IBS CTPU-CGA 2024 Summer School and Workshop for Particle Physics and Cosmology in Korea

21 Jul 2024, 12:00 → 26 Jul 2024, 18:00 Asia/Seoul

Muju Deogyusan Resort

The purpose of this school and workshop is to encourage IBS CTPU-CGA and Korean Particle Physics and Cosmology Community to exchange ideas, to promote collaboration, and to discuss recent developments in the fields through review talks, lectures, and general talks.

The list of invited speakers is

Kyujung Bae (Kyungpook NU)
Ki Young Choi (Sungkyunkwan U)
Jinn-Ouk Gong (Ewha Womans U)
Donghui Jeong ( Penn state U & KIAS)
Dong-Won Jung (IBS CTPU-CGA)
Kenji Kadota (HIAS)
Takeshi Kobayashi (SISSA)
Hyung Do Kim (Seoul NU)
Junhan Kim (KAIST)
Kang Young Lee (Gyeongsang NU)
Wanil Park (Jeonbuk NU)
Minho Son (KAIST)
Masahide Yamaguchi (IBS CTPU-CGA)

and many postdocs and students.

Sunday, 21 July

16:00 → 22:00 Check-In

Monday, 22 July

09:50 → 10:00 Opening: Welcome Remark

Convener: Prof. Masahide Yamaguchi

10:00 → 11:25 Presentations

Convener: Prof. Masahide Yamaguchi (IBS CTPU-CGA)

10:00 Monopoles and Cosmological Magnetic Fields

Studies of the magnetic fields in our universe provide an excellent opportunity to confront monopoles with astrophysical observations. I will discuss two projects along this line: (1) New Parker-type bounds on the monopole flux in light of the modern understanding of cosmological magnetic fields, including galactic, intergalactic, and primordial fields. I will also present constraints on minicharged monopoles and magnetic black holes. (2) Monopole-antimonopole pair production in primordial magnetic fields. Here I will show that even superheavy monopoles can be produced in the early universe.

Speaker: Prof. Takeshi Kobayashi (SISSA)

10:35 Topologically Stable Monopoles from Metastable Cosmic Strings

We present a novel mechanism for the formation of topologically stable monopoles with two unit of Dirac magnetic charges from the decay of metastable string network in an SO(10) GUT. Superheavy monopoles (masses ~ 1015 – 1017 GeV) can be produced with an observable flux from a string network with the dimensionless tension Gμ ~ 10-9-10-5 (μ is the string tension and G the Newton’s constant) in the scaling regime. They are accompanied by a high-frequency gravitational wave background from the metastable strings. We discuss the possibility of the production of relativistic intermediate mass monopoles (masses ~ 108 – 1014 GeV) which could be observed in neutrino detectors such as IceCube and KM3NeT.

Speaker: Dr Rinku Maji (IBS CTPU-CGA)

11:00 Topological defects and parametric resonance

This study presents a detailed investigation into the formation and dynamics of topological defects, particularly domain walls, resulting from the breaking of discrete symmetries within scalar field theories. By focusing on the effects of parametric resonance, we investigate the mechanisms driving symmetry restoration and the subsequent emergence of domain walls. Through the analysis of a scalar field Lagrangian exhibiting Z_2 symmetry, we establish comprehensive conditions for the stability and decay of these domain walls.

Speaker: Mr Taegyu Kang (SNU)

14:00 → 15:50 Presentations

Convener: Prof. Kang Young Lee (Gyeongsang NU)

14:00 Probing the mixing between sterile and tau neutrinos in the SHiP experiment

TBA

Speaker: Prof. Ki Young Choi (SKKU)

14:35 Higgs boson precision analysis of the full LHC run 1 and run 2 data

We perform global fits of the Higgs boson couplings to the full Higgs datasets collected at the LHC with the integrated luminosities per experiment of approximately 5/fb at 7 TeV, 20/fb at 8 TeV, and up to 139/fb at 13 TeV. Our combined analysis based on the experimental signal strengths used in this work and the theoretical ones elaborated for our analysis reliably reproduce the results in the literature. We reveal that the LHC Higgs precision data are no longer best described by the Standard Model (SM) Higgs boson taking account of extensive and comprehensive CP-conserving and CP-violating scenarios found in several well-motivated models beyond the SM. Especially, in most of the fits considered in this work, we observe that the best-fitted values of the normalized Yukawa couplings are about 2σ below the corresponding SM ones with the 1σ errors of 3%–5%. On the other hand, the gauge-Higgs couplings are consistent with the SM with the 1σ errors of 2%–3%. Incidentally, the reduced Yukawa couplings help to explain the excess of the H → Zγ signal strength of 2.2±0.7 recently reported by the ATLAS and CMS collaborations.

