UMass Nuclear Physics Group is part of the Primex Primakoff Experiment Collaboration which will perf

1
UMass Nuclear Physics Group is part of the Primex
(Primakoff Experiment) Collaboration which will
perform a high precision measurement (1.4) of
the two photon decay width of the neutral pion
(p) in order to get an accurate value of its
lifetime and test one of the most fundamental
predictions of low energy quantum chromo dynamics.
Created by Pjerin Luli UMass-Amherst /
Jlab-Summer 2004 E-mail pluli_at_student.umass.edu
Introduction
Primex Setup
Primex Theory
Primakoff Effect
Photons from a photon tagging facility will
collide with the virtual photons produced by the
Coulomb Field of a nucleus to produce neutral
pions. This is called the Primakoff Efect. The
neutral pion will decay into two photons p???
This is Primakoff cross section where ? is the
p decay width and the rest are kinematic
variables.
In order to identify and extract the Primakoff
amplitude, the experimental setup must have
sufficient angular resolution for forward p
production. Both photons from the p??? decay
will be detected in the electromagnetic hybrid
calorimeter (HYCAL). Therefore the p angular
resolution will depend strongly on the position
and energy measurement accuracy of the
calorimeter. HYCAL is a state of the art detector
designed to provide the needed precise
measurements. It will consist of an array of 576
modules of lead glass detectors located in the
peripheral part of the calorimeter, and 1152
modules of lead tungstate scintillating crystals
in the central region with a hole in the middle
for the passage of the photon beam through the
calorimeter. (Summer 2003)
Predictions Evaluations
Primex Trigger Electronics
The electronic trigger developed for HYCAL
requires the detection of two photons with
energies greater than 0.5 GeV. This provides high
acceptance for p events and high efficiency for
background rejection. Since the p production
rate is small, the fast trigger will provide a
quick and accurate evaluation of potential p
events.
Kinematical considerations enable one to separate
the Primakoff effect from other p production
mechanisms. This is the angular behavior of the
electromagnetic and nuclear p cross sections for
Pb at 5.6 GeV. Primakoff cross section peaks at
extremely small angles!
Several experimental techniques have been used
previously to measure the p width, but they
stand far from the theoretical one. The dashed
horizontal line is the leading order prediction.
The shaded band is the recent next-to leading
order prediction. The expected value from Primex
agrees with the theoretical level!
The core of the trigger electronics system is a
modular electronic system called CAMAC which has
built in a power unit and a bus. A computer
program at the Counting Room communicates with
CAMAC in the experimental hall through an
Ethernet device controller. The signals from
HYCAL will be discriminated by remotely
programmable Digital to Analog Converter and
Phillips Scientific Discriminator modules.
Memory Logic Unit modules are used to identify
specific patterns in the HYCAL. They can be set
to a mode that contains 256 memory addresses and
programmed to determine the two particle
coincidence.
Jefferson Lab in Virginia

• Primex will take place at Thomas Jefferson
  National Accelerator Facility or Jefferson lab,
  which is a basic research laboratory built to
  probe the nucleus of the atom to learn more about
  the quark structure of matter. The Continuous
  Electron Beam accelerator Facility (CEBAF) is the
  heart of Jefferson Lab.

Acknowledgements Special thanks to UMass group
for the support David Lawrence (who helped in
creating the poster), Eric Clinton, Rory
Miskimen, Daniel Pomeroy, and to my family for
always being close to me yet so far, and my
girlfriend for her loving and caring support.