Physics Colloquium Series - Spring 2020

Example Abstract

TBA

Since the discovery of the European Muon Collaboration (EMC) effect, where it was observed that the quark longitudinal momentum distributions in a nucleus are different from those of free nucleons (protons or neutrons), there has been a long standing question in nuclear physics as to how the structure of a free nucleon might change when bound in a nucleus or embedded in a nuclear medium. One of the cleanest signatures of in-medium modification can be achieved by testing the Coulomb Sum Rule through quasi-elastic electron scattering; where one counts the expected total nuclear charge by integrating the longitudinal response function of the nucleus and comparing it to the incoherent sum of electric form factors of the constituent nucleons. Standard nuclear effects that quench the charge response of the nucleus are well understood, leaving any additional quenching to be interpreted as a signature of modification of these form factors in-medium. Preliminary results from a recent analysis of data from experiment E05-110 at Jefferson Lab will be presented. Another novel method of probing in-medium nucleon structure can be achieved by accessing their 3D partonic distributions. Upcoming experiments at Jefferson Lab and the EIC that use coherent heavy vector meson production to probe the spatial gluonic density of whole nuclei will also be discussed.

The proton is commonplace in the world but contains rich information about how matter is constructed by the strong force. It is well described by the parton picture, namely a system made of quarks, antiquarks and gluons, when it is observed in short range such as high-energy scattering. The proton is the best object to investigate the dynamics of the strong force (i.e. QCD) since it is the simplest stable system bound by the strong force. Particularly the antiquarks in the proton are of great interest because they are all dynamically created by the strong force. They are thus sensitive to mechanisms beyond the simple quark model where the proton is composed of only three quarks. SeaQuest is a fixed-target experiment using the 120-GeV proton beam at Fermilab. It detects the Drell-Yan process in proton + proton and proton + deuteron scatterings; quark + antiquarks -> gamma^* -> mu^+ + mu^-. It primarily aims at measuring the flavor asymmetry of light antiquarks, namely the difference between antiup and antidown quarks in their distributions in the proton. Several mechanisms that can induce this asymmetry have been theoretically proposed. In this colloquium, the physics motivation and the measurement principle of SeaQuest are explained. Then the detector construction, data analysis and experimental results are presented.

The proton, discovered by Ernest Rutherford in 1917, was initially thought to be a fundamental particle. Now known to be composed of quarks, as are its fellow hadrons, its composition gives rise to additional internal degrees of freedom. A number of parameters, called polarizabilities, describe how this structure responds to electric and magnetic forces. A Compton scattering program has been ongoing in the A2 experimental hall at the Mainz Microtron to study these polarizabilities. A tagged Bremsstrahlung beam of polarized photons (with kinetic energies 150 million times that of a red laser pointer) is scattered off of polarized protons (cooled to 1/50th the `temperature' of deep space) and detected in the Crystal Ball and TAPS detectors (arrays of 1100 scintillation crystals). Different combinations of beam and target polarizations allow for the measurement of different polarization observables, which permit extractions of these polarizabilities, some of which had not been experimentally determined previously. This talk will discuss these polarizabilities as well as the status, and outlook, of the A2 Compton program.

The Tritium program in Hall A at Jefferson Lab utilized the mirror nuclei, Tritium and Helium-3, to study, in a model independent way, the d/u ratio at high-x, the EMC effect in the A=3 nuclei, probe nucleon-nucleon short-range correlations and their isospin dependence, and to investigate the three-body hyper-nuclear bound-system. By the end of November 2018, four highly rated Tritium experiments have successfully been completed. In this talk, I will discuss the physics motivation of the program, share the experimental details of these experiments, present the most recent results and introduce a plan of the new generation of Tritium experiments.

A broad program of parity-violation measurements in electron scattering from heavy nuclei will constrain nuclear structure models as well as search for new BSM physics. This program is comprised of PREX-II (Lead Radius Experiment), CREX(Calcium Radius Experiment), and MOLLER(Measurement of a Lepton-Lepton Electroweak Reaction) experiment. PREX-II is a PVES experiment which ran at Jefferson Laboratory in 2019 and CREX is currently running in 2020. The aim of PREX-II and CREX is to map the weak charge distribution in nuclei, with implications for the equation of state of highly dense matter, neutron stars, and gravitational waves produced in neutron star collisions. MOLLER is a future PVES experiment searching for new neutral currents, providing an unprecedented precision on the electron weak charge and electroweak mixing angle. One common crucial component of all these experiments is control of helicity correlated false asymmetries in the polarized electron beam. This talk will describe the development of the newly installed RTP Pockels cell system in the JLab injector source which enables this program of experiments. The summer 2019 run of PREX-II will be reviewed, including the demonstration of nm-level control of electron beam asymmetries.

Bottom quarks are the second-heaviest type of quark known to us. They are also unusually long-lived because their most common decay modes are suppressed by small mixing parameters. This combination of properties makes bottom quarks especially sensitive to physics beyond the Standard Model. Several tensions between Standard-Model expectations and measurements at particle colliders have emerged in semileptonic bottom-quark decays. I will discuss these processes and the Standard-Model predictions, which depend on nonperturbative calculations of the strong interactions of the quarks using lattice quantum chromodynamics. If confirmed by future data from LHCb and Belle II in combination with improved calculations, the tensions would point to the existence of new elementary particles or forces.