arXiv subjects

William Detmold

Publications and source records attributed to William Detmold.

At least 109 records · Page 6Linked to original sources

arXiv2012

Form factors for Lambda_b -> Lambda transitions from lattice QCD

The rare baryonic decays $Λ_b \to Λμ^+ μ^-$ and $Λ_b \to Λγ$ can complement rare $B$ meson decays in constraining models of new physics. In this work, we calculate the relevant $Λ_b \to Λ$ transition form factors at leading order in heavy-quark effective theory using lattice QCD. Our analysis is based on RBC/UKQCD gauge field ensembles with 2+1 flavors of domain-wall fermions, and with lattice spacings of $a\approx 0.11$ fm and $a\approx 0.08$ fm. We compute appropriate ratios of three-point and two-point correlation functions for a wide range of source-sink separations, and extrapolate to infinite separation in order to eliminate excited-state contamination. We then extrapolate the form factors to the continuum limit and to the physical values of the light-quark masses.

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arXiv2012

Lambda_b -> Lambda l+ l- form factors and differential branching fraction from lattice QCD

We present the first lattice QCD determination of the $Λ_b \to Λ$ transition form factors that govern the rare baryonic decays $Λ_b \to Λl^+ l^-$ at leading order in heavy-quark effective theory. Our calculations are performed with 2+1 flavors of domain-wall fermions, at two lattice spacings and with pion masses down to 227 MeV. Three-point functions with a wide range of source-sink separations are used to extract the ground-state contributions. The form factors are extrapolated to the physical values of the light-quark masses and to the continuum limit. We use our results to calculate the differential branching fractions for $Λ_b \to Λl^+ l^-$ with $l=e,μ,τ$ within the standard model. We find agreement with a recent CDF measurement of the $Λ_b \to Λμ^+ μ^-$ differential branching fraction.

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arXiv2012

Nuclear correlation functions in lattice QCD

We consider the problem of calculating the large number of Wick contractions necessary to compute states with the quantum numbers of many baryons in lattice QCD. We consider a constructive approach and a determinant-based approach and show that these methods allow the required contractions to be performed in computationally manageable amount of time for certain choices of interpolating operators. Examples of correlation functions computed using these techniques are shown for the quantum numbers of the light nuclei, He, Be, C, O and Si.

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arXiv2012

Calculation of the heavy-hadron axial couplings g_1, g_2, and g_3 using lattice QCD

In a recent letter [Phys. Rev. Lett. 108, 172003 (2012), arXiv:1109.2480] we have reported on a lattice QCD calculation of the heavy-hadron axial couplings $g_1$, $g_2$, and $g_3$. These quantities are low-energy constants of heavy-hadron chiral perturbation theory (HH$χ$PT) and are related to the $B^*Bπ$, $Σ_b^*Σ_bπ$, and $Σ_b^{(*)}Λ_bπ$ couplings. In the following, we discuss important details of the calculation and give further results. To determine the axial couplings, we explicitly match the matrix elements of the axial current in QCD with the corresponding matrix elements in HH$χ$PT. We construct the ratios of correlation functions used to calculate the matrix elements in lattice QCD, and study the contributions from excited states. We present the complete numerical results and discuss the data analysis in depth. In particular, we demonstrate the convergence of $SU(4|2)$ HH$χ$PT for the axial-current matrix elements at pion masses up to about 400 MeV and show the impact of the nonanalytic loop contributions. Finally, we present additional predictions for strong and radiative decay widths of charm and bottom baryons.

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arXiv2012

Lattice QCD at non-zero isospin chemical potential

Quantum chromodynamics (QCD) at non-zero isospin chemical potential is studied in a canonical approach by analyzing systems of fixed isospin number density. To construct these systems, we develop a range of new algorithms for performing the factorially large numbers of Wick contractions required in multi-hadron systems. We then use these methods to study systems with the quantum numbers of up to 72 $π^+$'s on three ensembles of gauge configurations with spatial extents $L\sim$ 2.0, 2.5 and 3.0 fm, and light quark masses corresponding to a pion mass of {390 MeV}. The ground state energies of these systems are extracted and the volume dependence of these energies is utilized to determine the two- and three- body interactions amongst $π^+$'s. The systems studied correspond to isospin densities of up to $ρ_I\sim 9\ {\rm fm}^{-3}$ and probe isospin chemical potentials, $μ_I$, in the range $m_π \lsim μ_I\ \lsim 4.5\ m_π$, allowing us to investigate aspects of the QCD phase diagram at low temperature and for varying isospin chemical potential. By studying the energy density of the system, we provide numerical evidence for the conjectured transition of the system to a Bose-Einstein condensed phase at $μ_I\ \gsim m_π$.

