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First Determination of the Weak Charge of the Proton

First Determination of the Weak Charge of the Proton
Author: Fang Guo (Ph. D.)
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Total Pages: 188
Release: 2016
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The Qweak experiment measures the parity violating asymmetry in elastic e[upper script right arrow]p scattering at Q2 = 0.02455 (GeV/c)2 with a 180 [mu]A and 88.7% longitudinally polarized electron beam of 1.165 GeV and a 34.4 cm liquid hydrogen target in experimental Hall C at Jefferson Lab. As the Q2 at the Qweak experiment is much lower than all previous experiments, the Qweak measurement can be considered as the first direct determination of the weak charge of the proton, Qp/w, which is related to the weak mixing angle, sin2 [theta]w, an important Standard Model parameter. The preliminary and blinded asymmetry of the Run 2 data set, which constitutes approximately 60% of all the data collected in the experiment, is Aep/PV = -232.7 +/- 8.7 (stat) +/- 6.4 (syst) ppb. The value of Qp/w obtained by fitting this blinded asymmetry and earlier parity violating electron scattering (PVES) data at higher Q2 is Qp/w (PVES) = 0.0705 +/- 0.0051. When combining this result with the 133Cs atomic parity violation (APV) measurement, further constraints can be placed on the neutral weak quark coupling constants C1u and C1d. The combined PVES and APV analysis yields the blinded neutron's weak charge to be Qn/w (PVES+APV) = -0.9798 +/- 0.0065. The Qweak experiment also measures the parity violating asymmetry in nonresonance inelastic e[upper script right arrow]p scattering at Q2 ~/= 0.09 (GeV/c)2 with 3.35 GeV electron beams to provide inputs for the [gamma]Z box calculation. The preliminary measured inelastic asymmetry is Aep/inelastic-PV = 2.91 +/- 0.35 ppm.

The Qweak Experiment

The Qweak Experiment
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Total Pages:
Release: 2015
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The $Q_{weak}$ Collaboration has completed a challenging measurement of the parity-violating asymmetry in elastic electron-proton ($\vec{e}$p) scattering at the Thomas Jefferson National Accelerator Facility (Jefferson Lab). The initial result reported here is extracted from the commissioning part of the experiment, constituting about 4% of the full data set. The parity-violating asymmetry at a low momentum transfer $Q^2$=0.025 GeV$^2$ is $A_{ep}$ = -279 $\pm$ 35 (stat) $\pm$ 31 (syst) ppb, which is the smallest and most precise asymmetry ever measured in $\vec{e}$p scattering. This result allowed the first determination of the weak charge of the proton $Q_W^p$ from a global fit of parity-violating elastic scattering (PVES) results from nuclear targets, where earlier data at higher $Q^2$ constrain uncertainties of hadronic structure. The value extracted from the global fit is $Q_W^p$ (PVES) = 0.064 $\pm$ 0.012, in agreement with the standard model prediction $Q_W^p$ (SM) = 0.0710 $\pm$ 0.0007. The neutral weak charges of up and down quarks are extracted from a combined fit of the PVES results with a previous atomic parity violation (APV) measurement on $^$Cs. The analysis of the full $Q_{weak}$ data is ongoing and expected to yield a value for the asymmetry within 10 ppb of precision. Because of the suppression of $Q_W^p$, such a high precision measurement will place significant constraints to models of physics beyond the standard model.

Measuring the Weak Charge of the Proton Via Elastic Electron-Proton Scattering

Measuring the Weak Charge of the Proton Via Elastic Electron-Proton Scattering
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Total Pages: 251
Release: 2015
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The Qweak experiment which ran in Hall C at Jefferson Lab in Newport News, VA, and completed data taking in May 2012, measured the weak charge of the proton QpW via elastic electron-proton scattering. Longitudinally polarized electrons were scattered from an unpolarized liquid hydrogen target. The helicity of the electron beam was flipped at approximately 1 kHz between left and right spin states. The Standard Model predicts a small parity-violating asymmetry of scattering rates between right and left helicity states due to the weak interaction. An initial result using 4% of the data was published in October 2013 [1] with a measured parity-violating asymmetry of -279 ± 35(stat) ± 31 (syst) ppb. This asymmetry, along with other data from parity-violating electron scattering experiments, provided the world's first determination of the weak charge of the proton. The weak charge of the proton was found to be pW = 0.064 ± 0.012, in good agreement with the Standard Model prediction of pW(SM) = 0.0708 ± 0.0003[2].

Q Weak

Q Weak
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Release: 2017
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The Qweak experiment, which took data at Jefferson Lab in the period 2010 - 2012, will precisely determine the weak charge of the proton by measuring the parity-violating asymmetry in elastic e-p scattering at 1.1 GeV using a longitudinally polarized electron beam and a liquid hydrogen target at a low momentum transfer of Q2 = 0.025 (GeV/c)2. The weak charge of the proton is predicted by the Standard Model and any significant deviation would indicate physics beyond the Standard Model. The technical challenges and experimental apparatus for measuring the weak charge of the proton will be discussed, as well as the method of extracting the weak charge of the proton. Finally, the results from a small subset of the data, that has been published, will also be presented. Furthermore an update will be given of the current status of the data analysis.

Qweak

Qweak
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Total Pages: 8
Release: 2014
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The Qweak experiment at Hall C of Thomas Jefferson National Accelerator Facility has made the first direct measurement of the weak charge of the proton, QWp, through a precision measurement of the parity-violating asymmetry in elastic e-p scattering at low momentum transfer Q2= 0.025 (GeV/c)2 with incident electron beam energy of 1.155 GeV. The Qweak experiment, along with earlier results of parity violating elastic scattering experiments, is expected to determine the most precise value of QWp which is suppressed in the Standard Model. If this result is further combined with the 133Cs atomic parity violation (APV) measurement, significant constraints on the weak charge of the up quark, down quark, and neutron can be extracted. This data will also be used to determine the weak-mixing angle, sin2?W, with a relative uncertainty of

The Weak Charge of the Proton

The Weak Charge of the Proton
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Release: 2015
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The Qweak experiment, which completed running in May of 2012 at Jefferson Laboratory, has measured the parity-violating asymmetry in elastic electron-proton scattering at four-momentum transfer Q^2=0.025 (GeV/c)^2 in order to provide the first direct measurement of the proton?s weak charge, Qpw. The Standard Model makes firm predictions for the weak charge; deviations from the predicted value would provide strong evidence of new physics beyond the Standard Model. Using an 89% polarized electron beam at 145 microA scattering from a 34.4 cm long liquid hydrogen target, scattered electrons were detected using an array of eight fused-silica detectors placed symmetric about the beam axis. The parity-violating asymmetry was then measured by reversing the helicity of the incoming electrons and measuring the normalized difference in rate seen in the detectors. The low Q^2 enables a theoretically clean measurement; the higher order hadronic corrections are constrained using previous parity-violating electron scattering world data. The experimental method will be discussed, with recent results constituting 4% of our total data and projections of our proposed uncertainties on the full data set.