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At least 811 records · Page 45Linked to original sources

arXiv2010

Nucleon and $N^* (1535)$ Distribution Amplitudes

The QCDSF collaboration has investigated the distribution amplitudes and wavefunction normalization constants of the nucleon and its parity partner, the $N^* (1535)$. We report on recent progress in the calculation of these quantities on configurations with two dynamical flavors of $\mathcal{O}(a)$-improved Wilson fermions. New data at pion masses of approximately 270 MeV helps in significantly reducing errors in the extrapolation to the physical point.

hep-lat↗

arXiv2010

Hunting the static energy renormalon

We employ Numerical Stochastic Perturbation Theory (NSPT) together with twisted boundary conditions (TBC) to search for the leading renormalon in the perturbative expansion of the static energy. This renormalon is expected to emerge four times faster than the one for the gluon conden- sate in the plaquette. We extract the static energy from Polyakov loop calculations up to 12 loops and present preliminary results, indicating a significant step towards confirming the theoretical expectation.

hep-lat↗

arXiv2010

Heavy-light current-current correlators

The current-current correlator method has been used successfully to obtain very accurate results for quark masses and the coupling alpha_s. The calculations were done using Highly Improved Staggered Quarks (HISQ) and heavy-heavy meson correlators. We now extend this work to the significantly more challenging heavy-light case, reporting the first results here. The aim is to determine nonperturbative Z factors for NRQCD heavy-light currents, but first we test the method in the HISQ case where Z=1.

hep-lat↗

arXiv2010

Axial and electromagnetic observables of hyperons in 2-flavor chiral perturbation theory

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 in principle one can formulate two-flavor theories for the various hyperon multiplets, the nearness of kaon thresholds can seriously undermine the effectiveness of such two-flavor theories in practice. We investigate the importance of virtual kaon thresholds on hyperon properties, specifically their isovector axial charges and electromagnetic observables. In particular we uncover the underlying expansion parameter governing the description of virtual kaon thresholds. For spin-half hyperons, this expansion parameter is under theoretical control. As a result, the 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. We find that, despite the fact that higher-order corrections to the expansion parameter is necessary to ascertain whether the two-flavor theory of spin three-half hyperons remains perturbative, there is a useful perturbative expansion for isovector axial charges and magnetic moments of both spin-half and spin three-half hyperons.

hep-lat↗

arXiv2010

Charmonium-nucleon interaction from lattice QCD with a relativistic heavy quark action

Detailed information of the low-energy interaction between the charmonia (ηc and J/ψ) and the nucleon is indispensable for exploring the formation of charmonium bound to nuclei. In order to investigate the charmonium-nucleon interactions at low energies, we adopt two essentially different approaches in lattice QCD simulations. The charmonium-nucleon potential can be calculated from the equal-time Bethe-Salpeter amplitude through the effective Schrödinger equation. This novel method is based on the same idea originally applied for the nucleon force by Aoki- Hatsuda-Ishii. Another approach is to utilize extended Lüscher's formula with partially twisted boundary conditions, which allows us to calculate the s-wave phase shift at any small value of the relative momentum even in a finite box. We then extract model independent information of the scattering length and the effective range from the phase shift through the effective-range expansion. Our simulations are carried out at a lattice cutoff of $1/a \approx$ 2 GeV in a spatial volume of (3 fm)^3 with the non-perturbatively O(a)-improved Wilson fermions for the light quarks and a relativistic heavy quark action for the charm quark. Although our main results are calculated in quenched lattice calculations, we also present a preliminary full QCD result by using the 2+1 flavor gauge configurations generated by PACS-CS Collaboration. We have found that the charmonium-nucleon potential is weakly attractive at short distances and exponentially screened at large distances. We have also successfully evaluated both the scattering length and effective range from the charmonium-nucleon scattering phase shift.

hep-lat↗

arXiv2010

Extracting scattering parameters using the isospin chemical potential

Hadronic scattering mediated through the strong interaction has been an area of great interest for both theory and experiment. Recently, lattice QCD calculations of scattering processes from first principles have seen remarkable progress, including multiple calculations of two-pion scattering lengths that agree with experiment to within a few percent. However, there exists a certain class of scattering processes, such as pion-nucleon scattering, that contain annihilation diagrams, which are often prohibitively expensive to simulate on the lattice. In this talk, I will present a method to extract certain parameters from this class of scattering processes by employing an isospin chemical potential, which can be simulated on the lattice as a result of its positive-definite fermion determinant.

