First results from the Qweak measurement of the proton's weak charge

Conference · Mon Oct 01 00:00:00 EDT 2012

OSTI ID:1993492

Dalton, Mark

The Qweak experiment has made the first direct measurement of the weak charge of the proton, $$Q^p_W$$, through precision measurement of the parity-violating asymmetry in elastic scattering of longitudinally polarized electrons from unpolarized protons. The data were taken in Hall C at Jefferson Laboratory, during 12 months of running over a 2 year period, ending in May 2012. A beam current of up to 180 $$\mu$$A on a 35 cm long liquid-hydrogen target produced a scattered electron flux of $$\sim$$6.5 GHz incident on the detectors with Q$$^2\sim0.026\ (\mathrm{GeV}/c)^2$$. It is expected that the weak charge of the proton will be extracted with a 4.1\% combined statistical and systematic uncertainty. The proton weak charge is suppressed in the Standard Model so that these data may be used to determine the weak mixing angle, $$\sin^2\theta_W$$, with a relative uncertainty of 0.3\%, providing a competitive measurement of the running of $$\sin^2\theta_W$$ to low $Q^2$. In addition, these data will impose a strong constraint on a combination of the vector weak charges of the $$u$$ and $$d$$ quarks, $$C_{1u}$$ and $$C_{1d}$$, which is distinct from and complimentary to the combination from measurements of parity violation in atomic systems, and thereby provide an independent constraint on physics beyond the Standard Model. A first look at a subset of the data will be presented.

• Research Organization: Thomas Jefferson National Accelerator Facility (TJNAF), Newport News, VA (United States)
• Sponsoring Organization: USDOE Office of Science (SC), Nuclear Physics (NP)
• DOE Contract Number: AC05-06OR23177
• OSTI ID: 1993492
• Report Number(s): JLAB-PHY-12-1730; DOE/OR/23177-6848
• Resource Relation: Conference: 2012 APS DNP Fall Meeting, Newport Beach, CA, October 24, 2012
• Country of Publication: United States
• Language: English