arXiv subjects
Publications and source records attributed to William Detmold.
Dynamical 2+1 flavor lattice QCD is used to calculate the masses of bottom hadrons, including B mesons, singly and doubly bottom baryons, and for the first time also the triply-bottom baryon Omega_bbb. The domain wall action is used for the up-, down-, and strange quarks (both valence and sea), while the bottom quark is implemented with non-relativistic QCD. A calculation of the bottomonium spectrum is also presented.
Electromagnetic properties of hadrons can be computed by lattice simulations of QCD in background fields. We demonstrate new techniques for the investigation of charged hadron properties in electric fields. Our current calculations employ large electric fields, motivating us to analyze chiral dynamics in strong QED backgrounds, and subsequently uncover surprising non-perturbative effects present at finite volume.
The $π^+Σ^+$, $π^+Ξ^0$, $K^+p$, $K^+n$, and $\overline{K}{}^0 Ξ^0$ scattering lengths are calculated in mixed-action Lattice QCD with domain-wall valence quarks on the asqtad-improved coarse MILC configurations at four light-quark masses, and at two light-quark masses on the fine MILC configurations. Heavy Baryon Chiral Perturbation Theory with two and three flavors of light quarks is used to perform the chiral extrapolations. To the order we work in the three-flavor chiral expansion, the kaon-baryon processes that we investigate show no signs of convergence. Using the two-flavor chiral expansion for extrapolation, the pion-hyperon scattering lengths are found to be $a_{π^+Σ^+}=-0.197\pm0.017$ fm, and $a_{π^+Ξ^0}=-0.098\pm0.017$ fm, where the comprehensive error includes statistical and systematic uncertainties.
Charged and neutral, pion and kaon electric polarizabilities are extracted from lattice QCD using an ensemble of anisotropic gauge configurations with dynamical clover fermions. We utilize classical background fields to access the polarizabilities from two-point correlation functions. Uniform background fields are achieved by quantizing the electric field strength with the proper treatment of boundary flux. These external fields, however, are implemented only in the valence quark sector. A novel method to extract charge particle polarizabilities is successfully demonstrated for the first time.
We present the results of an exploratory Lattice QCD calculation of three-baryon systems through a high-statistics study of one ensemble of anisotropic clover gauge-field configurations with a pion mass of m_π~ 390 MeV. Because of the computational cost of the necessary contractions, we focus on correlation functions generated by interpolating-operators with the quantum numbers of the $Ξ^0Ξ^0 n$ system, one of the least demanding three baryon systems in terms of the number of contractions. We find that the ground state of this system has an energy of E_{Ξ^0Ξ^0n}= 3877.9\pm 6.9\pm 9.2\pm3.3 MeV corresponding to an energy-shift due to interactions of δE_{Ξ^0Ξ^0n}=E_{Ξ^0Ξ^0n}-2M_{Ξ^0} -M_n=4.6\pm 5.0\pm 7.9\pm 4.2 MeV. There are a significant number of time-slices in the three-baryon correlation function for which the signal-to-noise ratio is only slowly degrading with time. This is in contrast to the exponential degradation of the signal-to-noise ratio that is observed at larger times, and is due to the suppressed overlap of the source and sink interpolating-operators that are associated with the variance of the three-baryon correlation function onto the lightest eigenstates in the lattice volume (mesonic systems). As one of the motivations for this area of exploration is the calculation of the structure and reactions of light nuclei, we also present initial results for a system with the quantum numbers of the triton (pnn). This present work establishes a path to multi-baryon systems, and shows that Lattice QCD calculations of the properties and interactions of systems containing four and five baryons are now within sight.
