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R measurements at e + e - colliders

R measurements at e + e - colliders. Debora Leone (IEKP – Universität Karlsruhe) for the KLOE collaboration. IFAE Torino, 14 th -16 th April. M  hadr. d s ( e + e -  hadrons + g ) d M 2 hadrons. s ( e + e -  hadrons ) H ( M 2 hadr ). Definition.

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R measurements at e + e - colliders

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  1. R measurements at e+e- colliders Debora Leone (IEKP – Universität Karlsruhe) for the KLOE collaboration IFAE Torino, 14th-16th April

  2. Mhadr ds(e+ e- hadrons+ g) dM2hadrons s(e+ e- hadrons) H(M2hadr ) Definition R: very important quantity in particle physics: counts directly the number of quark flavours and their colours To measure R: • R scan: systematic variation of c.m. energy of the machine and • measurement stot(e+e-  hadrons) • Inclusive: directly measure tot • Exclusive: sum up all channels of tot=  I • Radiative return: trough ISR: D. Leone R measurements at e+e- colliders

  3. g* B field · q m m + + · · g* * g q amhad (e+e-) K(s) ~ 1/s Hadronic contribution to (g-2)m Magnetic moment: with amSM = amQED + amhad + amEW Due to quantum corrections: amhadincludes contribution from not evaluable in pQCD, but it can be provided by s(e+ e- hadrons) by means of dispersion relations: t  nt hadronsdecays provide complementary information: CVC + SUiso(2) amhad (t) R measurements at e+e- colliders D. Leone

  4. Status of (g-2): theory vs. experiment ComparisonExperimental ValuewithTheory - Prediction New cross section data have recently lowered theory error: a) CMD-2 (Novosibirsk/VEPP-2M) p+ p- channel with 0.6% precision < 1 GeV b) t-Data from ALEPH /OPAL/CLEO Theoretical values taken from M. Davier, S. Eidelman, A. Höcker, Z. Zhang hep-ex/0308213 THEORY ’02/‘03 e+ e- - Data: 2.7 s - Deviation t – Data: 1.4 s - Deviation Experiment BNL-E821 Values for m+(2002) and m-(2004) in agreement with each other. Precision: 0.5 ppm Experiment ’02/‘04 R measurements at e+e- colliders D. Leone

  5. 2 3 (+,) 4 > 4 (+KK) 1.8 - 3.7 3.7 - 5 (+J/, ) 5 - 12 (+) 12 -  < 1.8 GeV 2p contrib. ahad 8% [2mp - 0.5 GeV] 54% [0.6 - 1.0 GeV] 10% [Rest<1.8 GeV] r 2p contrib. dahad 8% [2mp - 0.5 GeV] 34% [0.6 - 1.0 GeV] 31% [Rest< 1.8 GeV] r > 1GeV Hadron contribution to am Calculations based on hep-ph/0308213 Davier, Eidelman, Höcker, Zhang ahad <1.8 GeV 2[ahad] <1.8 GeV 1% e+e- data only! R measurements at e+e- colliders D. Leone

  6. e p m The s(e+e- p+p-) analysis @ CMD-2 R.R. Akhmetshin et al., Phys.Lett.B578:285-289,2004 Energy scan analysis with 0.6< s < 1.0 GeV • selection of collinear e+e-e+e-, e+e- p+p-, e+e- m+m- • in the fiducial volume • separation of the three sample, by energy deposition • rejection of cosmic events by means of the spatial distribution • of the vertex • Sources of systematic errors: • Event separation 0.2% • Radiative correction 0.4% • Detection efficiency 0.2% • Fiducial volume 0.2% • Correction for p losses 0.2% • Beam energy determination 0.1% • TOTAL 0.6% R measurements at e+e- colliders D. Leone

  7. correction for misidentification of  p+p-p0 L= 317.3 nb-1 • due to pion loss: • by decays in flight • by nuclear interaction |Fp|2 2  E (MeV) Pion form factor r- interference Fit function according to Gounari – Sakurai model: 3 terms for , , ’ ……..and s(e+e- p+p-)  |Fp(s)|2 R measurements at e+e- colliders D. Leone

  8. ee   L= 119.6 nb-1 amhad @ VEPP-2M amhad(total) = 70.2  10-9 (60.21 ppm) amhad(VEPP-2M) = 60.16  10-9 (51.61 ppm) contributions to aabove VEPP-2M:8.6  0.6 ppm R measurements at e+e- colliders D. Leone

