arXiv subjects

Cao H. Nam

Publications and source records attributed to Cao H. Nam.

At least 19 recordsLinked to original sources

arXiv2026

Thermodynamic topology of black holes and an invariant of spacetime

We present a formalism for exploring the thermodynamic topology of black holes, without introducing the auxiliary variable $Θ\in[0,π]$ employed in previous studies. The formalism is based on an off-shell grand free energy appropriate to thermodynamic systems that can exchange both energy and matter with their environment, thereby allowing thermal and chemical equilibrium. We define a vector field as the gradient of the off-shell grand free energy, whose zeros correspond to equilibrium configurations and hence to black hole solutions, and then we construct a conserved topological tensor from its normalized components. The associated topological flux is given by the sum of local topological indices of all zeros contained within a hypersurface at fixed ensemble parameters. We further define the asymptotic topological flux as the limiting value of the conserved topological flux through a family of finite hypersurfaces as they approach the asymptotic boundary of the ensemble parameter space. We find that the asymptotic topological flux is determined by the asymptotic/background spacetime geometry. This motivates the conjecture that the asymptotic topological flux is an invariant associated with the asymptotic/background spacetime geometry. Within the spacetime classification based on the asymptotic topological flux, the flat-space limit of AdS is found to be nontrivial, providing a potentially complementary perspective on the AdS distance conjecture.

gr-qc↗

arXiv2026

Four-dimensional de Sitter cosmology on D-branes nucleated in an asymptotically $\text{AdS}_5\times T^{1,1}$ background

We find four-dimensional de Sitter (dS) vacuum solutions on probe D-branes nucleating in a background, which is a U(1) charged black hole solution of IIB supergravity, including stringy corrections. A sufficiently high chemical potential induced by the wrapped D3-brane charge, inducing an instability in the system, is essential to lead to the nucleation of the probe D-branes. We show that stringy corrections can yield a dS vacuum on spherical D3-branes consistent with the observations without fine-tuning. Motivated by the fact that the matter fields propagating in the compact extra dimensions can provide solutions for problems in particle physics and cosmology, we also construct a dS vacuum on a probe D5-brane that wraps on a two-torus of the internal manifold $T^{1,1}$. This construction requires turning on a worldvolume U(1) gauge field along the wrapped part of the D5-brane and dissolving some D3-branes in the D5-brane. The stringy corrections play an important role in yielding a dS vacuum, and the field strength of the U(1) gauge field must be sufficiently large to produce a tiny cosmological constant.

hep-th↗

arXiv2026

Microstates and statistical entropy of observed 4D black holes

We aim to provide a microscopic explanation of observed 4D black holes based on the compactification of 5D Einstein gravity plus a positive cosmological constant on a circle. The framework of the dimensional reduction in this work allows us to compute the statistical entropy of general 4D black holes independent of the symmetries of the black hole solution, such as the spherical symmetry, and going beyond the class of special black holes that are supersymmetric and (near-)extremal as well as have exotic charges. The statistical entropy of 4D black holes includes the Bekenstein-Hawking area term at leading order and sub-leading exponential corrections. We find a new exponential correction that is more meaningful than those previously found in the literature.

hep-th↗

arXiv2026

Physical implications of a double right-handed gauge symmetry

Guided by the flipping principle, we propose a novel extension of the Standard Model based on a double right-handed $U(1)$ gauge symmetry. In this framework, all left-handed fermions are neutral, while right-handed fermions of the third generation carry charges distinct from those of the first two generations. This structure naturally explains the observed Standard Model fermion mass hierarchy: the heavy masses of the third generation are generated at tree level, while the lighter masses of the first and second generations arise radiatively at the one-loop level. For the active neutrino sector, the tiny masses are generated through a combination of tree-level and two-loop seesaw mechanisms. Crucially, this approach successfully reproduces the observed neutrino mass hierarchy, with the atmospheric mass-squared difference generated at tree level and the solar neutrino mass squared difference emerging at the two-loop level. These hierarchical patterns stem from the interplay between gauge invariance and a residual parity symmetry that survives the spontaneous breaking of the extended gauge group. The same residual symmetry stabilizes a viable scalar singlet dark matter candidate, which we show can reproduce the observed relic abundance while remaining consistent with current direct detection bounds. After addressing constraints from electroweak precision tests and flavor-changing neutral currents, we explore the discovery prospects for the new neutral bosons at existing and future colliders, including the LEP, LHC, and a future ILC.

