Study of the reaction e+e-  p+p- at KLOE Debora Leone IEKP – Universität Karlsruhe DPG Tagung München, 24.03.2006 Study of the reaction e+e-  p+p- at KLOE Introduction Result on the small photon angle analysis (Phys. Lett. B606, 12 (2005)) Large photon angle analysis (in progress) Conclusion

Hadronic contribution to (g-2)m Magnetic momentum: with · m + q g * g* B field amSM = amQED + amhad + amEW Due to quantum corrections: amhad includes contributions not evaluable in pQCD, but it can be provided by s(e+ e-  hadrons) by means of dispersion relation: K(s) is a steady function that goes with 1/s, enhancing low energy contributions of shadr(s) In energy range <1 GeV, e+e-  p+p- contributes to more than 60% to amhad

s(e+e- p+p-) with Radiative Return DAFNE is designed for a fixed center-of-mass energy: s = mf = 1.02 GeV We can measure s(e+e-p+p-) as a function of the hadronic c.m. energy M2hadr (S.Binner, J.H. Kühn, K. Melnikov, Phys.Lett. B459,1999) 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)   +- Mhadr ds(e+ e-  hadrons + g ) dM2hadrons =s(e+ e-  hadrons) H(M2hadr ) This method is a complementary approach to the energy scan.

Small photon polar angle analysis SELECTION Pion tracks: 50o< p <130o Photons: g <15o or g >165o No photon tagging: high statistics for ISR low relative FSR contribution suppressed   p+p-p0 background threshold region not covered no kinematic closure of event PRO & CONTRA Mpp2 (GeV2) MC: FSR /(ISR + FSR) (%) KLOE result (140 pb-1 of 2001) Phys. Lett. B606 (2005) 12

KLOE result compared with CMD-2 1.3% Error 0.9% Error CMD-2 KLOE 0.4 0.5 0.6 0.7 0.8 0.9 45 40 35 30 25 20 15 10 5 s (GeV2) |Fp(s)|2 Mpp2 (GeV2) s(e+e-p+p-) result statistical error: negligible experimental systematic uncertainty: 0.9% theoretical systematic uncertainty: 0.9% Total systematic error: 1.3% Pion Form Factor comparison with result from CMD-2 experiment: KLOE and CMD-2 in fair agreement Dispersion integral evaluation KLOE: (375.6  0.8stat  4.9syst+theo) 10-10 CMD-2: (378.6  2.7stat  2.3syst+theo) 10-10 KLOE and CMD-2 in good agreement 365 370 375 385 380 ampp·1010

Theory (SM) - Experiment Muon anomaly comparison Theory - Experiment KLOE and CMD-2 results used to evaluate the hadronic contribution and therefore the muon anomaly am - 11 659 000 ∙ 10-10 140 150 160 170 180 190 200 210 DEHZ’03 [e+e- based] DEHZ’03 [t based] DEHZ‘03 Experiment E821 Standard model prediction: KLOE measurement Phys. Lett. B606 (2005) 12 New 4th order QED calculation (Kinoshita, Nio) Phys. Rev. D70 (2004) 113001 New ‘Light-by-light’ calculation (Melnikov, Vainshtein) Phys. Rev. D70 (2004) 113006 CMD-2 and KLOE averaged in hadronic contribution DEHZ’04 [e+e-] New Theory (SM) - Experiment amexp - amtheo = ( 25.2 ± 9.2 ) ·10-10 2.7 standard deviations difference

Large photon polar angle analysis - signal selection the threshold region is accessible one photon is detected (4-momentum constraints) lower signal statistics large FSR contributions irreducible background from f to p+p-g decays large   p+p-p0 background contamination PRO & CONTRA Pion tracks: 50o < p < 130o Photons: at least one with 50o <g< 130o and Eg > 50 MeV tagged measurement 500< qp,g <1300 p g 50o<qp<130o 50o<qg<130o p+p-g MC p+p-p0 MC Mpp2 [GeV2]

