Theory Seminar Series

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Matteo Broccoli (Max Planck Institute for Gravitational Physics)

The Third Way to Interacting p-forms Theories Jul 09, 2021 at 14:30

In three spacetime dimensions certain gravitational and gauge theories are `third way’ consistent. This means that their equations of motion are only on-shell consistent and do not come from the variation of an action which contains the dynamical field alone. Although this mechanism is not special to 3d, no higher dimensional third way consistent theory was known. In this talk, I will introduce the third way by presenting the 3d theories and show how we recover them by shifting a flat gauge connection. Applying the same method in higher dimensions, we find a new class of interacting (d-2)-form theories and I will discuss various generalisations of them. Our result proves that the third way can be realised in dimensions higher than three, and I will conclude by discussing the possibility of constructing new third way consistent theories of gravity.

Andreas Schachner (Cambridge University)

The Standard Model Quiver in de Sitter String Compactifications Jul 02, 2021 at 14:30

With the advent of the string landscape, the realisation of the Standard Model in general string theory compactifications to 4D has become a primary focus. This talk concerns novel constructions of the Standard Model in global set-ups of type IIB Calabi-Yau compactifications. We argue that the Standard Model quiver can be embedded into compact Calabi-Yau geometries through orientifolded D3-branes at del Pezzo singularities dP_n with n ≥ 5 in a framework including moduli stabilisation. To illustrate our approach, we explicitly construct a local dP_5 model via a combination of Higgsing and orientifolding. This procedure reduces the original dP_5 quiver gauge theory to the Left-Right symmetric model with three families of quarks and leptons as well as a Higgs sector to further break the symmetries to the Standard Model gauge group. We embed this local model in a globally consistent Calabi-Yau flux compactification with tadpole and Freed-Witten anomaly cancellations. The model features closed string moduli stabilisation with a de Sitter minimum from T-branes, supersymmetry broken by the Kähler moduli, and the MSSM as the low energy spectrum. We further discuss phenomenological and cosmological implications of this construction.

Viraf Mehta (Gottingen University)

Exploring the String Axiverse with Black Holes Jun 25, 2021 at 14:30

We analyse a large ensemble of axion data derived directly from string theory compactifications arising in the Kreuzer-Skarke database. We then use astrophysical measurements of spinning black holes to constrain our axion systems and, for the first time, exclude portions of the string axiverse.

Andrea Cavaglia (King’s College London)

Looking for the Separation of Variables in a 4D CFT Jun 18, 2021 at 14:30

Some higher-dimensional CFTs, such as N=4 Super Yang-Mills (SYM) theory, are believed to possess the mathematical structures of integrability – however, it is far from completely understood how to solve these models completely. In this talk, I will discuss the role of the Separation of Variables, an expected powerful property of integrable systems, in this context. I will present some recent theoretical advances on the SoV method, and how these new techniques can be employed to compute nontrivial observables in a 4D CFT, the “fishnet” theory – a close cousin of N=4 SYM. The talk is based on the work hep-th/2103.15800 in collaboration with N. Gromov and F. Levkovich-Maslyuk.

Alba Grassi (University of Geneva & CERN)

Painlevé equations, quantum operators and gauge theory Jun 11, 2021 at 14:30

Sante Carloni (Genova University)

Covariant relativistic stars Jun 4, 2021 at 14:30

In this talk, I will construct a dimensionless and covariant formulation of Tolman-Oppenheimer-Volkoff (TOV) equations. This new formulation makes evident the mathematical structure of the TOV equations and allows to devise strategies to obtain new, physically relevant, exact solutions. I will show that these covariant TOV equations can be extended to include anisotropic pressure terms and even additional fluids. I will then present some exact solutions for isotropic, anisotropic, and two-fluid isotropic objects composed of cold catalyzed matter. Finally, I will briefly sketch some perspectives for further generalization aimed to include heat fluxes, interactions, electromagnetic fields and modified gravity.

Justin Feng (Lisbon University)

Generalized coupling theories and the MEMe model May 28, 2021 at 15:30

Generalized coupling theories are characterized by a nontrivial coupling between the gravitational metric and matter, mediated by an auxiliary rank-2 tensor. The actions generating the field equations are constructed so that these theories are equivalent to general relativity in a vacuum, and only differ from Einstein’s theory within a matter distribution. This talk will focus on one of the simplest realizations of these theories, termed the MEMe model. The MEMe model admits an exact solution for the coupling for a single perfect fluid; features of the model include varying gravitational wave propagation speeds and inflationary behavior in sufficiently dense matter distributions. I discuss some recent work on the post-Newtonian limit and junction conditions for the MEMe model.

Sebastián Franchino-Viñas (La Plata University – Heidelberg University)

Too odd to be true? May 21, 2021 at 14:30

Anomalies are of great importance both because of their formal peculiarities and their practical consequences. In this talk I’ll first review some of their general aspects. Then, I’ll consider in more detail some recent results regarding the trace anomaly. I’ll devote particular attention to the trace anomaly of a chiral fermion, for which a possible odd-parity contribution has recently generated a controversy.

