arXiv subjects

Pieter Maris

Publications and source records attributed to Pieter Maris.

At least 73 records · Page 4Linked to original sources

arXiv2013

Nonperturbative Quantum Field Evolution

We introduce a nonperturbative, first-principles approach to time-dependent problems in quantum field theory. In this approach, the time-evolution of quantum field configurations is calculated in real time and at the amplitude level. This method is particularly suitable for treating systems interacting with a time-dependent background field. As a test problem, we apply this approach to QED and study electron acceleration and the associated photon emission in a time- and space-dependent electromagnetic background field.

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arXiv2013

Positronium in Basis Light-Front Quantization

We present a calculation of the mass spectrum of positronium within the framework of the recently developed Basis Light-Front Quantization approach to non-perturbative quantum field theory. In this calculation, we employ a two-body effective interaction for the photon exchange, neglecting self-energy effects. We demonstrate the cancellation of Light-Front small-{\it x} divergences within our non-perturbative approach. The resulting spectrum is compared to both non-relativistic quantum mechanics and previous work in Discretized Light-Cone Quantization.

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arXiv2013

Non-Perturbative Time-Dependent Quantum Field Evolution

We present a nonperturbative, first-principles numerical approach for time-dependent problems in the framework of quantum field theory. In this approach the time evolution of quantum field systems is treated in real time and at the amplitude level. As a test application, we apply this method to QED and study photon emission from an electron in a strong, time-dependent external field. Coherent superposition of electron acceleration and photon emission is observed in the nonperturbative regime.

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arXiv2013

Introduction to Basis Light-Front Quantization Approach to QCD Bound State Problems

Basis Light-Front Quantized Field Theory (BLFQ) is an $\textit{ab intio}$ Hamiltonian approach that adopts light-cone gauge, light-front quantization and state-of-the-art many-body methods to solve non-perturbative quantum field theory problems. By a suitable choice of basis, BLFQ retains the underlying symmetries to the extent allowed within light-front coordinates. In this talk, we outline the scheme for applying BLFQ to QCD bound state problems. We adopt a 2D Harmonic Oscillator with 1D plane wave basis that corresponds to the AdS/QCD soft-wall solution. Exact treatment of the symmetries will be discussed.

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arXiv2013

Non-perturbative quantum time evolution on the light-front

This paper introduces "time-dependent basis light-front quantization", which is a covariant, nonperturbative, and first principles numerical approach to time-dependent problems in quantum field theory. We demonstrate this approach by evaluating photon emission from an electron in a strong, time and space dependent external field. We study the acceleration of the electron in combination with the evolution of the invariant mass of the electron-photon system, in real time and at the amplitude level. Successive excitations of the electron-photon system are evident.

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arXiv2013

Scattering in Time-Dependent Basis Light-Front Quantization

We introduce a nonperturbative, first principles numerical approach for solving time-dependent problems in quantum field theory, using light-front quantization. As a first application we consider QED in a strong background field, and the process of non-linear Compton scattering in which an electron is excited by the background and emits a photon. We track the evolution of the quantum state as a function of time. Observables, such as the invariant mass of the electron-photon pair, are first checked against results from perturbation theory, for suitable parameters. We then proceed to a test case in the strong background field regime and discuss the various nonperturbative effects revealed by the approach.

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arXiv2013

Computational Nuclear Quantum Many-Body Problem: The UNEDF Project

The UNEDF project was a large-scale collaborative effort that applied high-performance computing to the nuclear quantum many-body problem. UNEDF demonstrated that close associations among nuclear physicists, mathematicians, and computer scientists can lead to novel physics outcomes built on algorithmic innovations and computational developments. This review showcases a wide range of UNEDF science results to illustrate this interplay.

