Hooke Royal Society Meeting, 4–5 March 2019.
The issue of topology in field theory is an important one. In layman's terms, a field configuration would be characterized as being topologically non-trivial if it exhibits the topology of an object like a ‘mathematical knot’ in some space, which could be either the real space or the space of internal quantum degrees of freedom that characterize the field. The ‘mathematical knot’ (or its higher dimensional generalizations, like a Moebius strip, etc.), as opposed to the knot formed by a material string, is closed, that is, it has no ends and cannot be continuously deformed to a topologically trivial configuration like the circle (or the sphere, in the case of higher dimensional objects).
Topologically, non-trivial solutions of quantum field theory have always constituted elegant subjects from a theoretical study point of view, covering all sorts of interesting and physically relevant extended (in space and/or time) field configurations, such as magnetic monopoles, sphalerons (i.e. topologically non-trivial configurations of Higgs-like fields), black holes, etc.
In the last few years, the physics of such objects have attracted significant attention due to the development of theoretical models predicting field configurations with mass range accessible to current or future colliders (e.g. teraelectron volt mass scale electroweak monopoles, or sphalerons with teraelectron volt-size potential barriers), the launch or design of dedicated experiments searching for them or indirect detection through observation of their consequences (e.g. gravitational waves from coalescing black holes observed by LIGO/VIRGO Collaborations).
Cosmological consequences of such objects, such as their role in the dark sector of the Universe (e.g. primordial black hole as dark matter), or the production and role of cosmic magnetic monopoles for baryogenesis and nucleosynthesis, are also topics of active debate in the contemporary scientific literature. Last but not least, understanding the nature of black holes might also lead to progress in understanding the yet elusive theory of quantum gravity.
The theoretical modelling and experimental searches, both on Earth and in Space, of such topologically non-trivial configurations constitutes the main theme of the current special volume, which consists of articles based on talks given by experts invited to the Royal Society Hooke meeting Topological Avatars of New Physics (4 and 5 March 2019), organized at the headquarters of the Royal Society in London, by N.E. Mavromatos (Chair, King's College London, UK), A. Bevan (Queen Mary University of London, UK), John Ellis (King's College London, UK), V. A. Mitsou (IFIC-Valencia, Spain), L Patrizii (INFN-Bologna University, Italy), J. Pinfold (University of Alberta, Canada) and A. Rajantie (Imperial College, UK).
Among the foci of interest in the meeting lie black holes, magnetic monopoles and sphalerons. There is the recent groundbreaking evidence for the indirect detection of coalescent black holes, through the gravitational waves observed by the LIGO/VIRGO Collaborations, which imply a novel role of such objects, especially primordial ones, produced after inflation, as dark matter components. Indeed, although the observed black holes by LIGO/VIRGO may simply be the endpoints of massive stars, an alternative, quite appealing, explanation is that these are primordial black holes, which are formed deep in the radiation-dominated era. This idea is especially intriguing as there remains the possibility that such objects could account for (part of) the dark matter in the observable Universe. In the meeting, this idea and the related evidence were put under scrutiny.
On the field-theory side, there are important relatively recent developments predicting finite mass magnetic monopoles with masses of up to a few teraelectron volts, which come from a variety of models beyond the Standard Model (e.g. the Cho–Maison monopole or variants thereof, like the one embedded in Born–Infeld nonlinear extension of the (weak) hypercharge gauge group, which may arise in string-inspired models). Non-suppressed production mechanisms of low-mass magnetic monopoles in environments with strong magnetic fields and high temperature, generalizing the pair production Schwinger mechanism in Quantum Electrodynamics, have been demonstrated theoretically recently. This opens up new directions in the search for such objects in heavy ion collisions at LHC, by developing magnetic trapping detectors of the type used in the MoEDAL-LHC experiment, which is dedicated to the search for magnetic monopoles (of teraelectron volt mass) producible, in principle, by particle collisions.
