Conference on geometric aspects of string theory – formerly known as the series “Physics and Geometry of F-theory” – this new instalment will include a wider range of recent developments in geometric aspects of string theory.
Conference on geometric aspects of string theory – formerly known as the series “Physics and Geometry of F-theory” – this new instalment will include a wider range of recent developments in geometric aspects of string theory.
Twistor theory was originally proposed by Roger Penrose as a geometric framework for physics that aims to unify general relativity and quantum mechanics. In this approach, spacetime is secondary with events being derived objects that correspond to compact holomorphic curves in a complex three–fold, the twistor space. The mathematics of twistor theory goes back to the 19th century Klein correspondence in projective geometry, but one of the unexpected spinoffs from twistor theory is its impact on modern pure mathematics, from differential geometry and representation theory to gauge theories and integrable systems.
Loop quantum gravity is a background-independent approach to the quantization of general relativity. It provides a compelling picture of quantum spacetime in terms of a collection of `atoms’ with discrete spectra, and the possibility of resolving the singularities of general relativity. Applied to cosmology and black hole physics, it has led to new ideas for the origin of the universe (a `Big Bounce’ replacing the Big Bang) and the final state of Hawking evaporation.
The communities working in these two theories share both technical and a conceptual pillars, however they have evolved independently for many years, with different methods and intermediate goals. Some recent developments have weaved a possible new path of interaction: Collaborations between researchers in the two fields have started, with the potential to enrich each other and find new synergies.
The aim of the proposed meeting is to bring together for the first time the two communities in a broad and comprehensive way, to strengthen this interdisciplinary overlap and foster new collaborations and developments, concentrating primarily on the geometric and general– relativistic aspects. Leading international researchers both in twistor theory and loop quantum gravity will have the opportunity to establish and consolidate the connections between the two areas of research, and to overcome problems at the forefront of both fields.
This series of conferences started in 1985 at Maryland, USA. It brings together experimentalists and theorists every other year to review the status and progress in hadron spectroscopy, structure and related topics and to exchange ideas for future explorations.
The main topics of this conference include:
· Meson spectroscopy
· Baryon spectroscopy
· Exotic hadrons and candidates
· Hadron decays, production and interactions
· Analysis tools
· QCD and hadron structure
· Hadrons in hot and nuclear environment including hypernuclei
The 29th International Symposium on Lepton Photon Interactions at High Energies follows the tradition of a long series of high-energy physics conferences. The program features plenary sessions covering topics of major interest to the particle physics community. New this year will be two (or three) tracks of parallel sessions for one day, that will provide an opportunity for additional presenters to give a more in-depth presentation of individual physics results. We will also organise poster sessions where additional researchers may present their work.
Since 1993 the Rencontres du Vietnam, which is an official partner of UNESCO, has organised international scientific conferences and schools to foster exchanges between Vietnamese or Asia-Pacific scientists and colleagues from other parts of the world.
2019 will be the 30th anniversary of the discovery that there are only three families of light active neutrinos. The theme of the XVèmes Rencontres du Vietnam will be centred on what we know today about these three families, what are the consequences, what physics might lie beyond, and how to access it.
MC Escher’s 1941 woodcut Plane-Filling Motif with Reptiles depicts two tessellating tetrapods, one black and one white, with intertwined legs and feet on the adjacent sides. Reflect the image horizontally and vertically, and the result is a photo negative: maximal parity violation. A black–white transformation – charge conjugation in the metaphor that inspired Steven Vigdor’s book – and, as in nature, we return to the original. Well, almost. We can tell the difference from the position and colouration of Escher’s stylised initials in the corner. The only imperfection is the signature of the artist.
Vigdor’s idea is that such deviations from perfect symmetry are not in fact “bugs”, but are beautiful and essential. In the case of CP violation – an essential ingredient in Sakharov’s baryogenesis recipe – the artist’s signature is indispensable to our very existence, and the subject of a glut of searches for physics beyond the Standard Model.
