Baryon Electromagnetic Form Factors at BESIII

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1 BESIII Baryon Electromagnetic Form Factors at BESIII Cristina Morales (Helmholtz-Institut Mainz) Introduction to electromagnetic form factors BESIII exeriment Measurement of Baryon electromagnetic form factors at BESIII Summary 55th International Winter Meeting on Nuclear Physics Bormio January 2017

2 The structure of baryons Baryons: simlest non-erturvative systems for which non-abelian nature of QCD is manifest PDG Strong QCD Perturbative QCD Mu,d~0.005 GeV Ms~0.095 GeV Mc~1.25 GeV Mb~4.2 GeV Mt~175 GeV Different quark masses robe different regions of the strong interaction Powerful observables to understand baryon structure (e.g. Form Factors, Distribution Amlitudes (GPD,TDA)) obtained from electromagnetic rocesses 2

3 Baryon electromagnetic form factors All hadronic structure and strong interactions in EM FFs but subject to QED Sace-like: Studied in and (elastic) scattering real functions Rigurously studied for nucleons since 1960's Related to charge and magnetization distributions Time-like: In and comlex functions Very few measurements SL and TL FFs related through disersion relations 3

4 Baryon electromagnetic form factors Angular analysis to extract EM form factors Sace-like: Rosenbluth searation, olarization observables Time-like: Total cross section does not allow searation of form factors effective form factor 4

5 Baryon electromagnetic form factors Sace-like: Studied in (elastic) scattering Playground and real numbers for theory and exeriments: Rigorously studied for nucleons since 1960's From the size of baryons (low q2) to QCD scaling (at high q2) Many hot toics: GE/GM, Charge Radius, Unhysical Region, Threshold Behaviour, Time-like: In Radiative Corrections, Two-Photon Exchange, Large Q2... and comlex numbers SL and TL FFs related through disersion relations 5

6 BESIII exeriment at BEPCII Double ring e+e- collider: Multi-urose detector: 93% of 4π 240m 1Tesla Solenoid e+ e- Beam energy: GeV Design luminosity: 1033 cm-2 s-1 Main MDC: Drift Chamber σ()/ < 0.5 % for 1 GeV tracks, σ(de/dx)/de/dx < 6%, σ(xy) = 130 μm Energy sread: Time of Flight σ(t) ~ 90 s Number of bunches: 93 EMCalorimeter σ(e)/e < 2.5 %, Total current: 0.91 A Bunch length: 1.5 cm σ(x) < 6mm for 1 GeV e- Muon Counter σ(xy) < 2 cm 6

7 BESIII data samles BEPCIIData BESIII Collider Samles From 2009 until 2016: From scan data: From data at the resonances: 4600 ~0.6 fb ~3 fb-1 7

8 Time-like roton EM form factors Effective form factor Ratio of GE and GM PS170 [PRL 114, (2015)] Precision: 11% to 28% Stee behaviour of the effective form factor at threshold Structures aeared in BaBar data [PRD 87, (2013)] - Resonances [PRD 92, (2015)] - Rescattering rocesses between few coherent sources [PRL 114, (2015)] At high q2 time-like FFs 2 times larger than sace-like FFs Ratio of absolute vales of form factors: discreancy between PS170 [NPB 411,3(1994)] and BaBar 8 1

9 + - e e at BESIII Phys. Rev. D91, (2015) Based on 157 b-1 collected in 12 scan oints with s= GeV in 2011/2012 and from vertex, in time, back to back, E, = ECM/2 GeV ECM (GeV) ECM = 2.4Ldt (b-1) Background negligible or subtracted Efficiencies around 60% Radiative corrections u to LO in ISR (ConExc) 3.05,3.06,3.08 Normalization to e+e- γγ (Babayaga 3.5) 3.40,3.50,3.54, BESIII XY- event 3.60,3.65, dislay Cross section 60.7 Effective form factor overall uncertainty imroved by ~30% overall uncertainty imroved by ~60% 9

10 + - e e at BESIII Phys. Rev. D91, (2015) Extraction of electromagnetic form factors from roton angular distribution: Proton angular distribution s = GeV s = 2.4 GeV s = GeV GM term GE term Magnetic form factor Ratio of form factors Only measurements in TL region GE / GM consistent with 1 10

