Download - Status of the 0 Experiment
Status of the 0 Experiment
A. GasparianNC A&T State University, Greensboro, NC
for the PrimEx Collaboration
Outline
Physics Motivation PrimEx-I experiment and results Improvements over Primex-II experiment PrimEx-II Summary
Symmetries in QCD and Light Pseudoscalar Mesons
)1()1()3()3( BARL UUSUSU
Classical QCD Lagrangian in Chiral limit is invariant under:
UA(1) is explicitly broken: (Chiral anomalies) Γ(0→), Γ(→), Γ(’→) Mass of 0
Chiral SUL(3)xSUR(3) spontaneously broken: 8 Goldstone bosons (π,K,η)
Massive quarks, SU(3) broken: Goldstone bosons are massive Mixing of π0 η η’
π0, η, η’ system provides a rich laboratory to study the symmetry structure of QCD.
PrimEx experimental program: Γ(0→), Γ(→), Γ(’→) decay widths F(*→ 0), F(* →), F(* →) transition form factors
0 mass is smallest less mixing effects Γ(0→) decay width is direct test of Chiral anomaly prediction in QCD at our energies
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0 decay width (Theory)
eVF
mNc 725.7576 23
3220
0→ decay proceeds primarily via the Chiral anomaly in QCD. The chiral anomaly prediction is exact for massless quarks:
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: less than 1%
Precision measurements of (0→) at the percent level will provide a stringent test of a fundamental prediction of QCD.
0→
Recent calculations in QCD sum rule: (B.L. Ioffe, et al. Phys. Lett. B647, p. 389, 2007)
Γ() is only input parameter 0- mixing includedΓ(0) = 7.93eV ± 1.5%
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Experimental Methods: Decay Length Measurement (Direct Method)
1x10-16 sec too small to measure
solution: Create energetic 0 ‘s, L = vE/m
But, for E= 1000 GeV, Lmean 100 μm very challenging experiments
Measure 0 decay length distribution
1984 CERN experiment: P=450 GeV proton beamTwo variable separation (5-250m) foilsResult:(0) = 7.34eV3.1% (total)
Major limitations of method unknown P0 spectrum needs higher energies for improvement
0→
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Primakoff Method
22
..4
43
3
2Pr
3
sin)(8
QFQ
E
m
Z
d
dme
ρ,ω
Challenge: Extract the Primakoff amplitude
12C target
Primakoff Nucl. Coherent
Interference Nucl. Incoh.
Primakoff coherent cross section measurement required
3/12
2
Pr
2
2 AEE
mNCpeak
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Previous Primakoff Experiments
DESY (1970) bremsstrahlung beam,
E=1.5 and 2.5 GeVTargets C, Zn, Al, Pb Result: (0)=(11.71.2) eV
10.%
Cornell (1974) bremsstrahlung beam: E=4 and 6 GeV targets: Be, Al, Cu, Ag, U Result: (0)=(7.920.42) eV
5.3%
All previous experiments used: Untagged bremsstrahlung beam Conventional Pb-glass calorimetry
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PrimEx Experiment
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)
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
PrimEx-I Status
New Experimental setup commissioned with beam: (10 days in Sept. 2004) Beamline commissioned All new detectors commissioned Electronics and new readout system commissioned New DAQ commissioned HyCal gain equalizing and calibration scans done Pair Spectrometer commissioned
Physics data runs started just after commissioning: (45 days, Oct. – Nov. 2004) Good data set on 12C target Good data set on 208Pb target good statistics for Compton runs good statistics on e+e- pair production
Final results have been submitted for publication
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Results from PrimEx-I Experiment
Nuclear targets: 12C and 208Pb; 6 GeV Hall B tagged beam; experiment performed in 2004
12
C
208Pb
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PrimEx-I Result
PrimEx-I achieved 2.8% precision (total):
(0) = 7.82 eV 1.8% (stat) 2.1% (syst.)
Task for PrimEx-II: to achieve 1.4% precision:
Projected errors:
0.5% (stat.) 1.3% (syst.)
PrimEx-I 7.82eV2.8%
PrimEx-II projected 1.4%
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Estimated Systematic Errors
Type of Errors Errors in PrimEx-I Estimated errors in PrimEx-II
Photon flux 1.0% 1.0%
Target number <0.1% <0.1%
Background subtractionEvent selection
1.6% 0.8%
HYCAL response function 0.5% 0.2%
Beam parameters 0.4% 0.4%
Acceptance 0.3% 0.3%
Model errors (theory) 1.0% 0.25%
Physics background 0.25% 0.25%
Branching ratio 0.03% 0.03%
Total 2.2% 1.3%
Improvements for PrimEx-II:
12
1.4 % Total Error
0.5 % Stat. 1.3 % Syst.
New DAQ with 5 kHz rate, (factor of 5 gain) Implement more stringent trigger
(factor of 2 gain) (better HyCal gain equalizing)
However, the allocated time was barely enough for one target with 0.5% stat error only.
