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First Determination of the Weak Charge of the Proton

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Using a unique technique, the Q-weak collaboration has found the value of the weak force to be in agreement with theory—so far.

The basic experimental design showing the target, collimation, magnet coils, electron trajectories, and detectors. Elastically scattered electrons (red tracks) focus at the detectors while inelastically scattered electrons (not shown), are swept away from the detectors (to larger radii). The distance along the beamline from the target center to the center of the quartz bar detector array is 12.2 m.

The basic experimental design showing the target, collimation, magnet coils, electron trajectories, and detectors.
Elastically scattered electrons (red tracks) focus at the detectors while inelastically scattered electrons (not shown), are swept away from the detectors (to larger radii). The distance along the beamline from the target center to the center of the quartz bar detector array is 12.2 m.

The Qweak experiment has measured the parity-violating asymmetry in polarized e-p elastic scattering at Q2 = 0.025(GeV/c)2, employing 145 microamps of 89% longitudinally polarized electrons on a 34.4cm long liquid hydrogen target at Jefferson Lab.
The results of the experiment’s commissioning run are reported here, constituting approximately 4% of the data collected in the experiment.
From these initial results the measured asymmetry is Aep = -279 ± 35 (statistics) ± 31 (systematics) ppb, which is the smallest and most precise asymmetry ever measured in polarized e-p scattering.
The small Q2 of this experiment has made possible the first determination of the weak charge of the proton, QpW, by incorporating earlier parity-violating electron scattering (PVES) data at higher Q2 to constrain hadronic corrections.
The value of QpW obtained in this way is QpW(PVES) = 0.064 ± 0.012, in good agreement with the Standard Model prediction of QpW(SM) = 0.0710 ± 0.0007.
When this result is further combined with the Cs atomic parity violation (APV) measurement, significant constraints on the weak charges of the up and down quarks can also be extracted.
That PVES+APV analysis reveals the neutron’s weak charge to be QnW(PVES+APV) = -0.975 ± 0.010.

Read more at http://arxiv.org/pdf/1307.5275v2.pdf

Read also: “Just how weak is the weak force?

Written by physicsgg

September 22, 2013 at 4:13 pm

Posted in NUCLEAR PHYSICS

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