Neutron skins from coherent pion photoproduction Dan Watts, University of Edinburgh 1

Talk Outline Why measure the neutron skin ? Basics of coherent p0 photoproduction process Apparatus - The Crystal Ball at MAMI Analysis & results for 208Pb Future plans

208Pb neutron skin from nuclear models Main features obtained from 2PF parameterisation Analytic relationship a, c and √<r2> ap=0.46 fm Diffuseness (a) Neutron Skin (Drnp) fm c

Neutron skin of 208Pb and neutron EOS Warda, Centelles, Vinas Roca-Masa, arxiv 1202.4622 (2012)

Neutron skins and neutron stars 208Pb Neutron skin and Neutron stars Thick neutron skin → Low transition density in neutron star New data from X-Ray telescopes → mass, radii, temp of neutron stars ! Liquid Solid Proton fraction as a function of density in neutron star Rutel et al, PRL 95 122501 (2005) Horowitz, PRL 86 5647 (2001) Horowitz, PRC 062802 (2001) Carriere, Astrophysical Journal 593 (2003) Tsuruta, Astrophysical Journal Lett. 571 (2002) Direct URCA Cooling n → p + e- + n e- + p → n + n Constrains gravitational wave emission from neutron stars – Frequency and damping modes!! PRC 80 025801 (2009)

Previous skin measurements for 208Pb Recent reviews Tsang PRC 015803 (2012) Fattoyev arxiv:1306.6034 (2013) } Droplet [PRL 108 052501] Analyses using theory, expt observation Nstar+QMC [PRL 108 081102] Latimer ARNPS 62 485] Tsang [PRC86 015803] PREX [PRL 108 112502] Pygmy dipole [PRC 76 051603] Electric dipole [PRL 107 062502] Heavy ion diffusion [PRC 72 064309] Antiprotonic atoms [PRC 76 0143301] Proton scattering [PRC 82 044601] Pion beam [NPA 896 46] Drnp Most experimental information from strongly interacting probes Information on shape of neutron distribution also desirable

Coherent pion photoproduction Photon probe  Interaction well understood p0 meson – produced with ~equal probability on protons AND neutrons. Reconstruct p0 from p0→2g decay Angular distribution of p0 → PWIA contains the matter form factor p0 final state interactions - use latest complex optical potentials tuned to p-A scattering data. Corrections modest at low pion momenta ds/dW(PWIA) = (s/mN2) A2 (qp*/2kg) F2(Eg*,qp*)2 |Fm(q)|2 sin2qp* I lead an experiment which will measure the matter distribution of the nucleus using pion photoproduction. The process is shown schematically here. A photon is incident on the target nucleus and interacts to produce a pi0 meson which occurs with ~ equal probabliity on both neutrons and protons. If the nucleus remains in its ground state the probabliiy or amplitude for producing the pi0 from each nucleon adds coherently and you get a diffraction pattern in the angle of the emitted pi0s which contains accurate information on the distribution of matter in the nucleus. From consideration of the theoretical model predictions and computer simulations of the detector apparatus we will get an accuracy in the rms matter radius close to what we presently know the charge radius. This will be sufficient to pin down any differences in the proton and neutron distributions.

 photoproduction - amplitude Basic production amplitude ~ equal for protons and neutrons in D region PWA (MAID,SAID) - close agreement Eg>180 MeV for p,n cross sections  M1 well established multipole Electromagnetic probe of the matter distribution! Isospin structure of amplitude A(p→0p) = √2/3 AV3 +√1/3(AVI –AIS) A(n→0n) = √2/3 AV3 +√1/3(AVI +AIS)  has I=3/2  AV3 only EM couplings identical for p,n As dominated by delta (I=3/2) only isosppin changing poart of amplitude contributes. Therefore no differnce in reaction on proton and neutron – only arise from isoscalar isovector componoents. 8

The MAMI facility  100% duty factor electron microtron MAMI-C 1.5 GeV upgrade (MAMI-B 0.85 GeV) One of the MAMI-C magnets  e 9

Upgraded Tagger Goniometer e- beam

Crystal Ball arrives at Frankfurt 11

Crystal Ball at MAMI g DEg ~ 2 MeV 108 g sec-1 g TAPS 528 BaF2 crystals Crystal Ball 672 NaI crystals

Coherent pion photoproduction - analysis E=175±5 MeV Ediff 208Pb Ediff = Ecalc- Edet E=210±10 MeV 208Pb Coherent maxima Ediff  theta (deg) Non-coherent contributions  theta (deg)

Extraction of coherent yield : Eg=210±10 MeV q =0.415 (1st maxima) q =0.655 (1st minima) Yield (au) Epdiff q =1.25 (2nd minima) q =0.845 (2nd maxima)

