Pion Form Factor Measurement at BESIII

Transcription

1 Pion Form Factor Measurement at BESIII Martin Ripka on behalf of the BESIII colaboration EINN - November, / 17

2 Motivation: Why Form Factor Measurements at BESIII? Muon anomalous magnetic moment a µ = (g µ 2)/2 Experimental measurement at BNL: aµ exp = (5.4)(3.3) [PRD (2006)] Theoretical calculation: aµ theo = (4.9) [J. Phys. G 38, (2011)] Theory and experiment not in agreement: aµ exp aµ theo (30 ± 8) σ deviation M. Davier, A. Hoecker, B. Malaescu and Z. Zhang, Eur. Phys. J. C (2011) 2 / 17

3 Theoretical calculation of a µ aµ theo = aµ QED + aµ weak + aµ QCD aµ QED = ( ± 0.0) [PRL 109, (2012)] aµ weak = (15.36 ± 0.1) [PRD 88, (2013)] aµ QCD = aµ LbL + aµ VP,LO + aµ VP,HO aµ VP,LO = (694.9 ± 4.2) [J. Phys. G 38, (2011)] aµ VP,HO = ( 98.4 ± 0.7) [J. Phys. G 38, (2011)] aµ LbL = (11.6 ± 3.9) [Phys. Rept. 477, 1 (2009)] B-field B-field B-field had had 3 / 17

4 The Vacuum Polarisation Contribution to a QCD µ Loop can not be calculated for low momentum hadrons Optical theorem connects VP amplitude with hadronic cross sections: σ(s) e + e hadrons = 4πα s Im Π γ (s) 2 had had photon self-energy function aµ VP,LO = 1 4π 3 0 ds K(s)σ e + e hadrons(s) Hadronic contributions to a QCD µ : hadronic cross-section M. Davier, A. Hoecker, B. Malaescu and Z. Zhang, Eur. Phys. J. C (2011) 4 / 17

5 Hadronic Final States contributing to a VP,LO µ D. Bernard [BaBar Collaboration], PoS Hadron 2013, 126 (2013) [arxiv: [hep-ex]]. Most important channels: π + π, KK, π + π π 0, π + π 2π 0 Largest contribution to uncertainty: π + π, π + π 2π 0, KK 5 / 17

6 π + π at Babar and Kloe M. Davier, A. Hoecker, B. Malaescu and Z. Zhang, Eur. Phys. J. C (2011) Babar and Kloe each claim sub-percent precision Measurements do not agree with each other Another high precision measurement needed BESIII 6 / 17

7 BEPCII τ-charm factory Energy range: GeV Design luminosity: cm 2 s 1 (at 3.77 GeV) Linac + double storage ring 7 / 17

8 BESIII Detector Multilayer Drift Chamber (MDC) Time of Flight system (ToF) Electromagnetic Calorimeter (EMC) Super Conducting magnet 1 Tesla (SC) Resistive Plate Chamber (RPC) for muon detection 8 / 17

9 Initial State Radiation Technique I Need σ had (s) in the entire energy range where pqcd fails had Initial State Radiation (ISR) reduces the effective CMS-energy of the collision: m 2 had = E 2 CMS 2E CMSE ISR Non radiative cross-section can be obtained by dσ (had+γ) dm had = 2m had W (s, E ISR, θ ISR )σ had s Radiator-function W (s, E ISR, θ ISR ) gives the amplitude to emit an ISR photon 9 / 17

10 Initial State Radiation Technique II Emission of ISR photons is suppressed by α/π High integrated luminosity needed for precision measurements Untagged analysis possible above 1 GeV Tagged analysis Untagged analysis 10 / 17

11 Event Selection and Particle Identification (PID) TMVA overtraining check for classifier: CFMlpANN dx (1/N) dn/ Signal (test sample) Signal (training sample) 22 Background (test sample) Background (training sample) 20 Kolmogorov-Smirnov test: signal (background) probability = (0.455) CFMlpANN response U/O-flow (S,B): (0.0, 0.0)% / (0.0, 0.0)% Kinematic Fit for π + π γ ISR final state Standard BESIII PID system for electron rejection Artificial Neuronal Network for muon-pion separation Before ANN After ANN events / 20 MeV π + π γ MC - µ + µ γ MC data events / 20 MeV π + π γ MC - µ + µ γ MC data m 2π [GeV] m 2π [GeV] 11 / 17

12 Systematic Uncertainties Source Uncertainty (%) Photon efficiency correction 0.2 Pion tracking efficiency correction 0.3 Pion ANN efficiency correction 0.2 Pion e-pid efficiency correction 0.2 ANN negl. Angular acceptance 0.1 Background subtraction 0.1 Unfolding 0.2 FSR correction δ FSR 0.2 Vacuum polarisation correction δ vac 0.2 Radiator function 0.5 Luminosity L 0.5 Sum / 17

13 π + π Cross Section )) [nb] FSR π + π - (γ e + σ bare (e s' [GeV] σ bare ( s ) = 1 2 s W (s,x)ɛ( dn s s )Lδ vacδ FSR d s ρ-ω interference clearly visible 13 / 17

14 π + π Form Factor (Gounaris-Sakurai Parametrisation) 45 BESIII fit 40 BESIII 35 2 π F s' [GeV] parameter BESIII value PDG 2014 m ρ [MeV/c 2 ] ± ± 0.25 Γ ρ [MeV] ± ± 0.9 m ω [MeV/c 2 ] ± ± 0.12 Γ ω [MeV] fixed to PDG 8.49 ± 0.08 c ρ [10 3 ] 1.7 ± φ ω [rad] 0.04 ± / 17

15 Comparison to Other π + π Measurements 2 2 / BESIII fit BESIII fit KLOE 08 KLOE 10 KLOE 12 / BESIII fit BESIII fit BaBar BESIII π F π F s' [GeV] s' [GeV] 0.15 BESIIIMfit M/MBESIIIMfitM-M1 2 π F SND CMD CMD New BESIII measurement agrees with KLOE and BaBar Small shift wrt. BaBar above ρ-ω interference s [GeV] 15 / 17

16 Final Result: Contribution to a VP,LO µ Precision competitive with previous measurements BESIII measurement between BaBar and KLOE a ππ,lo µ ( MeV) = (370.0 ± 2.5 stat ± 3.3 sys ) Confirms deviation of 3.4σ between experiment and theory arxiv: and submitted to PLB 16 / 17

17 Outlook Extend tagged π + π ISR study to threshold region Use Untagged ISR technique for π + π cross section at higher energies Analyse π + π form factor from R-scan data (130 points, L 1.3fb 1 ) Ongoing investigation of π + π π 0 and π + π π 0 π 0 Thank You 17 / 17