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ISR cross sections in e + e - and implications on muon g-2

ISR cross sections in e + e - and implications on muon g-2. Bogdan MALAESCU ( LAL – Orsay ) ( Representing the BaBar Collaboration ). Outlook. The muon magnetic anomaly The BaBar ISR (Initial State Radiation) analysis Test of the method: e + e   +   ()

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ISR cross sections in e + e - and implications on muon g-2

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  1. ISR cross sections in e+e- and implications on muon g-2 Bogdan MALAESCU ( LAL – Orsay ) ( Representing the BaBar Collaboration ) HEP 2010

  2. Outlook • The muon magnetic anomaly • The BaBar ISR (Initial State Radiation) analysis • Test of the method: e+e+() • Results on e+e+ () (PRL 103, 231801  (2009)) • Combination of all e+e data • Discussion and conclusion HEP 2010

  3. 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 HEP 2010

  4. 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 HEP 2010

  5. 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  Im[ ]  | hadrons |2  had   Dispersion relation HEP 2010

  6. The E-821 Direct a Measurement at BNL* Storage ring technique pioneered at CERN (Farley-Picasso…, 1970s), with μ+ and μ- data a obtained from a ratio of frequencies result updated with new value for /p (+0.9 1010) (see new review in RPP2009 (Hoecker-Marciano)) aexp = (11 659 208.9  5.4  3.3) 1010 ( 6.3) (0.54 ppm) precessionrotation *Bennet et al., 2006, Phys. Rev. D 73, 072003 HEP 2010

  7. Goals of the BaBar Analysis • Measure [e+ e +  (FSR)] with high accuracy for vacuum polarization calculations, using the ISR method e+ e +  ISR (add.) •  channel contributes 73% of ahad • Dominant uncertainty also from  • 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 in BABAR), with same detector and analysis • The ISR method needs a very large amount of data at an energy larger than the one at which we measure the cross section • Measure simultaneously +  ISR (add.) and +  ISR (add.) • Compare the measured +  ISR (add.) spectrum with QED prediction • 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 HEP 2010

  8. The Relevant Processes e+ e +  ISR (add.) and +  ISR (add.) measured simultaneously ISR FSR LO FSR negligible for pp at s(10.6 GeV)2 ISR + add. ISR ISR + add. FSR HEP 2010

  9. BaBar / PEP II BaBar EMC: • 6580 CsI(Tl) crystals, resolution ~1-2 % high E. • PEP-II is an asymmetric e+ecollider operating at CM energy of (4S). • Integrated luminosity = 531 fb-1 BaBar IFR: • resistive plate chambers • BaBar DIRC • particle ID up to 4-5 GeV/c • BaBar SVT and DCH • precision tracking HEP 2010

  10. The Measurement • ISR photon at large angle, detected in the EMC • detected tracks of good quality: 1 (for efficiency) or 2 (for physics) • identification of the charged particles (DIRC, DCH) • separate pp/KK/mm event samples • kinematic fit (not using ISR photon energy) including 1 additional photon (discussed later) • 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 LO FSR (|FSR|2) contribution in mmg(g) (QED, <1% below 1 GeV) • additional FSR photons measured otherwise ~2% syst error HEP 2010

  11. Analysis Steps Used an integrated luminosity of 232 fb-1 (U(4S) on-peak & off peak) • 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 and FSR) secondary interactions • Unfolding of mass spectra • Consistency checks for  (QED test, ISR luminosity) and  • Unblinding R  partial (√s`>0.5 GeV) preliminary results (Tau08, Sept. 2008) • Additional studies and checks • Final results on  cross section and calculation of dispersion integral (PRL 103, 231801  (2009)) HEP 2010

  12. 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 • (FSR) cross section obtained through () / () ratio, rather insensitive to detailed description of radiation in MC HEP 2010

  13. 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… • 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 HEP 2010

  14. Kinematic Fitting • First analysis to measure cross section • with additional photons (NLO) • 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 • Loose 2 cut (outside BG region in plot) for  and  in central  region • Tight 2 cut (ln(2+1)add.ISR<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) HEP 2010

  15. 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  cross section) BaBar (0.2  3 GeV) BaBar ee luminosity ISR  efficiency 3.4 syst. trig/track/PID 4.0 HEP 2010

  16. Obtaining the (FSR) cross section Acceptance from MC + data/MC corrections Unfolded spectrum Effective ISR luminosity from () analysis (similar equation + QED)  mass spectrum unfolded (B. M. 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) HEP 2010

  17. Systematic uncertainties s` intervals (GeV) errors in 103 Dominated by particle ID (-ID, correlated ‘’, -ID in ISR luminosity) HEP 2010

  18. BaBar results (arXiv:0908.3589, PRL 103, 231801  (2009)) e+ e  +  (FSR) bare (no VP) cross section diagonal errors (stat+syst) HEP 2010

  19. BaBar results in  region 2-MeV energy intervals HEP 2010

  20. BaBar vs. other experiments at larger mass HEP 2010

  21. VDM fit of the pion form factor  Running (VP) add. FSR BaBar Preliminary Fit BaBar Preliminary Fit BaBar Preliminary Fit HEP 2010