William J. Torres Bobadilla (Max Planck – Munich)

All loop causal representation of scattering amplitudes May 14, 2021 at 14:30

The numerical evaluation of multi-loop scattering amplitudes in the Feynman representation usually requires to deal with both physical (causal) and unphysical (non-causal) singularities. The loop-tree duality (LTD) formulation offers a powerful framework to easily characterise and distinguish these two types of singularities, and then analytically simplify the underlying expressions. On top of it, we elucidate that the causal representation of loop topologies depends only on the features of the latter, vertices and edges. In effect, by virtue of this relation, a straightforward classification of loop topologies in terms causal (physical) thresholds can be generated, regardless of the loop order. Finally, we introduce the Mathematica package Lotty that automates the LTD formulation at multi-loop level by means of dual and causal representation of scattering amplitudes (https://bitbucket.org/wjtorresb/lotty).

Paolo Meda (Genova University)

Evaporation of four-dimensional black holes induced by the trace anomaly in semiclassical gravity
May 7, 2021 at 15:30

A first approximation to describe the interplay between quantum matter and gravity is given by the quantum field theory on curved spacetimes and the semiclassical gravity, in which the backreaction of a quantum matter field propagating over a fixed spacetime geometry is studying using the so-called semiclassical Einstein equation. In this framework, the evaporation of four-dimensional spherically symmetric black holes can be explained by the appearance of an ingoing negative energy flux at the apparent horizon, which induces a negative variation of the black hole mass. I will show that this flux can be sourced by the trace anomaly of the quantum stress-energy tensor in case of a massless conformally-coupled scalar field once a certain averaged energy condition is assumed outside the horizon. As an example, I will compute the negative flux and the rate of evaporation in the Vaidya spacetime, which describes the exterior geometry of a null radiating star.

Tobias Denz (Heidelberg University)

Graviton spectral functions Apr 30, 2021 at 14:30

We reconstruct the Lorentzian graviton propagator in asymptotically safe quantum gravity from Euclidean data. The reconstruction is applied to both the dynamical fluctuation graviton and the background graviton propagator, where the background spectral function necessarily has negative parts. In turn, the spectral function of the dynamical graviton is positive. We argue that the latter enters cross sections and other observables in asymptotically safe quantum gravity. Hence, its positivity may hint at the unitarity of asymptotically safe quantum gravity

Xiaoling Cui‬ (Institute of Physics, Chinese Academy of Sciences)

Nature of polaron-molecule transition in Fermi polarons Apr 23, 2021 at 13:30

For a single impurity moving in the background of Fermi-sea atoms, whether there will be a first-order transition from polaron to molecule as the impurity-fermion attraction increases is a fundamental question. Many theories have supported the existence of such transition based on a separate treatment of polaron and molecule states. On the other hand, the experiments have never observed the signature of such an abrupt transition. In this talk, I will introduce our recent work on this problem by utilizing a unified variational ansatz on both states. The existence of polaron-molecule transition is confirmed in 3D and 2D impurity systems, and more importantly, the nature of such transition is identified as an energy competition between systems with different total momenta. Near the transition, both momenta states appear as local minima in the dispersion curve, providing the underlying mechanism for polaron and molecule coexistence in realistic experiments with a finite impurity density and at finite temperature. This theory produces quantitatively good fits to recent experimental data of 3D Fermi polarons in unitary regime.

Alexandre Dauphin‬ (ICFO Barcelona)

Phase Detection with Neural Networks: Interpreting the Black Box Apr 9, 2021 at 14:30

Neural networks (NNs) normally do not allow any insight into the reasoning behind their predictions. We demonstrate how influence functions can unravel the black box of NN when trained to predict the phases of the one-dimensional extended spinless Fermi-Hubbard model at half-filling. Results provide strong evidence that the NN correctly learns an order parameter describing the quantum transition. Moreover, we demonstrate that influence functions not only allow to check that the network, trained to recognize known quantum phases, can predict new unknown ones but even guide physicists in understanding patterns responsible for the phase transition. This method requires no a priori knowledge on the order parameter, the system itself, or even the architecture of the ML model. Finally, we will also how such a method, combined with unsupervised learning algorithms, can recover the phase diagram of the quantum anomalous Hall phase, a paradigmatic example of topological insulators, directly from experimental data.

Xavier Calmet (University of Sussex)

Bounds on Dark Matter Masses Mar 19, 2021 at 14:30

We review our current understanding of quantum gravity using effective field theory methods and then consider implications for dark matter. We show that quantum gravity leads to lower and upper bounds on the masses of dark matter candidates. These bounds depend on the spins of the dark matter candidates and the nature of interactions in the dark matter sector. For example, for singlet scalar dark matter, we find a mass range 10^{−3} eV≲m≲10^7 eV . The lower bound comes from limits on fifth force type interactions and the upper bound from the lifetime of the dark matter candidate.