Speaker: Dr Yongtae Heo (Jeonnam NU)

15:00 Model independent dark photon bound

I and collaborators revised model independent dark photon bound with recent observation. By assuming minimal assumption on dark photon, irreducible dark photon abundance is estimated. This irreducible abundance can provide dark photon bound with cosmological and astrophysical observation such as CMB, BBN, galactic Gamma ray. Also existence of dark photon can affect supernovae evolution so that there are bounds from it.

Speaker: Mr Jaeyoung Park (SNU)

15:25 Halo-independent bounds of WIMP-nucleon couplings from direct detection and neutrino telescope observations

I will discuss the halo-independent bounds on the WIMP-nucleon couplings of the non-relativistic effective Hamiltonian that drives the scattering process off nuclei of a WIMP of spin 1/2 combining direct detection experiments and neutrino telescopes data in order to cover the full WIMP incoming speed range.

In the elastic interactions, for most of the couplings the degree of relaxation of the halo-independent bounds compared to those obtained with the Standard Halo Model is relatively moderate in the low and high WIMP mass regimes while in the intermediate mass range it can be large. An exception with moderate bounds at all WIMP masses is observed in the case of several WIMP-proton couplings that depend on the nuclear spin and on the WIMP incoming velocity.

In the case of inelastic scattering, I show that a non-vanishing mass splitting modifies incoming WIMP speed range and that for particular combinations of WIMP mass and mass splitting the complementarity between two detection techniques is lost. In low-mass regime the neutrino telescope bound is sufficient alone to provide a halo-independent constraint. On the other hand at large WIMP masses the halo-independent bound is given by a combination of two kinds of experiments.

Speaker: Dr Sunghyun Kang (Sogang U)

15:50 → 16:10 Break

16:10 → 18:00 Presentations

Convener: Prof. Donghui Jeong (Penn. state U & KIAS)

16:10 Development of a 12-20 GHz CO Intensity Mapping Receiver for Capturing the Star-formation History in the Early Universe

Line-intensity mapping (LIM) is an observational technique that has recently been actively used in astronomy and astrophysics research. LIM utilizes relatively small aperture telescopes to perform large-area surveys, effectively analyzing the brightness distribution of galaxies and intergalactic medium through their spectral line emissions. Probing astrophysics and cosmology using LIM has been gaining attention, with several experiments already in operation or planning deployments.
Among these is the carbon monoxide (CO) Mapping Array Project (COMAP), which uses CO line emission to explore molecular gas that traces star formation. The COMAP Pathfinder currently observes at 26-34 GHz to measure the CO power spectrum at redshifts 2.4-3.4, near the peak of the star formation in cosmological history. In this presentation, I will introduce the planned development of a prototype receiver that will be operating in the 12-20 GHz range to study the 115.27 GHz CO line emission originating from z=4.8-8.6 near the epoch of reionization.

Speaker: Prof. Junhan Kim (KAIST)

16:45 WimPyDD: an object-oriented Python code for the calculation of WIMP direct detection signals

We introduce WimPyDD, a modular, object–oriented and customizable Python code that calculates accurate predictions for the expected rates in Weakly Interacting Massive Particle (WIMP) direct–detection experiments within the framework of Galilean–invariant non–relativistic effective theory in virtually any scenario, including inelastic scattering, an arbitrary WIMP spin and a generic WIMP velocity distribution in the Galactic halo. WimPyDD exploits the factorization of the three main components that enter in the calculation of direct detection signals: i) the Wilson coefficients that encode the dependence of the signals on the ultraviolet completion of the effective theory; ii) a response function that depends on the nuclear physics and on the main features of the experimental detector (acceptance, energy resolution, response to nuclear recoils); iii) a halo function that depends on the WIMP velocity distribution and that encodes the astrophysical inputs. In WimPyDD these three components are calculated and stored separately for later interpolation and combined together only as the last step of the signal evaluation procedure. This makes the phenomenological study of the direct detection scattering rate with WimPyDD transparent and fast also when the parameter space of the WIMP model has a large dimensionality.

