Hadron Form Factors in BESIII

Transcription

1 BESIII Hadron Form Factors in BESIII Cristina Morales (Helmholtz-Institut Mainz) on behalf of BESIII-Collaboration Outline: Experiment Motivation Nucleon Electromagnetic Form Factors Hyperon Electromagnetic Form Factors Pion Form Factor Summary HADRON2015 September 13th 18th, 2015, Newport News 1

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3 BEPCII Double ring e+e- collider: Beam energy: GeV Crossing angle: 22 mrad Design Luminosity at Ψ(3770): 1033 cm-2 s-1,85% achieved Optimum energy: 1.89 GeV Energy spread: Number of bunches: 93 Bunch length: 1.5 cm Total current: 0.91 A MDC: TOF: time of flight (two layers plastic scintillator): σ(t) ~ 90 ps EMC: Cs I(Tl), barrel+2 end caps: σ(e)/e < 2.5 %, σ(x) < 6mm for 1 GeV emuc: time of flight (RPC): σ(xy) < 2 cm 1T 3

4 BESIII Data Samples BESIII Data BEPCII Collider Samples ~130 scan points (~1.3 fb-1) 4

5 Motivation 5

6 Hadron Form Factors All hadronic structure and strong interactions in form factors but subject to QED corrections Hadronic vector current: (2s+1) form factors. For baryons 2 electromagnetic FFs: Vector current. Dirac and Pauli FFs: B Sachs parametrization: B B = p, n, Λ, Σ... GE(4M2) = GM(4M2) PQCD and analiticity: asymptotic behaviour PQCD and analiticity: asymptotic behaviour Form Factors real unphysical region Form Factors complex 6 1

7 Hadron Form Factors Fundamental properties of hadrons Contain information on charge, magnetization distribution Connected to distribution, dynamics of quarks Crucial testing ground for models of hadron internal structure Necessary input for experiments probing hadronic structure, or trying to understand modification of hadronic structure in hadronic medium Driving renewed activity on theory side: Models trying to explain all four EM FFs of Nucleons Trying to explain data at both low and high q2 Progress on QCD based calculations... 7

8 Baryon EM FFs in BESIII BEPCII: e+e- -annihilation: access to time-like form factors from Direct annihilation Initial State Radiation B B B B B = p, n, Λ, Σ... Coulomb correction factor: Effective form factor (assume GE = GM ): Separation of GE and GM through angular analysis: with 1 8

9 Data Samples for Hadron FFs Data available for Hadron FFs measurements: Direct annihilation Initial State Radiation BaBar (500 fb-1) ΛΛ ΣΣ ΞΞ ΩΩ ΛcΛc Phokhara MC v8.0 Visible luminosity! Simulations with Phokhara v8.0 2 q (GeV/c ) 2 q (GeV/c ) Complementary approaches: High accuracy in q2 Continuous q2-range available: m2th < q2 < s High geometrical acceptance for hadron pair (93%) Full angular distribution in hadronic center-of-mass Low background Detection efficiency independent of q2 and hadronic angular distribution Acceptance at threshold 0 9

10 Nucleon EM FFs in BESIII 10

11 + - e e pp Phys. Rev. D91, (2015) Analysis based on 157 pb-1 collected in 12 scan points between GeV in 2011 and 2012 Main features: Born cross section p and p from vertex, in time, back to back, E p,p = ECM/2 Background negligible, ~4 GeV Bhabha subtracted effective form factor Efficiencies 60% (2.23 GeV) 3% (~4 GeV) Radiative corrections from ConExC (NLO in ISR) Angular analysis: Extraction of Rem and GE,M Normalization to e+e- e+e-, e+e- γγ (Babayaga 3.5) ε ε No steps observed in cross section. Overall uncertainty improvement by 30% Rem = GE / GM and GM for three ECM. Rem consistent with BaBar and R=1. GM extracted for first time! PRD87,092005(2013) G Unphysical region PRD91,112004(2015) Nucl. Phys. B411,3 (1994) q (GeV/c2 ) Arxiv: (2015) 11

