Some of the Research Highlights from our users in 2018:


Light quark flavour breaking effects in B meson decay constants

In the standard model of particle physics, there are six types of quarks whimsically called up u, down d, strange s, charm c, bottom b and top t in order of increasing mass. These bind together to form two classes of hadrons, mesons consisting of a quark – antiquark pair and baryons with three quarks. An example of a baryon is the proton (uud) while examples of mesons are the pion (ud), kaon (us), B (ub) and Bs (sb). These mesons decay to a lepton (usually the electron) and a neutrino. The decays are parametrised by ‘meson decay constants’, from which, together with experimentally determined leptonic decay rates elements of the CKM matrix can be found. These are fundamental parameters of the Standard Model and their determination can help to isolate possible new physics. Presently recent B physics experiments have left us with a number of heavy flavour physics puzzles which will be more intensively investigated by Belle II.

Figure 1: Left panel: A ‘fan’ plot for the light quark ud and us decay constants (normalised with the average ud, us decay constant) for one lattice spacing against the pion mass; Right panel: The heavy b quark ub, sb decay constants, again normalised, for several lattice spacings also plotted against the pion mass.

In our Lattice QCD programme we have developed a method by expanding about a point in the light quark mass parameter space where the u, d, s quark masses are initially set equal, and then extrapolating to the physical quark masses while keeping the average quark mass constant. The original expansions were for hadron masses, and lead to a ‘fan’ plot where the different hadron masses radiate from their common point. They were extended to decay constants for the pion and kaon mesons, [1], as shown in the left panel of the figure. We have now further extended this programme to the heavier quark masses and in particular the b, [2]. In the right panel of the figure we give the equivalent plot for the B and Bs mesons, together with the present (FLAG16) values.

[1] V.G. Bornyakov et al., Phys. Lett. B 767 (2017) 366.

[2] S. Hollitt et al., PoS(LATTICE2018)268, arXiv:1811.06677 [hep-lat].


Gravitational Waves 2018

The Advanced LIGO and Virgo detectors were undergoing an upgrade during 2018, so the year was devoted to completing analyses of data from the first two observing runs (O1 and O2), and preparations for the third observing run (O3), due to start in April 2019.

The chief results from the LIGO and Virgo collaborations were the final estimates of the physical properties of the binary-neutron-star merger, GW170817, including stronger constraints on the neutron star tidal deformability, and therefore its equation of state and radius; and the release of the first catalogue of observations from O1 and O2, based on a re-analysis of all data, which in turn had undergone improved characterisation and cleansing. This re-analysis uncovered new signals in the data, and doubled the number of observed black hole binary mergers from five to ten. This included re-classifying the marginal “LIGO-Virgo transient” from October 12, 2015 as a bona-fide GW signal, and among the new signals, GW170729, which is likely the most massive binary yet observed, at around 85 solar masses. All of this work used the waveform models that were calibrated to numerical-relativity simulations performed on DiRAC.

Figure 2: The eleven gravitational-wave signals observed in the first and second LIGO-Virgo observing runs (O1 and O2).

Modelling work also continued. The first inspiral-merger-ringdown model to include higher signal harmonics (PhenomHM) was published in Physical Review Letters, and is now being used to update the measurements of the properties of GW170729. The bulk of the most extensive and systematic set of simulations of precessing binaries was completed on Cosma6, and a preliminary model of the precession effects through merger and ringdown (the first of its kind) was completed, with publication expected in 2019, for use in analysis of O3 observations.


Exomol: Computational spectroscopy of Exoplanets

Molecular spectra play a critical role for the retrieval of atmospheres of exoplanets and cool starts. However, the lack of laboratory data on many of molecular species severely limits models for objects as diverse as Jupiter, exoplanets and brown dwarfs. The UCL ExoMol team is world leader in providing molecular line lists for exoplanet and other hot atmospheres. The ExoMol procedure uses a mixture of ab initio calculations and available laboratory data. These line lists form the input for opacity models for cool stars and brown dwarfs as well as for radiative transport models involving exoplanets. So far ExoMol has collected molecular opacities for more than 50 molecules (130 isotopologues): 30 line lists have been generated using the ExoMol methodology and other line lists were taken from external sources or from our work predating the ExoMol project.

Ethylene and Acetylene are important absorber in hot Jupiter exoplanets, brown dwarfs and cool carbon stars. As part of our Hydrocarbons DiRAC project, we have new line lists for these molecules, each of which contain over 30 billion transitions. In order to accomplish this data intensive task, some of the UK’s most advanced supercomputers have been used, provided by the DiRAC project. Even larger line lists have been completed. For example, the methyl chloride’s line list contains over 300 billion lines.

Many of those exoplanets discovered thus far are categorized as rocky objects with an atmosphere. Most of these objects are however hot due to their short orbital period. Models suggest that water is the dominant species in their atmospheres. The hot temperatures are expected to turn these atmospheres into a (high pressure) steam bath containing remains of melted rock. The spectroscopy of these hot rocky objects will be very different from that of cooler objects or hot gas giants. Molecules suggested to be important for the spectroscopy of these objects are reviewed together with the current status of the corresponding spectroscopic data. We have started building a comprehensive database of linelist/cross sections applicable for atmospheric models of rocky super-Earths as part of the ExoMol project.