Speaker: Mr Injun Jeong

17:10 Local Universe with Cosmicflows-4

The large-scale structures in our local Universe emerge from the rivalry between gravitation and the expansion of the Universe, akin to a cosmic tug-of-war. Peculiar velocities of galaxies reflect their motion primarily governed by gravitational interactions, making them unbiased dynamical tracers of the total matter in the Universe (including dark and luminous matter). These velocities serve as crucial tools for testing the LCDM cosmological model. The Cosmicflows collaboration prepares catalogs of galaxy distances, enabling the derivation of radial peculiar velocities of galaxies. In this talk, I will demonstrate how deep learning techniques can reconstruct the local dark matter density and three-dimensional peculiar velocity fields from line-of-sight galaxy peculiar velocities.

Speaker: Dr Alexandra Dupuy (KIAS)

17:35 Quintom cosmology and modified gravity after DESI 2024

The newly released data of DESI has suggested an unexpected dynamical evolution of our Universe that indicates new physics beyond the standard cosmological paradigm of the LCDM.
We reconstructed the H(z) function to get the effective dark-energy equation-of-state parameter w(z) by virtue of the Gaussian Process method using the DESI-BAO + Previous BAO data. We discovered that w(z) exhibits a surprising behavior of crossing -1 from the phantom to quintessence regime along with the cosmic expansion, dubbed as quintom-B scenario. Theoretically, however, such a scenario is one of the most difficult phenomenon that could be explained by the knowledge of traditional field theories, and hence poses new challenges to the current theoretical paradigm. As a demonstration of probing new physics, we further reconstruct the actions for a series of modified gravity theories including f(R), f(T), and f(Q) gravity appropriately by applying the data-driven H(z) function. Our results have delicately illustrated the reasonable interpretations of such a nontrivial dynamic of the Universe from gravitational physics beyond General Relativity.

Speaker: Dr Xin Ren (Tokyo Tech.)

Tuesday, 23 July

10:00 → 12:00 Presentations

Convener: Prof. Wanil Park (Jeonbuk NU)

10:00 One-point correlators of conserved and non-conserved charges in QCD

I will discuss about theoretical and phenomenological aspects of one-point correlators of conserved and non-conserved charges in QCD

Speaker: Prof. Minho Son (KAIST)

10:35 Axion-Mediated Dark Matter

We consider the axion-mediated scattering processes between dark matter (DM) and nucleus. The substantial contributions are made via the CP-odd gluonic current which induces the spin-dependent process. Since the QCD axion is too feebly coupled to the visible particles, non-QCD axions are necessary for the current DM experiments to accomplish the ample sensitivity. In the case of multi-component DM models, the inelastic scattering processes also make sizable contributions to the direct detection. The supersymmetry (SUSY) and clockwork (CW) mechanism provide a realistic model for the QCD and non-QCD axions and the axion-mediated DM scattering processes. In the SUSY CW axion model, the lightest axino is the DM particle and the axions mediate the elastic and inelastic scattering processes. We show that the current and future XENONnT can produce relevant constraints for some parameter space of the model.

Speaker: Prof. Kyu Jung Bae (Kyungpook NU)

11:10 Axion Dark Matter from Cosmic String Network

We perform lattice simulations to estimate the axion dark matter abundance radiated
from the global cosmic strings in the post-inflationary scenario. The independent
numerical confirmation on the recently observed logarithmic growth in both the
number of strings per Hubble patch and the spectral index of the power law scaling
for the axion spectrum is reported. These logarithmic scalings are checked against
two different prescriptions for generating initial random field configurations,
namely fat-string type and thermal phase transition. We discuss a possible strong
correlation between the axion spectrum and the string evolutions with different
initial conditions to support the insensitivity of scaling behaviors against different
initial data and we provide a qualitative understanding of it.
The impact of various combinations of the power law of the axion spectrum,
nonlinearities around the QCD scale, and average inter-string distances on the axion
abundance are discussed.
Additionally, we introduce a new novel string identification method, based on the
tetrahedralization of the space, which guarantees the connectedness of the strings
and provides a convenient way of assigning the core location.
Finally we derive a lower bound on the axion mass.

