Lepton flavor violation (LFV) offers a powerful probe of physics beyond the Standard Model, particularly in models addressing neutrino masses and the baryon asymmetry of the universe. In this study, we investigate LFV processes within the framework of type II seesaw leptogenesis, where the Standard Model is extended by an $S U(2)_L$ triplet Higgs field. We focus on key LFV processes, including $\mu^+\to e^+\gamma$ , $\mu^+ \to e^+e^-e^+$ , and $\mu \rightarrow e$ conversion in nuclei, deriving stringent constraints on the parameter space from current experimental data. We scan the 3$\textit{σ}$ range of neutrino oscillation parameters and identify the most conservative bounds consistent with existing measurements. Our results reveal that the MEG experiment currently provides the most stringent constraints in the normal ordering (NO) scenario, while the SINDRUM experiment offers comparable sensitivity in the inverted ordering (IO) case. Future experiments, such as MEG II, Mu3e, Mu2e, and COMET, are predicted to significantly improve the sensitivity, testing larger regions of the parameter space.

China

Chengcheng Han

Yijun Han

Sihui Huang

Zhanhong Lei

Probing type II seesaw leptogenesis through lepton flavor violation * Chengcheng Han acknowledges supports from the National Key R&D Program of China (2023YFA1606100), the Sun Yat-Sen University Science Foundation, the Fundamental Research Funds for the Central Universities at Sun Yat-sen University (24qnpy117) and the Key Laboratory of Particle Astrophysics and Cosmology (MOE) of Shanghai Jiao Tong University

The deflection of light in the gravitational field of a massive body can be analyzed through diverse theoretical approaches. The null geodesic approach is commonly employed to calculate light deflection within strong and weak field limits. Alternatively, several studies have explored the gravitational deflection of light using the material medium approach. For a static, non-rotating spherical mass, the deflection in a Schwarzschild field can be determined by expressing the metric in an isotropic form and evaluating the refractive index to trace the light ray’s trajectory. In this study, we extend the above-mentioned approach to the Kerr–Sen black hole spacetime in heterotic string theory, a solution representing a rotating, charged solution in heterotic string theory. The frame-dragging effects inherent to the Kerr–Sen geometry are incorporated to compute the velocity of light rays, enabling the derivation of the refractive index in this field. Considering the far-field approximation, we calculate the deflection of light in the Kerr–Sen spacetime and compare our results with those obtained for the Kerr and Schwarzschild black hole solution in GR.

India

Saswati Roy

Shubham Kala

Atanu Singha

South Africa

Hemwati Nandan

Asoke Sen

Deflection of light due to Kerr Sen black hole in heterotic string theory using material medium approach

This paper investigates the observable properties of a Kerr–Sen black hole surrounded by a thin accretion disk, focusing on the impact of the black hole’s spin and charge on the image. Using ray-tracing techniques, we conduct a detailed analysis of the black hole’s image, redshift distribution, and intensity distributions at different observation frequencies. The results demonstrate that spin has a more significant effect on the distortion of the inner shadow than charge, and the observer’s inclination angle plays a critical role in shaping the redshift distribution, especially near the innermost stable circular orbit. Additionally, the intensity is found to be higher at 86 GHz than at 230 GHz. This study highlights the crucial role of the accretion disk’s geometry in determining the black hole’s image and redshift effects, thereby providing a refined theoretical framework to guide future observational efforts targeting the Kerr–Sen black hole and its electromagnetic signals.