hep-ph↗

arXiv2025

A novel force from the curved field space: Lepton flavor violation and $g-2$

We find that the nonzero curvature extension of the field space for the charged leptons induces a peculiar force, leading to an elegant and simple mechanism for generating the charged lepton flavor violation (CLFV) and their anomalous magnetic moment, which has not been explored in the literature. This novel force corrects the electromagnetic vertex, leading to an effective coupling which is flavor off-diagonal at tree level. Consequently, it yields the CLFV decays with a very strongly amplified sensitivity, allowing for the CLFV probes in the small flavor-violating parameter region which is out of reach of the previous models. We point out a new type of contribution for the anomalous magnetic moment of the charged leptons, coming from the electric form factor corrected by the force emerging from the curved field space.

hep-ph↗

arXiv2025

Scoto-seesaw model implied by flavor-dependent Abelian gauge charge

Assuming fundamental fermions possess a new Abelian gauge charge that depends on flavors of both quark and lepton, we obtain a simple extension of the Standard Model, which reveals some new physics insights. The new gauge charge anomaly cancellation not only explains the existence of just three fermion generations as observed but also requires the presence of a unique right-handed neutrino $ν_R$ with a non-zero new gauge charge. Further, the new gauge charge breaking supplies a residual matter parity, under which the fundamental fermions and $ν_R$ are even, whereas a right-handed neutrino $N_R$ without the new charge is odd. Consequently, light neutrino masses in our model are generated from the tree-level type-I seesaw mechanism induced by $ν_R$ and from the one-loop scotogenic contribution accommodated by potential dark matter candidates, $N_R$ and dark scalars, odd under the matter parity. We examine new physics phenomena related to the additional gauge boson, which could be observed at colliders. We analyze the constraints imposed on our model by current experimental limits on neutrino masses, neutral meson oscillations, $B$-meson decays, and charged lepton flavor violating processes. We also investigate the potential dark matter candidates by considering relic density and direct detection.

hep-ph↗

arXiv2024

Topological equivalence and phase transition rate in holographic thermodynamics of regularized Maxwell theory

Utilizing the holographic dictionary from the proposal that treats Newton's constant as a thermodynamic variable, we establish a thermodynamic topological equivalence between the AdS black holes in the bulk and the thermal states in the dual CFT. The findings further reveal that the thermodynamic topological characteristics of the RegMax AdS black holes are strongly influenced by the characteristic parameter of the regularized Maxwell theory. Additionally, we investigate the phase transition between low and high entropy thermal states within a canonical ensemble in the dual CFT. Our observations indicate that the phase transition behavior of the thermal states mirrors that of the black holes. By modeling the phase transition process as a stochastic process, we are able to calculate the rates of phase transition between the thermal states. This result enhances our understanding of the dominant processes involved in the phase transition of the thermal states in the dual CFT.

hep-th↗

arXiv2024

Prediction of non-SUSY AdS conjecture on the lightest neutrino mass revisited

We study the constraint of the non-SUSY AdS conjecture on the three-dimensional vacua obtained from the compactification of the Standard Model coupled to Einstein gravity on a circle where the three-dimensional components of the four-dimensional metric are general functions of both non-compact and compact coordinates. From studying the wavefunction profile of the three-dimensional metric in the compactified dimension, we find that the radius of the compactified dimension must be quantized. Consequently, the three-dimensional vacua are constrained by not only the non-SUSY AdS conjecture but also the quantization rule of the circle radius, leading to both upper and lower bounds for the mass of the lightest neutrino as $\sqrt{2}\leq m_ν/\sqrt{Λ_4}<\sqrt{3}$ where $Λ_4\simeq5.06\times10^{-84}$ GeV$^2$ is the observed cosmological constant. This means that the lightest neutrino should have a mass around $10^{-32}$ eV or it would be approximately massless. With this prediction, we reconstruct the light neutrino mass matrix that is fixed by the neutrino oscillation data and in terms of three new mixing angles and six new phases for both the normal ordering and inverted ordering. In the situation that the light neutrino mass matrix is Hermitian, we calculate its numerical value in the $3σ$ range.

hep-th↗

arXiv2024

Non-singular cosmology from non-supersymmetric AdS instability conjecture

We show that the non-supersymmetric AdS instability conjecture can point to how quantum gravity removes the initial Big Bang singularity, leading to a potential resolution for the past-incomplete inflationary universe. From the constraints on the dynamics of the universe realized as the nucleation of a thin-wall bubble mediating the decay of the non-supersymmetric AdS vacuum, we find the critical temperature $T_c$ and the critical scale factor $a_c$ for which the universe exists. These critical quantities are all finite and determined in terms of the parameters specifying the stringy 10D AdS vacuum solutions. Additionally, we derive the prediction of quantum gravity for $T_c$ and $a_c$ relying on the inflationary observations.

