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Low energy e + e –  hadrons in Novosibirsk

Low energy e + e –  hadrons in Novosibirsk. Logashenko On behalf of the CMD-2 and SND Collaborations. Budker Institure of Nuclear Physics (Novosibirsk, Russia) Boston University (Boston, USA). 10 th International Workshop On Tau Lepton Physics Novosibirsk, Russia, Sep 22 – 25, 2008.

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Low energy e + e –  hadrons in Novosibirsk

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  1. Low energy e+e– hadrons in Novosibirsk • Logashenko • On behalf of the CMD-2 and SND Collaborations Budker Institure of Nuclear Physics (Novosibirsk, Russia) Boston University (Boston, USA) 10th International Workshop On Tau Lepton Physics Novosibirsk, Russia, Sep 22 – 25, 2008

  2. Cross-section e+e- hadrons • Measurement of the cross-section e+e-  hadrons in VEPP-2M energy range is interesting for: • measurement of R(s) • measurement of parameters of light vector mesons ρ, ω, φ, ρ’, ρ’’, ω’, ω’’ • comparison with spectral functions of the hadronic tau decays At low s R(s) has to be measured. The value and the error of the hadronic contribution to muon’s (g-2) are dominated by low energy R(s).

  3. R(s) measurements at low s Babar/Belle (ISR) KLOE (ISR) VEPP-2000 VEPP-2M At VEPP-2M the cross-sections of each final state are measured exclusively

  4. 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: 70 pb-1 collected in 1993-2000

  5. CMD-2 detector

  6. SND detector 1 - beam pipe, 2 - drift chambers, 3 - coincidence counter, 4 - fibre light guide, 5 - PMTs, 6 - NaI(Tl) crystals, 7 – phototriodes, 8 - iron absorber, 9 - muon tubes, 10 - 1cm iron plate, 11 - muon counters, 12 - magnetic lenses, 13 - bending magnets

  7. Example of CMD-2 and SND events e+e-π+π- in CMD-2 e+e-K+K- in SND

  8. Overview of the results

  9. How cross-section is measured 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 (sort of) • Radiative corrections account for ISR and FSR effects • Formfactor is measured to better precision than L (1-2%)

  10. e+e-π+π-

  11. Experimental data CMD-2 • Events signature: • two back-to-back tracks, originated near the interaction point • Data sample includes: • e+e-, μ+μ-, π+π-, cosmic muons • There is almost no background at √s <1 GeV • Data were taken in 6 separate runs between 1994 and 2000 95,98 96 97 SND 98,2000 96,98

  12. Event separation (CMD-2) <0.6 GeV >0.6 GeV Momentum Energy Energy Momentum Likelihood minimization: • e// separation using particles momentum • can measure N()/N(ee) and compare to QED • e// separation using energy deposition • N()/N(ee) is fixed according to QED

  13. Event separation (SND) Re/π E1(1,2,3), E2(1,2,3),Θ • Event separation is based on neural network: • 7 input parameters: energy deposition in each layer for both clusters and polar angle • 2 hidden layers 20 neurons each • 1 output parameter – Re/π • Trained on simulated events • Checked on experimental 3π and e+e- events Distribution by separation parameter Misidentification ~ 0.5--1%

  14. Radiative corrections ISR+FSR ISR+FSR+VP Radiation terms • CMD-2 uses custom Monte-Carlo generator to calculate RC • ee, ,  final states: 1  at large angle, multiple ’s along initial or final particles (≤0.2%) • CMD-2 calculation is consistent with independent calculations (BHWIDE, KKMC) • SND uses BHWIDE for ee final state and CMD-2 generator for ,  final states Vacuum polarization

  15. Comparison with other calculations of the radiative corrections Comparison with BHWIDE Comparison with KKMC

  16. Reconstruction efficiency correction (CMD-2) All selected final states produce very similar signal in the drift chamber. In the first order, the reconstruction efficiency is cancelled out from the formfactor calculation Due to drift chamber malfunction, the correction for reconstruction efficiency reach up to 4% percent for 1998 data set. Measured ratio εππ/εee

  17. Pion formfactor - results SND only CMD2 only CMD2 0.7% 0.6% (95)/ 0.8% (98) 1.2-4.2% SND 3.2% 1.3%

  18. Internal Cross-checks CMD2, 2 independent scans of rho-region Δ(95-98)≈0.7%±0.5% CMD2 vs SND √s<0.52 GeV CMD2 vs SND 0.6<√s<1.0 GeV Δ(SND-CMD2)≈1.2%±3.6% Δ(SND-CMD2)≈-0.53%±0.34% CMD2 √s<0.52 GeV