Pei Wang

Sen Guo

Li-Fang Li

Zhan-Feng Mai

Bo-Feng Wu

Wen-Hao Deng

Qing-Quan Jiang

Optical images of the Kerr–Sen black hole and thin accretion disk

The present study unfolds the relative importance of <math altimg='si1.svg'><mi>N</mi><mo stretchy='false'>/</mo><mi>Z</mi></math>, shell effects, and thermal fluctuations in shaping the temperature dependence of the isovector giant dipole resonance (IVGDR) width. To this end, we measured the <italic>&#947;</italic>-ray spectra from excited <math altimg='si2.svg'><mmultiscripts><mrow><mi mathvariant='normal'>Zn</mi></mrow><mprescripts /><none /><mrow><mn>62</mn><mo>,</mo><mn>68</mn></mrow></mmultiscripts></math> populated through 4He <math altimg='si3.svg'><msup><mrow><mo linebreak='badbreak' linebreakstyle='after'>+</mo></mrow><mrow><mn>58</mn><mo>,</mo><mn>64</mn></mrow></msup></math>Ni reactions at beam energies of 28 MeV and 40 MeV, and compared the corresponding IVGDR widths with those for nuclei close to the doubly magic 56Ni and moderately away from it. The large-area modular BaF <inf>2</inf> detector array was used to detect high-energy <italic>&#947;</italic> rays ( <math altimg='si4.svg'><msub><mrow><mi>E</mi></mrow><mrow><mi>&#947;</mi></mrow></msub><mo linebreak='goodbreak' linebreakstyle='after'>&gt;</mo><mn>4</mn></math> MeV). The Bayesian inference approach was combined with the statistical model analysis to extract the IVGDR parameters from the measured spectra. Calculation of IVGDR widths was performed using various theoretical approaches, specifically the thermal shape fluctuation model (TSFM) with microscopic energy density functional inputs. TSFM calculation was also performed using the free energy surfaces from the deformed liquid drop model. For 68Zn, the width is observed to increase closely following the TSFM prediction. In contrast, suppression of the width is noticed for 62Zn at low temperatures, similar to other nearby nuclei with <italic>N</italic> and/or <italic>Z</italic> closer to 28. Moreover, 68Zn indicates an early onset of the saturation in the IVGDR width, mimicking structural changes appropriately comprehended by the microscopic calculations. These findings suggest that the relative influence of microscopic effects and thermal broadening in the IVGDR width is strongly governed by the proximity of the decaying nucleus to magicity.

Chandrani Sen

Debasish Mondal

Gourab Banerjee

S. Mukhopadhyay

Jhilam Sadhukhan

Deepak Pandit

Surajit Pal

Sujan Kumar Roy

Balaram Dey

Srijit Bhattacharya

A. De

Pratap Roy

T.K. Rana

Rajkumar Santra

Saumanti Sadhukhan

Study of the giant dipole resonance around the magic number 28

We investigate the stability of test scalar fields in the region inside the extremal dyonic Kerr–Sen black hole (DKSBH) horizon, where closed timelike curves exist. We successfully find and present the novel exact solutions to the Klein–Gordon equation in the extremal DKSBH spacetime in terms of the double confluent Heun functions. The spacetime stability is explored by investigating the scalar’s quasiresonance (QS) frequencies obtained from polynomial condition of the double confluent Heun function. We found that both massive and massless scalar quasiresonances are double branched, having purely positive and negative imaginary frequencies, therefore, do not propagate, prohibiting time travel and suggesting no violation of Hawking’s Chronology Protection Conjecture (CPC). However, only the positive branch with $$0\le \Omega _0<2(n+1)$$ and negative branch with $$\Omega _0>2(n+1)$$ that grow exponentially has the ability to destroy spacetime. Remarkably, a new mass scale $$M_{p}^{2}/M$$ , where $\textit{M}$ is the black hole mass, is found to play a crucial role. The QS zeroth modes flip sign between purely damping and purely growing when the scalar mass is at this mass scale.

Thailand

David Senjaya

Teephatai Bunyaratavej

Piyabut Burikham

Scalar instabilities inside the extremal dyonic Kerr–Sen black hole: novel exact solutions and Chronology Protection Conjecture

The chronology protection conjecture (CPC) was first introduced by Hawking after his semi-classical investigation of the behaviour of a spacetime with closed timelike curves (CTCs) in response to scalar perturbations. It is argued that there would be instabilities leading to amplification of the perturbation and finally causing collapse of the region with CTCs. In this work, we investigate the CPC by exactly solving the Klein–Gordon equation in the region inside the inner horizon of the non-extremal Dyonic Kerr–Sen (DKS) black hole, where closed timelike curves exist. Successfully find the exact radial solution, we apply the polynomial condition that turns into the rule of energy quantization. Among the quasi-resonance modes, only certain modes satisfy the boundary conditions of quasinormal modes (QNMs). QNMs in the region inside the inner horizon of the rotating black hole with nonzero energy have only positive imaginary parts which describe states that grow in time. The exponentially growing modes will backreact and deform the spacetime region where CTC exists, hence the CPC is proven to be valid in the non-extremal Dyonic Kerr–Sen black hole spacetime. Since the Dyonic Kerr–Sen black hole is the most general axisymmetric black hole solution of the string inspired Einstein–Maxwell-dilaton-axion (EMDA) theory, the semiclassical proof in this work is also valid for all simpler rotating black holes of the EMDA theory. The structure of the Dyonic KS spacetime distinctive from the Kerr–Newman counterpart is also explored.

