Department of Physics & Astronomy Colloquia

Hilbun 150, 125 Hilbun Hall, Mississippi State University

Mondays at 3:30 PM

Apr. 25, 2016 Dr. Dipangkar Dutta, Mississippi State University

Straight outta Compton: Lorentz invariance tests with Compton Scattering

Host: Dr. Lamiaa El Fassi

Abstract:

      Lorentz invariance (LI) is the corner-stone of the two accepted field theoretical descriptions of nature; General relativity (GR) and the standard Model (SM) of strong and electro-weak interactions. It is commonly regarded that both SM and GR are the low-energy limits of a more fundamental theory that consistently merges the two at the Plank-scale. Thus tests of Lorentz symmetry are tests for new physics.
     Recently, a new Compton polarimeter was built in Hall~C at Jefferson Lab and it achieved the highest precision ever for low-energy (about 1 GeV) electron beams. The polarimeter incorporates key innovations such as ensuring that the laser beam is 100% polarized and a novel diamond-based electron detector that images the paths of the scattered electrons and can withstand very high radiation doses.
     We have used the Compton scattering asymmetry measured by the new Compton polarimeter to verify the constancy and anisotropy of the speed of light by searching for deviations of the vacuum refractive index from unity. For photon energies in the range of 9 - 48 MeV we find a new limit . Further, the absence of any sidereal variation in the measurement over a six month period puts constraints on anisotropies in the speed of light. This constitutes the first study of Lorentz symmetry using Compton asymmetry and demonstrates the feasibility of the technique. Future parity violating electron scattering experiments at Jefferson Lab will use higher energy electrons enabling more competitive constraints.

Apr. 18, 2016 Dr. Misty C. Bentz, Georgia State University

Black Hole Masses in Active Galaxies

Host: Dr. Angelle Tanner

Abstract:

      One of the more unexpected results from 20 years of Hubble Space Telescope observations is the discovery that supermassive black holes inhabit the centers of all massive galaxies. Furthermore, these black holes appear to have a symbiotic relationship with their host galaxies, in which each regulates the growth of the other. One of the keys to understanding this relationship relies upon knowing the masses of the black holes involved. However, black hole masses are difficult measurements to carry out because they require directly observing the gravitational influence of the black hole on a luminous tracer. A few different techniques have been developed over the last 20 years to meet these goals. One of these techniques, known as reverberation mapping, is exclusively applicable to active supermassive black holes but may be used on even the most distant quasars in our universe, providing a way to study black holes across history. On the other hand, the most widely used technique in the local universe is based on observations of the bulk motions of stars deep in the center of a (usually inactive) galaxy. I will introduce these two techniques and describe our program to identify a small sample of galaxies where BOTH techniques can be applied to every galaxy. This program will allow us to directly test the results of these independent mass measurement techniques and to investigate whether all black hole masses are on the same scale. The results of this work will have implications for our understanding of the evolution of galaxies across the ~13 billion year history of the universe

Apr. 11, 2016 Dr. Yoshitaka Taira, National Institute of Advanced Industrial Science and Technology

Inverse Compton scattering between laser and electron beam carrying orbital angular momentum

Host: Dr. Dipangkar Dutta

Abstract:

     Particle accelerators have contributed for evolving field of high energy physics, generating an intense and tightly focused electromagnetic radiation of X-rays as well as ultraviolet and infrared radiation in the past few decades. High energy X-rays (gamma-rays) in the MeV or GeV region can be generated via inverse Compton scattering (ICS) of laser photons with a relativistic beam of sub-GeV or GeV electrons. ICS gamma-rays are tunable in energy, quasi-monochromatic, intense, and highly polarized, and used in nuclear science, polarized positron generation, and electron beam diagnostics.
     In 1992, a generation of laser light carrying orbital angular momentum (OAM) was proposed. The laser light possesses helical wavefronts and a phase singularity at the center of helical wavefronts and is expected to provide new physics and applications. At present, the beam carrying OAM is widely developed in 10 keV X-rays, 300 keV electrons, terahertz radiation, and thermal neutron.
     Collaborator consists of Dipangkar Dutta at MSU, Yoshitaka Taira at AIST, and other researchers at university and laboratory will investigate a development of a high energy electron carrying OAM and gamma-rays carrying OAM. An acceleration to more than 1 MeV and measurement of the high energy electron carrying OAM will be investigated. It is theoretically predicted that spatially structured gamma-rays are generated by ICS between the high energy electrons carrying OAM and a plane wave laser. This phenomena is one candidate for measurement method of the high energy electron carrying OAM. The gamma rays carrying OAM can be generated by ICS of the laser photons carrying OAM with plane wave electrons. In this colloquium, I will present on details.

