Status of the PrimEx Experiment:A Precision Measurement of the Neutral
Pion Lifetime
7th European Conference on “Electromagnetic Interactions with Nucleons and Nuclei”
Milos Island, GreeceSeptember 12-15, 2007
Dan DaleIdaho State University
Participating Institutions
North Carolina A&T State University University of MassachusettsUniversity of Kentucky Idaho State UniversityJefferson Lab Hampton UniversityMIT University of North Carolina at WilmingtonCatholic University of America Arizona State UniversityUniversity of Virginia ITEP, RussiaIHEP, Russia Norfolk State UniversityUniversity of Illinois Kharkov Inst. of Physics and TechnologyChinese Institute of Atomic Energy IHEP, Chinese Academy of Sciences George Washington University Yerevan Physics Institute, ArmeniaSouthern University at New Orleans Tomsk Polytechnical University, RussiaUniversity of Sao Paulo
A High Precision Measurement of
• Physics motivation• The Primakoff effect• Experimental setup• Towards high precision• Current analysis event selection radiative width extraction • Summary
The axial anomaly
Leading order corrections (Donoghue, 1989)
Corrections O(p4)
Current experimental and theoretical status
Expected PrimEx error bar,arbitrarily projected to theleading order chiral anomaly
decay
wid
th (
eV
)
Next to LeadingOrder, +- 1%
Sum rule approach to axial-vector-vector (AVV) form factor.
f – f caused by strong interaction shown to be small.
Eta width only input parameter. o mixing included.
o eV
Recent Theoretical Advances(B.L. Ioffe and A.G. Oganesian, Phys. Lett. B647, p. 389, 2007)
The Primakoff Effect
Radiative width Known, measured quantities
Angular distributionenables separation ofamplitudes
For high precision:
• Measure angular distribution well to separate various amplitudes
new state of the art o detector
• Measure on a variety of targets to test validity of background extraction (Primakoff
• Tight photon energy (peak4and flux control
photon tagging
• Measure QED processes (Compton and pair production) to validate setup and analysis techniques.
incident electrons
Tagged photon beam
Bremsstrahlung photon tagging
Electron beam
Post-brems electrons
Tagging Ratios
Photon Calorimetry – the HYCAL calorimeter
1152 PbWO4 and 576 lead glass modules
1.6% energy resolution
1-2 mm position resolution
In-beam calibration
Effect of periodicityof detector modules.
Energy resolution – lead tungstate
Luminosity analysis efforts
• Electron counting (absolute flux)
• Pair spectrometer (relative tagging ratios)
• TAC (absolute tagging ratios)
• Target thickness
• Known cross sections (pair production,
Compton)
Online Flux Monitoring
e+ - e - pair spectrometer
dipolemagnet
plastic scintillators
Stability of relative tagging ratios
Monitored during production data taking.
Stability of relative tagging ratios
Stable to 1%.
Relative tagging ratios versus rate
Independent of rate!
Pair production cross section
(1)Bethe-Heitler (modified by nuclear form factor).
(2)Virtual Compton scattering.
(3)Radiative effects.
(4)Atomic screening.
(5)Electron field pair production.
Compton Scattering--Stability of Setup
Angular distribution for CarbonPrimakoff Peak
UMass Analysis
HYCAL – Tagger timing
Invariant Mass Elasticity
Total Probability = Timing x Mass x Elasticity
Event selection:
Normalized probabilitydistributions for
Elasticity versus invariant mass
Hybrid mass
UMass Analysis: Radiative Width for Carbon
Primakoff peak
Nuclear coherent andPrimakoff – NC interference
MIT/JLab Analysis: Event Selection
Invariant mass
HYCAL – Tagger timing
Elasticity
Angular distributions and background
Carbon Lead
MIT/JLab Analysis: Radiative Widths for Carbon and Lead
Carbon: = 8.20 ± 0.15 eV ± 0.20 eV Lead: = 8.11 ± 0.16 eV ± 0.20 eV
ITEP/NCA&T Analysis: Radiative Width for Carbon
= 7.98 ± 0.18eV
All possible cluster combinations.Elasticity imposed.HYCAL – tagger timing < 4 nsec.
ITEP/NCA&T Analysis: Radiative Width for Lead
= 7.94 ± 0.18eV
ITEP/NCA&T Analysis: Radiative Width Combined Fit for Carbon and Lead
= 7.95 ± 0.13eV
Carbon
Lead
Summary IUMass AnalysisCarbon 8.41 eV ± 0.15 eV ± 1.8%
MIT/JLab Analysis: Carbon: = 8.20 eV ± 0.15 eV ± 0.20 eVLead: = 8.11 eV ± 0.16 eV ± 0.20 eV
ITEP/NCA&T Analysis:Carbon: = 7.98 eV ± 0.18 eVLead: = 7.94 eV ± 0.18 eVCombined fit, Carbon and Lead: = 7.95 eV ± 0.13 eV
Summary
• The neutral pion lifetime is a stringent test of confinement scale QCD.
• High precision of the experiment has pushed the limits of photon calorimetery, luminosity monitoring, and QED calculations.
• Experimental systematic errors controlled by pair production and Compton scattering.
• High quality data taken in late 2004. Three quasi-independent analyses.
• Our preliminary result is:
= 7.93 eV ± 2.1% (stat) ± 2.0% (stat)
• In good agreement with ChPT predictions.• Further studies of backgrounds underway.
Extra Slides
PDF1: HYCAL – Tagger timing
PDF 3: Invariant mass
PDF 2: Elasticity
Previous Experiment: The Direct Method(Phys. Lett., vol 158B, no. 1, 81, 1985)
o`s produced by 450 GeV protons in tungsten foils.
o decays observed by detection of 150 GeV/c positrons produced by decay rays converting in foils.
Y(d) = N[A + B(1-e-d/)]
Dalitz decay, conversion in first foil