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

D. LeoneNovosibirsk, , 2006Pion Form KLOE Signal selection 50 0 <   <130 0   Pion tracks: 50 o <    < 130 o Photons: at least one with 50 o <   < 130 o and E  > 50 MeV tagged measurement the threshold region is accessible one photon is detected (4-momentum constraints) lower signal statistics large FSR contributions irreducible background from  to     decays large       background contamination PRO & CONTRA      MC       MC 50% of final sample statistic 50 o <   <130 o 50 o <   <130 o M  2 [GeV 2 ]

D. LeoneNovosibirsk, , 2006Pion Form KLOE Reducible background rejection three sources: Radiative Bhabhas e + e -  e + e - , muon pairs  +  -   +  -  and  +  -  0 Radiative Bhabhas are separated by means of a particle-ID (signature of EmC-Clusters and time of flight of particles) Particle ID      – MC       – MC      - MC M  2 [GeV 2 ] M trk [MeV] mm mm mm To reject      and (partially)       background a cut in the plane M trk vs. M  2 is applied. M trk is the kinematical variable obtained by solving in the assumption of  x + x -  TrackMass

D. LeoneNovosibirsk, , 2006Pion Form KLOE Kinematic fit in the       background hypothesis Two tracks in 40  <   < 140  At least two photons in time, one of them with E  > 40 MeV and 40  <   < 140  4-momenta conservation M inv (  ) = m(  0 ) Reducible background rejection Two further dedicated cuts to       rejection Kinematic fit      Angle Angle between the missing momentum and the detected photon momentum [o][o]      MC (due to NLO  events)       MC 22 22 Data      MC

D. LeoneNovosibirsk, , 2006Pion Form KLOE Residual background evaluation –      Muons sample selected asking 80 < TrackMass < 110 MeV, in the same angular region as the      sample 80 <TrackMass<110MeV Up tp 10% difference between data and MC for a maximum of 10% contamination (excluding the threshold region...see later) M  2 [GeV 2 ] % Muons contamination M  2 [GeV 2 ] Absolutely normalized 1% error on the knowledge of      in the final      sample Preliminary MC: Phokhara 5 DATA MC

D. LeoneNovosibirsk, , 2006Pion Form KLOE Residual background evaluation –       In order to select a sample of       from data, we have applied after the angular cuts a rigid cut on the  2 of the        kinematic fit (  2 <20) Absolutely normalized Data       MC M  2 [GeV 2 ] 3 pions contamination % Difference data-MC of the order of 20%, contamination in the final sample smaller than 10 % accuracy on       subtraction at per mil level.  2 <20 E  [MeV]  [ o ] Data       MC Data       MC

D. LeoneNovosibirsk, , 2006Pion Form KLOE Irreducible background      and       background channels well under control… but FSR events as e + e -  FSR   f 0     all of them with      final state, indistinguishable from the signal signature At low M  2, ISR and FSR are not the only contributions to the mass spectrum and to the charge asymmetry  model dependence for the additional contributions Three processes of the same family: their amplitudes interfere    ffff      FSR f0f0  More phenomenological input nedeed concerning the hadronic models.

D. LeoneNovosibirsk, , 2006Pion Form KLOE 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. Single track efficiency: above 96% for 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 <       

D. LeoneNovosibirsk, , 2006Pion Form KLOE The spectrum extends down to the 2-pions threshold dN/dM  2 spectrum 50 o 50MeV Both the particles not identified as electrons Cut on  2  Cut on TrackMass vs. M  2 Cut on  angle 2002 Data L = 240 pb -1 M  2 [GeV 2 ] KLOE preliminary M  2 [GeV 2 ]

D. LeoneNovosibirsk, , 2006Pion Form KLOE Efficiency of the selection The signal selection efficiency is never below 80% even in the threshold region, where the ratio signal/background is low. Further checks proves that the reducible background contribution in the data sample after the selection is negligible.  [o][o] Data      MC Selection efficiency M  2 [GeV 2 ] Data      MC

D. LeoneNovosibirsk, , 2006Pion Form KLOE Forward-backward asymmetry  +  - system: A(ISR)  C-odd A(FSR)  C-even  an asymmetry is expected in the variable: Pion polar angle [ o ] 90 o MC test of sQED via comparison data/MC Issue: to distinguish the effect of the interference (described in our MC by sQED ) and the effect of f 0 (980). Czyż, Grzelińska, Kühn, Phys.Lett.B 611(116)2006 M  [GeV] 20 o <   <160 o 45 o <   <135 o f 0 kk model f 0 ‘no str’    f 0 ‘no str’    /2 no f 0 A(f 0 )  C-even

D. LeoneNovosibirsk, , 2006Pion Form KLOE Forward-backward asymmetry Using the f 0 amplitude from Kaon Loop model, good agreement data-MC* both around the f 0 mass and at low masses. M  (MeV) zoom data  MC: ISR+FSR  MC: ISR+FSR+f 0 (KL) Phys.Lett.B634 (06), 148 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. * G. Pancheri, O. Shekhovtsova, G. Venanzoni, hep-ph/ for more details see C. Di Donato’s talk

D. LeoneNovosibirsk, , 2006Pion Form KLOE 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  -peak and at high M  2 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

D. LeoneNovosibirsk, , 2006Pion Form KLOE P > 250 MeV: P(  + ) = P(  - ) = 0.96  The probability to not trigger is 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  P<250 MeV: lower single track efficiency, but the dinamic makes the overall probabily negligible. Sample: N(LA)= (in our acceptance region) MC stand alone N=20981 (5.4% of N(LA)) N=37 (0.09% of N(LA)) N=368 (0.09% of N(LA)) And it becomes completely negligible if we consider both the pions at low momenta