  19. Other modes

  20. “Narrow” resonances Mass and width are measured to ≈0.1 MeV, Γee to ≈2-3%

  21. Cross-section e+e-  4π Systematic error: Systematic error ≈5-7% SND = 8% CMD2= 15% (discrepancy 15-25%) problem with efficiency determination!CMD2-reanalysis preliminary = 8% Efficiency determination gives main contribution to the systematic error

  22. Cross-section e+e-  3π • Systematic error: • ≈6% (>1GeV) • 1-2% on omega • 2.5% on phi • Fit:

  23. Cross-section e+e-  2K CMD2(prel.) SND CMD2 SND Systematic error ≈5-8% Systematic error ≈3-9% Systematic error is ≈2-3% at φ resonance

  24. Overview of the results Systematic error: ~0.6-0.7% 1.0% 0.6% 1.5% 1.5 -- 3.5 % Error of R(s) Total error: ~ 6 -- 1% 1.5% 1--2% 2.0% 2.5 -- 3.5 %

  25. Future measurements at VEPP-2000

  26. VEPP-2000 storage ring • ≈100 1/pb per detector per year • Status: • construction is finished • had first beams and collisions • 2009 – start of experiments • Up to 2 GeV c.m. energy • Factor >10 in luminosity • L=1031 cm-2c-1,√s=1.0 GeV • L=1032 cm-2c-1,√s=2.0 GeV

  27. CMD-3 Detector • Advantages compared to CMD-2: • new drift chamber with x2 better resolution • better tracking • thicker barrel calorimeter • better separation • LXe calorimeter • - much better spatial resolution for γ’s • - shower profile 1 – vacuum tube, 2 – drift chamber, 3 – calorimeterBGO (680 crystals),4 – Z–chamber, 5 – CMD-3 superconducting solenoid, 6 – calorimeterLXe (400 liters), 7 – calorimeterCsI (1152 crystals), 8 – magnet yoke, 9 – solenoids of VEPP-2000

  28. SND 2000 1 – beam pipe 2 – tracking system 3 – aerogel 4 – NaI(Tl) crystals 5 – phototriodes 6 – muon absorber 7–9 – muon detector 10 – focusing solenoid • Advantages compared to “old” SND: • new system - cherenkov counter (n=1.05, 1.13) • e/π separation E<450 MeV • π/K separation E<1 GeV • new drift chamber • better tracking • better determination of solid angle

  29. How can we improve measurement of R? • We expect to get the following improvements: • high statistics! (x10-x100) – current measurement is still statistics-limited • radiative corrections to 0.1% - add photon jet + large angle γ • measure radiative tails and compare them to calculations (ISR?) • luminosity to 0.5%, use γγ in addition to Bhabha for cross-check • much better separation (LXe @CMD-3, Cerenkov @SND) – smaller systematic error, try to measure e+e-μ+μ- • precise trigger efficiency monitoring • better drift chambers – higher resolution, efficiencies • We expect to: • measure 2 mode to 0.3-0.4%, 4 mode to 2% • overall improvement in R precision by factor 2-3

  30. CMD-3 CMD-2 Examples of improvements (CMD-3) Separation by momentum Separation by energy deposition CMD-2 2*0.26 GeV CMD-3 2*0.32 MeV 4σ √s/2, GeV Because of better resolution of the drift chamber, the separation by momentum should work up to √s ≈ 0.65 GeV

  31. Conclusion • CMD-2 and SND data analyses are nearly complete. Cross sections of all major modes of e+e- hadrons are measured at energy range √s < 1.4 GeV . These are the best direct (energy-scan) measurements at the moment. • There is good agreement between Novosibirsk results, in particular: pion formfactor CMD-2 (94,95) vs CMD-2 (98), CMD-2 vs SND. • Over the last few years new indirect high precision measurements of R were performed: new tau-decay data and ISR. The question of agreement between different methods is still open. • New more precise measurement of R is scheduled at VEPP-2000.

  32. Backup slides

  33. Pion formfactor calculation Master formula: Nππ/Nee is measured, other values are calculated: • σB – Born cross-section (Fπ=1) • δ – radiative correction • ε – reconstruction efficiency • ΔN – correction for nuclear interactions • ΔD – correction for decay in flight • Δbg – correction for e+e-3π,4π,2K background • Δcorr – correction for E+E- correlation

  34. 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

  35. Systematic errors

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