1. A Precision Measurement of the η Radiative Decay Width via the Primakoff Effect
A. Gasparian
NC A&T State University, Greensboro, NC
for the PrimEx Collaboration
Outline
PrimEx GeV
Previous Γ(→) experiments
The proposed experiment
Control of systematic errors
Summary

2. PrimEx Project @ 12 GeV Input to Physics: precision tests of Chiral
Experimental program
Precision measurements of:
Two-Photon Decay Widths: Γ(0→), Γ(→), Γ(’→)
Transition Form Factors at low Q2 ( GeV2/c2): F(*→ 0), F(* →), F(* →)
Input to Physics:
precision tests of Chiral
symmetry and anomalies;
determination of quark
mass ratio
-’ mixing angle
0, and ’ interaction
electromagnetic radii
is the ’ an approximate
Goldstone boson?
A. Gasparian
PAC34, Jan 27, 2009

3. Symmetries in QCD
Classical QCD Lagrangian in Chiral limit is invariant under:
Chiral SUL(3)xSUR(3) spontaneously broken:
8 Goldstone Bosons (π,K,η,η/)
UA(1) is explicitly broken:
(Chiral anomalies)
Γ(0→), Γ(→), Γ(’→)
Mass of ’
Massive quarks, SU(3) broken:
GB are massive
Mixing of π0 η η/
The π0, η, η’ system provides a rich laboratory to study the symmetry structure of QCD at low energy.
A. Gasparian
PAC34, Jan 27, 2009

4. PrimEx 12 GeV Project History
This program had been reviewed by 3 special high energy PACs:
PAC18 (2000)
PAC23 (2003)
PAC27 (2005)
It is included in the 12 GeV CDR under:
“Test of the Standard Model of electro-weak interactions and the
determination of fundamental Parameters of this model”
With the following statement in the CDR Abstract:
“… Precision measurements of the two-photon decay widths and
transition form factors of the three neutral pseudoscalar mesons
via the Primakoff effect will lead to a significant improvement on
our knowledge of chiral symmetry in QCD, in particular on the
ratios of quark masses and on chiral anomalies.”
A. Gasparian
PAC34, Jan 27, 2009

5. PrimEx-I Experiment: Γ(0) Decay Width
Nuclear targets: 12C and 208Pb;
6 GeV Hall B tagged beam;
experiment performed in 2004
12C
208Pb
A. Gasparian
PAC34, Jan 27, 2009

6. PrimEx-I Result () = 7.93eV2.3%1.6% A. Gasparian
PAC34, Jan 27, 2009

7. PrimEx-II Planned Run 1.4% Approved by PAC33
for 20 days to get high statistics
1.4%
A. Gasparian
PAC34, Jan 27, 2009

8. → Decay Width: e+e- Collider Experiments
e+e-  e+e-** e+e- η  e+e- 
e+, e- scattered at small angles
(not detected);
Only  detected;
Error in individual experiments:
from 7.6% to 25%
η→
PDG average for collider experiments:
Γ(η) = ± keV ( ± 5.1%)
Major limitations of method
unknown q2 for **
knowledge of luminosity
A. Gasparian
PAC34, Jan 27, 2009

9. Primakoff Method η Difficulties of → experiment:
ρ,ω
Difficulties of → experiment:
cross section is smaller
larger overlap between Primakoff and
hadronic processes
Challenge: Separate Primakoff amplitude
from hadronic processes.
larger momentum transfer:
We propose to use hydrogen target to address all tose issues.
A. Gasparian
PAC34, Jan 27, 2009

10. Cornell Primakoff Experiment
Cornell (PRL, 1974)
brems.  beam, E=5.8, 9.0, GeV
targets: Be, Al, Cu, Ag, U
Result: (η)=(0.3240.046) keV (14.2%)
A. Gasparian
PAC34, Jan 27, 2009

11. Comments on Cornell Experiment
Untagged bremsstrahlung  beam
Conventional Pb-glass calorimetor:
(size: 4.5x4.5x49. cm3
target to calorimeter dist.: 4.7m)
As a result:
insufficient resolutions in experimental
parameters;
hard to resolve Primakoff from hadronic
contribution;
model dependent calculation for
hadronic contributions.
A. Gasparian
PAC34, Jan 27, 2009

12. Precision Measurement of → decay width
We propose to measure Γ(→) with 2.2% total error
all-decay widths are
normalized to  decay
width and experimental
Branching Ratios (B.R.):
Γ(η→all-decays) =Γ(→)/B.R.
Improvement in
Γ(→)
will change the whole
-sector in PDG
A. Gasparian
PAC34, Jan 27, 2009

13. Physics Outcome from  Experiment
light quark mass ratio
( - ’) mixing angle:
Γ(η→3)=Γ(→)×B.R.
A. Gasparian
PAC34, Jan 27, 2009

