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 1.Motivation 2.Measurements of R with energy scans 3.Tau decays and R 4.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  (M Z ) 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 Experimental challenges are similar: luminosity, efficiencies, background, radiative corrections. Small systematic error is crucial! Exclusive approach: measure each final state separately and calculate the sum Inclusive approach: select events with any hadron(s) in the final state

8. Measurement of R in Novosibirsk VEPP-2M collider VEPP-2M collider: 0.36-1.4 GeV in c.m., L  10 30 1/cm 2 s 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 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 All modes except 2  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 22

11. What is really measured? 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 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 VP FSR

12. The radiative corrections Vacuum polarizationInitial 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 ISR+FSR ISR+FSR+VP 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 e/  /  separation using particles momentum can measure N(  )/N(ee) and compare to QED <0.6 GeV >0.6 GeV e/  /  separation using energy deposition N(  )/N(ee) is fixed according to QED 1-2%0.6-1%1-5% Systematic error 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%

16. Systematic errors 2  (at rho-meson) Radiative corrections0.4% “Experimental” (event selection, separation, efficiencies,…) 0.4% Total0.6% Other final states Radiative corrections1% Luminosity1-2% Efficiency1-5% Total1.5-7%

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 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 Most likely explanation: isospin effects, e.g. 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’ Main idea: explore wide energy range using the hard photons emitted from the initial particles Requires high luminosity to overcome  /  factor  meson factories or FSR background 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 M  2 (GeV 2 )      number of events (x10 3 ) 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 10 20 30 40 0.50.70.9 Systematic errors Radiation function H  1% (FSR) Luminosity0.6% Efficiency 1%1% Total  1.5% 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 Detection efficiencies Corrections for final state radiation “effective c.m. energy squared” dL(s’) ISR luminosity Cross-section to final state f :

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

28. Measurement of 2  final state at BaBar 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 Number of ,  events Radiative corrections at CMD-2

29. Implication to a  Final state Today 2004 CMD-2+ISR 2005-2006 +VEPP-2000 2010 22 5.9<4.0<2.0 ,  1.41.0<0.7 Other, <2 GeV2.31.51.0 >2 GeV1.8 Total7.2<5.0<3.0 BaBar VEPP-2000 Uncertainty of the hadronic contribution to a , 10 -10 My estimation

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

31. Conclusion Measurement of R is still very active and important field Important for interpretation of g-2 experiment, evaluation of  (M Z ), 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)