### Information

#### Date

Thursday, January 26, 2023 – Friday, January 27, 2023

#### Venue

• Akiu Yumoto City Center [Access]

• Online

* On–site participation is open only for those who have a presentation, due to COVID preventive measures.

#### Invited Speakers

Chunshan Lin (Jagiellonian University)

Mainak Mukhopadhyay (Pennsylvania State University)

George Zahariade (Barcelona IFAE)

#### Registration (Online only)

Registration deadline: Friday, January 20, 2023 15:00 (JST)

– Closed

*For the online participants, we will send you the link of the online information before the workshop.
Please do not share it with others.

#### Lecture Abstracts

**Sound Speed Resonance: A Novel Mechanism of Matter–Graviton Conversation**

Chunshan Lin (Jagiellonian University)

The matter–to–graviton conversion and the graviton–to–matter conversion can occur via parametric resonance, which can be realized by the oscillatory sound speed of the graviton and the matter fields respectively. This mechanism is in fact quite general. It can occur in a large class of theories, including the Einstein gravity minimally coupled with the standard model in particle physics, the Horndeski theory, and the theories beyond Horndeski.

**Kink–antikink scattering in a quantum vacuum**

Mainak Mukhopadhyay (Pennsylvania State University)

In this talk, I will discuss kink–antikink scattering in the sine–Gordon and λφ^{4} models in the presence of interactions with an additional scalar field, in its quantum vacuum.
In contrast to the classical scattering, here there would be quantum radiation and backreaction.
I will elaborate how this changes the various expected outcomes like the formation of bound states for the parameters of the theory and the rate of radiation.
In particular, I will show how the bound states of the sine–Gordon model resemble typical breathers of the model which radiate,
decay and finally form a long–lived oscillon and how the interesting fractal dependence of the final state on the initial velocity of the kink–antikink configuration in the λφ^{4} model,
get modified with the additional quantum field and backreaction.

**Quantum formation of topological defects**

George Zahariade (Barcelona IFAE)

In this talk I will describe recent work (in collaboration with M.Mukhopadhyay, O. Pujolas and T. Vachaspati) about the quantum formation of kinks and antikinks during a quantum phase transition in 1+1 dimensions. I will evaluate the number density of kinks as a function of time and show that it scales as t^{-1/2} in the late time limit (and independently of the details of the phase transition). This decay can be interpreted in terms of ballistic annihilation of kink–antikink pairs. Finally I will discuss the extension of the model to the case of asymmetric initial conditions, as well as higher dimensions.

#### Time Schedule

- Thursday, January 26, 2023
- 10:00 – 12:00
- Free Discussion about gravitational wave

- 12:00 – 13:30
- Break

- 13:30 – 14:30
- Chunshan Lin (Jagiellonian University)
- Sound Speed Resonance: A Novel Mechanism of Matter–Graviton Conversation (1)

- 14:30 – 15:30
- Mainak Mukhopadhyay (Pennsylvania State University)
- Kink–antikink scattering in a quantum vacuum (1)

- 15:30 – 16:30
- George Zahariade (Barcelona IFAE)
- Quantum formation of topological defects (1)

- Friday, January 27, 2023
- 10:00 – 11:00
- George Zahariade (Barcelona IFAE)
- Quantum formation of topological defects (2)

- 11:00 – 12:00
- Mainak Mukhopadhyay (Pennsylvania State University)
- Kink–antikink scattering in a quantum vacuum (2)

- 12:00 – 13:00
- Chunshan Lin (Jagiellonian University)
- Sound Speed Resonance: A Novel Mechanism of Matter–Graviton Conversation (2)

- 13:00 – 15:00
- Break

- 15:00 – 17:00
- Free discussion about cosmology

#### Organizers

Atsushi Naruko

(Center for Gravitational Physics, Kyoto University)

Rampei Kimura

(Waseda Institute for Advanced Study, Waseda University)

Naoya Kitajima

(The Frontier Research Institute for Interdisciplinary Sciences, Tohoku University)

Ryo Namba

(RIKEN)

Kenji Toma

(The Frontier Research Institute for Interdisciplinary Sciences, Tohoku University)

#### In cooperation with

- Waseda Institute for Advanced Study, Waseda University
- RIKEN iTHEMS