Publications (43)187.24 Total impact
 [Show abstract] [Hide abstract]
ABSTRACT: We show how lattice Quantum Monte Carlo can be applied to the electronic properties of carbon nanotubes in the presence of strong electronelectron correlations. We employ the pathintegral formalism and use methods developed within the lattice QCD community for our numerical work. Our lattice Hamiltonian is closely related to the hexagonal Hubbard model augmented by a longrange electronelectron interaction. We apply our method to the singlequasiparticle spectrum of the (3,3) armchair nanotube configuration, and consider the effects of strong electronelectron correlations. Our approach is equally applicable to other nanotubes, as well as to other carbon nanostructures. We benchmark our Monte Carlo calculations against the two and foursite Hubbard models, where a direct numerical solution is feasible. 
Article: Ab initio alphaalpha scattering
[Show abstract] [Hide abstract]
ABSTRACT: Processes involving alpha particles and alphalike nuclei comprise a major part of stellar nucleosynthesis and hypothesized mechanisms for thermonuclear supernovae. In an effort towards understanding alpha processes from first principles, we describe in this letter the first ab initio calculation of alphaalpha scattering. We use lattice effective field theory to describe the lowenergy interactions of nucleons and apply a technique called the adiabatic projection method to reduce the eightbody system to an effective twocluster system. We find good agreement between lattice results and experimental phase shifts for Swave and Dwave scattering. The computational scaling with particle number suggests that alpha processes involving heavier nuclei are also within reach in the near future.  [Show abstract] [Hide abstract]
ABSTRACT: Projection Monte Carlo calculations of lattice Chiral Effective Field Theory suffer from sign oscillations to a varying degree dependent on the number of protons and neutrons. Hence, such studies have hitherto been concentrated on nuclei with equal numbers of protons and neutrons, and especially on the alpha nuclei where the sign oscillations are smallest. We now introduce the technique of "symmetrysign extrapolation" which allows us to use the approximate Wigner SU(4) symmetry of the nuclear interaction to control the sign oscillations without introducing unknown systematic errors. We benchmark this method by calculating the groundstate energies of the $^{12}$C, $^6$He and $^6$Be nuclei, and discuss its potential for studies of neutronrich halo nuclei and asymmetric nuclear matter.  [Show abstract] [Hide abstract]
ABSTRACT: We explore the twobody spectra of spin$1/2$ fermions in isotropic harmonic traps with external spinorbit potentials and short range twobody interactions. Using a truncated basis of total angular momentum eigenstates, nonperturbative results are presented for experimentally realistic forms of the spinorbit coupling: a pure Rashba coupling, Rashba and Dresselhaus couplings in equal parts, and a Weyltype coupling. The technique is easily adapted to bosonic systems and other forms of spinorbit coupling. 
Article: Nuclear Reactions from Lattice QCD
[Show abstract] [Hide abstract]
ABSTRACT: One of the overarching goals of nuclear physics is to rigorously compute properties of hadronic systems directly from the fundamental theory of strong interactions, Quantum Chromodynamics (QCD). In particular, the hope is to perform reliable calculations of nuclear reactions which will impact our understanding of environments that occur during big bang nucleosynthesis, the evolution of stars and supernovae, and within nuclear reactors and high energy/density facilities. Such calculations, being truly ab initio, would include all twonucleon and three nucleon (and higher) interactions in a consistent manner. Currently, lattice QCD provides the only reliable option for performing calculations of some of the low energy hadronic observables. With the aim of bridging the gap between lattice QCD and nuclear manybody physics, the Institute for Nuclear Theory held a workshop on Nuclear Reactions from Lattice QCD on March 2013. In this review article, we report on the topics discussed in this workshop and the path planned to move forward in the upcoming years.  [Show abstract] [Hide abstract]
ABSTRACT: The energy spectra of two nucleons in a cubic volume provide access to the two phase shifts and one mixing angle that define the S matrix in the S13D13 coupled channels containing the deuteron. With the aid of recently derived energy quantization conditions for such systems, and the known scattering parameters, these spectra are predicted for a range of volumes. It is found that extractions of the infinitevolume deuteron binding energy and leading scattering parameters, including the SD mixing angle at the deuteron pole, are possible from lattice QCD calculations of twonucleon systems with boosts of P≤(2π)/(L)3 in volumes with 10fm≲L≲14fm. The viability of extracting the asymptotic D/S ratio of the deuteron wave function from lattice QCD calculations is discussed.  [Show abstract] [Hide abstract]
ABSTRACT: We explore the use of twisted boundary conditions in extracting the nucleon mass and the binding energy of twobaryon systems, such as the deuteron, from Lattice QCD calculations. Averaging the results of calculations performed with periodic and antiperiodic boundary conditions imposed upon the lightquark fields, or other pairwise averages, improves the volume dependence of the deuteron binding energy from ~exp(kappa*L)/L to ~exp(sqrt(2)kappa*L)/L. However, a twist angle of pi/2 in each of the spatial directions improves the volume dependence from ~exp(kappa*L)/L to ~exp(2kappa*L)/L. Twist averaging the binding energy with a random sampling of twist angles improves the volume dependence from ~exp^(kappa*L)/L to ~exp(2kappa*L)/L, but with a standard deviation of ~exp(kappa*L)/L, introducing a signaltonoise issue in modest lattice volumes. Using the experimentally determined phase shifts and mixing angles, we determine the expected energies of the deuteron states over a range of cubic lattice volumes for a selection of twisted boundary conditions. 
