Measurement of the hadronic cross section at KLOE

Transcription

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

2 a µ theory vs. experiment Status up to July 04 THEORY 03 EJ95 (e + e - ) 186.8±15.7 e + e - and τ data differ for π + π - channel in a specific energy window above 0.6 GeV 2 (above the ρ peak) DH98 (e + e - +τ+qcd) 176.8±7.2 DEHZ02 (e + e - based) 169.3±7.8 HMNT02 (e + e - based) 166.9±7.4 DEHZ03 (e + e - based) 180.9±8.0 DEHZ03 (τ based) 195.6±6.8 BNL-E Dispersion integral for hadronic contribution to a µ evaluated for: a) CMD-2 (Novosibirsk/VEPP-2M) π + π channel with 0.6% precision < 1 GeV b) τ-data from ALEPH /OPAL/CLEO e + e - Data: 2.7 σ -Deviation τ Data: 1.4 σ - Deviation Experiment 02/ 04 BNL-E µ + 203±8 BNL-E µ - 214±8.5 BNL-E ave. 208± a µ (10-10 ) a µ (10-10 ) Experiment BNL-E821 Values for µ + (2002) and µ - (2004) in agreement with each other. Precision: 0.5ppm

3 The Radiative Return The standard method to measure σ(e + e - hadrons) is the energy scan, i.e. the syst. variation of c.m. energy of the machine. Since at DAΦNE the collision energy is fixed, we use a complementary approach: looking for e + e - π + π - γ events, where the photon is emitted in the initial state (ISR), we have a continuous variation of s π, the invariant mass of the hadronic system 4m π2 < s π < m φ 2 Precise knowledge of ISR process Radiator function H(Q 2,θ γ,m 2 Φ) MC generator: Phokhara H. Czyz, A. Grzelinska, J.H. Kühn, G. Rodrigo dσ(e + e - hadrons + γ ) Μ 2 hadr dμ 2 hadrons = σ(e + e - hadrons) H(Μ 2 hadr )

4 π + π γ selection Pion Tracks at large angles 50 o < θ π < 130 o Photons at small angles θ γ < 15 o and θ γ > 165 o are masked by quadrupoles near the I.P. (no photon tagging) r r r r pγ = pmiss = (p+ + p ) Drift chamber EM calorimeter e + e- High Statistics for ISR Photons Low relative contribution of FSR Reduced background contamination

5 π + π γ cross section dn(π + π γ)/dm ππ 2 after acceptance cuts Event analysis: Efficiencies and background Normalize to Luminosity Differential cross section dσ(π + π γ)/dm ππ dn(π + π γ)/dm ππ2 [nb -1 /GeV 2 ] Divide by Radiator Function Radiative Corrections Cross section σ(e + e - π + π ) 141 pb -1 M ππ2 [GeV 2 ] No acceptance at threshold region

6 Background Rejection 1/2 Step 1: e + e - γ are separated from π + π γ by means of a Likelihood method (signature of EmC-clusters and TOF of particle tracks) M trk [MeV] π + π π 0 π + π γγ tail m π signal region m µ µ + µ γ + e + e γ Step 2: φ π + π π 0 and e + e - µ + µ γ rejected by means Trackmass m ρ 2 M ππ2 [GeV 2 ] (M φ r 2 r 2 2 r r 2 2 p1 + Mtrk p2 + Mtrk ) p 1 + p2 = q = 0 γ

7 Background Rejection 2/2 Step 3: fit data trackmass distributions with MC ones (signal + background) with free normalization parameters M ππ2 [0.32,0.37] GeV 2 M trk [MeV] M trk [MeV]

8 Luminosity Measurement KLOE uses large angle Bhabha events for the luminosity evaluation: L dt = σ N MC ( 1 δ Bkg ) N = events with 55 <θ<125 Experimental precision Excellent agreement data-mc 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 Entries Mean RMS data MC o 125 o Polar Angle [ ]