  22. 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 BaBar Preliminary Fit BaBar Preliminary Fit SND KLOE BaBar Preliminary Fit BaBar Preliminary Fit HEP 2010

  23. BaBar vs. IB-corrected  data (0.5-1.0 GeV) • relative comparison w.r.t. BaBar of • spectral functions corrected for isospin-breaking(IB) • IB corrections: radiative corr.,  masses, • - interference,  masses/widths • each  data normalized to its own BR ALEPH BaBar Preliminary Fit CLEO Belle BaBar Preliminary Fit BaBar Preliminary Fit HEP 2010

  24. Computing ampp 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 M. Davier et al. HEP 2010

  25. Including BaBar in the e+e Combination arXiv: 0908.4300 Davier-Hoecker-BM-Yuan-Zhang Improved procedure and software (HVPTools) for combining cross section data with arbitrary point spacing/binning, with full covariance matrices and error propagation HEP 2010

  26. Other hadronic contributions from Davier-Eidelman-Hoecker-Zhang (2006)  another large long-standing discrepancy in the +  20 channel ! HEP 2010

  27. The Problematic +-20 Contribution preliminary ISR BaBar data: A. Petzold, EPS-HEP (2007) e+e-  +20 data (CMD2 discarded) only statistical errors Preliminary old contribution 16.8  1.3 update 17.6  1.7 probably still underestimated (BaBar)  21.4  1.4 HEP 2010

  28. BaBar Multi-hadronic Published Results Statistical + systematic errors Still more channels under analysis: K+K, KK with K0 HEP 2010

  29. 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) 1010 • deviation (ee) (25.5  8.0) 1010 • (3.2 ) • updated  analysis • +Belle +revisited IB corrections • deviation () (15.7  8.2) 1010 • (1.9 ) *new proposal submitted to Fermilab to improve accuracy by a factor 4 HEP 2010

  30. Conclusions • BaBar analysis of p+p-and m+m-ISR processes completed • Precision goal has been achieved: 0.5% in  region (0.6-0.9 GeV) • Absolute m+m- cross section agrees with NLO QED within 1.1% • eep+p-()cross section very insensitive to MC generator • full range of interest covered from 0.3 to 3 GeV • 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 an accuracy (0.7%) comparable to combined previous results • Contribution from multi-hadronic channels will continue to be updated with • more results forthcoming from BaBar, particularly +-20 • Deviation between BNL measurement and theory prediction reduced using BaBar p+p- data a[exp] –a[SM] =(19.8 ±8.4)10–10+- from BaBar only 25.5  8.0 combined e+e- including BaBar HEP 2010

  31. Backup Slides HEP 2010

  32. Data on e+e  hadrons CMD-2 (2006) CMD-2 (2004) SND (2006) KLOE (2009) HEP 2010

  33. 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) HEP 2010

  34. 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 HEP 2010

  35. 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 HEP 2010

  36. Revisited Analysis using  Data: Belle + new IB Relative comparison of  and ee spectral functions ( green band) arXiv:0906.5443 M.Davier et al. slope… KLOE CMD-2, SND Global test of spectral functions: prediction of  BR using ee data  larger disagreement with KLOE HEP 2010

  37. Revisited Analysis using  Data: including Belle HEP 2010

  38. Revisited Analysis  Data: new IB corrections HEP 2010

  39. 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 HEP 2010

  40. 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 HEP 2010

  41. Data/MC PID corrections to mm and pp cross sections mmg Two running periods with different IFR performance ppg HEP 2010

  42. 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) HEP 2010

  43. 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) HEP 2010

  44. 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 HEP 2010

  45. Additional ISR mmgg Angular distribution of add. ISR /beams! Energy cut-off for add. ISR in AfkQed ppgg HEP 2010

  46. Additional FSR FSR Angle between add  and closest track 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 HEP 2010

  47. 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 HEP 2010

  48. Unfolding  Mass Spectrum • measured mass spectrum distorted by resolution effects and FSR (mpp vs. s’) • iterative unfolding method (B. M. arXiv:0907-3791) : one step is sufficient • 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 HEP 2010

  49. Obtaining the () cross section Acceptance from MC + data/MC corrections Unfolded spectrum Effective ISR luminosity from () analysis (similar equation + QED) 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 ratio mm ISR lumi / LO formula should behave smoothly (HVP effects on resonances cancel) Use measured lumi in 50-MeV bins averaged in sliding 250-MeV bins for smoothing HEP 2010

  50. Changes since preliminary results at Tau08 • preliminary results Sept. 2008: only 0.5-3 GeV (excess/expect. near threshold) • problem explored (Oct. 2008- Feb. 2009): trigger/BGFilter, ee background • `’ re-investigated  direct measurement achieved using ID probabilities before: model for correlated loss, no precise direct check  significant changes  efficiency for ISR lumi  +0.9%  contamination in `’ sample    cross section  1.8% 0.525 GeV 1.0% 0.775 GeV 1.4% 0.975 GeV • other changes: - MC unfolding mass matrix corrected for data/MC differences (small) - ISR lumi now used in 50-MeV sliding bins, instead of global fit - cancellation of add ISR in / ratio studied/corrected • extensive review HEP 2010

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