Conference Paper: The origin of mass
[Show abstract] [Hide abstract]
ABSTRACT: The origin of mass is one of the deepest mysteries in science. Neutrons and protons, which account for almost all visible mass in the Universe, emerged from a primordial plasma through a cataclysmic phase transition microseconds after the Big Bang. However, most mass in the Universe is invisible. The existence of dark matter, which interacts with our world so weakly that it is essentially undetectable, has been established from its galacticscale gravitational effects. Here we describe results from the first truly physical calculations of the cosmic phase transition and a groundbreaking firstprinciples investigation into composite dark matter, studies impossible with previous stateoftheart methods and resources. By inventing a powerful new algorithm, "DSDR," and implementing it effectively for contemporary supercomputers, we attain excellent strong scaling, perfect weak scaling to the LLNL BlueGene/Q two million cores, sustained speed of 7.2 petaflops, and timetosolution speedup of more than 200 over the previous stateoftheart.  [Show abstract] [Hide abstract]
ABSTRACT: I provide a short overview of the current status of nuclear physics calculations using lattice quantum chromodynamics (LQCD). I give an heuristic description of how LQCD calculations are performed and how nuclear scattering data are extracted from these calculations, emphasizing the overlap between traditional nuclear manybody theory and LQCD calculations. I look at the Ω −Ω − system as a concrete example, and in so doing demonstrate the predictive nature of LQCD calculations as applied to nuclear physics.  [Show abstract] [Hide abstract]
ABSTRACT: The quantization condition for interacting energy eigenvalues of the twonucleon system in a finite cubic volume is derived in connection to the nucleonnucleon scattering amplitudes. This condition is derived using an auxiliary (dimer) field formalism that is generalized to arbitrary partial waves in the context of nonrelativistic effective field theory. The quantization condition presented gives access to the scattering parameters of the twonucleon systems with arbitrary parity, spin, isospin, angular momentum and center of mass motion, from a lattice QCD calculation of the energy eigenvalues. In particular, as it includes all noncentral interactions, such as the twonucleon tensor force, it makes explicit the dependence of the mixing parameters of nucleonnucleon systems calculated from lattice QCD when there is a physical mixing among different partialwaves, e. g. SD mixing in the deuteron channel. We provide explicit relations among scattering parameters and their corresponding point group symmetry class eigenenergies with orbital angular momentum l smaller than or equal to 3, and for center of mass boost vectors of the form 2\pi (2n_1, 2n_2, 2n_3)/L, 2\pi (2n_1, 2n_2, 2n_3+1)/L and 2\pi (2n_1+1, 2n_2+1, 2n_3)/L. L denotes the special extent of the cubic volume and n_1,n_2,n_3 are integers. Our results are valid below inelastic thresholds up to exponential volume corrections that are governed by the pion mass.  [Show abstract] [Hide abstract]
ABSTRACT: The scattering lengths and effective ranges that describe lowenergy nucleonnucleon scattering are calculated in the limit of SU(3)flavor symmetry at the physical strangequark mass with Lattice Quantum Chromodynamics. The calculations are performed with an isotropic clover discretization of the quark action in three volumes with spatial extents of L \sim 3.4 fm, 4.5fm and 6.7 fm, and with a lattice spacing of b \sim 0.145 fm. With determinations of the energies of the twonucleon systems (both of which contain bound states at these up and down quark masses) at rest and moving in the lattice volume, Luscher's method is used to determine the lowenergy phase shifts in each channel, from which the scattering length and effective range are obtained. The scattering parameters, in the 1S0 channel are found to be m_pi a^(1S0) = 9.50^{+0.78}_{0.69}^{+1.10}_{0.80} and m_pi r^(1S0) = {4.61^{+0.29}_{0.31}^{+0.24}_{0.26}, and in the 3S1 channel are m_pi a^(3S1) = 7.45^{+0.57}_{0.53}^{+0.71}_{0.49} and m_pi r^(3S1) = 3.71^{+0.28}_{0.31}^{+0.28}_{0.35}. These values are consistent with the twonucleon system exhibiting Wigner's supermultiplet symmetry, which becomes exact in the limit of largeN_c. In both spin channels, the phase shifts change sign at higher momentum, near the start of the tchannel cut, indicating that the nuclear interactions have a repulsive core even at the SU(3)symmetric point. 
