## Celebrating the 70th birthday of László Palla

### Budapest, 14 - 15 June 2018

The aim of the workshop is to celebrate the 70th birthday of our friend and colleague László Palla. This is the booklet we gave to him.

### Venue

The workshop venue is the Physics Building of the Eötvös University located on the bank of river Danube: H-1117, Budapest, Pázmány Péter sétány 1/A . Talks will be held in the Novobátzky room 2.54.

### List of speakers

• János Balog
• Ed Corrigan
• Patrick Dorey
• Clare Dunning
• Péter Forgács
• Tamás Gombor
• Péter Horváthy
• Márton Kormos
• Rafael Nepomechie
• Gordon Semenoff
• Péter Vecsernyés
• Wojciech Zakrzewski
• Zoltán Bajnok
• Gábor Takács

### Programme

 Thursday Friday 9:15 Opening 9:15 Horváthy 9:30 Palla 10:00 Coffee 10:45 Coffee 10:30 Nepomechie 11:15 Corrigan 11:15 Balog 12:30 Lunch 12:30 Lunch 14:00 Semenoff 14:00 Dunning 14:45 Forgács 14:45 Dorey 15:30 Coffee 15:30 Coffee 16:00 Zakrzewski 16:00 Kormos 16:45 Vecsernyés 16:45 Gombor

### Lunch

Lunch will take place on the boat A38 in front of the University. We walk together from the discussion room 1.125 at 12:20.

### Conference dinner

The conference dinner will be held at Újbuda Trófea Grill on Thursday. The dinner starts at 18:00. We can walk together from the lecture room starting at 17:40.

### List of participants

• Michael Abbott
• János Balog
• Péter Bántay
• Dániel Berényi
• Tamás Bíró
• Ed Corrigan
• Gábor Cynolter
• Patrick Dorey
• Clare Dunning
• Péter Forgács
• Tamás Fülöp
• Árpád Hegedűs
• Dezső Horváth
• Péter Horváthy
• Tamás Gombor
• Géza Györgyi
• Sándor Katz
• Márton Kormos
• Zoltán Kökényesi
• András László
• Endre Lendvai
• Rafael Nepomechie
• Zoltán Rácz
• Alessandro Sfondrini
• Haryanto Siahaan
• Gábor Zsolt Tóth
• Enikő Regős
• Gordon Semenoff
• István Szécsényi
• Zsolt Szép
• Péter Vecsernyés
• Wojciech Zakrzewski
• Chao Wu

### Titles and abstracts

Gordon W. Semenoff : Entanglement and the Infrared

The subtle interplay of infrared singularities in quantum electrodynamics and perturbative quantum gravity and information theoretic issues such as quantum entanglement between soft and hard degrees of freedom will be discussed. It will be argued that the inevitable loss of soft photons and gravitons in a scattering experiment leads to decoherence of the out-going state, that this decoherence persists even when the incoming states are infrared-safe coherent states, and that mathematical issues in scattering theory with normalizable incoming wave packets distinguishes between competing methods and requires infrared-safe in-states.

Ed Corrigan : Aspects of defects, boundaries and networks in integrable field theories

Following a short summary introducing the main ideas and recent developments, some as yet unsolved problems and partly developed speculations will be described briefly.