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arXiv2013

Properties of trapped neutrons interacting with realistic nuclear Hamiltonians

We calculate properties of neutron drops in external potentials using both quantum Monte Carlo and no-core full configuration techniques. The properties of the external wells are varied to examine different density profiles. We compare neutron drop results given by a selection of nuclear Hamiltonians, including realistic two-body interactions as well as several three-body forces. We compute a range of properties for the neutron drops: ground-state energies, spin-orbit splittings, excitation energies, radial densities and rms radii. We compare the equations of state for neutron matter for several of these Hamiltonians. Our results can be used as benchmarks to test other many-body techniques, and to constrain properties of energy-density functionals.

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arXiv2012

Ab initio nuclear structure calculations of light nuclei

We perform no-core configuration interaction calculations for nuclei in the p-shell. We show that for typical light nuclei, a truncation on the total number of quanta in the many-body system converges much more rapidly than a full configuration interaction (FCI) truncation, which is a truncation on the single-particle basis space. We present new results for the ground state energies of the Be isotopes with the nonlocal two-body potential JISP16, and discuss emerging phenomena such as clustering and rotational band structures in 9Be. We also show that the anomalously suppressed beta decay of 14C to the ground state of 14N can be reproduced using two- and three-nucleon forces from chiral effective field theory. In particular the structure of the ground state of 14N is sensitive to the three-nucleon force.

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arXiv2012

Lithium isotopes within the ab intio no-core full configuration approach

We perform no-core full configuration calculations for the Lithium isotopes, 6Li, 7Li, and 8Li with the realistic nucleon-nucleon interaction JISP16. We obtain a set of observables, such as spectra, radii, multipole moments, transition rates, etc., and compare with experiment where available. We also present one-body density distributions for selected states. Convergence properties of these density distributions shed light on the convergence properties of one-body observables. We obtain underbinding by 0.5 MeV, 0.7 MeV, and 1.0 MeV for 6Li, 7Li, 8Li respectively. Magnetic moments are well-converged and agree with experiment to within 20%.

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arXiv2012

Structure of A = 7 - 8 nuclei with two- plus three-nucleon interactions from chiral effective field theory

We solve the ab initio no-core shell model (NCSM) in the complete Nmax = 8 basis for A = 7 and A = 8 nuclei with two-nucleon and three-nucleon interactions derived within chiral effective field theory (EFT). We find that including the chiral EFT three-nucleon interaction in the Hamiltonian improves overall good agreement with experimental binding energies, excitation spectra, transitions and electromagnetic moments. We predict states that exhibit sensitivity to including the chiral EFT three-nucleon interaction but are not yet known experimentally.

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arXiv2012

Convergence properties of {\itshape ab initio} calculations of light nuclei in a harmonic oscillator basis

We study recently proposed ultraviolet and infrared momentum regulators of the model spaces formed by construction of a variational trial wavefunction which uses a complete set of many-body basis states based upon three-dimensional harmonic oscillator (HO) functions. These model spaces are defined by a truncation of the expansion characterized by a counting number ($\mathcal{N}$) and by the intrinsic scale ($\hbarω$) of the HO basis; in short by the ordered pair ($\mathcal{N},\hbarω$). In this study we choose for $\mathcal{N}$ the truncation parameter $N_{max}$ related to the maximum number of oscillator quanta, above the minimum configuration, kept in the model space. The ultraviolet (uv) momentum cutoff of the continuum is readily mapped onto a defined uv cutoff in this finite model space, but there are two proposed definitions of the infrared (ir) momentum cutoff inherent in a finite-dimensional HO basis. One definition is based upon the lowest momentum difference given by $\hbarω$ itself and the other upon the infrared momentum which corresponds to the maximal radial extent used to encompass the many-body system in coordinate space. Extending both the uv cutoff to infinity and the ir cutoff to zero is prescribed for a converged calculation. We calculate the ground state energy of light nuclei with "bare" and "soft" $NN$ interactions. By doing so, we investigate the behaviors of the uv and ir regulators of model spaces used to describe $^2$H, $^3$H, $^4$He and $^6$He with $NN$ potentials Idaho N$^3$LO and JISP16. We establish practical procedures which utilize these regulators to obtain the extrapolated result from sequences of calculations with model spaces characterized by ($\mathcal{N},\hbarω$).

