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

D. Leone Hadronic cross 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  ( ) Status up to July ’04

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 KLOE The Radiative Return

D. Leone Hadronic cross 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

D. Leone Hadronic cross 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 Differential cross section d  (      )/dM  2 Divide by Radiator Function

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 KLOE Background Rejection 1/2 M trk [MeV] M  2 [GeV 2 ] Step 2 :       and e + e -      rejected by means “Trackmass ” mm mm mm      e  e   

D. Leone Hadronic cross 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]

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 KLOE Luminosity Measurement 55 o 125 o Polar Angle [°] data MC

D. Leone Hadronic cross 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 %

D. Leone Hadronic cross 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:

D. Leone Hadronic cross 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   ee ee } ss Invariant mass of the system    , s   invariant mass of the virtual photon s

D. Leone Hadronic cross 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 % Schwinger ‘90  = 0.2% ± 0.1% Rel. difference inclusive - exclusive approach 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%

D. Leone Hadronic cross 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 %

D. Leone Hadronic cross 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   = (  0.8 stat  3.5 syst  3.5 theo )

D. Leone Hadronic cross KLOE Conclusion KLOE has proven the feasibility to use initial state radiation to measure hadronic cross section (hep-ex ) 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

D. Leone Hadronic cross 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

D. Leone Hadronic cross KLOE Backup slices

D. Leone Hadronic cross 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

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

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

D. Leone Hadronic cross KLOE M TRK      e e e e  before cutting on particle ID after cutting on particle ID            mm Likelihood effect on M trk distribution F  =1  with F  =1 s  (GeV 2 ) Radiator Function

D. Leone Hadronic cross KLOE

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

D. Leone Hadronic cross KLOE KLOE & DA  NE e + e -  S = M  = MeV achieved peak Luminosity: 8  cm -2 s 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