Daniele Malafarina (Nazarbayev University)

Modern insights into black holes, collapse and singularities Mar 12, 2021 at 14:30

Analytical models for gravitational collapse are a fundamental tool for our understanding of the formation of black holes and space-time singularities. However, singularities should not occur in the real universe. How are collapse models modified once we introduce a mechanism to resolve the singularities? Could these models show the way towards a theory of gravity beyond General Relativity?

Riccardo Rossi (Institute of Physics, EPFL)

High-order renormalized perturbative approach for strongly-correlated fermions Feb 26, 2021 at 14:30

In this talk I show how perturbation theory can be turned into a viable computational approach for physical systems afflicted by the fermionic sign problem. This is accomplished by designing new numerical approaches to reach arbitrary-high orders for the bare [1] and the renormalized [2] expansion. I discuss the results that we have obtained for the doped square-lattice Hubbard model in the pseudogap regime, and in frustrated lattices. In particular, we numerically prove that the model’s strong antiferromagnetic correlations lead to Fermi surface reconstruction. Finally, I present the first unbiased diagrammatic computation in a broken-symmetry phase by discussing the s-wave superfluid transition in the spin-polarized cubic-lattice attractive Hubbard model.

[1] RR, Phys. Rev. Lett. 119, 045701 (2017)

[2] RR, Simkovic, Ferrero EPL 132, 11001 (2020)

Tim Schmitz (University of Cologne)

Exteriors to bouncing collapse models Feb 12, 2021 at 14:30

Bouncing collapse scenarios are a promising resolution of the singularities forming in classical gravitational collapse. In them, quantum gravitational effects in the high curvature regime cause the initially collapsing matter to re-expand. There are however a few open questions left to be answered before these scenarios can be considered fully consistent: How does the horizon behave during the bounce? What are the relevant timescales of this process, in particular from the point of view of a far-away observer? To shed some light on these issues we construct a large class of spacetimes that are smoothly matched to homogeneous, spherically symmetric clouds of matter. The evolution of the clouds is left arbitrary to allow for the incorporation of modifications by quantum effects, particularly bounces. We further discuss two simple yet illustrative examples of these spacetimes, both in general terms and for a specific form of the bounce, with a focus on horizon behavior and relevant timescales.

Ennio Salvioni (Theoretical Physics Department, CERN)

The Present and Future of Four Tops Feb 5, 2021 at 14:30

After discussing theoretical motivations for a strongly-interacting top quark, I will present its phenomenology at current and future colliders, showing that the characteristic four-top contact operators give rise to the most important effects. Hadron colliders can probe these operators directly, whereas lepton colliders have indirect sensitivity through renormalization group evolution. I will discuss the impact on composite Higgs models with a strongly-coupled top, and show that four-top probes provide the best sensitivity on the compositeness scale at the energy frontier. In addition, I will comment on the mild but persisting excesses observed by ATLAS and CMS in multilepton + jets final states, entertaining the possibility that they may arise from such new physics scenario.

Domenico Bonocore (University of Münster)

Soft emissions in the wordline representation Jan 29, 2021 at 14:30

The presence of soft radiation in scattering amplitudes poses a severe obstacle to the predictive power of perturbation theory. While various techniques have been available for a long time at leading power in the soft expansion, recently there has been growing interest (both from a phenomenological and a more formal perspective) in the extension of this framework at subleading power in the soft expansion. In this talk I will discuss how the worldline representation provides a powerful tool to tackle these issues.

Antonella Grassi (University of Bologna)

Singularities Jan 22, 2021 at 14:30

I will overview different aspects of ongoing projects at the interface of geometry, strings, gauge theory and representation theory. I will then illustrate the construction of new examples.

Mikel A. Urkiola (University of the Basque Country)

A complete mass spectrum of type-IIB flux vacua Dec 18, 2020 at 14:30

In this talk I will outline the results of our recent works (arxiv:2007.10381 and 2011.13953). I will give an overview of the moduli stabilization problem in type-IIB flux compactifications in string theory, and how this issue can be treated analytically through the use of symmetries of the moduli space. Applying these methods to no-scale vacua in the Large Complex Structure (LCS) region, we will be able to obtain a closed and analytical expression for the scalar spectrum of the theory at tree-level for the complex structure and dilaton sector. Finally, we will see how these results apply for a concrete example (the octic), by a direct numerical search for vacua in that model.

Roberto Bonezzi (Humboldt University – Berlin)

A worldline theory for gravity Dec 11, 2020 at 14:30

It is widely accepted that one of the most satisfactory features of String Theory is to reproduce Einstein’s equations (to lowest order in alpha prime) from consistency of the first-quantized string. I will try to show that this feature is not at all peculiar to String Theory. In particular, I will discuss some recent results in the quantization of the N=4 spinning particle coupled to a background metric, Kalb-Ramond field and dilaton. Quantum consistency of the particle is shown to “predict” the field equations for all its backgrounds.

Jacopo Fumagalli (IAP – Paris)

Turns in the inflationary landscape: hyper non-Gaussianities, primordial black holes and gravitational waves Dec 4, 2020 at 15:00

Contacts

If you would like to schedule a talk, or have any queries, please contact tiziano.peraro@unibo.it or davide.vodola@unibo.it.