Theorie Palaver

According to the standard model of cosmology, the Universe at its very beginning underwent a phase of rapid expansion, followed by a reheating period. During this epoch, the energy density, initially accumulated in the coherent oscillations of the inflaton field, was injected into the visible sector, eventually setting the initial conditions for the hot Big Bang. In this talk, I will discuss perturbative production of the Standard Model (SM) particles adopting a non-standard post-inflationary scenario with a generic equation-of-state parameter \(\bar{w}\). To specify the inflaton dynamics, I will employ the \(\alpha\)-attractor T-model of inflation, such that \(\phi\) has a monomial potential \(V(\phi) \propto \phi^{2n}\) about the minimum. Moreover, I will explore the Higgs boson-induced reheating, assuming that it is achieved through a cubic inflaton-Higgs coupling \(\phi |\mathcal{H}|^2\). In the presence of such interaction, the Higgs field acquires a \(\phi\)-dependent mass which generates a vacuum expectation value that oscillates in time and breaks the electroweak gauge symmetry. Interestingly, the non-zero Higgs mass leads to a time-dependent inflaton decay rate and generates a phase-space suppression of the reheating efficiency. This, in turn, has non-trivial consequences for the reheating dynamics, modifying the evolution of the SM radiation energy density or the duration of the reheating phase. Furthermore, the implications of the non-standard reheating for the dark sector will be discussed, exemplified by the UV freeze-in dark matter model.

A population of relativistic QCD axions can be produced in the early Universe, via scatterings with Standard Model particles. This can be searched for in cosmological datasets, which therefore provide the opportunity to discover/constrain the QCD axion, independently of astrophysical and/or laboratory probes. In this talk, after reviewing the subject, I present an improved calculation of the relic abundance of such “hot” axions from scatterings with pions below the QCD crossover, as well as the resulting upper bound on the QCD axion mass. I then discuss the exciting outlook of upcoming cosmological surveys, which may probe otherwise unexplored regions of the QCD axion parameter space. I highlight the need of a non-perturbative calculation of axion production rates throughout the QCD crossover, to fully exploit the reach of such datasets.

In the last years there has been a renewed interest in the particle physics theoretical and experimental communities on the study of exotic (non-standard) signatures at colliders, including Long-Lived Particles (LLPs). In this talk I will first give a brief overview of the theoretical motivations for long-lived particles. Later I will illustrate the impact of LLPs in the current LHC physics programme, including new LHC detectors specifically hunting for LLPs (MoEDAL, FASER, SND@LHC), detector proposals under consideration (MATHUSLA, ANUBIS, FPF) and relevant upgrades to the ATLAS, CMS and LHCb experiments. Finally, I will present snapshots of novel signatures and the implications of LLP searches in the context of neutrino, axion, dark matter and Higgs physics.

The direct integration of Feynman integrals can be a daunting task, in particular for increasing numbers of loops and external particles. The “symbol bootstrap” has proven to be a powerful tool in the calculation of certain polylogarithmic Feynman integrals and scattering amplitudes that bypasses this direct integration. In this approach one first writes an ansatz for the symbol of the integral and then fixes its degrees of freedom by imposing known mathematical and physical properties of the final result. In this talk I will discuss a generalisation of this approach to an elliptic case: the 12-point two-loop double-box integral. The bootstrapping ansatz is obtained from an elliptic generalisation of the so-called Schubert problem, and after imposing a sufficient number of constraints on this ansatz, we obtain a compact one-line formula for the (2,2)-coproduct of the double-box integral.

The Electroweak Symmetry Breaking Sector of the Standard Model is an active field where new physics is sought. In the absence of new higher-energy particles at colliders, Effective Field Theories in terms of the Standard Model particles (longitudinal gauge and Higgs bosons) seem a natural tool. New physics could then manifest itself as new forces modifying the couplings of those particles. Should such new forces be found, they could be used to confront (falsify?) the popular SMEFT: as it is not the most general possible EFT, we have managed to produce correlations among the parameters of the embedding HEFT that can be tested if that new physics is describable by SMEFT. We have also extended HEFT via the Inverse Amplitude Method to be able to address resonances in the electroweak sector directly from the low-energy particles and eventual BSM couplings, thus maintaining agnosticism about the nature of the eventual new physics, and I will discuss the systematic uncertainties of the unitarization procedure. Based on various works, e.g. https://inspirehep.net/literature/2120028 https://inspirehep.net/literature/2154526 https://inspirehep.net/literature/1826204 https://inspirehep.net/literature/1345172 https://inspirehep.net/literature/1310013

According to the inflationary theory of cosmology, most elementary particles in the current universe were created during a period of reheating after inflation. In this talk I will show how to self-consistently couple the Einstein-inflaton equations to a strongly coupled quantum field theory (QFT) that is described by holography. I will then use a specific example to demonstrate that this setup leads to an inflating universe, a reheating phase and finally a universe dominated by the QFT in thermal equilibrium. This talk is based on arXiv:2302.06618.

The Standard Model Higgs becomes tachyonic at high energy scales according to current measurements. This unstable regime of the Higgs potential can be realized in the early Universe during high scale inflation, potentially with catastrophic consequences. In this talk, we will discuss a crucial inherent feature of such configurations that has so far remained ignored: Higgs particle production out of vacuum induced by the rapidly evolving Higgs field, which gets exponentially enhanced due to the tachyonic instability. We will discuss various theoretical and observational implications of this effect.

