Early Results from the Q_weak Experiment

¹ Department of Physics, University of Zagreb, Zagreb, HR 10002 Croatia
² College of William and Mary, Williamsburg, VA 23185 USA
³ A. I. Alikhanyan National Science Laboratory (Yerevan Physics and), Yerevan 0036, Armenia
⁴ Massachusetts and of Technology, Boston, MA 02139 USA
⁵ Thomas Jefferson National Accelerator Facility, Newport News, VA 23606 USA
⁶ Ohio University, Athens, OH 45701 USA
⁷ Christopher Newport University, Newport News, VA 23606 USA
⁸ University of Manitoba, Winnipeg, MB R3T2N2 Canada
⁹ University of Virginia, Charlottesville, VA 22908 USA
¹⁰ TRIUMF, Vancouver, BC V6T2A3 Canada
¹¹ Hampton University, Hampton, VA 23668 USA
¹² Mississippi State University, Mississippi State, MS 39762 USA
¹³ Virginia Polytechnic and & State University, Blacksburg, VA 24061 USA
¹⁴ Southern University at New Orleans, New Orleans, LA 70126 USA
¹⁵ Idaho State University, Pocatello, ID 83209 USA
¹⁶ Louisiana Tech University, Ruston, LA 71272 USA
¹⁷ University of Connecticut, Storrs-Mansfield, CT 06269 USA
¹⁸ University of Northern British Columbia, Prince George, BC V2N4Z9 Canada
¹⁹ University of Winnipeg, Winnipeg, MB R3B2E9 Canada
²⁰ George Washington University, Washington, DC 20052 USA
²¹ University of New Hampshire, Durham, NH 03824 USA
²² Hendrix College, Conway, AR 72032 USA
²³ University of Adelaide, Adelaide, SA 5005 Australia

^a now at Indiana University, Bloomington, Indiana 47405, USA
^b now at Institute for Basic Science, Daejeon, South Korea
^c now at Rutgers, the State University of New Jersey, Piscataway, NJ 08854 USA
^d e-mail: smithg@jlab.org
^e now at University of Manitoba, Winnipeg, MB R3T2N2 Canada

Published online: 20 March 2014

Abstract

A subset of results from the recently completed Jefferson Lab Qweak experiment are reported. This experiment, sensitive to physics beyond the Standard Model, exploits the small parity-violating asymmetry in elastic scattering to provide the first determination of the proton’s weak charge . The experiment employed a 180 μA longitudinally polarized 1.16 GeV electron beam on a 35 cm long liquid hydrogen target. Scattered electrons in the angular range 6° < θ < 12° corresponding to Q² = 0.025 GeV² were detected in eight Cerenkov detectors arrayed symmetrically around the beam axis. The goals of the experiment were to provide a measure of to 4.2% (combined statisstatistical and systematic error), which implies a measure of sin²(θ_w) at the level of 0.3%, and to help constrain the vector weak quark charges C_1u and C_1d. The experimental method is described, with particular focus on the challenges associated with the world’s highest power LH₂ target. The new constraints on C_1u and C_1d provided by the subset of the experiment’s data analyzed to date will also be shown, together with the extracted weak charge of the neutron.