# Quantum Field Theory: new ideas in strings, lattice and LHC physics

Lead Research Organisation:
Swansea University

Department Name: College of Science

### Abstract

The standard model of particle physics encapsulates our current knowledge of the fundamental constituents of atoms and the nature of matter in the earliest moments following the Big Bang. However, our understanding of the dynamics of the standard model is limited by our ability to solve its strongly-interacting sector, known as quantum chromodynamics (QCD), which describes the interactions of quarks and gluons. The Swansea group is attacking this problem from two quite different perspectives. First, by approximating the spacetime continuum as a discrete lattice of points, it is possible to simulate QCD on high performance computers. The group will study lattice QCD in the extreme conditions of high temperature and density which existed following the Big Bang and which will be recreated in high-energy particle collisions at the Large Hadron Collider at CERN in Geneva, which is due to begin operation in 2008. The primary goal of the LHC is, however, to discover the new physics which is responsible for the generation of mass for all the known elementary particles. This `electroweak symmetry breaking' is the least understood part of the standard model. It may be due to the existence of a background field permeating spacetime, which gives mass to particles as they interact with it; this field would manifest itself at the LHC as the famous `Higgs boson'. On the other hand, mass generation may be due to new strong interaction physics at the TeV scale probed by the LHC, as described by a class of theories known in analogy with QCD as `technicolor'. The Swansea group is pursuing a variety of approaches to understand the dynamics and experimental signatures of this type of theory. Particle physicists do not, however, believe that the standard model is the ultimate theory. It is an example of a quantum gauge field theory, a theoretical framework which unifies quantum mechanics, special relativity and the fundamental symmetries which physicists have discovered through decades of experiments with particle accelerators. A deeper unification appears possible with superstrings, which contain both gauge theories and gravity together with a new type of spacetime symmetry known as `supersymmetry', and are widely believed to describe all of nature in a `Theory of Everything'. However, it has recently been discovered that in many ways gauge theories and superstrings may be simply different, complementary ways of viewing the same fundamental theory. Exploiting this `gauge-string duality' is one of the main activities of the Swansea group. This works in two ways. First, insights from string theory can be used to elucidate properties of strongly-interacting supersymmetric gauge theories with the ultimate aim of understanding the dynamics of QCD and technicolor from an alternative perspective to the lattice theorists. Second, insights from gauge theories can be used to study deep problems in quantum gravity, such as the nature of Hawking radiation from black holes and eventually how string theory may resolve the problem of the spacetime singularity at the Big Bang, throwing a completely new light on the origin of the universe.

### Publications

Aarts G
(2008)

*Effective convergence of the two-particle irreducible 1 / N expansion for nonequilibrium quantum fields*in Physical Review D
Aarts G
(2010)

*On the convergence of complex Langevin dynamics: the three-dimensional XY model at finite chemical potential*in Journal of High Energy Physics
Aarts G
(2010)

*Adaptive stepsize and instabilities in complex Langevin dynamics*in Physics Letters B
Aarts G
(2010)

*Holographic Roberge-Weiss transitions*in Journal of High Energy Physics
Aarts G
(2009)

*Complex Langevin dynamics at finite chemical potential: mean field analysis in the relativistic Bose gas*in Journal of High Energy Physics
Aarts G
(2010)

*Degenerate distributions in complex Langevin dynamics: one-dimensional QCD at finite chemical potential*in Journal of High Energy Physics
Aarts G
(2009)

*Two complex problems on the lattice: transport coefficients and finite chemical potential*in Nuclear Physics A
Aarts G
(2015)

