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Andre Walker-Loud

Publications and source records attributed to Andre Walker-Loud.

At least 37 records · Page 2Linked to original sources
arXiv2016

Massive photons: an infrared regularization scheme for lattice QCD+QED

Standard methods for including electromagnetic interactions in lattice quantum chromodynamics calculations result in power-law finite-volume corrections to physical quantities. Removing these by extrapolation requires costly computations at multiple volumes. We introduce a photon mass to alternatively regulate the infrared, and rely on effective field theory to remove its unphysical effects. Electromagnetic modifications to the hadron spectrum are reliably estimated with a precision and cost comparable to conventional approaches that utilize multiple larger volumes. A significant overall cost advantage emerges when accounting for ensemble generation. The proposed method may benefit lattice calculations involving multiple charged hadrons, as well as quantum many-body computations with long-range Coulomb interactions.

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arXiv2015

Photon mass term as an IR regularization for QCD+QED on the lattice

Inclusion of QED in lattice QCD calculations can lead to power-law volume artifacts as a consequence of the long-range nature of the interaction. Such artifacts must be removed by extrapolation in order to attain reliable infinite volume estimates of observables and quantities derived from them. As an alternative to this methodology, we consider the use of a photon mass term as an infrared regulator for QCD+QED, and explore the viability of its use in determining hadron mass shifts and splittings.

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arXiv2015

Baryon mass splittings and strong CP violation in SU(3) Chiral Perturbation Theory

We study $SU(3)$ flavor-breaking corrections to the relation between the octet baryon masses and the nucleon-meson CP-violating interactions induced by the QCD $\barθ$ term. We work within the framework of $SU(3)$ chiral perturbation theory and work through next-to-next-to-leading order in the $SU(3)$ chiral expansion, which is $\mathcal{O}(m_q^2)$. At lowest order, the CP-odd couplings induced by the QCD $\barθ$ term are determined by mass splittings of the baryon octet, the classic result of Crewther et al. We show that for each isospin-invariant CP-violating nucleon-meson interaction there exists one relation which is respected by loop corrections up to the order we work, while other leading-order relations are violated. With these relations we extract a precise value of the pion-nucleon coupling $\bar g_0$ by using recent lattice QCD evaluations of the proton-neutron mass splitting. In addition, we derive semi-precise values for CP-violating coupling constants between heavier mesons and nucleons with $\sim 30\%$ uncertainty and discuss their phenomenological impact on electric dipole moments of nucleons and nuclei.

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arXiv2015

Nuclear Parity Violation from Lattice QCD

The electroweak interaction at the level of quarks and gluons are well understood from precision measurements in high energy collider experiments. Relating these fundamental parameters to Hadronic Parity Violation in nuclei however remains an outstanding theoretical challenge. One of the most interesting observables in this respect is the parity violating hadronic neutral current: it is hard to measure in collider experiments and is thus the least constrained observable of the Standard Model. Precision measurements of parity violating transitions in nuclei can help to improve these constraints. In these systems however, the weak interaction is masked by effects of the seven orders of magnitude stronger non-perturbative strong interaction. Therefore, in order to relate experimental measurements of the parity violating pion-nucleon couplings to the fundamental Lagrangian of the SM, these non-perturbative effects have to be well understood. In this paper, we are going to present a Lattice QCD approach for computing the $ΔI{=}2$ parity violating matrix element in proton proton scattering. This process does not involve disconnected diagrams in the isospin symmetric limit and is thus a perfect testbed for studying the feasibility of the more involved calculation of the parity violating pion-nucleon coupling.

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arXiv2015

Two-nucleon scattering in multiple partial waves

We determine scattering phase shifts for S,P,D, and F partial wave channels in two-nucleon systems using lattice QCD methods. We use a generalization of Luscher's finite volume method to determine infinite volume phase shifts from a set of finite volume ground- and excited-state energy levels on two volumes, V=(3.4 fm)^3 and V=(4.5 fm)^3. The calculations are performed in the SU(3)-flavor limit, corresponding to a pion mass of approximately 800 MeV. From the energy dependence of the phase shifts we are able to extract scattering parameters corresponding to an effective range expansion.

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arXiv2015

High-Performance I/O: HDF5 for Lattice QCD

Practitioners of lattice QCD/QFT have been some of the primary pioneer users of the state-of-the-art high-performance-computing systems, and contribute towards the stress tests of such new machines as soon as they become available. As with all aspects of high-performance-computing, I/O is becoming an increasingly specialized component of these systems. In order to take advantage of the latest available high-performance I/O infrastructure, to ensure reliability and backwards compatibility of data files, and to help unify the data structures used in lattice codes, we have incorporated parallel HDF5 I/O into the SciDAC supported USQCD software stack. Here we present the design and implementation of this I/O framework. Our HDF5 implementation outperforms optimized QIO at the 10-20% level and leaves room for further improvement by utilizing appropriate dataset chunking.

