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Debora Leone Institut für Experimetelle Kernphysik Universität Karlsruhe on behalf of KLOE collaboration Measurement of the hadronic cross section at KLOE Tau04 Nara 14-17/09/2004

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D. Leone Hadronic cross section @ KLOE a theory vs. experiment THEORY ‘03 e + e - Data: 2.7 - Deviation – Data: 1.4 - Deviation Experiment ’02/‘04 Experiment BNL-E821 Values for + (2002) and - (2004) in agreement with each other. Precision: 0.5ppm Dispersion integral for hadronic contribution to a evaluated for: a) CMD-2 (Novosibirsk/VEPP-2M) channel with 0.6% precision < 1 GeV b) -Data from ALEPH /OPAL/CLEO e + e - and data differ for + - channel in a specific energy window above 0.6 GeV 2 (above the peak) a -11659000(10 -10 ) Status up to July ’04

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The standard method to measure (e + e - hadrons) is the energy scan, i.e. the syst. variation of c.m. energy of the machine. Since at DA NE the collision energy is fixed, we use a complementary approach: looking for e + e - + - events, where the photon is emitted in the initial state (ISR), we have a continuous variation of s , the invariant mass of the hadronic system hadr d ( e + e - hadrons + ) d hadrons ( e + e - hadrons) H( hadr ) Precise knowledge of ISR process Radiator function H(Q 2, ,M 2 ) MC generator: Phokhara H. Czyz, A. Grzelinska, J.H. Kühn, G. Rodrigo 4m 2 < s < m 2 D. Leone Hadronic cross section @ KLOE The Radiative Return

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D. Leone Hadronic cross section @ KLOE selection Pion Tracks at large angles 50 o < < 130 o Photons at small angles 165 o are masked by quadrupoles near the I.P. ( no photon tagging ) High statistics for ISR Photons Low relative contribution of FSR Reduced background contamination e+e+ e-e- EM calorimeter Drift chamber

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D. Leone Hadronic cross section @ KLOE dN( )/dM 2 after acceptance cuts Event analysis : Efficiencies and background Normalize to Luminosity Radiative Corrections Cross section (e + e - cross section M 2 [GeV 2 ] dN( )/dM 2 [nb -1 /GeV 2 ] 141 pb -1 No acceptance at threshold region 50000 40000 30000 20000 10000 Differential cross section d ( )/dM 2 Divide by Radiator Function

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signal region tail Step 1 : e + e - are separated from by means of a Likelihood method (signature of EmC-clusters and TOF of particle tracks) D. Leone Hadronic cross section @ KLOE Background Rejection 1/2 M trk [MeV] M 2 [GeV 2 ] Step 2 : and e + e - rejected by means “Trackmass ” mm mm mm e e

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D. Leone Hadronic cross section @ KLOE Background Rejection 2/2 Step 3 : fit data trackmass distributions with MC ones (signal + background) with free normalization parameters M 2 [0.32,0.37] GeV 2 M trk [MeV]

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KLOE uses large angle Bhabha events for the luminosity evaluation: Theory precision (radiative corr.) BABAYAGA event generator (Pavia group) syst. comparison among other generators (Bhagenf, BHWIDE, VEPP-2M ); max. = 0.7 % uncertainty 0.5% =BABAYAGA error Experimental precision Excellent agreement data-MC N = events with 55 < <125 D. Leone Hadronic cross section @ KLOE Luminosity Measurement 55 o 125 o Polar Angle [°] data MC

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D. Leone Hadronic cross section @ KLOE Analysis (e + e - ) EFFICIENCIES: Trigger + Cosmic Veto Tracking + Vertexing /e separation Reconstruction filter Trackmass cut Unfolding procedure Acceptance BACKGROUND e + e - e + e - e + e - LUMINOSITY Bhabha at large angles 0.9 % 0.3 % d (e + e - )/ dM [nb/GeV 2 ] M [GeV 2 ] Interference 140 pb -1 of 2001 data 1.5 millions events 0.6 %

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D. Leone Hadronic cross section @ KLOE FSR Contribution Two kinds of FSR There is no initial state radiation and the e + and the e - collide at the energy M the virtual has Q 2 = M 2 Background Simultaneous presence of a initial and final photon The cross section e + e - + - has to be inclusive with respect to NLO FSR events:

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D. Leone Hadronic cross section @ KLOE FSR Treatment “ FSR Inclusive ” approach ( e + e - FSR ) Event analysis Phokhara ISR+FSR Luminosity ( e + e - ISR FSR ) Radiator H N ( e + e - ISR FSR ) add back missing FSR Correction for unshifting s ee ee } ss Invariant mass of the system , s invariant mass of the virtual photon s

