Measurement of Polarization Observables Pz, P s z and P c z in Double-Pion Photoproduction off the Proton

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1 Measurement of Polarization Observables Pz, P s z and P c z in Double-Pion Photoproduction off the Proton Yuqing Mao Ph.D. Defense November 10, 2014 Dept. of Physics and Astronomy, USC Supported in part by NSF PHY

2 Outline Motivation The FROST experiment Data analysis Results Conclusion 2

3 Baryon spectroscopy Baryon resonance properties Invariant mass Charge Width Decay modes Quantum numbers (spin, isospin, parity) The "Triple-Scoop" Baryon archive.news.softpedia.com Baryon Experimental Observables Quark models of Baryons Baryon spectrum QCD 3 Nucleon excited states

4 Missing resonance problem Quark models predict more nucleon resonances than have been observed. Insufficient experimental data? or Incorrect model description of the nucleon? (Various models use different effective degrees of freedom) New resonances have been observed in recent analyses of new data. 4

5 Polarization observables in double-pion photoproduction The cross section is proportional to the transition amplitude squared, I / M 2 Nucleon resonances are broad and overlapping The unpolarized cross section is: I 0 / 4X ( M i 2 + M + i 2 ) i=1 Double observable P c z involved: I 0 P c z /<(M + 1 M 1 + M + 2 M 2 M + 3 M 3 M + 4 M 4 ) Advantage of polarization observables: Study phase differences of complex amplitudes Study small amplitudes 5

6 Double-pion photoproduction Example: P33(1600) Single-pion (10-25%) Double-pion (75-90%) xsect/, courtesy of the COMPAS group, IHEP, Protvino Energy range of interest in this work. Double-pion photoproduction allows the study of resonances, which decay through multiple intermediate states. Double-pion photoproduction channel dominates at photon energies above 600 MeV. 6

7 Outline Motivation The FROST experiment Data analysis Results Conclusion 7

8 Thomas Jefferson National Accelerator Facility Photoproduction experiment, p! p + with polarized beam and target at center of mass energies, W = GeV. Data were taken as part of the FROST program at Jefferson Lab, Hall B. 8

9 Linearly polarized photon beam Side view Downstream view y (lab) β Scattering plane x (lab) Parallel ( ) Perpendicular ( ) Photon tagger: - Diamond radiator for coherent bremsstrahlung - Production of linearly polarized photons - Detection of recoiling electrons allows determination of photon energy Linearly polarized photons: - Degree of polarization up to 80% - Polarization direction: perpendicular or parallel to the floor 9

10 FRozen Spin Target (FROST) Free protons in butanol target are polarized up to 90% The polarization direction is either parallel or anti-parallel to the beam direction Jun01& &Jun08& Negative(-) polarization Beam Positive(+) polarization Butanol (polarized) Carbon (unpolarized) C 12 10

11 CEBAF Large Acceptance Spectrometer (CLAS) Start counter Detection of final-state particles: p, π +, π - Drift chamber (3 layers) Torus coil Time-of-flight scintillators 11

12 Definition of kinematic variables p! p + π+π- plane Scattering Plane Particle angles in the center-of-mass frame Polarization observables are extracted as a function of the azimuthal angle Φ 12

13 Outline Motivation The FROST experiment Data analysis Results Conclusion 13

14 Entries e+07 Mean x Mean y RMS x RMS y Particle identification Example: positively charged particles Pion Relative speed mom_beta vs. Momentum Proton 6000 mom_beta Momentum (GeV) 0 14

15 Reaction ID: Missing-mass technique Shaded area Events of interest All detected γp π + π - (p) Proton missing γp π + π - (p) Missing Mass Squared (GeV 2 ) Missing Mass Squared (GeV 2 ) π + missing γp pπ - (π + ) π - missing γp pπ + (π - ) Missing Mass Squared (GeV 2 ) Missing Mass Squared (GeV 2 ) 15

16 Target selection Butanol target with free (polarized) and bound protons Unpolarized carbon 16 Reaction vertex z component (cm) Unpolarized CH2

17 Background determination Butanol-target yield is diluted by bound-nucleon background Topology1 (all particles detected) Butanol Scaled carbon Free protons Gaussian fit Dilution factor: Fraction of polarized free-proton events in the sample of butanol-target events Definition h = Y 0 Y 0 + Y b Calculation h N B N C N B 17

18 Polarized cross section Cross-section formula when (1) Target longitudinally polarized (2) Photon beam linearly polarized I = I 0 {(1 + P z + l [sin 2 (I s + P s z ) + cos 2 (I c + P c z )]} Observables of interest: Pz, P c z and P s z Target polarization vector Beam polarization degree I Polarized cross section Unpolarized cross section 18

19 From asymmetry to observables The yields Y k +,Y? +,Y k,y?, are normalized before asymmetry calculation Asy #1: Asy #2: (Y +? + Y+) k (Y? + Y k ) (Y +? + Y+)+(Y k? Y k ) = + hp z (Y +? Y+) k (Y? Y k ) (Y +? + Y+)+(Y k? Y k ) = + h(sin 2 P z s + cos 2 Pz c ) Asy #1 Example: W = 1.67 GeV, ɸ = 75 deg. Asy #2 The differences in the degrees of target and beam polarizations were accounted for in the asymmetry calculation 19

20 Angular distribution of observables 12 Φ bins for each W bin Fit with Fourier series Example: W=1.67 GeV Odd symmetry Odd symmetry Even symmetry Φ (deg) 20

21 Effective Lagrangian model Diagrams of N* for the reaction γn ππn N-Born terms Δ-Born terms Model includes N* resonances: Model calculates polarized cross sections from transition amplitudes and observables from polarized cross sections. To compare with experimental results, event-weighted averages of the polarization observables are determined for each experimental bin. 21

22 Outline Motivation The FROST experiment Data analysis Results Conclusion 22

23 Comparison with model: Pz G9a - preliminary Effective Lagrangian model 23

24 Comparison with model: P c z G9a - preliminary Effective Lagrangian model 24

25 Comparison with model: P s z G9a - preliminary Effective Lagrangian model 25

26 Further study: invariant mass (pπ + ) cut W: 2.26 GeV No cut N* W (GeV) Δ ++ p W: 2.26 GeV With cut Cut 26

27 Outline Motivation The FROST experiment Data analysis Results Conclusion 27

28 Summary Conclusion The FROST experiment at Jefferson Lab measured photoproduction with polarized beam and frozen-spin butanol target. Polarized yields for the double-charged pionphotoproduction reaction have been extracted. Angular distributions of the polarization observables Pz, P c z, and P s z have been obtained in the energy range, W = GeV. The results are compared to acceptance-corrected model predictions. The model reproduces main features of the data in most kinematic bins. Future - Estimate the systematic uncertainties of the data. - Further study of the observables in statistically rich kinematic bins. - Improve the model parameters in view of the constraints imposed by the new results. 28

29 Thanks! 29

30 Backup-Dilution Factor Yf : Yields from Free proton in Carbon d 1 Butanol Free 30