Probing Chiral Dynamics with
photons
Henry R. Weller
Duke University and Triangle Universities Nuclear Laboratory
HIgS PROGRAM
QCD Town Meeting-January 2007
A wide variety of physical processes can be used to
study Chiral Dynamics, guided mainly by the results of
ChPT, an expansion of the Lagrangian for low energy
QCD about the chiral limit, mq=0.
I want to mention a few of these today.
Reference:
International Workshop on Chiral Dynamics 2006
Organizers: H. Gao, B. Holstein, HRW
www.tunl.duke.edu/events/cd2006/proceeding.html
QCD Town Meeting-January 2007
Chiral Anomaly
The
p0
(from Yelena Prok for the PrimEx Collaboration)
decay rate is a fundamental prediction of confinement scale QCD.
Chiral Anomaly – the closed-loop triangle
diagram results in axial vector current
non-conservation, even in the limit of
vanishing quark masses.
The anomaly leads to the reduced decay
amplitude, in leading order (chiral limit)
αem
-1
Aγγ 
 0.02513GeV
4πFπ
where Fp = 92.42+/-0.25 MeV is the
pion decay constant.
QCD Town Meeting-January 2007
Decay Width
The p0ggg decay width is related to the amplitude:

Gp 0 gg  mp
Agg
2
64p
 7.725  0.044eV
is presently known to 10%
The goal of the PrimEx Experiment
is to measure the decay width to
an accuracy of 1.5%. ChPT calculations
Including effects of md-mu being non-zero
Increase G by 4.5%.
QCD Town Meeting-January 2007
Success of ChPT at pion-threshold
Linearly Polarized Photon asymmetry for the gpp0p reaction at an
average energy of 159.5 MeV
MAINZ 2001
QCD Town Meeting-January 2007
Difficulties!
e + p  e’ + p + p0 at low Q2 --MAINZ
QCD Town Meeting-January 2007
Precision Measurement of the Electroproduction of p0 Near
Threshold at JLAB: A Test of Chiral QCD Dynamics
Co-spokespersons J. Annand, D. Higinbotham, R. Lindgren, V. Nelyubin, and B. Norum,
)q
Goal: Extract high precision data in a fine grid of Q 2 and W
from Q 2  0.04  0.14 in steps of 0.01 (GeV/c)2 and
from W  0  20 MeV above threshold in steps of 1 MeV:
 T (qp* )   L L (qp* )
 LT (qp* )
 TT (qp* )
 LT (qp* )
'
QCD Town Meeting-January 2007
Pion Electro-Production in the US  BigBite Collaboration
Target
• 10 - 15 cm Liquid Hydrogen( LH2)
• 125 mm Al Foil
LH2
Beam
Dump
e
p
Electron Beam
15 mA
Luminosity
• 3 - 5 x 1037 Hz/cm2
HRS
Electron
BigBite
MWDC(15 Planes)
HRS
Luminosity
Monitor
Two Segmented
Scintillator Arrays
This experiment (now E04-007) is scheduled to be run in Nov 2007, but
may be delayed until April 2008 due to budget problems.
QCD Town Meeting-January 2007
New measurements of P – A for the proton and neutron at
LEGS using the (SPHICE) frozen-spin solid HD target
QCD Town Meeting-January 2007
LEGS data for the ‘neutron’
QCD Town Meeting-January 2007
LEGS
The LEGS group has now completed taking data for both
polarized p and polarized d targets.
They have used their recently commissioned TPC to
measure the charged pion channels.
This will provide very accurate results for the GDH and
the forward-spin-polarizability integrals for both p
and n. Measuring them simultaneously will provide
accurate values of the GDH n-p difference, where the
theoretical uncertainty is the smallest and the
discrepancy with multipole analysis of pi-photoproduction data the largest
QCD Town Meeting-January 2007
N/Delta Physics at Mainz (IASA (Athens), MIT,
Mainz,…)
g* + p  D  p  N
Detect:
e' + p
or
e' + p+
in coincidence
d
u
u
N
p
D
p
C2/M1 vs.
2
0
Q (p p
channel)
W=1232 MeV
Lattice QCD
Results
Dynamical Model
without pion cloud
Dynamical Model
with pion cloud
Effective Field Theory Calculations
Gail/Hemmert
Pascalutsa/Vanderhaeghen
HIgS –A free-electron laser generated
g-ray source
QCD Town Meeting-January 2007
Upgraded Facility
1.2-GeV Booster Injector
RF System with HOM Damping
QCD Town Meeting-January 2007
Upgrade Schedule
Commissioning of Booster and Ring with OK-4—underway now!
