Transcript The PrimEx Project at Jefferson Lab (a short overview)
Precision Measurements of Electromagnetic Properties of
0
, η and η’ Mesons at JLab
A. Gasparian NC A&T State University, Greensboro, NC and the PrimEx Collaboration Outline The project and physics motivation: The first experiment: Results for the 0 PrimEx at 12 GeV The proposed → Summary 0 lifetime lifetime experiment with GlueX
The QCD Lagrangian
chiral limit:
is the limit of vanishing quark masses m q → 0.
QCD classical Lagrangian with quark masses set to zero: (
o
)
L QCD
q L
iD
q L D
q R
iD g s
q R
/ 2
G
1 4
G
G
q
u d s
q R
,
L
1 2 ( 1 5 )
q
Has Large global symmetry group: A. Gasparian
SU L
( 3 )
SU R
( 3 )
U A
( 1 )
U B
( 1 ) INT 09-3, November 12, 2009 2
Physics Motivation
A. Gasparian INT 09-3, November 12, 2009 3
Physics Motivation (contin.)
A. Gasparian 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?
INT 09-3, November 12, 2009 4
The PrimEx Project
Experimental program
Precision measurements of: Two-Photon Decay Widths: Γ( 0 → ), Γ( → ), Γ( ’ → ) Transition Form Factors at low Q 2 (0.001-0.5 GeV 2 /c 2 ): F( * → 0 ), F( * → ), F( * → ) Test of Chiral Symmetry and Anomalies via the Primakoff Effect A. Gasparian INT 09-3, November 12, 2009 5
Transition Form Factors at Low Q
2 Direct measurement of slopes • Interaction radii: F γγ*P (Q 2 )≈1-1/6 ▪
0
Decay Width (the First Experiment)
0 → decay proceeds primarily via the chiral anomaly The chiral anomaly prediction is exact in QCD.
for massless quarks: 0 2
N c
2 576 3
m
3
F
2 7 .
725
eV
Corrections to the chiral anomaly prediction: (u-d quark masses and mass differences) Calculations in NLO ChPT: (J. Goity, at al. Phys. Rev. D66:076014, 2002) Γ( 0 ) = 8.10eV ± 1.0% ~4% higher than LO, uncertainty: <1% K. Kampf, B. Moussallam, PRD 79, 076005, 2009 QCD sum rule calculations: (B.L. Ioffe, et al. Phys. Lett. B647, p. 389, 2007) Γ( ) is only input parameter 0 mixing included Γ( 0 ) = 7.93eV ± 1.5% Precision measurements of ( 0 → ) at the percent level will provide a stringent test of a fundamental prediction of QCD.
A. Gasparian INT 09-3, November 12, 2009 7
Decay Length Measurements (Direct Method)
Measure 0 decay length 1x10 -16 sec too small to measure solution: Create energetic 0 L = v E /m ‘s, for E= 1000 GeV, L mean 100 μm very challenging experiment Recent CERN experiment, 1984: P=450 GeV proton beam Two variable separation (5-250 m) foils Result: ( 0 ) = 7.34eV
3.1% (total) Major limitations of method unknown P 0 spectrum needs higher energies for improvement A. Gasparian INT 09-3, November 12, 2009 8
Primakoff Method
ρ,ω
d
3
d
Pr 8
Z
2
m
3 3
E
4
Q
4
F e
.
m
.
(
Q
) 2 sin 2 12 C target Primakoff Nucl. Coherent Interference Nucl. Incoh.
Challenge: Extract the Primakoff amplitude
A. Gasparian INT 09-3, November 12, 2009 9
PrimEx Experiment
Requirements of Setup:
high angular resolution (~0.5 mrad)
high resolutions in calorimeter small beam spot size (‹1mm)
Background:
tagging system needed
Particle ID for (
-charged part.)
veto detectors needed JLab Hall B high resolution high intensity photon tagging facility New pair spectrometer for photon flux control at high intensities New high resolution hybrid multi-channel calorimeter (HYCAL) A. Gasparian INT 09-3, November 12, 2009 10
PrimEx-I Milestones
Proposal approved in 1999 by PAC15, re-approved by PAC22 (E02-103) in 2002 with A rating.
In 2000, NSF awarded a collaborative MRI grant of $1 M to develop the experimental setup.
Installation of setup in August, 2004.
