Stefan E. M üller Laboratori Nazionali di Frascati

1. A precise KLOE measurement of  (e+e-+-) and extraction of ampp between 0.35 and 0.95 GeV2 Stefan E. Müller Laboratori Nazionali di Frascati (for the KLOE collaboration) 14. International QCD Conference (QCD08) Montpellier, 7-12 July 2008

2. 1 (e+e-+-) with ISR: + - sp s =s(e+ e- p+p-)H(s,sp) s · ds(e+ e- p+p-+g) dsp Particle factories have the opportunity to measure the cross sections(e+ e- p+p-) as a function of the pionic c.m. energy sp = M2pp using initial state radiation (radiative returnto energies below the collider energy s). Neglecting FSR requires precise calculation of the radiator H(s,sp)  EVA + PHOKHARA MC Generator (S.Binner, J.H.Kühn, K.Melnikov, PLB459,1999) (H.Czyż, A.Grzelińska, J.H.Kühn, G.Rodrigo, EPJC27,2003) Advantages: • overall energy scale √s is well known and applies to all values of sp • syst. errors from luminosity, √s, rad. corrections enter only once, don’t have to be studied for each point of sp • data comes as together with the standard physics program of the experiment Requirements: • precise evaluation of radiator function • good suppression, or understanding, of Final State Radiation (FSR) • large integrated luminosity 1st KLOE publication (based on 140 pb-1) A. Aloisio et al., PLB606(2005)12

3. 2 DANE: A -Factory DEAR, , SIDDHARTA e+e--collider with =m1.0195 GeV Integrated Luminosity Peak Luminosity Lpeak= 1.4 • 1032cm-2s-1 Total KLOE int. Luminosity: L dt ~ 2500 pb1 (2001 - 05) New result is based on 240pb-1 from 2002 data • 2006: • Energy scan with 4 points around m-peak • 225pb-1 at = 1 GeV

4. 3 KLOE Detector Driftchamber sp/p = 0.4% (for 900 tracks) sxy 150mm, sz 2 mm Excellent momentum resolution

5. 4 KLOE Detector sE/E = 5.7% /E(GeV) sT= 54 ps /E(GeV)  50 ps (Bunch length contribution subtracted from constant term) Excellent timing resolution Electromagnetic Calorimeter

6. 5 Event Selection 2 pion tracks at large angles 50o<p <130o Photons at small angles < 15oor > 165o  photon momentum from kinematics: • high statistics for ISR events • low relative FSR contribution • suppression of  +-0background statistics: 242pb-1 3.5 Million Events

7. 6 Event selection • Experimental challenge: control • backgrounds from •  • ee ee • ee , • removed using kinematical cuts in trackmass MTrkand Missing Mass Mmiss MTrk: defined by 4-momentum conservation under the hypothesis of 2 tracks with equal mass and oneg To further clean the samples from radiative Bhabha events, we use a particle ID estimator for each charged track based on Calorimeter Information and Time-of-Flight. Mmiss: defined by 4-momentum conservation under the hypothesis of e+e-+-x

8. 7 Radiative corrections Radiator: Radiator-Function H(s,sp) (ISR): • ISR-Process calculated at NLO-level • PHOKHARAgenerator • (H.Czyż, A.Grzelińska, J.H.Kühn, G.Rodrigo, EPJC27,2003) • Precision: 0.5% H(s,sp) Vac. Pol. corr: Radiative Corrections: i) Bare Cross Section divide by Vacuum Polarisation d(s)=(a(s)/a(0))2  from F. Jegerlehner ii) FSR Cross section spp must be incl. for FSRfor use in the dispersion integral of am FSR corrections have to be taken into account in the efficiency eval. (SA Acceptance, MTrk) and in the passage ss FSR contr. (sQED): Net effect of FSR is ca. 0.8% s*>sp s* (H.Czyż, A.Grzelińska, J.H.Kühn, G.Rodrigo, EPJC33,2004)

9. 8 Luminosity: e+ e-  KLOE measures L with Bhabha scattering F. Ambrosino et al. (KLOE Coll.) Eur.Phys.J.C47:589-596,2006 55° < < 125° acollinearity < 9° p 400 MeV generator used foreff BABAYAGA (Pavia group): C. M.C. Calame et al., NPB584 (2000) 459 C. M.C. Calame et al., NPB758 (2006) 22 New: new version (BABAYAGA@NLO) gives 0.7% decrease in cross section, and better accuracy: 0.1%