Nuclear Physics Program begins—March, 2007
March 07  Aug 07
Dec. 07 
Linear Pol.- Below 65 MeV, >2x108 g/s
Circ. Pol. Up to 110 MeV, >108 g/s
These are TOTAL intensities. Beam on target is:
TOTAL x 1.5 x % resolution (ex. 5% res. at 100 MeV: 7.5 x 106 g/s)
Expect to have energies up to 160 MeV by Spring 09
QCD Town Meeting-January 2007
The GDH integrand for deuterium
below pion threshold @ HIgS
A 400 hour run will allow us to measure the GDH
integrand between 5 and 100 MeV to an overall
accuracy of about 3% or better, assuming a beam
of 1 x 107 g/s with ~5% energy spread.
An experiment to measure the GDH integrand for
3He below pion threshold is also being developed
by Haiyan Gao et al.
QCD Town Meeting-January 2007
The upgraded BLOWFISH array as of January, 2005.
QCD Town Meeting-January 2007
QCD Town Meeting-January 2007
Compton@HIgS Collaboration
(see www.tunl.duke.edu/~mep/higs/compton.pdf)
Use the intense polarized beams at HIgS to
obtain very precise values of the electric and
magnetic polarizabilities of the proton and
the neutron.
Perform double polarization experiments to
obtain precise values of the spinpolarizabilities of the proton and the neutron.
QCD Town Meeting-January 2007
The HINDA Array
(HIgS NaI Detector Array)
Recently 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@HIgS Collaboration
QCD Town Meeting-January 2007
QCD Town Meeting-January 2007
QCD Town Meeting-January 2007
Electric and Magnetic Polarizability of the proton
• Recent results (B. Pasquini) of a free fit to data yield:
 = 11.52 +/- 2.4 x 10-4 fm3
b = 3.42 +/- 1.70 x 10-4 fm
(with Baldin sum rule value of 13.82
11.0 +/- 1.4 ; b 2.8 +/-1.4 )
A ~50% error in b – which will impact future measurements.
QCD Town Meeting-January 2007
100% Linearly polarized beams at HIgS can
improve this
Simulation (Blaine Norum) assumed:
Eg = 120 MeV
Target: 80 mg/cm2
107 g/s
280 hours
QCD Town Meeting-January 2007
Determination of the electric and magnetic polarizabilities of the proton
using 100% linearly polarized gammas@HIgS –a ~300 hr experiment
will yield ~5% errors on both  ( now~20%) and b (now~50%).
QCD Town Meeting-January 2007
Compton scattering of g rays from the deuteron
at LUND
G. Feldman, M. Kovash, A. Nathan, B. Schroder, H.R.
Weller et al.
• This will give us the so called isoscalar
polarizabilities. Since the proton is
“known”, this gives the values for the
neutron.
• The following shows ChPT calculations
O(p4) (from Phillips and Choudhury)
QCD Town Meeting-January 2007
Solid curve->N = 12; bN  
Dashed curve->N = 6; bN = 9
QCD Town Meeting-January 2007
Presently, results from 55 and 66 MeV disagree with those obtained
from 94 MeV data. eg. 94 MeV (SAL) data yield N  bN ~ 2.5 while
we expect a value of ~10 if the proton and neutron have the same
values for this difference (as expected from Chiral Symmetry).
QCD Town Meeting-January 2007
The LUND experiment will make ~5% measurements at five angles and
3 energies between 40 and 110 MeV. The goal is to obtain errors for
the isoscalar polarizabilities comparable to those which exist for the
proton. This experiment will be run in 2007.
QCD Town Meeting-January 2007
Spin polarizabilities.
Measuring these requires polarized beams and
polarized targets. They are predicted (ChPT) to
be different for the n and the p.
There are four “dipole” spin polarizabilities: g1,2,,4
which can be written in terms of gE1E1, E1M2, M1E2, M1M1.
g1 (~gE1E1) and g4(=gM1M1) are the largest.
QCD Town Meeting-January 2007
HIgS Proposal for measuring the proton spinpolarizabilities
Spokesperson – Rory Miskimen
•A 200 hr. run at 120 MeV will give helicity dependent cross sections at
the 3% level, which translates into ~10% measurments of spinpolarizabilities using the HINDA array.
Sensitivity estimates for all four spin-polarizabilities are based upon
calculations of Hildebrandt, Griesshammer and Hemmert.
QCD Town Meeting-January 2007
QCD Town Meeting-January 2007
Projection for double-polarized Compton scattering from proton
Total beam time for proton 100 hrs for each
measurement: 450 hrs
target spin orientation
R. Miskimen, theory curves: Hildebrandt, Griesshammer, Hemmert,
Nucl-th/0308054
QCD Town Meeting-January 2007
Experiments are being developed by Dr. Haiyan
Gao at Duke/HIgS to measure the spinpolarizabilities of the neutron.
