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SUMMARY:Tunneling in QFT
DTSTART:20240111T130000Z
DTEND:20301231T130000Z
DTSTAMP:20260311T063600Z
UID:indico-event-5685@indico.global
CONTACT:wenyuan.ai@oeaw.ac.at\;oliver.gould@nottingham.ac.uk\;blaurent@per
 imeterinstitute.ca\;miha.nemevsek@ijs.si
DESCRIPTION:Seminar series devoted to tunneling in QFT\nMonthly on Thursda
 ys\, usually @ 14:00 for Central Europe (CEST in European summer and CET i
 n winter)\nWhether it be vacuum stability\, phase transitions\, or analogu
 e 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.\n\nPlease register on this Indico page to receive Zoom j
 oining instructions plus a reminder for each talk.\nUpcoming seminars\n\nM
 iguel Vanvlasselaer (U. Barcelona) 21 May 2026TBA\nKarthik Rajeev (U. Edin
 burgh) 23 April 2026TBA\nGianluca Lagnese (IJS) 19 March 2026False vacuum 
 decay in quantum spin chains and quantum simulators\nConfinement and false
  vacuum decay are cornerstone non-perturbative phenomena in quantum field 
 theory. Both emerge when an explicit symmetry breaking field lifts the deg
 eneracy of a vacuum. In this framework\, confined states act as droplets o
 f the "false" vacuum\, while decay occurs through the nucleation of "true"
  vacuum bubbles via quantum tunneling. Recent breakthroughs in controllabl
 e quantum platforms\, including trapped ions and Rydberg atom arrays\, hav
 e moved these concepts from theory to the lab. While confinement has alrea
 dy been observed\, real time investigations of false vacuum decay are now 
 on the horizon. I will discuss how these phenomena manifest in quantum spi
 n chains\, focusing on real time dynamics following a quantum quench and e
 xperimental setups that provide a controlled environment to study quantum 
 tunneling and bubble nucleation.\n\n\nPrevious seminars\n(Previous talks a
 re uploaded at www.youtube.com/@TunnelingQFT )\n\nJavier Subils (Utrecht 
 University) 26 February 2026 (slides\, video)Microscopic Description of Cr
 itical BubblesFirst-order phase transitions in the early Universe or in ne
 utron stars can leave observable gravitational wave imprints. But predicti
 ng these signals requires precise understanding of bubble dynamics and\, i
 n particular\, the nucleation of critical bubbles of the stable phase with
 in a metastable plasma. Using holography\, I present a fully microscopic d
 escription of these bubbles in a strongly coupled four-dimensional gauge t
 heory at finite temperature. In the gravitational dual\, the bubbles appea
 r as static\, inhomogeneous\, and unstable black-brane solutions featuring
  localized horizon deformations. I will outline the construction of these 
 solutions and show how they allow the nucleation rate to be extracted acro
 ss the entire metastable branch. Time permitting\, I will also compare thi
 s rate—and other microscopic properties—with those obtained from an ef
 fective field-theory description.\n\n\nJorinde van de Vis  (CERN) 29 Janu
 ary 2026 (slides\, video)\nWall velocities and friction in cosmological ph
 ase transitions\nThe bubble wall velocity is an essential quantity in firs
 t order phase transitions. It determines the gravitational wave spectrum a
 nd\, in certain mechanisms\, the baryon asymmetry or the dark matter abund
 ance. The first part of this talk focuses on computing the wall velocity i
 n the static limit\, using the Boltzmann equation for the plasma particles
 . I will start by summarizing the state-of-the art computation of the bubb
 le wall velocity. Then\, I will present a recent study of the theoretical 
 uncertainties in the computation\, motivating further theoretical studies.