hep-th↗

arXiv2024

Brane-vector dark matter and its connection to inflation and primordial gravitational waves

The scalar mode describing the fluctuation of the 3-brane (the observable universe) in a five-dimensional bulk spacetime compactified on a circle is absorbed by the Kaluza-Klein U(1) gauge field, leading to a massive brane-vector living on the 3-brane. The brane-vector can be responsible for dark matter because it is odd under a $\mathrm{Z}_2$ symmetry, neutral under the Standard Model (SM) symmetries, and couples extremely weak to the SM particles due to its gravitational origin. Interestingly, the brane-vector dark matter could leave particular imprints on the cosmic microwave background (CMB) and the primordial gravitational waves. Hence, the precise measurements of the CMB and the observations of the primordial gravitational waves generated during the inflation can provide a potential way to probe the extra-dimensions and branes which are the main ingredients of string/M theory.

hep-ph↗

arXiv2024

Generalized free energy and thermodynamic phases of black holes in the gauged Kaluza-Klein theory

In the context of the generalized (off-shell) free energy, we explore the phase emergence and corresponding phase transitions of charged dilaton $\text{AdS}$ black holes in the gauged Kaluza-Klein (KK) theory where the KK vector field is gauged such that the fermionic fields are charged under the U(1)$_{\text{KK}}$ gauge group. The black hole solutions are asymptotic to the AdS$_D$ geometry and can be realized as the dimensional reduction of the gauged supergravities on the compact internal manifolds, leading to the restriction as $4\leq D\leq 7$. By studying the behavior of the generalized free energy under the change of the ensemble temperature, we determine the thermodynamic phases and the corresponding phase transitions of black holes. This is confirmed by investigating the heat capacity at the constant pressure and the on-shell free energy. In the canonical ensemble, the thermodynamics of black holes can be classified into three different classes as follows: (i) $D=4$, (ii) $D=5$, and (iii) $D=6,7$. Whereas, in the grand canonical ensemble, the thermodynamics of black holes is independent of the number of spacetime dimensions and the pressure, but depends on the chemical potential $Φ$. The thermodynamic behavior of black holes can be classified into three different classes as follows: (i) $Φ<1$, (ii) $Φ>1$, and (iii) $Φ=1$.

gr-qc↗

arXiv2023

Phenomenology of a minimal extension of the standard model with a family-dependent gauge symmetry

We consider a gauge symmetry extension of the standard model given by $SU(3)_C\otimes SU(2)_L\otimes U(1)_X\otimes U(1)_N\otimes Z_2$ with minimal particle content, where $X$ and $N$ are family dependent but determining the hypercharge as $Y=X+N$, while $Z_2$ is an exact discrete symmetry. In our scenario, $X$ (while $N$ is followed by $X-Y$) and $Z_2$ charge assignments are inspired by the number of fermion families and the stability of dark matter, as observed, respectively. We examine the mass spectra of fermions, scalars, and gauge bosons, as well as their interactions, in presence of a kinetic mixing term between $U(1)_{X,N}$ gauge fields. We discuss in detail the phenomenology of the new gauge boson and the right-handed neutrino dark matter stabilized by $Z_2$ conservation. We obtain parameter spaces simultaneously satisfying the recent CDF $W$-boson mass, electroweak precision measurements, particle colliders, as well as dark matter observables, if the kinetic mixing parameter is not necessarily small.

hep-ph↗

arXiv2023

Implications for the hierarchy problem, inflation and geodesic motion from fiber fabric of spacetime

In this paper, we represent a resolution for the hierarchy problem where the inverse size of the extra dimension and the fundamental Planck scale would all be of the order of the TeV scale by proposing a fiber fabric of spacetime. The origin of the large hierarchy is essentially due to the $\cosh$ function which has a physical origin from the dynamics of the horizontal metric in the vacuum of non-zero energy. In addition, the fiber fabric of spacetime allows us to resolve elegantly and naturally the problems of the chirality fermions and stabilizing potential for the size of the extra dimension, which are usually encountered in the higher dimensional theories. Then, we explore the inflation with the modulus of the extra dimension identified as the inflaton where our slow-roll inflationary model belongs to the E-model class with $n=1$. We calculate the main inflationary observables which are consistent with the present experiments. Finally, we study how the geodesic motion of neutral test particles gets modified from the extension of spacetime. We compute the radius of the photon sphere, the innermost stable circular orbit, the perihelion shift, the light bending angle, and the observables of the strong gravitational lensing and the retrolensing phenomenon. By comparing the predicted values with the experimental observations, we determine the constraints on the fiber fabric of spacetime.