Haiyan Gao has built a high pressure spinpolarized 3He target. Target thickness will be
about 1022 atoms/cm2 with a length of 40 cm.
Polarizations of ~40% have been achieved.
• Effect of the reduced target thickness is
offset by the increased sensitivity in the
observables.
QCD Town Meeting-January 2007
QCD Town Meeting-January 2007
Proposed set-up using the NaI detector array (HINDA) and the 88neutron detector array for quasi-elastic Compton scattering studies
using the polarized 3He target.
QCD Town Meeting-January 2007
Present proposed experiments
• New theoretical calculations by
Choudhury, Nogga and Phillips make
extraction of spin-polarizabilities possible
from elastic scattering data from 3He.
• With a gamma intensity of 2 x 107/sec and
the target and detector system just
described, a 350 hour experiment will give
neutron spin polarizabilities with errors of
about +/- 0.5 x 10-4 fm4.
QCD Town Meeting-January 2007
Estimate of the experimental uncertainties in
the individual spin polarizabilities
For example, at 90o, the longitudinal cross section
difference is sensitive to g1 , while the transverse
polarization cross section difference is sensitive
to g4.
The value of g0 for the proton was fixed from the
Mainz experiment with an error of +/- 13%.
(Could improve the resutls by considering additional
constraints (B. Pasquini))..
QCD Town Meeting-January 2007
•Projected HIgS measurement on Nucleon Spin
•Polarizabilities (quasifree) (all in 10-4 fm4)
•Proton HIgS
•Neutron HIgS
projected
projected uncertainties
uncertainties
•gp1=1.1
0.25 •gn1=3.7
0.40
•gp2=-1.5
0.36
•gn2=-0.1
0.50
•gp3=0.2
0.24
•gn3=0.4
0.50
•gp4=3.3
0.11
•gn4=2.3
0.35
•McGovern et al. NLO heavy baryon Chiral Perturbation Theory
QCD Town Meeting-January 2007
QCD Town Meeting-January 2007
Threshold pion-photoproduction from the
proton @ HIgS
p(g,p0)p
Co-spokesperson: Aron Bernstein
The first experiment:
A measurement of the Target analyzing power at Eg  158 MeV.
QCD Town Meeting-January 2007
These experiments will provide
stringent tests of
• The predictions of Chiral Perturbation
Theory
• Predictions of isospin breaking due to the
mass differences of the up and down
quarks.
QCD Town Meeting-January 2007
The Neutral Meson Spectrometer (NMS)
•14 Plastic
•Scintillators
•All Phases
•Veto Scint.
•2 BGO Layers +
•2 Sets of MWPC
•Phase II
•Tracking
•60 CsI Crystals
•Phase I
•Calorimetry Only
QCD Town Meeting-January 2007
QCD Town Meeting-January 2007
Motivation
Isospin Symmetry Breaking
A measurement of the imaginary part of the s-wave production
amplitude (E0+) provides a determination of the charge exchange
scattering length acex(pnp0p).
Requires measurement of the polarized target analyzing power T(q).
QCD Town Meeting-January 2007
Simulations
(Bernstein et al.)
The results indicate that we can measure ImE0+ with a
statistical uncertainty of 3.7% in 200 hours of actual
data taking at 158 MeV.
This gives us the value of acex (p+n -> p0p).
Isospin conservation implies
acex (p+n -> p0p) = -acex (p-p -> p0n).
The latter is well known from the width of pionic hydrogen
(0.1301 +/- 0.0059) after a decade of work. Our result will
give a comparable accuracy for acex (p+n -> p0p).
QCD Town Meeting-January 2007
Resources at HIgS
Mirror development is the key to pion threshold
Physics at HIgS.
Present mirrrors take us up to 110 MeV.
Although a development plan is in place for 165 nm mirrors (140 MeV),
additional resources are needed to assure that 160 MeV is
reached (150 nm mirrors) with full flux in a timely manner.
Funding presently limits operations to 1000 hrs/yr. In order to execute
this program we would like to increase this to 3000 hours per
year. This requires significant additional $upport.
QCD Town Meeting-January 2007
A wide variety of process can be used to study Chiral Dynamics,
guided mainly by the results of CHPT, an expansion of the
Lagrangian for low energy QCD about the chiral limit, mq=0.
EXAMPLES:
1.
PrimEx at JLAB—a precision measurement of the p0 lifetime.
2.
Pion-electroproduction from the proton near threshold at Mainz and JLAB. ChPT at finite Q2.
