We study classical background electric fields and the Schwinger effect in de Sitter space. We show that a constant electric field in de Sitter requires the photon to have a tachyonic mass proportional to the Hubble scale. This has physical implications for the induced Schwinger current that affect its IR behaviour. To study this, we recompute the Schwinger current in de Sitter space for charged fermions and minimally coupled scalars, imposing a physically consistent renormalization condition. We find a $\textit{finite and positive}$ Schwinger current even in the massless limit. This is in contrast to previous calculations in the literature, which found a negative IR divergence. We also obtain the first result for the Schwinger current of a non-minimally coupled scalar, including the conformally coupled case, which we find exhibits behaviour very similar to that of the fermion current. Our results may have physical implications for both magnetogenesis and inflationary dark matter production.

Spain

Mar Bastero-Gil

Portugal

Paulo B. Ferraz

António Torres Manso

Slovenia

Italy

Lorenzo Ubaldi

Roberto Vega-Morales

Schwinger current in de Sitter space * This work has been partially supported by Junta de Andalucía Projects P21-00199 and A-FQM-472-UGR20 (FEDER funds); SRA (10.13039/501100011033) and ERDF (PID2022-139466NB-C21) (R.V.M.); PID2022-140831NB-I00, funded by MICIU/AEI/10.13039/501100011033 and FEDER, UE (M.B.G., A.T.M.); by FCT CERN (10.54499/2024.00252.CERN) (M.B.G., A.T.M., P.F.); FCT Grant No. SFRH/BD/151475/2021 and 10.54499/SFRH/BD/151475/2021 (P.F.). LU is supported by the Slovenian Research Agency under research core funding No. P1-0035 and by research grants J1-60026 and J1-4389. This article is based upon work from COST Action COSMIC WISPers CA21106, supported by COST (European Cooperation in Science and Technology). R.V.M. thanks the Mainz Institute for Theoretical Physics (MITP) of the Cluster of Excellence PRISMA$^{+}$ (Project ID 390831469) for its hospitality and partial support during the completion of this work

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 investigate the direct $\rm CP $ violation in the decay $ D^\pm \to \pi^\pm \pi^+ \pi^- $ incorporating the $ a_0^0(980) $ - $ f_0(980) $ mixing mechanism. The integrated mixing intensities $ \overline \xi_{fa} $ and $ \overline \xi_{af} $ are calculated using meson masses and coupling constants extracted from various theoretical models and experimental data, yielding values of appreciable magnitude. We find that when the invariant mass of the $ \pi^+\pi^- $ pair lies near the $ f_0(980) $ resonance, this isospin-breaking mechanism can enhance the $\rm CP $ asymmetry. The enhancement is particularly pronounced when the $ f_0(980) $ carries a significant $ n\bar{n} $ quark component and the $ f_0(980) $ and $ \sigma(600) $ mixing angle is approximately $ 26^\circ $ . After accounting for non-factorizable effects, we find these corrections tend to partially cancel the leading-order contributions, resulting in a suppression of the $\rm CP $ violations relative to the naive factorization predictions. It is emphasized that the $ a_0^0(980) $ - $ f_0(980) $ mixing mechanism should be taken into account in both theoretical and experimental studies of $\rm CP $ violation in $\textit{B}$ or $\textit{D}$ meson decays.

Shi-Ji Cao

Jing-Juan Qi

Yi-Fan Zhao

Chao Wang

Zhen-Yang Wang

Xin-Heng Guo

Direct CP violation in ${ {\boldsymbol D}^{\bf\pm }\to {\boldsymbol\pi}^{\bf\pm} {\boldsymbol\pi}^{\bf +} {\boldsymbol\pi}^{\bf -}} $ with ${{\boldsymbol a}_{\bf 0}^{\bf 0}{\bf (980)-}{\boldsymbol f}_{\bf 0}{\bf (980)}} $ mixing * Supported by the National Natural Science Foundation of China (12405115, 12105149, 11805153, 12275024), and the High-Level Scientific Research Fund of Ningbo University (GJPY2026032)

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, the strong decays of the discovered $ P_c(4380) $ , $ P_c(4440) $ , $ P_c(4457) $ and their possible isospin cousins are systematically studied via the assignment that they are meson-baryon molecular states. In detail, the strong decay constants and partial decay widths of their decay channels are calculated under the framework of QCD sum rules. The decay widths of the discovered $ P_c(4380) $ , $ P_c(4440) $ , and $ P_c(4457) $ are in good agreement with the experiments. The predictions of the decays of these three related possible isospin cousins are presented, which would shed light on their findings in experiments. In return, this may testify to the assignments of the discovered $ P_c $ states.