11 - Prosects for e e at BESIII + From e e- γ at the main charmonium resonances (7.4 fb-1) + Effective form factor Ratio of form factors Stat. recision 1.5% to 13% Stat. recision 16% to 34% Wide continuous range from threshold to 3.8 GeV/c From e e- at 22 scan oints between s = GeV in 2015 (630 b-1) + Ratio of form factors Magnetic form factor Comarable accuracies Stat. in SL and TL recision regions for 1% to 9% similar Q2 values Stat. recision 3% to 35% 11

12 - Prosects for e e nn(γ) at BESIII + Measurement of the total cross section and the effective FF in wide q2-region Effective form factor Possibility for a first time measurement of GE and GM Challenges: Large background contribution form neutral channels (γ -final γ state) and beam associated rocesses Low trigger efficiencies for urely neutral channels Current analysis based only on EMC information low signal efficiency (few %) Significant enhancement is exected from using TOF and MUC information n/n interactction with BESIII EMC calorimeter CsI(Tl): 15X0, 52% n/n interact in EMC MUC: Iron + resistive lates 56 cm Fe thickness in barrel ~96 % n/n interact in MUC TOF: 2 lastic scintillator layers Total width: 10 cm Pn = 55%, Pn=13.5% 12

13 Baryon air roduction: the threshold Annihilation cross section (one-hoton exchange) Coulomb interaction as FSI Coulomb factor [Sommerfeld,Sakharov,Schwinger,Fadin,Khoze] Enhancement factor: Ste at threshold: cross section at threshold Resummation factor: Few MeV above threshold: For a wide energy range (200 MeV) roton behaves like a oint-like article!! [Ferroli, R. B., Pacetti, S., & Tomasi-Gustafsson, E. Threshold henomenology of nucleon form factors,arxiv: v1]. 13

14 + - Baryon air roduction at BESIII: ΛcΛc Based on b-1 collected in 4 scan oints around Λc roduction threshold in Λc decay modes indeendently reconstructed ~1.6 MeV away from threshold! Unrecedented statistical uncertainty (~2% at 4.6 GeV) First measurement of GE/GM for Λc extracted at 2 energies: test of isotroy (analiticity of the FFs) PRELIMINARY PRELIMINARY PRELIMINARY 14

15 Baryon air roduction at BESIII: ΛΛ Based on 40.5 b-1 collected at s= , 2.4, 2.8, 3.08 GeV in 2012 mini scan Lowest energy only 1 MeV above theshold! ECM = GeV Decay channels at GeV: Λ π- & Λ π+, Λ nπ0 & Λ inclusive decay Decay channels at higher energies: Λ π-, Λ π+ π- Annihilation cross section BESIII + π Conexc MC Non-vanishing threshold cross section observed!! PRELIMINARY PRELIMINARY 15

16 Baryon air roduction at BESIII: ΛΛ Based on 40.5 b-1 collected at s= , 2.4, 2.8, 3.08 GeV in 2012 mini scan Lowest energy only 1 MeV above theshold! ECM = GeV Decay channels at GeV: Λ π- & Λ π+, Λ nπ0 & Λ inclusive decay Decay channels at higher energies: Λ π-, Λ π+ π- Annihilation cross section BESIII + π Conexc MC Non-vanishing threshold cross section observed!! more comlicated hysics scenario underlying: [arxiv: v1, v1 Eur.Phys.J. A39,315 (2009)] PRELIMINARY PRELIMINARY 15

17 Prosects for hyerons FFs at BESIII What haens if we relace one of the light quarks in the nucleon with one or many heavier quark(s)? Can we derive the roerties of the hyerons from what we know about nucleons. i.e. to which extent is SU(3) symmetry broken? BESIII scan data Advantages of hyerons: Easier to trace the heavier quark Polarization accessible thanks to weak, arity violating decay BESIII 2015 energy scan: Unrecedented statistics between GeV At 5 energies large enough samles to extract R Λ= GΛE/GΛM At GeV comlete sin decomosition ossible: first determination of GΛE-GMΛ-hase Also accessible: 16