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arXiv2013

The Scalar Strange Content of the Nucleon from Lattice QCD

The scalar strange-quark matrix element of the nucleon is computed with lattice QCD. A mixed-action scheme is used with domain-wall valence fermions computed on the staggered MILC sea-quark configurations. The matrix element is determined by making use of the Feynman-Hellmann theorem which relates this strange matrix element to the change in the nucleon mass with respect to the strange-quark mass. The final result of this calculation is m_s < N | s-bar s| N > = 49 +-10 +- 15 MeV and, correspondingly f_s = m_s < N | s-bar s |N > / m_N = 0.051 +- 0.011 +- 0.016. Given the lack of a quantitative comparison of this phenomenologically important quantity determined from various lattice QCD calculations, we take the opportunity to present such an average. The resulting conservative determination is f_s = 0.043 +- 0.011.

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arXiv2013

Baryons in/and Lattice QCD

Anchoring our understanding of low-energy nuclear and hadronic physics to the fundamental theory of strong interactions, QCD, remains and outstanding challenge for physicists. Lattice QCD and chiral perturbation theory are the most powerful theoretical tools we have at our disposal to make this connection. These tools share a symbiotic relationship as chiral perturbation theory is used to understand the light quark mass dependence of observables, while lattice QCD is used to determine the values of the unknown operator coefficients appearing in the chiral Lagrangian. In this talk, I review our present understanding of select single baryon properties from lattice QCD and chiral perturbation theory, highlighting some of the challenges and discussing some unresolved puzzles.

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arXiv2012

Cottingham formula for the electromagnetic self-energy contribution to M_p - M_n

We provide an update of the determination of the electromagnetic self-energy contribution to M_p - M_n based upon Cottingham's Formula. A technical oversight in the literature is uncovered: the application of the Cottingham Formula requires the use of a subtracted dispersion integral; an argument to evade the subtraction function was presented; the argument was based on false assumptions about the scaling violations of the parton model, a point first mentioned by J. C. Collins. We elucidate this point and utilize low-energy effective theory to relate the unknown subtraction function to the nucleon isovector magnetic polarizability. This allows us to provide the first reliable determination of dM = 1.30(03)(47) MeV [1].

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arXiv2012

The Electromagnetic Self-Energy Contribution to M_p - M_n and the Isovector Nucleon Magnetic Polarizability

We update the determination of the isovector nucleon electromagnetic self-energy, valid to leading order in QED. A technical oversight in the literature concerning the elastic contribution to Cottingham's formula is corrected and modern knowledge of the structure functions is used to precisely determine the inelastic contribution. We find δM_{p-n}^γ= 1.30(03)(47) MeV. The largest uncertainty arises from a subtraction term required in the dispersive analysis, which can be related to the isovector magnetic polarizability. With plausible model assumptions, we can combine our calculation with additional input from lattice QCD to constrain this polarizability as: β_{p-n} = -0.87(85) x 10^{-4} fm^3.

nucl-th↗
arXiv2011

Evidence for chiral logarithms in the baryon spectrum

Using precise lattice QCD computations of the baryon spectrum, we present the first direct evidence for the presence of contributions to the baryon masses which are non-analytic in the light quark masses; contributions which are often denoted "chiral logarithms". We isolate the poor convergence of SU(3) baryon chiral perturbation theory to the flavor-singlet mass combination. The flavor-octet baryon mass splittings, which are corrected by chiral logarithms at next to leading order in SU(3) chiral perturbation theory, yield baryon-pion axial coupling constants D, F, C and H consistent with QCD values; the first evidence of chiral logarithms in the baryon spectrum. The Gell-Mann--Okubo relation, a flavor-27 baryon mass splitting, which is dominated by chiral corrections from light quark masses, provides further evidence for the presence of non-analytic light quark mass dependence in the baryon spectrum; we simultaneously find the GMO relation to be inconsistent with the first few terms in a taylor expansion in m_s - m_l, which must be valid for small values of this SU(3) breaking parameter. Additional, more definitive tests of SU(3) chiral perturbation theory will become possible with future, more precise, lattice calculations.

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arXiv2011

Evidence for non-analytic light quark mass dependence in the baryon spectrum

Using precise lattice QCD computations of the baryon spectrum, we present the first direct evidence for the presence of contributions to the baryon masses which are non-analytic in the light quark masses; contributions which are often denoted "chiral logarithms". We isolate the poor convergence of SU(3) baryon chiral perturbation theory to the flavor-singlet mass combination. The flavor-octet baryon mass splittings, which are corrected by chiral logarithms at next to leading order in SU(3) chiral perturbation theory, yield baryon-pion axial coupling constants D, F, C and H consistent with QCD values; the first evidence of chiral logarithms in the baryon spectrum. The Gell-Mann--Okubo relation, a flavor-27 baryon mass splitting, which is dominated by chiral corrections from light quark masses, provides further evidence for the presence of non-analytic light quark mass dependence in the baryon spectrum; we simultaneously find the GMO relation to be inconsistent with the first few terms in a taylor expansion in m_s - m_l, which must be valid for small values of this SU(3) breaking parameter. Additional, more definitive tests of SU(3) chiral perturbation theory will become possible with future, more precise, lattice calculations.

