Physics at Virginia
"Parity Violating Electron Scattering Experiments"Dr. Rakitha Beminiwattha , Syracuse University [Host: Nilanga Liyanage]
ABSTRACT:
Parity Violating Electron Scattering (PVES) is an extremely successful precision frontier tool that have been using for testing the Standard Model (SM) and understanding nucleon structure. Historically, 1978 pioneering Prescott experiment at SLAC was the first successful PVES experiment that confirmed the electroweak theory of particle physics developed by S. Glashow, S. Weinberg and A. Salam as the SM of the particle physics. Several generations of highly successful PVES programs (SAMPLE, A4, HAPPEX, G0 and SLAC E158 programs) have contributed to understanding of nucleon structure and testing the SM. But missing phenomena like matter antimatter asymmetry, neutrino flavor oscillations, and dark matter and energy suggest that the SM is only a “low energy” effective theory. Precision PVES measurements of SM predicted quantities can be used to constrain or discover new physics models beyond the SM. In nuclear physics the “EMC effect” has not yet been properly explained. Therefore an important question in hadronic physics is how protons and neutrons are modified when they are bound in a nucleus. A precise measurement of the neutron skin thickness which is a fundamental test of nuclear theory, will pin down the density dependence of the symmetry energy of neutron rich nuclear matter which has impacts on heavy ion collisions and neutron stars. The current and next generation PVES experiments will provide answers to these questions. Qweak and SoLIDPVDIS will provide a precision test of the Standard Model through an unique constraints on neutral electronquark effective couplings while MOLLER will provide same through neutral electronelectron coupling. The Lead Radius Experiment PREX will provide a clean measurement of the neutron skin thickness. I will introduce PVES experimental techniques and discuss current generation PVES Qweak and PREX experiments and next generation PVES SoLIDPVDIS and MOLLER experiments. REX experiments and next generation PVES SoLIDPVDIS and MOLLER experiments. The “EMC effect,” was some of the first data observed that there is a significant change on the quark level and that the identities of the nucleons are different. It showed a depletion of quarks in the valence region and despite sophisticated modeling, cannot be described by simple binding effects. Despite decades of theoretical efforts, a rigorous explanation has been elusive. A precise measurement of neutron skin thickness which is a fundamental test of nuclear theory, will pins down the density dependence of the symmetry energy of neutron rich nuclear matter which has impacts on heavy ion collisions and neutron star structure.and understanding the nuclear medium effects on structure of quarks. PVES program at Jefferson Lab For Qweak After a decade of preparations, the \Qweak experiment at Jefferson Lab is making the first direct measurement of the weak charge of the proton, $\mathrm{Q^p_W}$. This quantity is suppressed in the \acl{acrosm} making a good candidate for search for new physics beyond the \acs{acrosm} at the TeV scale. Operationally, we measure a small (about $\rm 0.200\ ppm $) parityviolating asymmetry in elastic electronproton scattering in integrating mode while flipping the helicity of the electrons 1000 times per second. Commissioning took place Fall 2010, and we finished taking data in early summer 2012. This dissertation is based on the data taken on an initial two weeks period (Wien0). It will provide an overview of the \Qweak apparatus, description of the data acquisition and analysis software systems, and final analysis and results from the Wien0 data set. The result is a $\mathrm{16\%}$ measurement of the parity violating electronproton ($\vec{\rm e}\rm p$) scattering asymmetry, $\rm A = 0.2788 \pm 0.0348 (stat.) \pm 0.0290 (syst.)\ ppm$ at $\rm Q^2 = 0.0250 \pm 0.0006\ (GeV)^2$. From this a $\rm 21\% $ measurement of the weak charge of the proton, $\rm Q_w^p(msr)=\ +0.0952 \pm 0.0155 (stat.) \pm 0.0131 (syst.) \pm 0.0015 (theory) $ is extracted. From this a $\rm 2\% $ measurement of the weak mixing angle, $\rm sin^2\hat{\theta}_W(msr)= +0.2328 \pm 0.0039 (stat.) \pm 0.0033 (syst.) \pm 0.0004 (theory)$ and improved constraints on isoscalar/isovector effective coupling constants of the weak neutral hadronic currents are extracted. These results deviate from the \acl{acrosm} by one standard deviation. The Wien0 results are a proof of principle of the \Qweak data analysis and a highlight of the road ahead for obtaining full results. For SoLID A new proposal to measure parity violating deep inelastic asymmetry (PVDIS) will provide a precision test of the Standard Model through an unique constraints on axialvector neutral electronquark effective couplings and a precision measurement of the weak mixing angle. We plan to measure the asymmetry to about $\rm 0.5 \% $ precision over the Bjoken x from 0.3 to 0.7 where the PV asymmetry is approximately independent of hadronic structure. If any deviations observed, the residual hadronic and new physics contributions in the asymmetry could be separated by kinematic dependence of the PVDIS asymmetry. At the proposed precision level, asymmetry will be sensitive to novel hadronic structure effects including charge symmetry violation (CSV) effects and higher twist effects. Observation of these effects itself will be beneficial to building better theoretical models. A deuterium target will be used for Standard Model tests while a d/u quark distribution ratio measurement is proposed using a hydrogen target. The broad reach in kinematic and high statistical precision at which PVDIS asymmetries are planned to measure, a large angle acceptance, high luminosity, and large azimuthal acceptance device is a necessity. We are proposing a new spectrometer based on a solenoid magnet called Solenoidal Large Intensity Device (SoLID) that will provide broad range of electroweak and QCD physics measurements to be conducted at the Jefferson Laboratory. The apparatus will have set of tracking gas electron multiplier (GEM) detectors, a gas Cerenkov detector, and a sampling electromagnetic calorimeter to provide particle identification. The experiment will be conducted as a high rate counting mode experiment. SLIDESHOW:
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Nuclear Physics Seminar Tuesday, November 10, 2015 3:30 PM Physics Building, Room 204 Note special room. |
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