We present the results of high-statistics calculations of correlation functions generated with single-baryon interpolating operators on an ensemble of dynamical anisotropic gauge-field configurations generated by the Hadron Spectrum Collaboration using a tadpole-improved clover fermion action and Symanzik-improved gauge action. A total of 292,500 sets of measurements are made using 1194 gauge configurations of size 20^3 x 128 with an anisotropy parameter ξ= b_s/b_t = 3.5, a spatial lattice spacing of b_s=0.1227\pm 0.0008 fm, and pion mass of m_π~ 390 MeV. Ground state baryon masses are extracted with fully quantified uncertainties that are at or below the ~0.2%-level in lattice units. The lowest-lying negative-parity states are also extracted albeit with a somewhat lower level of precision. In the case of the nucleon, this negative-parity state is above the Nπthreshold and, therefore, the isospin-1/2 πN s-wave scattering phase-shift can be extracted using Luescher's method. The disconnected contributions to this process are included indirectly in the gauge-field configurations and do not require additional calculations. The signal-to-noise ratio in the various correlation functions is explored and is found to degrade exponentially faster than naive expectations on many time-slices. This is due to backward propagating states arising from the anti-periodic boundary conditions imposed on the quark-propagators in the time-direction. We explore how best to distribute computational resources between configuration generation and propagator measurements in order to optimize the extraction of single baryon observables.
Dynamical 2+1 flavour lattice QCD is used to calculate the splittings between the masses of mesons and baryons containing a single static heavy quark and domain-wall light and strange quarks. Our calculations are based on the dynamical domain-wall gauge field configurations generated by the RBC and UKQCD collaborations at a spatial volume of (2.7 fm)^3 and a range of quark masses with a lightest value corresponding to a (partially-quenched) pion mass of 275 MeV. When extrapolated to the physical values of the light quark masses, the results of our calculations are generally in good agreement with experimental determinations in the bottom sector. However, the static limit splittings between the Omega_b^- baryon and other bottom hadrons tend to slightly underestimate those obtained using the recent D-zero measurement of the Omega_b^-.
In this contribution, I summarise the studies of the properties of Bose-Einstein condensed systems composed of up to twelve pions or kaons carried out by the NPLQCD collaboration. These investigations have provided precise determination the I=2 pi-pi and I=1 K-K scattering lengths and the first determination of three-hadron interactions from QCD, finding a repulsive three-pion interaction of size consistent with naive dimensional analysis and a three kaon interaction consistent with zero. We have also determined the isospin (strangeness) density dependence of the isospin (strangeness) chemical potential, finding results in surprisingly good agreement with chiral perturbation theory.
Lattice QCD is used to calculate the potential between a static quark and anti-quark in the presence of a finite density of pi^+'s. Correlation functions of multiple pi^+'s are used in conjunction with Wilson-loop correlators to determine the difference between the Q\overline{Q}-potential in free-space and in the presence of a pion condensate. The modifications to the potential are found to have significant dependence on the Q\overline{Q} separation over the range r < 1 fm explored in this work. Our results are consistent with the pion-condensate behaving as a (nonlinear) chromo-dielectric.
The response of hadrons to electromagnetic probes is highly constrained by chiral dynamics; but, in some cases, predictions have not compared well with experimental data. The lattice can be used to test the chiral electromagnetism of hadrons and ultimately confront experiment. We use background field techniques to study the electromagnetic polarizabilities of hadrons. Focusing on simulations in background electric fields, we present preliminary results for both charged and neutral particle polarizabilities. The former are extracted using a novel method.
Kaon condensation may play an important role in the structure of hadronic matter at densities greater than that of nuclear matter, as exist in the interior of neutron stars. We present the results of the first lattice QCD investigation of kaon condensation obtained by studying systems containing up to twelve negatively charged kaons. Surprisingly, the properties of the condensate that we calculate are remarkably well reproduced by leading order chiral perturbation theory. In our analysis, we also determine the three-kaon interaction from the multi-kaon systems and update our results for pion condensates.