Commissioning: September, 2004 (10-15 days) Data taking: September-November (45 days) data on two targets: 12 C 208 Pb, Total number of π 0 ~3.2 M Total elastic π 0 : ~ 300 K Total Primakoff π 0 : ~ 3-5 K First preliminary results released at the April, 2007 APS meeting with AIP press conference. Publication of results is expected this winter A. Gasparian INT 09-3, November 12, 2009 11
PrimEx-I Experiment
PrimEx commissioned and took data: August-November, 2004 We measure: initial photon energy: E and time energies of decayed photons: E 1 , E 2 and time X,Y positions of decayed photons Kinematical constrains: Conservation of energy; Conservation of momentum; m invariant mass A. Gasparian INT 09-3, November 12, 2009 12
elastic
0 s(signal)
0
Event selection
elastic non-
0 background Inelastic
0 s(background)
Tagger-HyCal timing ( Δt) ; Energy conservation: Elasticity ((E γ1 +E γ2 )/E tagger ) Invariant mass (M γγ ) ; A. Gasparian INT 09-3, November 12, 2009 13
Fit to Extract
0
Decay Width
Theoretical angular distributions smeared with experimental resolutions are fit to the data
Γ( 0 )
7.82 eV
2.2% (stat. error, including fit error)
A. Gasparian INT 09-3, November 12, 2009 14
Systematic Errors in
(
0 →
)
The sources of Systematic Errors:
Instrumental (experimental setup) Model errors (from fit)
Independent verifications of the extracted cross sections are needed
The data for the following QED processes had been taken periodically in this experiment:
e + e pair production Compton scattering
A. Gasparian INT 09-3, November 12, 2009 15
Systematic Uncertainty
Control of Systematic Errors: Compton Cross Section
e
0.085
0.080
Uncertainties: Statistical Systematic Compton Forward Cross Section Klein-Nishina Primex Compton Data P R E L I M I N A R Y 0.075
0.070
0.065
0.060
5.0
5.1
5.2
Energy (GeV)
Average stat. error: 0.6% Average syst. error: 1.5% Total error: 1.6%
5.3
5.4
5.5
Cross sections are in agreement with theory at percent level
A. Gasparian INT 09-3, November 12, 2009 16
Model Errors in Fitting Procedure
Primakoff: magnitude kept free; form factor is calculated Nucl. Coherent: magnitude kept free; form factor is calculated Interference: Phase angle kept free; Nucl. Incoherent: magnitude kept free; form factor is calculated A. Gasparian INT 09-3, November 12, 2009 17
Γ(
0
) Model Sensitivity
Incoherent Production A → 0 A´ Two independent approaches: Glauber theory Cascade Model (Monte Carlo) Photon shadowing effect
Deviation in Γ(
0
) less than 0.2%
Overall model error in Γ(
0
) extraction is controlled at 0.3%
A. Gasparian INT 09-3, November 12, 2009 18
Estimated Systematic Errors
Contributions Photon flux Target Yield extraction HYCAL eff.
Beam parameters Trigger eff.
VETO eff.
Acceptance Model errors (theory) Physics background Branching ratio Total Error, [%] 1.0
0.1
1.6
0.5
0.4
0.1
0.4
0.3
0.3
0.25
0.03
2.1
A. Gasparian INT 09-3, November 12, 2009 19
Final Result from PrimEx-I
A. Gasparian INT 09-3, November 12, 2009 20
Planned PrimEx-II Experiment
Statistical error: 0.44%
Contributions Photon flux Target Yield extraction HYCAL eff.
Beam parameters Trigger eff.
VETO eff.
Acceptance Model errors (theory) Physics background Branching ratio Total (syst.) A. Gasparian Error, [%] 1.0
0.1
0.5
0.2
0.4
0.1
0.3
0.3
0.3
0.25
0.03
1.3
INT 09-3, November 12, 2009
Projected PrimEx-II (1.4%)
21
Proposed Experiment for
→
Decay Width Measurement in Hall D
We propose to measure Γ( → ) with 3% total error using GlueX standard setup 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 INT 09-3, November 12, 2009 22
→
Decay Width Experiments: e
+
e
-
Collider Results
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% PDG average for collider experiments: Γ(η ) = 0.510 ± 0.026 keV ( ± 5.1%) Major limitations of method unknown q 2 for * * knowledge of luminosity A. Gasparian INT 09-3, November 12, 2009 23
η
The Primakoff Method
ρ,ω
d
3
d
Pr 8
Z
2
m
3 3
E
4
Q
4
F e
.
m
.
(
Q
) 2 sin 2 Difficulties of
→
experiment: cross section is smaller larger overlap between Primakoff and hadronic processes
Challenge: Separate Primakoff amplitude
Pr
peak
m
2 2
E
2
NC
E
2
A
1 / 3 larger momentum transfer:
from hadronic processes.
We propose to use hydrogen and 4 He targets to address all those issues.