Another topic that was discussed at the meeting is the tantalizing new possibility that, despite common belief, sphaleron configurations can have a potential barrier height of a few teraelectron volts, thereby being producible in principle at current or future colliders. If true, this would imply spectacular signatures of baryon-number violation!
The meeting was unique in its topics and very timely. It brought closer together experts working in diverse fields such as: Theoretical Particle Physics—Field Theory, Experimental High Energy Physics, General Relativity, and Theoretical and Observational Cosmology-Astrophysics.
The programme consisted of invited talks and a poster session, in which senior and early-stage researchers were given the opportunity of presenting their latest research on the topics of the event. The scientific part of the meeting was opened by G. ’t Hooft, who gave an inspiring talk (not included in this volume) on formulating a black hole in a way consistent with quantum mechanics and time reversal symmetry. In a different spirit, but on the same topic, S. Giddings described his version of formulating a quantum theory of gravity, consistently with quantum mechanics. Other highlights of the workshop included talks by N. Manton on the inevitability of sphalerons in field theory, followed by two talks (not included in this volume) with somewhat contradictory points of view, as far as the possibility of sphaleron production at colliders is concerned: one by S. H. Henry Tye, claiming that such sphaleron configurations can in principle be produced at the (upgraded) LHC or future linear colliders, and the other by S. Demidov, who argued that such a production will be strongly suppressed at colliders, thus restricting the important role of the sphalerons only in the early Universe.
A big part of the workshop was devoted to theoretical ideas about magnetic monopoles. The existence of light (of mass of order of teraelectron volt) monopoles in the Standard Model, together with their cosmological consequences, was discussed in a talk by Y. M. Cho. A. Rajantie discussed relatively unsuppressed thermal production of monopole–antimonopole pairs at high temperatures and strong magnetic fields, generalizing early ideas by Affleck and Manton on Schwinger-type vacuum production of such pairs at zero temperature. D. Tong gave a thought-provoking talk (not included in this volume) on ambiguities in the gauge group of the Standard Model and how these could affect monopoles that could be admissible solutions of such gauge field theories. He also discussed unresolved paradoxes on scattering of massless (chiral) fermions off monopoles, pointing towards some problems in defining the magnetic charge in the presence of such chiral matter.
Other quite interesting talks on computer simulations of monopole–antimonopole scattering as well as numerical studies of cosmic string and other topologically non-trivial defects in field theories of relevance to cosmology were presented by T. Vachaspati and A. Achucarro, respectively.
On the experimental side, there were quite informative talks on searches and prospects of detection of such objects at colliders and in Space, by A. Bevan, L Patrizii, J Pinfold and A de Roeck. Monopoles are highly ionizing particles, and this property is currently being exploited in a small but innovative LHC experiment, the MoEDAL-LHC Experiment, which operates simultaneously, but in a complementary way, with the other large LHC experiments in searches of highly ionizing avatars of new physics, including supersymmetric partners of ordinary matter, if supersymmetry exists.
In this latter front, John Ellis gave a complete overview of prospects of detecting supersymmetry in current and future colliders. Supersymmetry is a theoretical framework which can accommodate consistently all of the aforementioned topologically non-trivial field theory configurations, and in this sense, prospects of its detection constituted a quite relevant but complementary part of the workshop.
Last but not least, we had two inspiring talks, in the last section of the workshop, on the role of primordial black holes as dark matter and their potential detection by means of gravitational wave observations, by J. García Bellido and Marc Kamionkowski (not included in this volume).
The workshop ended with a vivid and thought-provoking round table discussion on the future prospects of the field between a panel of experts, chaired by Annarita Margiotta (who also contributed to this volume with a review talk on searches for exotica and dark matter with neutrino telescopes), and comprised of John Ellis, Albert De Roeck, Steven Giddings, Marc Kamionkowski, Vasiliki Mitsou, James Pinfold, Mairi Sakellariadou, Veronica Sanz and Tanmay Vachaspati. During this session, there were also interventions by the audience or some invited speakers, on topics that had been the focus of discussion in the previous two days.