Neutron skins from coherent pion

photoproduction

Dan Watts, University of Edinburgh

• Why measure the neutron skin ?

• Basics of coherent 0 photoproduction process

• Apparatus - The Crystal Ball at MAMI

• Analysis & results for 208Pb

• Future plans

Talk Outline

208Pb neutron skin from nuclear models

Neutron Skin (rnp) fm

Diff

usen

ess

(a)

• Main features obtained from 2PF

parameterisation

• Analytic relationship a, c and √<r2>

• ap=0.46 fm

c

Neutron skin of 208Pb and neutron EOS

Warda, Centelles, Vinas Roca-Masa, arxiv 1202.4622 (2012)

Proton fraction as a function of density in neutron star

Thick neutron skin→ Low transition density in neutron star

New data fromX-Ray telescopes→ mass, radii, tempof neutron stars !

208Pb Neutron skin and Neutron stars

Liquid

Solid

Direct URCA Cooling

n → p + e- +

e- + p → n +

Neutron skins and neutron stars

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)

Constrains gravitational wave emission from neutron stars – Frequency and damping modes!! PRC 80 025801 (2009)

Pion beam [NPA 896 46]

PREX [PRL 108 112502]

Previous skin measurements for 208Pb

•Most experimental information from strongly interacting probes •Information on shape of neutron distribution also desirable

rnp

Proton scattering [PRC 82 044601]

Antiprotonic atoms [PRC 76 0143301]

Heavy ion diffusion [PRC 72 064309]

Electric dipole [PRL 107 062502]

Pygmy dipole [PRC 76 051603]

}Recent reviewsTsang PRC 015803 (2012)Fattoyev arxiv:1306.6034 (2013)

Droplet [PRL 108 052501]

Nstar+QMC [PRL 108 081102]

Latimer ARNPS 62 485]

Tsang [PRC86 015803]

Analyses usingtheory, exptobservation

Angular distribution of 0 → PWIA contains the matter form factor

0 final state interactions - use latest complex optical potentials tuned to -A scattering data. Corrections modest at low pion momenta

Coherent pion photoproduction

Photon probe Interaction well understood 0 meson – produced with

~equal probability on protons AND neutrons.

Reconstruct

from →decay

d/dPWIA) = (s/mN2) A2 (q*/2k) F2(E

,)2 |F

m(q)|2 sin2

photoproduction - amplitude • Basic production amplitude ~ equal for protons and neutrons in region

• PWA (MAID,SAID) - close agreement E>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

• 100% duty factor electron microtron

• MAMI-C 1.5 GeV upgrade

(MAMI-B 0.85 GeV)

The MAMI facility

One of the MAMI-C magnets

e

TAPS

Crystal Ball528 BaF2 crystals

672 NaI crystals

E~ 2 MeV108 sec-1

Crystal Ball at MAMI

208Pb

Coherent maxima

208Pb

Ediff

Ediff

theta (deg)

Non-coherent contributions

E=210±10 MeV

theta (deg)

Coherent pion photoproduction - analysis

E=175±5 MeV

Ediff = E

calc- Edet

Extraction of coherent yield : E=210±10 MeV

Ediff

Yie

ld (

au

) q =0.415 (1st maxima)

q =0.655(1st minima)

q =0.845(2nd maxima)

q =1.25 (2nd minima)

E=185 5 MeV

E=195 5 MeV

E=210 10 MeV

E=230 10 MeV

-- PWIA calculation − Full calculationDrechsel et. al. NPA 660 (1999)

Momentum transfer distributionsS

qu

are

ro

ot

sca

le

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 E limit: dominatesHigh E limit: FSI not too large (p-wave interactions set in)

The extracted skin properties

•Systematics: i) Normalisation parameter within ±5% of unity for all bins i) E dependences – an high E bin 3.5 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

ap

Pion scattering

PREX

Comparison with previous measurements

•New result in general agreement with other methods

rnp

Proton scattering

Antiprotonic atomsHeavy ion diffusionElectric dipolePygmy dipole

}Coherent pion

DropletNstar + QMC

Latimer

Tsang

Analyses using theory, expt, observation.

Comparison with theory

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

Experimental data

E=180 5 MeV

Rat

io

11

6S

n/1

24S

n

(A

rbit

rary

un

its) •Very early stage analysis

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 0 productionR

atio

0

prod

uctio

n c

ross

sec

tion

(

neu

tron

/ pr

oton

)

Photon energy (MeV)

• New data on 0 production from p,n will improve amplitudes away from the e.g. Krusche Phys. Rev. Lett. 112, 142001 2014 New PWA fit – unaffected - large changes in N* couplings

Early results from tin isotope dataR

atio

1

16S

n/1

24S

n (

Arb

itra

ry u

nit

s)

Experimental data

Theoretical prediction(without exp resolution)

• Assuming SKM* neutron distribution

• Early stage analysis

E=175 5 MeV

2PF sum2PF single

E=185 5 MeV

E=195 5 MeV

E=210 10 MeV

E=230 10 MeV

-- PWIA calculation − Full calculationDrechsel et. al. NPA 660 (1999)

Estimate of systematics from -A potentialS

qu

are

ro

ot

sca

le

FSI shift in minima/maxima ~0.13 fm-1

Reproduces shift in data to <10%

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=185 5 MeV

E=195 5 MeV

E=210 10 MeV

E=230 10 MeV

-- PWIA calculation − Full calculationDrechsel et. al. NPA 660 (1999)

2 for fitsS

qu

are

ro

ot

sca

le

2=0.33

2=0.38

2=0.59

2=1.0

Note: expt error bars Increased to give more weight to minima in fit.

- hence values <1

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 GaussiansConstrain signal from fit to coherent peak (below)First iteration leave background parameters freeSecond iteration constrain from fits to first iteration parameters

5 MeV wide bin

Simple cut on M(p0)

Targets and test holder

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)

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?

Isotopically pure targets

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

Diffuseness

• 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

Summary

672 NaI crystals

342 BaF2 crystals