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