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arXiv2010

Kaon Thresholds and Two-Flavor Chiral Expansions for Hyperons

Two-flavor chiral expansions provide a useful perturbative framework to study hadron properties. Such expansions should exhibit marked improvement over the conventional three-flavor chiral expansion. Although one can theoretically formulate two-flavor theories for the various hyperon multiplets, the nearness of kaon thresholds can seriously undermine the effectiveness of the perturbative expansion in practice. We investigate the importance of virtual kaon thresholds on hyperon properties, specifically their masses and isovector axial charges. Using a three-flavor expansion that includes SU(3) breaking effects, we uncover the underlying expansion parameter governing the description of virtual kaon thresholds. For spin-half hyperons, this expansion parameter is quite small. Consequently virtual kaon contributions are well described in the two-flavor theory by terms analytic in the pion mass-squared. For spin three-half hyperons, however, one is closer to the kaon production threshold, and the expansion parameter is not as small. Breakdown of SU(2) chiral perturbation theory is shown to arise from a pole in the expansion parameter associated with the kaon threshold. Estimating higher-order corrections to the expansion parameter is necessary to ascertain whether the two-flavor theory of spin three-half hyperons remains perturbative. We find that, despite higher-order corrections, there is a useful perturbative expansion for the masses and isovector axial charges of both spin-half and spin three-half hyperons.

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arXiv2010

Towards a direct lattice calculation of m_d - m_u

We describe an independent method for determining the strong-isospin breaking mass parameter, 2delta = m_d - m_u, which utilizes the baryon spectrum. We use a prudent partially quenched choice of splitting the valence quark masses symmetrically about the light sea quark mass. This choice has the consequence of mitigating the most severe partial quenching artifacts. We also discuss the most significant hurdle to this method which is determining the electromagnetic self-energy of the neutron-proton mass splitting, a challenge which lacks a satisfactory answer. Despite these issues, the phenomenologically interesting dependence of m_n - m_p on delta can be determined.

hep-lat↗
arXiv2010

Nucleon Magnetic Moments and Electric Polarizabilities

Electromagnetic properties of the nucleon are explored with lattice QCD using a novel technique. Focusing on background electric fields, we show how the electric polarizability can be extracted from nucleon correlation functions. A crucial step concerns addressing contributions from the magnetic moment, which affects the relativistic propagation of nucleons in electric fields. By properly handing these contributions, we can determine both magnetic moments and electric polarizabilities. Lattice results from anisotropic clover lattices are presented. Our method is not limited to the neutron; we show results for the proton as well.

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arXiv2010

Heavy-Baryon Spectroscopy from Lattice QCD

We use a four-dimensional lattice calculation of the full-QCD (quantum chromodynamics, the non-abliean gauge theory of the strong interactions of quarks and gluons) path integrals needed to determine the masses of the charmed and bottom baryons. In the charm sector, our results are in good agreement with experiment within our systematics, except for the spin-1/2 $Ξ_{cc}$, for which we found the isospin-averaged mass to be $Ξ_{cc}$ to be $3665\pm17\pm14^{+0}_{-78}$ MeV. We predict the mass of the (isospin-averaged) spin-1/2 $Ω_{cc}$ to be $3763\pm19\pm26^{+13}_{-79}$ {MeV}. In the bottom sector, our results are also in agreement with experimental observations and other lattice calculations within our statistical and systematic errors. In particular, we find the mass of the $Ω_b$ to be consistent with the recent CDF measurement. We also predict the mass for the as yet unobserved $Ξ^\prime_b$ to be 5955(27) MeV.

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arXiv2010

Extracting Nucleon Magnetic Moments and Electric Polarizabilities from Lattice QCD in Background Electric Fields

Nucleon properties are investigated in background electric fields. As the magnetic moments of baryons affect their relativistic propagation in constant electric fields, electric polarizabilities cannot be determined without knowledge of magnetic moments. This is analogous to the experimental situation, for which determination of polarizabilities from the Compton amplitude requires subtraction of Born terms. With the background field method, we devise combinations of nucleon correlation functions in constant electric fields that isolate magnetic moments and electric polarizabilities. Using an ensemble of anisotropic gauge configurations with dynamical clover fermions, we demonstrate how both observables can be determined from lattice QCD simulations in background electric fields. We obtain results for the neutron and proton, however, our study is currently limited to electrically neutral sea quarks. The value we extract for the nucleon isovector magnetic moment is comparable to those obtained from measuring lattice three-point functions at similar pion masses.

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arXiv2009

A Lattice Test of 1/N_c Baryon Mass Relations

1/N_c baryon mass relations are compared with lattice simulations of baryon masses using different values of the light-quark masses, and hence different values of SU(3) flavor-symmetry breaking. The lattice data clearly display both the 1/N_c and SU(3) flavor-symmetry breaking hierarchies. The validity of 1/N_c baryon mass relations derived without assuming approximate SU(3) flavor-symmetry also can be tested by lattice data at very large values of the strange quark mass. The 1/N_c expansion constrains the form of discretization effects; these are suppressed by powers of 1/N_c by taking suitable combinations of masses. This 1/N_c scaling is explicitly demonstrated in the present work.

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