hep-lat↗

arXiv2010

Meson and baryon masses with low mode averaging

We describe and test a method known in the literature as low mode averaging to improve Euclidean two-point functions in lattice QCD using the low-lying eigenmodes of the Wilson-Dirac operator D. The contribution from the low modes is averaged over all positions of the quark sources while the contribution from high modes is calculated in the traditional way using one source point per lattice. We apply this method to different baryon and meson two-point functions and we compare the improvements using the eigenmodes of the non-hermitian operator D and the eigenmodes of the hermitian operator Q=gamma5 D. The convergence strongly depends on the parity of the states.

hep-lat↗

arXiv2010

Systematic errors in extracting nucleon properties from lattice QCD

Form factors of the nucleon have been extracted from experiment with high precision. However, lattice calculations have failed so far to reproduce the observed dependence of form factors on the momentum transfer. We have embarked on a program to thoroughly investigate systematic effects in lattice calculation of the required three-point correlation functions. Here we focus on the possible contamination from higher excited states and present a method which is designed to suppress them. Its effectiveness is tested for several baryonic matrix elements, different lattice sizes and pion masses.

hep-lat↗

arXiv2010

A worm-inspired algorithm for the simulation of Abelian gauge theories

We present an algorithm in which the all-order strong coupling expansion of the Abelian U(1) gauge theory with Wilson plaquette action is sampled. In addition to the vacuum closed surface graphs of the partition function we propose to also allow for a class of defects (boundaries) related to Wilson loops in the ensemble. The efficiency of our scheme in estimating various observables is compared to a standard Metropolis algorithm.

hep-lat↗

arXiv2010

Curvature of the QCD phase transition line in a finite volume

The curvature which characterizes the QCD phase transition at finite temperature and small values of the chemical potential is accessible to lattice simulations. The results for this quantity which have been obtained by several different lattice simulation methods differ due to different numbers of flavors, different pion masses and different sizes of the simulation volume. In order to reconcile these results, it is important to investigate finite-volume effects on the curvature. We investigate the curvature of the chiral phase transition line at finite temperature and chemical potential in a finite volume. We use a phenomenological model for chiral symmetry breaking and apply non-perturbative functional renormalization group methods which account for critical long-range fluctuations at the phase transition. We find an intermediate volume region in which the curvature of the phase transition line is actually reduced relative to its infinite-volume value, provided periodic spatial boundary conditions are chosen for the quark fields. Size and location of this region depend on the value of the pion mass. Such an effect could account for differences in the curvature between lattice simulations in differently sized volumes and from functional methods in the infinite volume limit. We discuss implications of our results for the QCD phase diagram.

hep-ph↗

arXiv2010

Can Lorentz-breaking fermionic condensates form in large N strongly-coupled Lattice Gauge Theories?

The possibility of Lorentz symmetry breaking (LSB) has attracted considerable attention in recent years for a variety of reasons, including the attractive prospect of the graviton as a Goldstone boson. Though a number of effective field theory analyses of such phenomena have recently been given it remains an open question whether they can take place in an underlying UV complete theory. Here we consider the question of LSB in large N lattice gauge theories in the strong coupling limit. We apply techniques that have previously been used to correctly predict the formation of chiral symmetry breaking condensates in this limit. Generalizing such methods to other composite operators we find that certain LSB condensates can indeed form. In addition, the interesting possibility arises of condensates that 'lock' internal with external symmetries.

hep-lat↗

arXiv2010

Fisher's zeros as boundary of RG flows in complex coupling space

We discuss the possibility of extending the RG flows to complex coupling spaces. We argue that the Fisher's zeros are located at the boundary of the complex basin of attraction of IR fixed points. We support this picture with numerical calculations at finite volume for2D O(N) models in the large-N limit and the hierarchical Ising model using the two-lattice matching method. We present numerical evidence supporting the idea that, as the volume increases, the Fisher's zeros of 4-dimensional pure gauge SU(2) lattice gauge theory with a Wilson action, stabilize at a distance larger than 0.1 from the real axis in the complex beta=4/g^2 plane. We show that when a positive adjoint term is added, the zeros get closer to the real axis. We compare the situation with the U(1) case. We discuss the implications of this new framework for proofs of confinement and searches for nontrivial IR fixed points in models beyond the standard model.

hep-lat↗

arXiv2010

An application of the variational analysis to calculate the meson spectral functions

