The Sen action for self-dual fields has been generalised by Hull to include two metrics which allows it to be defined on generic manifolds. In this paper we consider Kaluza-Klein compactifications of this action. The existence of two metrics presents novel challenges as there are two Kaluza-Klein towers of fields. We show that to find a consistent truncation one must include zero-modes associated to each of the two towers. Although this naively leads to a doubling of the massless degrees of freedom we show that on-shell this is not the case. We also discuss a deformation of the Sen action to include an additional form-field but which does not lead to new degrees of freedom on-shell but which arises naturally upon compactification.

United Kingdom

Neil Lambert

Yuchen Zhou

Compactifying the Sen Action: Six Dimensions

The weak decays of charmonium states such as $ J/\psi $ and $ \psi(2S) $ are instrumental in probing both nonperturbative QCD dynamics and the flavor structure of the Standard Model (SM). The extreme rarity of charmonium weak decays renders them highly sensitive to physics beyond the SM, particularly in channels that are heavily suppressed in the SM, such as flavor-changing neutral-current (FCNC) decays. This review highlights the critical role of the BESIII experiment, which leverages an unprecedented data sample of over $ 10^{10} $ $ J/\psi $ and $ 2.7\times10^{9} $ $ \psi(2S) $ events to achieve leading sensitivity in searches for charmonium weak decays. We present the latest and most stringent upper limits established by BESIII on various semileptonic, nonleptonic, and FCNC charmonium weak decay channels.

China

Xu-Ze Li

Kai-Xin Fan

Zheng-Yun You

Yu Zhang

Minggang Zhao

Mini-review of charmonium weak decays at BESIII * Supported in part by the National Key R&D Program of China (2023YFA1606000); the National Natural Science Foundation of China (NSFC) (12035009, 12175321, U1932101), and the National College Students' Science and Technology Innovation Project of Sun Yat-sen University

We constructed a four-Higgs-doublet model (4HDM) invariant under $\textit{D}$ $_{5}$ symmetry and investigated its complete neutral vacuum structure in detail. Assuming explicit CP conservation in the scalar potential, we examined whether CP symmetry can be spontaneously broken. We provided a complete list of all possible real and complex vacua, along with the constraints on the potential parameters required for each vacuum solution to exist. We also discussed the positive-definiteness conditions that the Hessian must satisfy for each vacuum to be a local minimum of the potential. The results show that, after spontaneous symmetry breaking, some complex vacua lead to spontaneous CP violation in the potential, whereas the remaining complex vacua still preserve CP conservation. Among these CP-violating complex vacua, one can be regarded as the most general form. Furthermore, we discussed the relationship between real and complex vacua.

Dong-Ping Fu

Michihisa Takeuchi

Spontaneous CP violation in the $\textit{D}$ $_{5}$-symmetric four-Higgs-doublet models * Supported by the Fundamental Research Funds for the Central Universities, the One Hundred Talent Program of Sun Yat-sen University, China, and the Guangdong Natural Science Foundation (2026A1515012641)

In the present work, we numerically investigate the horizon structure of the Kerr–Sen black holes in anti-de Sitter (AdS) spacetime. Further, we investigate the phase transitions and critical phenomena in Kerr–Sen–AdS black holes at the critical points. Such black holes are characterized by its mass ($\textit{M}$), the dilaton charge ($\textit{Q}$), and the negative cosmological constant, $$\Lambda (<0)$$ . We define a dimensionless parameter $$\epsilon =\bar{J}/{\bar{Q}^2}$$ and express the mass, temperature, volume, and Gibbs free energy in terms of $$\epsilon $$ and its polynomials. Moreover, we numerically fit the data for the critical points and find that in the appropriate limit, the expressions for critical points would correspond to the respective critical points of the Kerr–AdS black hole thermodynamics. Such a study involves a systematic analysis of temperature, Gibbs free energy, and volume in the extended phase space. We provide an analytical verification of the nature of the phase transitions at the critical points by introducing the Ehrenfest equations. We find that the Ehrenfest equations are satisfied in accordance with the phase behaviour. We also check that all three quantities, e.g., the specific heat at constant pressure, $$C_P$$ , the volume expansion coefficient, $$\alpha $$ , and the isothermal compressibility, $$\kappa _T$$ , diverge at the critical points. We find the $\textit{Prigogine}$-$\textit{Defay}$ ratio using the expressions of $$C_P$$ , $$\alpha $$ , and $$\kappa _T$$ , and find that it identically equals unity. Hence, the phase transition behavior of the Kerr–Sen–AdS black holes at their critical points is of second order. In addition, we propose investigating the energy extraction process via the Penrose process by deriving work efficiency. Later, we calculate the speed of sound and adiabatic compressibility for the rotating Kerr–Sen–AdS black holes. Finally, on a specific note, we calculate the thermodynamic quantities of the boundary conformal field theory (CFT) dual to the extended phase space.

