Tunneling in QFT

Seminar series devoted to tunneling in QFT

Monthly on Thursdays, usually @ 14:00 for Central Europe (CEST in European summer and CET in winter)

Whether it be vacuum stability, phase transitions, or analogue quantum systems, tunneling is part and parcel of quantum field theory. In this seminar series we explore new developments in our understanding of these phenomena.

Please register on this Indico page to receive Zoom joining instructions plus a reminder for each talk.

Upcoming seminars

• Thomas Steingasser (MIT and Harvard) 17 April 2025
  TBC
  
  
• Matthew Johnson (Perimeter Inst. Theor. Phys.) 22 May 2025
  TBC

Previous seminars

(Previous talks are uploaded at www.youtube.com/@TunnelingQFT )

• Matthias Carosi (Munich, Tech. U.) 20 February 2025 (slides)
  False vacuum decay beyond the quadratic approximation 
  Our Standard Model might be living in a metastable state, and a precise determination of its lifetime may hint at new Physics. In this talk, I review some of the efforts made towards computing the lifetime of the Standard Model. I focus, in particular, on the presence of an IR divergence due to instantons of any size contributing equally to the partition function. Extending our calculation beyond the quadratic approximation, thus retaining the leading quantum corrections, cures the IR divergence. I show how casting false vacuum decay in the language of the 2PI effective action provides a natural framework to do so.
  
  
• Andrey Shkerin (Perimeter Inst. Theor. Phys.) 23 January 2025 (slides, video)
  Thermal false vacuum decay is not what it seems
  We study the decay of a thermally excited metastable vacuum in classical field theory using real-time numerical simulations. We find a lower decay rate than predicted by standard thermal theory. The discrepancy is due to the violation of thermal equilibrium during the critical bubble nucleation. It is reduced by introducing dissipation and noise. We propose a criterion for the system to remain in equilibrium during the nucleation process and show that it is violated in the Hamiltonian evolution of a single field. In the case of many fields, the fulfillment of the criterion is model-dependent.
  
  

• Joonas Hirvonen (University of Nottingham) 12 December 2024 (slides, video)
  Real-Time Nucleation and Off-Equilibrium Effects in High-Temperature Quantum Field Theories
  We will discuss real-time nucleation in weakly coupled, high-temperature quantum field theories (QFTs), with a particular focus on the effects of off-equilibrium plasma on nucleation rates. We begin with an overview of high-temperature nucleation, highlighting its similarities and differences with the well-known scenario of vacuum tunneling. A key feature of the thermal system is its division into two quasi-classical components: nucleating fields and thermal particles interacting with the nucleating bubbles.
  To compute the nucleation rate, we construct a Hamiltonian description for the system consisting of the fields and off-equilibrium particles. We derive the form of the off-equilibrium corrections to the nucleation rate and demonstrate that Langer's paradigm, augmented with these QFT-specific corrections, holds over Linde's thermal rate. We also establish a connection to Euclidean (imaginary-time) treatments of thermal nucleation, particularly with the effective field theory approach. Additionally, we find that the off-equilibrium effects of bosonic particles are subdominant to those arising from the corresponding off-equilibrium bosonic fields.
  This presentation is primarily based on the article arXiv:2403.07987.
  
  

• Mark Hindmarsh (University of Helsinki and University of Sussex) 14 November 2024 (video)
  Cosmology and the AB transition in superfluid 3He
  Phase transitions in the early universe are expected in many extensions of the Standard Model of particle physics, and could provide the departure from equilibrium needed for a dynamical explanation of the baryon asymmetry of the Universe. A phase transition could also produce gravitational waves of a frequency observable by future space-based detectors such as the Laser Interferometer Space Antenna (LISA). All calculations of the gravitational wave power spectrum rely on a relativistic version of classical nucleation theory. Yet when the A phase of superfluid He-3 is supercooled, the B phase appears far faster than classical nucleation theory would predict. If the appearance of B phase is due to a new rapid intrinsic mechanism, gravitational wave production could be rendered negligible. I discuss how the nucleation rate influences gravitational wave production at a first order phase transition in the early Universe, and make quantitive analogies with the AB transition. I outline current experiments designed to eliminate extrinsic nucleation mechanisms in superfluid 3He, and how they can be used to test cosmological nucleation theory.
  
  
• Simone Blasi (DESY) 17 October 2024 (video)
  Impurities in cosmological phase transitions
  Topological defects can play an important role in cosmology. In this talk I will discuss the impact of defect formation in the context of first order phase transitions, by which defects can act as local impurities catalyzing the decay of the false vacuum. As concrete particle physics realizations I will consider one of the simplest extensions of the Standard Model, the xSM, where domain wall configurations associated to the new singlet scalar are shown to enhance the tunneling rate (2203.16450), as well as the minimal KSVZ implementation of the QCD axion in the post-inflationary scenario where axion strings can affect the electroweak phase transition in a similar way (2405.08060). I will finally comment on the ongoing work aiming to perform real-time simulations of catalyzed bubble nucleation.
  
  
• Rosemary Zielinski (MIT and Australian National University) 19 September 2024 (video)
  Quantum field-theoretic approaches to quantum tunnelling through external potentials
  Though theoretical treatments of quantum tunnelling within single-particle quantum mechanics are well-established, at present, there is no quantum field-theoretic description (QFT) of tunnelling. Due to the single-particle nature of quantum mechanics, many-particle effects arising from quantum field theory are not accounted for. Such many-particle effects, including pair-production, have proved to be essential in resolving the Klein-paradox. This talk addresses the question of how quantum corrections affect the tunnelling probability through an external field. We investigate a massive neutral scalar field, which interacts with an external field in accordance with relativistic quantum mechanics. To consider QFT corrections, we include another massive quantised neutral scalar field coupling to the original via a cubic interaction. This study formulates an all-order recursive expression for the loop-corrected scalar propagator, which contains only the class of vertex-corrected Feynman diagrams. This equation applies for general external potentials. Though there is no closed-form analytic solution, we also demonstrate how to approximate the QFT corrections if a perturbative coupling to the quantised field is assumed.
  