  9. 4040 4160 4415 3770 R scan @ BES R scan between 2 and 5 GeV: March-May, 1998: 6 energy points at 2.6, 3.2, 3.4, 3.55, 4.6, 5.0 GeV Feb.- June, 1999: 85 energy points at 2.0-4.8 GeV • kinematic cuts to select the event • use vertex-fitting procedure to remove • residual background • luminosity determined using large • angle Bhabha Nll= lepton pairs Ngg= g pairs correction to to ISR average error ~6.6% R measurements at e+e- colliders D. Leone

  10. Mhadr ds(e+ e- hadrons + g) dMhadrons s(e+ e- hadrons) H(Mhadr ) Radiative Return Standard method for cross section measurement is the energy scan, i.e. the syst. variation of the the c.m. energy of the accelerator. Particle factories (fixed c.m. energy) have the opportunity to measure the cross sections(e+ e-hadrons)as a function of the hadronic c.m. energy M 2hadrons by using the radiative return. Precise knowledge of ISR – process: Radiator function MC generator: Phokhara (H. Czyz, A. Grzelinska, J.H. Kühn, G. Rodrigo, hep-ph/0308312) “Radiative Return” to (w) resonance e+e-  (w)   +- This method is a complementary approach to the energy scan R measurements at e+e- colliders D. Leone

  11. Analysis overview Drift Chamber EM Calorimeter 50o<<130o > 1650 < 150 50o < p < 130o, g< 15o and g> 165o No photon tagging Obs. Spectrum dN/dMpp2 141 pb-1 of 2001 data Event analysis Divide by luminosity Analysis Flow Diff. Cross Section ds(p+p-g)/dMpp2 Divide by H function Diff. Cross Section ds(p+p-)/dMpp2 • High statistics for • ISR photons • Low relative • contribution from FSR • Low relative background VP, FSR corrections R measurements at e+e- colliders D. Leone

  12. knowledge of radiative corrections Precision determined by precision in collecting large angle Bhabha 55o 125o Luminosity measurement KLOE uses large angle Bhabha events for the luminosity evaluation: effective cross section, given by theoretical generator interfaced with our simulation detector program • 2 independent generators used for radiative corrections: • BABAYAGA (Pavia group): seff = (428.80.3stat) nb • BHAGENF (Berends modified): seff = (428.50.3stat) nb claimed by generator’s authors Experimental systematic error are determined by comparing data and MC angular and momentum distribution Polar Angle [°] R measurements at e+e- colliders D. Leone

  13. e+e-g spectrum • Efficiencies: • - Trigger & Cosmic veto • Tracking, Vertex • p- e- separation • Reconstruction filter • Trackmass-cut • Unfolding resolution • Acceptance Statistics:141pb-1 of 2001-Data 1.5 Million Events Errors 1.0 % • Background: • e+ e- e+e-g • e+ e- m+ m- g • f p+ p- po r-w interference 0.5 % Luminosity:0.6% R measurements at e+e- colliders D. Leone

  14. g p+ Fp(Q2) e+ g* p- e- p+ g 1 e+ g* e- p- nb sF=1, Phokhara Final spectrum To get the cross section for e+e-p+p-: - division by radiator function H  ISR-process calculated at NLO-level - radiative corrections: i) bare cross section  subtraction of vacuum polarization contribution ii) FSR corrections Radiator function R measurements at e+e- colliders D. Leone

  15. Pion Formfactor after FSR corrections line = approach 1 + = approach 2 Ratio = approach 1 / approach 2 • 2 approaches for FSR corrections: • Approach 1: “exclusive NLO-FSR“ • - Correcting measured sppg for FSR • - Use MC with pure ISR in analysis • - Add FSR-contrib. to spp (ca. 0.8%) by hand • Approach 2: “inclusive NLO-FSR“ • - Correct for „unshifting“, i.e. s‘  Mpp • - Use MC with ISR + FSR in analysis Both methods in excellent agreement! For the error on the model dependence of FSR (scalar QED) we take 0.5% R measurements at e+e- colliders D. Leone

  16. Summary on systematic error • Theory • Radiator Function H 0.5% • Vacuum Polarization 0.2% • Luminosity 0.6% • FSR resummation 0.5% • TOTAL 1.0% Thesystematic errorhas contributions from: • Experiment TOTAL 1.2% Calculating the dispersion integral: amhad-pp(0.35< Mpp< 0.95 GeV2) = (389.2  0.8stat  4.7syst  3.9theo) 10-10 (376.5  0.8stat  5.9syst+theo) 10-10KLOE amhad-pp(0.37< Mpp2< 0.93GeV2) = (378.6  2.7stat  2.3syst+theo) 10-10CMD-2 • both measurement are in agreement within the errors • e+e- - t discrepancy for am is confirmed R measurements at e+e- colliders D. Leone

