photopion photoproduction neutron radii and neutron stars dan watts, claire tarbert university of...
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Photopion photoproduction neutron radii and neutron stars
Dan Watts, Claire TarbertUniversity of Edinburgh
Crystal Ball and A2 collaboration at MAMI
CB meeting, Mainz, September 2008
• Neutron skins, EOS and Neutron stars (covering recent PREX neutron skins workshop)
• Present status of coherent and incoherent measurement
• Possible future measurements
Talk Outline
Why measure the matter form factor?
e.g. 208Pb RMS charge radius accuracy < 0.001 fm RMS neutron radius accuracy ~0.2 fm !!
• Our knowledge of the shape of stable nuclei is presently incomplete
Horowitz et al. PRC63 025501 (2001)Piekarewicz et al. NPA 778 (2006)
Relativistic mean field
Skyrme HF
Mass Number (A)
Neutron skins over the nuclear chart
Nuclear equation of state
Equation of state for n rich matter
Nuclear equation of state
Correlation of neutron skin and EOS
Correlation of neutron skin and EOS
Nuclear EOS from Chiral EFT
208Pb Neutron skin and Neutron starsMatter form factor and neutron stars
Formed supernovae of massive star (>8Mo)
Typical ly ~12km radius, ~1.5Mo
Correlation to nuclei not Obvious – 18 OOM in size and 5 OOM in density !
But have shared dependence on the equation of state
EOS and neutron stars
EOS and neutron stars
Neutron star astronomy
Neutron star astronomy
Neutron star astronomy – future plans
Measure mass and radii from hydrogen atmosphere
Parallax measurement to get distances to 1%
Neutron star astronomy – future plans
Direct URCARule out last non-”exotic”Cooling mechanism?
Requires high proton fraction so that PF(p) + PF(e) > PF(n)
• 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.
Select reactions which leave nucleus in ground state
Reconstruct
from →decay
d/dPWIA) = (s/mN2) A2 (q*/2k) F2(E
,)2 |F
m(q)|2 sin2
208Pb
Coherent maxima
208Pb
Ed
iff
Ed
iff theta (deg)
Non-coherent contributions
E=210±10 MeV
theta (deg)
Coherent pion photoproduction - analysis
E=175±5 MeV
Ediff = E
measured- Ecalc
208Pb : Momentum transfer distributions
▬ Unitary isobar model () with complex optical potential Dreschel et. al. NPA 660 (1999)
|q| = p-p0 (fm-1)
E=180-190 MeV E=180-190 MeVE=190-200 MeV E=200-220 MeV
E=220-240 MeV E=240-260 MeV
208Pb: 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=200-220 MeV
208Pb neutron skin – preliminary assessment
• See effects of a neutron skin of ~0.1 fm (preliminary!!)• More detailed analysis in progress ▬ implement various predicted FF ▬ “model independent” analysis
No Skin
0.1 fm skin
0.2 fm skin
Pro
ton
sca
tter
ing
NP
A 7
78 (
200
6) 1
0
An
tip
roto
nic
ato
mP
RC
76
034
305
(20
07)
0.3 fm skin
208Pb form factors from model predictions
Implications of a small skin
~MAMI preliminary
+ rule out Direct URCA as viable cooling mechanism ??
Skin thicknesses in isotopic chain
• Isotopic chain e.g. Stable Ca isotopes (40,42,44,46,48) ? • Stable Sn isotopes ? (112 -124)
• Common problem – need isotopically pure Target loan/preparation
Incoherent nuclear 0 photoproduction
• 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 decay photons
• Important if want to extract matter form factorsfor light nuclei (Coherent/incoherent goes as A2/A = A)
Ex, q
In coincidence with 12C(0)
E=200-220 MeV
• Strong sin2(2) distribution for 4.4 MeV photons - Spin independent amplitude dominant () (Tryasuchev and Kolchin Phys. At. Nuc. 70 827 (2007))
• Spin dependent predicted to give cos2(2) – use to separate the in medium amplitude?