Speaker: Mr Heejoo Kim (KAIST)

11:35 Flavor Leptogenesis during the reheating era

We show that the process of non-instantaneous reheating during the post-inflationary period can have a sizable impact on the charged lepton Yukawa equilibration temperature in the early Universe. This suggests relooking the effects of lepton flavors in the leptogenesis scenario where the production and decay of right-handed neutrinos take place within this prolonged era of reheating. We find this observation has the potential to shift the flavor regime(s) of leptogenesis compared to the standard thermal scenario.

Speaker: Dr Arghyajit Datta (Kyungpook NU)

14:00 → 15:50 Presentations

Convener: Prof. Kenji Kadota (HIAS)

14:00 Prefactor of quantum tunneling rate in curved spacetime

TBA

Speaker: Prof. Masahide Yamaguchi (IBS CTPU-CGA)

14:35 Gravitational Wave Sourced by Decay of Massive Particle from Primordial Black Hole evaporation

I will discuss the stochastic gravitational waves (GWs) spectrum, resulting from the emission of gravitons through bremsstrahlung, in the decay of particles produced by Hawking radiation. Although particle decays inevitably entail the emission of graviton due to bremsstrahlung, the associated decay width is notably suppressed due to the Planck scale suppression in the coupling of matter fields to gravitons. Consequently, the relic abundance of such GWs constituted of these gravitons undergoes a corresponding reduction. However, we demonstrate that super-heavy particles, reaching masses as high as Planck scale, can emerge naturally in the Hawking radiation of evaporating primordial black holes (PBHs) and can compensate for this suppression. In addition, we also discuss the stochastic gravitational waves constituted out of the gravitons directly radiated from such evaporating PBHs. When the super-heavy particle decays promptly after its production, then the corresponding GW spectrum remains subdominant to the one arising from direct PBH evaporation. However, if this particle is long-lived and decays after PBH evaporation, then the resulting GWs produced in these two processes have two distinct spectra with their peaks at extremely high frequencies, providing avenues for proposed ultra-high frequency gravitational wave detectors. We also show that such gravitational waves contribute significantly to substantial dark radiation, well within the anticipated sensitivity thresholds of future experiments like CMB-S4 and EUCLID.etic field is significantly smaller than its equipartition estimation.

Speaker: Dr Satyabrata Mahaptra (SKKU)

15:00 Gravitational waves induced by scalar-tensor mixing

This talk explores second-order gravitational waves (GWs) originating from
scalar-tensor perturbation interactions during the radiation-dominated phase
of the Universe. We investigate the unique features and detectability of
these GWs compared to the scalar-induced ones. Unlike scalar-scalar
induced GWs, scalar-tensor induced GWs lack resonances or logarithmic
running in the low frequency spectrum with peaked primordial spectra.
However, they exhibit inheritance of primordial parity violations from tensor
modes, particularly in the ultraviolet (UV) region due to chirality in primordial
GWs. We also address potential divergences in our GWs analysis and
explore solutions. This study significantly contributes to understanding GWs
in the early Universe, with implications for cosmology and GWs detection.

Speaker: Dr Pritha Bari (IBS CTPU-CGA)

15:50 → 16:10 Break

16:10 → 18:00 Presentations

Convener: Prof. Kyu Jung Bae (Kyungpook NU)

16:10 Freeze-in Inelastic Dirac Dark Matter and H0 tension

The discrepancy in the estimation of the Hubble constant, as measured by the local distance ladder and the Cosmic Microwave Background (CMB) observations, has become a significant concern, reaching a $4\sigma$ to $6\sigma$ level of significance. In this study, we investigate a particle physics model featuring an inelastic Dirac dark matter framework with a gauged $U(1)_D$ symmetry, aimed at mitigating this tension in a cosmologically consistent manner while maintaining feeble coupling between the dark and visible sectors. Our model proposes the production of dark sector particles through a freeze-in mechanism, with the two sectors retaining distinct temperatures due to minimal kinetic mixing. Leveraging the presence of a light mediator, the dark matter particles undergo late-stage annihilation into these mediators via resonantly enhanced Sommerfeld cross-sections. Subsequently, these mediators annihilate into self-interacting dark radiations, selectively influencing the dynamics during the CMB era without perturbing Big Bang Nucleosynthesis (BBN), thus offering a dual solution by alleviating both the Hubble constant ($H_0$) and $\sigma_8$ tensions concurrently.

