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| - New physics is expected at higher energy scales. Like Weak processes in the standard model, effects at low energies will be suppressed by the mass scale. - This suppression makes these effects much smaller than Strong or E+M processes and would be invisible without impossibly precise measurements if they did not also have a signature of a fundamentally different character. For Standard Model Weak interactions a chiral coupling gives a parity-violating psuedo-vector current, while Stong and E+M processes preserve this chiral symmetry. So any measure of parity violation is a direct measure of Weak physics. - Similarly new physics can violate P or CP symmetry, so looking for these violations provides direct evidence of new fundamental processes. Since P violation also occurs from the Weak sector of the Standard Model, these measurements require high precision to detect new effects. CP violation for low energy processes is expected to be unmeasurably low from Standard Model effects, so any detection of such CP (or T) violation is more likely new physics. These effects are also predicted to be very small, so precision measurements are again required here. Even high-energy experiments, though now able to directly probe weak scale processes, are performed at an energy scale far below that of new physics and therefore must also precisely study symmetry-breaking effects for new insights. - New effects should be largest in corrections to the Z propagator, the oblique corrections (such as loops involving new, possibly supersymetric, SU(2) doublets), and in direct corrections, that are possible new tree level interactions, such as a new Z boson , SU(5) leptoquarks, or other possible new gauge bosons. Propagator corrections are given by a modified self energy with typical contributions conventionally paramaterized by S, T, and U. |  | |||||||||||
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| For example, for a new SU(2) doublet with small mass splitting, the 1-loop diagram gives: | ||||||||||||
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| Notice that U is much smaller than T, where N is a new particle and Z is the familiar Weak boson. Also both T and U vanish in the case of full isospin symmetry. | ||||||||||||