Exploiting the Proton

1
Exploiting the Protons Weakness
L. Cobus, A. Micherdzinska, J. Pan, P.Wang, and
J.W. Martin
University of Winnipeg, Winnipeg, R3B 2E9, Canada
New Physics??
Introduction
Focal Plane Scanner
The Standard Model is still being tested. One
of the main ways to test the theory is to make
precise measurements of the properties of a
particle, and to compare the experimental results
with the theory. At the Thomas Jefferson
National Accelerator Facility in Virginia, US.,
an experiment called Qweak will precisely measure
the weak charge of the proton. If experimental
results are different from the Standard Model
predictions, we could be seeing new physics for
example, the existence of a new particle! And if
experimental results agree with the theory, we
have placed stringent restraints on new particle
physics theories.
The Standard Model is a theory that describes
fundamental particles and how they interact. The
theory is very successful, but it is still
incomplete. The Qweak experiment at the Thomas
Jefferson National Accelerator Facility in
Virginia, US, will test the Standard Model by
using electron-proton scattering to infer the
weak charge of the proton. The University of
Winnipeg is building a scanner to monitor
Jefferson Labs Cherenkov electron detectors. I
constructed a laser position detection system to
precisely track scanner position the sensor
system succeeded in tracking position to
one-tenth of a millimeter.
The Cherenkov bars that detect the scattered
electrons at Jefferson Lab must be monitored to
ensure that they are operating correctly. The
University of Winnipeg is building a small focal
plane scanning detector to do this. A 2D motion
robot will move the scanning detector around to
check the rate of particles hitting the Cherenkov
bars.
Cherenkov Detector
Scanning Detector
Laser Positioning System
The Qweak Experiment
The Standard Model of Physics
To ensure that the scanner position is known
accurately at all times, a laser system was
constructed to test and calibrate existing
positioning software.
Q The weak force is weak. How can it be
measured when all the other forces are always
dominating? A By exploiting the fact that the
weak force is the only force with
parity-violating asymmetry.
The Standard Model is a theory of fundamental
particles and how they interact. Developed in
the 1970s, it has since successfully predicted
the results of many particle physics experiments.
Several aspects of the theory remain to be
tested.
Scanner passes between laser and photodiode
(light sensor)
Parity violation when an interaction between
particles does not have the same strength as its
mirror-image interaction.
For example electrons that are mirror-images of
each other (below) do not interact with protons
in exactly the same way, due to the weak force.
The laser was tested by gradually blocking the
laser light as shown above. The plot of voltage
versus position (right) shows that 8V indicates
precisely when the blocker is at a certain
position.
electron with right-handed spin
electron with left-handed spin
Particles are categorized based on properties
such as mass and charge. Charge denotes how
strongly a particle interacts with other
particles through a specific force
mirror
Although the weak force is weak compared to the
other forces, we can separate it out by finding
the asymmetric processes. In the Qweak
experiment, electrons with a particular spin are
scattered off protons and the scattering rate
measured. The electron spin direction is then
reversed and the experiment done again. This is
equivalent to performing the mirror experiment
the two results will be slightly different due to
the weak force. From the resulting asymmetry
(A), we can extract the protons weak charge
The Strong Force particles called hadrons (e.g.
protons) are made up of particles called quarks,
which are bound together by the strong
force. The Electromagnetic Force responsible
for the attraction of electrons to protons to
form atoms. Individual atoms can also be
attracted or repelled. The Weak Force although
it is extremely weak, this force acts on almost
all types of particles. It is responsible for
nuclear decays. Gravitation Force it is
currently unknown how to incorporate gravity into
the Standard Model, however many new theories aim
at doing so.
Focal plane scanner installation at Jefferson
Lab 2009 Running of Qweak 2010 2012
For more information
UWinnipeg subatomic physics group website
http//nuclear.uwinnipeg.ca The Qweak website
http//www.jlab.org/qweak