12 - Prospects for e e pp, ppγisr + PRD87,092005(2013) PRD91,112004(2015) Nucl. Phys. B411,3 (1994) Arxiv: (2015) e+e- pp Expected statistical accuracies or Rem= GE / GM =1 between 9 % and 35% (similar to space-like region for same q2-region) Expected accuracies for GM between 3 to 9%, 9 to 35 % for GE Unphysical region BESIII 2015: 21 scan points between 2.0 and 3.08 GeV (552 pb -1) e+e- ppγisr Data samples (ECM): ψ(3770),ψ(4040), 4230, 4260, 4360, 4420, Total: 7.4 fb-1 Analysis for each ECM and q, then combine statistics ISR kinematics: photon and pp-system with small opposite polar angles Efficiencies: ~20% -γ-untagged, ~6% γ-tagged analysis From 2.1 GeV up untagged-photon analysis possible ECM = GeV, Phokhara MC v8.0 cosθp,p < % Photon tagged cosθγ < % Photon untagged cosθγ > % Remaining e+e- ppπ0 subtracted from data Final statistics competitive with BaBar 12

13 - Prospects for e e nn, nnγisr + Only two direct measurements of neutron effective FF Nucl. Phys.B517,3 (1998) BESIII data cover wide range ( GeV) with unprecedented statistics PRD90, (2014) measurement of cross section and G in wide q 2-region could provide the first measurement of Rem BESIII data region Strategy: First identification of n and γisr: EMC shower information neutron identification event kinematics (geometry) 2 q (GeV/c ) e+e- nn n/n detection efficiencies of ~20/30% (efficiencies up to % level) Main background from beam background processes Unprecedented statistics above 2.0 GeV (~300 evts at 2.4 GeV) e+e- nnγisr Only tagged analysis possible (efficiencies at per mille level) Increase detection efficiency using TOF, MUC Main background from e+e- nnπ0 and e+e- γγ(γ) (Neural Network) BESIII EMC calorimeter CsI(Tl): 15X0, λi = g/cm2, ρ = 4.53 g/cm3 52% n/n interact in EMC MUC: Iron + resistive plates λi = g/cm2, ρ = g/cm3 56 cm Fe thickness in barrel ~96 % n/n interact in MUC 13

14 Hyperon EM FFs in BESIII 14

15 + - e e ΛΛ (BESIII Preliminary!!) Analysis based on 40.5 pb-1 collected in 4 scan points between GeV in 2012 test run BESIII event display ECM = GeV at ECM = GeV (mth = GeV) ΛΛ From Λ pπ- and Λ pπ+ (BRpπ= 64%) well defined pπ, possible p-annihilation From Λ nπ0 (BRnπ0 = 36%) n-annihilation in EMC (Neural Network), well defined p π0 - at ECM 2.4 GeV, from Λ pπ and Λ pπ π- + p, p, π- and π+ from interaction vertex, in time, ΛΛ back to back, EΛ,Λ = ECM/2... p π+ e+e- ΛΛ pπ-pπ+ ConExc MC Current results improve uncertainty by at least 10% for low q 2 and even more for ECM > 2.4 GeV III Pr el im in a * ** ry BE S G BE S III Pr el * im in a q Cross section does not vanish at threshold!! ry *PRD76, (2007) ** Z Phys C48, 23(1990) q Coulomb interaction at quark level? 15

16 - From 2015 scan full determination of lambda- FFs possible: Imaginary part of FFs leads to polarization observables: Parity violating decay: Λ pπ Rem = GE / GM Prospects for e e Hyperons + MC Rem=1 MC Rem=2 and Θp : Angle between proton and polarization axis in Λ-CM ΘΛ: Λ polar angle in CM Φ : relative phase between GE and GM Expected statistical accuracies for Pn between 6 and 17% Phokhara v8.0 Expected statistical accuracies for Rem = GE / GM =1 between 14 and 29% 2 q (GeV/c ) Also available from threshold (2015, 2014, 2011 data): ee ΛΣ0, Σ0Σ0,Σ-Σ+, Σ+Σ-, Ξ0Ξ0, Ξ+Ξ-, Ω+Ω-, Λ-cΛ+c measurements of effective FF and Rem and Pn at single energy points possible measurements of effective FF, Rem and GM at threshold possible ΣΣ ΞΞ ΩΩ ΛcΛc ΛΛ ee ΛΣ0, Σ0Σ0 previously measured by BaBar, no Rem extraction possible 2 q (GeV/c ) 16

