The axial anomaly is one of the most profound and fundamental examples of spontaneous symmetry break

1
A Precision Measurement of the Neutral Pion
Lifetime Via the Primakoff Effect
Poster Presenter Eric R. Clinton University Of
Massachusetts Amherst, USA Primakoff Experiment
(PrimEx) at Thomas Jefferson National Accelerator
Facility Newport News, VA
Forming p0 Yields
The Chiral Anomaly the Primakoff Effect
Experimental Set-up
Analysis Methods and Results
The data can be binned as a function of p0
production angle. There are three backgrounds,
at varying intensities, at all angles. A
horizontal continuum at unit Elasticity, a
vertical mass resonant background at the p0 mass
and a diffuse background. Filtering events
through a p0 invariant mass cut and requiring
good Elasticity, the diffuse background has been
doubly subtracted. A sideband analysis reveals
the amount of over-subtraction. Elasticity
(Energy of cluster pair)/(Energy of Photon)
The axial anomaly is one of the most profound and
fundamental examples of spontaneous symmetry
breaking in quantum mechanics. It arises from
the coupling of quarks and gluons to the U(1)
symmetry of the classical Lagrangian in QCD in
the chiral limit where quark masses approach
zero. If the discussion is limited to the axial
anomaly that couples quarks to photons, the two
lightest quarks flavors (u and d) are considered,
and if still using exact isospin symmetry, the
p0 only couples to the electromagnetic current
qI3gmg5q.
In order to be successful, the PrimEx
Collaboration has very specific, important
experimental requirements. 1.) Photon flux.
a.) Must be known to 1 or better for luminosity
control. 2.) Well understood nuclear targets
a.) rt known to 0.07 or better 3.) A highly
segmented calorimeter for measuring the energy
and angle of decay photons 4.) Semi
mono-chromatic beam of photons, energy 5.75 or
less a.) Knowledge of photon energy and time
of creation. To satisfy these requirements, the
PrimEx collaboration monitored beam luminosity
with the recently installed and commissioned Hall
B pair spectrometer. Absolute photon flux was
monitored by a Total Absorption Counter, and the
collaboration manufactured and commissioned the
HyCal for precise angular and energy measurements
of p0 decay gammas. Veto Counters positioned in
front of the HyCal enables off-line charged
particle rejection of events in HyCal. One of
the nuclear targets employed was Highly
Ordered/Oriented Pyrolytic 12Carbon. The last
major requirement was met by using the Tagged
Photon Facility (Tagger) in Hall B. The tagger
is capable of analyzing a high rate of post
bremmstrahlung electrons, recording both energy
and time information. Electrons in the Tagger
can now be correlated with (hopefully lots of p0)
downstream events.
Photon Flux and Luminosity
Absolute Tagging Ratio. How often does the
tagger measure the electron when the Total
Absorption Counter (TAC) does not. The TAC is
assumed 100 efficient. 5 Xo 12C target in beam.
When coupling to a photon, the isosinglet and
isotriplet electromagnetic currents explicitly
break the symmetry with the axial-vector current.
This symmetry breaking directly affects how a p0
decays into two photons. The resulting decay
amplitude is. Agg aNc/(3pFp). The resulting
zero order p0 lifetime is G(p0-gtgg)
(mp3Agg2)/(64p). The PrimEx Collaboration has
proposed using the Primakoff Effect, photo-pion
production in the Coulomb field of a high Z
nucleus to measure the p0 lifetime.
The Chiral Anomaly
Elasticity vs Invariant Mass for p0 prodcution
angle of 0.00o-0.04o, 0.02o bin center.
Relative Tagging Ratio. Photon flux (luminosity)
at data production (50-150 nA) electron beam
current.
The Primakoff Effect
Preliminary Yields
Aim
HyCal Reconstruction
HyCal reconstruction, at its foundations is very
simple. Using cluster location, energy, and
timing information clusters are paired in all
possible combinations and matched against Tagger
photons in a given event. Only the most likely
candidate per event is selected.
The latest next-to-leading-order Chiral
Perturbation Theory calculation puts the lifetime
of the p0 at 8.10 eV 1. (hep-ph/0206007), well
within the error of the PDG value of 7.74 eV
7.1. However, there is no experimental
measurement commensurate with the precision of
the latest theory. The PrimEx Collaboration
hopes to fill this important gap in our knowledge
of low energy QCD to a precision of 1.0.
Number of p0s
Preliminary
The PrimEx Collaboration has designed,
constructed, commissioned and taken data with a
state-of-the-art Hybrid Calorimeter (HyCAL).
PrimEx recorded data in Hall B at Jefferson Lab
in the Fall of 2004. Preliminary yields from
12Carbon will only be presented.
p0 production angle (degrees)
Detector acceptance corrections, final photon
flux numbers, systematic error calculations must
be made to the data to extract Pi0 lifetime.
Acknowledgements
Advisor Rory Miskimen, PhD. University of
Massachusetts, Amherst, USA This research
supported by MRI grant PHY-0079840
HyCal. The interior array is Lead Tungstate.
The outer array is Lead glass. To first order, a
5x5 clustering algorithm determines total cluster
energy and shower position. Sample clusters
shown as black dots.
Conceptual rendering of PrimEx set-up