Revisiting chronology protection conjecture in the Dyonic Kerr–Sen black hole spacetime

In this study, we investigate the thermodynamic topology of the Kerr-Sen-Ads black hole in restricted phase space. In the restricted phase space, a new parameter, central charge <italic>C</italic>, and its conjugate parameter <italic>&#956;</italic> are introduced, omitting the well-known <italic>PdV</italic> term in the first law of black hole thermodynamics. We study the local and global topology of the black hole by considering the black hole solution as topological defects in the free energy landscape. We compute the winding number and the total topological number at the thermodynamic defects. For our analysis, we have considered five ensembles of Kerr-Sen-Ads black holes in restricted phase space: fixed <math altimg='si1.svg'><mo stretchy='false'>(</mo><mi>Q</mi><mo>,</mo><mi>J</mi><mo>,</mo><mi>C</mi><mo stretchy='false'>)</mo></math>, fixed <math altimg='si2.svg'><mo stretchy='false'>(</mo><mi>&#981;</mi><mo>,</mo><mi>J</mi><mo>,</mo><mi>C</mi><mo stretchy='false'>)</mo></math>, fixed <math altimg='si3.svg'><mo stretchy='false'>(</mo><mi>Q</mi><mo>,</mo><mi mathvariant='normal'>&#937;</mi><mo>,</mo><mi>C</mi><mo stretchy='false'>)</mo></math>, fixed <math altimg='si4.svg'><mo stretchy='false'>(</mo><mi>Q</mi><mo>,</mo><mi>J</mi><mo>,</mo><mi>&#956;</mi><mo stretchy='false'>)</mo></math>, and fixed <math altimg='si5.svg'><mo stretchy='false'>(</mo><mi>&#981;</mi><mo>,</mo><mi mathvariant='normal'>&#937;</mi><mo>,</mo><mi>C</mi><mo stretchy='false'>)</mo></math>, where <italic>Q</italic> is the electric charge, <italic>J</italic> is the angular momentum, <italic>C</italic> is the central charge, <italic>&#981;</italic> is the electric potential conjugate to charge, &#937; is the angular frequency conjugate to <italic>J</italic>, and finally, <italic>&#956;</italic> is the chemical potential. In the fixed <math altimg='si1.svg'><mo stretchy='false'>(</mo><mi>Q</mi><mo>,</mo><mi>J</mi><mo>,</mo><mi>C</mi><mo stretchy='false'>)</mo></math>, fixed <math altimg='si2.svg'><mo stretchy='false'>(</mo><mi>&#981;</mi><mo>,</mo><mi>J</mi><mo>,</mo><mi>C</mi><mo stretchy='false'>)</mo></math>, and fixed <math altimg='si4.svg'><mo stretchy='false'>(</mo><mi>Q</mi><mo>,</mo><mi>J</mi><mo>,</mo><mi>&#956;</mi><mo stretchy='false'>)</mo></math> ensembles, we find a topological charge of +1. In the fixed <math altimg='si3.svg'><mo stretchy='false'>(</mo><mi>Q</mi><mo>,</mo><mi mathvariant='normal'>&#937;</mi><mo>,</mo><mi>C</mi><mo stretchy='false'>)</mo></math> and fixed <math altimg='si5.svg'><mo stretchy='false'>(</mo><mi>&#981;</mi><mo>,</mo><mi mathvariant='normal'>&#937;</mi><mo>,</mo><mi>C</mi><mo stretchy='false'>)</mo></math> ensembles, depending on the values of the thermodynamic parameters, we find topological charges of &#8722;1, 0, and +1. Interestingly, in ensembles where we find the topological charge to be 0, we observe both Hawking-Page and Davies type phase transitions. We show that both types of these phase transitions can be studied using a common vector field, and the topological charges associated with Davies type and Hawking-Page phase transitions are &#8722;1 and +1, respectively.

