1. Exploiting the Proton’s Weakness L. Cobus, A. Micherdzinska, J. Pan, P.Wang, and J.W. Martin The Standard Model of Physics New Physics??IntroductionFocal Plane Scanner University of Winnipeg, Winnipeg, R3B 2E9, Canada 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. 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 proton’s weak charge: 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 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. 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: 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 Qweak Experiment Parity violation: when an interaction between particles does not have the same strength as its mirror-image interaction. electron with left-handed spin 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 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) For more information… 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. 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. specific scanner position mirror electron with right- handed spin UWinnipeg subatomic physics group website: http://nuclear.uwinnipeg.ca The Qweak website: http://www.jlab.org/qweak 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. Focal plane scanner installation at Jefferson Lab: 2009 Running of Qweak: 2010 – 2012