Taking no position on the existence of a creator artist per se, Vigdor’s aim is rather to complement books that speculate on new theories with an exposition of the “painstaking and innovative” efforts of generations of experimentalists to establish the weird and wonderful physics we know. His book is a romp from quantum mixing to the apparent metastability of the vacuum (given current measurements of the Higgs and top masses), with excursions into cosmology, biology and metaphysics. The intended audience is university students. As they cut their way through a jungle of mathematical drills in 19th-century physics, many lose sight of the destination. Cheerful and down to earth, this book offers an invigorating glimpse through the foliage.
The seventh edition of the Large Hadron Collider Physics (LHCP) conference took place in Puebla, Mexico, from 20 to 25 May, hosted by the Benemérita Universidad Autónoma de Puebla (BUAP). With almost 400 participants, the week involved dynamic discussions between experimentalists and theorists on an assortment of topics related to LHC research. These ranged from heavy-ion physics to precision measurements of the Standard Model (SM), including Higgs-sector constraints and searches for hints of physics beyond the SM such as supersymmetry and model-independent high-mass resonance searches.
Results from the wealth of LHC data collected at 13 TeV during Run 2 (from 2015–2018) are beginning to be published. The ATLAS and CMS collaborations presented new results in the search for supersymmetry, setting new limits on supersymmetric parameters. The latest CMS search for top squarks in events with two tau leptons in the final state excludes top-squark masses above 1 TeV for nearly massless neutralinos. The first ATLAS Run 2 measurement for the production of tau sleptons was also presented, excluding masses between 120 and 390 GeV for a massless neutralino. Both of these challenging analyses contain a high amount of missing momentum, originating from the lightest supersymmetric particle and the neutrinos from the tau decays.
Studies involving unusual signatures were popular at the Mexico conference. Disappearing tracks, emerging jets, displaced vertices and out-of-time decays, which would each be indications of new processes or particles being present in the event, were all discussed. These signatures also provide a challenge for detector and algorithm designs, especially at the high-luminosity LHC (HL-LHC).
The recent observation of CP violation in charm quarks (CERN Courier May/June p7) published by the LHCb Collaboration in March was presented. “Long awaited, finally observed!” was the statement from LHCb-spokesperson Giovanni Passaleva. This result, which shows the different decay rates of charm quarks and charm anti-quarks, opens up new avenues of investigation for testing the SM.
The final two days of the conference featured open discussions on recent progress in the upgrades of the LHC and the detectors for the HL-LHC, and on various proposals and design challenges for future colliders. The HL-LHC will be a very challenging environment in which to distinguish particles of interest, as the average number of proton–proton collisions will increase from around 50 to about 200 each time the bunches in the LHC beams cross. For future colliders, circular and linear, delegates agreed that the community must better communicate the motivations and goals for such future machines with governments and the public.
The next edition of the conference will take place in Paris in 2020. Though also taking place during the current long shutdown, many new results with the full LHC Run-2 statistics will be presented, as well as progress on preparing the detectors and the accelerator for Run 3.
The first dedicated workshop on searches for new physics in heavy-ion collisions took place at the Université Catholique de Louvain, Belgium, on 4–5 December 2018. The meeting was inspired by several recent proposals to take advantage of the unique environment of heavy-ion collisions at the LHC to search for new phenomena. A key topic was the exploration of “hidden” or “dark” sectors that couple only feebly to ordinary matter and could explain the dark-matter puzzle, neutrino masses or the matter–antimatter asymmetry of the universe. This is currently a hot topic in the search for physics beyond the Standard Model that has gained increasing interest in the heavy-ion community. The purpose of this workshop was to spark ideas and initiate exchanges between theorists, experimentalists and accelerator physicists.
A key question was how to optimise the choice of ions and the beam parameters for new-physics searches without compromising the study of the quark–gluon plasma
Discussions at the workshop first focused on particle production mechanisms unique to heavy-ion collisions. Simon Knapen from the IAS at Princeton University and Oliver Gould of the University of Helsinki emphasised the strongly enhanced production cross-sections for axion-like particles and magnetic monopoles in ultra-peripheral heavy-ion collisions compared to proton–proton collisions. This enhancement is due to the collective action of up to 82 charges (for lead ions), thereby generating the strongest electromagnetic fields ever produced in the laboratory, as the heavy ions pass each other at ultra-relativistic energies. David d’Enterria of CERN discussed the experimental potential to exploit such unique opportunities in searches for new physics by using the LHC as a “photon–photon collider”. In contrast to these studies of ultra-peripheral collisions, Glennys Farrar of New York University motivated interest in head-on collisions: thermal production in the quark–gluon plasma could be used to search for non-conventional dark-matter candidates such as “sexaquarks”.
