About The HAPPEX Collaboration

Since the mid-1990s, the HAPPEX collaboration has carried out numerous experiments utilizing the technique of parity violating electron scattering.

Thanks to the high-polarization electron beam provided by the Continuous Electron Beam Accelerator Facility (CEBAF) at Jefferson Labs, the HAPPEX collaboration has been able to make very precise measurements in relatively short periods of time.

The main science program of the collaboration aimed at measuring the strange quark contribution to the electromagnetic properties of nucleons. When combined with results from other experiments such as G0, SAMPLE and A4, the HAPPEX measurements have constrained such contributions to a few percent, but are consistent with zero to within 2-sigma.

Over time, the science goals evolved to include measurements of the neutron distribution of heavy nuclei (PREX) and the weak charge of the electron (PVDIS and MOLLER).

  • Thomas Jefferson National Accelerator Facility (Jefferson Lab)

    JLab is one of 17 national laboratories funded by the U.S. Department of Energy. The lab also receives support from the City of Newport News and the Commonwealth of Virginia.

    The lab’s primary mission is to conduct basic research of the atom's nucleus using the lab’s unique particle accelerator, known as the Continuous Electron Beam Accelerator Facility (CEBAF). Jefferson Lab also conducts a variety of research using its Free-Electron Laser, which is based on the same electron-accelerating technology used in CEBAF.

    In addition to its science mission, the lab provides programs designed to help educate the next generation in science and technology, and to engage the public.

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  • Hall A Collaboration

    The present base instrumentation in Hall A has been used with great success for experiments which require high luminosity and high resolution in momentum and/or angle for at least one of the reaction products.

    The Hall A spectrometers have been designed for detailed investigations of the structure of nuclei, often utilizing the (e, e'p) reaction.

    An extensive program has been developed to study the spin structure of the neutron using a polarized 3He target. The strange quark contributions to the charge and magnetization distributions of the nucleons have been investigated via very precise parity-violating electron scattering experiments. Together these experiments provide stringent tests of nucleon structure models.

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  • Experiments Planned for the Near Future

    The MOLLER experiment at JLab proposes to measure the parity-violating asymmetry in electron-electron (Møller) scattering. The measurement will be carried out with a longitudinally polarized electron beam that has been accelerated to 11 GeV at CEBAF, and observing the resulting fractional difference in the probability of electrons scattering off atomic electrons in a liquid hydrogen target.

    The asymmetry is proportional to the weak charge of the electron, which in turn is a function of the electroweak mixing angle, a fundamental parameter of the electroweak theory. The accuracy of the proposed measurement will provide a value of the mixing angle with a precision on par with the two single best measurements of the same parameter at electron-positron colliders.

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