Rbc UkqcdEdit
RBC-UKQCD is a joint international effort in lattice quantum chromodynamics (QCD) that brings together researchers affiliated with the RBC collaboration and the UKQCD collaboration. The aim is to perform nonperturbative, first-principles calculations of hadronic physics by simulating QCD on spacetime lattices. By using chiral-preserving actions, carefully tuned lattice spacings, and physical or near-physical quark masses, the RBC-UKQCD program seeks to connect the underlying theory of quarks and gluons with observable properties of hadrons, including masses, decay processes, and mixing phenomena. The work integrates high-performance computing, advances in lattice techniques, and connections to experimental results in particle physics.
Beneath the surface, the project is part of the broader effort of lattice QCD to study strong interactions in regimes where perturbation theory fails. The collaboration emphasizes precision in weak matrix elements, CP-violation parameters, and multi-hadron final states, topics that require careful control of systematic effects such as finite-volume corrections and discretization errors. The RBC-UKQCD initiative also serves as a bridge between theoretical developments in nonperturbative QCD and the interpretation of experiments in kaon physics and beyond.
History
The RBC-UKQCD collaboration emerged from the convergence of ideas and resources from the RBC group at Brookhaven National Laboratory and the UKQCD consortium in the United Kingdom. Early efforts focused on developing lattice formulations that preserve chiral symmetry to a high degree, which is essential for reliably computing weak transition amplitudes. The partnership rapidly expanded to include multiple European and North American institutions, pooling expertise in physics, algorithms, and high-performance computing.
A defining feature of the program has been the deployment of lattice formulations based on domain-wall fermions, which maintain near-exact chiral symmetry at finite lattice spacing. This choice reduces certain systematic uncertainties in weak matrix elements and helps control operator mixing under renormalization. The collaboration also makes use of improved gauge actions, such as the Iwasaki action, to reduce lattice artifacts and enable reliable extrapolations to the continuum limit.
The program has advanced through generations of gauge-field ensembles that span a range of lattice spacings and volumes, including configurations with light up and down quarks and a strange quark with masses tuned toward their physical values. A major milestone has been achieving simulations with near-physical pion masses, enabling more direct comparisons with experimental data and a cleaner path to phenomenological quantities such as CP-violation parameters in the kaon system.
Scientific program and methods
Lattice QCD framework: RBC-UKQCD operates within the standard lattice QCD paradigm, discretizing spacetime to numerically evaluate the path integral for QCD. This approach makes it possible to compute nonperturbative quantities from first principles, providing tests of the Standard Model and inputs for hadronic physics. See lattice QCD.
Chiral-preserving actions: The collaboration emphasizes chiral symmetry on the lattice, primarily through the use of domain-wall fermions in many of its calculations. This property is crucial for controlling operators relevant to weak decays and for reducing unwanted operator mixing under renormalization.
Gauge actions and discretization: Employing improved gauge actions, notably the Iwasaki gauge action, helps to minimize discretization errors and improve the approach to the continuum limit. This choice interacts with the fermion action to shape the size of lattice artifacts.
Quark content and masses: Early ensembles featured 2+1 flavors (up, down, strange), with ongoing efforts to extend to 2+1+1 flavors by including the charm quark in a controlled way. These configurations underpin calculations of hadron properties and weak matrix elements. See 2+1 flavor QCD and 2+1+1 flavor QCD.
Finite-volume and multi-hadron states: The RBC-UKQCD program uses finite-volume techniques to relate lattice matrix elements to physical decay amplitudes. Methods such as the Lellouch-Lüscher framework are employed to extract real-world decay observables from finite-volume simulations. See Lellouch-Lüscher.
Renormalization and matching: Nonperturbative renormalization schemes (e.g., RI-MOM-type schemes) are used to connect lattice operators to their continuum counterparts, enabling comparisons with experimental results and phenomenological analyses. See renormalization group and non-perturbative renormalization.
Major results and ongoing work
Kaon to two-pion amplitudes: A central goal has been to determine K -> pi pi decay amplitudes from first principles. The RBC-UKQCD collaboration has made substantial progress in computing the amplitudes for kaon decays with both isospin channels, using the finite-volume formalism to relate lattice matrix elements to physical decay rates. These efforts are part of an ongoing program to test the Standard Model’s description of CP violation in the kaon sector.
CP violation in kaon decays: Progress on calculating the direct CP-violating parameter epsilon'/epsilon from lattice QCD provides a stringent test of the interplay between weak interactions and strong dynamics. The results from RBC-UKQCD contribute to the broader effort to compare Standard Model predictions with experimental measurements and to understand the roles of strong interaction effects in CP violation. See epsilon'/epsilon.
Hadron spectrum and structure: In addition to decay amplitudes, the collaboration contributes to the computation of hadron masses, decay constants, and form factors, strengthening the connection between QCD and observable hadron properties. See hadron spectroscopy.
BK and neutral kaon mixing: Lattice determinations of neutral kaon mixing parameters, such as the BK parameter, provide essential inputs for flavor physics analyses and for constraining physics beyond the Standard Model. See BK parameter.
Methodological advancements: The RBC-UKQCD program has driven improvements in algorithms for simulations with chiral fermions, noise-reduction techniques, and strategies for controlling finite-volume effects. These methodological advances have influenced lattice QCD more broadly lattice QCD.
Collaborations and institutions
The RBC component traces back to the collaboration between researchers at Brookhaven National Laboratory and partner institutions, focusing on lattice formulations that preserve important symmetries of QCD. See Brookhaven National Laboratory.
The UKQCD side comprises several universities and research centers in the United Kingdom contributing to algorithm development, ensemble generation, and phenomenology. See UKQCD collaboration.
Other collaborating centers and researchers across Europe and North America contribute to computing resources, software infrastructure, and cross-checks against other lattice programs. See lattice QCD collaborations.