Topological Origin of Equatorial Waves

Topology sheds new light on the emergence of unidirectional edge waves in a variety of physical systems, from condensed matter to artificial lattices. Waves observed in geophysical flows are also robust to perturbations, which suggests a role for topology. We show a topological origin for two celebrated equatorially trapped waves known as Kelvin and Yanai modes, due to the Earth's rotation that breaks time-reversal symmetry. The non-trivial structure of the bulk Poincar\'e wave modes encoded through the first Chern number of value $2$ guarantees existence for these waves. This invariant demonstrates that ocean and atmospheric waves share fundamental properties with topological insulators, and that topology plays an unexpected role in the Earth climate system

Lattice calculation of the pion transition form factor π0→γ∗γ∗\pi^0 \to \gamma^* \gamma^*

We calculate the $\pi^0\to \gamma^*\gamma^*$ transition form factor ${\cal F}_{\pi^0\gamma^*\gamma^*}(q_1^2,q_2^2)$ in lattice QCD with two flavors of quarks. Our main motivation is to provide the input to calculate the $\pi^0$-pole contribution to hadronic light-by-light scattering in the muon $(g-2)$, $a_\mu^{\rm HLbL;\pi^0}$. We therefore focus on the region where both photons are spacelike up to virtualities of about $1.5~$GeV$^2$, which has so far not been experimentally accessible. Results are obtained in the continuum at the physical pion mass by a combined extrapolation. We reproduce the prediction of the chiral anomaly for real photons with an accuracy of about $8-9\%$. We also compare to various recently proposed models and find reasonable agreement for the parameters of some of these models with their phenomenological values. Finally, we use the parametrization of our lattice data by these models to calculate $a_\mu^{\rm HLbL;\pi^0}$.Comment: 31 pages - Minor text improvements. Published versio

Exploratory studies for the position-space approach to hadronic light-by-light scattering in the muon g−2g-2

The well-known discrepancy in the muon $g-2$ between experiment and theory demands further theory investigations in view of the upcoming new experiments. One of the leading uncertainties lies in the hadronic light-by-light scattering contribution (HLbL), that we address with our position-space approach. We focus on exploratory studies of the pion-pole contribution in a simple model and the fermion loop without gluon exchanges in the continuum and in infinite volume. These studies provide us with useful information for our planned computation of HLbL in the muon $g-2$ using full QCD.Comment: 8 pages, 11 figures, 1 table, Lattice 2017 proceedings, Granada, Spai

Lattice calculation of the pion transition form factor with Nf=2+1N_f=2+1 Wilson quarks

We present a lattice QCD calculation of the double-virtual neutral pion transition form factor, with the goal to cover the kinematic range relevant to hadronic light-by-light scattering in the muon $g-2$. Several improvements have been made compared to our previous work. First, we take into account the effects of the strange quark by using the $N_f=2+1$ CLS gauge ensembles. Secondly, we have implemented the on-shell $\mathcal{O}(a)$-improvement of the vector current to reduce the discretization effects associated with Wilson quarks. Finally, in order to have access to a wider range of photon virtualities, we have computed the transition form factor in a moving frame as well as in the pion rest-frame. After extrapolating the form factor to the continuum and to physical quark masses, we compare our results with phenomenology. We extract the normalization of the form factor with a precision of 3.5\% and confirm within our uncertainty previous somewhat conflicting estimates for a low-energy constant that appears in chiral perturbation theory for the decay $\pi^0 \to \gamma\gamma$ at NLO. With additional input from experiment and theory, we reproduce recent estimates for the decay width $\Gamma(\pi^0 \to \gamma\gamma)$. We also study the asymptotic large-$Q^2$ behavior of the transition form factor in the double-virtual case. Finally, we provide as our main result a more precise model-independent lattice estimate of the pion-pole contribution to hadronic light-by-light scattering in the muon $g-2$: $a_{\mu}^{\mathrm{HLbL}; \pi^0} = (59.7 \pm 3.6) \times 10^{-11}$. Using in addition the normalization of the form factor obtained by the PrimEx experiment, we get the lattice and data-driven estimate $a_{\mu}^{\mathrm{HLbL}; \pi^0} = (62.3 \pm 2.3) \times 10^{-11}$.Comment: 29 pages, 14 figures. v2: minor corrections to match the published version. A file with the transition form factor data at the physical pion mass and in the continuum is included in the submissio

Importance of interorbital charge transfers for the metal-to-insulator transition of BaVS3_3

The underlying mechanism of the metal-to-insulator transition (MIT) in BaVS$_3$ is investigated, using dynamical mean-field theory in combination with density functional theory. It is shown that correlation effects are responsible for a strong charge redistribution, which lowers the occupancy of the broader \a1g band in favor of the narrower $E_g$ bands. This resolves several discrepancies between band theory and the experimental findings, such as the observed value of the charge-density wave ordering vector associated with the MIT, and the presence of local moments in the metallic phase.Comment: improved discussion, new figure, added reference

Theory of double resonance magnetometers based on atomic alignment

We present a theoretical study of the spectra produced by optical-radio-frequency double resonance devices, in which resonant linearly polarized light is used in the optical pumping and detection processes. We extend previous work by presenting algebraic results which are valid for atomic states with arbitrary angular momenta, arbitrary rf intensities, and arbitrary geometries. The only restriction made is the assumption of low light intensity. The results are discussed in view of their use in optical magnetometers

APC Communiqué

This is the archive of the summer 2012 APC Communiqué, a bi-annual newsletter, which features a special report entitled, "President Rupiah Banda defends democracy in Zambia and Africa". Banda was APC's eighth President-in-Residence