Speaker: Mr Wonsub Cho (SKKU)

16:35 Constraints on Inelastic Dark Matter imposed by Capture in White Dwarves and Left-Right gauge symmetry

Weakly Interacting Massive Particles (WIMPs) can be captured in
compact stars such as white dwarves (WDs). If they are in a Dark Matter
(DM)-rich environment, this process can lead to an increase in the star
luminosity because of the DM annihilation process. N-body simulations
indicate that the core of the Messier 4 globular cluster (where plenty of
WDs are observed) is rich of DM. We make this assumption to analyze
the Inelastic Dark Matter (IDM) scenario, showing that when the DM
mass exceeds a few tens GeV DM mass splittings as large as 40 MeV can
be probed. This largely exceeds the sensitivity to the IDM mass splitting
from direct detection in terrestrial detectors, which does not exceed 200
keV. We also apply such constraint to the specific DM scenario of self
conjugate bi-doublet in the left-right symmetric model, where the
standard SU(2) left group with coupling gL is extended by an additional
SU(2) right group with coupling gR. In such scenario, we show that
bounds from WDs significantly reduce the cosmologically viable
parameter space, especially requiring gR>gL.

Speaker: Ms Hyomin Kim (Sogang U)

17:00 The imprint of dark matter spike on the merger rate history of primordial black hole binaries

We show that the merger rate of primordial black hole (PBH) binaries in the dark matter (DM) spike can greatly change the merger rate history of PBHs in late universe, since the high DM density in DM spike enhances the merger rate of PBHs, especially for the DM profile index gamma, larger than 1.7. This PBH merger rate from DM spike causes a bump structure in its merger rate history at redshift around 5, due to the larger matter density and rapid halo formation at that era, which is consistent with the star formation rate. Such an additional merger rate contribution could be strongly affected by two extra phenomena, two body relaxation and loss cone refilling. We calculate their impacts on PBH merger rate in DM spike for various galaxy models, and find the modification on PBH merger rate remains for f_{PBH} around 10^{-4}.

Speaker: Dr Qianhang Ding (IBS CTPU-CGA)

17:25 Gauss-Bonnet Cosmology: WIMP phenomenology, large-temperature behaviour and bounds from Gravitational Waves

The Weakly Interacting Massive Particle (WIMP) thermal
decoupling scenario can be used to probe modified Cosmologies. We
apply this idea to the specific example of dilatonic Einstein
Gauss-Bonnet (dEGB) gravity, where the Gauss–Bonnet term is
non–minimally coupled to a scalar field with vanishing potential. We
show that when the WIMP relic density is constrained to match the
observed DM abundance in the Universe the ensuing modified
cosmological scenario can drive the required WIMP annihilation cross
section to Standard Model particles beyond the present bounds from DM
indirect detection searches, allowing to constrain the model
parameters. Moreover, at temperatures much higher than those relevant
for WIMP decoupling, dEGB exhibits only very few asymptotic
behaviors, characterized by a few values of the equation of state
w. We provide a transparent explanation of this peculiar behaviour in
terms of only three attractors (stable critical points) of the set of
autonomous differential equations that describe the evolution of the
Friedmann equations. Compared to standard Cosmology dEGB can show a
strong enhancement of the expected Gravitational Wave stochastic
background produced by the primordial plasma of relativistic particles
of the Standard Model. This is due to the very peculiar fact that dEGB
allows to have an epoch when the energy density of the relativistic
plasma dominates the energy of the Universe while at the same time the
rate of dilution with the temperature of the total energy density is
slower than what usually expected during radiation dominance. The
ensuing bounds are complementary to late-time constraints from compact
binary mergers.

Speaker: Dr Arpan Kar (Sogang U)

Wednesday, 24 July

10:00 → 12:00 Presentations

Convener: Prof. Minho Son (KAIST)

10:00 Unexpected possible connection between cosmology and particle physics

I'll introduce two ideas connecting the big problem in particle physics with cosmology.
The first example is 'Nnaturalness' in which multiple copies of the Standard Model explain the smallness of the Higgs mass.
Dark radiation and dark neutrino would be the main prediction of the setup.
The second example is 'Weak scale as a trigger' in which the smallness of the cosmological constant is tied up with the weak scale.
Extremely light scalar fields are the essential ingredients and can be a candidate of dark matter.
Seemingly unrelated observation might give a strong hint on the big problem we pursue for a long time.

