The 5th workshop on Symmetry and Structure of the Universe (SSU 2026 Busan)

23 Aug 2026, 09:00 → 27 Aug 2026, 21:50 Asia/Seoul

The 5th workshop on Symmetry and Structure of the Universe (SSU 2026 Busan) is to be held in Bolero Hall (4F) at Lavalse Hotel, Yeongdo, Busan on Aug 23 - 27, 2026. The workshop aims to cover the present status and recent issues of high energy physics and cosmology, and so to establish close collaborations for probing the fundamental symmetries and the structure of the Universe. A broad range of topics including collider phenomenology, dark matter, neutrino physics, early universe cosmology and astroparticle physics will be extensively covered and intensively discussed.

There is no registration fee. Accommodation at Lavalse Hotel during the workshop (check-in on Aug 23, check-out on Aug 27) will be provided for invited speakers and graduate students. Other participants can also stay at the hotel at a discounted rate. If you would like the organizing committee to arrange your hotel reservation, please indicate your check-in and check-out dates during registration.

SSU 2026 Busan is generously supported by IBS-CTPU-PTC, Jeonbuk National University, Pusan National University, and Chungnam National University.

The registration period is July 6 (Mon) – July 14 (Tue). Please note that abstract submissions and additional talk requests cannot be accommodated. As the number of hotel rooms is limited, registration may close before the deadline once the available rooms are fully booked. We encourage you to register as early as possible.

SSU 2026 Busan is preceded by 12th Workshop on Flavor Symmetries and Consequences in Accelerators and Cosmology (FLASY 2026) on Aug 17 - 22 at Gyeongju which is just about an hour distance by car.

Invited Speakers

 Gongjun Choi (University of Minnesota)

 Peizhi Du (University of Science and Technology of China)     

 Patrick Foldenauer (Instituto de Fisica Teorica, Madrid)

 Patrick Fox (Fermilab)

 Sudhakantha Girmohanta (IBS-CTPU-PTC)

 Andre De Gouvea (Northwestern University)

 Kylar Greene (Seoul National University)

 Tae Hyun Jung (IBS-CTPU-PTC)

 Boris Betancourt Kamenetskaia (IBS-CTPU-CGA)

 Dong Woo Kang (Jeonbuk National University)

 Heejoo Kim (KIAS)

 Seong Sik Kim (Chungnam National University)

 Chui-Fan Kong (IBS-CTPU)

 Enrico Lunghi (Indiana University) 

 Yeji Park (IBS-CTPU-PTC)

 Tanmay Kumar Poddar (IPPP Durham)

 Michael Shamma (KIAS)

 Arunansu Sil (Indian Institute of Technology Guwahati)   

 Fuminobu Takahashi (Tohoku University)

 Tanner Trickle (University of Illinois Urbana Champaign)

 Edoardo Vitagliano (University of Padua)

 Huangyu Xiao(Boston University / Harvard University)

 Yue Zhao (Hong Kong University of Science and Technology)

 

Invited Students

 Junghoon Joh (Sungkyunkwan University)

 Ju Hyeong Kang (Pusan National University)

 Sang Hwan Kim (Yonsei University)

 Min-Gwa Park (Northwestern University)

 

Organizing Committee

Jae Hyeok Chang (Seoul National University)

Kwang Sik Jeong (Pusan National University)

Chang Sub Shin (Chungnam National University)

Seodong Shin (Jeonbuk National University / KIAS / IBS-CTPU)

Seokhoon Yun (Kyungpook National University / IBS-CTPU)

 

Sunday 23 August

Discussion

Monday 24 August

08:30 Morning Coffee

Opening Remarks

Convener: Prof. Seodong Shin (Jeonbuk National University)

SSU2026-opening.pdf

S1

Convener: Prof. Seodong Shin (Jeonbuk National University)

1 Looking for light dark states with taus and/or time

Axion light particles are a well-motivated example of light, weakly coupled BSM states. I will investigate the constraints on such particles through their couplings to tau leptons. I will discuss situations where the ALPs are long lived and the best search strategy is through beam dumps (both past and future experiments) and the situation where the ALP decays promptly which can result in exotic multi-lepton final states at colliders -- many of which have not yet been searched for. Finally, I will discuss a situation where light dark matter can induce time-modulation in rare decays.