9 Analysis σ(e + e - π + π γ) EFFICIENCIES: Trigger + Cosmic Veto Tracking Vertex π/e separation Reconstruction filter Trackmass cut Unfolding procedure Acceptance BACKGROUND e + e - e + e - γ e + e - µ + µ γ φ π + π π 0 LUMINOSITY Bhabha at large angles 0.9 % 0.3 % dσ(e + e - π + π γ)/dm ππ 2 [nb/gev 2 ] 0.6 % dσ(e + e - π + π - γ)/ds π [nb/gev 2 ] 140 pb -1 of 2001 data 1.5 millions events ρ ω Interference s π [GeV 2 ] M ππ2 [GeV 2 ]

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

11 FSR Treatment FSR Inclusive approach N(e + e - π + π γ ISR γ FSR ) add back missing FSR Event analysis Phokhara ISR+FSR Luminosity σ(e + e - π + π γ ISR γ FSR ) e + e s π + π } s π Invariant mass of the system π + π, s π invariant mass of the virtual photon s Correction for unshifting Radiator H σ(e + e - π + π γ FSR )

12 FSR uncertainty FSR Exclusive approach N(e + e - π + π γ ISR γ FSR ) subtract FSR contribution estimated by MC Event analysis Phokhara ISR Luminosity σ(e + e - π + π γ ISR ) Radiator H γ FSR % Schwinger 90 σ(e + e - π + π ) Relative difference exclusive/inclusive approach fit = 0.02 ± 0.01 m 2 ππ (GeV2 ) Rel. difference inclusive - exclusive approach = 0.2% ± 0.1% 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%

13 Cross Section σ(e + e - π + π - ) Final spectrum: after the correction for vacuum polarization σ bare α(0) (s) = σ(s) α(m 2 ππ ) α had (s) from F. Jegerlehner, July σ(e + e - π + π - ) [nb] s s π ππ [GeV 2 ] Total error = 1.3 %

14 2π contribution to a µ hadr We have evaluated the dispersion integral for 2π channel in the energy range 0.35 <s ππ <0.95 GeV 2 F π (s) CMD-2 KLOE a 0.95GeV ππ 1 + µ = ds σ ( e e 3 4π GeV 2 π + π ) K( s) a µ ππ = (388.7 ± 0.8 stat ± 3.5 syst ± 3.5 theo ) Comparison with CMD-2 in the energy range 0.37<s ππ <0.97 GeV 2 KLOE CMD-2 (375.6 ± 0.8 stat ± 4.8 syst+theo ) 1.3% Error (378.6 ± 2.7 stat ± 2.3 syst+theo ) 0.9% Error At large values of s π (>m ρ2 ) we are consistent with CMD-2 and we confirm the deviation from τ-data. s π [GeV 2 ]

15 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

16 Outlook Large angle photon analysis to explore the threshold region tagged measurement 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) A(θ) = N N π π + + (θ) N (θ) + N π π Asymmetry (θ) (θ) preliminary = data = MC θ π + θ π - θ π [ ] Polar angle [ ] Integrating asymmetry we get a difference data-mc of (8.5 ± 1.2)%

17 Backup slices

18 Unfolding the Mass Revolution 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). s π (obs) [GeV 2 ] s π, obs. (GeV 2 ) Issues: - Reliability of MC simulation - Correct choice of the regularization parameter s π, true (GeV 2 ) s π (true) [GeV 2 ] Systematics studied by varying meaningful values of the regularization parameter: Due to nature of the dispersion integral the effect on a µ is almost negligible Trkmass(MeV) Trackmass [MeV]

19 Terms not included in Phokhara Unshifting correction

20 Relative contribution of LO-FSR Relative contribution of NLO-FSR

21 dσ/dq 2 (nb/gev 2 ) σ (ππγ) with F π = dσ/dq 2 ππγ F π = events Radiator Function F π =1 e + e γ before cutting on particle ID π + π γ after cutting on particle ID Q 2 (GeV 2 ) s π (GeV 2 ) µ + µ γ m π π + π π 0 M TRK Likelihood effect on M trk distribution

22

23 reconstructed kine M ππ2 [GeV 2 ] M ππ2 [GeV 2 ]

24 KLOE & DAΦNE e + e - S = M Φ = MeV achieved peak Luminosity: cm -2 s data set: 500 pb -1 pb -1 KLOE detector designed for CP violation studies good time resolution σ t = 57 ps / E(GeV) 54 ps in calorimeter and high resolution drift chamber (σ p /p is 0.4% for θ > 45 ), ideal for the measurement of M ππ.