M.Davier BaBar pi pi Tau /9/ 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         discussion

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

M.Davier BaBar pi pi Tau /9/  Measure R  =     pt (also R KK ) 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  (M Z 2 ) (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 Goals of the Analysis

M.Davier BaBar pi pi Tau /9/ Situation at ICHEP-Tau06 a  had [ee ]= (690.9 ± 4.4)  10 –10 a  [ee ]= ( ± 4.4 had ± 3.5 LBL ± 0.2 QED+EW )  10 –10 Hadronic HO – ( 9.8 ± 0.1)  10 –10 Hadronic LBL + (12.0 ± 3.5)  10 –10 Electroweak (15.4 ± 0.2)  10 –10 QED ( ± 0.1)  10 –10 inclu- ding: a  [exp ] – a  [SM ]= (27.5 ± 8.4)  10 –10  3.3 „standard deviations“ Observed Difference with BNL using e + e  : BNL E821 (2004): a  exp = (  6.3) 10  10 Knecht-Nyffeler, Phys.Rev.Lett. 88 (2002) Melnikov-Vainshtein, hep-ph/ Davier-Marciano, Ann. Rev. Nucl. Part. Sc. (2004) Kinoshita-Nio (2006)

M.Davier BaBar pi pi Tau /9/ ISR FSR ISR + add. ISRISR + add. FSR The Relevant Processes

M.Davier BaBar pi pi Tau /9/   ISR photon at large angle in EMC  1 (for efficiency) or 2 (for physics) tracks of good quality  identification of the charged particles  separate  /KK/  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  to  cross sections to cancel ee luminosity additional ISR vacuum polarization ISR photon efficiency  still need to correct for |FSR| 2 contribution in  and additional FSR, both calculated in QED, but also checked in data (ISR-FSR interference, additional detected photons) otherwise 3-4% syst error The Measurement

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

M.Davier BaBar pi pi Tau /9/ fb -1  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 Analysis Steps

M.Davier BaBar pi pi Tau /9/ 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 MC Generators

M.Davier BaBar pi pi Tau /9/ tag particle (track, ID) candidate (p, , φ) γ ISR   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  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) Particle-related Efficiency Measurements

M.Davier BaBar pi pi Tau /9/ m  m  and similarly for   single track efficiency  correlated loss probability f 0  probability to produce more than 2 tracks f 3 Data/MC Tracking Correction to  cross sections

M.Davier BaBar pi pi Tau /9/ 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 (E cal, dE/dx) All ID efficiencies measured  x  I  a tighter  ID (  h ) is used for tagging in efficiency measurements and to further reject background in low cross section regions. Particle Identification Barrel Backward EndcapForward Endcap Z IFR  IFR X IFR Y IFR * isolated muons M  > 2.5 GeV  efficiency maps (p,v 1,v 2 ) impurity (1.1  0.1) 10  3 * correlated efficiencies/close tracks  maps (dv 1,dv 2 )

M.Davier BaBar pi pi Tau /9/ Data/MC PID corrections to  and  cross sections Two running periods with different IFR performance  

M.Davier BaBar pi pi Tau /9/ PID separation and Global Test All ‘xx’  solve for all xx (0) and compare with no-ID spectrum and estimated syst. error hist: predicted from PID dots: measured (no ID) m   (GeV)  N (o) ii

M.Davier BaBar pi pi Tau /9/ 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 

M.Davier BaBar pi pi Tau /9/ Backgrounds  background larger with loose  2 cut used in 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 m  (GeV) multi-hadrons  Fitted BG/predicted =  BG fractions in at m  values

M.Davier BaBar pi pi Tau /9/ Additional ISR   Angular distribution of add. ISR /beams! Energy cut-off for add. ISR in AfkQed

M.Davier BaBar pi pi Tau /9/ Additional FSR   Angle between add  and closest track Large-angle add.ISR in data  AfkQed Evidence for FSR data  AfkQed FSR ISR

M.Davier BaBar pi pi Tau /9/  2 cut Efficiency Correction  depends on simulation of ISR (FSR), resolution effects (mostly ISR  direction) for  and    2 cut efficiency can be well measured in  data because of low background  main correction from lack of angular distribution for additional ISR in AfkQed  common correction: 1% for loose  2, 7% for tight     additional loss for  because of interactions studied with sample of interacting events agreement data/MC loose    1.00  0.16 loose   0.11 tight  2 syst error  10 -3