  In the second part of the talk\, I will discuss the situation where obtai
 ning a solution to the Boltzmann equation is not realistic\, for example i
 n numerical simulations of gravitational waves from a phase transition. He
 re\, one typically uses a phenomenological friction term. I will discuss t
 he derivation of the friction term from the full set of equations\, includ
 ing Boltzmann terms\, and I will discuss the range of applicability.\n\n\n
 \nPeter Samuel Athron (Nanjing Normal University) 11 December 2025 (slides
 \, video)Cosmological phase transitions & automation: Towards end to end a
 utomation from the effective potential to gravitational waves with the be
 st possible perturbative treatmentTransitionSolver and PhaseTracer are sof
 tware packages for first order cosmological phase transitions that provide
  an end to end calculation from the effective potential to gravitational w
 ave signals. These calculations  have many subtleties  and every step co
 ntains significant challenges and/or open questions that are still activel
 y investigated.  In this talk I will describe these steps and the treatme
 nt of them in TransitionSolver/Phasetracer\, focusing on progress towards 
 achieving the best possible perturbative treatment and future improvements
  needed to fully realise this goal.\n\n\nJosé Ramon Espinosa (Madrid\, IF
 T) 20 November 2025 (slides\, video)Surprises in Vacuum Decay  In the sta
 ndard picture\, the decay of a false vacuum in a single-field potential is
  described by a semiclassical Euclidean bounce\, which we can find using o
 vershoot/undershoot algorithms. Sometimes no such bounce exists and the de
 cay proceeds through a different kind of field configurations\, called pse
 udo-bounces. I’ll highlight the key properties of pseudo-bounces and whe
 n they can be useful. Then I’ll use them to show that the landscape of v
 acuum decay can be much richer than just a single bounce solution. For ins
 tance\, in steep potentials we can also encounter antibounces—configurat
 ions with the “wrong” behavior for overshoot/undershoot algorithms tha
 t can miss them. I’ll also discuss how all these different decay channel
 s—bounces\, pseudo-bounces\, and antibounces—fit together. Most of my 
 discussion will follow the Euclidean approach\, but I’ll end by showing 
 how pseudo-bounces appear in a particularly natural way when we look at th
 e problem from the tunneling potential perspective.\n\n\nAnna Kormu (Hels
 inki U.) 19 June 2025 (slides\, video)Trouble in the Bubble - Testing Bubb
 le Nucleation Calculations for Strong Phase TransitionsNucleation rate com
 putations are of broad importance in particle physics and cosmology. Previ
 ously\, these nucleation rates have generally been calculated perturbative
 ly\, but those calculations depend on the semiclassical picture of the bub
 ble and its fluctuations\, and different orders of perturbative calculatio
 n yield very different results. In this talk\, I will give you an update o
 n the results of our lattice calculations of the nucleation rate. We focus
 ed on a real scalar theory with a tree-level potential barrier and perform
 ed nonperturbative simulations to determine the nucleation rate\, computin
 g a final result extrapolated to the thermodynamic and continuum limits. A
 lthough the system in question should be well-described by a complete one-
 loop perturbative calculation\, we find only qualitative agreement with th
 e full perturbative result. Our result motivates further testing of the cu
 rrent nucleation paradigm.\nMatthew Johnson (Perimeter Inst. Theor. Phys. 
 and York U.\, Canada) 22 May 2025 (slides\, video)Towards quantum simulati
 on of vacuum decayScalar quantum field theories can possess metastable vac
 uum states which decay through thermal and quantum fluctuations. In space\
 , the decay of the metastable vacuum proceeds through the creation and sub
 sequent expansion of bubbles containing a new phase\, which coalesce and e
 ventually complete the phase transition. Vacuum decay is a non-perturbativ
 e and non-linear dynamical problem\, making it the perfect candidate for q
 uantum simulators. Understanding this process has phenomenological implica
 tions for early Universe cosmology\, including observables such as a stoch
 astic gravitational wave background\, meaning there may be novel ways to u
 se quantum simulators to make predictions for observational cosmology. In 
 this talk I will first describe theoretical work developing observables fo
 r vacuum decay\, including nucleation site correlation functions and bubbl
 e nucleation pre-cursors. I will then describe experimental efforts within
  the Quantum Simulators for Fundamental Physics consortium to develop anal
 og quantum simulators of vacuum decay with cold atomic gasses. Such experi
 ments will allow us to empirically test early Universe physics in tabletop
  experiments.\nThomas Steingasser (MIT and Harvard) 17 April 2025 (slides\
 , video)The steadyon picture for excited state tunneling It is well-known 
 that tunneling rates can be calculated through imaginary-time techniques: 
 To leading order\, the exponent of the tunneling rate is determined by the
  euclidean action of so-called instanton solutions. Despite the great impo
 rtance of these results\, it was only recently attempted to obtain a more 
 general\, non-perturbative understanding of these results using path integ
 rals. Most importantly in the context of my talk\, it was suggested that t
 he length of the imaginary-time interval on which the instanton is defined
  is related to the physical time of the tunneling through a Wick-rotation.