Alignment of recoiling 12C nucleus
Alpha (deg)
Co
un
ts [
arb
. Un
its]
q
sin2(2)
cos2(2) Incl..detector acceptance
12C(0)12C(4.4MeV)E=300 10 MeV
Spin dependent / independent in medium
12C(0)12C(4.4MeV)E=310 10 MeV
12C(0)12C(4.4MeV)E=420 20 MeV
Decay angular distributions – other targets
Ca40(0)40Ca*E=230 10 MeV
O160)16O*E=230 10 MeV
16O40Ca
Takaki -hole model (NPA 443 p570 (1985)) ▬ Full calculation --- Without -N interaction
Tryasuchev model (Phys At.Nuc. 70 827 (2007))
--- Full calculation
CM Tarbert et. al., PRL 100 132301 (2008)
Transition matter form factors12C(0)12C(4.4MeV)E=220 10 MeV
0
• New high quality nuclear 0 photoproduction data will give timely constraints on nuclear structure and neutron stars – topic of high current interest
• Complementary measurement to PREX experiment at JLAB with different systematic uncertainties
• Nuclear decay photon detection to tag incoherent processes -> accurate matter form factors for lighter nuclei
• Potential for important new measurements in the future
Summary
342 BaF2 crystals
Symmetrised Fermi distributions
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 +
Matter form factor 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, Asttrophysical Journal Lett. 571 (2002)
Skx-s15
Neutron skin effects in diff nuclei show correlation
Coherent production on Oxygen
Skx-s15
Coherent production on Calcium
208Pb: Total coherent cross sections
▬ PWIA▬ DWIA▬ DWIA+self energy.
CB@MAMI
TAPS99@MAMI
Theoretical prediction
Dreschel, Tiator, Kamalov & Yang - NPA 660 (1999)
production amplitude from Unitary Isobar Model
interaction treated with momentum space optical potential supplemented with self-energy
Skx-s20
J.Brudvik, J. Goetz, B.M.K.Nefkens, S.N.Prakhov, A.Starostin, I. Saurez, University of California, Los Angeles, CA, USA
J.Ahrens, H.J.Arends, D.Drechsel, D.Krambrich, M.Rost, S.Scherer, A.Thomas, L.Tiator, D. von Harrach and Th.Walcher Institut fur Kernphysik, University of Mainz, Germany
R. Beck, M. Lang, A. Nikolaev, S. Schumann, M. Unverzagt, Helmholtz-Institut fur strahlen und Kernphysik, Universitat Bonn,
S.Altieri, A.Braghieri, P.Pedroni, A.Panzeri and T.Pinelli INFN Sezione di Pavia and DFNT University of Pavia, Italy
J.R.M.Annand, R.Codling, E.Downie, J. Kellie, K.Livingston, J.McGeorge, I.J.D.MacGregor, R. Owens D.Protopopescu and G.Rosner Department of Physics and Astronomy, University of Glasgow, Glasgow, UK
C.Bennhold and W.Briscoe George Washington University, Washington, USA
S.Cherepnya, L.Fil'kov, and V.Kashevarow Lebedev Physical Institute, Moscow, Russia
V.Bekrenev, S.Kruglov, A.Koulbardis, and N.Kozlenko Petersburg Nuclear Physics Institute, Gatchina, Russia
B.Boillat, B.Krusche and F.Zehr, Institut fur Physik University of Basel, Basel, Ch
P. Drexler, F. Hjelm, M. Kotulla, K. Makonoyi, R.Novotny, M. Thiel and D. Trnka II. Phys. Institut, University of Giessen, Germany
D.Branford, K.Foehl, D. Glazier, T. Jude, C.Tarbert and D.P.Watts, School of Physics, Univ. of Edinburgh, Edinburgh, UK
V.Lisin, R.Kondratiev and A.Polonski Institute for Nuclear Research, Moscow, Russia
J.W. Price California State University, Dominguez hills, CA, USA
D.Hornidge Mount Allison University, Sackville, Canada
P. Grabmayr and T. Hehl Physikalisches Institut Universitat Tubingen, Tubingen, Germany
D.M. Manley Kent State University, Kent, USA
M. Korolija and I. Supek Rudjer Boskovic Institute, Zagreb, Croatia
D. Sober, Catholic University, Washington DC
M. Vanderhaeghen, College of William and Mary, Williamsburg, USA CB@MAMI
Potential future measurements
Isotopic chain
e.g. Stable Ca isotopes (40,42,44,46,48)
rms neutron radii ?? Stable Sn isotopes (112,114,115,116,117,118,119,120,122,124)As expect to also see skin of similar size to 208Pb
Common problem – target loan/preparation
208Pb : Momentum transfer distributions
photoproduction - amplitude
• Basic production amplitude ~ equal for protons and neutrons
• Dominated by production
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
• Strong sin2(2) distribution for 4.4 MeV photons
• 2+ to 0+ transition from aligned residual nucleus
• Degree of alignment - new experimental observable!!