Péter Vecsernyés : Phase structure of $G$-spin chains

$G$-spin chains are quantum chains connected to a finite group $G$: the local monomials in the Hamiltonian generate the group algebra and the function algebra on $G$. If $G$ is the cyclic group $\mathbf{Z}_N$ one arrives at the $N$-state Potts models. $G=\mathbf{Z}_2\times\mathbf{Z}_2$ leads to the spin-$\frac{1}{2}$ $XYZ$ chain. Instead of examining the behavior of the ground state of the Hamiltonian $H(G,\mathbf{J})$ in the thermodynamic limit as a function of a parameter space $\{\mathbf{J}\}$ we describe the (translation invariant) phases as essentially different pure (translation invariant) states of the limit algebra $\mathcal{A}$ generated by local monomials in the Hamiltonian. Namely, the phases are given by non-equivalent algebra extensions of $\mathcal{A}$ in its irreducible representations provided by the pure states. The expectation values of the new algebra elements serve as local order/disorder parameters being non-analytic in different phases. Our main result is that the (translation invariant) phases of a $G$-spin chain are in one-to-one correspondence with the cohomology classes of (bosonic) intermediate coproduct cocycles of the Drinfeld double Hopf algebra $\mathcal{D}(G)$ of $G$. Classifying these cocycles, for example, for $G=\mathbf{Z}_N$ with $N$ prime and $N=4$, and for $G=\mathbf{Z}_2\times\mathbf{Z}_2$ we can describe the phase structure of the corresponding Potts models and the spin-$\frac{1}{2}$ $XYZ$ chain. The trace of the inequivalent algebra extension structure is illustrated by the phase diagram on the finite dimensional subset of ferromagnetic states.

Péter Horváthy : Eisenhart-Duval lift: a unified framework of Galilei and Carroll symmetry

While the usual Wigner-Inönü contraction $c\to\infty$ of the Poincaré group yields the Galilei group, another $c\to0$ contraction yields the Carroll group" of Lévy-Leblond. Both boost-invariant theories are conveniently unified within the Eisenhart-Duval'' framework. Plane gravitational waves carry a non-trivially implemented distorted'' Carroll symmetry with broken rotations.

Rafael Nepomechie : Symmetries and degeneracies of spin chains

The spectrum of a quantum spin chain can have degeneracies, which can be understood from the model's symmetries. We consider here the symmetries and degeneracies of anisotropic integrable open spin chains. Based on recent work with A. Retore (arXiv 1802.04864) and R. Pimenta (arXiv 1805.10144).

Wojtek Zakrzewski: BPS equations for interacting scalar field theories in (1+1) dimensions

We study BPS systems, in (1+1) dimensions, involving more than one field. We discuss the conditions that have to be imposed on various terms in Lagrangians involving many fields to produce BPS systems and then look in detail at the simplest of such cases. We analyse in detail a BPS system involving 2 sine-Gordon like fields and study its properties when we take its solitonic static solutions and use them as initial conditions for their evolution in Lorentz covariant versions of the model. We also discuss other, more general, such systems and relate the properties of their solitonic solutions to the vacua of such models."

János Balog : Effective potential for relativistic scattering

We consider quantum inverse scattering with singular potentials and calculate the Sine-Gordon model effective potential in the laboratory and centre-of-mass frames. The effective potentials are frame dependent but closely resemble the zero-momentum potential of the equivalent Ruijsenaars-Schneider model.

Clare Dunning : On the exceptional spectrum of the asymmetric quantum Rabi model

The quantum Rabi model is the simplest model used in quantum optics to describe the interaction of light and matter. The Hamiltonian couples a two-level atom and a single-mode bosonic field; the asymmetric version allows tunneling between the two atomic states. We discuss the exceptional part of the spectrum of the asymmetric quantum Rabi model. Based on recent work with K-L Guan, Z-M Li and M T Batchelor (arXiv:1804.03878).

Péter Forgács : Gravitational breathers ("geons") in asymptotically anti-de Sitter space-times

A review is given on recent results on spatially localized, time periodic solutions of Einstein's equations with a negative cosmological constant.

Tamás Gombor : Lax representations of sigma models on the half line

The Lax description is very useful for integrable models because it can be used to find the action-angle variables, in which the dynamics is trivial. For theories on the half line these representations have classical reflection matrices. A summary of boundary sigma models with Lax representations will be described including spectral parameter and field dependent classical reflection matrices.

Márton Kormos : Quantum systems out of equilibrium

In the talk I will review some of my recent works on the non-equilibrium dynamics of integrable and non-integrable quantum many-body systems.

The scientific developments of László Palla

Péter Horváthy, Péter Forgács, János Balog, Gábor Takács and Zoltán Bajnok summarize the scientific life of Laci.

### Accommodation

We booked a block of rooms at Hotel Gellért: , (1111 Budapest, Szent Gellért tér 1), 15 minutes walk from the conference venue.

Zoltán Bajnok
Gábor Takács