  As the latter can be argued to be significantly larger than the typical d
 ynamical time scale of the system of interest\, this reproduces the famili
 ar leading order results for tunneling out of a false vacuum.This identifi
 cation obviously fails for tunneling out of excited states\, for which res
 ults obtained through the WKB method suggest that the instanton be defined
  on a finite euclidean-time interval. In my talk\, I will explain how this
  tension can be resolved through the introduction of a new formalism\, whi
 ch can be understood as a real-time version of the familiar instanton-pict
 ure. After reviewing some relevant concepts of the existing literature I w
 ill introduce the most important concepts of this new formulation and demo
 nstrate how they can be used to calculate tunneling rates at NLO for tunne
 ling out of arbitrary initial states. This includes\, in particular\, the 
 calculation of vacuum decay/bubble nucleation rates in realistic systems w
 ith finite temperature and time-dependent backgrounds.\nMaciej Kierkla (Wa
 rsaw U.) 20 March 2025 (slides\, video) Finite-temperature (supercooled) 
 bubble-nucleation with shifting scale hierarchiesFocusing on supercooled p
 hase transitions in models with classical scale symmetry\, we investigate 
 the limitations of derivative expansions in constructing a thermal EFT des
 cription for bubble nucleation. We demonstrate that derivative expansions 
 for gauge field fluctuations diverge after the two leading orders because 
 the gauge field mass varies strongly between the high- and low-temperature
  phases. By computing such contributions directly using the fluctuation de
 terminant\, we show that these effects can be captured while at the same t
 ime accounting for large explicit logarithms at two loops utilising the ex
 act renormalisation group structure of the EFT. We show how this construct
 ion can be used to improve nucleation rate calculations\, providing a more
  robust framework for describing gravitational waves from supercooled phas
 e transitions in models like the SU(2)cSM.\nMatthias Carosi (Munich\, Tec
 h. U.) 20 February 2025 (slides\, video)False vacuum decay beyond the quad
 ratic approximation Our Standard Model might be living in a metastable st
 ate\, 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 li
 fetime of the Standard Model. I focus\, in particular\, on the presence of
  an IR divergence due to instantons of any size contributing equally to th
 e partition function. Extending our calculation beyond the quadratic appro
 ximation\, thus retaining the leading quantum corrections\, cures the IR d
 ivergence. I show how casting false vacuum decay in the language of the 2P
 I effective action provides a natural framework to do so.\nAndrey Shkerin 
 (Perimeter Inst. Theor. Phys.) 23 January 2025 (slides\, video)Thermal fa
 lse vacuum decay is not what it seems We study the decay of a thermally ex
 cited metastable vacuum in classical field theory using real-time numerica
 l simulations. We find a lower decay rate than predicted by standard therm
 al theory. The discrepancy is due to the violation of thermal equilibrium 
 during the critical bubble nucleation. It is reduced by introducing dissip
 ation and noise. We propose a criterion for the system to remain in equili
 brium during the nucleation process and show that it is violated in the Ha
 miltonian evolution of a single field. In the case of many fields\, the fu
 lfillment of the criterion is model-dependent.\nJoonas Hirvonen (Universit
 y of Nottingham) 12 December 2024 (slides\, video)Real-Time Nucleation and
  Off-Equilibrium Effects in High-Temperature Quantum Field TheoriesWe will
  discuss real-time nucleation in weakly coupled\, high-temperature quantum
  field theories (QFTs)\, with a particular focus on the effects of off-equ
 ilibrium plasma on nucleation rates. We begin with an overview of high-tem
 perature nucleation\, highlighting its similarities and differences with t
 he well-known scenario of vacuum tunneling. A key feature of the thermal s
 ystem is its division into two quasi-classical components: nucleating fiel
 ds and thermal particles interacting with the nucleating bubbles.To comput
 e the nucleation rate\, we construct a Hamiltonian description for the sys
 tem consisting of the fields and off-equilibrium particles. We derive the 
 form of the off-equilibrium corrections to the nucleation rate and demonst
 rate that Langer's paradigm\, augmented with these QFT-specific correction
 s\, holds over Linde's thermal rate. We also establish a connection to Euc
 lidean (imaginary-time) treatments of thermal nucleation\, particularly wi
 th the effective field theory approach. Additionally\, we find that the of
 f-equilibrium effects of bosonic particles are subdominant to those arisin
