Photopion photoproduction and neutron radii

Transcription

1 Photopion photoproduction and neutron radii Dan Watts, Claire Tarbert University of Edinburgh Crystal Ball and A2 collaboration at MAMI Jefferson Lab PREX workshop, August 2008

2 Talk Outline Nuclear (π 0 ) photoproduction γ E x, q π 0 Coherent - Accurate Matter form factors Neutron skins of stable nuclei (neutron stars) Incoherent - Transition matter form factors New observable for production amplitude

3 Why measure the matter form factor? Our knowledge of the shape of stable nuclei is presently incomplete Relativistic mean field Skyrme HF e.g. 208 Pb RMS charge radius accuracy < fm RMS neutron radius accuracy ~0.2 fm!! Mass Number (A) Horowitz et al. PRC (2001) Piekarewicz et al. NPA 778 (2006)

4 208 Pb Matter Neutron form skin factor and and Neutron neutron stars stars Thick neutron skin Low transition density in neutron star New data from X-Ray telescopes mass, radii, temp of neutron stars! Solid Liquid Proton fraction as a function of density in neutron star Direct URCA Cooling n p + e - + ν Rutel et al, PRL (2005) e - Horowitz, PRL (2001) + p n + ν Horowitz, PRC (2001) Carriere, Astrophysical Journal 593 (2003) Tsuruta, Asttrophysical Journal Lett. 571 (2002)

5 Coherent pion photoproduction Photon probe Interaction well understood π 0 meson produced with ~equal probability on protons AND neutrons. Select reactions which leave nucleus in ground state Reconstruct π 0 from π 0 2γ decay Angular distribution of π 0 PWIA contains the matter form factor dσ/dω(pwia) = (s/m N2 ) A 2 (q π */2k γ ) F 2(E γ,θ π ) 2 F m (q) 2 sin 2 θ π π 0 final state interactions - use latest complex optical potentials tuned to π-a scattering data. Corrections modest at low pion momenta

6 Coherent pion photoproduction γ Eγ ~ 2 MeV 10 8 γ sec -1 γ TAPS 528 BaF 2 crystals Crystal Ball 672 NaI crystals

7 Coherent pion photoproduction - analysis E γ =175±5 MeV E π diff = E π measured - E π calc E π diff 208 Pb E γ =210±10 MeV 208 Pb E π diff Coherent maxima π 0 theta (deg) Non-coherent contributions π 0 theta (deg)

8 208 Pb : Momentum transfer distributions E γ = MeV E γ = = MeV E γ = MeV q = p γ -p π0 (fm -1 ) E γ = MeV E γ = MeV Unitary isobar model (γ,π γ,π) with complex optical potential Dreschel et. al. NPA 660 (1999)

9 208 Pb: Simple correction for distortion For first preliminary assessment 1) Carry out simple correction of q shift using the theory 2) Analyse corrected minima - fit with Bessel fn. E γ = MeV

10 208 Pb neutron skin preliminary assessment No Skin 0.1 fm skin 0.2 fm skin Proton scattering NPA 778 (2006) 10 Antiprotonic atom PRC (2007 7) 0.3 fm skin See effects of a neutron skin of ~0.1 fm (preliminary!!) More detailed analysis in progress implement various predicted FF model independent analysis

11 Incoherent nuclear π 0 photoproduction γ E x, q π 0 Measurement of neutral pion production to a discrete excited nuclear state has proven elusive for many decades Detect nuclear decay photon in the same detector as the π 0 decay photons Counts [arb. units] E γ = MeV In coincidence with 12 C(γ,π 0 ) Important if want to extract matter form factors for light nuclei (Coherent/incoherent goes as A 2 /A = A) [MeV] E γ

12 γ Alignment of recoiling 12 C nucleus γ α q Counts [arb. Units] 12 C(γ,π 0 ) 12 C(4.4MeV) E γ =300 ± 10 MeV sin 2 (2α) cos 2 (2α) Incl..detector acceptance π 0 Alpha (deg) Strong sin 2 (2α) distribution for 4.4 MeV photons - Spin independent amplitude dominant ( (1232)) (Tryasuchev and Kolchin Phys. At. Nuc (2007)) Spin dependent predicted to give cos 2 (2α) use α to separate the in medium amplitude?

13 Eγ dependence of spin dependent / independent in medium 12 C(γ,π 0 ) 12 C(4.4MeV) E γ =175± 5 MeV 12 C(γ,π 0 ) 12 C(4.4MeV) E γ =310± 10 MeV 12 C(γ,π 0 ) 12 C(4.4MeV) E γ =420± 20 MeV

14 Decay γ angular distributions other targets Counts Eγ = ( )MeV Ca 2.5 deg9 Entries Mean χ 2 / ndf / 75 Counts E γ = ( )MeV O deg9 Entries Mean RMS Underflow 0 Overflow Height ± 28.9 Centroid ± Sigma ± p ± p ± χ 2 / ndf / 65 Height 3332 ± 48.2 Centroid 6.38 ± 0.01 Sigma ± p ± p ± E γ [MeV] E γ [MeV] Ca 40 (γ,π 0 ) 40 Ca* E γ =230± 10 MeV O 16 γ,π 0 ) 16 O* E γ =230± 10 MeV

15 Transition matter form factors 12 C(γ,π 0 ) 12 C(4.4MeV) E γ =220 ± 10 MeV θ π0 Takaki -hole model (NPA 443 p570 (1985)) Full calculation --- Without -N interaction Tryasuchev model (Phys At.Nuc (2007)) --- Full calculation CM Tarbert et. al., PRL (2008)

16 Summary New high quality nuclear π 0 photoproduction data will give timely constraints on nuclear structure and neutron stars Complementary measurement to PREX with different systematic uncertainties 342 BaF2 crystals Nuclear decay photon detection to tag incoherent processes -> accurate matter form factors for lighter nuclei 672 NaI crystals