The scattering of energetic electrons from atomic nuclei can be described to a good approximation by the process in which a single photon is exchanged between the electron and the target. It was through this process that experimenters at SLAC during the 1960's uncovered the existence of quarks inside the proton, an important step in the development of the Standard Model. Today the Standard Model is accepted as the fundamental theory of the structure and interactions of matter (excluding gravity) up to the highest energies that have been accessible to experiment up to the present. Electron-proton scattering at the high-energy frontier from the DESY laboratory in Germany have recently yielded hints of new physics - new kinds of particles and interactions - but until these results are confirmed, the Standard Model stands as a concise summary of all that is known about physics at the most fundamental level.
Electron scattering is called a neutral current process because the reaction leaves the charge of the electron unchanged. In the Standard Model, there are two kinds of fields that couple to neutral currents, the photon and the Z0 boson. Because the Z0 is much heavier than the photon, it is classified under the weak interactions. Weak neutral currents are generally obscured by the dominant electromagnetic process in reactions like electron scattering where both contribute. It is possible however, by comparing the scattering rate of electrons with left-handed polarisation with that of right-handed electrons, to detect small differences due to Z0 exchange. This is because the photon couples in a symmetrical way to electrons of both helicities, whereas the weak neutral ``charge'' depends upon helicity. This dependence on helicity is what is responsible for parity violation in electron scattering.
Using the known electric and weak charges of quarks and leptons in the Standard Model, a measurement of the parity-violating asymmetry in polarised electron scattering yields direct information about the distribution of the different flavours of quarks inside the nucleon. Alternatively, what we know about the total quark content of the nucleon can be used to extract the weak neutral charges of the quarks from a parity-violation measurement. If the measurement is made with sufficient precision, a comparison of the extracted weak neutral charges with the Standard Model values can be used to probe for certain kinds of new physics. Assuming that the present generation of parity experiments at the Bates, Jefferson and SLAC laboratories reach their design goals, this approach can test many of the interesting extensions to the Standard Model, including additional Z0 bosons, leptoquarks and quark/lepton compositeness, at a level that is competitive with collider experiments. Click on the link below to find out more about work going on at Jefferson Lab towards the preparation of a proposal.
| Qweak | (JLab E02-020) | Standard Model test using parity-violating elastic ep scattering |
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