Alignment of recoiling 12C nucleus
Nuclear decayphoton
Momentum transferdirection
sin2(2) distributionPassed through Detector acceptance
Alpha (deg)
Co
un
ts [
arb
. Un
its]
• Nuclear (0) photoproduction Coherent - Accurate Matter form factors Neutron skins of stable nuclei (neutron stars) Incoherent - Transition matter form factors New observable for production amplitude
Talk Outline
E~ 2 MeV108 sec-1
672 NaI crystals
528 BaF2 crystals
40Ca208Pb
Nuclear decay photons in the Crystal Ball !!
12C 16O
Neutron density for 208Pb
Shows the shell layers
(Dashed line is the proton density)
208Pb: Preliminary evaluation of Neutron skin effects
▬ No neutron skin▬ 0.1 fm skin▬ 0.2 fm skin▬ 0.3 fm skin
photoproduction as a nuclear probe
Precision test of our understanding of the nucleon interaction
Access matter form factorand matter transition formfactors
Test specific aspects of the pion production amplitude
• Gives information on the matter distribution with EM probe• 0 production ~ identical probability from protons & neutrons
• Strong sin2(2) distribution for 4.4 MeV photons - Spin independent amplitude dominant () (Tryasuchev and Kolchin Phys. At. Nuc. 70 827 (2007))
• observable has potential to separate spin dependent and independent parts of amplitude !!
Alignment of recoiling 12C nucleus
sin2(2) distributionPassed through Detector acceptance
Alpha (deg)
Co
un
ts [
arb
. Un
its]
q
Takaki -hole model (NPA 443 p570 (1985))
▬ Full calculation --- Without -N interaction
▬ Tryasuchev (Phs At. Nuc. 70 827 (2007))
CM Tarbert, DP Watts et. al., Phys. Rev. Lett (2008)
Transition matter form factors
E=220-240 MeV
E=240-260 MeV
Theta 0
Why is neutron radius hard to establish?
“Insensitivity of the elastic proton–nucleus reaction to the neutron radius of 208Pb”Piekarewicz and PieperNPA 778 10 (2006)
Maybe mention antiproton stuff?
208Pb: Preliminary evaluation of Neutron skin effects
▬ No neutron skin▬ 0.1 fm skin▬ 0.2 fm skin▬ 0.3 fm skin
• Strong sin2(3) component
• Expected from 3- to 0+ transition
Alignment of recoiling 16O nucleus
Nuclear decayphoton
Momentum transferdirection
Equation of state for n rich matter EOS is a function of density () and isospin asymmetry ()
Can be expanded in a Taylor Series about the nuclear Saturation density (0) and =0 (symmetric matter)
Asymmetry energy
First derivative of symmtyry energy with respect to density
Coherent → Gaussian with (E) extracted from coherent maximum)
Smeared step function at A(,N)A-1 threshold
For light nuclei with well separated 1st excited state(s) Include second gaussian centered at appropriate energy
Fitting the pion energy difference Spectra
How do we get the Coherent part?
