Search

We determine the $\mathrm{\ensuremath{\Delta}}(1232)$ resonance parameters using lattice QCD and the L\"uscher method. The resonance occurs in elastic pion-nucleon scattering with ${J}^{P}=3/{2}^{+}$ in the isospin $I=3/2$, $P$-wave channel. Our calculation is performed with ${N}_{f}=2+1$ flavors of clover fermions on a lattice with $L\ensuremath{\approx}2.8\text{ }\text{ }\mathrm{fm}$. The pion and nucleon masses are ${m}_{\ensuremath{\pi}}=255.4(1.6)\text{ }\text{ }\mathrm{MeV}$ and ${m}_{N}=1073(5)\text{ }\text{ }\mathrm{MeV}$, respectively, and the strong decay channel $\mathrm{\ensuremath{\Delta}}\ensuremath{\rightarrow}\ensuremath{\pi}N$ is found to be above the threshold. To thoroughly map out the energy dependence of the nucleon-pion scattering amplitude, we compute the spectra in all relevant irreducible representations of the lattice symmetry groups for total momenta up to $\stackrel{\ensuremath{\rightarrow}}{P}=\frac{2\ensuremath{\pi}}{L}(1,1,1)$, including irreps that mix $S$ and $P$ waves. We perform global fits of the amplitude parameters to up to 21 energy levels, using a Breit-Wigner model for the $P$-wave phase shift and the effective-range expansion for the $S$-wave phase shift. From the location of the pole in the $P$-wave scattering amplitude, we obtain the resonance mass ${m}_{\mathrm{\ensuremath{\Delta}}}=1378(7)(9)\text{ }\text{ }\mathrm{MeV}$ and the coupling ${g}_{\mathrm{\ensuremath{\Delta}}\ensuremath{-}\ensuremath{\pi}N}=23.8(2.7)(0.9)$.

Generalized transverse momentum-dependent parton distributions (GTMDs) provide a comprehensive framework for imaging the internal structure of the proton. In particular, by encoding the simultaneous distribution of quark transverse positions and momenta, they allow one to directly access longitudinal quark orbital angular momentum, and, moreover, to correlate it with the quark helicity. The relevant GTMD is evaluated through a lattice calculation of a proton matrix element of a quark bilocal operator (the separation in which is Fourier conjugate to the quark momentum) featuring a momentum transfer (which is Fourier conjugate to the quark position), as well as the Dirac structure appropriate for capturing the quark helicity. The weighting by quark transverse position requires a derivative with respect to momentum transfer, which is obtained in unbiased fashion using a direct derivative method. The lattice calculation is performed directly at the physical pion mass, using domain wall fermions to mitigate operator mixing effects. Both the Jaffe-Manohar as well as the Ji quark spin-orbit correlations are extracted, yielding evidence for a strong quark spin-orbit coupling in the proton., Comment: 8 pages, 2 figures, to appear in the proceedings of the 38th International Symposium on Lattice Field Theory, LATTICE2021, PoS LATTICE2021, 413

We present a lattice-QCD determination of the elastic isospin-$1/2$ $S$-wave and $P$-wave $K\ensuremath{\pi}$ scattering amplitudes as a function of the center-of-mass energy using L\"uscher's method. We perform global fits of $K$-matrix parametrizations to the finite-volume energy spectra for all irreducible representations with total momenta up to $\sqrt{3}\frac{2\ensuremath{\pi}}{L}$; this includes irreducible representations (irreps) that mix the $S$- and $P$-waves. Several different parametrizations for the energy dependence of the $K$-matrix are considered. We also determine the positions of the nearest poles in the scattering amplitudes, which correspond to the broad $\ensuremath{\kappa}$ resonance in the $S$-wave and the narrow ${K}^{*}(892)$ resonance in the $P$-wave. Our calculations are performed with $2+1$ dynamical clover fermions for two different pion masses of 317.2(2.2) and 175.9(1.8) MeV. Our preferred $S$-wave parametrization is based on a conformal map and includes an Adler zero; for the $P$-wave, we use a standard pole parametrization including Blatt-Weisskopf barrier factors. The $S$-wave $\ensuremath{\kappa}$-resonance pole positions are found to be $[0.86(12)\ensuremath{-}0.309(50)i]\text{ }\text{ }\mathrm{GeV}$ at the heavier pion mass and $[0.499(55)\ensuremath{-}0.379(66)i]\text{ }\text{ }\mathrm{GeV}$ at the lighter pion mass. The $P$-wave ${K}^{*}$-resonance pole positions are found to be $[0.8951(64)\ensuremath{-}0.00250(21)i]\text{ }\text{ }\mathrm{GeV}$ at the heavier pion mass and $[0.8718(82)\ensuremath{-}0.0130(11)i]\text{ }\text{ }\mathrm{GeV}$ at the lighter pion mass, which corresponds to couplings of ${g}_{{K}^{*}K\ensuremath{\pi}}=5.02(26)$ and ${g}_{{K}^{*}K\ensuremath{\pi}}=4.99(22)$, respectively.

We use lattice QCD and the L\"uscher method to study elastic pion-nucleon scattering in the isospin $I = 3/2$ channel, which couples to the $\Delta(1232)$ resonance. Our $N_f=2+1$ flavor lattice setup features a pion mass of $m_\pi \approx 250$ MeV, such that the strong decay channel $\Delta \rightarrow \pi N$ is close to the threshold. We present our method for constructing the required lattice correlation functions from single- and two-hadron interpolating fields and their projection to irreducible representations of the relevant symmetry group of the lattice. We show preliminary results for the energy spectra in selected moving frames and irreducible representations, and extract the scattering phase shifts. Using a Breit-Wigner fit, we also determine the resonance mass $m_\Delta$ and the $g_{\Delta-\pi N}$ coupling.