17 Pion FF in BESIII 17

18 - + - e e π π γisr + arxiv: (submitted to PLB) Goal: hadronic vacuum polarization contribution to ɑμ = ɑ SM μ =ɑ QED μ +ɑ weak μ +ɑ (gμ-2) 2 hadr μ most relevant contribution to ɑμhadr below 1 GeV: σ(e+e- π+π-) Disagreement between existing measurements limits knowledge of ɑ μ 2 q (GeV/c ) Features of BESIII analysis: 2.9 fb-1 from Ψ(3770) studied range between MeV only tagged analysis possible below 1 GeV main background from e+e- μ+μ-γisr prefectly understood (<1%o ) luminosity from BhaBha events 0.5% accuracy (Babayaga NLO) FF fit function: Gounaris-Sakurai parametrization radiative corrections from Phokhara v8.0 2 q (GeV/c ) Syst. uncertainty in cross section 0.9% Compatible with prev. measurements (1σ) More than 3σ deviation wrt (gμ-2)sm prediction confirmed Data from untagged analysis and above Ψ(3770) will be used Analysis will be extended below 600 MeV and above 900 MeV 18

19 Summary 19

20 Summary & Outlook BESIII excellent laboratory for hadron form factor measurements: scan data + ISR tecnique Proton Form Factors and their ratio have been measured using a small amount of data Preliminary results on Λ just released New high statistics data between 2.0 and 3.1 GeV will significantly improve FFs measurements for protons, neutrons, lambdas and other hyperons. Also from ISR measurements exciting results for nucleon FFs expected ISR technique allows access to energies below 2 GeV: the first result is the charged pion, more to come Other related topics being studied (not reviewed here): Baryon production threshold measurements Space-like transition FFs of mesons (light-by-light contributions to (gμ-2) ) Thank you! 20

21 Backup 21

22 BEPCII Collider BEPCII Collider Symmetric e+e--collider Beam Energy: GeV Design Luminosity 1033 cm-2 s-1 Achieved Luminosity

23 BESIII Detector BEPCII Collider Magnet yoke SC Magnet (1 T) Beam pipe R inner: 63mm R outer: 810 mm Length: 2582 mm 43 Layers 6240 CsI(Tl) crystals: 28cm (15X0) Barrel: cosθ <0.83 Endcap: 0.85 < cosθ <0.93 BTOF: two layers; ETOF: 48 crys. for each

24 Experimental Situation: proton FFs Initial direct measurements of σborn(ee--> pp) had very poor statistics: only extraction of effective form factor, G, possible (Assumption: G = GE = GM ) New measurements by BaBar (ISR) and pp-experiments: Steep rise at threshold G = G Steps near 2.25 and 3.0 GeV Asymptotic behavior in SL and TL regions differ: GMTL(10 GeV2) = 2 GMSL(10 GeV2) q= Only BaBar and PS170 with statistics for angular analysis extraction of R = GE / GM possible Precision between 11% and 43%, (<1% precision in SL) Strong tension between Babar and PS170 No individual determination of G and G E M q=

25 e+e- pp (Phys. Rev. D91, ) Analysis based on157 pb-1 collected in 12 scan points between GeV in 2011 and 2012 Selection: Only 2 oppositely charged tracks in detector: ECM = 2.4 GeV e+e- pp MC identified as p and p: de/dx + Tof +E/pp < 0.5 from interaction vertex: Rxy < 1cm, Rz < 10 cm same time of flight window: tofp tofp < 4ns back to back signature sharing ECM for ECM > 2.4 GeV low polar angles rejected (Bhabha) BESIII event display (XY-view) Background: Beam background, ee--> two-body and multi-body with p and p negligible Only remaining Bhabha at ECM> 3.40 GeV corrected for ECM = GeV ECM = GeV

26 e+e- pp (Phys. Rev. D91, ) Results: from σborn(ee--> pp) measurement, extraction of effective form factor, G : G = GE = GM G Nobs : observed signal events G L : integrated luminosity Nbkg : estimated background (from MC) ε : detection efficiency (from MC) 1 +δ: radiative factor (from MC) Agreement with previous measurements and overall uncertainty improved by 30% q= from proton angular distribution in CM: extraction of R = G E / GM and GM : fit differential cross section with: ε GM 2 from second moment of cosθp: ;

