Neutral Weak Charge Measurements

In the weak interaction, both charged (W^+-) and neutral (Z⁰) bosons may be exchanged. But while the strength of the weak charged current is related to G_f, the Fermi coupling constant, the strength of the weak neutral current is further dependent on a parameter called the weak mixing angle. Precision measurements of the muon decay rate determine G_f, while precision asymmetry measurements with on-shell Z⁰ intermediate states largely determine our knowledge of the weak mixing angle.

If the Standard Model of electroweak interactions is complete, then it must be self-consistent. For example, measurements of the weak mixing angle made at low energies in atomic parity violation experiments must be consistent with those made at the very high energies using Z⁰ particles. We say "consistent" and not "the same" because the couplings of particles are influenced by virtual particles in the vacuum, and the size of such perturbations is energy dependent. The most familiar example of such small pushes and pulls by ghostly particles occurs in the case of the electromagnetic fine structure constant, alpha. This coupling is 1/137 at low energies, but is 7% stronger at energies where the Z⁰ has been produced. Lower energies correspond to longer distances, where a particle's charge is more screened by a cloud of virtual e⁺e^- pairs. With increasing energy, the reaction occurs further inside this cloud and one sees more of a particle's bare charge. But while the variation of the coupling with energy for the electromagnetic force is reasonably well established, for the weak coupling this was only convincingly demonstrated recently by the SLAC E158 experiment. A low energy program of measurements will help better determine the weak mixing angle and achieve a better understanding of theoretical corrections for the extremely heavy virtual particles which modify weak interactions.

But the most important motivation for such neutral weak-charge measurements is their potential sensitivity to physics beyond the Standard Model. It is fascinating to realize that all the particles of nature are present in the vacuum around us, all the time. Thus, very heavy (TeV-scale) particles which have not yet been observed in high energy accelerator experiments may reveal themselves by their subtle effects in precision low energy measurements. In the JLab Qweak experiment we plan to measure the weak mixing angle in the coupling of an electron to the proton at relatively low energies. Following that experiment and the JLab upgrade to 12 GeV, we plan to remeasure the weak mixing angle in an electron-electron scattering experiment called e2ePV.

The JLab Qweak experiment was approved by the JLab Program Advisory Committee with high priority, has received substantial funding from the US NSF and DOE, the College of William and Mary, as well as NSERC of Canada, and is currently under construction.

Overview Talks (Qweak and e2ePV Experiments):

Qweak: A Measurement of the Proton's Weak Charge*,   August 2007 DNP Meeting    PPT

e2ePV Toroidal Conceptual Design*,   December 2006 ElectroWeak Workshop at JLab    PDF

e2ePV at 12 GeV*,   August 2006 Hall C Workshop    PPT

e2ePV: Next Step in the Evolution of the Qweak Experiment?,   July 2006 Qweak Collaboration Meeting at JLab    PPT

Outlook for an Improved Moeller Measurement*,   May 2006 PAVI06    PPT    (.PS of Proceedings )

New Physics and 0+ --> 0+,   March 2006 Qweak Collaboration Meeting    PPT

The Weak Charge Program at JLab*,   December 2005 grad student pizza seminar    PPT

Measuring the Proton's Weak Charge,   September 2005 DNP    PPT

Seminar on Qweak(proton)*   PS (1st half)   PS (2nd half)

Technical Talks (fused silica detectors, beam diagnostics, etc.):

Main Detector Status,   July 2008 Qweak Collaboration Meeting    PDF and PPT

All 1st Order Beam Modulation in the 3C Line,   July 2008 Qweak Collaboration Meeting    PPTX

Beam Current Monitor Performance in the June 2008 Beam Test,   July 2008 Qweak Collaboration Meeting    PPTX

Recent WBS1 Activities,   January 2008 Qweak Collaboration Meeting    PDF

Main Detector Status,   July 2007 Qweak Collaboration Meeting    PDF and PPT

Main Detector Status,   April 2007 APS    PPT

Main Detector Status,   January 2007 Qweak Collaboration Meeting    PDF

e2ePV Toroidal Magnet Concept*,   December 2006 Electroweak Workshop at TJNAF    PPT

Toroidal e2ePV Briefly,   July 2006 e2ePV organizational meeting    PPT

Bar QA, Glue prototyping, and Overall Status,   July 2006 Qweak Collaboration Meeting    PPT

Proposed Spot Size Monitor (Qweak Goes Nonlinear),   March 2006 Qweak Collaboration Meeting    PPT

Main Detector Status,   March 2006 Qweak Collaboration Meeting    PPT

Fused Silica Detectors for Hall C*,   January 2006 Hall C Meeting    PPT

Main Detector Status,   October 2005 Qweak Collaboration Meeting    PDF

Voltage Divider Design and Overall Status,   April 2005 Qweak Collaboration Meeting and APS talk    PPT

Detector Beam Test Results,   August 2004 Qweak Collaboration Meeting    PPT

Transmission/Prototype Update,   October 2003 Qweak Collaboration Meeting    PS

Proposed Quartz Bar Tests in the HMS,   May 2003 Qweak Collaboration Meeting    PDF

First Quartz Cerenkov talk,   March 2002 Qweak Collaboration Meeting    PS    PDF

Technical Documents (fused silica detectors, beam diagnostics, experiment requirements, etc.):

If There is Unique Neutral Axis, How Do We Find It?,   June 2008 Technical Note    PDF

Proposed Beam Time Reduction Scenarios for Qweak,   January 2008 draft response to final Jeopardy PAC    PS

Qweak Lightguide Specifications,   June 2006 (Procurement Document)    PS

Microwave Cavity Spot Size Monitor for Qweak,   January 2006 Tech Note    PS

TRIUMF Preamplifier Irradiation Test,   December 2005 Tech Note    PS

How Should We Calculate the Asymmetry for a Single Bar?,   November 2005 Tech Note    PS

Qweak Fused Silica Radiator Specifications,   July 2005 (Procurement Document)    PS

First Look at Test Ticket Data for Qweak D753WKB PMT's,   June 2005 Tech Note    PS

PMT Selection and First Prototype Divider Design,   May 2005 Tech Note    PS

Error Update,  March 2005 Tech Note    PS

A Future JLab Moeller experiment,   December 2004 (internal e2ePV working group report, update of 2002 version)    PS

Detector Nonlinearity Specification,   September 2004 Tech Note    PS

For further information about our JLab proton experiment (proposal, publications, collaboration, etc.), see the JLab Qweak(proton) home page