We describe aspects of the Chroma software system for lattice QCD calculations. Chroma is an open source C++ based software system developed using the software infrastructure of the US SciDAC initiative. Chroma interfaces with output from the BAGEL assembly generator for optimised lattice fermion kernels on some architectures. It can be run on workstations, clusters and the QCDOC supercomputer.

A new quark-field smearing algorithm is defined which enables efficient calculations of a broad range of hadron correlation functions. The technique applies a low-rank operator to define smooth fields that are to be used in hadron creation operators. The resulting space of smooth fields is small enough that all elements of the reduced quark propagator can be computed exactly at reasonable computational cost. Correlations between arbitrary sources, including multihadron operators can be computed a posteriori without requiring new lattice Dirac operator inversions. The method is tested on realistic lattice sizes with light dynamical quarks.

The axial coupling of the nucleon, g, is the strength of its coupling to the weak axial current of the standard model of particle physics, in much the same way as the electric charge is the strength of the coupling to the electromagnetic current. This axial coupling dictates the rate at which neutrons decay to protons, the strength of the attractive long-range force between nucleons and other features of nuclear physics. Precision tests of the standard model in nuclear environments require a quantitative understanding of nuclear physics that is rooted in quantum chromodynamics, a pillar of the standard model. The importance of g makes it a benchmark quantity to determine theoretically-a difficult task because quantum chromodynamics is non-perturbative, precluding known analytical methods. Lattice quantum chromodynamics provides a rigorous, non-perturbative definition of quantum chromodynamics that can be implemented numerically. It has been estimated that a precision of two per cent would be possible by 2020 if two challenges are overcome: contamination of g from excited states must be controlled in the calculations and statistical precision must be improved markedly. Here we use an unconventional method inspired by the Feynman-Hellmann theorem that overcomes these challenges. We calculate a g value of 1.271 ± 0.013, which has a precision of about one per cent.

We present a spectrum of highly excited charmonium mesons up to around 4.5 GeV calculated using dynamical lattice QCD. Employing novel computational techniques and the variational method with a large basis of carefully constructed operators, we extract and reliably identify the continuum spin of an extensive set of excited states, states with exotic quantum numbers (0+-, 1-+, 2+-) and states with high spin. Calculations are performed on two lattice volumes with pion mass approximately 400 MeV and the mass determinations have high statistical precision even for excited states. We discuss the results in light of experimental observations, identify the lightest 'supermultiplet' of hybrid mesons and comment on the phenomenological implications of the spectrum of exotic mesons.Comment: 33 pages, 17 figures; v2: minor changes to reflect published versio

We present evidence for the existence of a bound H-dibaryon, an I = 0, J = 0, s = −2 state with valence quark structure uuddss, at a pion mass of mπ ∼ 389 MeV. Using the results of Lattice QCD calculations performed on four ensembles of anisotropic clover gauge-field configurations, with spatial extents of L ∼ 2.0, 2.5, 3.0 and 3.9 fm at a spatial lattice spacing of bs ∼ 0.123 fm, we find an H-dibaryon bound by B H ∞ = 16.6 ± 2.1 ± 4.6 MeV at a pion mass of mπ ∼ 389 MeV.It is now well established that quantum chromodynamics (QCD), the theory describing the dynamics of quarks and gluons, and the electroweak interactions, underlie all of nuclear physics, from the hadronic mass spectrum to the synthesis of heavy elements in stars. To date, there have been few quantitative connections between nuclear physics and QCD, but fortunately, Lattice QCD is entering an era in which precise predictions for hadronic quantities with quantifiable errors are being made. This development is particularly important for processes which are difficult to explore in the laboratory, such as hyperon-hyperon and hyperon-nucleon interactions for which knowledge is scarce, primarily due to the short lifetimes of the hyperons, but which may impact the late-stages of supernovae evolution. In this letter we report strong evidence for a bound H-dibaryon, a six-quark hadron with valence structure uuddss, from n f = 2 + 1 Lattice QCD calculations at light-quark masses that give the pion a mass of m π ∼ 389 MeV.The prediction of a relatively deeply bound system with the quantum numbers of ΛΛ (called the H-dibaryon) by Jaffe [1] in the late 1970s, based upon a bag-model calculation, started a vigorous search for such a system, both experimentally and also with alternate theoretical tools. Experimental constraints on, and phenomenological models of, the H-dibaryon can be found in Refs. [2,3,4]. While experimental studies of doublystrange hypernuclei restrict the H-dibaryon to be unbound or to have a small binding energy, the most recent constraints on the existence of the H-dibaryon come from heavy-ion collisions at RHIC, from which it is concluded that the H-dibaryon does not exist in the mass region 2.136 < M H < 2.231 GeV [5], effectively eliminating the possibility of a loosely-bound H-dibaryon at the physical light-quark masses. Recent experiments at KEK suggest there is a resonance near threshold in the H-dibaryon channel [6].The first study of baryon-baryon interactions with Lattice QCD was performed more than a decade ago [7,8]. This calculation was quenched and with m π > ∼ 550 MeV. The NPLQCD collaboration performed the first n f = 2+ 1 QCD calculations of baryon-baryon interactions [9,10] at low-energies but at unphysical pion masses. Quenched and dynamical calculations were subsequently performed by the HALQCD collaboration [11,12]. A number of quenched Lattice QCD calculations [13,14,15,16,17,18] have searched for the H-dibaryon, but to date no definitive results have been reported. Earlier work concluded that the H-dibaryon does not exi...