14.  Experiment with GlueX Setup
Advantages of Hall D/GlueX setup:
High energy tagged photon beam
Eγ=10 – 11.7 GeV
High acceptance Pb-glass
calorimeter (FCAL)
LH2 and LHeTargets (30 cm)
Pair Spectrometer
75 m
Counting
House
Disadvantages of standard Hall D/GlueX setup
for Γ(→) precision measurement:
Beam line: 3.4 mm collimator on 75 m distance:
very low tagging efficiency (~15%);
Radiation damage effects of diamond crystal on
endpoint spectrum?
Compton cross section measurement
to control overall systematic errors:
FCAL with its central hole unable to detect double
arm Compton (need more than 10 m distance)
A. Gasparian
PAC34, Jan 27, 2009 14

15. Propositions for Existing Setup
5.0 mm collimator
double the tagging efficiency
from 15% to ~30%
help to control photon flux stability
within 1%
5.0mm coll.
Design
limit
Amorphous radiator (~10-4 r.l. Au)
better control of photon flux at endpoint
HyCal Calorimeter
enables Compton detection
factor of 2 better experimental resolutions:
better event selection;
extraction of Primakoff process from
hadronic background.
A. Gasparian
PAC34, Jan 27, 2009 15

16. Proposed Experiment with GlueX Setup
General characteristics of proposed experiment:
Incoherent bremsstrahlung photon beam
Eγ =10.5 – 11.7 GeV
(~10-4 r.l. Au radiator, 5.0 mm beam collimator)
High resolution, high segmentation HyCal
Calorimetor
30 cm LH2 target (~3.6 r.l.)
Advantages of Hydrogen target:
no inelastic hadronic contribution;
no nuclear final state interactions;
proton form factor is well known;
better separation between Primakoff and
nuclear processes;
new theoretical developments of Regge
description of hadronic processes.
A. Gasparian
PAC34, Jan 27, 2009 16

17. Experimental Resolutions: Prod. Angle
Precision Primakoff measurement
requires high resolutions in:
Production angle (fit);
Invariant mass (background)
Energy (elasticity)
A. Gasparian
PAC34, Jan 27, 2009 17
A. Gasparian, Hall D, May 9, 2008 17

18. Experimental Resolutions (contd.)
γγ invariant mass
Energy conservation (elasticity)
High resolution, high granularity calorimeter is critical in:
event selection;
extraction of Primakoff from hadronic processes (fitting stage).
A. Gasparian
PAC34, Jan 27, 2009 18
A. Gasparian, Hall D, May 9, 2008 18

19. Statistics and Beam Time Request
Target: 30 cm (3.46% r.l.) LH2, Np=1.28x1024 p/cm2
Photon intensity: 7.6x106 γ/sec in Eγ = 10.5–11.7 GeV
Total cross section on P for θη= ,
Δσ = 1.1x10-5 mb (10% is Primakoff).
N(evts) = Np x Nγ x Δσ x ε(eff.)x(Br. Ratio)
= 1.28x1024x7.6x106x1.1x10-32x0.6x0.4
= 2.6 x 10-2 events/sec
= 2200 events/day
= Primakoff events/day
Beam time request:
LH2 target run
45 days
Empty target run
5 days
Tagger efficiency, TAC
3 days
Setup calibration and checkout
7 days
Total
60 days
Statistics: 45 days of run on LH2:
1% stat. error
A. Gasparian
PAC34, Jan 27, 2009

20. Estimated Error Budget
Systematical errors:
Contributions
Estimated Error
Photon flux
1.0%
Target number
0.5%
Background subtraction
Event selection
0.8%
Acceptance, misalign.
Beam energy
0.2%
Branching ratio (PDG)
0.66%
Total Systematic
1.9%
Total estimated error:
Statistical error
1.0%
Systematic error
1.9%
Total Error
2.2%
A. Gasparian
PAC34, Jan 27, 2009

21. Control of Overall Systematics: Compton Cross Section
PrimEx-I Result
Δσ/ΔΩ (mb/6.9 msrad)
Data with radiative corrections
Total error: 1.3%
reached in PrimEx-I
run
PrimEx-I Result
A. Gasparian
PAC34, Jan 27, 2009

