1 / 31

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

alec
Download Presentation

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

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. 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)

More Related