Federico Nguyen - IEKP Uni Karlsruhe for the KLOE Collaboration DAΦNE 2004: Physics at meson factories Measurement of the e+e- → π+π–(γ) cross section below 1 GeV with the KLOE detector Federico Nguyen - IEKP Uni Karlsruhe for the KLOE Collaboration 10th June 2004

Outline: measuring s(e+e-  p+p- g) with ISR events selection and identification of e+e-  p+p- g events background rejection and estimates the cross section dsppg /dMpp2 extraction of s(e+e-  p+p-) and FSR corrections results and conclusions Federico Nguyen

The radiative return standard way: Advantages: Additional issues: measuring se+ e-  hadrons(s) by varying e beam energies alternative approach: given a fixed s1/2, by studying Initial State Radiation events Advantages: uncertainties related to beam energy and luminosity are the same for the full spectrum it doesn’t require a dedicated set-up (measurement performed in parallel with typical KLOE programs) H = radiation function qmin = emitted g min. ang. Additional issues: it requires a precise knowledge of ISR rad. corr. FSR effects need to be under control for H we used the MC code Phokhara with the full NLO ISR corrections [H.Czyz et al., Eur Phys J C24 (2002) 71] Federico Nguyen

The selection of e+e- p+ p- g events 2 charged tracks in 50o < qp < 130o with a vertex in r < 8 cm, |z| < 7 cm g acceptance: for enhancing ISR wrt FSR events with g emitted at small angle are selected  only 2 p’s are detected and a cut on the direction of the missing momentum is applied: qS < 15o ( 165o < qg < 180o), qS = dipion system polar angle, pg = - pS = - ( pp++pp- ) high statistics for ISR g events f  p+p-p0 contamination is reduced Federico Nguyen

Pion identification /e separation is performed using a Likelihood function based on: 0.016 0.014 0.012 0.01 0.008 0.006 0.004 0.002 TOF of charged clusters Shape and quantity of energy deposition of the “charged” cluster along the EMC - before any cut p+p-p0 events e+e-g events the event is selected if at least one of the 2 tracks is recognized as a pion (log L > 0) it leads to a rejection power > 92% for e+ e-  e+ e- events keeping a selection efficiency ~ 99% for e+ e-  p+ p- g events -10 -7.5 -5 -2.5 0 2.5 5 7.5 10 log L Federico Nguyen

Background identification and rejection p+p-p0 m+m-g + e+e-g signal region mtrk (MeV) mp p+p-gg tail mm Mr2 Mpp2 (GeV2) 3500 3000 2500 2000 1500 1000 500 N. of events before Pion ID after Pion ID e+ e- g m+ m- g p+ p-p0 0 50 100 150 200 250 300 mtrk (MeV) the effect is seen in “track mass” distr., mtrk, defined by 4-mom. conserv. under the hypothesis of 2 equal mass tracks and a g Federico Nguyen

Estimates of the residual background (I) i) evaluate mtrk histograms in Mpp2 slices for MC signal and background samples ii) fit data mtrk histograms with MC ones to get the relative weights of background N. of events / MeV data  ee L = Pi P(di | wS Si+wB Bi ) only free parameters data MC signal, MC background Mpp2  [0.82,0.87] GeV2 c2 = 27.9/42 - index i runs over mtrk bins - P = Poisson probability distribution - minimize c2 = -2 ln L data sum of  and ee Federico Nguyen

Estimates of the residual background (II) N. of events / 0.5 MeV N. of events / MeV data  p+p-p0 data  mmg Mpp2  [0.32,0.37] GeV2 c2 = 87.0/88 Mpp2  [0.38,0.42] GeV2 c2 = 104.4/88 data sum of  and mmg data sum of  and p+p-p0 Federico Nguyen

The observed spectrum and efficiencies 100 80 60 40 20 eselection [%] N. of events / (0.01 GeV2) Mpp2 (GeV2) Mpp2 (GeV2) High statistics: 1.55  106 events out of 141 pb-1 of data taken in 2001 High resolution: bin width = 0.01 GeV2 Tracking + Vertex Acceptance Trigger Reconstr. filter Pion ID Trackmass Federico Nguyen

The luminosity measurement in KLOE Large angle Bhabha: * Data Babayaga (MC) (Pavia theory group) Bhagenf (MC) (Berends adapted by E. Drago and G. Venanzoni) track momentum (MeV) 55° < q < 125° acollinearity < 9° p  400 MeV BABAYAGA: sbha = ( 428.8 ± 0.3stat ) nb BHAGENF: sbha = ( 428.5 ± 0.3stat ) nb e- e+ g of s Theoretical Systematic Error = 0.5% Total Error = 0.6% Federico Nguyen

The cross section ds(e+e- p+ p- g) fbkg = ( Np+p-p0 + Nmmg + Neeg ) / Ntot ds/dMpp2 (nb/GeV2) Mpp2 (GeV2) r-w Interference Mpp2 (GeV2) Federico Nguyen

From ds(e+e- p+ p- g) to s(e+e- p+ p-) PHOKHARA (Kühn et al.) agrees with KKMC (Jadach et al.) at the level of the claimed accuracy: 0.5% for evaluating am we must get the bare cross section: Dahad(s) from F. Jegerlehner, July 2003 Mpp2 (GeV2) Federico Nguyen

it is coherently included FSR corrections Fp (exclusive FSR) Fp (inclusive FSR) the cross section in the dispersion integral must be inclusive in FS g’s spp(g)  A0 = 1.002 ± 0.001 our approach: spp(g) inclusive in FSR Mpp2 (GeV2) it is suppressed by acceptance cuts 0.4 0.5 0.6 0.7 0.8 0.9 another approach: spp(g) exclusive in FSR subtract FSR (by MC) from the observed sppg efficiencies are evaluated from MC with only ISR add FSR (Schwinger) to the measured Fp it is coherently included in the MC simulation of the signal [Czyz et al., Eur. Phys. J C27 (2003) 563] Federico Nguyen

Comparison with CMD-2 KLOE 50 45 40 35 30 25 20 15 10 5 |Fp | 2 CMD-2 KLOE Comparison with CMD-2 only statistical errors are shown we have evaluated the dispersion integral for the pp channel in the range 0.35 GeV2 < Mpp2 < 0.95 GeV2 ampp = ( 388.7  0.8stat  3.5syst  3.5theo )  10-10 KLOE PRELIMINARY Mpp2 (GeV2) comparison with CMD-2 in the range 0.37 GeV2 < Mpp2 < 0.93 GeV2 0.4 0.5 0.6 0.7 0.8 0.9 1 KLOE ampp = (375.6  0.8stat  4.9syst+theo)  10-10 1.3% Error CMD-2 ampp = (378.6  2.7stat  2.3syst+theo)  10-10 0.9% Error Federico Nguyen

How the plot from M. Davier et al. (2003) modifies… …including KLOE result Federico Nguyen

Conclusions and outlook the measurement of the cross section s(e+e-  p+p-) via ISR events using a sample of 141 pb-1 of data taken in 2001 has been presented high statistics and high mass resolution have allowed to release results competitive with CMD-2 for Mpp2 > Mr2 our results agree with CMD-2, confirming the difference between t decays and e+e- data estimates our ampp value also confirms the 2.7s deviation between amSM(e+e-) and the recent amexp value we started the large g angle analysis, allowing to probe the threshold region, and also Particle ID studies for measuring R Federico Nguyen