Article: Nuclear Physics in Box
[Show abstract] [Hide abstract]
ABSTRACT: I provide a short overview of the current status of nuclear physics calculations using lattice Quantum Chromodynamics (LQCD). I demonstrate, at a very high level, how LQCD calculations are performed and how nuclear scattering data are extracted from these calculations, emphasizing the overlap between traditional nuclear manybody theory and LQCD calculations. I look at the ΩΩ system as a concrete example, and in so doing demonstrate the predictive nature of LQCD calculations as applied to nuclear physics. 
Article: HyperonNucleon Interactions from Quantum Chromodynamics and the Composition of Dense Nuclear Matter
[Show abstract] [Hide abstract]
ABSTRACT: The lowenergy nΣ^{} interactions determine, in part, the role of the strange quark in dense matter, such as that found in astrophysical environments. The scattering phase shifts for this system are obtained from a numerical evaluation of the QCD path integral using the technique of lattice QCD. Our calculations, performed at a pion mass of m_{π}∼389 MeV in two large lattice volumes and at one lattice spacing, are extrapolated to the physical pion mass using effective field theory. The interactions determined from lattice QCD are consistent with those extracted from hyperonnucleon experimental data within uncertainties and strengthen modeldependent theoretical arguments that the strange quark is a crucial component of dense nuclear matter. 
Article: Light Nuclei and Hypernuclei from Quantum Chromodynamics in the Limit of SU(3) Flavor Symmetry
[Show abstract] [Hide abstract]
ABSTRACT: The binding energies of a range of nuclei and hypernuclei with atomic number A <= 4 and strangeness s <= 2, including the deuteron, dineutron, Hdibaryon, 3He, Lambda 3He, Lambda 4He, and Lambda Lambda 4He, are calculated in the limit of flavorSU(3) symmetry at the physical strange quark mass with quantum chromodynamics (without electromagnetic interactions). The nuclear states are extracted from Lattice QCD calculations performed with n_f=3 dynamical light quarks using an isotropic clover discretization of the quarkaction in three lattice volumes of spatial extent L ~ 3.4 fm, 4.5 fm and 6.7 fm, and with a single lattice spacing b ~ 0.145 fm.  [Show abstract] [Hide abstract]
ABSTRACT: The π+π+ swave scattering phase shift is determined below the inelastic threshold using lattice QCD. Calculations were performed at a pion mass of mπ∼390 MeV with an anisotropic nf=2+1 clover fermion discretization in four lattice volumes, with spatial extent L∼2.0, 2.5, 3.0 and 3.9 fm, and with a lattice spacing of bs∼0.123 fm in the spatial direction and bt∼bs/3.5 in the time direction. The phase shift is determined from the energy eigenvalues of π+π+ systems with both zero and nonzero total momentum in the lattice volume using Lüscher’s method. Our calculations are precise enough to allow for a determination of the threshold scattering parameters, the scattering length a, the effective range r, and the shape parameter P, in this channel and to examine the prediction of twoflavor chiral perturbation theory: mπ2ar=3+O(mπ2/Λχ2). Chiral perturbation theory is used, with the lattice QCD results as input, to predict the scattering phase shift (and threshold parameters) at the physical pion mass. Our results are consistent with determinations from the Roy equations and with the existing experimental phase shift data.  [Show abstract] [Hide abstract]
ABSTRACT: We explore the interactions of two strangeness 3 baryons in multiple spin channels with lattice QCD. This system provides an ideal laboratory for exploring the interactions of multibaryon systems with minimal dependence on light quark masses. Model calculations of the two$\Omega^$ system in two previous works have obtained conflicting results, which can be resolved by lattice QCD. The lattice calculations are performed using two different volumes with $L\sim2.5$ and 3.9 fm on anisotropic clover lattices at $m_\pi \sim 390$ MeV with a lattice spacing of $a_s \sim 0.123$ fm in the spatial direction and $a_t\sim{a}_s/3.5$ in the temporal direction. Using multiple interpolating operators from a nondisplaced source, we present scattering information for two ground state $\Omega^$ baryons in both the S=0 and S=2 channels. For S=0, $k\cot\delta$ is extracted at two volumes, which lead to an extrapolated scattering length of $a^{\Omega\Omega}_{S=0}=0.16 \pm 0.22 \ \text{fm}$, indicating a weakly repulsive interaction. Additionally, for S=2, two separate highly repulsive states are observed. We also present results on the interactions of the excited strangeness 3, spin1/2 states with the ground spin3/2 states for the spin1 and spin2 channels. Results for these interactions are consistent with attractive behavior. 