• One technique is to use energy difference analysis
(k, E) (0, E)(kN, EN)
(k, E)(k, E)
(k, E)
Ediff = E(E)E(1,2)
Best previous measurements → segmented arrays
Reliable coherent extraction limited due to sharply dependent systematic effects in E determination
Combined p0 and decay g detection efficiency
208Pb: 0 angular distributions
▬ DWIA + SE
CB@MAMI
TAPS99@MAMI
E=160-170 MeV E=170-180 MeV
E=200-220 MeV
..... + spectra up to E=700 MeV
208Pb(,0)
E=185±5 MeV E=195±5 MeV208Pb(,0)
E=205±5 MeV208Pb(,0)
Theta pion (deg)
No Skin0.2 fm Skin
Also evident in Nuclear Decay photon spectraPicked up by CB!!
E=185±5 MeV E=195±5 MeV
40Ca(,0)40Ca(,0)
40Ca shows large Incoherent contaminationAt larger 0 angles
• Proton scattering
Seminal analysis by Hoffman for all data (0.3 - 1 GeV). rnp (208Pb) = -0.02 →0.5 fm
• Pickup reactions. Recent analysis of p and n pickup gave
rnp~0.5fm for 208Pb
• Antiprotonic atomsrnp ~ 0.15fm for 208Pb.
D
p
p
N
p
Neutron Skins - present situation
Coherent pion photoproduction
Nuclear decay photons 12C
Background predominantly from split-off clusters from pi0 detection
Pion – Nucleus interactions
• Diffraction pattern distorted due to effects of
-A interactions (FSI)
• Optical potential constructed from pi-N amplitude in p space
• Intermediate also included (important at higher P)
• Accurately describes wealth of A(/) data
• If (FSI) ~ 10% ~ (0.07)×(±2o)×0.1 = ±0.014 fm
• Each q occurrs for different P at different incident E – check predicted FSI effects
208Pb
40Ca
40Ca: Total coherent cross sections
CB@ MAMI
TAPS99@MAMI
▬ PWIA▬ DWIA▬ DWIA + mod.
40Ca: 0 angular distributions
▬ DWIA + SE.
E=160-170 MeV E=170-180 MeV
E=200-220 MeV E=220-240 MeV
CB@MAMI
TAPS99@MAMI
12C: Total coherent cross sections
CB@MAMI
TAPS99@ MAMI
▬ PWIA▬ DWIA▬ DWIA + mod.
Photon Tagger upgrade
GoniometerUpgraded Tagger
e- beam
Crystal Ball arrives at Frankfurt
Good angular and energy resolution, close to 4acceptance
Setup at MAMI
Tracker & Particle-ID
GeV)
~41cm
~25cm
sin
d/dA2(q/k)P32|F
m(q)|2sin2
Ags(0)Ags coherent 0 photoproduction
208Pb: 0 angular distributions
▬ DWIA + SE
CB@MAMI
TAPS99@MAMI
E=160-170 MeV E=170-180 MeV
E=200-220 MeV E=300-320 MeV
• 100% duty factor electron microtron
• MAMI-C 1.5 GeV upgrade (Completed!!)
(MAMI-B 0.85 GeV)
The MAMI facility
One of the MAMI-C magnets
e
MWPC & Particle-ID in situ
photoproduction amplitude
• Basic production amplitude ~ equal for protons and neutrons
• Dominated by production
Isospin structure of amplitude
A(p→0p) = √2/3 AV3 +√1/3(AIV –AIS)A(n→0n) = √2/3 AV3 +√1/3(AIV +AIS)
has I=3/2 -- AV3 only
production in the nucleus
“Clean” test of 0-nucleus interaction & effect of medium on properties
Access matter form factorand matter transition form factorwith EM probe
Test more specific aspects of the basic production amplitude
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