An efficient usage of line lists comprising billions of lines is extremely challenging. To address this challenge of Big Data, we have developed a new Fortran 2003 program ExoCross. ExoCross takes line lists as input and returns pressure- and temperature-dependent cross sections as well a variety of other derived molecular properties which depend on the underlying spectroscopic data. These include state-dependent lifetimes, temperature-dependent cooling functions, and thermodynamic properties such as partition functions and specific heats and is designed to work with the recently proposed method of super-lines. It is capable of simulating non-LTE spectra using a simple two-temperature approach.

Supported by the DiRAC high performance facilities, we have organised and run a workshop “Digital Exoplanets” in Prague, January 2019 (www.digitalexoplanets.org). The workshop gathered over 50 computational exoplanetary scientists (with more than half career researchers) and focused on the data for atmospheric models of exoplanets and cool stars. The main theme of the workshop was the open access to the data and computer codes. This was the first workshops of this kind, focusing not only on observations and theory, but mostly on ‘how to actually run these codes’. The participants had the opportunity to learn how to use (install, run, debug etc) different main stream atmospheric and spectroscopic codes during the hands-on sessions, which were the central part of the workshop. These practical sessions were run on the resources provided by DiRAC team, with the help of the DiRAC team, which is greatly appreciated.


UK MHD Consortium: 1 Solar and Planetary Interiors

One of the great challenges in the study of planetary interiors is to understand the generation of magnetic fields in the interiors of the giant gas planets. Wieland Dietrich and Chris Jones from the University of Leeds have been addressing this problem through high resolution magnetohydrodynamic models, with the novel feature of incorporating realistic radially-dependent profiles for the electrical conductivity (Icarus 305 (2018) 15-32). This allows the study of a range of models in which the conductivity ranges from Jupiter-like, with only a thin outer non-conducting shell to Saturn-like, with a thick non-conducting shell. They find Jupiter-like steady dipolar fields, quadrupolar dynamos for profiles between those of Jupiter and Saturn, and dipolar fields again for Saturn-like profiles. The non-axisymmetric components of the Saturn model are, though, more pronounced than in Saturn itself, prompting the suggestion that a stably stratified region – postulated to exist in Saturn, but not yet incorporated in the model – may be of significance.



Figure 3: The figure shows (from left to right) time-averaged meridional sections of the azimuthal flow, meridional stream function, kinetic helicity, radial and azimuthal magnetic fields; the right hand plots are typical snapshots of the radial field and azimuthal flow at the surface. The top row is for the case of uniform electrical conductivity; the bottom row is for a Jupiter-like profile, in which the electrical conductivity drops off sharply at 90% of the planetary radius.

Leicester Highlight: Planet-driven warps in protoplanetary discs

Since their discovery in the 1990s our knowledge of extra-solar planets has increased at a staggering rate, and we now know that most, perhaps all, stars host planetary systems. These planets form in cold discs of dust and gas around young stars, and in recent years our observations of these protoplanetary discs have also advanced dramatically. We typically treat planet-forming discs as flat, smooth structures but recent observations have revealed a wealth of sub-structures in these discs, including spiral arms, gaps, misalignments and warps. While we can explain the origin of some of these features, many are still open questions. One of the best-studied protoplanetary discs is around the nearby star TW Hya, and this almost face-on disc shows evidence of gaps (possibly due to planets) as well as a localised dark region in the outer disc. Observations with the Hubble Space Telescope found that this dark region is moving around the outer disc, but it is seen to move much more quickly than gas orbits at that radius. This strongly suggests that the dark region is actually a shadow cast by something in the inner disc (where the gas orbits faster). This was interpreted as evidence of a disc warp, such that the inner part of the disc is inclined with respect to the outer disc. The relative misalignment between the inner and outer disc then results in a shadow being cast on the outer disc. Such a configuration could be caused if a misaligned planet is present in this system, where the planets orbit is tilted away from the mid-plane of the disc. The motion of the inner, tilted disc and hence the shadow would thus be governed by the gravity of this misaligned planet, and it might be possible to infer properties of the planet from the observed shadow.

Figure 4: 3-D numerical simulation of en inclined planet in a protoplanetary disc, performed on the DiRAC Data Intensitve @ Leicester system. Our simulations showed that an inclined giant planet can create an observable warp in the disk. (Figure adapted from Nealon et al. 2018).

Simulating such a configuration is technically challenging, because we need to resolve the out-of-plane interactions between the disc and the planet, as well as including the influence of the outer disc. In practice this requires high resolution 3-D hydrodynamic simulations covering a large dynamic range. Using DiRAC, we were able to run a suite of simulations to study the interaction between misaligned planets and their parent discs in detail, investigating the importance of different planet masses and inclinations on the subsequent disc structure. For planets that were massive enough to carve the disc into an inner disc and an outer disc (as seen in Fig.1), we confirmed it was possible to drive the configuration predicted from the observations of TW Hya. Importantly, the large spatial scales we were able to simulate allowed us to show that this was possible even for small planet inclinations. We subsequently ran detailed radiative transfer simulations to calculate how our simulated discs would appear to our telescopes, and confirmed that such a configuration is broadly consistent with what is observed in TW Hya. New observations are finding that misalignments of this type are common in planet-forming discs, and in the coming years DiRAC will allow us to understand the dynamics and evolution of these nascent planetary systems in detail.