Momentum transfer distributions E=185  5 MeV -- PWIA calculation − Full calculation Drechsel et. al. NPA 660 (1999) E=195  5 MeV Square root scale Fitting procedure Calculate grid cn=6.28-7.07 fm an=0.35-0.65 fm Predictions smeared by q resolution Interpolated fit to experimental data (q = 0.3 - 0.9) Free param. : norm, cn, an, Fixed param. : cp=6.68 ap= 0.447 (PRC 76 014211 (2011)) Low Eg limit: D dominates High Eg limit: p FSI not too large (p-wave interactions set in) E=210 10 MeV E=230  10 MeV

The extracted skin properties  ap Systematics: i) Normalisation parameter within ±5% of unity for all bins i) Eg dependences – an high Eg bin 3.5s away from average ii) Vary yield fitting procedure iii) 10% variation relative p,n amplitudes in the model (mainly affects diffuseness) iv) Different fit ranges 

Comparison with previous measurements Coherent pion } Droplet Nstar + QMC Analyses using theory, expt, observation. Latimer Tsang PREX Pygmy dipole Electric dipole Heavy ion diffusion Antiprotonic atoms Proton scattering Pion scattering Drnp New result in general agreement with other methods

Comparison with theory Diffuseness of neutron equal to proton would be an-ap = 0.0 Relativistic mean field models tend to give larger skins (and Cn-Cp) Experimental evidence that difueseness of neutron distribution is larger than for protons in stable heavy nuclei has been lacking PRL 112 242502 (2014)

Future plans Data under analysis for 116Sn, 120Sn, 124Sn & 56Ni Plans for 48Ca, 40Ca in future Discussions on Xenon isotopic chain E=180  5 MeV Ratio 116Sn/124Sn (Arbitrary units) Experimental data Very early stage analysis qp

Summary Neutron skin powerful observable for nuclear structure and the equation of state Coherent pion photoproduction  complimentary measurement with electromagnetic probe First results for 208Pb agree with previous data for neutron skin & additionally constrain the neutron diffuseness More data to come !

Comparison of amplitudes for p0 production Ratio p0 production cross section ( neutron / proton ) Photon energy (MeV) New data on p0 production from p,n will improve amplitudes away from the D(1232) e.g. Krusche Phys. Rev. Lett. 112, 142001 2014 New PWA fit – D unaffected - large changes in N* couplings

Early results from tin isotope data E=175  5 MeV Experimental data Assuming SKM* neutron distribution Early stage analysis Ratio 116Sn/124Sn (Arbitrary units) qp Theoretical prediction (without exp resolution) qp

2PF sum 2PF single

Estimate of systematics from p-A potential E=185  5 MeV -- PWIA calculation − Full calculation Drechsel et. al. NPA 660 (1999) 1st min/max shifted by ~0.01fm-1 10% accuracy -> 0.001fm-1  ~0.01fm-1 systematic skin (0.1fm skin~0.01fm-1 min/max position) E=195  5 MeV Square root scale E=210 10 MeV FSI shift in minima/maxima ~0.13 fm-1 Reproduces shift in data to <10% E=230  10 MeV

c2 for fits -- PWIA calculation − Full calculation c2=0.33 c2=0.38 E=185  5 MeV -- PWIA calculation − Full calculation Drechsel et. al. NPA 660 (1999) c2=0.33 E=195  5 MeV c2=0.38 Note: expt error bars Increased to give more weight to minima in fit. - hence values <1 Square root scale E=210 10 MeV c2=0.59 E=230  10 MeV c2=1.0

10% change in weighting of amplitudes

Form factors – 1st minima and 2nd maxima Momentum transfer (fm-1)

Proton scattering data

Background fit parameters: data

Background fit parameters: Quasi free model

Extraction of coherent yield

Fit signal + background with 2 Gaussians Constrain signal from fit to coherent peak (below) First iteration leave background parameters free Second iteration constrain from fits to first iteration parameters

Targets and test holder 5 MeV wide bin Simple cut on M(p0) Want to achieve similar stats as 208Pb data ---> ~65k in 200 +-2.5 MeV bin ~3 hours data Sn gave ~2.5k – need ~3 days get 45k Need ~6 days for ½ mm thick target (similar to the proposal)

Isotopically pure targets New enriched Sn targets – obtained from Russian company (Concettina) 5.2 g 112Sn and tin 124Sn – targets ½-1mm thick dependent on diameter. Other targets UK money available to buy further targets 6Li target (Edinburgh) 14C target (Basel) – compare with 12C 48Ca target in Mainz?

Form factors – 1st minima and 2nd maxima Momentum transfer (fm-1)

Form factors approach to 1st minima Momentum transfer (fm-1)

Pion elastic and inelastic scattering

Effect of diffuseness parameter on heights of maxima   Diffuseness  

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