3.
N/D Physics at Mainz: the pion-cloud to quark-parton transition.
4.
Compton scattering from the deuteron at LUND—neutron polarizabilities.
5.
Precision measurements of the polarizabilities of the proton at HIgS.
Obtain ~5% measurements of p and bp.
6.
Double-polarization measurements at LEGS using the HD target.
7.
Spin-polarizability measurements for both p and n at HIgS using polarized p, d and 3He targets.
Test ChPT and Lattice QCD results
.
8.
Pion-threshold measurements at HIgS using polarized beam and target.
QCD Town Meeting-January 2007
•HIgS –A
free electron laser generated
EXTRA SLIDES
g-ray source
QCD Town Meeting-January 2007
HIgS
Anticipated Schedule for 2007
Linearly polarized beams below ~65 MeV will be available in early
2007.
Measure Compton scattering from the deuteron at ~50 MeV using
a scintillating target (unpolarized).
Circularly polarized beams (OK-5) will be available in late 2007 up to
~110 MeV.
Measure GDH on the deuteron up to 50 MeV using Blowfish and
the frozen-spin target.
QCD Town Meeting-January 2007
•Spin-exchange optical pumping
•Optical pumping of alkali atoms (Rubidium)
•Spin-exchange of 3He with Rubidium
•High pressure target (~10 bar), 3He pol’n ~40 to 50%
•Target will be 40 cm in length giving 1 x 1022 atoms/cm2
QCD Town Meeting-January 2007
•Extending Gamma Energy Range (4 kA Wiggler Op)
•Extending Wiggler Current 4 kA max
•Operational Concerns
•Saturated magnetic fields
•Additional power supplies
•Filter/bassbar system upgrades
•1.2 GeV operation to reach 158 MeV with 150
nm mirrors
QCD Town Meeting-January 2007
•158 MeV
Schedule for FEL Cavity Mirror R&D
190 nm mirrors have been proven to work in our
environment…these will produce 110 MeV gammas.
165 nm mirrors will produce 140 MeV beams once OK-5 is
operating at 4kA.
Date
Milestone
July 2006
Delivery of 1st mirror sets with coatings for 190, 180 and
165 nm
May 2007
Intracavity evaluation of 1st set of 190, 180 and 165 nm mirrors
Oct. 2007
Delivery of 2nd mirror sets with coatings for 180 and 165 nm
Nov. 2007
Intracavity evaluation of 2nd set of 180 and 165 nm mirrors
QCD Town Meeting-January 2007
Recent results from Choudhury, Nogga and
Phillips for elastic scattering from 3He
QCD Town Meeting-January 2007
QCD Town Meeting-January 2007
Primakoff Effect
• The p0 photoproduction
from Coulomb field of the
nucleus.
• Production (gg*!p0) and
decay (p0!gg)
mechanisms imply the
Primakoff cross section is
proportional to the p0
lifetime.
8 em Z 2 b 3 E 4
d P
2
2
(
)
(qp )
 Gp 0 gg
F
Q
sin
em
3
4
d
mp
Q
QCD Town Meeting-January 2007
Events/0.04 deg
p0 production on 208Pb
QCD Town Meeting-January 2007
Frozen Spin Polarized Deuterium Target
Butanol
Polarization ~ 80 %
Polarizing Field ~ 2.5 T
Holding Field ~ 0.6 T
~4 x 1023 d/cm2
QCD Town Meeting-January 2007
COMPTON ON d WITH A SCINTILLATING TARGET
Vertical axis : light output from target detector
Horizontal axis: Missing energy (binding energy)
Courtesy of Rory Miskimen
QCD Town Meeting-January 2007
Our measurement will determine b to +/- 0.10, where
Im[E0+(gp -> p0p)] = b pp/mp
and
b  Re[E0+ (gp -> p+n)] acex (p+n -> p0p)
Re[E0+ (gp -> p+n)] is well measured (=28.06 +/- 0.27 +/- 0.45), giving us
acex (p+n -> p0p).
Isospin conservation implies acex (p+n -> p0p) = -acex (p-p -> p0n).
The latter is well known from the width of pionic hydrogen (-0.1301 +/- 0.0059)
after a decade of work. Our measurement will give a comparable accuracy for acex
(p+n -> p0p).
QCD Town Meeting-January 2007
Recent (PRC C71, 044002 (2005)) HBChPT calculations of Choudhury
and Phillips indicate appreciable sensitivity of Sx observed in Compton
scattering from the deuteron to g1n at 135 MeV. Test for consistency!
QCD Town Meeting-January 2007
QCD Town Meeting-January 2007