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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