India

Md Ali

Arindam Mondal

South Africa

Sushant Ghosh

The extended phase space thermodynamics and Ehrenfest scheme for the Kerr–Sen AdS black holes

<p>Four-dimensional chiral gauge theory can be formulated as the boundary theory on a five-dimensional manifold in a manner that may be realized on a finite lattice. There are interesting features of these theories that defy a purely four-dimensional conception of universality. We find that QCD when embedded in a chiral gauge theory (the Standard Model) and regulated this way can simultaneously avoid both the <math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><mi>U</mi><mo stretchy="false">(</mo><mn>1</mn><msub><mo stretchy="false">)</mo><mi>A</mi></msub></math> problem and the strong <math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><mi>C</mi><mi>P</mi></math> problem, with a central role played by fermion zero modes localized far away in the fifth dimension. In this way it differs from conventional lattice QCD formulated as a stand-alone theory, universality being violated by inaccessible light modes in the five-dimensional bulk. Our analysis builds on recent work by others that highlights the role of global <math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><mi>U</mi><mo stretchy="false">(</mo><mn>1</mn><mo stretchy="false">)</mo></math> symmetries in five-dimensional formulations of four-dimensional chiral gauge theories, and the generic appearance of fermion zero modes in the bulk.</p>

United States

David B. Kaplan

Srimoyee Sen

Regulated chiral gauge theory and the strong <math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><mi>C</mi><mi>P</mi></math> problem

In this paper, we examine the dynamics of test particles in the Kerr-Sen black hole (BH) spacetime and its properties. In this case, the study concentrates on equatorial circular orbits and their stability properties using the effective potential formalism approach. We derive conditions for stable circular motion and examine the influence of BH parameters on the observed dynamics properties of the orbital structure. Furthermore, we investigate small harmonic perturbations around circular orbits and calculate the associated oscillation frequencies measured by distant asymptotic observers. In this context, these frequencies provide a direct probe of the underlying geometry and deviation from the Kerr limit. We also perform a numerical analysis to illustrate the dependence of orbital stability and oscillatory behavior on the rotation and charge parameters of the Kerr–Sen solution. In addition, our general relativistic hydrodynamical simulations of two-dimensional equatorial planar accretion show that increasing the Kerr–Sen spacetime parameter <italic>q</italic> enhances the mass-accretion rate and modifies the local density and radial-velocity profiles near the black hole, with the strongest deviation from the Kerr case appearing for the largest value of <italic>q</italic>.

Shahid Hussain Gurmani

Kuwait

Orhan Donmez

Algeria

Abdelmalek Bouzenada

Imtiaz Khan

Azerbaijan

G. Mustafa

Equatorial circular orbits, epicyclic frequencies, and planar accretion in Kerr–Sen black hole spacetimes

We study the equation of state (EoS) of QCD matter in a bottom-up holographic setup that combines an Einstein-Maxwell-dilaton (EMD) sector with an improved Karch-Katz-Son-Stephanov (KKSS) flavor action. In the probe approximation, we perform an inverse reconstruction of the model functions by parameterizing them with neural networks and solving the EMD equations via a differentiable ODE solver (a neural ODE framework), calibrating the model to a reference thermodynamic table for $(2+1)$ -flavor QCD at finite temperature and finite baryon chemical potential. The reconstructed model functions are then parameterized and kept fixed across thermodynamic states. Next, viewing the EMD sector as an effective description of pure Yang-Mills theory, we fix its parameters by fitting the $\mu_B=0$ lattice pure-glue EoS using a hybrid optimization strategy. Finally, we go beyond the probe limit and solve the coupled EMD+KKSS equations with back-reaction, using the pure-glue-calibrated EMD sector as a fixed input and varying the KKSS couplings to compare with the $\mu_B=0$ two-flavor lattice EoS. We find a visible mismatch and a high-temperature behavior in which the back-reacted dimensionless ratios approach a nearly $\beta_1$ -insensitive plateau close to the pure-glue baseline, providing a simple structural diagnostic for the present flavor-sector truncation.