 [Show abstract] [Hide abstract]
ABSTRACT: Results of a highstatistics, multivolume Lattice QCD exploration of the deuteron, the dineutron, the Hdibaryon, and the XiXi system at a pion mass of m ~ 390 MeV are presented. Calculations were performed with an anisotropic n_f = 2+1 Clover discretization in four lattice volumes of spatial extent L ~ 2.0, 2.5, 3.0 and 4.0 fm, with a lattice spacing of b_s ~ 0.123 fm in the spatialdirection, and b_t ~ b_s/3.5 in the timedirection. The XiXi is found to be bound by B_{XiXi} = 14.0(1.4)(6.7) MeV, consistent with expectations based upon phenomenological models and lowenergy effective field theories constrained by nucleonnucleon and hyperonnucleon scattering data at the physical lightquark masses. We find weak evidence that both the deuteron and the dineutron are bound at this pion mass, with binding energies of B_d = 11(05)(12) MeV and B_{nn} = 7.1(5.2)(7.3) MeV, respectively. With an increased number of measurements and a refined analysis, the binding energy of the Hdibaryon is B_H = 13.2(1.8)(4.0) MeV at this pion mass, updating our previous result.  [Show abstract] [Hide abstract]
ABSTRACT: An analysis of the pion mass and pion decay constant is performed using mixedaction Lattice QCD calculations with domainwall valence quarks on ensembles of rooted, staggered n_f = 2+1 MILC configurations. Calculations were performed at two lattice spacings of b~0.125 fm and b~0.09 fm, at two strange quark masses, multiple light quark masses, and a number of lattice volumes. The ratios of light quark to strange quark masses are in the range 0.1 <= m_l / m_s <= 0.6, while pion masses are in the range 235 < m_\pi < 680 MeV. A twoflavor chiral perturbation theory analysis of the Lattice QCD calculations constrains the GasserLeutwyler coefficients bar{l}_3 and bar{l}_4 to be bar{l}_3 = 4.04(40)(+7355) and bar{l}_4 = 4.30(51)(+8460). All systematic effects in the calculations are explored, including those from the finite lattice spacetime volume, the finite lattice spacing, and the finite fifth dimension in the domainwall quark action. A consistency is demonstrated between a chiral perturbation theory analysis at fixed lattice spacing combined with a leading order continuum extrapolation, and the mixedaction chiral perturbation theory analysis which explicitly includes the leading order discretization effects. Chiral corrections to the pion decay constant are found to give f_\pi / f = 1.062(26)(+4240) where f is the decay constant in the chiral limit. The most recent scale setting by the MILC Collaboration yields a postdiction of f_\pi = 128.2(3.6)(+4.46.0)(+1.23.3) MeV at the physical pion mass.  [Show abstract] [Hide abstract]
ABSTRACT: We present evidence for the existence of a bound H dibaryon, an I = 0, J = 0, s = 2 state with valence quark structure uuddss, at a pion mass of m(pi) similar to 389 MeV. Using the results of lattice QCD calculations performed on four ensembles of anisotropic clover gaugefield configurations, with spatial extents of L similar to 2.0, 2.5, 3.0, and 3.9 fm at a spatial lattice spacing of b(s) similar to 0.123 fm, we find an H dibaryon bound by B(infinity)(H) = 16.6 +/ 2.1 +/ 4.6 MeV at a pion mass of m(pi) similar to 389 MeV.
Publication Stats
1k  Citations  
187.24  Total Impact Points  
Top Journals
Institutions

2015

University of Bonn
Bonn, North RhineWestphalia, Germany


2013

Forschungszentrum Jülich
 Institute for Advanced Simulation (IAS)
Jülich, North RhineWestphalia, Germany


20062013

Lawrence Livermore National Laboratory
 • Physical & Life Sciences Directorate
 • Physics Division
Livermore, California, United States