*Developments in lattice quantum chromodynamics for matter at high temperature and density*in PramanaDescription | The `Gauge Theories and Strings' sub-group has developed and extended the principle of gauge-gravity duality in a number of directions, both to improve our understanding of the fundamental structure of gauge and string theory and to develop new tools to analyse strong-coupling dynamics of relevance to the LHC experimental programme. Some highlights of this work include: Significant progress has been made in the problem of including a large number of quark flavours Nf ~ Nc in the framework of AdS/CFT and the resolution of the difficult problem of flavour back-reaction. Applications include the phase structure of QCD-like theories with fundamental flavours at non-zero temperature and density, complementing the group's lattice programme and relevant to the quark-gluon plasma produced in heavy-ion collisions at the LHC. `Holographic technicolor' models have been constructed and their implications for the interpretation of the Higgs boson as a psudo-dilaton investigated. Gauge-gravity duals have been found which exhibit `walking' behaviour of the coupling constant, as required for a phenomenologically viable technicolor model. Important progress has been made in understanding the quantum integrability underlying the AdS/CFT correspondence, which is an important step in the key problem of finding a consistent quantisation of string theory in AdS x S5 backgrounds. A complete resolution was found of the problem of reconciling causality with the existence of low-frequency superluminal photon propagation in QED in curved spacetime. This revealed many novel generic features of QFT in curved spacetime, notably new analytic structures of Green functions induced by the geometry of geodesic congruences and modified dispersion relations. Progress in Amplitudes included the development of augmented recursion techniques to calculate certain one-loop pure gravity amplitudes and related studies of the systematics of UV divergence cancellations in N < 8 supergravities. The `Lattice' sub-group has focused on two main research programmes: understanding the phase structure and properties of gauge theories at non-zero temperature and/or density, and studying strongly-coupled BSM gauge theories of importance to possible strong TeV physics at the LHC. Highlights include: The first lattice studies of heavy quarkonia at finite temperature using non-relativistic QCD, showing that some states such as the Upsilon survive deconfinement and persist up to T ~ 2.1 Tc, whereas the chi_b has clearly dissociated at these temperatures. Simulations of QCD with gauge group SU(2) and two quark flavors at non-zero baryon chemical potential show evidence for an exotic `quarkyonic' phase in which the behaviour of thermodynamic observables resemble that of a degenerate quark gas but in which coloured degrees of freedom remain confined. Investigations have been made of the use of the complex Langevin equation to study theories such as QCD with a complex Euclidean action at non-zero chemical potential. It was shown that the phase transition from vacuum to superfluid in an interacting relativistic Bose gas as the chemical potential is increased is reproduced by this technique in accord with theoretical expectations. The Lattice group has made important progress in understanding the dynamics of BSM gauge theories, in particular the study of minimally walking Technicolor and near-conformal dynamics in QCD-like theories with non-standard color and flavor content. The analysis of the hadron spectrum (including glueballs) in SU(2) gauge theory with 2 adjoint flavors shows evidence for a light dynamically-generated scale, of particular importance to the phenomenology of electroweak symmetry breaking. |

Exploitation Route | This research is an integral part of the ongoing UK and international activity in particle theory. The topics addressed and resolved will inform the work of particle theorists working across a spectrum from fundamental mathematical theory to LHC phenomenology. |

Sectors | Digital/Communication/Information Technologies (including Software),Education |

Description | Knowledge Exchange: High Performance Computing Meeting the computational demands of Lattice Gauge Theory has been a powerful driver of innovation in HPC, motivating the development of ever faster machines. The Swansea group is part of the UKQCD collaboration, a consortium of 8 U.K. universities with research groups in lattice gauge theory. Its collaboration with IBM can be traced back to the acquisition in 2005 by UKQCD of the 12,000 processor, 10 Tflop QCDOC (QCD on a chip) computer. QCDOC is a massively parallel, special purpose machine which was designed by physicists from Columbia, Brookhaven National Laboratory (BNL) and UKQCD using IBM Blue Logic technology. It was one of the first multi-Tflop facilities available to scientists in the U.K. As part of this effort the Swansea group has established a close contact with IBM Research in Yorktown Heights, running lattice code as a test application to provide valuable information on computing performance and contributing to machine development. This collaboration between IBM and UKQCD was further enhanced by the installation in 2011 of a 30 Tflop IBM BlueGene/P system in Swansea, followed in Edinburgh by an early version of a new machine, the IBM BlueGene/Q of around 800 Tflops. The Q system, the latest in the Blue Gene series, shares many features of its architecture with the original QCDOC machine and has again been designed as a result of a collaborative partnership between IBM Research, Columbia, BNL and UKQCD. This promises to propel the U.K. into the era of Petaflop computing. The Lattice group has also been closely involved since the early stages in a project to establish a High Performance Computing capability in Wales. The HPC Wales project has raised substantial funding from the Welsh Assembly Government and from the U.K. Government Strategic Investment Fund (SIF), with further matched funding from EU Convergence. Its mission is ``to build a gobally recognised and networked HPC capability across Wales, providing an enabling technology and building a skills base to support research and development projects that deliver significant economic benefits across Wales and beyond as the economy returns to growth''. |

