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Probing Chiral Perturbation Theory from realistic two-flavour Lattice QCD simulations

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Fundamental particles and the ‘glue’ that binds them

EU-funded researchers developed novel numerical algorithms to investigate the behaviour of light quarks, among the 12 fundamental matter particles, and their governance by the strong force, one of the 4 fundamental forces of the Universe.


The nucleus of an atom consists of positively charged protons and neutral neutrons. Positive charges repel so there must be a theory explaining why the nucleus does not blow apart but instead remains glued together. In order to understand what is happening inside the nucleus, one must delve a bit deeper into particle physics as it has evolved according to the so-called Standard Model with a particular focus on quarks. According to the Standard Model of particle physics, the Universe is defined by 12 elementary matter particles (fermions) governed by 4 fundamental force particles (bosons, including the well known Higgs boson). Quarks are a six-member sub-family of fermions. They exist only in groups called hadrons, never individually, and the strong force holding them together is carried by a boson appropriately named ‘gluon’ because it so tightly glues the quarks together. Quantum chromodynamics (QCD) is the field of theoretical physics dealing with the interactions between quarks and gluons that make up some hadrons such as the proton or neutron. Hadrons come in heavy, middle-weight and light forms. European researchers initiated the ‘Probing chiral perturbation theory from realistic two-flavour lattice QCD simulations’ (LATQCD-CHIPT) project to develop numerical simulations describing the behaviours of light hadrons. Researchers focused on the problem of ergodicity, an attribute of stochastic or random dynamical systems whereby the system forgets its initial state and almost all sequences are revisited given a very long time interval. Scientists developed a new simulation algorithm of light quark behaviour represented by instability and non-ergodicity phenomena. It is expected to overcome the lack of ergodicity caused by the emergence of disconnected sectors in field space (represented by topological charge) by studying the nature of the transition between sectors. In addition, they accurately calculated so-called topological susceptibility, defining its limit and universality in the continuum limit. Every piece of information relating to behaviour of fundamental matter and force particles brings us one step closer to understanding and explaining the Universe and EU-funded researchers have done just that.

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