Physics
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Item Measurement of the Strange Quark Contribution to Nucleon Structure Through Parity-Violating Electron Scattering(2010) Ellis, Colleen L.; Beise, Elizabeth J.; Physics; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)The G0 backward angle experiment, completed in Hall C of the Thomas Jefferson National Accelerator Facility (TJNAF), measured parity-violating asymmetries in elastic electron-proton and quasielastic electron-deuteron scattering at Q2 = 0.22 and 0.63 (GeV/c)2 . The asymmetries are sensitive to strange quark contributions to currents in the nucleon and the nucleon axial-vector current. The results indicate strange quark contributions of < 10% of the charge and magnetic nucleon form factors at these four-momentum transfers. This was also the first measurement of the anapole moment effects in the axial-vector current at these four-momentum transfers.Item Transport coefficients and universality in hot strongly coupled gauge theories(2010) Cherman, Aleksey; Cohen, Thomas D; Physics; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)The gauge/gravity duality provides a valuable opportunity to study the behavior of relativistic fluids described by some strongly-interacting non-Abelian gauge theories. However, as yet no gravity duals are known for the field theories that are currently used to describe nature. Thus, it is particularly interesting to search for universal properties of theories with gravity duals. This dissertation discusses a broad class of theories with gravity duals, and it is shown that at high temperatures, the speed of sound squared is bounded from above by one-third of the speed of light squared. It is conjectured that this may be a universal property of theories with gravity duals. It is also shown that the temperature dependence of a number of transport coefficients takes a universal form in the high-temperature limit. In particular, in a high-temperature expansion, the power law of the leading correction away from the infinite temperature limit is universal for all of the transport coefficients, and is the same as that of the speed of sound squared.Item Topics in Lattice QCD and Effective Field Theory(2010) Buchoff, Michael Ireland; Bedaque, Paulo F; Physics; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Quantum Chromodynamics (QCD) is the fundamental theory that governs hadronic physics. However, due to its non-perturbative nature at low-energy/long distances, QCD calculations are difficult. The only method for performing these calculations is through lattice QCD. These computationally intensive calculations approximate continuum physics with a discretized lattice in order to extract hadronic phenomena from first principles. However, as in any approximation, there are multiple systematic errors between lattice QCD calculation and actual hardronic phenomena. Developing analytic formulae describing the systematic errors due to the discrete lattice spacings is the main focus of this work. To account for these systematic effects in terms of hadronic interactions, effective field theory proves to be useful. Effective field theory (EFT) provides a formalism for categorizing low-energy effects of a high-energy fundamental theory as long as there is a significant separation in scales. An example of this is in chiral perturbation theory (χPT ), where the low-energy effects of QCD are contained in a mesonic theory whose applicability is a result of a pion mass smaller than the chiral breaking scale. In a similar way, lattice χPT accounts for the low-energy effects of lattice QCD, where a small lattice spacing acts the same way as the quark mass. In this work, the basics of this process are outlined, and multiple original calculations are presented: effective field theory for anisotropic lattices, I=2 ππ scattering for isotropic, anisotropic, and twisted mass lattices. Additionally, a combination of effective field theory and an isospin chemical potential on the lattice is proposed to extract several computationally difficult scattering parameters. Lastly, recently proposed local, chiral lattice actions are analyzed in the framework of effective field theory, which illuminates various challenges in simulating such actions.Item A STUDY OF EFFECTIVE THEORY APPROACHES TO PROBLEMS IN NUCLEAR PHYSICS(2010) Chen, Panying; Ji, Xiangdong; Physics; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)The effective field theory is an efficient tool in dealing with physics involving separate scales. A typical effective field theory approach involves calculating the matching conditions for the desired parameters at some cutoff scale and obtain the renormalization group equation for the effective theory. In this thesis we study a collection of effective theories including the non-relativistic QED, the chiral perturbation theory, the soft-collinear effective theory, and their