Speaker: Prof. Hyung Do Kim (SNU)

10:35 The MCSM (the minimal cosmological standard model)

I will introduce the minimal cosmological standard model(MCSM) which we proposed recently as a minimal BSM scenario to address various theoretical/phenomenological/cosmological big puzzles in a simple framework.

Speaker: Prof. Wanil Park (Jeonbuk NU)

11:10 The stochastic formalism in the presence of a curvaton

Cosmological perturbations generated during inflation are the seeds of the inhomogeneity and anisotropy of our universe. In contrast with the usual perturbation theory explaining these perturbations at large scale well, the stochastic formalism is an effective theory of long-wavelength modes that incorporates probabilistic noise from short-wavelength modes as they exit the horizon. This is expected to have non-perturbative properties, particularly useful in the context of primordial black hole (PBH) formation. Indeed, PBHs are formed from large perturbations exceeding a threshold when they re-enter the horizon.
In this presentation, I will review the stochastic delta-N formalism, which combines stochastic inflation and the delta-N formalism to relate curvature perturbations to fluctuations in e-foldings. This framework allows us to calculate curvature perturbations from each stochastic realization using mathematical tools, particularly solving the adjoint Fokker-Planck equation. Additionally, I will demonstrate the application of the stochastic formalism for the multi-field case, especially in the presence of a curvaton, and discuss the relationship between perturbations from the inflaton and curvaton and the possibilities of PBH formation in this scenario.

Speaker: Mr Tomotaka Kuroda (IBS CTPU-CGA)

11:35 Prospects of constraining WIMP using black hole low-mass X-ray binaries

The fast orbital decay observed in some of the Galactic black hole Low-Mass X-ray binaries (BH-LMXBs) (of a few solar masses) can be explained by the dynamical friction between Dark Matter (DM) and the companion stars orbiting around the BHs. In these cases the values of the index of the DM spike surrounding the BHs can be pinned down with an accuracy of a few percent, way better than that for much bigger systems such as the super massive BHs (SMBHs). I will show that the observational data from such a BH-LMXB, named XTE J1118+480, allow to constrain the WIMP DM annihilation cross section times velocity ⟨σv⟩ below its thermal value for DM masses up to the TeV scale. The constraints are driven mainly by the radio synchrotron signal produced by the DM induced e+- propagating in the magnetic field near the BH. The bound from XTE J1118+480 is stronger than those from SMBHs when the very large uncertainties on the spike indices corresponding to the SMBHs are taken into account. I will show that taken at face value the bound from XTE J1118+480 on ⟨σv⟩ is the most constraining compared to those obtained from other experiments for DM mass < 1 TeV, unless the intensity of the magnetic field is significantly smaller than its equipartition estimation.

Speaker: Dr Kar Arpan (Sogang U)

12:00 → 18:00 Lunch and Free Discussion

Thursday, 25 July

10:00 → 12:00 Presentations

Convener: Prof. Hyung Do Kim (Seoul NU)

10:00 Physics of SND@LHC / SHiP Experiments

We explain the physics potential of the present SND@LHC experiment and the future SHiP experiment.

Speaker: Prof. Kang Young Lee (Gyeongsang NU)

10:35 Scalar-induced one-loop tensor power spectrum during inflation

We calculate the one-loop corrections due to the curvature perturbation to the tensor power spectrum during inflation. This requires the quartic order action of both scalar and tensor perturbations.