  17. f = hadrons or e- (9 GeV) e+ (3 GeV) gISR ISR method @ BABAR • up to now very few data with large errors for s 1.4 GeV: DR ~ 10-15% • data from different experiments and different systematics s’ is the effective energy • e+e- fmany channels: • , KK, pp, 0 , 4, 5, 6, , KK, • KK, 2K2K, KK • R will be reconstructed from the sum of exclusive • channels up to 2.5 GeV • inclusive approach also studied, limited by photon • energy resolution for lower recoil masses R measurements at e+e- colliders D. Leone

  18. 4p invariant mass distribution data events/0.025 GeV ISR bkgd non-ISR bkgd e+e-→ g +p+ p- p+ p- @ BABAR perspectives on R measurements by BABAR: • comparable accuracy w.r.t. CMD-2 and KLOE for s < 1 GeV • few % accuracy for 1 < s < 3 GeV • ISR photon + 4 charged hadrons • 1C fit in 4p hypothesis BABAR PRELIMINARY R measurements at e+e- colliders D. Leone

  19. CONCLUSION • CMD2 and SND experiments at VEPP-2M and KLOE at DAFNE have improved the accuracy on R respectively in the regions E < 1.4 GeV and E < 1 GeV, the first one using the energy scan and the last one the ISR method. Their results are in agreement • BESII at BEPC has improved quite a lot • the accuracy on R in the region 2<E<5 GeV • Other improvements are expected by BABAR in the region 1-3.5 GeV, using ISR R measurements at e+e- colliders D. Leone

  20. Riserva R measurements at e+e- colliders D. Leone

  21. The VEPP-2M collider VEPP-2M parameters: • Data collected by CMD-2 : • 2107f decays • 5106 p+ p- events • 3106w decays • 2107 multi-hadrons Beam energy 2(180-700) MeV Peak luminosity 31030 cm-2 s-1 Interaction regions 2 Years of activity 1974-2000 R measurements at e+e- colliders D. Leone

  22. The CMD-2 detector Drift chamber: sR-f=250 mm sz=5 mm sq = 1.5 10-2 sf = 7 10-3 sdE/dx = 0.2  E sp/p=(90  p2[GeV] +7)1/2 % Calorimeter: Barrel: sE/E = 8% Eg 100700 MeV sq,f=0.02 radiant Endacap: sE/E = 4 8 % Eg 100700 MeV sq,f=0.02  0.03 radiant Solid angle covered by cal:  = 0.92  4p steradiant R measurements at e+e- colliders D. Leone

  23. Energy resolution: Angular resolution: SND detector 1-vacuum chamber; 2 – drift chambers; 3 – internal scintillating counter; 6-NaI crystals; 7-vacuum phototriodes; 8-absorber; 9-streamer tubes; 11- scintillator plates; R measurements at e+e- colliders D. Leone

  24. The BEPC complex e+e- machine in 2-5 GeV • the only e+e- machine in 2-5 GeV since 1989 • L ~ 51030 cm-2 s-1 at the J/y peak R measurements at e+e- colliders D. Leone

  25. The BES detector R measurements at e+e- colliders D. Leone

  26. PEP II & BABAR Design Performance Peak Lumi 31033cm-2s-1 4.51033cm-2s-1  Lumi/day 135 pb-1 303 pb-1 Tracking: (pt)/pt = 0.13% pt 0.45% pt in GeV EMC: sE/E = 2.30 % E-1/4 1.35 % D. Leone R measurements at e+e- colliders

  27. FSR treatment 1/2 Two complementary procedures to correct for FSR FSR – exclusive approach FSR – inclusive approach N(e+e-  p+p-gISRgFSR) Subtract FSR contribution Add back FSR contribution Event analysis – Phokhara3 ISR+FSR Event analysis – Phokhara3 ISR Division by Luminosity s(e+e- p+p-gISR) s(e+e- p+p-gISRgFSR) Division by radiator function H s(e+e- p+p-) Map M2ppg(FSR) to M2pp using MonteCarlo Add back FSR by hand (sQED, Schwinger 1990) s(e+e- p+p-gFSR) R measurements at e+e- colliders D. Leone

  28. pb-1 DANE & KLOE • e+e-collider @ S = M = 1019.4 MeV • achieved peak Luminosity: 8  1031 cm-2 s-1 • 2000-2002 data set: 500 pb-1 KLOE detector designed for CP violation studies  good time resolution st= 57 ps /E(GeV)  50 ps in calorimeter and high resolution drift chamber (p/p is 0.4 % for  > 45°), ideal for the measurement of M. D. Leone R measurements at e+e- colliders

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