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Measurement of the total hadronic cross-section at e+e- machines

Measurement of the total hadronic cross-section at e+e- machines. I. Logashenko. Boston University (Boston, USA) Budker Institure of Nuclear Physics (Novosibirsk, Russia). Heavy Quarks and Leptons – 2004 San Juan, Puerto Rico, June 1-5, 2004. Outline. Motivation

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Measurement of the total hadronic cross-section at e+e- machines

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  1. Measurement of the total hadronic cross-section at e+e- machines I. Logashenko Boston University (Boston, USA) Budker Institure of Nuclear Physics (Novosibirsk, Russia) Heavy Quarks and Leptons – 2004 San Juan, Puerto Rico, June 1-5, 2004

  2. Outline • Motivation • Measurements of R with energy scans • Tau decays and R • ISR experiments This talk is done by experimental physicist

  3. R, the definition R(s) is defined as: • R(s) is one of the most fundamental quantities in high energy physics: • its global structure reflects number of quarks and theirs colors; used for QCD tests and as a source of QCD parameters • plays special role in precision measurements

  4. Role of R in the precision measurements • R(s) is essential for interpretation of the results of some precision measurements • anomalous magnetic moment of the muon • global electroweak fit – value of (MZ) At high s R(s) can be calculated, at low s R(s) has to be measured.

  5. Role of R in the precision measurements

  6. R, the current status

  7. Low energy vs High energy • Exclusive approach: • measure each final state separately and calculate the sum • Inclusive approach: • select events with any hadron(s) in the final state Experimental challenges are similar: luminosity, efficiencies, background, radiative corrections. Small systematic error is crucial!

  8. Measurement of R in Novosibirsk VEPP-2M collider • VEPP-2M collider: 0.36-1.4 GeV in c.m., L1030 1/cm2s at 1 GeV • Detectors CMD-2 and SND: 50 pb-1 collected in 1993-2000 • All major hadronic modes are measured:

  9. Measurement of R in Novosibirsk Total hadronic cross-section measured by CMD-2 and SND

  10. The approach All modes except 2 2 • Luminosity L is measured using Bhabha scattering at large angles • Efficiency  is calculated via Monte Carlo + corrections for imperfect detector • Radiative correction  accounts for ISR effects only • Ratio N(2)/N(ee) is measured directly  detector inefficiencies are cancelled out • Virtually no background • Analysis does not rely on simulation • Radiative corrections account for ISR and FSR effects • Formfactor is measured to better precision than L

  11. What is really measured? • CMD-2 published 2 cross-sections e+e-2: • radiative correction take into account part of FSR, allowed by the event selection (thus remove FSR completely from the measured cross-section); VP is left untouched. • Used to get rho-meson mass, width, … • VP is removed, all FSR is added. • Used for R calculation FSR VP Definition of (e+e-hadrons) depends on the application • Hadron spectroscopy: vacuum polarization (VP) is the part of the cross-section (“dressed”), final state radiation (FSR) is not • “Bare” cross-section used in R: vice versa – FSR is the part of the cross-section, VP is not • Measured number of events include VP and part of FSR allowed by the event selection

  12. The radiative corrections ISR+FSR ISR+FSR+VP Vacuum polarization Initial and final state radiation • the correction factor |1-(s)|2 is the same for all final states • R(s) itself is used for (s) evaluation  iterations • ee, ,  final states: 1  at large angle, multiple ’s along initial or final particles (0.2%) • CMD-2 results are consistent with independent calculations (BHWIDE, KKMC) • Other final states: multiple ’s along initial particles (1%)

  13. Rho-meson >0.6 GeV <0.6 GeV • e// separation using energy deposition • N()/N(ee) is fixed according to QED • e// separation using particles momentum • can measure N()/N(ee) and compare to QED Systematic error 1-2% 0.6-1% 1-5% 4 separate runs over 5 years, >1 million  events, 100k published

  14. Systematic error  5-7% CMD-2/SND discrepancy recently resolved

  15. Narrow resonances Syst. error  2% Syst. error  2%

  16. Systematic errors 2 (at rho-meson) Other final states

  17. Future measurements at VEPP-2000 • Factor >10 in luminosity • Up to 2 GeV c.m. energy • CMD-3: major upgrade of CMD-2 (new drift chamber, LXe calorimeter) • measure 2 mode to 0.2-0.3% • measure 4 mode to 1-2% • overall improvement in R precision by factor 2-3 Under construction. Data taking is expected to start is 2006-2007.

