# Theoretical Particle Physics and Cosmology

Lead Research Organisation:
Swansea University

Department Name: College of Science

### Abstract

Research in particle physics and cosmology connects the largest scales, those of the Universe as a whole, with the smallest, namely those of fundamental particles. The Swansea Particle Physics and Cosmology Theory group works on a wide range of problems relevant for our understanding of cosmology and gravitational waves, black holes, the physics of the Standard Model and beyond the Standard Model, and fundamental aspects of quantum field and string theory.

It is commonly understood that the early Universe underwent a period of rapid expansion, called inflation. However, many open questions remain, on the mechanism of cosmological inflation and on possible links to the theory of quantum gravity, which is yet to be defined. A tantalising question is whether gravitational waves arising from inflation can be detected, leading to the field of gravitational wave cosmology. The nature of dark matter and dark energy may provide an additional window into the early universe, with consistency checks between observations and expectations from quantum gravity, supergravity and string theory. Black holes and Hawking radiation remain a source of inspiration in attempts to reconcile quantum mechanics with gravity. A detailed understanding of entanglement and quantum correlations provides new insights here, using concepts familiar from quantum information. A new mathematical equivalence between colliding black holes and seemingly completely different calculations of two quantum point particles undergoing quantum gravitational scattering will give complementary information on gravitational wave emission. The framework to describe all of the above combines quantum field theory, geometry and gravity. Theoretical advances explore dualities, holography and geometry, linking hitherto unrelated theories and uncovering new structures in M-theory, the overarching concept of gravity, strings and fields.

At the scale of elementary particles, hadrons are formed out of quarks and gluons. Properties of newly discovered 'exotic' hadrons and of quarks and gluons in new phases of matter, such as the quark-gluon plasma, are under intense investigation at current and future particle colliders. To obtain full scientific value from the wealth of data generated by the Large Hadron Collider requires high-precision theoretical predictions, e.g. in the case of multi-loop processes at high multiplicity. Heavy-ion collision experiments probe the QCD [Quantum Chromodynamics] transition at which light hadrons cease to exist, by briefly recreating conditions prevalent in the early universe. The phenomenology of heavy-ion collisions requires quantitative predictions on how the QCD spectrum changes with temperature. Due to the strongly coupled nature of QCD, numerical methods need to be relied on, using the largest supercomputers available. A new connection with Machine Learning may provide fruitful here, transferring knowledge on data generation and interpretation from that community to computational particle physics. In reverse, insight from computational particle physics may shed light on the yet unexplained success of ML, in learning from large data sets and generating ensembles with desirable features. Understanding strongly-coupled dynamics beyond QCD links the model-building literature with phenomenological and experimental international programmes probing physics beyond the Standard Model, covering a range of extensions including Composite Higgs models, top compositeness, strongly-interacting dark matter, and possibly more speculative early-universe phenomena, which in some cases are testable at the Large Hadron Collider.

It is commonly understood that the early Universe underwent a period of rapid expansion, called inflation. However, many open questions remain, on the mechanism of cosmological inflation and on possible links to the theory of quantum gravity, which is yet to be defined. A tantalising question is whether gravitational waves arising from inflation can be detected, leading to the field of gravitational wave cosmology. The nature of dark matter and dark energy may provide an additional window into the early universe, with consistency checks between observations and expectations from quantum gravity, supergravity and string theory. Black holes and Hawking radiation remain a source of inspiration in attempts to reconcile quantum mechanics with gravity. A detailed understanding of entanglement and quantum correlations provides new insights here, using concepts familiar from quantum information. A new mathematical equivalence between colliding black holes and seemingly completely different calculations of two quantum point particles undergoing quantum gravitational scattering will give complementary information on gravitational wave emission. The framework to describe all of the above combines quantum field theory, geometry and gravity. Theoretical advances explore dualities, holography and geometry, linking hitherto unrelated theories and uncovering new structures in M-theory, the overarching concept of gravity, strings and fields.

At the scale of elementary particles, hadrons are formed out of quarks and gluons. Properties of newly discovered 'exotic' hadrons and of quarks and gluons in new phases of matter, such as the quark-gluon plasma, are under intense investigation at current and future particle colliders. To obtain full scientific value from the wealth of data generated by the Large Hadron Collider requires high-precision theoretical predictions, e.g. in the case of multi-loop processes at high multiplicity. Heavy-ion collision experiments probe the QCD [Quantum Chromodynamics] transition at which light hadrons cease to exist, by briefly recreating conditions prevalent in the early universe. The phenomenology of heavy-ion collisions requires quantitative predictions on how the QCD spectrum changes with temperature. Due to the strongly coupled nature of QCD, numerical methods need to be relied on, using the largest supercomputers available. A new connection with Machine Learning may provide fruitful here, transferring knowledge on data generation and interpretation from that community to computational particle physics. In reverse, insight from computational particle physics may shed light on the yet unexplained success of ML, in learning from large data sets and generating ensembles with desirable features. Understanding strongly-coupled dynamics beyond QCD links the model-building literature with phenomenological and experimental international programmes probing physics beyond the Standard Model, covering a range of extensions including Composite Higgs models, top compositeness, strongly-interacting dark matter, and possibly more speculative early-universe phenomena, which in some cases are testable at the Large Hadron Collider.