Yutian Deng

Mei Huang

Lin Zhang

Holographic QCD equation of state constrained by lattice QCD: neural-ODE for probe-limit and a back-reaction test * Start-up funding from Sun Yat-sen University (SYSU) (Y. Deng and L. Zhang); National Natural Science Foundation of China (NSFC) (12235016) (L. Zhang and M. Huang) and 12221005 (M. Huang)

We analytically investigate scalar superradiant scattering in the rotating dyonic Kerr–Sen black hole of Einstein–Maxwell-dilaton–axion theory. Starting from the separable Klein–Gordon equation for a massive neutral scalar field, we work in the low-frequency and slow-rotation regime and employ the analytical asymptotic matching (AAM) method to compute the reflection coefficient and the associated superradiant amplification factor. Since an exact global scattering solution is not available in this four-charge geometry, the AAM framework enables a controlled analytic treatment of the near-and far-region dynamics. We provide detailed and systematic derivations of the matching procedure leading to the closed-form amplification formula. The superradiant condition is obtained explicitly and we demonstrate that energy extraction occurs exclusively for co-rotating modes satisfying $$\Omega < m \Omega _H$$ . We show that the presence of electric and magnetic charges suppresses the amplification relative to the Kerr limit, whereas lighter co-rotating scalar fields broaden the superradiant window and enhance the efficiency of rotational energy extraction.

Thailand

Supakorn Katewongveerachart

David Senjaya

Dyonic Kerr–Sen black hole’s resonant scattering: absorption and superradiance

In this study, we investigate the possibility of enhancing the di-jet resonance searches by tagging the final state radiation (FSR) jet using an event-level deep neural network. It is found that solely relying on the 4-momenta of the leading three jets allows the algorithm to achieve good discriminating power that can identify the hardest FSR jet in the signal while rejecting other soft jets. Once the invariant mass is corrected with the tagged FSR jet, the mass resolution of the signal is greatly enhanced, and the sensitivity of the search is also improved by more than 10%. By crafting the input variables carefully, the algorithm introduces minimal mass sculpting for the background, and its applicability extends to a broad mass range. This work proves that FSR jet tagging can potentially enhance di-jet resonance searches, suiting various stages of the physics programmes at the Large Hadron Collider (LHC) and High-Luminosity LHC (HL-LHC).

Bingxuan Liu

Yuxuan Shen

Yuanshunzi Sui

Enhancing di-jet resonance searches via a final-state radiation jet tagging algorithm * Bingxuan Liu is supported by Shenzhen Campus of the Sun Yat-sen University under project 74140-12255011, and by the Young Scientists Fund (C Class) of the National Natural Science Foundation of China (12405122). Both Yuxuan Shen and Yuanshunzi Sui are supported by College Students’ Innovative Entrepreneurial Training Plan Program

This study presents a detailed investigation of the dynamics of massless test particles (photons) to analyze the stability of null circular geodesics around the Kerr–Sen black hole (KSBH) spacetime. KSBH is a new solution arising in the low-energy effective field theory of heterotic string theory. The analysis proceeds by obtaining the radial effective potential, which serves as the foundation for calculating the Lyapunov exponent, a central quantity used to describe the stability or instability of geodesic motion in dynamical systems. The study employs the Lyapunov exponent approach to determine the stability or instability of circular orbits under varying values of the charge parameter $\textit{Q}$ and Kerr parameter $\textit{a}$. Additionally, the analytical expressions for the radius of circular photon orbits and the corresponding impact parameter, defined as the ratio of conserved angular momentum to energy are also derived explicitly. To further analyze the orbital behavior, techniques such as Poincaré maps, the Lyapunov Exponent, and FLI are utilized to distinguish between regular and chaotic trajectories. The sign and magnitude of the Lyapunov exponent provide a rigorous criterion to distinguish between stable and unstable circular orbits. These results contribute to a deeper understanding of the geodesic structure and perturbative stability in string-inspired rotating charged black hole backgrounds. The findings are also compared with those of the KBH case to highlight the impact of the BH charge on the stability of null geodesics. This study provides deeper insights into the dynamical properties of null geodesics in charged rotating BH spacetimes in GR and alternative theory of gravity.

Pradeep Singh

Hemwati Nandan

Shubham Kala

Nidhi Handa

Lokesh Joshi

Probing strong-gravity chaos and stability of circular photon orbits around the Kerr–Sen black hole