Wolfgang Gradl. on behalf of the BABAR collaboration. Tau 2016, Beijing 23 rd September 2016

Transcription

1 ISR K u K u π (π ) Wolfgang Gradl on behalf of the collaboration Tau 216, Beijing 23 rd September 216

2 Motivation γ µ QED is precision physics u l 1 2 (u 2) l testbed for QED experimentally: electron ok muon: currently > 3u discrepancy btw. theory and experiment u theo u 1 1 = ± 4.9 u exp u 1 1 = ± 6.3 Δu u 1 1 = 28.7 ± 8. Test SM new physics? u u u u u (u ) ISR from W. Gradl 2

3 Motivation γ γ µ µ had Muon anomalous magnetic moment sensitive to hadronic vacuum polarisation ab-initio calculations difficult experimental input required: u (u + u hadrons) u had,lo u 1 1 = ± 4.2 Davier et al., EPJ C71, 1515 u had,lo u 1 1 = ± 4.3 u had,nlo u 1 1 = 9.84 ±.6 ±.4 Hagiwara et al., JP G38, 853 u u u u u (u ) ISR from W. Gradl 3

4 Motivation µ γ had dispersion relation e e + γ had u (u ) : analytically known kernel function a had,lo u = α2 () 3π 2 4u 2 u ds K(s) s R(s) dominated by low-energy cross sections improve precision by measuring exclusive final states u (u ) = u (u + u hadrons) u (u + u u + u ) experimental input u u u u u (u ) ISR from W. Gradl 4

5 e + e hadrons cross sections from cross sections, nb π + π γ π + π - π π + π - π π - - π + π π + π π + π - π + π - π 2π + 2π - 2π - 3π + 3π + - K K γ K K K K K + K - π K + K - π π K K π + π K + K - π + π - π K K π + π π + π K S K L - K S K L π + π K K +- π -+ S ± K S K π η ± K π π S K + - K S K S K K - K S K S π + π + - ηk K (notφ) ηk K π + π + - ηπ π ηπ π π + π ppbar ± ± E, GeV u u u u u (u ) ISR from W. Gradl 5

6 The experiment PEP-II: u + u collider, GeV 2 u = 1.58 GeV [Υ(4u )] Asymmetric beam energies c.m. lab boost u u =.56 Asymmetric detector acceptance in c.m..9 u.85 detects 15% of ISR u contains 5% of events with fwd/bwd u ISR excellent performance Good tracking, mass resolution Good u, u reco. Full PID for u, u, u, u, u Cherenkov Detector (DIRC) Particle identification π/k separation > 3.4σ at p < 3.5 GeV Solenoid 1.5 T Instrumented Flux Return Identification of muons and neutral hadrons μ efficiency > 85%, π misid ~ 4% at p > 1.5 GeV e - (< 9 GeV) e + (3.1 GeV) Silicon Vertex Tracker 5 layers of double-sided Si-strip detectors Vertex reconstruction, tracking + de/dx Efficiency ~ 97% High luminosity Drift chamber 4 layers, momentum measurement for charged particles and de/dx σ(p T)/ p T=.13% p T.45% L peak = cm 2 s (18) fb 1 accumulated (1.7 billion u + u u u events) Electromagnetic Calorimeter 658 CsI(Tl) crystals Electron and photon energy measurement σ(e)/e = 1.4% E -1/4 2.2% u u u u u (u ) ISR from W. Gradl 6

7 Initial state radiation in e + e u + u u ISR u + u u ISR u u is any allowed (hadronic) system, e.g. e + s m X a resonance with u u u = 1 2 particle system with appropriate quantum numbers e γ ISR Cross section factorises into du (u ; u, u u ) du d u u = u (u ; u, u u ) u u (u ) Radiator function known to.5% cross section u u (u + u u ) Use u or normalise to u + u u + u u (many systematics cancel) u u u u u (u ) ISR from W. Gradl 7

8 ISR at Υ(4S) energies Rely on tagged (= measured) photon to identify ISR events Excellent momentum resolution by means of kinematic fit High fiducial efficiency: hadronic system forced into detector fiducial region Harder momentum spectrum due to boost fewer problems with soft particles measure down to threshold Simultaneous access to wide range of u in single experiment very small point-to-point systematic errors Large integrated luminosity Hadronic system u ISR u u u u u (u ) ISR from W. Gradl 8

9 K S K L π (π )

10 General event selection and reconstruction Using 469 fb 1 of data near Υ(4u ) At least two charged tracks and at least four neutral clusters u u u + u, pointing back to IP u u reconstruction: cluster in EMC with u 2 MeV, take direction from cluster, and energy from kinematic fit Apply kinematic fits for different signal hypotheses Invariant mass resolution for the hadronic system 25 MeV Events/.7 GeV/c m(π + π - ) GeV/c 2 u u u u u (u ) ISR from W. Gradl 1

