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R Measurement from BES

R Measurement from BES. Z.G. Zhao (Representing BES Collaboration) University of Michigan, Ann Arbor, MI, USA IHEP of CAS, Beijing, China. Motivation R Scan in 2-5 GeV QCD Test Topics with R scan Data Summary. The B eijing E lectron P ositron C ollider ( BEPC ).

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R Measurement from BES

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  1. R Measurement from BES Z.G. Zhao (Representing BES Collaboration) University of Michigan, Ann Arbor, MI, USA IHEP of CAS, Beijing, China • Motivation • R Scan in 2-5 GeV • QCD Test Topics with R scan Data • Summary Zhengguo Zhao

  2. The Beijing Electron Positron Collider (BEPC) A unique e+e- machine operating in Ecm~2-5 GeV since 1989. L~51030 /cm2s at J/ peak Zhengguo Zhao

  3. BESII Detector(General Purpose Collider Detector) VC: xy = 100 m TOF: T = 180 ps  counter: r= 3 cm MDC: xy = 200 m BSC: E/E= 22 % z = 5.5 cm dE/dx= 8.4 %  = 7.9 mr B field: 0.4 T p/p=1.8%(1+p2) z = 2.3 cm Zhengguo Zhao

  4. Experimentally Nhad: observed hadronic events Nbg: background events L: integrated luminosity had: detection efficiency for Nhad : radiative correction Definition of R R: one of the most fundamental quantities in particle physics, counts directly the number of quarks, their flavor and colors Zhengguo Zhao

  5. R/R ~ 15% below 5 GeV Unclear & complex structure in 3.7-5 GeV R Values Below 10 GeV 15% Zhengguo Zhao

  6. Reducing the uncertainty of (MZ2) essential for precision tests of the SM • calculated measured • At MZ2 0.03142 0.02800.0009 • Hunting for new physics from a(g-2)/2  Interpretation of E821 at BNL New physics Motivations Zhengguo Zhao

  7. The indirectly determination of mH depends critically on the precision of (MZ2) Relative uncertainties of three input parameters in the SM fit Zhengguo Zhao

  8. BEPC energy region Relative Contributions to the Uncertainties of a and (MZ2) Zhengguo Zhao

  9. BES R Scan March-May, 1998: • 6 energy points at 2.6, 3.2, 3.4, 3.55, 4.6, 5.0 GeV + separated beam operation at each Feb.- June, 1999: • 85 energy points at 2.0-4.8 GeV + 24 energy points separated beam operation Zhengguo Zhao

  10. Events collected at BESII N N Events Recorded by BESII Event rate Zhengguo Zhao

  11. Hadronic Events Selection Select Nhad(e+e-hadrons) from all other possible contaminations. e- Zhengguo Zhao

  12. Data MC Typical Cuts Used for Selecting Nhad Zhengguo Zhao

  13. Trigger Efficiency (trg) • trg measured by comparing the responses to different trigger requirements in - special runs taken at J/ - R at 2.6, 3.0, 3.5 GeV with larger data sample • trg = 99.96%, 99.33%, 99.76% for Bhabha, dimuon and hadrons with errors of 0.5% Zhengguo Zhao

  14. Background Subtraction Background sources: 1. Cosmic ray 2. Lepton pair production subtracted by event 3. Two-photon processes selection and MC 4. Beam associated  most serious • fit the event vertex distribution with Gaussian for hadronic events, polynomial(degree 2) for BG b) separated beam and single beam operation data Nbg = f  Nsep Both a) and b) are consistent Zhengguo Zhao

  15. Subtraction of the Beam Associated Background Gaussican for Nhad P2 for BG Zhengguo Zhao

  16. Rely on large-angle Bhabha e+e- e+ e- () e+e- cross check Bhabha events Bhabha events Luminosity Measurement Zhengguo Zhao

  17. Detection Efficiency for the Hadronic Events • Rely on hadronic events generatormodel dependent • LUARLW is developed to improve JETSET for low energy region, particularly for Ecm< 3 GeV - final states are simulated exclusively - less parameters in the fragmentation function - remove high energy assumption • D, D*, Ds, Ds* productions are simulated according to Eichiten Model • A generator is built into LUARLW to handle decays of resonances in the radiative return processes e+e- J/ or (2S) Zhengguo Zhao