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D. Leone Hadronic cross section @ KLOE FSR uncertainty “ FSR Exclusive ” approach N ( e + e - ISR FSR ) ( e + e - ISR ) ( e + e - ) Radiator H Event analysis Phokhara ISR Luminosity subtract FSR contribution estimated by MC FSR 0.8.. 0.9% Schwinger ‘90 = 0.2% ± 0.1% Rel. difference inclusive - exclusive approach 0.4 0.50.60.7 0.80.9 1 1.04 1.02 0.98 0.97 0.96 0.95 0.99 1.01 1.03 1.05 1. The 2 methods are in excellent agreement Higher order FSR corrections negligible Proof of Factorization Ansatz FSR systematic = 0.3%, coming from 2 contributions 0.2% difference incl-excl correction upper limit of 20% for scalar QED model (point-like pions): 20% 1% = 0.2%

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D. Leone Hadronic cross section @ KLOE Cross Section (e + e - + - ) Final spectrum: after the correction for vacuum polarization had (s) from F. Jegerlehner, July 2003 s [GeV 2 ] Total error = 1.3 %

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D. Leone Hadronic cross section @ KLOE 2 contribution to a hadr We have evaluated the dispersion integral for 2 channel in the energy range 0.35 <s <0.95 GeV 2 Comparison with CMD-2 in the energy range 0.37<s <0.97 GeV 2 KLOE (375.6 0.8 stat 4.8 syst+theo ) 1.3% Error CMD-2 (378.6 2.7 stat 2.3 syst+theo ) 0.9% Error At large values of s (>m 2 ) we are consistent with CMD-2 and we confirm the deviation from -data. CMD-2 KLOE s [GeV 2 ] |F (s)| a = ( 388.7 0.8 stat 3.5 syst 3.5 theo ) 10 -10

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D. Leone Hadronic cross section @ KLOE Conclusion KLOE has proven the feasibility to use initial state radiation to measure hadronic cross section (hep-ex 0407048) We expect to reduce the systematic error below 1% by repeating the analysis with 2002 data. Improvements from theory are also expected. The analysis at large photon angle to study the region near the threshold is going on. Evaluation of ratio R – the analysis has already begun

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D. Leone Hadronic cross section @ KLOE = data = MC preliminary Polar angle [ ] Asymmetry Test of sQED model : at large photons angles the amount of FSR is large. Here it is possible to study the charge asymmetry. It comes out from the interference between ISR (C-odd) and FSR (C-even) [ ] ++ -- Outlook Integrating asymmetry we get a difference data-MC of (8.5 1.2)% Large angle photon analysis to explore the threshold region tagged measurement

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D. Leone Hadronic cross section @ KLOE Backup slices

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D. Leone Hadronic cross section @ KLOE Unfolding the Mass Revolution s (true) [GeV 2 ] s (obs) [GeV 2 ] Trackmass [MeV] The smearing matrix “almost” diagonal Inversion of smearing matrix possible A more sophisticated unfolding technique is obtained by means of the unfolding package GURU (A. Höcker et.al./ALEPH). Issues :- Reliability of MC simulation - Correct choice of the regularization parameter Systematics studied by varying meaningful values of the regularization parameter: Due to nature of the dispersion integral the effect on a is almost negligible

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D. Leone Hadronic cross section @ KLOE Terms not included in Phokhara Unshifting correction

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D. Leone Hadronic cross section @ KLOE Relative contribution of LO-FSR Relative contribution of NLO-FSR

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D. Leone Hadronic cross section @ KLOE M TRK e e e e before cutting on particle ID after cutting on particle ID mm Likelihood effect on M trk distribution F =1 with F =1 s (GeV 2 ) Radiator Function

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D. Leone Hadronic cross section @ KLOE

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D. Leone Hadronic cross section @ KLOE reconstructed kine M 2 [GeV 2 ]

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D. Leone Hadronic cross section @ KLOE KLOE & DA NE e + e - collider @ S = M = 1019.4 MeV achieved peak Luminosity: 8 10 31 cm -2 s -1 2000-2002 data set: 500 pb -1 KLOE detector designed for CP violation studies good time resolution t = 57 ps / E(GeV) 54 ps in calorimeter and high resolution drift chamber ( p /p is 0.4% for > 45° ), ideal for the measurement of M . pb -1

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