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arXiv2011

Electron Anomalous Magnetic Moment in Basis Light-Front Quantization Approach

We apply the Basis Light-Front Quantization (BLFQ) approach to the Hamiltonian field theory of Quantum Electrodynamics (QED) in free space. We solve for the mass eigenstates corresponding to an electron interacting with a single photon in light-front gauge. Based on the resulting non-perturbative ground state light-front amplitude we evaluate the electron anomalous magnetic moment. The numerical results from extrapolating to the infinite basis limit reproduce the perturbative Schwinger result with relative deviation less than 0.6%. We report significant improvements over previous works including the development of analytic methods for evaluating the vertex matrix elements of QED.

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arXiv2009

{\it Ab initio} nuclear structure - the large sparse matrix eigenvalue problem

The structure and reactions of light nuclei represent fundamental and formidable challenges for microscopic theory based on realistic strong interaction potentials. Several {\it ab initio} methods have now emerged that provide nearly exact solutions for some nuclear properties. The {\it ab initio} no core shell model (NCSM) and the no core full configuration (NCFC) method, frame this quantum many-particle problem as a large sparse matrix eigenvalue problem where one evaluates the Hamiltonian matrix in a basis space consisting of many-fermion Slater determinants and then solves for a set of the lowest eigenvalues and their associated eigenvectors. The resulting eigenvectors are employed to evaluate a set of experimental quantities to test the underlying potential. For fundamental problems of interest, the matrix dimension often exceeds $10^{10}$ and the number of nonzero matrix elements may saturate available storage on present-day leadership class facilities. We survey recent results and advances in solving this large sparse matrix eigenvalue problem. W also outline the challenges that lie ahead for achieving further breakthroughs in fundamental nuclear theory using these {\it ab initio} approaches.

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arXiv2004

Electromagnetic properties of diquarks

Diquark correlations play an important role in hadron physics. The properties of diquarks can be obtained from the corresponding bound state equation. Using a model for the effective quark-quark interaction that has proved successful in the light meson sector, we solve the scalar diquark Bethe-Salpeter equations and use the obtained Bethe-Salpeter amplitudes to compute the diquarks' electromagnetic form factors. The scalar ud diquark charge radius is about 8% larger than the pion charge radius, indicating that these diquarks are somewhat larger in size than the corresponding mesons. We also provide analytic fits for the form factor over a moderate range in Q^2, which may be useful, for example, in building quark-diquark models of nucleons.

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arXiv2003

Ladder Dyson-Schwinger calculation of the anomalous gamma-3pi form factor

The anomalous processes, γ\to 3 πand γπ\to ππ, are investigated within the Dyson-Schwinger framework using the rainbow-ladder approximation. Calculations reveal that a complete set of ladder diagrams beyond the impulse approximation are necessary to reproduce the fundamental low-energy theorem for the anomalous form factor. Higher momentum calculations also agree with the limited form factor data and exhibit the same resonance behavior as the phenomenological vector meson dominance model.

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arXiv2002

Massless QED_3 with explicit fermions

We study dynamical mass generation in QED in (2+1) dimensions using Hamiltonian lattice methods. We use staggered fermions, and perform simulations with explicit dynamical fermions in the chiral limit. We demonstrate that a recently developed method to reduce the fermion sign problem can successfully be applied to this problem. Our results are in agreement with both the strong coupling expansion and with Euclidean lattice simulations.

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arXiv2002

QCD based quark description of pi-pi scattering up to the sigma and rho region

We study forward and backward pi-pi scattering within a QCD model based on the Dyson--Schwinger, Bethe--Salpeter equations truncated to the rainbow-ladder level. Our microscopic relativistic quark formulation preserves chiral symmetry and reproduces the observed scattering lengths for total isospin zero, one and two. At higher energies both scalar and vector meson resonances naturally occur in the scattering amplitudes. We also report a comparative study with phenomenological meson-exchange models and find such approaches are reasonable especially near pi-pi resonances.

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