Double-Polarization Results with Polarized HD at LEGS

Transcription

1 st Pion Photoproduction Double-Polarization Results with Polarized HD at LEGS A. Lehmann (for the LEGS-Spin Collaboration) NStar LEGS Laser Electron Gamma Source Physics Motivation Experiment (SASY detector, HD target) Analysis and first results of polarized data Immediate and future plans

2 LEGS-Spin Collaboration Brookhaven National Laboratory C. Cacace, A. Caracappa, S. Hoblit, O.C. Kistner, A. Kuczewski, F. Lincoln, M. Lowry, L. Miceli, A.M. Sandorfi, C. Thorn, X. Wei James Madison University A. Lehmann, C.S. Whisnant Norfolk State University M. Khandakar Ohio University K. Ardashev, C. Bade, R. Deininger, K. Hicks, M. Lucas, J. Mahon Syracuse University A. Honig Universita di Roma II - Tor Vergata A. D'Angelo, A. d'angelo, R. Di Salvo, D. Moricciani, C. Schaerf Universitè de Paris - Sud, ORSAY C. Commeaux, J.-P. Didelez University of South Carolina I. Danchev, C. Gibson, B.M. Preedom University of Virginia A. Cichocki, B. Norum, K. Wang Virginia Polytechnic Institute & State University M. Blecher, T. Kageya, H. Meyer, T. Saitoh

3 What is LEGS? Compton backscattering facility = + 4 l 4 l E e m e Scattered laser photon ε l gains energy from the moving electron E e in a storage ring (E e /m e ) + θ (E e /m e ) E e =.8 GeV ε l = ev } 5 MeV < < 47 MeV Polarization determined by laser Linear and circular polarization states By superposition of different lines always >8% Linear polarization nm.58 GeV 35 nm.58 GeV 35 nm.8 GeV 63 nm.8 GeV Eγ (MeV) "Random" flipping through all linear and circular polarizations possible

4 Motivation for N(, ) Multipole Amplitudes Double polarization observables Asymmetries E and G Neutron channels π o n and π - p Nucleon Spin Sum Rules Gerasimov-Drell-Hearn - α m κ Nucleon spin structure at Q = LEGS covers ~65% Measurement down to pion threshold is important Forward Spin-Polarizability γ = = 4π 4π m π m π σ / σ / 3 - σ 3/ - σ 3/ d d Test of chiral perturbation theory LEGS covers ~9% Measurement down to pion threshold is important

5 Other Physics Goals with polarized p and D Nucleon Spin-Polarizabilities Double polarized Compton scattering N N Forward and backward spin-polarizability Multipole amplitudes Nucleon-Delta Interaction Photo-disintegration D p n Multipole amplitudes

6 Current Status of GDH ( Only Q = ) Proton [ b] Neutron [ b] m Experiment: m π < <. GeV MAMI / ELSA. GeV < <.9 GeV -55 ( 5) --- >.9 GeV Total (with extrapolations) --- Multipoles: -5 ( 5) SAID multipoles m π < <. GeV MAID multipoles m π < <.8 GeV MAID multipoles (with extrapolations) Experiment: Proton high Neutron no data Multipole Calculations: Proton overestimated Neutron underestimated

7 Spin-ASYmmetry Array (SASY) All four pion-photoproduction channels can be measured simultaneously (in principle): γ p π + n γ p π o p γ n π - p γ n π o n Phase I: Spin sum rules at the proton Inclusive measurement of π + n and π o p Spin sum rules at the neutron Upgrade with additional neutron detectors ("neutron barrel") Exclusive measurement of π o n Phase II: Spin sum rules at the neutron Upgrade with TPC magnetic analysis of π + /π - Inclusive measurement of π - p

8 SASY Current Setup LEGS Spin ASYmmetry array (SASY). LEGS / 3 MeV SAID Σ γ + HD π ο θ CM (deg) Σ γ + HD π ο φ (deg) γ

9 Strongly Polarized Hydrogen and Deuterium ICE Target (SPHICE) Frozen-spin hydrogen and deuterium target HD molecules in solid phase Simple: only two protons and one neutron H(p free ) + D(p bound + n bound ) High purity Contains only % (weight) aluminium wires for cooling purposes Flexibility and high quality Polarization of either H or D or both Independent spin orientations 8% proton and 5% deuteron polarization possible Thickness up to ~ g/cm Relaxation times > days for H and >3 days for D Storable and transportable Long relaxation times under storage

