chiral dynamics 2012 compton scattering at the high intensity -ray source henry r. weller duke...

Chiral Dynamics 2012

Compton Scattering at the High Intensity -ray Source

Henry R. Weller Duke University and Triangle Universities Nuclear Laboratory

HIS PROGRAM


HIS

Nearly Mono-energetic -rays from 2 to 160 MeV•Up to 100 MeV now

•Up to ~160 MeV by 2015

~100% Linearly and Circularly Polarized -rays

High Beam Intensities(2x108 on target at 15 MeV as of June 2009)


S -ray beam generation•Circularly and linearly polarized, nearly monoenergetic rays from 2 to 100 MeV

•Utilizes Compton backscattering of FEL light to generate rays

•Booster Injector

•LINAC

•RF Cavity

•Mirror

•Optical Klystron

•FEL


HIS –A free-electron laser generated -ray source


GDR IVGQR

ISGQR

Measuring Giant Resonances Giant resonances arise from the collective motion of the

nucleons within the nucleus Isovector Giant Dipole (GDR) -- well known Isoscalar Giant Quadrupole (IVGQR) -- well known Isovector Giant Quadrupole (IVGQR) -- poorly known

Precision measurements of the IVGQR would be useful for:– Establishing the A-dependence of the parameters of the IVGQR– Determining the density dependence of the nuclear symmetry energy

Needed to predict the behavior of matter under extreme conditions (e.g. within a neutron star)


Study of the Isovector Giant Quadrupole Resonance in Nuclei(Dissertation of Seth Henshaw, PhD, 2010)

Phys. Rev. Lett. 107, 222501 (2011)

Linearly polarized Compton scattering

Exploits the 100% polarization of the S beam along with the realization that the E1-E2 interference term flips sign when going from a forward to a backward angle.


•NSF/MRI funded project—a high resolution-high acceptance gamma-ray spectrometer consisting of eight 10”x12” NaI detectors in

3” thick segmented NaI shields.

•The Compton@HIS Collaboration

•The HINDA Array(S NaI Detector Array)


One of eight detectors which form the HINDA ARRAY

Pb Collimator

Paraffin n Shield 10”x10” NaI Core Detector

8 3” thick Optically Isolated NaI Shield Segments


HINDA Setup

209Bi Scattering Target 2” Diameter x 1/8” thick 9*1021 nuclei/cm2

12mm collimated S beam 3 x 107 ’s/sec E/E=2.5 % = MeV

6 Detectors 3@ 60(55) (Left,

Right,Down)3@ =120(125) (Left,

Right, Down) msr


Backward angle detectors were background free, but forward angle detectors required background subtraction.


Scattering TheoryAssumptions: (GDR Dominates)

Modified Thomson Amp included in E2 strength due to IVGQR


RESULTS FOR 209Bi

E=23+/-0.13 MeV =3.9 +/- 0.7 MeV SR=0.56 +/- 0.04 IVQ-EWSRs


A second target—89Y—was studied in February, 2012.


Preliminary parameters for the IVGQR in 89Y

Eres = 28.0 +/- 0.4 MeV

=+/- 0.9 MeV

IVQ-EWSR = 0.93 +/- 0.11


The A-dependence of the IVGQR parameters is beginning to take shape.


Proposed experiments

IVGQR survey of 4 additional targets

Planning to study additional targets of 51V, 120Sn, 142Nd, and 152Sm in the future. These were chosen to cover a range of A values, and are required to be spherical nuclei in order to minimize inelastic (Raman) contributions.

E ~ 14–40 MeV.I ~ 107 /s with E ~ 2-3 %. 100% Linearly polarized.Complete program in 2013-14.


The Compton@HIS ProgramPAC approved experiments include:

1. Use the 100% linearly polarized beam at ~80 MeV to perform a model/sum-rule independent measurement of the proton’s electric polarizability.

2. Use a scintillating polarized proton target and determine the proton spin-polarizabilities with circularly polarized beam at 100 – 140 MeV.


The electric and magnetic polarizabilities of the proton

The values of the polarizabilities of the proton given by the PDG are:p = 12.0 +/- 0.6 x 10-4 fm3

p = 1.9 +/- 0.5 x 10-4 fm3

Lensky and Pascalutsa (BPT) with pion, Dirac nucleons and dof find (EPJ C65, 195 (2010)):

p = 10.8 +/- 0.7 x 10-4 fm3

p = 4.0 +/- 0.7 x 10-4 fm3

While a recent re-analysis of (selected) proton Compton scattering data using Chiral Effective Field theory gives:

p = 10.7 +/- 0.3stat +/- 0.2Bald +/- 0.8theory x 10-4 fm3

p = 3.1 +/- 0.3stat +/- 0.2Bald +/- 0.8theory x 10-4 fm3

(Griesshammer et al., RPNP (2012); see also Beane et al., Phys. Letts.B567, 200 (2003))


Comments

Measurements of Compton scattering from the proton using linearly polarized beams at 90o with the detectors perpendicular to the plane of polarization of the beam will provide a direct determination of , independent of .

If contributions of order M)4 are significant, they will show up as a deviation in the measured value of the cross section in a 900 detector parallel to the plane of polarization wrt. the Born (point) value.


Linearly polarized s allow for independent measurements of the electric () and the magnetic () polarizabilities of the proton.


The HIS ExperimentPAC approved for 2013

Will use 100% linearly polarized beam (2 x 107 /s) at 80 MeV.

Plastic scintillating target, shown to be able to separate scattering from protons from that from 12C, etc. 5 x 1022 protons/cm2.