Bidyut Hazarika

Prabwal Phukon

Topology of restricted phase space thermodynamics in Kerr-Sen-Ads black holes

We consider Klein–Gordon equation in the Dyonic Kerr–Sen black hole background, which is the charged rotating axially symmetric solution of the Einstein–Maxwell–Dilaton–Axion theory of gravity. The black hole incorporates electric, magnetic, dilatonic and axionic charges and is constructed in 3+1 dimensional spacetime. We begin our investigations with the construction of the scalar field’s governing equation, i.e., the covariant Klein–Gordon equation. With the help of the ansatz of separation of variables, we successfully separate the polar part, and find the exact solution in terms of Spheroidal Harmonics, while the radial exact solution is obtained in terms of the Confluent Heun function. The quantization of the quasibound state is done by applying the polynomial condition of the Confluent Heun function that gives rise to discrete complex-valued energy levels for massive scalar fields. The real part is the scalar field relativistic quantized energy, while the imaginary part represents the quasibound states’s decay. We present all of the sixteen possible exact energy solutions for both massive and massless scalars. We also present the investigation the Hawking radiation of the Dyonic Kerr–Sen black hole’s apparent horizon, via the Sigurd–Sannan method by making use of the obtained exact scalar wave functions. The radiation distribution function, and the Hawking temperature are also obtained.

Romania

Tiberiu Harko

The exact relativistic scalar quasibound states of the dyonic Kerr–Sen black hole: quantized energy, and Hawking radiation

The Sen formulation for chiral (2p)-form in $$4p+2$$ <math> <mrow> <mn>4</mn> <mi>p</mi> <mo>+</mo> <mn>2</mn> </mrow> </math> dimensions describes a system with two separate sectors, one is physical while the other is unphysical. Each contains a chiral form and a metric. In this paper, we focus on the cases where the self-duality condition for the unphysical sector is linear while for the physical sector can be nonlinear. We show the decoupling at the Hamiltonian and Lagrangian levels. The decoupling at the Hamiltonian level follows the idea in the literature. Then by an appropriate field redefinition of the corresponding first-order Lagrangian, the separation at the Lagrangian level follows. We derive the diffeomorphism of the theory in the case where the chiral form in the physical sector has nonlinear self-dual field strength and couples to external $$(2p+1)$$ <math> <mrow> <mo>(</mo> <mn>2</mn> <mi>p</mi> <mo>+</mo> <mn>1</mn> <mo>)</mo> </mrow> </math> -form field. Explicit forms of Sen theories are also discussed. The Lagrangian for the quadratic theory is separated into two Henneaux–Teitelboim Lagrangians. We also discuss the method of generating explicit nonlinear theories with $$p=1$$ <math> <mrow> <mi>p</mi> <mo>=</mo> <mn>1</mn> </mrow> </math> . Finally, we also show that the M5-brane action in the Sen formulation is separated into a Henneaux–Teitelboim action in unphysical sector and a gauge-fixed PST in physical sector.

Sujiphat Janaun

Anajak Phonchantuek

Pichet Vanichchapongjaroen

Nonlinear chiral forms in the Sen formulation

In the present study, we investigate the topological properties of black holes in terms of Rényi statistics as an extension of the Gibbs-Boltzmann (GB) statistics, aiming to characterize the non-Boltzmannian thermodynamic topology of Kerr-Sen and dyonic Kerr-Sen black holes. Through this research, we discover that the topological number derived via Rényi statistics differs from that obtained through GB statistics. Interestingly, although the nonextensive parameter λ changes the topological number, the topological classification of the Kerr-Sen and dyonic Kerr-Sen black holes remains consistent under both GB and Rényi statistics. In addition, the topological numbers associated with these two types of black holes without cosmological constant using Rényi entropy processes are the same as the AdS cases of them by considering the GB entropy, as further evidenced by such a study found here. This indicates the cosmological constant has some potential connections with the nonextensive parameter from the perspective of thermodynamic topology.