Speaker: Patrick Fox (Fermilab)

Tau_ALP_SSU2026.pdf

2 Gravitational Wave for New Physics

Speaker: Yue Zhao (HKUST)

DPDM - GeV - mid.ppt

3 Searching for ultralight bosons with compact stars

Ultralight bosons are well-motivated candidates for dark matter and new long-range interactions. Their tiny masses can generate macroscopic coherence and long-range effects, making compact objects powerful probes of extremely weak new physics. In this talk, I will review searches for ultralight bosons using neutron stars, pulsars, black holes, and compact binaries. I will highlight signatures in binary dynamics, orbital decay, exotic radiation, boson-photon conversion, polarisation, timing, and multimessenger observations involving electromagnetic, gravitational wave, and neutrino signals.

Speaker: Tanmay Kumar Poddar (IPPP, Durham University)

Tanmay-SSU.pdf

11:30 Lunch

S2

Convener: Jae Hyeok Chang (Seoul National University)

4 Reconstructing the Dark Matter Equation of State with Compact Object Inspirals

Can gravitational waves reveal the properties of dark matter? Compact binaries embedded in extended dark matter configurations provide a unique opportunity to probe the macroscopic behavior of the dark sector. If a neutron star or black hole is surrounded by a sufficiently massive and extended dark matter envelope, an inspiraling companion experiences dynamical friction in addition to the standard gravitational-wave energy loss. This environmental effect can be isolated through a new gravitational-wave observable, the $D$-function, which directly measures the extra dissipative power induced by the surrounding medium. Since the density profile of the envelope is determined by the dark matter equation of state, the frequency dependence of the $D$-function encodes direct information about the equation of state itself. Through a regression-based framework, we may reconstruct both the density profile and the dark matter equation of state from gravitational-wave observations. This method accurately recovers the input equation of state while remaining largely independent of the microscopic nature of dark matter. These results illustrate how future gravitational-wave observations could provide a new window into the fundamental properties of dark matter.

Speaker: Boris Betancourt Kamenetskaia (Institute for Basic Science CTPU-CGA)

5 A Natural Dark Matter–Dark Energy Interaction from Axion–Monopole Dynamics

We propose a natural framework for interacting dark matter and dark energy based on axion–monopole dynamics in a dark gauge sector. Dark matter is composed of dark ’t Hooft–Polyakov monopoles, while dark energy is an ultralight axion whose coupling to the dark gauge field induces an axion-dependent dyon mass through the Witten effect. This generates a direct dark matter–dark energy interaction. We show that generic models with a significant dark matter–dark energy coupling suffer from a naturalness problem: the Coleman–Weinberg potential induced by the interaction is typically much larger than the dark-energy scale. In our model, the corrections arise from a monodromy tower of dyon states, allowing them to be exponentially suppressed. In the minimal setup, the interaction strength is predicted to be $\sim 0.1\%$, below the sensitivity of current observations. We discuss scenarios in which cosmological population of higher-charge dyons can enhance the interaction to the percent level, producing an apparent phantom dark-energy equation of state.

Speaker: Huangyu Xiao

Axion monopole interaction_Huangyu.pptx

14:50 Coffee Break

S3

Convener: Seokhoon Yun (KNU)

6 Cornering axions with astrophysical transients

While it is way too early to make a final judgement, it is noteworthy that searches at LHC and direct detection experiments reported null results, therefore boosting the ever growing interest in sub-GeV dark sector candidates. Owing to their high temperature and density, the cores of proto-neutron stars, remnant of core-collapse supernovae and neutron star mergers, can be factories of axions and other feebly interacting particles with mass of up to several hundreds MeV. In this talk, we will summarize the state of the art of transient-based searches, and identify open questions and future directions.

Speaker: Edoardo Vitagliano (Max Planck Institute for Physics)

talk_busan2026.pdf

7 CSM for BSM

Core-collapse supernovae are powerful probes of physics beyond the Standard Model (BSM). In this talk, I will show how the circumstellar medium (CSM) affects supernova-based BSM searches using dark photons as an example. The decay product of dark photon can heat up CSM and lead to extra blackbody emission. Using the early-time observations of SN 2023ixf, we derive the strongest constraint on the DP parameter space.