Eight HINDA detectors at 90o (l,r,u,d), using two independent setups.

Measure both parallel (x) and perpendicular (y) cross sections. Also, determine asymmetries A = (y-x)/(y+x).

A 300 hour run is expected to give A to 0.25%.

The electric polarizability will be determined to within 5% uncertainty

(eg 10.8 +/- 0.5 x 10-4 fm3)


While the proton polarizabilities are “well-known”, the neutron values come from Compton scattering from the deuteron which determines the average of the n and p polarizabiities. After almost 20 years of effort,

the data are still in need of improvement.We decided to begin with Compton on 6Li.


Compton scattering from 6Li at 60 MeVNo previous measurements


Easy to use 5.8 g/cm2 target and ~10 times the cross section of the deuteron lead to clean spectra in a 50 hr experiment


Theoretical treatment

A full theoretical treatment is required in order to extract absolute values of the polarizabilities from these data.

At present, work is underway at Trento (W. Liedemann et al.) using the Lorentz Integral Transform method. They have previously calculated the total absorption cross section for 6Li, and their recent success in treating Compton scattering on the deuteron is encouraging.


Meanwhile, we have performed a phenomenological model calculation to examine the sensitivity of the data to the

isoscalar polarizabilities.

Main ingredients:

Modified Thomson scattering—

E1 and E2 Giant Resonances---

Quasi-deuteron contribution—

Polarizabilities enter the amplitude at order E2. Set and =3.6, fit data by varying Giant Resonance strengths, then vary and


Angular distribution at 60 MeV along with calculations to demonstrate the sensitivity to the isoscalar polarizabilities. The results indicate that the statistical uncertainties in and are +/- 0.7, better than the 0.9

from 20 years of deuterium data.


Hoped for results! Will run at 80 MeV early year. Increased sensitivity (a factor of 2-to-3) will reduce the uncertainty in and to around 0.3.


Spin polarizabilities.

• They tell us about the response of the spin of the nucleon to the polarization of the photons. The stiffness of the spin can be thought of as arising from the nucleon’s spin interacting with the pion cloud.

• Measuring these requires circularly polarized beams and polarized targets – ideally suited to HIS.

• Polarized protons will be provided by our frozen-spin target.


jjij2M1Ejjij2E1M1M1M1E1E

spin),3(eff EH2HE2BBEE4

21

H

• At O(3) four new nucleon structure terms that involve nucleon spin-flip operators enter the RCS expansion.

• A rotating electric field will induce a precession of the proton spin around the direction of the polarized photon, with a rate proportional to the spin-polarizability.

The spin-polarizabilities of the nucleon


Circularly polarized photons moving in the z-direction incident on a proton initially polarized in the x-direction

J

E

p x

y


d4

1

m3

232120

Experiments

2E1M1M1M2M1E1E1E0

2E1M1M1M2M1E1E1E

The GDH experiments at Mainz and ELSA used the Gell-Mann, Goldberger, and Thirring sum rule to evaluate the forward S.-P. 0

440 fm10)10.008.000.1(

Backward spin polarizability from dispersive analysis of backward angle Compton scattering

44 fm10)8.17.38(


•Proton spin-polarizabilities will be measured using a scintillating frozen-spin polarized target

Rory Miskimen et al., U. Mass. Simulations have been performed. A working prototype is

under construction.

The initial experiment will run near 120 MeV.The first experiment will determine E1E1 by measuring 2x

using a transverse polarized target and 8 HINDA detectors near 90 degrees ---4 left and 4 right.

Simulations indicate little sensitivity to M1M1, and we can use o and to fix the other two.


S Frozen Spin Polarized Target (HIFROST)

•Butanol •Polarization ~ 80 %

•Polarizing Field ~ 2.5 T•Holding Field ~ 0.6 T


Low temp APD •Quartz chamber

•Polarized scintillating disks 5 mm thick, BC-490 doped with Tempo

•Overall light transport efficiency ≈ 2%

•Light capture with wavelength shifting fibers

•BCF-92 blue to green wavelength shifting fiber, 1 mm square, double

clad, wrapped around clear shell


Four HINDA detectors will be located clustered around 90o in the horizontal plane on both the right and left sides of the beam.


Count rate calculations and projections

Target thickness: 2.6 x 1023 p/cm2 ; 80% polarizationBeam intensity: 5 x 106 /s @ 100 MeVThe HINDA array with dets. 75 cm from the targetRunning time: 800 hrs. transverse target polarization, 400

hrs. with +1 photon helicity, 400 hrs. with -1.


Anticipated result for E1E1 is -1.4 +/- 0.4 x 10-4 fm4

(theory value from PRL 85 (2000)14).


Summary:

• Measuring the spin-polarizabilities of the nucleon is an important next step. These are fundamental structure constants of the nucleons.

• Can measure the polarizabilities at S with a precision of ~ 0.4 x 10-4 fm4 , which is sufficient to test and differentiate between theoretical models. Full Lattice QCD calculations are imminent.


ACKNOWLEDGEMENTS

Thanks to my colleagues at TUNL/HIS for their invaluable contributions to these experiments.

Special thanks to --

Mohammad Ahmed (NCCU)

Seth Henshaw (Duke)

Luke Myers (Duke)

Jerry Feldman (GWU)

Mark Sikora (GWU)

Rory Miskimen (UMass)

Don Crabb (UVa)

chiral dynamics 2012 compton scattering at the high intensity -ray source henry r. weller duke...

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