Xiu-Wu Wang

Xin Li

Zhi-Gang Wang

Systematic study of the strong decays of the $\textit{P}$ $\textit{c}$ states and their possible isospin cousins via the QCD sum rules * Supported by the National Natural Science Foundation of China (12575083) and the Natural Science Foundation of Hebei Province (A2024502002)

In this work, we investigate three representative new-physics resonances that couple to Standard Model (SM) quarks through flavor-changing interactions involving the top quark. We identify the possible SMEFT operators at the electroweak scale and analyze their phenomenology.

Min Huang

Yandong Liu

Hao Zhang

Exploring new resonances with direct top flavor changing interactions * Min Huang and Hao Zhang is supported in part by the National Science Foundation of China (12575114, 12235001, 12075257)

We present a comprehensive study of warm hybrid inflation within the framework of $\textit{α}$-attractor models, where an axionic inflaton is coupled to a waterfall field in the presence of thermal dissipation. The model is analyzed for both linear ( $ \Upsilon \propto T $ ) and cubic ( $ \Upsilon \propto T^{3} $ ) dissipation regimes. Confronting the theoretical predictions with the latest observational data from Planck+BICEP/Keck, P-ACT-LB-BK18, and SPT, we find that in the weak dissipative regime ( $ Q_{*} \lesssim 10^{-5} $ ), the scalar spectral index $ n_{s} \simeq 0.965 $ lies at the boundary of the combined P-ACT-LB-BK18 constraints, while the tensor-to-scalar ratio $\textit{r}$ remains within observable ranges. For stronger dissipation ( $ Q_{*} \gtrsim 10^{-2} $ ), the model predicts values of $ n_{s} $ well within the $ 1-2\sigma $ confidence region of all datasets, with tensor modes remaining fully observable in both dissipation scenarios. These results indicate that forthcoming CMB polarization experiments may be capable of detecting primordial gravitational waves, thereby providing a robust observational test of warm hybrid inflation across different dissipative regimes.

Waqas Ahmed

Pakistan

Waqar Ahmad

Ahsan Illahi

M. Junaid

Warm hybrid axion inflation in $\textit{α}$-attractor models constrained by ACT and future plan experiments

Hadron-hadron interactions, being nonperturbative in nature, play a significant role in addressing phenomenological questions in particle physics. Femtoscopy is a powerful tool in heavy-ion collision experiments, enabling the extraction of hadron-hadron interactions via momentum-correlation functions (CFs). These CFs are typically expressed as a convolution of source functions and hadron-hadron wave functions, with the latter encoding information about the interactions. However, source functions remain poorly constrained and are commonly approximated by a Gaussian form. Reconstructing source functions from experimental correlation data constitutes an 'inverse problem.' To address this, we propose a toy model based on Tikhonov regularization. Using a square-well potential with four distinct strengths, we calculate the CFs for inputs of single Gaussian source functions and mixed Gaussian source functions. The resulting CFs are then used to reconstruct the source functions via Tikhonov regularization. Our results show that the Gaussian source function can be successfully reconstructed, highlighting the potential of this approach for extracting realistic source functions from hadron pairs of interest.

