## 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

**Wen-yuan Ai**(King's College London):*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.**Laura Batini**(University of Heidelberg): 11 July at 14:00 CEST (07:00 Chicago, 20:00 Shanghai)*Tunneling in string breaking and implications for hadronization*

In this work, we delve into the role of tunneling in understanding string-breaking dynamics and the implications for hadronization temperature in high-energy collisions. We use the massive Schwinger Model (QED1+1) as an effective model for exploring QCD, focusing on field dynamics and pair production phenomena. We have discovered that the computation of the asymptotic total particle density per rapidity interval can be described by a Boltzmann factor. This finding allows us to establish a connection between the obtained temperature and the hadronization temperature in QCD, providing quantitative insights into string-breaking processes. Lastly, we explore the possibility of an analog quantum simulation of the massive Schwinger Model using ultracold atoms.

### Previous seminars

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

**Lorenzo Ubaldi**(Jožef Stefan Institute and Ljubljana University): 16 May 2024*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*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*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*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*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

- Andreas Ekstedt (Uppsala University)
- Oliver Gould (University of Nottingham)
- Miha Nemevšek (Jožef Stefan Institute and University of Ljubljana)

Registration

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