The Q-weak Experiment – A Precision Test of the Standard Model: Determination of the Weak Charges of the Quarks through Parity-Violating Electron Scattering

Dr. Roger Carlini

Jefferson National Laboratory

 The Electroweak Standard Model (SM) has to date been enormously successful. The search for a fundamental description of nature which goes beyond the SM is driven by two complementary experimental strategies. The first is to build increasingly energetic colliders, such as the Large Hadron Collider (LHC) at CERN, to excite matter into a new form. The second approach is to perform high precision measurements where an observed discrepancy with the SM would reveal the signature of new forms of matter. The “Qweak experiment”, currently under construction, is Jefferson Laboratory's contribution to this program. The experiment will provide the first precision measurement of the proton's weak charge, QpWeak = 1 - 4sin2qW and when combined with other PV measurements a determination of the weak charges of the quarks. The measurement will constitute one of the most rigorous tests of the Standard Model at low Q2. The Standard Model makes a firm prediction of the proton's weak charge, based on the running of the weak mixing angle from the Z0 pole down to low energies, corresponding to a 10 sigma effect in our measurement. A deviation from this prediction will be a signal of new physics, whereas agreement will place constraints on possible Standard Model extensions. A recent global analysis using existing modern PVES and APV measurements has already led to the most precise determination of the weak charges of the quarks hitherto possible and constrains the possibility of new parity violating physics to an energy scale of order one TeV or higher — a factor of two above previous limits, which were dominated by atomic parity violation (APV) data. The Qweak experiment will push up this energy scale limit by yet another TeV.