11 K u K u π reconstruction Select events with kinematic fit u 2 < 25 Estimate backgrounds from control region in u 2 data Events/.5 GeV/c signal MC m(k S K L π ) (GeV/c 2 ) u u u u u (u ) ISR from W. Gradl 11

12 K u K u π cross section σ(k S K L π ) (nb) 3 2 stat. uncertainties only E c.m. (GeV) Systematic uncertainties include Background subtraction: 1% for u (u u u u u ) < 2.2 GeV, increasing to 8-1% above 3.2 GeV Efficiency corrections overall data-mc difference of ( 9.5 ± 1.6)% u u u u u (u ) ISR from W. Gradl 12

13 K u K u π resonant substructure Events/.3 GeV/c Events/.3 GeV/c u 2 (143) 1 2 m(k S π ) (GeV/c 2 ) 1 2 m(k L π ) (GeV/c 2 ) background-subtracted u u u and u u u mass distributions. Fit: red line coherent resonant; blue histogram non-resonant component Dominated by u u + u.u.: dominant contribution from u (892) u + u.u. small u 2 (143) u + u.u. u u u u u (u ) ISR from W. Gradl 13

14 K u K u π resonant substructure: K K Events/.5 GeV/c background subtraction K* component from fit to K pi mass m(k S K L π ) (GeV/c 2 ) Dominated by u u + u.u.: dominant contribution from u (892) u + u.u. small u 2 (143) u + u.u. u (892) u + u.u. almost saturates cross section u u u u u (u ) ISR from W. Gradl 14

15 K u K u π resonant substructure: φπ σ(φπ ), nb.2 u + u u, PRD 77, E c.m., GeV Small contribution from u u ; compatible with cross section measured in u + u u Isospin u = 1, OZI suppressed Possible resonant structure around 1.6 GeV u u u u u (u ) ISR from W. Gradl 15

16 K u K u π π cross section Events/.1 GeV/c 2 1 σ(k S K L π π ) (nb) m(k S K L π π ) (GeV/c 2 ) E c.m. (GeV) u u u u u (u ) ISR from W. Gradl 16

17 K u K u π π resonant substructure Some u (892) evident (19 ± 44 u u u, 171 ± 32 u u u ) but statistics too low to study further No indication for u (892) u (892) Consistent with u (892) u u + u.u. as expected from u + u u + u u u u u u (u ) ISR from W. Gradl 17

18 Charmonium K u K u π (π ) Previous measurements of u /u decays: Not all possible isospin combinations in final state included in PDG u u u u (u + u u ) Γ(u /u u + u ) u (u /u u ) u u u u u (u ) ISR from W. Gradl 18

19 Charmonium region Events/.2 GeV/c Events/.2 GeV/c m(k S K L π ) (GeV/c 2 ) m(k S K L π π ) (GeV/c 2 ) Fit with MC signal shape + second-order polynomial (background) u /u u (2u ) u u u u u 182 ± 21 u u u u u u 47 ± 11 u u u u u < 8 u u u u u u 14 ± 6 u u u u u (u ) ISR from W. Gradl 19

20 Charmonium region Events/.2 GeV/c Events/.2 GeV/c m(k S K L π ) (GeV/c 2 ) m(k S K L π π ) (GeV/c 2 ) u /1 3 prelim. PDG 214 u /u u u u u u 2.6 ±.24 ±.1 u /u u u u u u u 1.86 ±.43 ± ±.41 (from u + u u u ) u (2u ) u u u u u <.3 u (2u ) u u u u u u 1.24 ±.54 ±.6 u u u u u (u ) ISR from W. Gradl 19

21 Charmonium decays to K K Events/.2 GeV/c m(k * (892) K ) (GeV/c 2 ) Events/.2 GeV/c m(k 2 *(142) K ) (GeV/c 2 ) See significant yields for u /u Events u (u /u u ) u (u u u ) 1 3 u (892) u + u.u. u u u u u 16 ± ± 15 ±.6 u 2 (143) u + u.u. u u u u u 37 ± ±.12 ±.2 u u u u u (u ) ISR from W. Gradl 2

22 Summary Measure cross sections for u + u u u u u u (u ) Resonant substructure explored with u (1 2 ) events Contribution to u u : u u u u u u (u CM < 2 GeV) 1 1 = 3.31 ±.58 HLMNT 211 u allu u u u u (u CM < 2 GeV) 1 1 = 2.41 ±.11 All u u u and u u u u now directly measured by no isospin relations needed any more for cross sections and dispersion relation! Branching fractions for u /u and u to u u u u u (u ) improved precision, first measurements Final word from for these channels. More progress: BESIII, Belle II, VEPP-2 u u u u u (u ) ISR from W. Gradl 21