First Year Of Impact | 2010 |

Sector | Digital/Communication/Information Technologies (including Software) |

Impact Types | Economic |

Description | EPSRC |

Amount | £14,969 (GBP) |

Organisation | Engineering and Physical Sciences Research Council (EPSRC) |

Sector | Public |

Country | United Kingdom |

Start | 10/2009 |

End | 09/2010 |

Description | EU network |

Amount | £22,500 (GBP) |

Organisation | European Commission |

Department | Seventh Framework Programme (FP7) |

Sector | Public |

Country | European Union (EU) |

Start | 10/2009 |

End | 09/2010 |

Description | STFC Consolidated Grant |

Amount | £3,060,000 (GBP) |

Funding ID | ST/G000506/1 |

Organisation | Science and Technologies Facilities Council (STFC) |

Sector | Public |

Country | United Kingdom |

Start | 10/2008 |

End | 09/2011 |

Description | UKQCD |

Organisation | UKQCD |

Country | United Kingdom |

Sector | Academic/University |

PI Contribution | The Swansea Lattice Gauge Theory research group is a member of the UKQCD lattice consortium. |

Collaborator Contribution | Collaboration on physics content of research papers and use of HPC facilities. |

Impact | 30 of the refereed publications listed here (authors Aarts, Allton, Hands, Lucini) are in teh field of Lattice Gauge Theory and are related to some extent with UKQCD. The Swansea share of UKQCD's STFC HPC equipment grant procured a £1.2M IBM Blue Gene/P computer located in Swansea. The relation with IBM and software development formed part of the Department's submission in the 2010 REF Impact Pilot exercise. |

Description | Newspaper articles |

Form Of Engagement Activity | A magazine, newsletter or online publication |

Part Of Official Scheme? | No |

Geographic Reach | National |

Primary Audience | Public/other audiences |

Results and Impact | Many newspaper and magazine articles featuring research at CERN, especially antihydrogen. Not possible to quantify. |

Year(s) Of Engagement Activity | 2015 |

Description | Particle Physics Masterclasses |

Form Of Engagement Activity | Participation in an open day or visit at my research institution |

Part Of Official Scheme? | Yes |

Geographic Reach | Regional |

Primary Audience | Schools |

Results and Impact | Approx 120 pupils annually attend two or three separate day-long events featuring lectures and hands-on computer exercises featuring CERN, the LHC, particle physics, ATLAS software and the ALPHA antihydrogen collaboration. Around 10 schools attend each year. Most have asked to be re-invited on a regular basis to future year's events. |

Year(s) Of Engagement Activity | 2015 |

Description | Popular lectures |

Form Of Engagement Activity | A talk or presentation |

Part Of Official Scheme? | No |

Geographic Reach | Regional |

Primary Audience | Public/other audiences |

Results and Impact | Swansea Theoretical Particle Physics Group members give numerous popular talks to the general public and school, nationally and internationally. Examples include talks on the LHC, CERN AntimatterPhysics and the Einstein Centenary at the Swansea Science Cafe and Black Holes at local astronomical societies. Popular interest in lectures and requests for follow-up activity, e.g. interviews for local media. |

Year(s) Of Engagement Activity | 2015 |

Description | Radio Interviews |

Form Of Engagement Activity | A press release, press conference or response to a media enquiry/interview |

Part Of Official Scheme? | No |

Geographic Reach | National |

Primary Audience | Public/other audiences |

Results and Impact | Interviews and participation on science programmes for BBC Radio Wales, including Science Cafe programmes on string theory, particle physics and cosmology, the anniversary of Hawking's "Brief History of Time" and the Centenary of Einstein's General Relativity. Not possible to quantify. |

Year(s) Of Engagement Activity | 2015 |