2nd APCTP-INPP Demokritos meeting

24 Aug 2025, 08:00 → 30 Aug 2025, 20:00 Asia/Seoul

503 (Asia Pacific Center for Theoretical Physics (APCTP))

In 2024, the National Centre for Scientific Research (NCSR) “Demokritos” and the Asia Pacific Center for Theoretical Physics (APCTP) launched a cooperation aiming to strengthen the ties between Greece and South Korea in the field of Theoretical Physics.

This is the second in a series of meetings between the two institutes. The first meeting took place at the Institute of Nuclear and Particle Physics (INPP) in Athens on 30 September – 4 October 2024.

The present meeting will take place at the the headquarters of the Asia Pacific Center for Theoretical Physics (APCTP), in Pohang (South Korea) on 24 – 30 August 2025, with speakers from both institutes, along with a number of invited speakers from the Asia-Pacific region.

Invited speakers
Chiang-Mei Chen (NCU Taiwan)
Imtak Jeon (Huzhou U.)
Saebyeok Jeong (IBS-CGP)
Hee-Cheol Kim (POSTECH)
Ki-Seok Kim (POSTECH)
Sang-Pyo Kim (Kunsan U.)
Bum-Hoon Lee (Sogang U.)
Norton Lee (IBS-CGP)
Noboyoshi Ohta (Osaka Metropolitan U.)
Ioannis Papadimitriou (Athens U.)
Sang-Jin Sin (Hanyang U.)
Junggi Yoon (Kyung Hee U.)

INPP speakers
Christos Markou
Giuseppe Bevilacqua

APCTP speakers
Bayu Hartanto
Matti Jarvinen
Kyoung-Min Kim
Kanghoon Lee*
Zhesen Yang
Young-Dae Yoon

Junior speakers
Kostas Filippas (INPP)
Susmita Jana (APCTP)
Debangshu Mukherjee (APCTP)
Kunal Pal (APCTP)
Tabasum Rahnuma (APCTP)
Patricio Salgado-Rebolledo (APCTP)

Local Organizers
Misao Sasaki
Jae-Hyung Jeon
Bayu Hartanto
Georgios Linardopoulos

The APCTP is supported by the Korean Government through the Science and Technology Promotion Fund and Lottery Fund and strives to maximize public value through its various activities.

아시아태평양이론물리센터는 정부의 과학기술진흥기금 및 복권기금 지원으로 공익적 가치 제고에 힘쓰고 있습니다.

APCTP meeting webpage

Sunday 24 August

09:00 Discussions

19:30 Welcome dinner

Monday 25 August

1 Welcome by the APCTP President

Speaker: Misao Sasaki (Asia Pacific Center for Theoretical Physics (APCTP))

2 Presentation of INPP "Demokritos"

Speaker: Christos Markou (INPP)

3 KM3NeT: Neutrino telescopy in the abyss

The KM3NeT Collaboration is incrementally building two underwater Cherenkov neutrino telescopes in the Mediterranean Sea. Both telescopes share the same technology for neutrino detection, by studying Cherenkov radiation from secondary charged particles produced in neutrino interactions. Photomultipliers are a common choice for the detection of Cherenkov radiation, but the hostile underwater environment, affected by sea currents and bioluminescence demands innovative solutions in KM3NeT.

The KM3NeT design is modular and allows for data taking with the telescope still in the construction stage. Early technical and scientific results are enticing. In particular, KM3NeT recently discovered a neutrino of unprecedented energy from outer space.

The speaker will cover KM3NeT telescope design and operation, as well as give a brief overview of the latest results.

Speaker: Christos Markou (INPP)

4 Thermodynamics of various black holes

After the motivation for the extended gravity, such as higher curvatures beyond Einstein, various thermodynamic black holes will be studied. Black holes, with their fundamental structure, play a crucial role in guiding us towards the quantum domain. Their thermodynamic properties across different theories, particularly through holography, will be examined with the negative cosmological constant considered mostly. The theory with the Gauss-Bonnet term will also be mentioned. A notable feature of dilaton-Einstein-Gauss-Bonnet gravity in 4 dimensions, unlike Einstein's gravity, is the presence of a minimum mass threshold below which a black hole cannot form. Phase diagrams for various black holes will be treated.