Speaker: Chui-Fan Kong (IBS-CTPU-PTC)

KONG_SSU.pdf

8 Core-Collapse Supernova Cooling in the Dark Z Model

We investigate supernova cooling bounds on an MeV-scale dark Z boson with mass mixing. For a dark Z much lighter than the Standard Model Z boson, the kinetic mixing contribution to its coupling with the neutral current is strongly suppressed, while the mass mixing contribution remains sizable. This allows the dark Z to be efficiently produced in the dense medium of a core-collapse supernova through the neutral current interactions, particularly through axial-vector couplings. We evaluate the production and absorption of dark Z bosons in the proto-neutron star, including in-medium effects relevant to vector interactions, and obtain cooling bounds based on the observed neutrino signal from SN 1987A.

Speaker: Mr Ju Hyeong Kang (Pusan National University)

SSU2026_CCSN_Zprime.pdf

Panel Discussion 1

Convener: Seokhoon Yun (KNU)

Tuesday 25 August

08:30 Morning Coffee

S4

Convener: Chang Sub Shin (Institute for Basic Science)

9 Minicharged Particles from Quantum Consistency and Number Theory

We show that anomaly cancellation, usually imposed as a condition for quantum consistency, can also serve as a principle for organizing particle spectra in chiral gauge theories. For a broad class of spectra charged under both a vector-like gauge symmetry and a chiral gauge symmetry, the anomaly equations are exactly equivalent to the degree-3 Prouhet–Tarry–Escott problem in number theory. This correspondence turns charge consistency into a classification principle for particle spectra. In a minimal realization involving light minicharged particles, it implies a robust lower bound of four mass eigenstates and identifies paired states with the same minicharge and nearby masses. These spectra provide concrete targets for laboratory, astrophysical, and cosmological searches, while assigning physical significance to distinguished Prouhet–Tarry–Escott solutions. Our results show that quantum consistency can determine observable features of hidden charged matter, linking anomaly cancellation, number theory, and particle phenomenology.

Speaker: Fuminobu Takahashi (Tohoku University)

Takahashi_v3.pdf

10 Beyond Delta m^2: Absolute Mass Sensitivity in Neutrino Oscillations

We discuss the next-to-leading-order corrections to the neutrino oscillation phase and applying the result to reactor antineutrinos. At next-to-leading order, the oscillation phase includes terms proportional to m_i^4-m_j^4 = Delta m^2_{ij}(m_i^2+m_j^2), and is therefore sensitive to the absolute mass scale. We estimate the sensitivity of JUNO to the absolute neutrino mass scale.

Speaker: Andre de Gouvea (Northwestern University)

SSU_Busan_2026.pdf

11 Exploring Leptogenesis in the Era of First Order Electroweak Phase Transition

We propose a low-scale leptogenesis scenario within the neutrino seesaw framework, where CP-violating decays of right-handed neutrinos (RHN) generate the required lepton asymmetry even for RHN masses below the Standard Model Higgs mass. This is achieved by delaying the sphaleron decoupling through a first-order electroweak phase transition, allowing efficient conversion of lepton asymmetry into the observed baryon asymmetry at temperatures below the conventional sphaleron freeze-out temperature. The framework is particularly relevant for cosmologies with low reheating temperatures and predicts observable gravitational wave signals from the phase transition, together with promising collider signatures from light right-handed neutrinos.

Speaker: Arunansu Sil (Indian Institute of Technology Guwahati)

Talk-Leptogenesis-FOPT.pdf

Group photo

11:40 Lunch

S5

Convener: Kwang Sik Jeong (Pusan National University)

12 Closing in on Vector Portals to Dark Matter

Vector portal interactions provide a simple and theoretically well-motivated framework for connecting dark sectors to the Standard Model. In particular, dark photons and anomaly-free gauge bosons arise naturally in extensions of the Standard Model and can play a central role in dark matter phenomenology. In this talk, I first discuss scenarios in which dark photons mediate the interactions of GeV-scale thermal dark matter. Despite strong experimental constraints, viable regions of parameter space remain that can be tested at collider and direct detection experiments. If dark matter is only very feebly coupled, it can still be produced through the freeze-in mechanism. While the extremely weak interactions involved make these scenarios quite challenging to probe, I will show how they can nevertheless lead to observable signals in both electron and nuclear recoils at direct detection experiments, particularly if freeze-in occurs at low reheating temperatures.