  18. Some Hadronic Event Shapes at Ecm=2.2 GeV (LUARLW) Tune the parameters to reproduce 14 distributions of the observed kinematic variables and hadronic event shape. Hatched region: MC Histogram: Data Zhengguo Zhao

  19. Some Hadronic Event Shapes at Ecm=3.55 GeV Zhengguo Zhao

  20. ISR: remove high order effects from Leading order [O(2), diagram (a)] All the high order contributions Born term vertex Vacuum polarization Bremsstrahlung Initial States Radiative Correction Zhengguo Zhao

  21. Initial States Radiative Correction • Four schemes are compared to calculate ISR. (1) G. Bonneau and F. Martin, NP B 27(1971)387 (2) F.A. Berends and R. Kleiss, NP B 178(1981)14 (3) E. A. Kuraev and V.S. Fadin, Sov. J. NP 41(1985)3 (4) A. Osterheld et al., SLAC-PUB-4160, 1986. (T/E) • (2) + multi-soft photon = (4) • (1) – (4) are consistent within1% in the continuum region, 1-3% above charm threshold Zhengguo Zhao

  22. Syst. error Variation of Detection Efficiency and ISR in 2-5 GeV Zhengguo Zhao

  23. Some Values at a Few Typical Energy Points to Determine R Systematic Errors (in %) at a Few Typical Energy Points Zhengguo Zhao largest

  24. R Values in 2-5 GeV PRL 88:101802,2002 (3770) Preliminary results reported at ICHEP2000 are the same as the final published ones in 2002 Zhengguo Zhao

  25. R Below 10 GeV Before BES R Scan After BES R Scan Zhengguo Zhao

  26. CLEO CMD,SND KROE BESIII CLEOC had(MZ2) and Its Relative Contribution in Magnitude and Uncertainty Zhengguo Zhao

  27. With BES data Without BES data (95% C.L.) (95% C.L.) The Impact of BES R-value on SM Fit to mH Zhengguo Zhao

  28. R(pQCD) and R(BES) pQCD calculation agree well withBESdata (D. Michel et.al.) under current uncertainty Zhengguo Zhao

  29. Evaluation of s and s(MZ) from R(BES) Exhibits QCD correction known to O(s3(s)) so far. Precision measurement of R can determine s and thus evaluate s(MZ). Using R at 3.0 and 4.8 GeV, one has: Which gives: PDG2000: J.H. Kuhn, M.Rreinhauser Nucl. Phys. B619(2001)588 consistent Zhengguo Zhao

  30. A wonderful laboratory to study E.W. and strong interaction. Unique place to investigate pQCD, hadron production mechanism With BES R scan data  distribution (MLLA/LPHD) Multiplicity: second binomial moment R2 (NLO) e+e-(K) + X processes (polarized parton density) Hadron Production from e+e- Annihilation MLLA: Modified Leading Log Approximation LPHD: Local Parton Hadron Duality Zhengguo Zhao

  31.  Distribution: =-ln(2p/s) MLLA/LPHD calculations BES data can be reasonably well described by MLLA/LPHD Zhengguo Zhao

  32. *: Peak Position of  Distribution MLLA/LPHD predict: eff: free parameter C=0.2915/0.3190/0.3513 For nf = 3/4/5 BES data support MLLA/LPHD prediction Zhengguo Zhao

  33. BES KLPHD and eff Zhengguo Zhao

  34. Average multiplicity NLO Multiplicity Zhengguo Zhao

  35. Second Binomial Moment NLO calculation c=0.5603/0.3196/0.3513 for nf=3,4,5 BES data systematically lower than what NLO predicted, and consistent with the tendency of that from high energy e+e- data Zhengguo Zhao

  36. Summary • BES measured R in 2-5 GeV with average uncertainties of 6.6% (a factor of 2-3 improvement) • R scan data at continuum is used to test QCD. However precision test of QCD need more accuracy data (~1-3% level) • BES is planning to collect hadronic event sample with high statistics at a few energy points between 2-3 GeV • Further meaningful improvement on R measurement, and understand the broad structure between 4-4.5 GeV need BESIII at BEPCII. Zhengguo Zhao

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