Download ppt - Experimental Subatomic Physics

Exploiting the Proton’s Weakness
L. Cobus, A. Micherdzinska, J. Pan, P.Wang, and J.W. Martin
University of Winnipeg, Winnipeg, R3B 2E9, Canada
Introduction
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 Lab’s
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.
New Physics??
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.
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
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.
Scanning Detector
Detector
Laser Positioning System
The Qweak Experiment
The Standard Model of Physics
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.
Q: The weak force is… weak. How can it be measured when all
the other forces are always dominating?
To ensure that the scanner position is known accurately at all times, a
laser system was constructed to test and calibrate existing positioning
software.
Scanner passes
between laser
and photodiode
(light sensor)
A: By exploiting the fact that the weak force is the only force
with parity-violating asymmetry.
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.
electron with right-
electron with
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:
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.
left-handed spin
mirror
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.
Las e r be am c o mple te ly blo c ke d by s c anne r
8 vo lts indic ate s a
scanner
position
s pe specific
c ific s c anne
r
po s itio n
Error in pos ition
± 10 microns ,
or be tte r
Although the weak force is weak compared to the other forces,
we can separate it out by finding the asymmetric processes.
Las e r be am unblo c ke d
10 microns
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 proton’s weak charge:
 R  L
A
 R  L


  GF  2 p
4
2

Q Q weak  Q BQ 

 4 2 
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