22. Responses to Theory and TAC Comments
Theory comments:
“… How do hadronic contribution effect the decay width?”
The suggested hydrogen target will minimize the hadronic contributions: (a) no nuclear incoherent processes; (b) no final state interaction: better control of the measured nuclear coherent amplitude at relatively large angles;
“… new article with refitting the Cornell data …”
(a) the data from the Cornel experiment are not available for reanalysis; (b) this article fitted the data without the angular resolutions in; (c) it demonstrates the need for a new optimized Primakoff experiment to address the long standing discrepancy between the two data sets.
TAC comments:
About the “… DAQ readout electronics and trigger…”
It was discussed with Hall D leadership how to best organize the replacement of FCAL with HyCal for the proposed experiment. We agreed that building an “interface module” to connect the HyCal output cables into the FCAL output cables is the most cost effective and straightforward solution. It may cost ~$10K and will solve all questions for signal processing, trigger and DAQ.
A. Gasparian
PAC34, Jan 27, 2009

23. Summary
PrimEx-12 experimental program has been developed to perform precision tests of
chiral symmetry and anomaly effects in the light pseudoscalar meson sector.
This program has been a part of the “Science Driving the 12 GeV Upgrade”.
It is included in the “Abstract” of the CDR as a part of the physics program.
The first experiment, the 0 lifetime measurement, has been performed in fall, 2004
in Hall B with 6 GeV beam.
(0) has been extracted with high precision at 3% level.
A new PrimEx-II run is approved to reach the projected 1.4% precision.
New Primakoff precision experiment for Г(→) will:
Potentially solve the discrepancy between collider and Primakoff results;
Improve all  decay widths in PDG by more than a factor of 2;
Determine the light quark mass ratio in model independent way;
Significantly improve the (-’) mixing angle determination.
Hall D/GlueX experimental facility with its high resolution and high intensity 12 GeV
photon tagger is uniquely suited for the Г() Primakoff measurement with high accuracy.
A. Gasparian
PAC34, Jan 27, 2009

24. Summary (contd.)
To reach the 2.2% accuracy in Γ(→) measurement, we propose to:
Use hydrogen target to minimize the effects of hadronic processes:
No incoherent contribution;
No final state interactions: better control of nuclear coherent
contribution;
Use high resolution HyCal calorimeter to significantly improve the systematic
errors in this experiment.
Use larger (5 mm) beam collimator to control the photon flux at 1% level with PS.
Simultaneous measurement of Compton cross section to control the overall error in the experiment.
Requesting 60 days of beam time to measure the Γ(→) with 2.2% accuracy.
A. Gasparian
PAC34, Jan 27, 2009

25. The End
A. Gasparian
PAC34, Jan 27, 2009

26. Electromagnetic Background
A. Gasparian
PAC34, Jan 27, 2009

27. Geometrical Acceptance
Coverage of γγ events in HyCal
vs. distance
Coverage of γγ events in HyCal
vs. production angle
HyCal with its moderate size will provide a good (~60%) at 6.5 m
for these energies.
A. Gasparian
PAC34, Jan 27, 2009 27
A. Gasparian, Hall D, May 9, 2008 27

28. Comments on Cornell’s Result on η (I will redo this slide)
Major concerns:
Large overlap between the Primakoff production and the nuclear background.
Uncertainty of the theoretical calculations on the charge form factors and hadronic distributions.
A conventional lead glass calorimeter
a poor 2γ invariant mass resolution for the η events selection, and a poor angular resolution to discern the characteristic shapes of the different production mechanisms.
Untagged photon beam.
M2γγ (GeV2)

29. Compton as Stability Control (maybe to question section)
σ (mb)
A. Gasparian
PAC34, Jan 27, 2009

30. Physics Motivation precision tests of chiral anomaly
Fundamental input to Physics:
precision tests of chiral anomaly
determination of quark
mass ratio
-’ mixing angle
0, and ’ interaction
electromagnetic radii
is the ’ an approximate
Goldstone boson?
A. Gasparian
PAC34, Jan 27, 2009

31. The Need for a Dedicated Run
Instability of the photon flux at the endpoint of the bremsstrahlung spectrum due to radiation damage of the diamond radiator.
Propose to use an amorphous metallic radiator (~10-4 R. L. Au)
Low tagging efficiency (~15%) due to a 3.4 mm diameter collimator.
Propose to use 5.0 mm diameter collimator to double the tagging efficiency and control of the photon flux at 1% level.
Unable to detect the atomic Compton scattering simultaneously with the η production to control the overall systematic errors.
The new HyCal calorimeter with its small beam hole size (6x6 cm2) will access the Compton electrons at small angles (0.450 at 6.5 m distance)
A. Gasparian
PAC34, Jan 27, 2009

32. Advantages of the proton Target (Will work on this one)
The incoherent hadronic contribution vanishes within our kinematical range.
The proton form factor is well known, and is free of corrections from the nuclear final state interactions.
The coherent hadronic photoproduction on proton is peaked at much larger angles than the Promakoff peak.
New theoretical development of a Regge description of meson photoproduction in the proton strong field.
A. Gasparian
PAC34, Jan 27, 2009