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BaBar results on e + e   +   () and the muon g-2 prediction

BaBar results on e + e   +   () and the muon g-2 prediction. Michel Davier ( LAL – Orsay) (BaBar Collaboration). g-2 context physics goals analysis steps e + e   +   () e + e  p + p  () discussion.

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BaBar results on e + e   +   () and the muon g-2 prediction

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  1. BaBar results on e+e+() and the muon g-2 prediction Michel Davier (LAL – Orsay) (BaBar Collaboration) • g-2 context • physics goals • analysis steps • e+e+() • e+ep+p() • discussion Results still preliminary! BNL 29/1/2009

  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   had • need good data on e+ehadrons at low energies   BNL 29/1/2009

  3. Situation since 2006 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: BNL 29/1/2009

  4. Connecting and ee spectral funct. with CVC W: I=1 &V,A CVC: I=1 &V : I=0,1 &V  e+   hadrons W e– hadrons fundamental ingredient relating long distance (resonances) to short distance description (QCD) Hadronic physics factorizes inSpectral Functions : CVC: SU(2) branching fractionsmass spectrum kinematic factor (PS) SU(2) breaking (EM): short/long distance and mass corrections, r-w interference BNL 29/1/2009

  5. e+e-  Data Comparison since 2006 • problems: overall normalization shape (especially above ) BNL 29/1/2009

  6. 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) 2005 1.3% some deviation in shape 2008 0.8% deviation smaller • Big advantage of ISR: all mass spectrum covered at once, from threshold to 4-5 GeV, with same detector and analysis • Compare to spectral functions from  decays discrepancy /e+e evaluations (3.0  1.1)%  aim for a measurement with <1% accuracy 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 BNL 29/1/2009

  7. ISR method at BaBar : e+e-   f Arbuzov 98’, Binner 99’, Benayoun 99’ f = hadrons or  e+ (3 GeV) e- (9 GeV) gISR  detected at large angle in BaBar at lowest order Cross Section for final state f (normalized to radiative dimuons) “effective c.m. energy-squared” = s(1-x) FSR dL(s’) FSR Corrections for final state radiation ISR luminosity Detection efficiencies BNL 29/1/2009

  8. The relevant processes ISR FSR LO FSR negligible for pp at s(10.6 GeV)2 ISR + additional ISR ISR + additional FSR BNL 29/1/2009

  9. The measurement • ISR photon at large angle in EMC (CM energy >3 GeV) • 1 (for efficiency) or 2 (for physics) tracks of good quality (P>1 GeV) • 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 2-3% syst error BNL 29/1/2009

  10. PEP-II is an asymmetric e+ecollider operating at CM energy of (4S). Integrated luminosity = 531 fb-1 BaBar at 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 BNL 29/1/2009

  11. Analysis steps 230.8 fb-1 (U(4S) on-peak & off peak) • 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 • Geometrical acceptance • Consistency checks for  (QED test, ISR luminosity) and  • Unblinding R • Results on  cross section and calculation of dispersion integral Preliminary Final BNL 29/1/2009

  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 QED with additional radiation: ISR with structure function method,  assumed collinear to the beams and with limited energy FSR using PHOTOS similar to EVA (Phokhara ancestor) • Phokhara 4.0: (almost) exact second-order QED matrix element (2 FSR missing), limited to one extra photon • Studies comparing Phokhara and AfkQed at 4-vector level with fast simulation BNL 29/1/2009

  13. 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) BNL 29/1/2009

  14. 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 BNL 29/1/2009

  15. 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 BNL 29/1/2009

  16. Data/MC PID corrections to mm and pp cross sections Runs 1-2 Runs 3-4 mmg Two running periods with different IFR performance ppg BNL 29/1/2009

  17. PID separation and Global Test (small pp contribution subtracted statistically) 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) BNL 29/1/2009

  18. 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) BNL 29/1/2009

  19. Backgrounds in ppg • 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) BNL 29/1/2009

  20. Additional ISR mmgg Angular distribution of add. ISR /beams! Energy cut-off for add. ISR in AfkQed ppgg BNL 29/1/2009

  21. Additional FSR Angle between add  and closest track FSR mmgg ISR Large-angle add.ISR in data  AfkQed ppgg Evidence for FSR data  AfkQed BNL 29/1/2009

  22. 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 correction data/MC mmgg (J/ region excluded) BNL 29/1/2009

  23. 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 BNL 29/1/2009

  24. 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 • agreement with QED within 1.2% J/y excluded (0.2 – 5.0 GeV) BaBar ee luminosity ISR  efficiency 5.2 syst. trig/track/PID 4.0 BNL 29/1/2009

  25. Unfolding the pp 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) BNL 29/1/2009

  26. Systematic Uncertainties • mm ISR lumi • fitted w.r.t. • LO formula pp increased to 20. for preliminary results, pending investigations BNL 29/1/2009