The Hamiltonian approach to quantum gravity initiated by Bergman, Dirac, DeWitt, Komar, Wheeler et al has a long tradition and many quantum gravity programmes rest on it. While there has been progress in the past, the current theory is still not predictive because the Einstein-Hilbert Lagrangian is not even polynomial in the metric field which triggers many quantisation ambiguities. To eliminate those, renormalisation methods suggest themselves, preferrably directly in the Hamiltonian rather than the path integral language. After an introduction to those concepts, in this talk we present such a Hamiltonian renormalisation scheme which is derived from Wilson's notion of non-perturbative renormalisation of path integrals together with methods from constructive QFT. As a test we study Hamiltonian renomalisation of parametrised field theory in 2D which is a toy model for 4D quantum gravity in the sense that both theories are subject to the hypersurface deformation algebroid shared by all generally covariant field theories.

In cross sections involving angular cuts, intricate patterns of enhanced higher-order corrections known as non-global logarithms arise. These corrections do not exponentiate and to resum them to all orders one has to resort to numerical techniques. The resummation of the leading non-global logarithms was achieved twenty years ago, but higher-logarithmic resummations remained elusive. In my talk, I will show how such resummation can be performed using renormalization group methods in effective field theory. I will demonstrate how the associated evolution equations can be implemented into a Monte-Carlo framework and will present first numerical results for higher-logarithmic resummations at both lepton and hadron colliders.

The AdS3/CFT2 correspondence provides the best arena to test the holographic duality. This is because there is a better understanding of how to quantise strings on AdS3, compared with the higher dimensional cases, and the relative tractability of two-dimensional CFTs. In spite of this, little effort has been made to construct and classify supersymmetric AdS3 solutions. With a focus in their CFT interpretation, in this talk I will show you new AdS3 solutions in massive type IIA supergravity preserving small N=(4,0) supersymmetry. From the geometry, we engineer the dual CFT with well-known tools and propose a duality with a precise family of quivers. Additionally, we compute field theory and holographic central charges showing a clean matching in both descriptions.

In this talk, I will discuss different types of integrals over moduli space of punctured Riemann surfaces. First, I start with string scattering amplitudes and progress to the intersection number of twisted forms over the Riemann surface. I will discuss the equivalency between the two integrals and how this relation can be used to produce QFT (tree-level) amplitudes. In particular, I explore the double copy construction in amplitudes, which states that gravitational amplitudes can be expressed in terms of two sets of Yang-Mills amplitudes i.e.gravity=(gauge X gauge). I will motivate a formal construction of double copy in terms of the twisted cohomology and explain its relation to other forms of the double copy such as BCJ double copy and color kinematic duality. I will finish by discussing the possibility of extending this formalism into the loop level in both string and QFT amplitudes.

In our current understanding of the universe, the fundamental nature of one of its most abundant constituents, the dark matter (DM), is still a mystery. Among the many theorized candidates to play the role of DM, in this talk I will focus on sterile neutrinos with mass of O(keV) and in particular on their production in the early universe and phenomenology in terrestrial experiments today. The simplest mechanism able to produce sterile neutrino DM in the early universe is named Dodelson-Widrow mechanism after its inventors. Although very fascinating due to its extreme simplicity, if we assume that sterile neutrinos constitute the entire abundance of DM today this vanilla solution is, on the one hand, far from the region of the parameter space in which near future experiments will be sensitive to such particles and, on the other hand, mostly excluded by X-ray observations. After introducing the standard/vanilla scenario, I will discuss three minimal modifications to the standard scenario that change drastically the perspectives of detection of this DM candidate in the near future. They have to do with the following questions. What if before Big Bang Nucleosynthesis the universe evolved differently with respect to what is usually assumed? Should we consider the X-ray bound to be absolute or model dependent? What if active neutrinos interact among each other also with non-standard interactions?

Searches for long-lived particles (LLPs) are a rapidly expanding frontier at the LHC and other collider experiments. Still, many gaps remain in the current search program, in particular for LLPs with masses at the GeV or sub-GeV scale and with very large decay lengths. In this talk, I will illustrate two different approaches to filling this gap by discussing two models of light LLPs and their associated collider signals. First, I will show that the dominant decay mode of vectorlike leptons can be a very long-lived pseudoscalar and a tau lepton and argue that the muon chambers of CMS or ATLAS are ideal places to search for this final state. Second, I will illustrate the excellent sensitivity of Belle II to light LLPs with meter-scale decay lengths using the example of displaced vertex signals from strongly interacting dark sectors.

I will introduce a formalism for abelian lattice gauge theories called the Modified Villain Action formalism, which has been popular in recent years for constructing abelian gauge theories with correct symmetries and anomalies of their continuum counterparts. I will then show that this formalism can be used to write down a \(U(1)\) Chern-Simons theory on the lattice for even levels. The theory we construct suffers from the peculiar zero modes of the Gaussian differential operator which are infamous in the literature for plaguing the Chern-Simons theory with unphysical modes. I will show that such modes are related to a peculiar subsystem symmetry of a space-time lattice. This symmetry causes almost all Wilson loops to vanish. However Wilson loop-like operators with strip-like topology survive the pernicious symmetry and are topological, exactly like one expects in the continuum. Further the strip-like topology of the loops is a lattice realization of a well known phenomenon in continuum called the framing anomaly. I will further argue, time permitting, that the pernicious symmetry is really a form of gauge symmetry, projecting out certain states from the Hilbert space, and that it is not associated with any physical significance.

I would like to explain how the construction of a Wilson action in the exact renormalization group (ERG) formalism is related to diffusion equations for the fields. By making the diffusion equations gauge covariant, we have managed to linearize the BRST invariance of U(1) gauge theories. But with YM theories, the BRST invariance remains non-linear, and we have not been able to solve the constraint. All this has been done in collaboration with Hiroshi Suzuki (Kyushu University).