27 e+e- pp (Phys. Rev. D91, )

28 e+e pp BESIII Data Samples (Phys. Rev. D91, )

29 Proton threshold Xiaorongs protons-paper Xiaorongs lambda memo S. Pacetti

30 - Prospects for e e ppγisr + Available data samples (ECM): ψ'',ψ(4040), Y(4230), Y(4260), Y(4360), Y(4420), Y(4600). Total: 7.4 fb-1 LISR dn(ppγ)/dq Efficiencies, background, radiative factor, (functions dependent on q) Strategy: for each ECM tagged and untagged analysis MC simulations Add all corrected data from different ECM for each q-bin Normalize with LISR Born cross section, Effective form factor ECM = GeV, MC simulation cosθp,p < % Angular analysis: Extraction of R and GE,M Photon tagged cosθγ < % Photon untagged cosθγ > % For q > 2.1 GeV: Large efficiencies (~20%) from untagged photon analysis provide large statistics and better GE / GM accuracies For q < 2.1 GeV: Only tagged measurement possible for ECM GeV. Low efficiencies (~6%), lower statistics than BaBar. Perhaps untagged analysis of J/ψ and ψ(3686) possible?! 1

31 + - e e ΛΛ (BESIII Preliminary!!) Analysis based on 40.5 pb-1 collected in 4 scan points between GeV in 2012 ECM = GeV e+e- ΛΛ MC Selection at ECM = GeV (mλλ = GeV) Reconstruction of ee-->λλ from Λ -->pπ- and Λ -->pπ+ (BR 64%) only pi and pi in detector pion-momentum well defined Anti-proton annihilation--> secondary particles R r ~3 cm 43 ± 7 events observed Reconstruction of ee-->λλ from Λ -->nπ0:(br 36%) Multivariate Analysis (boosted decision tree) to identify n pπ0 well defined 22 ± 6 events observed At ECM = 2.4, 2.8 and 3.08 GeV II S BE re P I ry a in m li Data Signal MC Bkg MC Reconstruction of ee-->λλ from Λ -->pπ- and Λ -->pπ+ Only 4 oppositely charged tracks in detector: identified as p, p, π- and π+ from interaction vertex ΛΛ invariant mass cut ΛΛ back to back signature sharing ECM II S BE re P I ry a in m li Data Signal MC Beam bkg qq bkg

32 + - e e ΛΛ (BESIII Preliminary!!) Results: from σborn(ee--> ΛΛ) measurement, extraction of effective form factor, G : G = GE = GM Pr e * ** lim in ar BE SI II G BE SI II G y * Pr e lim in ar q y *PRD76, (2007) ** Z Phys C48, 23(1990) q Cross section expected to vanish at threshold (q =2mΛ ): with Λ q 2 Coulomb interaction at quark level? 320 ± ± 5.7

33 - Prospects for e e ΛΛ, ΣΣ, ΞΞ... + From BESIII 2015: 15 points above llbar threshold! Parity violating decay: Λ -->pπ, emission of proton depends on Λpolarisation ΣΣ (*) (*) Angle between proton and ΞΞ ΩΩ ΛcΛc ΛΛ polarization axis in mother's rest frame Imaginary part of FFs leads to polarization observables: Λ Expected statistical accuracies for Pn between 6 and 17% Expected statistical accuracies for R = GE / GM =1 between 14 and 29% R = GE / GM Λ (2015) MC R=1 (2015) MC R=2 Complete determination of TL Ffs possible!! Also available from threshold: ee ΛΣ0, Σ-Σ+, Σ0Σ0, Σ-Σ+, Ξ0Ξ0, Ξ+Ξ-, Ω+Ω-, Λ-cΛ+c 0 pπ, pπ, γλ, nπ-, Λπ0, Λπ, ΛK, Λπ, 64%, 52%, 100%, 100%, 100%, 100%, 68%, 1% q = Ecm (GeV)

34 Hyperon TL EM FF in BESIII

35 e+e Hyperons BESIII Data Samples

36 e+e Hyperons BESIII Data Samples

37 e+e Hyperons BESIII Data Samples

38 2.6464