLOE discrepancy: • - origin of the ‘slope’ (was very pronounced with the 2004 KLOE results, • reduced now with the 2008 results) • - normalization difference on  peak (most direct effect on a) • - Novosibirsk results in-between, closer to BaBar • - slope also seen in KLOE/ comparison; BaBar agrees with  • further checks of the KLOE results are possible: as method is based on MC • simulation for ISR and additional ISR/ISR probabilities  long-awaited test • with  analysis • contribution from multi-hadronic channels will continue to be updated with • more results forthcoming from BaBar, particularly 2 20 • more ee data expected from VEPP-2000 in Novosibirsk • experimental error of E-821 direct a measurement is a limitation, already now •  new proposal submitted to Fermilab to improve accuracy by a factor 4 •  project at JPARC Oxford 27/10/2009

  36. Conclusions • BaBar analysis of pp and mm ISR processes completed • Precision goal has been achieved: 0.5% in  region (0.6-0.9 GeV) • Absolute mm cross section agrees with NLO QED within 1.1% • eepp()cross section very insensitive to MC generator • full range of interest covered from 0.3 to 3 GeV • Structures observed in pion form factor at large masses • Comparison with data from earlier experiments • fair agreement with CMD-2 and SND, poor with KLOE • agreement with t data • Contribution to am from BaBar is (514.1 2.23.1)1010 in 0.28-1.8 GeV • BaBar result has comparable accuracy (0.7%) to combined previous results • Deviation between BNL measurement and theory prediction reduced using BaBar pp data a[exp] –a[SM] =(19.8 ±8.4)10–102 from BaBar only 25.5  8.0 combined ee including BaBar Oxford 27/10/2009

  37. Backup Slides Oxford 27/10/2009

  38. Data on e+e  hadrons CMD-2 (2006) CMD-2 (2004) SND (2006) KLOE (2009) Oxford 27/10/2009

  39. Revisited Analysis using  Data: including Belle Oxford 27/10/2009

  40. Revisited Analysis  Data: new IB corrections Oxford 27/10/2009

  41. 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 • correct for |FSR|2 contribution in mmg(g) (QED, <1% below 1 GeV) • additional FSR photons measured otherwise ~2% syst error Oxford 27/10/2009

  42. 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 Oxford 27/10/2009

  43. 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 Oxford 27/10/2009

  44. Data/MC PID corrections to mm and pp cross sections mmg Two running periods with different IFR performance ppg Oxford 27/10/2009

  45. 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 10-2 at mpp values Fitted BG/predicted = 0.9680.037 BaBar pp multi-hadrons mpp (GeV) Oxford 27/10/2009

  46. 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 much better study now, 2 independent methods mmgg secondary interactions data/MC 1.51  0.03 syst error 0.3 - 0.9 10-3 Oxford 27/10/2009

  47. Additional ISR mmgg Angular distribution of add. ISR /beams! Energy cut-off for add. ISR in AfkQed ppgg Oxford 27/10/2009

  48. Additional FSR Angle between add  and closest track FSR mmgg ISR Large-angle add.ISR in data  AfkQed Evidence for FSR data  AfkQed ppgg data/MC  0.960.06  1.210.05 Oxford 27/10/2009

  49. 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 Oxford 27/10/2009

  50. Unfolding  Mass Spectrum • measured mass spectrum distorted by resolution effects and FSR (mpp vs. s’) • iterative unfolding method (B. Malaescu arXiv:0907-3791) • mass-transfer matrix from simulation with corrections from data • 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) BaBar Absolute difference unfolded(1)  raw data unfolded(2)  unfolded(1) Statistical errors (band) Mass (GeV) BaBar Relative difference Oxford 27/10/2009

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