 g from the corresponding off-equilibrium bosonic fields.This presentation 
 is primarily based on the article arXiv:2403.07987.\nMark Hindmarsh (Unive
 rsity of Helsinki and University of Sussex) 14 November 2024 (video)Cosmol
 ogy and the AB transition in superfluid 3HePhase 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 d
 ynamical explanation of the baryon asymmetry of the Universe. A phase tran
 sition could also produce gravitational waves of a frequency observable by
  future space-based detectors such as the Laser Interferometer Space Anten
 na (LISA). All calculations of the gravitational wave power spectrum rely 
 on a relativistic version of classical nucleation theory. Yet when the A p
 hase of superfluid He-3 is supercooled\, the B phase appears far faster th
 an 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 influence
 s gravitational wave production at a first order phase transition in the e
 arly Universe\, and make quantitive analogies with the AB transition. I ou
 tline current experiments designed to eliminate extrinsic nucleation mecha
 nisms in superfluid 3He\, and how they can be used to test cosmological nu
 cleation theory.\nSimone Blasi (DESY) 17 October 2024 (video)Impurities in
  cosmological phase transitionsTopological defects can play an important r
 ole in cosmology. In this talk I will discuss the impact of defect formati
 on in the context of first order phase transitions\, by which defects can 
 act as local impurities catalyzing the decay of the false vacuum. As concr
 ete particle physics realizations I will consider one of the simplest exte
 nsions of the Standard Model\, the xSM\, where domain wall configurations 
 associated to the new singlet scalar are shown to enhance the tunneling ra
 te (2203.16450)\, as well as the minimal KSVZ implementation of the QCD ax
 ion in the post-inflationary scenario where axion strings can affect the e
 lectroweak phase transition in a similar way (2405.08060). I will finally 
 comment on the ongoing work aiming to perform real-time simulations of cat
 alyzed bubble nucleation.\nRosemary Zielinski (MIT and Australian National
  University) 19 September 2024 (video)Quantum field-theoretic approaches t
 o quantum tunnelling through external potentialsThough theoretical treatme
 nts of quantum tunnelling within single-particle quantum mechanics are wel
 l-established\, at present\, there is no quantum field-theoretic descripti
 on (QFT) of tunnelling. Due to the single-particle nature of quantum mecha
 nics\, many-particle effects arising from quantum field theory are not acc
 ounted 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 q
 uantum mechanics. To consider QFT corrections\, we include another massive
  quantised neutral scalar field coupling to the original via a cubic inter
 action. This study formulates an all-order recursive expression for the lo
 op-corrected scalar propagator\, which contains only the class of vertex-c
 orrected Feynman diagrams. This equation applies for general external pote
 ntials. Though there is no closed-form analytic solution\, we also demonst
 rate how to approximate the QFT corrections if a perturbative coupling to 
 the quantised field is assumed.\nLaura Batini (University of Heidelberg) 1
 1 July 2024 (video)Tunneling in string breaking and implications for hadro
 nizationWe study the pair production\, string breaking\, and hadronization
  of a receding electron-positron pair using the bosonized version of the m
 assive Schwinger model in quantum electrodynamics in 1+1 space-time dimens
 ions. Specifically\, we study the dynamics of the electric field in Bjorke
 n coordinates by splitting it into a coherent field and its Gaussian fluct
 uations. We find that the electric field shows damped oscillations\, refle
 cting pair production. Interestingly\, the computation of the asymptotic t
 otal particle density per rapidity interval for large masses can be fitted
  using a Boltzmann factor\, where the temperature can be related to the ha
 dronizationtemperature in QCD. Lastly\, we discuss the possibility of an a
 nalog quantum simulation of the massive Schwinger model using ultracold at
 oms\, explicitly matching the potential of the Schwinger model to the effe
 ctive potential for the relative phase of two linearly coupled Bose-Einste
 in condensates.\nWenyuan Ai (King's College London) 13 June 2024 (video)Se
 lf-consistent bounces in bubble nucleationStudies on false vacuum decay ar
 e usually based on Callan and Coleman's semi-classical formula. This formu
 la does not tell us how to compute quantum corrections to the classical bo
 unce\, i.e.\, classical critical bubble. Even worse\, there are situations
  in which bounce solutions do not exist at the classical level but only em
