Transcript Slide 1
International Workshop e+e- collision from to Novosibirsk February 27th- March 2nd 2006 Debora Leone (IEKP – Universität Karlsruhe) for the KLOE collaboration Progress on Pion Form Factor at KLOE (large photon polar angle) D. Leone Pion Form Factor @ KLOE Novosibirsk, 27.02- 02.03, 2006 PRO & CONTRA Signal selection Pion tracks: qp < Photons: at least one with 50o <qg< 130o and Eg > 50 MeV tagged measurement 50o < 130o 500< qp,g <1300 the threshold region is accessible one photon is detected (4-momentum constraints) lower signal statistics large FSR contributions irreducible background from to p+p- decays large p+p-p0 background contamination 50o<qp<130o 50o<qg<130o p+p-g MC p+p-p0 MC 50% of final sample statistic g p Mpp2 [GeV2] D. Leone Pion Form Factor @ KLOE Novosibirsk, 27.02- 02.03, 2006 Reducible background rejection three sources: Radiative Bhabhas e+e- e+e- g, muon pairs m+m- m+m- g and p+p-p0 Particle ID Mtrk [MeV] Radiative Bhabhas are separated by means of a particle-ID (signature of EmC-Clusters and time of flight of particles) mp mm 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 2 2 2 2 2 M - | p + | + M trk - | p- | + M trk - | p + + p- |2 0 p+p-p0 – MC p+p-g – MC in the assumption of x+ x- g m+m-g - MC 2 2 mr2 Mpp [GeV ] D. Leone Pion Form Factor @ KLOE Novosibirsk, 27.02- 02.03, 2006 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 < qp < 140 At least two photons in time, one of them with Eg > 40 MeV and 40 < qg < 140 4-momenta conservation Minv(gg) = m(p0) W Angle p+p-g Data p+p-g MC Angle between the missing momentum and the detected photon momentum MC p γ .pmiss W acos( ) | p γ | | pmiss | (due to NLO events) p+p-p0 MC pγ 0 10 D. Leone 20 30 40 50 60 70 W[o] Pion Form Factor @ KLOE 2 p+ g W 2 p- pmiss Novosibirsk, 27.02- 02.03, 2006 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] % Up tp 10% difference between data and MC for a maximum of 10% contamination (excluding the threshold region...see later) Muons contamination 1% error on the knowledge of m+m-g in the final p+p-g sample Mpp2 [GeV2] D. Leone Pion Form Factor @ KLOE Novosibirsk, 27.02- 02.03, 2006 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 Absolutely the p+p-p0 kinematic fit normalized (2 <20) Data MC p+p-p0 Data MC p+p-p0 2 <20 Mpp2 [GeV2] Eg [MeV] Data MC p+p-p0 W [o] D. Leone 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. Pion Form Factor @ KLOE % 3 pions contamination Mpp2 [GeV2] Novosibirsk, 27.02- 02.03, 2006 Irreducible background m+m-g and p+p-p0 background channels well under control… but FSR events as e+e- ppgFSR f0 g pp g r p pg p all of them with p+p-g final state, indistinguishable from the signal signature p r p FSR g g & f0 p & f0 p p rp g r p 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. D. Leone Pion Form Factor @ KLOE Novosibirsk, 27.02- 02.03, 2006 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. ep+ ep- 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 eg D. Leone Pion Form Factor @ KLOE Novosibirsk, 27.02- 02.03, 2006 dN/dMpp2 spectrum 50o<qp,g<130o, Eg>50MeV Both the particles not identified as electrons Cut on 2ppp Cut on TrackMass vs. Mpp2 Cut on W angle Mpp2 [GeV2] 2002 Data L = 240 pb-1 Mpp2 [GeV2] The spectrum extends down to the 2-pions threshold D. Leone Pion Form Factor @ KLOE Novosibirsk, 27.02- 02.03, 2006 Efficiency of the selection e Selection efficiency 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 W[o] D. Leone Mpp2 [GeV2] Pion Form Factor @ KLOE Novosibirsk, 27.02- 02.03, 2006 Forward-backward asymmetry 90o p+p- system: A(ISR) C-odd A(FSR) C-even an asymmetry is expected in the variable: θπ θ π+ - test of sQED via comparison data/MC MC f0 kk model f0 ‘no str’ ap f0 ‘no str’ ap/2 no f0 A(f0) C-even Pion polar angle [o] 20o<q o p<160 45o<qg<135o Issue: to distinguish the effect of the interference (described in our MC by sQED ) and the effect of f0(980). Mpp [GeV] D. Leone Czyż, Grzelińska, Kühn, Phys.Lett.B 611(116)2006 Pion Form Factor @ KLOE Novosibirsk, 27.02- 02.03, 2006 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, Mpp (MeV) Mpp (MeV) for more details see C. Di Donato’s talk zoom D. Leone G. Venanzoni, hep-ph/0506332 Pion Form Factor @ KLOE Novosibirsk, 27.02- 02.03, 2006 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 D. Leone Forward-backward asymmetry Pion Form Factor @ KLOE Novosibirsk, 27.02- 02.03, 2006 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 810-4 P<250 MeV: lower single track efficiency, but the dinamic makes the overall probabily negligible. Sample: N(LA)=390000 (in our acceptance region) MC stand alone N=37 (0.09% of N(LA)) N=20981 (5.4% of N(LA)) N=368 (0.09% of N(LA)) D. Leone And it becomes completely negligible if we consider both the pions at low momenta Pion Form Factor @ KLOE Novosibirsk, 27.02- 02.03, 2006