  27. BaBar results BNL 29/1/2009

  28. BaBar results in  region BNL 29/1/2009

  29. BaBar vs. other experiments at large mass • structures observed at large mass: deep dip (1.6 GeV), wider dip (2.2 GeV) • interferences between higher-mass vector mesons BNL 29/1/2009

  30. BaBar vs.CMD-2 and SND (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 both CMD-2 and SND determine e+e+ cross section through the ratio (pp+mm)/ee with assumed QED leptonic cross sections published and revised (rad. corr.) SND BNL 29/1/2009

  31. BaBar vs.KLOE (0.5-1.0 GeV) ISR method from f(1020) only p+p- detected no m+m- analysis yet ISR luminosity from Phokhara KLOE 2005 KLOE Dec. 2008 new data, found bias in cosmic veto for 2005 (superseded) smaller systematic errors BNL 29/1/2009

  32. BaBar vs.CMD-2 and SND (r-w interference region) • mass calibration of BaBar checked with J/ mm (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 BNL 29/1/2009

  33. 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 Belle 2008: large statistics BNL 29/1/2009

  34. Computing ampp !preliminary! ALEPH-CLEO-OPAL (DEHZ 2006) (DEHZ 2003) (2008) ampp(1010) FSR correction was missing in Belle, published 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 BNL 29/1/2009

  35. Next steps in BaBar analysis • still preliminary results (presented at Tau Workshop, Sept. 2008) • data not shown below 0.5 GeV: problems noticed (near threshold) • also new consistency check between loose and tight 2 not • satisfactory (systematic uncertainty enlarged) • problems now understood • new results in progress • BaBar review • final results in a couple of months BNL 29/1/2009

  36. Multihadronic channels: BaBar ISR measurements many ISR BaBar results already published on e+ehadrons for larger multiplicities only statistical errors syst. 5-10% to complete the R measurement in the energy range 1-2 GeV the processes +30, +40, K+K, KSKL, KSKL, KSK++0 are being measured BNL 29/1/2009

  37. Contributions to amhad from multihadronic modes • The BaBar results are the most precise measurements to date for CM 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 reminder: total 690.9  4.4 (DEHZ 2006) BNL 29/1/2009

  38. 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 • some discrepancy with CMD-2 and SND mostly below  • larger 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 would be reduced using preliminary BaBar pp data a[exp] –a[SM]=(27.5 ±8.4)  10–10  (14.0  8.4)  1010 BUT • Wait for final results and contributions of all multi-hadronic modes BNL 29/1/2009

  39. Outlook for muon g2 SM prediction Hadronic LO contribution • potentially existing e+e data can reach a precision Damhad = 2.5 (1010) • hopeful that remaining discrepancies will be brought close to quoted systematic uncertainties • use of t data limited by knowledge of isospin-breaking corrections: present precision 2.5 (1010), total error 3.9 (1010), work in progress • new data expected from VEPP-2000 with CMD-2 and SND Hadronic LBL contribution • relies only on phenomenological estimates, precision DamhadLBL = 3.5 (1010) • more progress? lattice? Other contributions • QED, electroweak, HO hadronic: Damothers = 0.2 (1010) g2 experimental error • BNL E821 (2004) Damexp = 6.3 (1010) • a new measurement will be needed to match the ‘theory’ uncertainty BNL 29/1/2009

  40. Backup Slides BNL 29/1/2009

  41. PID correction to mm cross section Two running periods with different IFR performance BNL 29/1/2009

  42. 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 BNL 29/1/2009

  43. PID correction to pp cross section BNL 29/1/2009

  44. 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) • comparison data/MC syst error at 10-3 level Beam pipe tight c2 BNL 29/1/2009

  45. Mass calibration mass calibration using J/y --> mm, scaled to r region : (0.16  0.16) MeV) mass resolution 6 MeV effect of a 1 MeV mass scale shift mpp (GeV) BNL 29/1/2009

  46. SU(2) breaking in t and ee spectral functions Electromagnetism does not respect isospin and hence we have to consider isospin breaking when dealing with an experimental precision of better than 1% • Corrections for SU(2) breaking applied to  data for dominant  – + contrib.: • Electroweak radiative corrections: • dominant contribution from short distance correction SEW to effective 4-fermion coupling  (1 + 3(m)/4)(1+2Q)log(MZ /m) • subleading corrections calculated and small • long distance radiative correction GEM(s) calculated [ add FSRto the bare cross section in order to obtain  – + () ] • 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 [not corrected !] • Electromagnetic decays, like:     ,    ,    ,   l+l – • Quark mass difference mu  mdgenerating “second class currents” (negligible) Marciano-Sirlin’ 88 Braaten-Li’ 90 Cirigliano-Ecker-Neufeld’ 02 Lopez Castro et al. 06 Alemany-Davier-Höcker’ 97, Czyż-Kühn’ 01 Lopez Castro et al. 08 BNL 29/1/2009

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