 erge from radiative corrections. In this talk\, I will show how the n-part
 icle-irreducible (nPI) effective action formalism can perfectly deal with 
 these problems. Although I will mostly focus on how to compute quantum cor
 rections to the bounce at zero temperature using the 1PI effective action\
 , I will also briefly discuss how to study the backreactions from the boun
 ce to the plasma at finite temperature using the 2PI effective action.\nLo
 renzo Ubaldi (Jožef Stefan Institute and Ljubljana University) 16 May 202
 4 (video)False vacuum decay from thin to thick wallsThe 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 w
 e depart from the near degeneracy of the vacua\, the approximation quickly
  ceases to work. This statement is known to practitioners of the field\, b
 ut it has a caveat: it is true only if one works at the leading order of t
 he thin wall approximation. In this talk I will describe how to organize t
 he calculation to systematically include higher order corrections in the t
 hin wall parameter expansion. The first few orders can be computed analyti
 cally. 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 reg
 imes\, where the vacua are far apart. I plan to also briefly discuss how t
 o deal in these cases with the calculation of the functional determinant\,
  also known as the prefactor\, in order to have the full proper understand
 ing of the vacuum decay rate. In the whole talk I will only consider flat 
 spacetime\, without gravity.\nPatrick Draper (Illinois University\, Urban
 a) 18 April 2024 (video)Tunneling with Time DependenceI will discuss semic
 lassical 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 no
 n-Euclidean rotation is necessary in order to find the most relevant tunne
 ling solution. The method is straightforward to implement for thin wall bu
 bble nucleation. I also describe some examples where gravitational effects
  are included\, and give one example of an exact instanton solution in a t
 ime-dependent Kaluza-Klein cosmology.\nYutaro Shoji (Jožef Stefan Institu
 te and Ljubljana University) 14 March 2024 (video)Gauge invariance and gau
 ge zero modes of bubble nucleation ratesA precise computation of a vacuum 
 decay rate requires the determination of the prefactor in front of the exp
 onential suppression factor. When the decay is driven by charged scalar fi
 eld(s)\, the prefactor includes the functional determinants of the gauge b
 oson and the Faddeev-Popov fluctuation operators. They depend on the gauge
  fixing parameter non-trivially and it has not been clear how this depende
 nce cancels out in the final results. We have explicitly proven that the f
 unctional determinants become independent of the gauge parameter for a gen
 eral setup with multiple scalar and gauge fields. The subtraction of the 
 gauge zero modes is necessary to make the functional determinant finite. T
 he path integral over these zero modes is translated into an integration o
 ver the collective coordinates\, requiring a Jacobian. Due to the gauge fi
 xing terms\, the Jacobian becomes non-trivial and gauge-dependent. We have
  determined the correct Jacobian\, which makes it possible to calculate th
 e prefactor for general scalar and gauge fields.\nSilvia Pla Garcia (King
 s College\, London) 8 February 2024 (video)Can quantum tunnelling induce a
  cosmic bounce?If we imagine rewinding the universe to early times\, the s
 cale 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 p
 urely generated by quantum fluctuations without the need for exotic matter
  or modified gravity. The quantum-induced bounce emerges when we allow a s
 calar field to tunnel between two degenerate minima. I will also explain h
 ow the picture can change in the presence of anisotropies\, identifying a 
 criterium for a successful bounce in terms of the size of the closed unive
 rse and the properties of the quantum field.\nIan Moss (Newcastle Univers
 ity): 11 January 2024 (video)The hot big bang in a cold gasFalse vacuum de
 cay is a fascinating theoretical prediction of non-perturbative quantum fi
 eld theory\, and one that has many implications for the early universe\, r
 anging from the formation of matter to the origin of the universe as a qua
 ntum event. I’ll explain some of the basic ideas\, and discuss the first
  observations of thermal vacuum decay in an atomic condensate.\n\nOrganise
 rs\n\nWenyuan Ai (OAW\, Vienna)\nOliver Gould (University of Nottingham)\n
 Benoit Laurent (Perimeter Institute for Theoretical Physics)\nMiha Nemevš
 ek (Jožef Stefan Institute and University of Ljubljana)\n\nAlso previousl
 y organised by Andreas Ekstedt.\n\nhttps://indico.global/event/5685/
URL:https://indico.global/event/5685/
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