In this project, we will compute the form factors relevant for $B \to K^*(\to K \pi)\ell^+\ell^-$ decays. To map the finite-volume matrix elements computed on the lattice to the infinite-volume $B \to K \pi$ matrix elements, the $K \pi$ scattering amplitude needs to be determined using Luscher's method. Here we present preliminary results from our calculations with $2+1$ flavors of dynamical clover fermions. We extract the $P$-wave scattering phase shifts and determine the $K^*$ resonance mass and the $K^* K \pi$ coupling for two different ensembles with pion masses of $317(2)$ and $178(2)$ MeV.

We investigate the fate of the Roberge-Weiss endpoint transition and its connection with the restoration of chiral symmetry as the chiral limit of ${N}_{f}=2+1$ QCD is approached. We adopt a stout staggered discretization on lattices with ${N}_{t}=4$ sites in the temporal direction; the chiral limit is approached maintaining a constant physical value of the strange-to-light mass ratio and exploring three different light quark masses, corresponding to pseudo-Goldstone pion masses ${m}_{\ensuremath{\pi}}\ensuremath{\simeq}100$, 70 and 50 MeV around the transition. A finite size scaling analysis provides evidence that the transition remains second order, in the 3D Ising universality class, in all the explored mass range. The residual chiral symmetry of the staggered action also allows us to investigate the relation between the Roberge-Weiss endpoint transition and the chiral restoration transition as the chiral limit is approached: our results, including the critical scaling of the chiral condensate, are consistent with a coincidence of the two transitions in the chiral limit; however we are not able to discern the symmetry controlling the critical behavior, because the critical indices relevant to the scaling of the chiral condensate are very close to each other for the two possible universality classes [3D Ising or $O(2)$].

We report on our investigation of low-energy pion-nucleon scattering from lattice QCD with Wilson type fermions. Our focus is on the isospin I = 3/2 channel, which couples to the Delta resonance. We discuss our calculations aiming at the extraction of the Delta resonance mass, width and effective coupling at pion mass 250 MeV, using the Lüscher finite-volume method to extract the phase shift δJ=3/2,l=1.

This paper describes a state-of-the-art parallel Lattice QCD Monte Carlo code for staggered fermions, purposely designed to be portable across different computer architectures, including GPUs and commodity CPUs. Portability is achieved using the OpenACC parallel programming model, used to develop a code that can be compiled for several processor architectures. The paper focuses on parallelization on multiple computing nodes using OpenACC to manage parallelism within the node, and OpenMPI to manage parallelism among the nodes. We first discuss the available strategies to be adopted to maximize performances, we then describe selected relevant details of the code, and finally measure the level of performance and scaling-performance that we are able to achieve. The work focuses mainly on GPUs, which offer a significantly high level of performances for this application, but also compares with results measured on other processors., 22 pages, 8 png figures

Varying from multi-core CPU processors to many-core GPUs, the present scenario of HPC architectures is extremely heterogeneous. In this context, code portability is increasingly important for easy maintainability of applications; this is relevant in scientific computing where code changes are numerous and frequent. In this talk we present the design and optimization of a state-of-the-art production level LQCD Monte Carlo application, using the OpenACC directives model. OpenACC aims to abstract parallel programming to a descriptive level, where programmers do not need to specify the mapping of the code on the target machine. We describe the OpenACC implementation and show that the same code is able to target different architectures, including state-of-the-art CPUs and GPUs.

In this contribution we measure the computing and energy performance of the recently developed DAVIDE HPC-cluster, a massively parallel machine based on IBM POWER CPUs and NVIDIA Pascal GPUs. We use as an application benchmark the OpenStaPLE Lattice QCD code, written using the OpenACC programming framework. Our code exploits the computing performance of GPUs through the use of OpenACC directives, and uses OpenMPI to manage the parallelism among several GPUs. We analyze the speed-up and the aggregate performance of the code, and try to identify possible bottlenecks that harm performances. Using the power monitor tools available on DAVIDE we also discuss some energy aspects pointing out the best trade-offs between time-to-solution and energy-to-solution.

The present panorama of HPC architectures is extremely heterogeneous, ranging from traditional multi-core CPU processors, supporting a wide class of applications but delivering moderate computing performance, to many-core GPUs, exploiting aggressive data-parallelism and delivering higher performances for streaming computing applications. In this scenario, code portability (and performance portability) become necessary for easy maintainability of applications; this is very relevant in scientific computing where code changes are very frequent, making it tedious and prone to error to keep different code versions aligned. In this work we present the design and optimization of a state-of-the-art production-level LQCD Monte Carlo application, using the directive-based OpenACC programming model. OpenACC abstracts parallel programming to a descriptive level, relieving programmers from specifying how codes should be mapped onto the target architecture. We describe the implementation of a code fully written in OpenACC, and show that we are able to target several different architectures, including state-of-the-art traditional CPUs and GPUs, with the same code. We also measure performance, evaluating the computing efficiency of our OpenACC code on several architectures, comparing with GPU-specific implementations and showing that a good level of performance-portability can be reached., Comment: 26 pages, 2 png figures, preprint of an article submitted for consideration in International Journal of Modern Physics C