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arXiv2012

Axial couplings and strong decay widths of heavy hadrons

We calculate the axial couplings of mesons and baryons containing a heavy quark in the static limit using lattice QCD. These couplings determine the leading interactions in heavy hadron chiral perturbation theory and are central quantities in heavy quark physics, as they control strong decay widths and the light-quark mass dependence of heavy hadron observables. Our analysis makes use of lattice data at six different pion masses, 227 MeV < m_π< 352 MeV, two lattice spacings, a=0.085, 0.112 fm, and a volume of (2.7 fm)^3. Our results for the axial couplings are g_1=0.449(51), g_2=0.84(20), and g_3=0.71(13), where g_1 governs the interaction between heavy-light mesons and pions and g_{2,3} are similar couplings between heavy-light baryons and pions. Using our lattice result for g_3, and constraining 1/m_Q corrections in the strong decay widths with experimental data for Σ_c^{(*)} decays, we obtain Γ[Σ_b^{(*)} \to Λ_b π^\pm] = 4.2(1.0), 4.8(1.1), 7.3(1.6), 7.8(1.8) MeV for the Σ_b^+, Σ_b^-, Σ_b^{*+}, Σ_b^{*-} initial states, respectively. We also derive upper bounds on the widths of the Ξ_b^{\prime(*)} baryons.

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arXiv2012

Axial couplings of heavy hadrons from domain-wall lattice QCD

We calculate matrix elements of the axial current for static-light mesons and baryons in lattice QCD with dynamical domain wall fermions. We use partially quenched heavy hadron chiral perturbation theory in a finite volume to extract the axial couplings g_1, g_2, and g_3 from the data. These axial couplings allow the prediction of strong decay rates and enter chiral extrapolations of most lattice results in the b sector. Our calculations are performed with two lattice spacings and with pion masses down to 227 MeV.

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arXiv2011

Medium Effects in Parton Distributions

Understanding the effects of a background hadronic medium on hadronic observables is important in the context of hadron structure. Many experiments probing nucleon structure make use of nuclear targets and unraveling the modifications that ensue is a complex task. Using lattice QCD, we investigate the ab initio computation of hadron structure in a medium, focusing on the structure of the pion in a Bose-condensed gas of pions.

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arXiv2011

Application of Quadrature Methods for Re-Weighting in Lattice QCD

Re-weighting is a useful tool that has been employed in Lattice QCD in different contexts including, tuning the strange quark mass, approaching the light quark mass regime, and simulating electromagnetic fields on top of QCD gauge configurations. In case of re-weighting the sea quark mass, the re-weighting factor is given by the ratio of the determinants of two Dirac operators $D_a$ and $D_b$. A popular approach for computing this ratio is to use a pseudofermion representation of the determinant of the composite operator $Ω=D_a(D_b^\dagger D_b)^{-1} D_a^\dagger$. Here, we study using quadrature methods together with noise vectors to compute the ratio of determinants. We show that, with quadrature methods each determinant can be computed separately using the operators $Ω_a=D_a^\dagger D_a$ and $Ω_b=D_b^\dagger D_b$. We also discuss using bootstrap re-sampling to remove the bias from the determinant estimator.

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arXiv2011

Axial couplings in heavy hadron chiral perturbation theory at the next-to-leading order

We present calculations of axial-current matrix elements between various heavy-meson and heavy-baryon states to the next-to-leading order in heavy hadron chiral perturbation theory in the p-regime. When compared with data from lattice computations or experiments, these results can be used to determine the axial couplings in the chiral Lagrangian. Our calculation is performed in partially-quenched chiral perturbation theory for both SU(4|2) and SU(6|3). We incorporate finite-size effects arising from a single Goldstone meson wrapping around the spatial volume. Results for full QCD with two and three flavours can be obtained straightforwardly by taking the sea-quark masses to be equal to the valence-quark masses. To illustrate the impact of our chiral perturbation theory calculation on lattice computations, we analyse the SU(2) full QCD results in detail. We also study one-loop contributions relevant to the heavy hadron strong-decay amplitudes involving final-state Goldstone bosons, and demonstrate that the quark-mass dependence of these amplitudes can be significantly different from that of the axial current matrix elements containing only single hadron external states.