Apr. 4, 2016 Dr. Andreas Klein, Los Alamos National Laboratory

Trying to solve the nucleon spin puzzle with polarized Drell Yan

Host: Dr. Lamiaa El Fassi

Abstract:

      One of the large puzzles today in nuclear physics is the composition of the nucleon spin. For the last few decades, many experiments have been performed to pinpoint the origin of the spin. While the naïve quark model predicts that this quantity should be the sum of the spins of the partons, experiments show that this picture can explain only roughly 50% of the nucleon spin, with the reminder being a mystery. In order to achieve a more complete picture, the angular contribution of the partons has to be considered as well.
      In this talk I will present experiment E1039 at Fermilab, which will measure a possible sea quark angular momentum continuation. I will discuss the Drell-Yan process and the polarized target used in this experiment.

Mar. 21, 2016 Dr. Rakitha Beminiwattha, Syracuse University

Parity Violating Electron Scattering at JLab

Host: Dr. Dipangkar Dutta

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 at Jefferson Lab will provide answers to these questions. I will introduce PVES experimental techniques and discuss current generation PVES experiments (Qweak and PREX) and next generation PVES experiments (SoLIDPVDIS).

Jan. 14, 2016 Prof. Jie Meng, School of Physics, Peking University

Covariant Density Functional Theory For Nuclear Structure

Host: Dr. Anatoli Afanasjev

Abstract:

      During the last decades, the covariant density functional theory with a minimal number of parameters allows a very successful description of nuclear structure properties range from ground state to excited state all over the nuclear chart. CDFT, implemented with self-consistency and taking into account various correlations by spontaneously broken symmetries, provides an excellent description for the ground-state properties. With pairing correlations and the continuum effect properly taken into account, the self-consistent microscopic descriptions and predictions of the neutron halo phenomena in both spherical and deformed nuclei become possible. Constrained and cranking calculations, CDFT in a static external field, are powerful tools to investigate the shape evolution, shape isomers, shape- coexistence, fission landscapes, and rotational spectra in both near spherical and deformed nuclei. RPA calculation based on CDFT provides a successful description of the mean energies of nuclear giant resonances. The restoration of symmetries and configuration mixing to take into account fluctuations around the mean-field equilibrium based on CDFT as well as its simplification, collective Hamiltonian, describe well the nuclear low-lying states and shape transitions well. Future perspective on CDFT application for nuclear astrophysics and its future.

Nov. 9, 2015 Prof. Wang-Kong Tse, University of Alabama

Light-Matter Interaction and Transport in Quantum Materials: A Spintronics Perspective

Host: Dr. Jinwu Ye

Abstract:

      Spintronics is a discipline that concerns the role of electron spins in transport and magnetic phenomena. Recently discovered quantum materials, including atomically thin two-dimensional crystals and topological insulators, have interesting optical and transport properties because their electrons behave in a quasi-relativistic fashion governed by the Dirac equation. In this talk I will discuss a number of light-matter interaction and transport properties in these emerging quantum materials from the perspective of spintronics. Because of considerable spin-orbit coupling and the emergence of the valley pseudospin degrees of freedom, they exhibit interesting spin/valley dynamics and strong magneto-optical responses.

2015-2016 Committee