Speaker: Bum-Hoon Lee (Sogang)

12:00 Lunch break

5 TBA

Speaker: Hee-Cheol Kim (POSTECH)

6 Near-extremal dynamics away from the horizon

Near-extremal black holes are usually studied by zooming into the throat that describes their near-horizon geometry. Within this throat, one can argue that two-dimensional JT gravity is the appropriate effective theory that dominates at low temperature. Here, we discuss how to capture this effective description by standing far away from the horizon. Our strategy is to construct a phase space within gravitational theories in AdS that fixes the radial dependence while keeping the transverse dependence arbitrary. This allows us to implement a decoupling limit directly on the phase space while keeping the coordinates fixed. With this, we can relate the effective description in JT gravity to the CFT description at the boundary of AdS, which we do explicitly in AdS and non-rotating configurations in AdS. From the perspective of the dual CFT, our decoupling limit should be understood as a flow between a CFT and a near-CFT. Our analysis shows that local counterterms can be constructed in the near-CFT, which arise from the anomalies (or absence of them) in the CFT. We show that one of these counterterms is the Schwarzian effective action, making this sector a scheme-dependent choice. This illustrates the delicate interplay between a far and a near analysis of near-extremal black holes.

Speaker: Ioannis Papadimitriou (U. of Athens)

15:20 Coffee break

7 TBA

Speaker: Imtak Jeon (Huzhou U.)

8 Einstein gravity with cosmological constant and scalar fields is nonperturbatively renormalizable

Speaker: Nobuyoshi Ohta (Kinki University)

17:00 Discussions

18:00 Dinner

Tuesday 26 August

9 ER=EPR and strange metal from field theory wormhole.

We give an understanding how strange metals arise from the spatially random Yukawa-SYK model based on the wormhole picture and find a parallelism between the disorder theory and quantum gravity. We start from the observation that the Gaussian average over the spatial random coupling gives a wormhole, defined as a mechanism for long range interaction without causal suppression outside the lightcone. We find that the large-
N
N limit equivalence of the quenched and annealed averages provides a field theory version of the ER=EPR. Since the wormhole establishes momentum exchanges over arbitrary distance without causal suppression, it provides a mechanism of the planckian dissipation. It also tells us why SYK-like models describe strongly interacting systems even in the small coupling case. We classify the disorder samples into two classes: I) spatially random coupling with wormholes and no information loss, II) spatially uniform coupling with decoherence.

Speaker: Sang-Jin Sin (Hanyang U.)

10 Dual holography from a non-perturbative generalization of the Wilsonian RG framework

In my opinion, physics is to aim understanding the macroscopic universal IR physics from the microscopic individual UV physics based on the first principle. The Wilsonian renormalization group (RG) framework would be the first organization principle. Unfortunately, this theoretical framework has its limitation in describing many long-standing physics problems, where the RG flow from UV to IR is nonperturbative in nature and beyond the paradigm of spontaneous symmetry breaking. In this talk, we discuss how to generalize the Wilsonian RG framework in a nonperturbative way. First, we introduce a brute-force way of RG transformations. Remarkably, the resulting Wilsonian effective action contains a particular class (most singular) of quantum corrections in the all-loop order, not exact but completely nonperturbative in nature. We confirm this nonperturbative physics explicitly from the Kondo problem, which shows the asymptotic freedom (decoupled local moment fixed point) at UV and confinement (local Fermi liquid fixed point) at IR. Although this brute-force way of RG transformations is explicit, the intermediate procedure looks dirty and hidden, involved with the heat-kernel calculation in a general background. We realize that there exists a topological structure in this brute-force construction. Second, we discuss an elegant reformulation of the previous nonperturbative RG theoretical framework, referred to as a cohomological-type topological field theory construction a la Witten. We demonstrate that this topological reformulation is essentially the same as the previous explicit derivation. Interestingly, we observe that this cohomological construction gives a deep connection to the path integral representation of the exact functional RG equation, where the RG flow of the probability distribution function is governed by the Fokker-Planck-type equation. Based on this novel reformulation, we discuss Weyl anomaly inflow and cancellation for the resulting renormalized effective action, regarded to be the consistency equation for the nonperturbative RG flow description. We see that all these mathematical structures are consistent with the holographic duality conjecture.