M.Davier BaBar pi pi Tau /9/ Checking Known Distributions Cos  * in XX CM /   flat at threshold 1+cos 2  *     sin 2  *    P>1 GeV track requirement  loss at cos  *  1

M.Davier BaBar pi pi Tau /9/ QED Test with  sample (0.2 – 5.0 GeV) ISR  efficiency 5.2 syst. trig/track/PID 4.0 BaBar ee luminosity  absolute comparison of  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)  full statistics J/  excluded

M.Davier BaBar pi pi Tau /9/ Unfolding Mass Spectrum  measured mass spectrum distorted by resolution effects and FSR (m  vs. s’)  unfolding uses mass-transfer matrix from simulation  2 MeV bins in GeV mass range, 10 MeV bins outside  most salient effect in  -  interference region (little effect on a   )

M.Davier BaBar pi pi Tau /9/ Systematic Uncertainties increased to 20. for preliminary results, pending investigations   ISR lumi  fitted w.r.t.  LO formula 

M.Davier BaBar pi pi Tau /9/ BaBar results

M.Davier BaBar pi pi Tau /9/ BaBar results in  region

M.Davier BaBar pi pi Tau /9/ BaBar vs. other experiments at large mass

M.Davier BaBar pi pi Tau /9/ BaBar vs.other ee data ( GeV) CMD-2 SNDKLOE 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

M.Davier BaBar pi pi Tau /9/ BaBar vs.other ee data  interference region) CMD-2SND  mass calibration of BaBar checked with ISR-produced J/    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

M.Davier BaBar pi pi Tau /9/ BaBar vs. IB-corrected  data ( GeV) relative comparison w.r.t. BaBar of isospin-breaking corrected  spectral functions BaBar data averaged in wider  bins and corrected for  -  interference

M.Davier BaBar pi pi Tau /9/ Computing a   FSR correction was missing in Belle, new value  3.0  2.5 ALEPH-CLEO-OPAL (DEHZ 2006) (DEHZ 2003) (2008) Direct comparison GeV BaBar  0.8  2.2 CMD  2.4  2.2 CMD  1.7  2.9 SND  1.2  4.7 a    (  10  10 )

M.Davier BaBar pi pi Tau /9/ Conclusions  BaBar analysis of  and  ISR processes completed  Precision goal has been achieved: 0.6% in  region ( GeV)  Absolute  cross section agrees with NLO QED within 1.2%  Preliminary results available for  in the range 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  results, especially Belle   Contribution to a  from BaBar agrees better with  results   Deviation between BNL measurement and theory prediction significantly reduced using BaBar  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

M.Davier BaBar pi pi Tau /9/ Backup Slides

M.Davier BaBar pi pi Tau /9/ PID correction to  cross section Two running periods with different IFR performance

M.Davier BaBar pi pi Tau /9/ ‘  ’ 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 level Measurement of  -ID efficiencies

M.Davier BaBar pi pi Tau /9/ PID correction to  cross section

M.Davier BaBar pi pi Tau /9/  2 cut Efficiency Correction: Interactions loose   tight    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 (R xy )  agreement data/MC 1.00  0.16 loose  1.07  0.11 tight  syst error  Beam pipe 

M.Davier BaBar pi pi Tau /9/ Mass Calibration Effect of a 1 MeV mass scale shift (calibration with J/  : (0.16  0.16) MeV) m  (GeV)

M.Davier BaBar pi pi Tau /9/ Present BaBar Measurements to obtain R in the energy range 1-2 GeV the processes              K + K -, K S K L, K S K L  K S K +     remain to be measured only statistical errors syst. 5-10%

M.Davier BaBar pi pi Tau /9/ BaBar Impact on Multi-Hadronic Modes Using ISR method the cross sections of e + e        ,    , 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  0.87 exp  0.23 rad from all  data  0.96 exp  0.40 SU(2) from BaBar  0.64 exp  0.13 rad from  +    0 (1.055  1.8 GeV) from all e + e  exp  0.26 exp  0.03 rad from BaBar 3.31  0.13 exp  0.03 rad More modes to come Davier-Eidelman- Hoecker-Zhang 2003 total  7.2