hep-th↗

arXiv2023

Topology in thermodynamics of regular black strings with Kaluza-Klein reduction

We study the topological defects in the thermodynamics of regular black strings (from a four-dimensional perspective) that is symmetric under the double Wick rotation and constructed in the high-dimensional spacetime with an extra dimension compactified on a circle. We observe that the thermodynamic phases of regular black strings can be topologically classified by the positive and negative winding numbers (at the defects) which correspond to the thermodynamically stable and unstable branches. This topological classification implies a phase transition due to the decay of a thermodynamically unstable regular black string to another which is thermodynamically stable. We confirm these topological properties of the thermodynamics of regular black strings by investigating their free energy, heat capacity, and Ruppeiner scalar curvature of the state space. The Ruppeiner scalar curvature of regular black strings is found to be always negative, implying that the interactions among the microstructures of regular black strings are only attractive.

gr-qc↗

arXiv2023

Probing an emergent $U(1)$ extension of the Standard Model at colliders

We explore the potential of probing for a new neutral gauge boson that emerges from a topologically nontrivial structure of spacetime, focusing on its couplings to the fermions of the Standard Model. We analyze the current experimental constraints on the mass of the new gauge boson and the radius of the fifth dimension, using the LEP bound and the LHC with 140 $\text{fb}^{-1}$ luminosity. In addition, we investigate the indirect search of the new gauge boson and its discrimination from other hypothetical gauge bosons like those predicted in the $U(1)_{B-L}$ and $U(1)_R$ models by studying the forward-backward, left-right, and left-right-forward-backward asymmetries.

hep-ph↗

arXiv2023

Compactified extra dimension and entanglement island as clues to quantum gravity

We show that the compactified extra dimension and the emergence of the island can provide clues about quantum gravity because their combination can solve the deepest puzzles of black hole physics. Suppose that the time dimension and the extra dimension compactified on a circle are symmetric under \emph{double Wick rotation}, the curvature singularity would be removed due to the end of spacetime as a smooth bubble hidden behind the event horizon. The smooth bubble geometries can also be interpreted as microstates leading to the Bekenstein-Hawking entropy because the smooth bubble geometries live in the same region of mass and charge as the black string. In addition, by applying the quantum extremal surface prescription, we show the emergence of the island at late times of the black string evaporation where it is located slightly outside the event horizon. Due to the dominant contribution of the island configuration, the entanglement entropy of the radiation grows no longer linearly in time but it reaches a finite value that is twice the Bekenstein-Hawking entropy at the leading order. This transition shows the information preservation during the black string evaporation. Furthermore, we calculate the Page time which determines the moment of the transition between the linearly growing and constant behaviors of the entanglement entropy as well as the scrambling time corresponding to the information recovery time of the signal falling into the black string.

hep-th↗

arXiv2023

Radiative stability of tiny cosmological constant from the Swampland and quantized compactification

We address a quantization mechanism that can allow us to understand why the cosmological constant is not large under the quantum corrections from studying the circle compactification solution of the Standard Model coupled to Einstein gravity which is subject to the constraint of the Swampland conjectures. A novel result in the present work compared to the previous investigations in the literature is that the radius of the compactified dimension and the 4D cosmological constant $Λ_4$ must in fact be quantized. The quantization rule of the cosmological constant is given by $Λ_4\propto n^2$ with $n=1,2,3,4,\ldots$, which means that the values of $Λ_4$ are not arbitrary but only its specific values are allowed. In general, the quantum corrections as well as other effects would break this quantization rule. Hence, it could prevent the quantum fluctuations from generating zero-point energy contributions to the cosmological constant.

hep-th↗

arXiv2023

4D dS vacua from AdS vacua of type IIB string theory and AdS distance conjecture

In order for string theory to be made compatible with the low-energy observations of a positive cosmological constant, there have been attempts to construct dS vacua in string theory which are particularly difficult to realize. Instead of attempting to find de Sitter (dS) vacuum solutions, we point to a new way to make string theory consistent with low-energy dS cosmology. In this way, string theory lives in an anti-de Sitter (AdS) vacuum (which is simple to construct) that exists only in the high-energy regime; however, as going to the low-energy scales where the heavy string excitations and Kaluza-Klein modes are integrated out, we show that the effective picture of string theory in lower dimensions would exhibit a 4D dS vacuum without needing to add additional structures such as anti-D3 branes. Additionally, we point to evidence from bottom-up physics for the strong version of the AdS distance conjecture realized from AdS vacua in string theory. This evidence hence supports the sharpening of the AdS distance conjecture as one of the universal features of quantum gravity.

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