QCD

Large scale computer simulations of the strong nuclear force, or Quantum Chromodynamics (QCD) allow us to test the Standard Model of particle physics, understand quark confinement, explore extremes of density and temperature and constrain models of new physics. The fundamental constituent quarks and gluons are not seen experimentally. Instead we must understand how the particles we do detect emerge as complicated bound states of the constituents. Computations on a discretised chunk of space-time, the lattice, are the only known way to establish this connection from first principles and can involve the largest national and international HPC facilities. We are part of the RBC-UKQCD collaboration which designed and built special-purpose QCDOC computers for lattice simulations. QCDOC design features were taken up in IBM's BlueGene systems. Members of our collaboration are now working on successor systems in collaboration with IBM.

Case Study: weak matrix elements

The weak interaction can change the identities of quarks, but because of strong interactions between quarks and gluons the fundamental quark interactions lead to decays of bound-state particles into other particles. Lattice QCD simulations can evaluate the strengths of these decays and thereby probe the underlying quark interactions. Calculations of kaon decays to pairs of pions allow us to test CP-violation, the breaking of the combination of particle-antiparticle and mirror reflection symmetry, whose understanding is bound up with the dominance of matter over antimatter in the Universe. Lattice calculations of weak decays involving bottom quarks will be tested with unprecedented precision at the recently-restarted Large Hadron Collider in Geneva and could provide clues to new physics beyond the current Standard Model.

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Projects

B-meson coupling with relativistic heavy quarks

Jonathan Flynn (Investigator), Patrick Fritzsch, Dirk Broemmel

We non-perturbatively compute the coupling between B* and B pi meson states relying on relativistic heavy quarks and domain wall light fermions. The coupling is of importance for an effective description of hadronic heavy meson decays.

Hadronic structure on the computer

Jonathan Flynn (Investigator), Dirk Broemmel, Thomas Rae, Ben Samways

In experiments at the Large Hadron Collider (LHC) at CERN, Geneva, the interactions that occur between the colliding particles (protons in this case) can be factorised into a simple scattering between two constituent particles, called quarks, followed by a hadronisation process, which describes the dynamics of forming the bound proton states. Quarks are particles within the proton that bind to form composite particles (hadrons) such as a proton. The scattering process can be computed relatively easily, but hadronisation is intrinsically non-perturbative and hard to calculate. Lattice QCD (computer simulation of QCD on a discrete space-time lattice) provides our only known first-principles and systematically-improvable method to address problems like hadronisation. This project uses Iridis to extract parton distribution amplitudes which are experimentally inaccessible, but needed to describe the quark structure of hadrons.

Kaon to two pion decays in lattice QCD

Jonathan Flynn (Investigator), Elaine Goode, Dirk Broemmel

We calculate kaon decay amplitudes on the lattice so we may compare the Standard Model to experiment.

Non-Perturbative Renormalisation on the Lattice

Jonathan Flynn (Investigator), Dirk Broemmel, Thomas Rae

In this project we compute renormalisation factors for various physical observables in a non-perturbative lattice framework. Renormalisation hereby arises due to a fundamental scale dependence of the physical processes.

People

Jonathan Flynn
Professor, Physics & Astronomy (FPAS)

Neil Broderick
Lecturer, Optoelectronics Research Centre

Dirk Broemmel
Research Fellow, Physics & Astronomy (FPAS)

Elaine Goode
Postgraduate Research Student, Physics & Astronomy (FPAS)

Thomas Rae
Postgraduate Research Student, Physics & Astronomy (FPAS)

Alvaro Ruiz-Serrano
Postgraduate Research Student, Chemistry (FNES)

Ben Samways
Postgraduate Research Student, Physics & Astronomy (FPAS)

Elena Vataga
Technical Staff, iSolutions

Petrina Butler
Administrative Staff, Research and Innovation Services