Better control of Background:
Add timing information in HyCal Improve photon beam line Improve PID in HyCal (add more veto counters, X and Y direction)
Measure HyCal detection efficiency (with TAC module)
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The PrimEx-I Beam line
Improvements in PrimEx-II Photon Beam Line
Make the primary collimator “tapered”: (with 1 mm thick and 10 cm long insertion, diameter 12.7 mm). Upgrade the Permanent Magnet from 1 unit to 3 units.
Extend distance from primary coll. to Pb-shielding wall by 32 cm (moving PS with production targets).
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Make the Pb-shielding wall’s hole diameter smaller (from 1.53” to 0.8”).
Total relative gain: PrimEx-I config. 100 % suggested PrimEx-II config. 19 %
Expect ~5 times less background events !!!
Monte Carlo Simulations
Add Timing in HyCal
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PrimEx Current Result
PrimEx-II Run Status
Experiment run period: Sep. 27 to Nov. 10.
Physics data collected: First target, 28Si, completed at required level of statistics (~0.6%). Second target, 12C, data taking completed in half only (~1.2%) Good statistics for Compton is collected Good statistics for e+e- pair production is collected
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PrimEx-II: Event Selection
PrimEx-II: 0 Event Distribution vs. Production Angle
Very preliminary (90% of data) Very preliminary
(30% of data)
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PrimEx-II: By-Products, 0+0+0
Detection of produced in He-bag close to HyCalZ-vertex resolution
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PrimEx-II: By-Products, 0+ (crystal part only)
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PrimEx-II: By-Products, / 0+0+
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PrimEx-II: By-Products, a0 + 0
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Summary
PrimEx-I achieved 2.8% total error for decay width.
Expected PrimEx-II result at 1.5% level
The 0 lifetime is one of the few precision predictions of QCD. Percent level measurement is a stringent test of QCD at these energies.
Compton and pair-production cross section measurements demonstrate that the systematic errors are controlled at the 1.3% level.
The PrimEx setup is capable of percent level cross section measurements.
High resolution, high intensity tagging facility together with recent developments in calorimetry make the Primakoff method one of the best way to reach the required accuracy in 0 decay width.
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The End
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0 Event selection (cont.)
Three groups analyzed the data independently
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Electromagnetic Calorimeter: HYCAL Energy resolution Position resolution Good photon detection efficiency @ 0.1 – 5 GeV; Large geometrical acceptance
PbWO4 crystals resolutionPb-glass budget
HYCALonly
Kinematicalconstraint
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PrimEx Current Result
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Luminosity Control: Pair Spectrometer
Scint. Det.
Measured in experiment:
absolute tagging ratios: TAC measurements at low intensities
Uncertainty in photon flux at the level of 1% has been reached
Verified by known cross sections of EM processes
Compton scattering e+e- pair production
relative tagging ratios: pair spectrometer at low and high intensities
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Beam Time Request12C target 7 days208Pb target 6 days
Compton and pair prod.
4 days
Empty target 2 days
Calibrations/checkout
6 days
HYCAL config. change
2 days
Tagging efficiency 1 days
Total 28 days
Response to TAC comment on statistical error:2% statistical error on current result Primakoff stat. (1.46%)
+ fit error 0.44% error from proposal requires (1.46/0.44)2 = 11 times more events:
Increase DAQ rate from 1 kHz to 5 kHz Implement more stringent trigger, (factor of 2-3)
Will provide 0.44% Primakoff statistics on each target
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PrimEx-II Gain calibration
Use the PrimEx-I HyCal Trigger Configuration (Version #1)
Each dynode output goes to one of 36 Inputs of the UVA120A Sum module.
Two linear output signals from UVA120A go to “electronics area”
One of them used for further summation, The second one was not used in the PrimEx-1 configuration.
Version #1Use the second signal (cable) for the timing;Needs 1)61 Discriminator channels;2)61 channels TDC.
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Verification of Overall Systematics: Compton Cross Section
4.9 5.0 5.1 5.2 5.3 5.4 5.5
Energy (GeV)
0.055
0.060
0.065
0.070
0.075
0.080
0.085
Systematic Uncertainty
P R E L I M I N A R Y
Uncertainties:StatisticalSystematic
Compton Forward Cross Section
Klein-NishinaPrimex Compton Data
4.9 5.0 5.1 5.2 5.3 5.4 5.5
Energy (GeV)
-10
-5
0
5
10
Devi
atio
n (%
)
P R E L I M I N A R Y
Uncertainties:Statistical
Experiment To Theory Comparison
No DeviationExperiment / Theory Average stat. error:
0.6% Average syst. error: 1.2%
Total: 1.3%
Δσ
/ΔΩ
(m
b/6
.9 m
sra
d)
e e
Data with radiative corrections
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PrimEx-II: HyCal Gain Re-Calibration (by 0 mass )