Progress on Pion Form Factor at KLOE (large photon polar angle) Debora Leone (IEKP – Universität Karlsruhe) for the KLOE collaboration International Workshop e+e- collision from  to  Novosibirsk February 27th- March 2nd 2006 Progress on Pion Form Factor at KLOE (large photon polar angle)

Signal selection PRO & CONTRA 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- 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 50% of final sample statistic Mpp2 [GeV2]

Reducible background rejection three sources: Radiative Bhabhas e+e- e+e- , muon pairs m+m- m+m-  and +-0 Particle ID Radiative Bhabhas are separated by means of a particle-ID (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 in the assumption of  x+ x- 

Reducible background rejection Two further dedicated cuts to p+p-p0 rejection Kinematic fit Kinematic fit in the p+p-p0 background hypothesis Two tracks in 40 < p < 140 At least two photons in time, 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 (due to NLO events) p+p-p0 MC p+ p- W g

Residual background evaluation – m+m-g Muons sample selected asking 80 < TrackMass < 110 MeV, in the same angular region as the p+p-g sample Preliminary 80 <TrackMass<110MeV MC: Phokhara 5 Absolutely normalized DATA MC Mpp2 [GeV2] % Muons contamination Up tp 10% difference between data and MC for a maximum of 10% contamination (excluding the threshold region...see later) 1% error on the knowledge of m+m-g in the final p+p-g sample Mpp2 [GeV2]

Residual background evaluation – p+p-p0 In order to select a sample of p+p-p0 from data, we have applied after the angular cuts a rigid cut on the 2 of the p+p-p0 kinematic fit (2 <20) Absolutely normalized Data p+p-p0 MC Data p+p-p0 MC 2 <20 Mpp2 [GeV2] % Difference data-MC of the order of 20%, contamination in the final sample smaller than 10 % accuracy on p+p-p0 subtraction at per mil level. 3 pions contamination Eg [MeV] Data p+p-p0 MC  [o] Mpp2 [GeV2]

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 nedeed concerning the hadronic models.

Trigger efficiency In KLOE, in order to trigger, an event has to release energy over a certain threshold in two different regions of the calorimeter. We have evaluated the event trigger efficiency by data combining the probability that the single particle triggers, when the other two have already triggered the event. e(p+) e(p-) e(g) Single track efficiency: above 96% for p(p) > 270 MeV above 99% for the photon in almost the whole energy range Sub-per mil probability to have an event with low energy photon and low momenta pions Trigger EVENT inefficiency < 10-3

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

Efficiency of the selection Selection efficiency e The signal selection efficiency is never below 80% even in the threshold region, where the ratio signal/background is low. p+p-g MC Mpp2 [GeV2] Data p+p-g MC Further checks proves that the reducible background contribution in the data sample after the selection is negligible. Data p+p-g MC [o] Mpp2 [GeV2]

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 str’ af=p f0 ‘no str’ 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) * G. Pancheri, O. Shekhovtsova, G. Venanzoni, hep-ph/0506332 Mpp (MeV) Mpp (MeV) for more details see C. Di Donato’s talk zoom

Conclusion The measurement of the hadronic cross section with tagged photons is in an advanced status. The threshold region requires more studies. 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

negligible. Sample: N(LA)=390000 (in our acceptance region) P > 250 MeV: P(p+) = P(p-) = 0.96  The probability to not trigger is 0.077 This probability has to be combined with the trigger probabilty of the photon i.e. > 0.99 Combining the two, the probability that the event does not trigger is 810-4 P<250 MeV: lower single track efficiency, but the dinamic makes the overall probabily negligible. Sample: N(LA)=390000 (in our acceptance region) N=37 (0.09% of N(LA)) MC stand alone N=20981 (5.4% of N(LA)) N=368 (0.09% of N(LA)) And it becomes completely negligible if we consider both the pions at low momenta