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

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

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Shore G (2013) Polarised structure functions and two-photon physics at Super-B in The European Physical Journal C

Prem Kumar S (2011) Towards holographic walking from $\mathcal{N} = 4$ super Yang-Mills in Journal of High Energy Physics

Piai M (2010) Mass-degenerate heavy vector mesons at hadron colliders in Journal of Physics G: Nuclear and Particle Physics

Piai M (2010) Lectures on Walking Technicolor, Holography, and Gauge/Gravity Dualities in Advances in High Energy Physics

Núñez C (2010) Unquenched Flavor in the Gauge/Gravity Correspondence in Advances in High Energy Physics

Núñez C (2010) Wilson loops in string duals of walking and flavored systems in Physical Review D

NÚÑEZ C (2012) WALKING DYNAMICS FROM STRING DUALS in International Journal of Modern Physics A

Nelson A (2009) Protecting unparticles from the MSSM Higgs sector in Physical Review D

Mück W (2010) Gauge/String Duality in Advances in High Energy Physics

Description 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

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
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