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arXiv2011

Investigations of QCD at non-zero isospin density

We investigate the QCD phase diagram as a function of isospin chemical potential at a fixed temperature by directly putting large numbers of π^+s into the system. Correlation functions of N π^+s systems involves N!N! contractions, and become extremely expensive when N is large. In order to alleviate this problem, a recursion relation of correlation functions has been derived in Ref. [1] that substantially reduces the number of independent contractions needed and makes the study of many pions systems be possible. In this proceeding this method is investigated numerically. We have also constructed a new method that is even more efficient, enabling us to study systems of up to 72 π^+s.

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arXiv2011

Lattice QCD study of mixed systems of pions and kaons

The different ground state energies of N-pion and M-kaon systems for N+M <=12 are studied in lattice QCD. These energies are then used to extract the various two- and three- body interactions that occur in these systems. Particular attention is paid to additional thermal states present in the spectrum because of the finite temporal extent. These calculations are performed using one ensemble of 2+1 flavor anisotropic lattices with a spatial lattice spacing a_s ~ 0.125 fm, an anisotropy factor ξ=a_s/a_t=3.5, and a spatial volume L^3 ~ (2.5 fm)^3. The quark masses used correspond to pion and kaon masses of m_π ~ 383 MeV and m_K ~ 537 MeV, respectively. The isospin and strangeness chemical potentials of these systems are found to be in the region where chiral perturbation theory and hadronic models predict a phase transition between a pion condensed phase and a kaon condensed phase.

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arXiv2011

New developments in multi-meson systems

New developments in the study of multi-meson systems are reviewed. We highlight a new recursive algorithm for generating the requisite contractions needed for studying complex systems of mesons involving large numbers of particles or multiple species of particles. First results on mixed species systems involving pions and kaons are also presented.

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arXiv2010

A method to study complex systems of mesons in Lattice QCD

Finite density systems can be explored with Lattice QCD through the calculation of multi-hadron correlation functions. Recently, systems with up to 12 $π^+$'s or $K^+$'s have been studied to determine the 3-$π^+$ and 3-$K^+$ interactions, and the corresponding chemical potentials have been determined as a function of density. We derive recursion relations between correlation functions that allow this work to be extended to systems of arbitrary numbers of mesons and to systems containing many different types of mesons, such as $π^+$'s, $K^+$'s, $\bar{D}^0$'s and $B^+$'s. These relations allow for the study of finite-density systems in arbitrary volumes, and for the study of high-density systems.

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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

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

High Statistics Analysis using Anisotropic Clover Lattices: (III) Baryon-Baryon Interactions

Low-energy baryon-baryon interactions are calculated in a high-statistics lattice QCD study on a single ensemble of anisotropic clover gauge-field configurations at a pion mass of m_π~ 390 MeV, a spatial volume of L^3 ~ (2.5 fm)^3, and a spatial lattice spacing of b~0.123 fm. Lüscher's method is used to extract nucleon-nucleon, hyperon-nucleon and hyperon-hyperon scattering phase shifts at one momentum from the one- and two-baryon ground-state energies in the lattice volume. The isospin-3/2 NΣinteractions are found to be highly spin-dependent, and the interaction in the ^3S_1 channel is found to be strong. In contrast, the NΛinteractions are found to be spin-independent, within the uncertainties of the calculation, consistent with the absence of one-pion-exchange. The only channel for which a negative energy-shift is found is ΛΛ, indicating that the ΛΛinteraction is attractive, as anticipated from model-dependent discussions regarding the H-dibaryon. The NN scattering lengths are found to be small, clearly indicating the absence of any fine-tuning in the NN-sector at this pion mass. This is consistent with our previous Lattice QCD calculation of NN interactions. The behavior of the signal-to-noise ratio in the baryon-baryon correlation functions, and in the ratio of correlation functions that yields the ground-state energy splitting is explored. In particular, focus is placed on the window of time slices for which the signal-to-noise ratio does not degrade exponentially, as this provides the opportunity to extract quantitative information about multi-baryon systems.

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arXiv2009

Charged Hadron Properties in Background Electric Fields

We report on a lattice calculation demonstrating a novel new method to extract the electric polarizability of charged pseudo-scalar mesons by analyzing two point correlation functions computed in classical background electric fields.

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