1 / 58

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   +   ()

walden
Download Presentation

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

An Image/Link below is provided (as is) to download presentation Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author. Content is provided to you AS IS for your information and personal use only. Download presentation by click this link. While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server. During download, if you can't get a presentation, the file might be deleted by the publisher.

E N D

Presentation Transcript


  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

More Related