Speaker: Patrick Foldenauer (IFT UAM-CSIC Madrid)

SSU_PF_compressed.pdf

13 A General Perspective on Boosted Dark Matter from Semi-annihilation

We develop a systematic approach for analyzing any dark matter (DM) model that produces boosted dark matter (BDM) through semi-annihilation. We emphasize three points. First, any BDM analysis must be grounded on the correct relic density. Second, semi-annihilation intrinsically yields boosted by-products, whose contribution to indirect detection is generally non-negligible and should be treated on the same footing as that of the BDM. Finally, the detection of the semi-annihilation by-product must be investigated.
As a demonstration, we apply our approach to an electrophilic, single-component $Z_3$ fermionic DM model. We find that this model is severely constrained by both model-independent and model-specific experimental bounds, and that the BDM direct-detection rate is unobservably small and is everywhere overwhelmed by the detectability of the neutrino by-product.

Speaker: Seongsik Kim (Chungnam National University)

2026 Aug SSU_SSK_v2.pdf

14:50 Coffee Break

S6

Convener: Jae Hyeok Chang (Seoul National University)

14 The Search for Light Dark Matter

Recent years have seen a dramatic expansion in ideas regarding the nature of dark matter, extending beyond the weakly-interacting massive particle (WIMP) paradigm. Many of these theories predict minuscule couplings between light (sub-GeV mass) dark matter and the Standard Model, which direct detection experiments can search for. In this talk we will begin by reviewing the status of current electron-based experiments which are achieving extraordinary sensitivity to the theoretically well-motivated freeze-in dark matter models. We'll then explore how phonons, collective excitations of lattice vibrations, will be essential for searching for lighter dark matter beyond the reach of these electron-based experiments. Lastly, we'll discuss how magnons, collective excitations of spin waves, can broaden our search to dark matter models with primarily spin-dependent interactions.

Speaker: Prof. Tanner Trickle (University of Illinois Urbana-Champaign)

08_25_SSU.pdf

15 Quantum Entanglement and Bell Nonlocality for two-qubit system at Colliders

TBA

Speaker: Dong Woo Kang (Yonsei University)

SSU2026_DWK.pdf

16 Analysis of Fermi-LAT data for cascade annihilation of dark matter

Galactic Center Excess (GCE) remains one of the most interesting unknown signals in the domain of few GeV gamma-rays. Since its morphology prefers spherical symmetry, dark matter annihilation can be an intriguing explanation of the GCE. In this work, we tested the possibility of cascade annihilation of dark matter, such as into four-b quarks with boost effect. At the same time, utilizing the observation data towards dwarf galaxies and observation of anti-proton flux of AMS-02 experiment, we set strong bound on the explanation of GCE in terms of dark matter annihilation.

Speaker: SangHwan Kim (Yonsei University)

5th_SSU_talkfile.pdf

Panel Discussion 2

Convener: Prof. Seodong Shin (Jeonbuk National University)

18:00 Banquet

Wednesday 26 August

08:30 Morning Coffee

S7

Convener: Chang Sub Shin (Institute for Basic Science)

17 Suppressing Extra-Dimensional Axion Isocurvature Dynamically

TBA

Speaker: Gongjun Choi (University of Minnesota)

SSU2026_Gongjun Choi_modified.pdf

18 What Does Cosmology Actually Measure? The FFAT Symmetry and Cosmological Concordance

Many cosmological observables are sensitive primarily to ratios of physical scales rather than their absolute values. This motivates the FFAT scaling symmetry, in which correlated changes to the expansion history, primordial fluctuations, and scattering rates can leave much of the CMB phenomenology nearly unchanged while shifting the inferred absolute expansion scale. I will discuss how this symmetry provides a useful framework for understanding the Hubble tension and for constructing extensions of ΛCDM that preserve the key ratios measured by cosmological data. More broadly, I will explore a ratio-preserving approach to cosmological concordance and the observations that ultimately break these degeneracies.