Jan Hajer of the Université Catholique de Louvain stressed that not only the production mechanisms but also the backgrounds are qualitatively different in heavy-ion collisions. This can, for example, allow searches for long-lived particles in parameter regions that are hard to probe in proton collisions due to limitations related to the high pile-up during future LHC runs.
A key question that emerged from the workshop was how to optimise the choice of ions and the beam parameters for new-physics searches without compromising the study of the quark–gluon plasma. The discussion was extremely helpful for elucidating the hard engineering restrictions within which any novel proposals must fit, such as the capacity of the injectors and the beam lifetime.
The workshop was very successful and triggered many discussions, including the proposal to submit an input for the update of the European Strategy for Particle Physics and for a follow-up event in 2020. The topic is still young, and we are very much looking forward to input from the wider community.
Topologically non-trivial solutions of quantum field theory have always been a theoretically “elegant” subject, covering all sorts of interesting and physically relevant field configurations, such as magnetic monopoles, sphalerons and black holes. These objects have played an important role in shaping quantum field theories and have provided important physical insights into cosmology, particle colliders and condensed-matter physics.
In layman’s terms, a field configuration is topologically non-trivial if it exhibits the topology of a “mathematical knot” in some space, real or otherwise. A mathematical knot (or a higher-dimensional generalisation such as a Möbius strip) is not like a regular knot in a piece of string: it has no ends and cannot be continuously deformed into a topologically trivial configuration like a circle or a sphere.
One of the most conceptually simple non-trivial configurations arises in the classification of solitons, which are finite-energy extended configurations of a scalar field behaving like the Higgs field. Among the various finite-energy classical solutions for the Higgs field, there are some that cannot be continuously deformed into the vacuum without an infinite cost in energy, and are therefore “stable”. For finite-energy configurations that are spherically symmetric, the Higgs field must map smoothly onto its vacuum solution at the boundary of space.
The ’t Hooft–Polyakov monopole, which is predicted to exist in grand unified theories, is one such finite-energy topologically non-trivial solitonic configuration. The black hole is an example from general relativity of a singular space–time configuration with a non-trivial space–time topology. The curvature of space–time blows up in the singularity at the centre, and this cannot be removed either by continuous deformations or by coordinate changes: its nature is topological.
Such configurations constituted the main theme of a recent Royal Society Hooke meeting “Topological avatars of new physics”, which took place in London from 4–5 March. The meeting focused on theoretical modelling and experimental searches for topologically important solutions of relativistic quantum field theories in particle physics, general relativity and cosmology, and quantum gravity. Of particular interest were topological objects that could potentially be detectable at the Large Hadron Collider (LHC), or at future colliders.
Gerard ’t Hooft opened the scientific proceedings with an inspiring talk on formulating a black hole in a way consistent with quantum mechanics and time-reversal symmetry, before Steven Giddings described his equally interesting proposal. Another highlight was Nicholas Manton’s talk on the inevitability of topological non-trivial unstable configurations of the Higgs field – “sphalerons” – in the Standard Model. Henry Tye said sphalerons can in principle be produced at the (upgraded) LHC or future linear colliders. A contradictory view was taken by Sergei Demidov, who predicted that their production will be strongly suppressed at colliders.
One of the exemplars of topological physics receiving significant experimental attention is the magnetic monopole
A major part of the workshop was devoted to monopoles. The theoretical framework of light monopoles within the Standard Model, possibly producible at the LHC, was presented by Yong Min Cho. These “electroweak” monopoles have twice the magnetic charge of Dirac monopoles. Like the ’t Hooft–Polyakov monopole, but unlike the Dirac monopole, they are solitonic structures, with the Higgs field playing a crucial role. Arttu Rajantie considered relatively unsuppressed thermal production of generic monopole–antimonopole pairs in the presence of the extreme high temperatures and strong magnetic fields of heavy-ion collisions at the LHC. David Tong discussed the ambiguities on the gauge group of the Standard Model, and how these could affect monopoles that are admissible solutions of such gauge field theories. Importantly, such solutions give rise to potentially observable phenomena at the LHC and at future colliders. Anna Achucaro and Tanmay Vachaspati reported on fascinating computer simulations of monopole scattering, as well as numerical studies of cosmic strings and other topologically non-trivial defects of relevance to cosmology.
