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

A. Gasparian 2Hall D Coll. meeting, Feb. 4, 2011

A. Gasparian Hall D Coll. meeting, Feb. 4, 2011 3

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%


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→


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


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


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


Results from PrimEx-I Experiment

Nuclear targets: 12C and 208Pb; 6 GeV Hall B tagged beam; experiment performed in 2004

12

C

208Pb


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%



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)

A. Gasparian Hall D Coll. meeting, Feb. 4, 2011

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).


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



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



PrimEx-II: Event Selection

PrimEx-II: 0 Event Distribution vs. Production Angle

Very preliminary (90% of data) Very preliminary

(30% of data)



PrimEx-II: By-Products, 0+0+0

Detection of produced in He-bag close to HyCalZ-vertex resolution


PrimEx-II: By-Products, 0+ (crystal part only)


PrimEx-II: By-Products, / 0+0+


PrimEx-II: By-Products, a0 + 0


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.


The End


0 Event selection (cont.)

Three groups analyzed the data independently


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


PrimEx Current Result


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


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


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.


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


PrimEx-II: HyCal Gain Re-Calibration (by 0 mass )

Documents

Status of the 0 Experiment