Organisation des séminaires et calendrier

Abstract: 

All the information we will ever obtain from the early universe is imprinted in the spatial correlations of primordial fluctuations at the hot Big Bang. I will explain how an influx of ideas from various areas of fundamental physics is providing us with new conceptual and practical tools to decode the physics of these primordial fluctuations. A thorough understanding of the fluctuations will give us insight into particle physics at the highest energies and may provide a window into the nature of spacetime itself.

In these lectures I will review the current status of the theory and phenomenology of cosmological correlators — the observable which encodes the statistics of primordial fluctuations.

• I will give a broad overview of inflationary perturbation theory, the current observational status and future prospects for constraining inflation;
• Motivate the EFT perspective on primordial fluctuations, and propose “cosmological collider physics” as a way to probe the UV completion of inflation;
• Explain the various methods to compute cosmological correlators: the in-in formalism, the wavefunction of the universe, and the cosmological bootstrap;
• Give a survey of recent theoretical developments that draw inspiration from the scattering amplitude and conformal field theory literature.

Abstract: 

All the information we will ever obtain from the early universe is imprinted in the spatial correlations of primordial fluctuations at the hot Big Bang. I will explain how an influx of ideas from various areas of fundamental physics is providing us with new conceptual and practical tools to decode the physics of these primordial fluctuations. A thorough understanding of the fluctuations will give us insight into particle physics at the highest energies and may provide a window into the nature of spacetime itself.

In these lectures I will review the current status of the theory and phenomenology of cosmological correlators — the observable which encodes the statistics of primordial fluctuations.

• I will give a broad overview of inflationary perturbation theory, the current observational status and future prospects for constraining inflation;
• Motivate the EFT perspective on primordial fluctuations, and propose “cosmological collider physics” as a way to probe the UV completion of inflation;
• Explain the various methods to compute cosmological correlators: the in-in formalism, the wavefunction of the universe, and the cosmological bootstrap;
• Give a survey of recent theoretical developments that draw inspiration from the scattering amplitude and conformal field theory literature.

I will discuss ultralight new physics as an alternative candidate to explain some of the puzzles in modern particle physics and cosmology. I will review how it could alleviate some of the fine tuning problems in the Standard Model and how it naturally predicts dark matter in the present universe. Then, I will discuss several different ways to experimentally and observationally test the existence of ultralight new physics. I will show that a wide variety of such scenarios can be tested currently and in near future, based on recent advancements on precision astrophysics (pulsar timing array, astrometry, range measurements) as well as quantum sensors (atomic clock, gravity wave detectors). I will conclude with a few future directions and challenges for ultralight new physics searches.

Abstract: 

All the information we will ever obtain from the early universe is imprinted in the spatial correlations of primordial fluctuations at the hot Big Bang. I will explain how an influx of ideas from various areas of fundamental physics is providing us with new conceptual and practical tools to decode the physics of these primordial fluctuations. A thorough understanding of the fluctuations will give us insight into particle physics at the highest energies and may provide a window into the nature of spacetime itself.

In these lectures I will review the current status of the theory and phenomenology of cosmological correlators — the observable which encodes the statistics of primordial fluctuations.

• I will give a broad overview of inflationary perturbation theory, the current observational status and future prospects for constraining inflation;
• Motivate the EFT perspective on primordial fluctuations, and propose “cosmological collider physics” as a way to probe the UV completion of inflation;
• Explain the various methods to compute cosmological correlators: the in-in formalism, the wavefunction of the universe, and the cosmological bootstrap;
• Give a survey of recent theoretical developments that draw inspiration from the scattering amplitude and conformal field theory literature.

We are interested in the problem of understanding the structure of the Bethe roots for the periodic spin-1/2 Heisenberg XXZ chain. To probe this structure it is fruitful to study what happens as one varies the parameters. We start with the (ferromagnetic) Ising limit ∆ → ∞, in which ​the Bethe roots become explicit, and strings of Bethe roots become directly visible on the lattice, already examined by Gaudin. Despite the simplicity of the limiting model, there are some subtleties in the limit. Next we study what happens with the explicit Ising Bethe roots as ∆ is lowered, where bound states may form or unbind. I will present a ‘critical equation’ that determines when Bethe roots switch between real and complex for the sector with M = 2 magnons, generalising Essler–Korepin–Schoutens (1992, ∆ = 1) and Imoto–Sato–Deguchi (2019, ∆ ≥ 1), and discuss some results for M > 2. If time allows, I will outline how a similar game can be played by turning on long-range interactions, going from Heisenberg XXX, via the Inozemtsev chain, to the Haldane-Shasatry chain.

This is based on joint work J.-S. Caux and R. Koch, to appear, and with R. Klabbers (2022).

We study the long-time, large-distance asymptotic behaviour of dynamical two-point correlation functions of the Lieb–Liniger model in the limit of infinite repulsion (the one-dimensional impenetrable Bose gas). Starting with exact representations of correlation functions in terms of Fredholm determinants of an integrable integral operator, we perform the rigorous asymptotic analysis, using Riemann–Hilbert techniques. The integral operator
depends parametrically on time \(t\), distance \(x\) and on the filling fraction — a function that characterizes the thermal or non-thermal equilibrium conditions. We consider a large class of non-thermal equilibrium conditions, extending previous results established for the thermal equilibrium case. The long-time and large-distance asymptotic behaviour is derived for two classes of filling fractions. These classes are characterized by the number of poles on the real axis (a generalization of Fermi points) that, together with the unique saddle point, contribute to the asymptotic expansion. For each class, we derive the long-time, large-distance symptotic behaviour as a series in \(x\)^\(−1/2\) as \(x\) and \(t\) go to infinity for a fixed ratio \(x/t\). We provide explicit closed-form expressions for the leading and sub-leading terms, logarithmic corrections, and overall constants in terms of special functions and simple integrals. For the impenetrable Bose gas in thermal equilibrium, we verify the derived asymptotic expansions by comparing them with the existing results in the literature and with numerical data.
This talk is based on joint work with Frank Göhmann, Karol K. Kozlowski, and Alexander Weiße.

After describing elastic interactions and the discovery of the odderon by the TOTEM and D0 Collaborations, we will discuss new kinematical regimes where the gluon density in the proton or Pb can be very large which leads to new equations in Quantum Chromodynamics.  We will also present the latest expectations on beyond standard model physics and especially the production of axion-like particles using intact protons after the collision.  We will finish by describing briefly the Low Gain Avalanche Detectors and their applications in medicine for cancer treatment and with NASA to measure cosmic rays.