A. Gasparian INT 09-3, November 12, 2009 24
Cornell Primakoff Experiment
Cornell (PRL, 1974) brems. beam, E =5.8, 9.0, 11.45 GeV targets: Be, Al, Cu, Ag, U Result: (
η
)=(0.324
0.046) keV ( 14.2%) A. Gasparian INT 09-3, November 12, 2009 25
Proposed
Experiment with GlueX Standard Setup
The GlueX setup will provide:
Counting House High energy tagged photon beam E γ =10 – 11.7 GeV High acceptance FCAL calorimeter LH2 and LHeTargets (30 cm) Pair Spectrometer 75 m
Dedicated Run with:
5 mm diameter beam line collimator amorphous radiator (~10 -4 R.L.Au) small detector behind the FCAL for overall control of systematic errors A. Gasparian INT 09-3, November 12, 2009
Proposed
Experiment with GlueX Standard Setup (contn.)
A. Gasparian INT 09-3, November 12, 2009
Photon Flux Stability and Resolutions
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 Designed limit A. Gasparian
Angular resolution vs. collimator diameter
INT 09-3, November 12, 2009
Acceptances and Resolutions
Acceptance of FCAL vs. prod. angle Angular resolution vs. target length Our interest A. Gasparian INT 09-3, November 12, 2009 Designed length
Statistics and Beam Time Request
Target: 30 cm (3.46% r.l.) LH 2 , Np=1.28x10
24 p/cm 2 Photon intensity: 7.6x10
6
γ
/sec in E γ = 10.5–11.7 GeV Total cross section on P for
θ η
=0 - 3.5
0 ,
Δσ
= 1.1x10
-5 mb (10% is Primakoff).
N(evts) = N p x N γ x Δσ x ε(eff.)x(Br. Ratio)
= 1.28x10
24
x7.6x10
6 x1.1x10
-32
x0.7x0.4
= 3.0 x 10
-2
events/sec = 2592 events/day = 259 Primakoff events/day
Beam time request: LH 2 target 4 He target Empty target run Tagger efficiency, TAC Setup calibration and checkout Total 40 days 30 days 6 days 4 days 8 days 88 days
A. Gasparian
Statistics:
1% stat. error on each LH 2 LHe4 target and INT 09-3, November 12, 2009 30
Estimated Error Budget
Systematical errors: Contributions
Photon flux Target number Background subtraction Event selection Acceptance, misalign.
Beam energy Model error Branching ratio (PDG)
Total Systematic Total estimated error:
Statistical error Systematic error
Total Error
A. Gasparian INT 09-3, November 12, 2009
Estimated Error
1.0% 0.5% 1.8% 1.7% 0.5% 0.2% 0.3% 0.66%
2.8%
1.0% 2.8%
3.0%
31
The Expected Result for
→
Decay Width
We propose to measure Γ( → ) with 3% total error using GlueX standard setup 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 INT 09-3, November 12, 2009 32
Physics Outcome from
Experiment
( ’) mixing angle: light quark mass ratio
Q
2
m s
2
m
2
d
m
2
m u
2 , whe re 1 2 (
m u
m d
) Γ(η → 3 )=Γ( → ) × B.R.
A. Gasparian INT 09-3, November 12, 2009 (
m K
0
m K
)
e
.
m
.
Corr.
33
Summary and Outlook
PrimEx-12 experimental program has been developed to perform precision tests of chiral symmetry and anomaly effects in the light pseudoscalar meson sector.
Availability of modern tagged-photon beams and novel calorimetry made the Primakoff method a feasible tool to reach the required few percent accuracy in radiative decay width measurements.
The first experiment, the November 11, 2009November 11, 20092004 in Hall B with 6 GeV beam. (
0
0
decay width measurement, has been performed in fall, ) has been extracted with high precision at 3% level.
A new PrimEx-II run is approved to reach the projected 1.4% precision.
The model error in hadronic contributions in decay width extraction process is currently controlled on 0.5% level for light and medium nuclei.
New Primakoff high precision experiment for Г(
→
) in Hall D 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.
The GlueX experimental facility with its high resolution and high intensity 12 GeV photon tagger with high aperture FCAL calorimeter, is well suited for the Г( ) Primakoff measurement with a 3% precision.
A proposal for η/ experiment is being develop using the GlueX setup with 12 GeV.
A. Gasparian INT 09-3, November 12, 2009 34
The End
A. Gasparian INT 09-3, November 12, 2009 35
Cross section on Proton
A. Gasparian INT 09-3, November 12, 2009 36
Γ(
0
) Model Sensitivity (an Example)
F S F A x F I F A F I x – Nuclear Form Factor – Intermediate state contribution - Shadowing parameter Variations in shadowing parameter x ΔΓ < 0.1% A. Gasparian INT 09-3, November 12, 2009 37
0
Forward Photoproduction off Complex Nuclei: (theoretical models)
Coherent Production A → 0 A Leading order processes: (with absorption) Primakoff Nuclear coherent Next-to-leading order: (with absorption) 0 rescattering Photon shadowing A. Gasparian INT 09-3, November 12, 2009 38