One of the exemplars of topological physics currently receiving significant experimental attention is the magnetic monopole. The MoEDAL experiment at the LHC has reported world-leading limits on multiply magnetically charged monopoles, and Albert de Roeck gave a wide-ranging report on the search for the monopole and other highly-ionising particles, with Laura Patrizii and Adrian Bevan also reporting on these searches and the machine-learning techniques employed in them.
Supersymmetric scenarios can consistently accommodate all the aforementioned topologically non-trivial field theory configurations. Doubtless, as John Ellis described, the story of the search for this beautiful – but as yet hypothetical – new symmetry of nature, is a long way from being over. Last but not least, were two inspiring talks by Juan Garcia Bellido and Marc Kamionkowski on the role of primordial black holes as dark matter, and their potential detection by means of gravitational waves.
The workshop ended with a vivid round-table discussion of the importance of a new ~100 TeV collider. The aim of this machine is to explore beyond the historic watershed represented by the discovery of the Higgs boson, and to move us closer to understanding the origin of elementary particles, and indeed space–time itself. This Hooke workshop clearly demonstrated the importance of topological avatars of new physics to such a project.
A never-before-seen object with a cataclysmic past has been spotted in the constellation Cassiopeia, about 10,000 light years away. The star-like object has a temperature of 200,000 K, shines 40,000 times brighter than the Sun and is ejecting matter with velocities up to 16,000 km s–1. In combination with the chemical composition of the surrounding nebula, the data indicate that it is the result of the merger of two dead stars.
Astronomers from the University of Bonn and Moscow detected the unusual object while searching for circumstellar nebulae in data from NASA’s Wide-Field Infrared Survey Explorer satellite. Memorably named J005311, and measuring about five light years across, it barely emits any optical light and radiates almost exclusively in the infrared. Additionally, the matter it emits consists mostly of oxygen and does not have any signs of hydrogen or helium, the two most abundant materials in the universe. All this makes it unlike a normal massive star and more in line with a white dwarf.
White dwarfs are “dead stars” that remain when typical stars have used up all of their hydrogen and helium fuel, at which point the oxygen- and carbon-rich star collapses into itself to form a high-mass Earth-sized object. The white dwarf is kept from further collapse into a neutron star only by the electron degeneracy pressure of the elements in its core, and its temperature is too low to enable further fusion. However, if the mass of the white dwarf increases, for example if it accretes matter from a nearby companion star, it can become hot enough to restart the fusion of carbon into heavier elements. This process is so violent that the radiation pressure it produces blows the star apart. Such “type 1A” supernovae are observed frequently and, since they are unleashed when a white dwarf reaches a very specific mass, they have a standard brightness that can be used to measure cosmic distances.
Despite having the chemical signature of a white dwarf, such an object cannot possibly burn as bright as J005311. By comparing the characteristics of J005311 with models of what happens when two white dwarfs merge, however, the explanation falls into place. As two white dwarfs, likely produced billions of years ago, orbited one another they slowly lost momentum through the emission of gravitational waves. Over time, the objects came so close to each other that they merged. This would commonly be expected to produce a type 1A supernova, but there are also models in which carbon is ignited in a more subtle way during the merging process, allowing it to start fusing without blowing the newly formed object apart. J005311’s detection appears to indicate that those models are correct, marking the first observation of a white-dwarf merger.
The rejuvenated star is, however, not expected to live for long. Based on the models it will burn through its remaining fuel within 10,000 years or so, forming a core of iron that is set to collapse into a neutron star through a violent event accompanied by a flash of neutrinos and possibly a gamma-ray burst. Using the speed of the ejected material and the distance it has reached from the star by now, it can be calculated that the merger took place about 16,000 years ago, meaning that its final collapse is not far away.