Reducible background rejection three sources: Radiative Bhabhas e+e- e+e- , muon pairs e+e- m+m-  and +-0 Particle ID Radiative Bhabhas are separated by means of a particle-ID (likelihood function): signature of EmC-Clusters and time of flight of particles p+p-g – MC p+p-p0 – MC m+m-g - MC Mpp2 [GeV2] Mtrk [MeV] mp mm mr2 TrackMass To reject m+m-g and (partially) p+p-p0 background a cut in the plane Mtrk vs. Mpp2 is applied. Mtrk is the kinematical variable obtained by solving under the assumption of  x+ x- 

Reducible background rejection Two further dedicated cuts to p+p-p0 rejection Kinematic fit Kinematic fit under the p+p-p0 background hypothesis Two tracks in 40 < p < 140 At least two photons, one of them with Eg > 40 MeV and 40 < g < 140 4-momenta conservation Minv(gg) = m(p0) 2 Data p+p-g MC  Angle 2 Angle between the missing momentum and the detected photon momentum 10 20 30 40 50 60 70 [o] p+p-g MC p+p-p0 MC p+ p- W g

their amplitudes interfere Irreducible background m+m-g and p+p-p0 background channels well under control… but FSR events as e+e-  ppgFSR f  f0 g  pp g f  r p  pg p all of them with p+p-g final state, indistinguishable from the signal signature f f0 g p r p g f r p g & & FSR f0 rp Three processes of the same family: their amplitudes interfere At low Mpp2, ISR and FSR are not the only contributions to the mass spectrum and to the charge asymmetry  model dependence for the additional contributions More phenomenological input needed concerning the hadronic models.

dN/dMpp2 spectrum KLOE preliminary 50o<qp,g<130o, Eg>50MeV 2002 Data L = 240 pb-1 Mpp2 [GeV2] KLOE preliminary 50o<qp,g<130o, Eg>50MeV Both the particles not identified as electrons Cut on 2 Cut on TrackMass vs. Mpp2 Cut on  angle The spectrum extends down to the 2-pions threshold

Forward-backward asymmetry Pion polar angle [o] 90o MC +- system: A(ISR)  C-odd A(FSR)  C-even  an asymmetry is expected in the variable: test of sQED via comparison data/MC A(f0) C-even f0 KK model f0 ‘no structure’ af=p f0 ‘no structure’ af=p/2 no f0 20o<qp<160o 45o<qg<135o Issue: to distinguish the effect of the interference (described in our MC by sQED ) and the effect of f0(980). Czyż, Grzelińska, Kühn, Phys.Lett.B 611(116)2006 Mpp [GeV]

Forward-backward asymmetry At large photon angles, the amount of FSR is large and the interference between the two terms gives a sizeable effect. KLOE has already published a first measurement of the forward-backward asymmetry, and proven the sensitivity of this quantity to the presence of scalar mesons. Phys.Lett.B634 (06), 148 Using the f0 amplitude from Kaon Loop model, good agreement data-MC* both around the f0 mass and at low masses. data MC: ISR+FSR MC: ISR+FSR+f0(KL) Mpp (MeV) Mpp (MeV) * G. Pancheri, O. Shekhovtsova, G. Venanzoni, hep-ph/0506332 zoom

Conclusion KLOE has measured for the first time the hadronic cross section with radiative return method, proving the validity of it. The measurement of the hadronic cross section in a different angular region and with tagged photon is in an advanced state. The threshold region requires more studies, due to the irreducible background The analysis on the r-peak and at high Mpp2 is close to the conclusion  important check for the already published KLOE result. Selection cuts are fixed Evaluation of efficiencies is almost finished test of model scalar QED possible study of scalar mesons Forward-backward asymmetry

Backup slide

The KLOE detector at the DAFNE F-factory 12582 sense wires 52140 wires in total Track momentum resolution sp/p  0.4% (q > 45°) Vertex resolution sxy  150 mm, sz  2 mm Drift chamber Electromagnetic calorimeter Energy resolution sE/E = 5.7%/E(GeV) Time resolution sT = 54 ps/E(GeV)  50 ps Pb/Scint fibres 4880 PM Magnetic Field of 0.52 T