Speaker: Prof. Jinn-Ouk Gong (Ewha Women's U)

11:10 Decoherence of Primordial Perturbations in Geodesic Observer's View

Supported by observational evidence indicating that cosmological scalar perturbations were nearly Gaussian at the beginning of the universe, it is anticipated that the origin of these perturbations is quantum fluctuations. Consequently, cosmic inflation provides a valuable setting for testing the quantum nature with/of gravity. Quantumness is characterized by features such as quantum coherence, quantum entanglement, and quantum incompatibility of measurements, all of which are sensitive to the specific setup of the observation. In this work, we discuss quantum (de)coherence using the off-diagonal components of the reduced density matrices for curvature perturbations and gravitons of observable scales, particularly under the specification that we are geodesic observers, a treatment referred to as the local observer's effect. As a result, we show that the decoherence rate for comoving observers is predominantly influenced by gauge artefacts vanishing in the geodesic coordinate system, leading to the implications that the primordial perturbations would retain their quantum nature for longer than previously expected.

Speaker: Mr Fumiya Sano (IBS CTPU-CGA)

11:35 The Strong Coupling Issue of f(T) Gravity and Beyond

Recently, there has been increasing interest in the physical degrees of freedom (DoFs) of these generalized theories in the teleparallel framework and the strong coupling issues with the vanishing coefficient of kinetic terms at a linear level in homogeneous or static backgrounds. We take f(T) gravity as an example. On the one hand, such phenomena are frequently observed in backgrounds with high symmetry, which plays a vital role in reducing the number of propagating modes at lower orders. On the other hand, such strong coupling in f(T) gravity may only exhibit strong nonlinear effects at extremely high energy scales for these subhorizon modes. Furthermore, we expand the scope by introducing an extra scalar field non-minimally coupled to $f(T)$ gravity, aiming to address or alleviate the aforementioned strong coupling behavior.

Speaker: Dr Yu-Min Hu (USTC)

14:00 → 15:50 Presentations

Convener: Prof. Takeshi Kobayashi (SISSA)

14:00 nEPT power spectrum and bispectrum

TBA

Speaker: Prof. Donghui Jeong (Penn. state U & KIAS)

14:35 Sphericity of peaks with non-Gaussianity

Spherical collapse is assumed in most of the works on PBH formation from
the primordial curvature perturbation. According to the peak theory[1],
sufficiently high peaks of a Gaussian random scalar field statistically have
spherical symmetric shapes in the homogeneous and isotropic universe,
which guarantees the above assumption. However, the PBH formation is
related to highly non-linear regime of the perturbation and the non-
Gaussianity may play an important role. In the talk, I will explicitly show how
this argument can be modified if the curvature perturbation obeys non-
Gaussianity.

Speaker: Ms Michiru Uwabo (IBS CTPU-CGA)

15:00 Wave nature of gravitational wave lensing and its applications

Gravitational lensing of gravitational wave(GW), or GW lensing for short, is a unique tool that enables us to explore the ultra-small-scale matter distribution in our universe. One of the distinctive features of GW lensing is its wave nature, which arises from the long wavelength of typical GW sources. Fully utilizing the wave nature, GW lensing can probe O(0.1) ~O(10) parsec scales matter distribution, depending on the type of GW source and detector. Investigating such small scales can provide crucial insight into the dark matter properties. In this presentation, I will provide a brief overview of the essence of GW lensing and discuss its applications to dark matter structures.

Speaker: Dr Hangil Choi (IBS CTPU-CGA)

15:25 Testing Gravity Theories: Black Hole Quasinormal Modes and their Higher Overtones

The motivation of this talk is to discuss how to probe gravity theories using the quasinormal modes (QNMs) in the ringdown phase of binary black hole mergers.
In this talk, the speaker will introduce the parametrized black hole quasinormal ringdown formalism, a robust framework for analyzing the QNMs in systems resembling GR, with a focus on higher overtones. These overtones have been shown to provide critical insights during the early ringdown phases and to exhibit heightened sensitivity to the physics near black hole horizons. The speaker's findings indicate that larger deviations from general relativity typically appear in the quasinormal frequencies of the higher overtones. These deviations in quasinormal frequencies can be characterized using model-independent parameters, and an analytical method used to understand this trend will also be discussed. This talk will highlight how the QNMs and especially their higher overtones can be a powerful tool in testing gravity theories.

Speaker: Ms Jiale Zhang (IBS CTPU-CGA)

15:50 → 16:10 Break

16:10 → 17:35 Presentations

Convener: Prof. Jinn-Ouk Gong (Ewha Women's U)

16:10 The Radio Telescope Probes on Dark Matter Beyond WIMP

We explore the sensitivity of the radio telescope to light dark matter, such as sub-GeV dark matter and axion-like particles. We will also discuss the complementarity of these forthcoming radio signal bounds with current experimental constraints from other investigations, highlighting the synergistic potential of multi-messenger astronomy in understanding dark matter.