  
• Laura Batini (University of Heidelberg) 11 July 2024 (video)
  Tunneling in string breaking and implications for hadronization
  We study the pair production, string breaking, and hadronization of a receding electron-positron pair using the bosonized version of the massive Schwinger model in quantum electrodynamics in 1+1 space-time dimensions. Specifically, we study the dynamics of the electric field in Bjorken coordinates by splitting it into a coherent field and its Gaussian fluctuations. We find that the electric field shows damped oscillations, reflecting pair production. Interestingly, the computation of the asymptotic total particle density per rapidity interval for large masses can be fitted using a Boltzmann factor, where the temperature can be related to the hadronization
  temperature in QCD. Lastly, we discuss the possibility of an analog quantum simulation of the massive Schwinger model using ultracold atoms, explicitly matching the potential of the Schwinger model to the effective potential for the relative phase of two linearly coupled Bose-Einstein condensates.
  
  
• Wen-yuan Ai (King's College London) 13 June 2024 (video)
  Self-consistent bounces in bubble nucleation
  Studies on false vacuum decay are usually based on Callan and Coleman's semi-classical formula. This formula does not tell us how to compute quantum corrections to the classical bounce, i.e., classical critical bubble. Even worse, there are situations in which bounce solutions do not exist at the classical level but only emerge from radiative corrections. In this talk, I will show how the n-particle-irreducible (nPI) effective action formalism can perfectly deal with these problems. Although I will mostly focus on how to compute quantum corrections to the bounce at zero temperature using the 1PI effective action, I will also briefly discuss how to study the backreactions from the bounce to the plasma at finite temperature using the 2PI effective action.
  
  
• Lorenzo Ubaldi (Jožef Stefan Institute and Ljubljana University) 16 May 2024 (video)
  False vacuum decay from thin to thick walls
  The computation of the false vacuum decay rate in the situation with nearly degenerate vacua can be performed analytically in the well known thin wall approximation. If we depart from the near degeneracy of the vacua, the approximation quickly ceases to work. This statement is known to practitioners of the field, but it has a caveat: it is true only if one works at the leading order of the thin wall approximation. In this talk I will describe how to organize the calculation to systematically include higher order corrections in the thin wall parameter expansion. The first few orders can be computed analytically. It is enough to include them to improve the approximation and get a very accurate estimate of the bounce action also deep into thick wall regimes, where the vacua are far apart. I plan to also briefly discuss how to deal in these cases with the calculation of the functional determinant, also known as the prefactor, in order to have the full proper understanding of the vacuum decay rate. In the whole talk I will only consider flat spacetime, without gravity.
  
  
• Patrick Draper (Illinois University, Urbana) 18 April 2024 (video)
  Tunneling with Time Dependence
  I will discuss semiclassical methods for studying bubble nucleation in models with parameters that vary slowly in time. Introducing a more general rotation of the time contour allows access to a larger set of final states, and typically a non-Euclidean rotation is necessary in order to find the most relevant tunneling solution. The method is straightforward to implement for thin wall bubble nucleation. I also describe some examples where gravitational effects are included, and give one example of an exact instanton solution in a time-dependent Kaluza-Klein cosmology.
  
  
• Yutaro Shoji (Jožef Stefan Institute and Ljubljana University) 14 March 2024 (video)
  Gauge invariance and gauge zero modes of bubble nucleation rates
  A precise computation of a vacuum decay rate requires the determination of the prefactor in front of the exponential suppression factor. When the decay is driven by charged scalar field(s), the prefactor includes the functional determinants of the gauge boson and the Faddeev-Popov fluctuation operators. They depend on the gauge fixing parameter non-trivially and it has not been clear how this dependence cancels out in the final results. We have explicitly proven that the functional determinants become independent of the gauge parameter for a general setup with multiple scalar and gauge fields. 
  The subtraction of the gauge zero modes is necessary to make the functional determinant finite. The path integral over these zero modes is translated into an integration over the collective coordinates, requiring a Jacobian. Due to the gauge fixing terms, the Jacobian becomes non-trivial and gauge-dependent. We have determined the correct Jacobian, which makes it possible to calculate the prefactor for general scalar and gauge fields.
  
  
• Silvia Pla Garcia (Kings College, London) 8 February 2024 (video)
  Can quantum tunnelling induce a cosmic bounce?
  If we imagine rewinding the universe to early times, the scale factor shrinks and the existence of a finite spatial volume may play a role in quantum tunnelling effects in a closed universe. In this talk, I will present a novel mechanism to induce a cosmological bounce that is purely generated by quantum fluctuations without the need for exotic matter or modified gravity. The quantum-induced bounce emerges when we allow a scalar field to tunnel between two degenerate minima. I will also explain how the picture can change in the presence of anisotropies, identifying a criterium for a successful bounce in terms of the size of the closed universe and the properties of the quantum field.
  
  
• Ian Moss (Newcastle University): 11 January 2024 (video)
  The hot big bang in a cold gas
  False vacuum decay is a fascinating theoretical prediction of non-perturbative quantum field theory, and one that has many implications for the early universe, ranging from the formation of matter to the origin of the universe as a quantum event. I’ll explain some of the basic ideas, and discuss the first observations of thermal vacuum decay in an atomic condensate.

Organisers

• Oliver Gould (University of Nottingham)
• Miha Nemevšek (Jožef Stefan Institute and University of Ljubljana)