Speaker: Ki-Seok Kim (POSTECH)

11 TBA

Speaker: Junggi Yoon (Kyung-Hee)

12:00 Lunch break

12 Magnetogenesis via plasma processes

Spontaneous magnetic field generation, or magnetogenesis, is an important process in both cosmological and astrophysical contexts. In plasma physics, it is well known that dynamo processes can amplify a given magnetic field, but how the required seed magnetic field is spontaneously generated is relatively less understood. Here I present via analyzing the equation of motion of a magnetized fluid that a term due to the pressure tensor is responsible for seed magnetogenesis in a collisionless plasma. In relativistic regimes, there is an additional term due to a non-commutative relationship between momentum and velocity that further affects magnetogenesis.

Speaker: Young Dae Yoon (Asia Pacific Center for Theoretical Physics)

13 Emergence of Meron Kekulé lattices in twisted Néel antiferromagnets

In magnetic materials, topological solitons, such as merons, are often discovered as lattice elements within Bravais lattice structures. In this talk, we will present an intriguing alternative: in twisted bilayer antiferromagnets, merons can form an exotic, distorted non-Bravais lattice structure known as a Kekulé lattice. Specifically, we will demonstrate that the spatial modulation of interlayer coupling through moiré patterns stabilizes the meron cores into the Kekulé-O pattern, which features different intracell and intercell bond lengths across the moiré supercells, thereby forming a Meron Kekulé lattice. Furthermore, we will show that the two bond lengths of the Meron Kekulé lattice can be finely tuned by adjusting the twist angle and specifics of the interlayer exchange coupling, indicating extensive control over the meron lattice configuration compared to conventional magnetic systems. Lastly, we will discuss the fascinating future research outlook of how bond-dependent interactions of merons affect phonon-like collective excitation modes associated with their core vibrations.

Speaker: Kyoung-Min Kim (Asia Pacific Center for Theoretical Physics)

14 TBA

This talk provides an introduction to non-Hermitian physics, covering key consequences of non-Hermitian Hamiltonians such as complex eigenvalues, nonorthogonal eigenstates, and the non-Hermitian skin effect (NHSE). It presents our contributions to non-Hermitian topological band theory, including clarifying topological charges of exceptional points and classifying 3D exceptional line semimetals, as well as advancements in non-Bloch band theory like establishing the bulk-boundary correspondence between spectral winding number and NHSE and proposing dynamical degeneracy splitting. Additionally, the talk discusses experimental realizations and physical understandings of NHSE, and outlines future directions in non-Hermitian many-body theory with a call for collaborators.

Speaker: Zhesen Yang (APCTP)

16:00 Discussions

18:00 Banquet/Conference Dinner

Wednesday 27 August

09:00 Free day

Excursion to Gyeongju

18:00 Dinner

Thursday 28 August

15 TBA

Speaker: Kanghoon Lee (APCTP)

16 Phase transitions and domain walls with holography-inspired hydrodynamics

Studying phase transitions and domain walls using the gauge/gravity duality typically leads to challenging tasks in numerical gravity. However, such tasks can be dramatically simplified by considering hydrodynamics matched to the holographic model. In this talk, I discuss extended hydrodynamics (perfect fluid hydrodynamics plus a scalar, i.e. the order parameter of the transition) fitted to data from the holographic Witten model to study the physics of phase transitions. Among other things, we find a simple formula for the velocity of moving walls and surprising behavior of hot plasma remnants in the last stages of the transition in an expanding setup.

Speaker: Matti Jarvinen

17 Logarithmic corrections to near-extremal entropy of de Sitter black holes

I will briefly review logarithmic corrections to black hole entropy. Following that, I will focus on thermodynamics of de Sitter black holes and our recent work (https://arxiv.org/abs/2503.08617) on log T corrections to the entropy of de Sitter black holes. I will establish the result that at leading order, the small temperature corrections to the extremal entropy is universal in the cold and Nariai limit, paving the way for similar such computations and tests in higher dimensional dS black hole spacetimes, including rotating dS black holes.