10. 9 Improvements error table on exp. systematics in the published work improved machine conditions (luminosity and background) in the new data set PLB606(2005)12 improved offline-event filter reduces systematic uncertainty to < 0.1% improvements (vertex requirement dropped) in the selection 30% inefficiency (veto of cosmic rays) recovered by introducing 3rd level trigger new generatorBABAYAGA@NLO, error onBhabhafrom 0.5% to 0.1% Stotal,2005= 1.3%

11. 10 New result from 2002 data: Systematic errors on ampp: KLOE 2008 experimental fractional error on a = 0.9 % stat. error only STotal= 1.0%

12. 11 Evaluating ampp Dispersion integralfor2p-channel in energy interval 0.35 <Mpp2<0.95 GeV2 2005 published result (Phys. Lett. B606 (2005) 12): app(0.35-0.95GeV2) = (388.7  0.8stat 3.5sys3.5theo) ·10-10 Applying update for trigger eff. and change in Bhabha-cross section used for luminosity evaluation: app(0.35-0.95GeV2) = (384.4  0.8stat3.5sys3.0theo) ·10-10 2008: app(0.35-0.95GeV2) = (389.4  0.6stat3.3sys 2.0theo) · 10-10

13. 12 ampp Summary: Summary of the results:

14. 13 ampp comparison: Comparison withamppfrom CMD2 and SND in the range 0.630-0.958 GeV : Phys. Lett. B648 (2007) 28 KLOE result in agreement with CMD2 and SND

15. 14 |Fp|2comparison: |2CMD,SND/ |2KLOE  |Fp|2 only statistical errors are shown Light grey band: KLOE stat. error Dark grey band: KLOE total error (stat. + syst.) Data points: CMD and SND results compared to KLOE value extrapolated to the corresponding sp (only stat. error shown)

16. 15 Large angle analysis: p g f, r p g p f p f g f0 r p p  small!  important! Pion tracks: 50o<p <130o At least 1 photon with 50o<g <130o and Eg > 50 MeV  photon detected • independent complementary analysis • threshold region (2m)2 accessible • ISR photon detected (4-momentum constraints) • lower signal statistics • larger contribution from FSR events • large background contamination • irreducible background from  decays (f0) 2002 data 485000 events L = 240 pb-1 Preliminary Nr. of events / 0.01 GeV2 sp [GeV2] Threshold region non-trivial due to irreducible FSR-effects, which have to be estimated from MC using phenomenological models (interference effects unknown) & & f0 FSR rp

17. 16 Large angle analysis: p+ g p- W Dedicated selection cuts: • Exploit kinematic closure of the event •  Cut on angle btw. ISR-photon and • missing momentum • Kinematic fit inp+p-p0hypothesis using 4-momentum andp0-mass as contstraints • FSR contribution added back to • cross section (estimated from PHOKHARA • generator) 2002 Data L = 240 pb-1 • LA stat.+syst. error Preliminary Error on LA dominated by syst. uncertainty on f0 contr. • Reducible Background fromp+p-p0 • andµ+µ-g well simulated by MC sp[GeV2] Dominating uncertainty from model dependence of irreducible background ff0g p+p-g.Using different models for f0-decay and using input from dedicated KLOEff0ganalyses (withf0p+p-and f0 p0p0)  Difference between the MC models contributes to systematic uncertainty

18. 17 Conclusions and Outlook: We have evaluated the contribution to a in the range between 0.35 - 0.95 GeV2 using cross section data obtained via ISR events with photon emission at small angles. • The result from new data agrees with the updated result from the published KLOE analysis • KLOE results also agree with recent results onafrom the CMD2 and SND experiments at VEPP-2M in Novosibirsk • better agreement in spectrum between different experiments than in the past Independent analysis is in progress using detected photons emitted at large angle • Syst. uncertainty dominated by model-dependence of scalar mesons We are also measuring the pion form factor using the bin-by-bin ratio of pion over muon  (instead of using absolute normalization with Bhabhas). Finally we are measuring pion form factor from data taken at s= M - 20 MeV (1000 MeV) • suppression of background fromf-decays • determination of f0-parameters

19. Backup:

20. Comparing 2008 with 2005 result: stat. error only