applications in nuclear physics. We study the angular momentum operator in the non-relativistic QED (NRQED). We construct its gauge-invariant decomposition into spin and orbital angular momentum operators of electrons and photons and calculate the matching conditions of operators between QED and NRQED up to one loop. We perform the matching with both dimensional regularization and UV cutoff $\Lambda$. We apply the result in the Hydrogen-like system and calculate the radiation correction of the orbital angular momentum. We study the CP-violating operators in chiral perturbation theory. We apply our general, model-independent result onto the left-right symmetric model and relate the desired operator to the standard model penguin operators through $SU(3)_L$ × $SU(3)_R$ chiral symmetry. We use the lattice result for the standard model and acquire a more strict lower bound for the mass of right-hand boson in LRSM. We study the polarized gluon distribution $\Delta g(x)$ in a longitudinally polarized proton. The first result from the MIT bag model as well as the non-relativistic quark model shows that $\Delta g(x)$ is positive at all $x$. The total gluon helicity $\Delta G$ from the bag model is about $0.3\hbar$ at the scale of 1 GeV, considerably smaller than previous theoretical expectations. We study deep-inelastic scattering factorization on a nucleon in the end-point regime $x_B \sim 1-{\cal O}( \Lambda_{\rm QCD}/Q)$ with an approach in soft-collinear effective theory. Refactorization of the scale $(1-x_B)Q^2$ in the coefficient function can be made in the SCET and remains valid in the end-point regime. On the other hand, the traditional refactorization approach introduces the spurious scale $(1-x_B)Q$ in various factors, which drives them nonperturbative in the region of our interest. We show how to improve the situation by introducing a rapidity cut-off scheme, and how to recover the effective theory refactorization by choosing appropriately the cut-off parameter. Through a one-loop calculation, we demonstrate explicitly that the proper soft subtractions must be made in the collinear matrix elements to avoid double counting.Item Measurement of the Electric Form Factor of the Neutron at High Momentum Transfer(2009) Miller, Jonathan Andrew; Beise, Elizabeth J; Physics; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)The electric form factor of the neutron, $G_{E}^{n}$, provides key understanding of the structure of one of the basic building blocks of visible matter in the universe. Recent interest in this quantity is the result of the improved quality of data provided by double polarization experiments, which have substantially improved in the last decade. This thesis presents precision measurements of $G_{E}^{n}$ by the E02-013 collaboration at $Q^{2}$ of 1.7, 2.5, and 3.5 GeV$^{2}$. This measurement used a double polarization technique, a highly polarized $^{3}$He target, a polarized electron beam, a large acceptance spectrometer to detect the scattered electrons, and a large neutron detector to detect the recoiling hadrons in the reaction $^{3}\vec{\mathrm{He}}(\vec{e},e'n)$. These measurements will be compared to a variety of models of the nucleon's internal structure, as well as used to extract individual contributions of the up and down quarks to the nucleon form factors.Item Excited Nucleon and Delta Spectra From Lattice QCD(2009) Engelson, Eric; Wallace, Stephen J; Physics; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)We calculate the nucleon and delta excited state spectra from lattice QCD. Operators which transform as irreducible representations of the lattice symmetry group are used as bases for variational calculations. We compute matrices of corre- lation functions between all the operators in the variational bases. From the time dependence of the eigenvalues of these matrices, we extract energy eigenvalues. By subducing the continuum SU(3) rotation group to the octahedral group, we can identify the spins of the continuum states which correspond to the lattice states. In the nucleon spectrum calculation, we use 24^3 × 64 anisotropic lattices with pion masses of 416 MeV and 576 MeV. The lattices have a spacing of about 0.1 fm and an anisotropy of 3. We use the Wilson gauge and the Wilson fermion actions with two flavors of dynamical light quarks. The low-lying spectrum has many of the qualitative features of the physical spectrum and we are able to identify the continuum states which correspond to several of the lattice states. This includes one of the first observations of a spin- 5 state on the lattice. For the delta spectrum calculation, we use 16^3 × 128 anisotropic lattices. The gauge action is the tree-level tadpole improved Wilson gauge action, while in the fermion sector we use the clover action. The pion mass is about 390 MeV and the anisotropy is 3.5. We have two flavors of dynamical light quarks as well as dynamical strange quarks. To