Speaker: Kylar Greene (Seoul National University)

Busan Symmetry.pdf

19 Can pre-inflationary axions form miniclusters?

The pre-inflationary axion dark matter scenario, with its uniform initial misalignment angle, is conventionally expected to yield smooth dark matter without substructure. We explore scenarios of pre-inflationary axion cosmology, where adiabatic temperature perturbations from inflation can source non-trivial axion density fluctuations through both the temperature-dependent QCD axion mass and anharmonic resonance with close-to-hilltop misalignment angles.

The former permits minicluster formation only in a narrow window; the latter produces O(0.1) density contrast at scales below the quantum Jeans momentum at matter–radiation equality, providing a clean initial condition for Schrodinger-Poisson simulations.

Speaker: Heejoo Kim (KIAS)

HeejooKim.pdf

Group photo

11:40 Lunch

S8

Convener: Kwang Sik Jeong (Pusan National University)

20 Isocurvature Induced Gravitational Waves at Pulsar Timing Arrays

The standard cosmological model (ΛCDM model) assumes adiabatic initial conditions for primordial density perturbations. However, many new physics scenarios can deviate from this assumption and generate isocurvature perturbations across a wide range of scales. In this talk, I will discuss how isocurvature can induce gravitational waves at second order in perturbation theory. Moreover, Pulsar Timing Arrays can set stringent limits on isocurvature around $10^6\textrm{Mpc}^{-1}$ via isocurvature-induced gravitational waves.

Speaker: Peizhi Du (University of Science and Technology of China)

iso_induced_GW_SSU.pdf

21 Quest for a Consistent Phase Transition Explanation of the PTA Signal

The discovery of a nano-Hz stochastic gravitational wave background by Pulsar Timing Array (PTA) collaborations can be interpreted as arising from a strongly supercooled phase transition in a nearly conformal dark sector. I will present a concrete model for this phase transition, and show that the strong supercooling it requires generically dilutes any pre-existing baryon asymmetry and dark matter abundance. I will then discuss how this apparent problem can be turned into a solution by exploiting the built-in out-of-equilibrium dynamics of the transition to generate both after the phase transition. I will present a neutron portal framework connecting the resulting dark baryon asymmetry to the visible sector, discuss its ultraviolet completion, and show how it naturally explains the baryon-dark matter coincidence problem. I will conclude with a discussion of baryon inhomogeneity effects and other phenomenological consequences of this framework.

Speaker: Sudhakantha Girmohanta (Institute for Basic Science, CTPU-PTC, Korea)

Girmohanta_ssu_busan.pdf

14:50 Coffee Break

S9

Convener: Prof. Seodong Shin (Jeonbuk National University)

22 Revisiting QCD-induced little inflation scenario

Pulsar timing arrays reported strong evidence for a stochastic gravitational-wave background whose peak frequency corresponds to the Hubble scale at a temperature of O(100) MeV in the early Universe, redshifted to the present. Such a signal had already been predicted in the QCD-induced little inflation scenario proposed by Boeckel and Schaffner-Bielich in 2009. In this talk, I revisit this scenario and argue that the amount of supercooling required to dilute the baryon number density is inconsistent with our current understanding of the critical endpoint of the QCD phase diagram. I then introduce the chiral density wave (CDW) phase, a hypothetical phase at high baryon density, and show that it can provide sufficient dilution. The reheating temperature after the transition from the CDW phase to the hadronic phase, however, is expected to be far below the BBN temperature, and we therefore conclude that the PTA signal cannot be explained by Standard Model QCD even in the presence of a large baryon density.

Speaker: Tae Hyun Jung (IBS CTPU)

QCD-induced PT.pdf

23 One mass function to fit them all

Primordial black holes form with an extended mass function whose shape encodes the physics of their formation. As the universe ages, Hawking evaporation reshapes that function, eroding it from below and thereby fixing the distribution that present observational constraints actually probe. An analytic parametrization of this object underlies every downstream analysis, yet the fitting forms in current use are tied to specific formation scenarios, lose accuracy in the tails that dominate observations, and surrender their shape the moment evaporation begins. In this talk I present a single analytic family with four parameters that spans the standard formation scenarios and repairs all of these failures at once. The mass functions of common practice arise as exact limits or boundaries of this family, so nothing already established is discarded. More surprisingly, the family is not merely compatible with evaporation but selected by it. When primordial black holes lose mass through Hawking emission governed by a power law, a subfamily fixed by that law is exactly invariant, evolution across it reduces to the linear drift of a single scale together with an elementary survival factor, and whatever accuracy a fit achieves at formation it retains at every later epoch. A numerical survey spanning lognormal, critical collapse and power law progenitors quantifies the general case and distills the results into a practical prescription for constraint work.