10 SPHICE -- Parameters

11 Target Polarization History Initial P T = { 7% for H 7% for D Many tests to study target under various conditions (transfers, different holding fields and temperatures) Start conditions in-beam 3 3% for H (T P T = { = 3d) 6 % for D (T = 36d) Calibration of Polarization: NMR line shift due to polarization decay (change in B field) Low-B/high-T thermal equilibrium NMR signal (H + HD) After days magnet quench P T dropped by about a factor of two After.5 days accelerator shutdown 3.5 days of net data taking <P H > = (P H dt)/ dt =.3% <P D > = (P D dt)/ dt = 5.%

12 C C N( N Observables d d, d d L cos E P z G P z P P y L sin L cos T P y F P x H P x L sin (P x,p y,p z ) target-; ( γ L, γ C ) beam-polarizations Four of eight observables needed for a model independent multipole analysis are measured simultaneously E(θ) GDH ; γ

13 Polarization Observables

14 Extraction of Asymmetries d d d d d d d d L P br L P br Q L L Q L Q L Q P z U L U L G cos P z U L U L G cos L P z Q Q L L L G U U P z Q L Q L L G U L U sin sin P P V L C br z V L E P P V L C br z V L E Fit istributions with, G, and E as the only free parameters! Asymmetry.4. +HD Circular Left/Right ο o = 33 MeV; Θ cm = 5 Best Fit same, but G= LEGS Data Linear -45/45 degree Linear /9 degree φ (deg)

15 Proton G Asymmetries + HD + HD p Asymmetry G.5 SAID γp π + n MAID γp π + n LEGS Data Belyaev 984 Asymmetry G.5 SAID γp MAID γp LEGS Data π p π p -.5 = 364 MeV -.5 = 364 MeV = 33 MeV = 85 MeV -.5 = 33 MeV = 85 MeV Θ cm (deg) Θ cm (deg)

16 Pion Missing Energy of +HD 3 MeV < < 35 MeV HD( 75 o < θ CM < 5 o D( Parallel (h = /) Parallel (h = /) counts 5 counts o < 5 o < Θ π o Missing Energy E miss (MeV) Antiparallel (h = 3/) γ n π n < 35 MeV 3 MeV < o < 5 o < Θ π o Missing Energy E miss (MeV) Antiparallel (h = 3/) counts 5 counts Missing Energy E miss (MeV) Missing Energy E miss (MeV) Parallel - Antiparallel (/ - 3/) Parallel - Antiparallel (/ - 3/) counts counts Missing Energy E miss (MeV) Missing Energy E miss (MeV)

17 Proton E Asymmetries + HD + HD p Asymmetry E E = 363 MeV γ SAID γp MAID γp LEGS Data π + n π + n Asymmetry E Eγ = 363 MeV SAID γp MAID γp LEGS Data π p π p - - E γ = 34 MeV = 37 MeV E γ = 34 MeV = 37 MeV - - = 95 MeV = 74 MeV = 95 MeV = 74 MeV Θ cm (deg) Θ cm (deg)

18 Integrands of GDH and γ at Neutron

19 LEGS GDH Program hν (~threshold < E γ BNL D < 47 MeV) γ γ π ng Feb'3 γ π γ π γ π π (.8 T) p π π + GDH Program at LEGS/TPCa/June

20 Summary and Outlook First double-polarization data at the proton (π + n, π ο p) and at the neutron (π p, π ο n) from a frozen-spin HD target Large solid angle coverage (full φ) H polarization 3% D polarization 6% Flipping through linear and circular beam polarizations Σ (θ), G(θ), E(θ) and σ(θ) measured simultaneously Good quality results after only 3.5 days of running (even for GDH integrals) New sets of HD targets D polarization ~% Double-polarization data on π ο n H polarization >7% Several weeks of running (in full energy range) (GDH; γ ) p down close to pion threshold Next phase D polarization 5% Detector upgrade with TPC magnetic analysis Complete exclusive measurements of neutron observables (GDH; γ ) n down close to pion threshold