Status of the  0  Experiment A. Gasparian NC 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  Classical QCD Lagrangian in Chiral limit is invariant under:  U A (1) is explicitly broken: (Chiral anomalies)  Γ(  0 →  ), Γ(  →  ), Γ(  ’→  )  Mass of  0  Chiral SU L (3)xSU R (3) spontaneously broken:  8 Goldstone bosons (π,K,η)  Massive quarks, SU(3) broken:  Goldstone bosons are massive  Mixing of π 0 η η ’ π 0, 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. Gasparian2Hall D Coll. meeting, Feb. 4, 2011

A. GasparianHall D Coll. meeting, Feb. 4,  0  decay width (Theory)   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 included Γ(  0  ) = 7.93eV ± 1.5%

A. GasparianHall D Coll. meeting, Feb. 4, Experimental Methods: Decay Length Measurement (Direct Method)     1x sec  too small to measure solution: Create energetic  0 ‘s, L = v   E  /m  But, for E= 1000 GeV, L mean  100 μm very challenging experiments  Measure  0 decay length distribution 1984 CERN experiment: 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  0 → 

A. GasparianHall D Coll. meeting, Feb. 4, Primakoff Method ρ,ωρ,ω Challenge: Extract the Primakoff amplitude 12 C target Primakoff Nucl. Coherent Interference Nucl. Incoh.  Primakoff coherent cross section measurement required

A. GasparianHall D Coll. meeting, Feb. 4, Previous Primakoff Experiments  DESY (1970)  bremsstrahlung  beam, E  =1.5 and 2.5 GeV  Targets C, Zn, Al, Pb  Result:  (  0  )=(11.7  1.2) eV  10.%  Cornell (1974)  bremsstrahlung  beam: E  =4 and 6 GeV  targets: Be, Al, Cu, Ag, U  Result:  (  0  )=(7.92  0.42) eV  5.3%  All previous experiments used:  Untagged bremsstrahlung  beam  Conventional Pb-glass calorimetry

A. GasparianHall D Coll. meeting, Feb. 4, 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 12 C target  Good data set on 208 Pb target  good statistics for Compton runs  good statistics on e + e - pair production Final results have been submitted for publication A. Gasparian8Hall D Coll. meeting, Feb. 4, 2011

Results from PrimEx-I Experiment  Nuclear targets: 12 C and 208 Pb;  6 GeV Hall B tagged beam;  experiment performed in C 208 Pb A. Gasparian9Hall D Coll. meeting, Feb. 4, 2011

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 7.82eV  2.8% PrimEx-II projected  1.4% A. Gasparian10Hall D Coll. meeting, Feb. 4, 2011

A. GasparianHall D Coll. meeting, Feb. 4, Estimated Systematic Errors Type of Errors Errors in PrimEx-IEstimated errors in PrimEx-II Photon flux1.0% Target number<0.1% Background subtraction Event selection 1.6%0.8% HYCAL response function0.5%0.2% Beam parameters0.4% Acceptance0.3% Model errors (theory)1.0%0.25% Physics background0.25% Branching ratio0.03% Total2.2%1.3%

Improvements for PrimEx-II: % 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. GasparianHall 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). A. Gasparian14Hall D Coll. meeting, Feb. 4, 2011  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 A. Gasparian15Hall D Coll. meeting, Feb. 4, 2011

A. GasparianHall D Coll. meeting, Feb. 4, PrimEx Current Result

PrimEx-II Run Status  Experiment run period: Sep. 27 to Nov. 10. Physics data collected:  First target, 28 Si, completed at required level of statistics (~0.6%).  Second target, 12 C, data taking completed in half only (~1.2%)  Good statistics for Compton is collected  Good statistics for e + e - pair production is collected A. Gasparian17Hall D Coll. meeting, Feb. 4, 2011

A. GasparianHall D Coll. meeting, Feb. 4, PrimEx-II: Event Selection

PrimEx-II:  0 Event Distribution vs. Production Angle Very preliminary (90% of data) Very preliminary (30% of data) A. Gasparian19Hall D Coll. meeting, Feb. 4, 2011

A. GasparianHall D Coll. meeting, Feb. 4, PrimEx-II: By-Products,   0 +  0 +  0 Detection of  produced in He-bag close to HyCal Z-vertex resolution

A. GasparianHall D Coll. meeting, Feb. 4, PrimEx-II: By-Products,    0 +  (crystal part only)

A. GasparianHall D Coll. meeting, Feb. 4, PrimEx-II: By-Products,  /   0 +  0 + 

A. GasparianHall D Coll. meeting, Feb. 4, PrimEx-II: By-Products, a 0   +  0

A. GasparianHall D Coll. meeting, Feb. 4, 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.

A. GasparianHall D Coll. meeting, Feb. 4, The End

A. GasparianHall D Coll. meeting, Feb. 4,  0 Event selection (cont.) Three groups analyzed the data independently

A. GasparianHall D Coll. meeting, Feb. 4, Electromagnetic Calorimeter: HYCAL  Energy resolution  Position resolution  Good photon detection 0.1 – 5 GeV;  Large geometrical acceptance PbWO4 crystals resolution Pb-glass budget HYCAL only Kinematical constraint

A. GasparianHall D Coll. meeting, Feb. 4, PrimEx Current Result

A. GasparianHall D Coll. meeting, Feb. 4, 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

A. GasparianHall D Coll. meeting, Feb. 4, Beam Time Request 12 C target7 days 208 Pb target6 days Compton and pair prod.4 days Empty target2 days Calibrations/checkout6 days HYCAL config. change2 days Tagging efficiency1 days Total28 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

A. GasparianHall D Coll. meeting, Feb. 4, 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 #1 Use the second signal (cable) for the timing; Needs 1)61 Discriminator channels; 2)61 channels TDC.

A. GasparianHall D Coll. meeting, Feb. 4, Verification of Overall Systematics: Compton Cross Section  Average stat. error: 0.6%  Average syst. error: 1.2%  Total: 1.3% Δσ/ΔΩ (mb/6.9 msrad) Data with radiative corrections

A. GasparianHall D Coll. meeting, Feb. 4, PrimEx-II: HyCal Gain Re-Calibration (by  0 mass )