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γ *p → Δ (1232) → π N Results from CLAS

γ *p → Δ (1232) → π N Results from CLAS. Cole Smith University of Virginia. Japan-US Workshop on Electromagnetic Meson Production and Chiral Dynamics Apr 2005 Osaka, Japan. e’. Δ 3/2+ (1232). 3/2. γ v. 1/2. e. λ Δ. -1/2. △ (1232). -3/2. Helicity conserving. N. N. A 1/2. S 1/2.

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γ *p → Δ (1232) → π N Results from CLAS

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  1. γ*p →Δ(1232) →πNResults from CLAS Cole Smith University of Virginia Japan-US Workshop on Electromagnetic Meson Production and Chiral Dynamics Apr 2005 Osaka, Japan

  2. e’ Δ3/2+(1232) 3/2 γv 1/2 e λΔ -1/2 △(1232) -3/2 Helicity conserving N N A1/2 S1/2 A3/2 Spin flip 1/2 λN Double spin flip -1/2 N1/2+(938) Δ(1232) Program at JLAB Physics Goals • Helicity amplitudes A3/2 A1/2 S1/2 vs. Q2 • Deformation of N and Δ (E2/M1, C2/M1). • Compare with quark models, lattice QCD. Reality • Resonant and non-resonant contributions require models to separate. • Theoretical tools: Effective Lagrangian (MAID,XPT), dynamical (Sato-Lee,DMT), Dispersion Relations (Aznauryan). • Experiment: Identify observables sensitive to Born and rescattering terms in models. Experiments at JLAB • Hall A – High luminosity, recoil polarization. • Hall B – Large acceptance and kinematic coverage, polarized target and beam. • Hall C – High Q2, search for onset of pQCD scaling (E2/M1→ 1). M1, E2, C2

  3. e / e / e e What do E2/M1 and C2/M1 ratios measure? E0+,S0+,M1-,S1- E1+,S1+,M1+,… M1, E2, C2 • Short Range Physics? • Gluon exchange → D-state admixtures • Deformation of N / Δ • Long Range Physics? • χSB → Light pion mass • Excite qq pairs from vacuum • Shape of pion cloud

  4. γ*p→Δ(1232) Magnetic Dipole – Quenched Lattice vs. Dynamical C. Alexandrou et al, PRL, 94, 021601 (2005) Pion Cloud Extrapolation of lattice GM* to Q2=0 is within 10% of experiment (where are the pions?). Predicted form factor falls with Q2 more slowly than data. Fits of dynamical pion models to π photoproduction data suggest 30-50% of M1 photocoupling strength near Q2=0 due to meson rescattering at EM vertex. Lattice shows a decreasing GM* coupling strength as quark mass decreases. Uncertainties: size of lattice volume (too small?), effects of unquenching (qq loops, Δ decay) Sato,Lee PRC 63, 055201 (2001)

  5. Quadrupole Transition – Lattice QCD vs. Pion Cloud Models C. Alexandrou et al, PRL, 94, 021601 (2005) γ*p→Δ(1232)→π0p Quenched lattice consistent with E2/M1 data ! E2/M1 (%) C2/M1 (%) Low Q2 behavior of C2/M1 strong test of calculations !

  6. Reaction Models – Need to better constrain non-resonant backgrounds Non-resonant multipoles not well determined in models. Sato, Lee nucl-th/0404025 Yang, Kamalov nucl-th/0303064 (DMT model)

  7. CLAS e1e Run (Nov-Dec 2002) • Beam energy 1.046 GeV • Q2=0.1-0.35 GeV2 W<1.4 GeV • Beam polarization ~70% • Current 10 nA • LH2 target thickness 2.0 cm Q2 (GeV2) Trigger Threshold Elastic Elastic and inelastic data taken simultaneously. Normalization error ~ 3-4% • Refined analysis since Aug 2004 • More accurate target window background subtraction. • Proton energy loss in target accounts for target geometry • Elastic Bethe-Heitler background rejection uses finer W steps. W (GeV) Fiducial Cut

  8. CEBAF Large Acceptance Spectrometer (CLAS) • Six identical sectors • 5 T toroidal B-field • Δθ=15-140 degrees • Δφ= 0-50 degrees • Δp/p = 10-2-10-3 • π0 electroproduction • Proton detected • Lorentz boost forms cone surrounding q vector. • Cone angle Q2,W dependent

  9. Pion Electroproduction Structure Functions M1+ dominance. Re(M1+)=0 d2σ(MAID) vsθ* and φ* at Δ(1232) peak. Grid shows experimental (cosθ*,φ*) bins.