compute the correlation functions, we use the distillation method in which operators are projected on the the low lying eigenmodes of the Laplacian operator, allowing for an exact computation of all-to-all propagators between the distilled source and sink operators. We are able to identify four low-lying states with continuum delta states.Item Phenomenological apsects of heavy quark systems(2008-04-29) Hohler, Paul M.; Cohen, Thomas D; Physics; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)The systems of heavy quarks are particularly interesting because they lend themselves to effective field theory techniques. Typically this involves considering an expansion about the limit of infinitely heavy quarks. In this limit, the phenomenology of heavy quark systems differs qualitatively from light quark systems; this provides a window into the workings of QCD. However, in the real world, heavy quarks have a finite mass. This dissertation will examine a number of heavy quark systems and describe the associated phenomenology It will also probe the extent to which realistic systems are well approximated by expansions about the heavy quark limit. This will be done with direct comparison with experimental data and models with a finite heavy quark. In the end, this study will show that, many of the systems considered here, it is unlikely that realistic heavy quarks can be accurately described by such expansions.Item The Pion Charge Form Factor Through Pion Electroproduction(2006-04-26) Horn, Tanja; Beise, Elizabeth; Physics; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)The goal of Jefferson Lab experiment E01-004 (Fpi-2) was the measurement of the longitudinal and transverse cross sections via pion electroproduction from hydrogen and deuterium for the purpose of extracting the charged pion form factor using pole dominance. The data were taken at two values of Q2 (1.60 and 2.45 (GeV/c)2). In order to attain full coverage in phi, charged pions were detected in parallel kinematics (along the direction of momentum transfer, q, and at +/- 3 degrees off the direction of momentum transfer. For each Q2 data were taken for two values of the virtual photon polarization, epsilon, respectively. All data were taken at a fixed center of mass energy, W=2.22 GeV. The longitudinal and transverse pieces of the cross section were separated using the Rosenbluth separation method.Item A Measurement of the Strange Quark Contributions to the Electromagnetic Form Factors of the Nucleon(2006-03-28) Liu, Jianglai; Beise, Elizabeth J; Physics; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Item Lattice QCD Simulations of Baryon Spectra and Development of Improved Interpolating Field Operators(2005-08-03) Sato, Ikuro; Wallace, Stephen J; Physics; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Large sets of baryon interpolating field operators are developed for use in lattice QCD studies of baryons with zero momentum. Because of the cubical discretization of space, the continuum rotational group is broken down to a finite point group. Operators are classified according to the irreducible representations of the double octahedral group. At first, three-quark quasi-local operators are constructed for each isospin and strangeness with suitable symmetry of Dirac indices. Nonlocal baryon operators are formulated in a second step as direct products of the quasi-local spinor structures together with lattice displacements. Appropriate Clebsch-Gordan coefficients of the octahedral group are used to form linear combinations of such direct products. The construction maintains maximal overlap with the continuum SU(2) group in order to provide a physically interpretable basis. Nonlocal operators provide direct couplings to states that have nonzero orbital angular momentum. Monte Carlo simulations of nucleon and delta baryon spectra are carried out with anisotropic lattices of anisotropy 3.0 with $\beta=6.1$. Gauge configurations are generated by the Wilson gauge action in quenched approximation with space-time volumes $(1.6\,\mbox{fm})^3\times 2.1\,\mbox{fm}$ and $(2.4\,\mbox{fm})^3\times 2.1\,\mbox{fm}$. The Wilson fermion action is used with pion mass $\simeq 500\,\mbox{MeV}$. The variational method is applied to matrices of correlation functions constructed using improved operators in order to extract mass eigenstates including excited states. Stability of the obtained masses is confirmed by varying the dimensions of the matrices. The pattern of masses for the low-lying states that we compute is consistent with the pattern that is observed in nature. Ordering of masses is consistent for positive-parity excited states, but mass splittings are considerably larger than the physical values. Baryon masses for spin $S \ge 5/2$ states are obtained in these simulations. Hyperfine mass splittings are studied for both parities. No significant finite volume effect is seen at the quark mass that is used.