Speaker: Yeji Park (IBS-CTPU-PTC)

slides-ssu2026-yeji.html

24 BSM Tau Neutrino Appearance at DUNE-PRISM

Accelerator neutrino experiments provide profound opportunities for Beyond the Standard Model physics searches. Among them, the near detector of DUNE experiment and its off-axis displacement, DUNE-PRISM, are widely studied utilizing energy and angular spectrum difference between SM and BSM interactions. There are other types of searches that utilize additional BSM neutrino flux, and tau neutrino appearance at DUNE near detector should be one of the stringent smoke-gun of BSM physics. In this work, we studied what we can learn at DUNE-PRISM, focusing on hard and wide neutrino spectrum from heavy BSM mediators, with demonstrative anomaly-free $B-3L_\tau$ models. We found that hard BSM $\nu_\tau$ spectrum allow us choose loose background selection keeping many signals to increase sensitivities, while wide spectrum give slight advantage of off-axis detector over on-axis at heavier $Z'$ mass region.

Speaker: Min-Gwa Park (Northwestern University)

mgpark_SSU2026.pdf

Panel Discussion 3

Convener: Sang Hui Im (Center for Theoretical Physics of the Universe, Institute for Basic Science, Korea)

Thursday 27 August

08:30 Morning Coffee

S10

Convener: Chang Sub Shin (Institute for Basic Science)

25 Inclusive vs exclusive $b\to s \mu\mu$: probing nonperturbative effects

Over the past decade, the LHCb and CMS experiments have reported persistent tensions in several branching fractions and angular observables of exclusive $b \to s \mu^+\mu^-$ decays. Taken together, these discrepancies exceed the five-standard-deviation level. However, their interpretation remains the subject of active debate, particularly regarding the size and treatment of nonperturbative QCD effects. In this talk, I present a new strategy that exploits forthcoming measurements to distinguish possible New Physics contributions from nonperturbative QCD effects.

Speaker: Enrico Lunghi (Indiana University)

t-260825-Busan-bsll ratios.pdf

26 On the Consistency of Dynamical Gauge Couplings

We study the low-energy behavior of gauge couplings in the presence of dynamical scalar backgrounds. In weakly coupled, renormalizable four-dimensional theories, gauge invariance restricts the dependence on the scalar background to be logarithmic. The gauge coupling evolution in these scenarios can be entirely described by the renormalization-group running with dynamical mass thresholds. Beyond one loop, other couplings can enter the gauge beta function, providing a different mechanism for variations. We discuss phenomenological implications for the dynamical evolution of the Standard Model gauge couplings.

Speaker: Michael Shamma (Korea Institute for Advanced Study)

Shamma_DynamicalGaugeCouplings_SSU.pdf

27 Photon axion enhancement around PBH

Dark matter (DM) can be gravitationally captured around a massive compact object, forming a dense halo. If primordial black holes (PBHs) exist, surrounding dark matter particles can be accreted through gravitational attraction, leading to the formation of a DM halo with a steep spike profile near the PBH.

If the dark matter consists of axions or axion-like particles (ALPs), the high number density in the spike region can significantly enhance photon production via stimulated decay. When the axion number density exceeds a critical threshold, photon enhancement becomes efficient, potentially generating a strong electromagnetic signal.

Photons emitted from the dense DM halo surrounding a PBH may propagate to Earth and could be detected in the radio frequency range, providing a possible observational signature of axion dark matter and primordial black holes.

Speaker: Junghoon Joh (SungKyunKwan University)

SSU_Junghoon Joh.pdf

Closing Remakrs

Convener: Kwang Sik Jeong (Pusan National University)