  10. π0pCenter-of-Mass Coverage - θLAB vs. φLAB of Detected Proton For Q2 < 1 GeV Torus coils block some angles, adjacent sectors provide additional coverage For Q2 > 1 GeV Full c.m. coverage is possible

  11. Acceptance Limitations at Low Q2 Magnet coils create acceptance asymmetry in φ*. Potential source of systematic error in σLT extraction. Mini-torus magnet coils Beam pipe hole exists in all low Q2 experiments. Removes yield near φ*=π which lowers sensitivity toσLT. Center-of-mass Acceptance (GEANT)

  12. Acceptance Monte Carlo Simulations • Generated ~25M events using MAID03 physics model. • W: 15 bins (1.10-1.40 GeV) • Q2: 6 bins (0.16-0.36 GeV2) • Cos θ*: 10 bins • φ*: 24 bins • GEANT simulation includes: • Accurate magnet coil geometry • Proton rescattering from coils. • Drift chamber resolution. • Bethe-Heitler radiation • Hadronic interactions (FLUKA) Comparison of Geant and Data φ* cos θ*

  13. Bethe-Heitler Background (elastic tail) A potential source of φ* asymmetry which can bias extraction of σLT • Largest background near φ*(proton)= 180o with tails extending beyond. • Suppressed using overconstrained elastic kinematic cuts on proton angle. • Residual BH tails subtracted using MC simulations. • MM2cut: 0.0056 < MM2 < 0.08 GeV2 After cut Missing Mass2 Missing Mass2

  14. Typical pπ0 Cross Sections Near Pion Threshold Q2 = 0.2 GeV2 W=1.10-1.12 GeV Preliminary Preliminary Preliminary Preliminary cos θ* φ*

  15. Analysis: Obtain Legendre Coefficients • Global Fits • Fit φ*,cos θ* simultaneously • More constrained fit • Smooths over missing bins. φ* Fits • Extract 3 structure functions for each cos θ* bin. • Fit cos θ* to obtain Legendre coefficients. • Works best with full angular distributions (no holes). Both methods used to check systematics of extracted Legendre coefficients.

  16. CLAS Results for W=1.10-1.16 GeV at Q2 = 0.2 GeV2 Structure functions from φ* fits Legendre coefficients from global fits Preliminary Preliminary

  17. Model Dependence in Legendre Coefficients: 1.10 < W < 1.16 GeV Preliminary E1+ S1+ Largest differences between dynamical models occur for A1 and D0. Largest difference between MAID and dynamical models is for D1, which is largely sensitive to Re(S1+)

  18. Q2 Dependence of Legendre Coefficients – D1

  19. Q2 Dependence of Legendre Coefficients – D0

  20. Q2 Dependence of Legendre Coefficients – D0’ Model sensitivity in D0/ maximized at W=1.22 GeV in σLT/ whereas for σLT it vanishes. Joo, PRC 70, 042201 (2004)

  21. Q2Dependence of Legendre Coefficients – A0

  22. Q2 Dependence of Legendre Coefficients – A1

  23. Q2 Dependence of Legendre Coefficients – A2

  24. Q2 Dependence of Legendre Coefficients – C0

  25. Typical Cross Sections vs cos θ* and φ* Q2 = 0.2 GeV2 W=1.22 GeV Preliminary Preliminary

  26. Resonant Multipoles W Dependence of Legendre Coefficients – A0,A1,A2,C0,D0,D1 Preliminary (M1+ dominance) Preliminary

  27. MAID2003 Sparveris Kunz Mertz DMT CLAS Preliminary E=1.046 Q2=0.16 Sato-Lee W=1.17 W=1.232 Response Functions - Comparison to Bates at Q2=0.127 Model variation negligible between Q2=0.127 and 0.16. Good agreement between Bates and CLAS for T+L and TT measurements. Disagreement with Kunz point (■) for LT. Global fit (-----) favors Sato-Lee model at W=1.232.

  28. Is the Siegert Theorem Relevant? Many models use the long wavelength limit q*=0 to constrain S1+ as Q2→0: Long wavelength limit Data so far do not satisfy this condition. Models show large variation.

  29. Summary and Future • Nearly complete angular distributions obtained for pizero electroproduction for Q2= 0.14 - 0.38 GeV2 and W = 1.10 - 1.30 GeV. • W and Q2 dependence of Legendre coefficients extracted from fits to φ* and cos θ* distributions. • Both previous and current preliminary CLAS data show clear disagreement with MAID in D0 and D1 (σLT) below the Δ(1232) resonance. • Preliminary E1+/M1+ in good agreement with MAID, while S1+/M1+ continues to show strong Q2 dependence. • Analysis of π+ and polarization data at low Q2 underway. Systematics analysis underway.

  30. Backup Slides

  31. Structure Functions vs Invariant Mass W

  32. Structure Functions vs cosθ* Preliminary

  33. Δ(1232) – Global Fit to CLAS pπ0 + nπ+data at Q2=0.4 and 0.65 GeV2 K. Joo et al., PRC 70, 042201 (2004) H. Egiyan et al., to be submitted to PRC

  34. Δ(1232) Multipoles – JANR Global Fit Results I.G. Aznauryan et al., PRC, 15201 (2005).

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