Speaker: Debangshu Mukherjee (Asia Pacific Center for Theoretical Physics (APCTP))

18 Holographic realms of quantum electrodynamics

We recently proposed a holographic duality between massive theories in 4D flat spacetime and massless ones on its conformal (lightlike) boundary, where Poincare maps to conformal symmetry. The dual boundary space is really the conformal class of $\mathbb{S}^2\times\mathbb{R}$ cylinders, which is also the conformal (timelike) boundary of AdS$_4$. This leads to a triality between three holographic realms $-\mathbb{R}^4$, $\mathbb{S}^2\times\mathbb{R}$ and AdS$_4-$ all supporting equivalent particle theories. As an illustration, we match the path integrals for free fermions across all three realms. We then identify the Landau levels of the $\mathbb{R}^4$ realm with a monopole problem on the cylinder, and with the spectrum of an extremal magnetic black hole in the AdS$_4$ realm. Finally, we suggest that QED with $N_f$ flavors of fermions is dual to an Abelian Chern-Simons-matter theory coupled to $2N_f$ massless fermions on the boundary, which in turn should be dual, for large $N_f$, to a higher-spin theory in AdS$_4$.

Speaker: Kostas Filippas (NCSR Demokritos, Institute of Nuclear and Particle Physics)

19 TBA

Speaker: Norton Lee (IBS-CGP)

12:50 Lunch break

20 TBA

Speaker: Patricio Salgado-Rebolledo (Asia Pacific Center for Theoretical Physics (APCTP))

21 TBA

Speaker: Susmita Jana (Asia Pacific Center for Theoretical Physics (APCTP))

22 TBA

Speaker: Kunal Pal (Asia Pacific Center for Theoretical Physics (APCTP))

23 Perturbative solutions of Einstein’s equations: recursive techniques and multipole expansions

Perturbative approaches to gravity, particularly within the post-Minkowskian (PM) expansion, provide a powerful framework for studying gravitational interactions beyond the weak-field limit. In this talk, I will highlight the role of off-shell recursion relations—formulated through the Perturbiner expansion—in systematically organizing gravitational perturbations. These recursive structures naturally generate all orders in Newton’s constant, offering new insight into both conservative dynamics and dissipative effects.

Compared to conventional Feynman diagram methods, the Perturbiner approach offers a far more efficient, computer-friendly framework for perturbative gravity, acting as a generating functional for off-shell currents in both gauge and gravity theories. By combining recursive structures with iterative loop integrals, this method transforms the overwhelming complexity of perturbative gravity into a systematic and tractable procedure.

This talk will be of interest to researchers in perturbative gravity, self-force dynamics, scattering amplitudes, quantum field theory, and mathematical physics, as well as those interested in computational methods for high-energy theory.

Speaker: Tabasum Rahnuma (Asia Pacific Center for Theoretical Physics (APCTP))

16:30 Discussions

18:00 Dinner

Friday 29 August

24 Progress towards numerical two-loop integrand reduction

We present a method for the integrand-level reduction of two-loop helicity amplitudes in both d=4−2ϵ and d=4 dimensions. The amplitude is expressed in terms of a set of Feynman integrals and their coefficients that depend on the external kinematics. The method presented in this talk, in conjunction with the ongoing development of the computational framework HELAC-2LOOP, paves the road for the construction of an automated program for numerical two-loop amplitude calculations.

Speaker: Giuseppe Bevilacqua (NCSR Demokritos)

25 TBA

Speaker: Heribertus Bayu Hartanto (Asia Pacific Center for Theoretical Physics (APCTP), Pohang, South Korea)

26 TBA

Speaker: Saebyeok Jeong (IBS-CGP)

12:00 Lunch break

27 In-out formalism for one-loop effective actions in QED and gravity

The in-out formalism is a systematic and powerful method for finding the effective actions in an electromagnetic field and a curved spacetime provided that the field equation has explicitly known solutions. The effective action becomes complex when pairs of charged particles are produced due to an electric field and curved spacetime. This may lead to a conjecture of one-to-one correspondence between the vacuum polarization (real part) and the vacuum persistence (imaginary part). We illustrate the one-loop effective action in a constant electric field in a Minkowski spacetime and in a uniform electric field in a two-dimensional (anti-) de Sitter space.

Speaker: Sang Pyo Kim (Kunsan National University)

28 Pair production in charged black holes: Monodromy approach

In this talk I will firstly review the Schwinger effect in charged black holes and then introduce a remarkable alternative approach by using the monodromy. The explicit elaboration of monodromy and the model calculations seem to reveal evidence that the monodromy can provide a practical technique to study the spontaneous pair production in general black holes and electromagnetic fields.

Speaker: Chiang-Mei Chen (National Central University)

29 TBA

Speaker: Young researcher

15:50 Discussions

18:00 Dinner

Saturday 30 August

09:00 Discussions