  18. Measurement of R at BES • March-May, 1998: • 6 energy points at 2.6, 3.2, 3.4, 3.55, 4.6, 5.0 GeV • ~ 1000 hadronic events at each point • Single beam and separated beam at each points • Feb.- June, 1999: • 85 energy points at 2.0-4.8 GeV • ~ 1000 at each point • 24 energy points separated beam operation

  19. Measurement of R at BES • R value is measured between 2 and 5 GeV. Typical error is 5-8%. • Inclusive measurement. Efficiency is calculated with LUARLW (Lund) Monte-Carlo generator to ~2%. The 14 experimental distributions were used to tune the LUARLW parameters. • The major source of the error is the event selection (background from , , beam)

  20. R and the tau decays Isospin symmetry and CVC allow to relate R(s) and spectral functions of hadronic decays of  1995: ALEPH published high precision measurement of 2 spectral function. Consistent with e+e-2. 1998: ALEPH result is used to improve e+e-2. Factor 1.5-2 improvement in a(had) 2002: CMD-2 published new high precision measurement of e+e-2 cross-section. There is clear discrepancy with the tau data

  21. e+e-/ discrepancy Most likely explanation: isospin effects, e.g. • Status today: •  data – ALEPH, CLEO, OPAL. Results are consistent. • e+e- data – CMD-2 data updated. KLOE preliminary result confirm CMD-2. Preliminary CMD-2 result with 100% data confirms previous one. • discrepancy firmly established There is no consistent theoretical explanation so far. At current level of understanding tau data cannot be used to improve R(s).

  22. ISR approach s s’ or FSR background Main idea: explore wide energy range using the hard photons emitted from the initial particles Requires high luminosity to overcome / factor  meson factories • Advantages: • “cheap” way to measure cross-section in the wide energy range • good detectors installed at meson factories • Major problems: • radiative corrections (calculation of H) • FSR background

  23. ISR at KLOE 1 548 000 events 50 40 number of events (x103) 30 20 10 Mpp2(GeV2) • working on 2 final state • ISR photon is not detected (small-angle)  reduce relative FSR contribution below 1% level, lose events below 600 MeV (c.m. energy) • statistics already similar to CMD-2 • normalization to large-angle Bhabha scattering • PHOKHARA Monte-Carlo generator

  24. Preliminary results from KLOE 40 30 20 10 0.5 0.7 0.9 Systematic errors Consistent with CMD-2: • Things to do: • study events with ISR at large angles (extend analysis below 0.6 GeV) • normalize to 

  25. ISR at BaBar • 200 fb-1 collected, 89 fb-1 analyzed • Unlike KLOE, ISR photon is detected • Very hard ISR photon  clear kinematic separation between photon and hadron state  large suppression of FSR background •  events provide excellent test • measure R up to ~2-4 GeV Working on exclusive channels: , KK, pp , 0, 4, 5, 6, , KK, KK, 2K2K, KK Inclusive approach is under evaluation

  26. Normalization to  events “effective c.m. energy squared” dL(s’) Corrections for final state radiation ISR luminosity Detection efficiencies Cross-section to final state f :

  27. Preliminary results from BaBar BaBarpreliminary J/ψ4 ψ(2S)J/ψ222 Systematic error 4-5% Systematic error <10% Already best measurements!

  28. Measurement of 2 final state at BaBar Radiative corrections at CMD-2 Number of ,  events • Normalization to  helps to cancel: • efficiencies • luminosity • initial state radiation (unlike (s), (s’) does not have radiative return structure) • vacuum polarization • Major challenge: • / separation • Goal: • <1% systematics

  29. Implication to a Uncertainty of the hadronic contribution to a , 10-10 VEPP-2000 BaBar My estimation

  30. Current/Future activities BaBar VEPP-2000 BES KLOE VEPP-2M CLEO-c CLEO

  31. Conclusion • Measurement of R is still very active and important field • Important for interpretation of g-2 experiment, evaluation of (MZ), tests of QCD • Recent improvements: VEPP-2M, BES • Lots of data are being analyzed: VEPP-2M, KLOE, BaBar, CLEO • Many future projects: VEPP-2000, BESIII, CLEO-c • ISR experiments have demonstrated impressive potential: KLOE, BaBar. • Expect to reach 0.3-5% precision over the whole 0-10 GeV range in few years (factor of 2 improvement)

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