The ground-state energies of systems containing up to twelve $π^+$'s in a spatial volume V ~ (2.5 fm)^3 are computed in dynamical, mixed-action lattice QCD at a lattice spacing of ~ 0.125 fm for four different values of the light quark masses. Clean signals are seen for each ground state, allowing for a precise extraction of both the $π^+π^+$ scattering length and $π^+π^+π^+$-interaction from a correlated analysis of systems containing different numbers of $π^+$'s. This extraction of the $π^+π^+$ scattering length is consistent with than that from the $π^+π^+$-system alone. The large number of systems studied here significantly strengthens the arguments presented in our earlier work and unambiguously demonstrates the presence of a low energy $π^+π^+π^+$-interaction. The equation of state of a $π^+$ gas is investigated using our numerical results and the density dependence of the isospin chemical potential for these systems agrees well with the theoretical expectations of leading order chiral perturbation theory. The chemical potential is found to receive a substantial contribution from the $π^+π^+π^+$-interaction at the lighter pion masses. An important technical aspect of this work is the demonstration of the necessity of performing propagator contractions in greater than double precision to extract the correct results.
The volume dependence of the ground-state energy of n identical bosons with short-range interactions in a periodic spatial volume with sides of length L is calculated at order L^{-7} in the large volume expansion. This result will enable a refined determination of the pi^+ pi^+ pi^+ interaction from lattice QCD calculations.
We investigate the light-quark mass and spatial volume dependence of matrix elements of Delta B = 2 and Delta C = 2 four-fermion operators. These operators are relevant for Neutral B(s) and D mixing in and beyond the Standard Model. An important conclusion of this work is that the chiral extrapolations for matrix elements of heavy-light meson mixing beyond the Standard Model are more complicated than that for the Standard Model mixing matrix elements.
The ground-state energies of 2, 3, 4 and 5 π^+'s in a spatial volume V (2.5 fm)^3 are computed with lattice QCD. By eliminating the leading contribution from three-π^+ interactions, particular combinations of these n-π^+ ground-state energies provide precise extractions of the π^+π^+ scattering length in agreement with that obtained from calculations involving only two π^+'s. The three-π^+ interaction can be isolated by forming other combinations of the n-π^+ ground-state energies. We find a result that is consistent with a repulsive three-π^+ interaction for m_π< 352 MeV.
We calculate the volume dependence of the ground-state energy of n identical bosons with short-range repulsive interactions in a periodic spatial volume of side L, up to and including terms of order 1/L^6. With this result, Lattice QCD calculations of the ground-state energies of three or more pions will allow for a systematic extraction of the three-pion interaction at this order in the volume expansion.
Using heavy meson chiral perturbation theory, we consider the light quark-mass and spatial volume dependence of the matrix elements of ΔB=2 and ΔC=2 four-quark operators relevant for B^{0}_{(s)}{-}\bar{B}^{0}_{(s)} and D^{0}{-}\bar{D}^{0} mixing, and the B_{s} meson width difference. Our results for these matrix elements are obtained in the N_{f}=2+1 partially quenched theory, which becomes full QCD in the limit where sea and valence quark masses become equal. They can be used in extrapolation of lattice calculations of these matrix elements to the physical light quark masses and to infinite volume. An important conclusion of this paper is that the chiral extrapolations for matrix elements of heavy-light meson mixing beyond the Standard Model, and those relevant for the B_s width difference are more complicated than that for the Standard Model mixing matrix elements.
The potentials between two B-mesons are computed in the heavy-quark limit using quenched lattice QCD at $m_π\sim 400~{\rm MeV}$. Non-zero central potentials are clearly evident in all four spin-isospin channels, (I,s_l) = (0,0) , (0,1) , (1,0) , (1,1), where s_l is the total spin of the light degrees of freedom. At short distance, we find repulsion in the $I\ne s_l$ channels and attraction in the I=s_l channels. Linear combinations of these potentials that have well-defined spin and isospin in the t-channel are found, in three of the four cases, to have substantially smaller uncertainties than the potentials defined with the s-channel (I,s_l), and allow quenching artifacts from single hairpin exchange to be isolated. The BB*πcoupling extracted from the long-distance behavior of the finite-volume t-channel potential is found to be consistent with quenched calculations of the matrix element of the isovector axial-current. The tensor potentials in both of the s_l = 1 channels are found to be consistent with zero within calculational uncertainties.