Speaker: Prof. Kenji Kadota (HIAS)

16:45 Photon region and shadow of a rotating 5D black string

Plenty of efforts have been made to explore the black string and its instability, but the fate of the black strings with fewer extra dimensions is still inconclusive. Now starting from the 5D uniform black string, we show that the EHT observations of M87 and SgrA can not only rule out the black string with an infinite extra dimension, but also constrain the length of a compact extra dimension, which is much smaller than the critical length given from the Gregory-Laflamme (GL) instability. Our findings support the hypothesis that the extra dimension is compact avoiding the GL instability.

Speaker: Dr Zi-Yu Tang (IBS CTPU-CGA)

17:10 Primordial Black Hole Formation from Power Spectrum with Finite-width

Primordial Black Holes (PBHs) may be produced by gravitational collapse in regions with a large amplitude of density contrasts in the early Universe. They may provide the seeds for galaxy formation, account for a population of the LIGO-Virgo events, and the candidates of cold dark matter. The main purpose of our study is to develop a more accurate estimation for PBH abundance. We address the calculation of the PBH abundance by applying peak’s theory to primordial field perturbations with a Gaussian probability distribution. Our study makes the application of peak’s theory in PBH abundance estimation no longer limited to monochromatic perturbations, power spectrum with any width or shape can be generally solved by handling the smoothing effects in a proper way.

Speaker: Ms Jianing Wang (ITP)

Friday, 26 July

10:00 → 11:25 Presentations

Convener: Prof. Ki Young Choi (SKKU)

10:00 Dark Z model and Higgs Phenomenology

We consider the phenomenology of the Higgs bosons in the hidden sector model mediated by an additional SU(2) scalar doublet. With the mediator scalar doublet, the Higgs sector of the model is extended to be that of the two Higgs doublet model of type I. The mediator scalar doublet is connected to the hidden sector with the hidden U(1)𝑋 gauge charge. Then the U(1)𝑋 gauge symmetry is spontaneously broken by the electroweak symmetry breaking and the hidden U(1) gauge boson gets the mass to give rise to a dark 𝑍 boson. Using the programs higgsbounds and higgssignals, we incorporate current experimental limits from LEP, Tevatron, and LHC to examine the Higgs sector in our model and derive constraints on model parameters together with the electroweak processes. As a result, our model favors the light dark 𝑍 boson. We also investigate the implications of the model on the dark matter phenomenology when the hidden fermion is a dark matter candidate.

Speaker: Prof. Dong-Won Jung (IBS CTPU-CGA)

10:35 How much is the lifetime of an oscillon affected by coupling to another field?

Oscillons are long-lived localized spherical solitons of real scalar field with potential flatter than quadratic. They are considered to be formed at the end of inflation and important in the early universe. Their lifetime can be greatly effected by the coupling of inflaton field to other fields. We consider an oscillon coupled with another real scalar field and investigate the effect on the oscillon's decay and lifetime. We use both semi-analytical method and numerical simulation to investigate the evolution and lifetime of the oscillon.

Speaker: Ms Siyao Li (IBS CTPU-CGA)

11:00 Cosmological correlators: more is difficult

Due to the extraordinarily high energy scales inherent to inflation, it naturally functions as a cosmological collider to probe physics at otherwise inaccessible energy regimes, and the information are encoded in cosmological correlators. After many efforts in recent years, the analytical structure of the correlators associated with single massive spinning field exchange is well understood. However, moving beyond that is highly challenging with the complexity increasing significantly. In this talk, the speaker will present their recent work towards understanding the analytical structure of cosmological correlators involving double massive exchanges, using the recently developed cosmological bootstrap method. He will show how to obtain analytical results valid in any kinematic configuration and free from spurious divergences. Then he will explore the phenomenology of such double massive exchange processes. Finally, if time permits, he will also mention their recent works on the factorization of correlators which can give a shortcut to calculate a type of correlators and provide us testable relations among cosmological observables.

Speaker: Dr Yuhang Zhu (IBS CTPU-CGA)

11:25 → 11:35 Adjourn

Convener: Masahide Yamaguchi