18 Summary Electromagnetic FFs are fundamental, Lorentz-invariant roerties of baryons that are rigurously defined, and rovide a benchmark for theoretical models BESIII is unique in its caability to measure baryon form factors, from nucleons to Λc and use two comlementary aroaches: energy scan and initial state radiation technique: Proton Form Factors have been measured using a test energy scan of 2012: - Effective form factors in good agreement with revious exeriments and imroving overall uncertainty by 30% - GE / GM and GM extracted at three energy oints with 25% to 50% uncertainty (dominated by statistics) Very exciting results from ISR on rotons exected very soon Preliminary results on cross sections and FFs from Λ and Λc close to threshold The measurements of baryon FFs will be significantly imroved and extended with the 2014 and 2015 high luminosity scan data: full determination of Λ and Λc FFs ossible, effective FFs accessible for neutrons, hyerons and multi-strange hyerons, neutron and Σ FFs accessible for the first time, and roton FFs with accuracies better than 10% 17

19 Backu Cristina Morales

20 Hyerons EM form factors Hyerons unstable cannot serve as targets: only TL EM FFs accessible e+e- collisions are currently the best way to study hyeron structure Through weak hyeron decays olarization observables accessible Very little data ublished so far: Lambda effective form factor ~200 evts ~20 evts ~25 evts Baryon magnetic(*) FF at 3770 GeV CLEO-c (*) [PLB 739,09006(2014)] GBE/GBM =0,1 [PRD 76,09006(2007)] Total error on GΛE/GΛM : % Relative hase: < sinøλ < 0.98 Cristina Morales

21 Structure of Hadrons Scattering Electromagnetic robes allow to factorize hadronic interactions in terms of 'non-erturbative' objects: Form Factors, Distribution Amlitudes... e- e- e- Annihilation Elastic scattering e+ e Annihilation into air of roton/antiroton Cristina Morales X Dee Virtual Comton scattering * e- e- e- Drell-Yan π Electroroduction Dee Inelastic Scattering Inverted wide angle Comton scattering Annihilation into gamma star * π

22 Exeriments: Now and Future GEn at -q2 = 1.5 GeV2 (Pol. 3He) Sace-like region Time-like region Mainz GE and GM for -q2 2.0 GeV2 n [Hall A] GE/GM u to 14 GeV2 [Hall B] GMn (2H) u to 14 GeV2 [Hall A] GMn u to 18 GeV2 [Hall C] GnE u to 7 GeV2 at VEPP-2000 e+e- collider N GNE / GMN, Geff (scan) q2 (4.0 GeV)2 at BEPCII e+e- collider B B GE, GM, GE/GM hase (?) (2.4 GeV)2 q2 (3.7 GeV)2 Λ Λ GE, GM, GE/GM hase (scan and ISR) q2 (3.5 GeV)2 at FAIR collider Cristina Morales n [Hall A] GE/GM u to 10.2 GeV2 at SuerKEKB e+e- collider q2 (4.5 GeV)2?

23 Two hoton exchange Exerimental access: angular distribution of Nucleon in e+e--center-of-mass Also interference between ISR and FSR could cause an asymmetry! Cristina Morales 1

24 Periodic interference near threshold [Phys. Rev. Lett. 114, ] A << 1 C = r < 1 fm B dam. ar. D thresh. shift S. Pacetti (PhiPsi'15) = roton momentum in rest frame Periodical behavior suggests an interference due to rescattering of roton and antiroton at low kinetic energy and distance ~1fm Proton and antiroton interact when they are almost henomenological Unitarity imlies a large imaginary art of form factors Cristina Morales

25 - From 2015 scan full determination of lambda- FFs ossible: Imaginary art of FFs leads to olarization observables: Parity violating decay: Λ π Rem = GE / GM Prosects for e e Hyerons + MC Rem=1 MC Rem=2 and Θ : Angle between roton and olarization axis in Λ-CM ΘΛ: Λ olar angle in CM Φ : relative hase between GE and GM Exected statistical accuracies for Pn between 6 and 17% Phokhara v8.0 Exected 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, GE,M and Pn at single energy oints ossible measurements of effective FF, Rem and GE,M at threshold ossible ΣΣ ΞΞ ΩΩ ΛcΛc ΛΛ ee ΛΣ0, Σ0Σ0 reviously measured by BaBar, no Rem extraction ossible 2 q (GeV/c ) Cristina Morales