We present a new method to calculate meson spectral functions (SPFs) on the lattice based on a variational method. Because, on a finite volume lattice, the meson SPFs have discrete spectra only, a suitable way to extract such discrete signals is needed. Using a variational method, we can calculate several discrete quantities such as the position and the area of spectral peaks for low-lying states. Moreover data accuracy can be improved by increasing the number of basis functions. In this report, we first confirm our method in the free quark case and show that our method works well. Then, we apply the method to a quenched lattice QCD simulation and calculate the charmonium SPFs for S and P-waves at zero temperature. Our results for the ground state are well consistent with the position and the area of the lowest peaks of charmonium SPFs calculated by the conventional maximum entropy method. For first excited states, the signals may be reliablly extracted with our method because the charmonium mass converges to a value close to the experimental one when the number of basis functions is increased. We also investigate the SPFs for S-wave charmonia at below and above $T_c$. Our results suggest that $J/ψ$ and $η_c$ may survive up to 1.4$T_c$.

hep-lat↗

arXiv2010

Exploring the chiral regime of $N_f=2$ QCD with mixed actions

We report on our simulations with Neuberger valence fermions on CLS $N_f=2$ configurations with non-perturbatively $O(a)$-improved Wilson sea quarks. We consider the matching of QCD to ChPT in the so called mixed-regime in which the sea quarks are in the $p$-regime while the valence quarks are in the $ε$-regime. From this matching, we can get information on $Σ$, $L_6$ and the combination $L_8+2L_6+2L_7$.

hep-lat↗

arXiv2013

The Thomas-Fermi Quark Model: Non-Relativistic Aspects

The first numerical investigation of non-relativistic aspects of the Thomas-Fermi (TF) statistical multi-quark model is given. We begin with a review of the traditional TF model without an explicit spin interaction and find that the spin splittings are too small in this approach. An explicit spin interaction is then introduced which entails the definition of a generalized spin "flavor". We investigate baryonic states in this approach which can be described with two inequivalent wave functions; such states can however apply to multiple degenerate flavors. We find that the model requires a spatial separation of quark flavors, even if completely degenerate. Although the TF model is designed to investigate the possibility of many-quark states, we find surprisingly that it may be used to fit the low energy spectrum of almost all ground state octet and decuplet baryons. The charge radii of such states are determined and compared with lattice calculations and other models. The low energy fit obtained allows us to extrapolate to the six-quark doubly strange {\it H}-dibaryon state, flavor symmetric strange states of higher quark content and possible six quark nucleon-nucleon resonances. The emphasis here is on the {\it systematics} revealed in this approach. We view our model as a versatile and convenient tool for quickly assessing the characteristics of new, possibly bound, particle states of higher quark number content.

hep-ph↗

arXiv2013

CDT and the Search for a Theory of Quantum Gravity

Causal Dynamical Triangulations provide a non-perturbative regularization of a theory of quantum gravity. We describe how this approach connects with the asymptotic safety program and Ho\vrava-Lifshitz gravity theory, and present the most recent results from computer simulations.

gr-qc↗

arXiv2013

The Sign Problem, PT Symmetry and Abelian Lattice Duality

Lattice field theories with complex actions are not easily studied using conventional analytic or simulation methods. However, a large class of these models are invariant under CT, where C is charge conjugation and T is time reversal, including models with non-zero chemical potential. For Abelian models in this class, lattice duality maps models with complex actions into dual models with real actions. For extended regions of parameter space, calculable for each model, duality resolves the sign problem for both analytic methods and computer simulations. Explicit duality relations are given for models for spin and gauge models based on Z(N) and U(1) symmetry groups. The dual forms are generalizations of the Z(N) chiral clock model and the lattice Frenkel-Kontorova model, respectively. From these equivalences, rich sets of spatially-modulated phases are found in the strong-coupling region of the original models.

hep-lat↗

arXiv2013

Charmonium Potentials at Finite Temperature

The charmonium states at non-zero temperature are studied on anisotropic lattices with 2 dynamical quark flavours. Non-local operators are used to determine the Nambu-Bethe-Salpeter (NBS) wavefunctions via both conventional fitting methods and the Maximum Entropy Method. The interquark potential is determined from the solution of the Schrodinger equation, given the NBS wavefunction as input following the HAL QCD method. We observe a temperature dependent potential which becomes steeper as the temperature decreases.

hep-lat↗