Ao-Sheng Xiong

Qi-Wei Yuan

Ming-Zhu Liu

Fu-Sheng Yu

Zhi-Wei Liu

Li-Sheng Geng

Solving the inverse source problem in femtoscopy with a toy model * Supported in part by the Fundamental and Interdisciplinary Disciplines Breakthrough Plan of the Ministry of Education of China (JYB2025XDXM204) and the National Natural Science Foundation of China (W2543006, 12435007). Ming-Zhu Liu acknowledges support from the National Natural Science Foundation of China (12575086). Zhi-Wei Liu acknowledges support from the National Natural Science Foundation of China (12405133) and from the Shenzhen Science and Technology Program (ZDSYS20230626091501002). Fu-Sheng Yu and Ao-Sheng Xiong acknowledge support from the Scientific Research Innovation Capability Support Project for Young Faculty (ZYGXQNJSKYCXNLZCXM-P2) and from the Fundamental Research Funds for the Central Universities (lzujbky-2023-stlt01, lzujbky-2024-oy02, lzujbky-2025-eyt01)

We present a quantum-field-theoretical study of bremsstrahlung, wherein an electron radiates a final-state photon as it propagates through the external electromagnetic field of a heavy ion. Crucially, this external field is linearly polarized and can be accurately described using the equivalent photon approximation via transverse-momentum-dependent photon distributions. Starting from the photon two-point correlator, the background-modified photon propagator is parameterized accordingly. Furthermore, the Coulomb correction is incorporated through a gauge-link formalism, and soft-photon radiation is resummed with a Sudakov factor, yielding an analytic form of the polarized Bethe–Heitler spectrum. As highlighted in our numerical figures for the EIC and EicC kinematics, this polarization manifests as a characteristic $$\cos 2\phi $$ azimuthal modulation, yielding a few percent at EicC and reaching up to approximately 50% at EIC in the differential cross-section. Our graphical results distinctly illustrate the interplay of these effects, demonstrating strong Sudakov suppression at low transverse momentum and a mild Coulomb-induced enhancement at larger $$q_\perp $$ .

Guofei Zhang

Longzhou Liao

Cong Li

Studying bremsstrahlung in polarized background field at EIC and EicC

<p>We develop tools of Bayesian inference on the moduli space of Calabi–Yau (CY) manifolds. We sample from the invariant Weil–Petersson (WP) measure using Markov Chain Monte Carlo and normalizing flows on Kähler moduli space with dimension up to <math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><msup><mi>h</mi><mrow><mn>1</mn><mo>,</mo><mn>1</mn></mrow></msup><mo>=</mo><mn>30</mn></math>, and present results on the spectrum of the CY volume and properties of divisors when the measure is restricted in physically meaningful ways. We furthermore present a theory-informed prior on axion masses and decay constants <math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><mo stretchy="false">(</mo><msub><mi>m</mi><mi>a</mi></msub><mo>,</mo><msub><mi>f</mi><mi>a</mi></msub><mo stretchy="false">)</mo></math> marginalized over the WP measure for <i>all</i> inequivalent CYs constructable from the Kreuzer–Skarke database with <math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><msup><mi>h</mi><mrow><mn>1</mn><mo>,</mo><mn>1</mn></mrow></msup><mo>≤</mo><mn>5</mn></math>. We then impose likelihoods based on axion physics. We demonstrate how detection of a relatively heavy QCD axion at small <math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><msup><mi>h</mi><mrow><mn>1</mn><mo>,</mo><mn>1</mn></mrow></msup></math>, e.g., by the Axion Dark Matter Experiment, provides detailed information about CY geometry and topology. Finally, we compute a full forward model incorporating likelihoods from the cosmic microwave background and Lyman-alpha forest and find the maximum posterior probability region on the moduli space of a given CY favored by a resolution of the tension in these data by an ultralight axion composing <math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><mi mathvariant="script">O</mi><mo stretchy="false">(</mo><mn>1</mn><mo>%</mo><mo stretchy="false">)</mo></math> of the dark matter. This demonstration serves as a blueprint for future statistical analyses within string phenomenology.</p>

United Kingdom

United States

Mudit Jain

Elijah Sheridan

David J.E. Marsh

Elli Heyes

Keir K. Rogers

Germany

Andreas Schachner

Bayesian inference on Calabi-Yau moduli spaces and the axiverse: Experimental data meets string theory