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

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.

China

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)

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)

The NUCLEUS experiment aims to detect coherent elastic neutrino–nucleus scattering of reactor antineutrinos using low-threshold, gram-scale cryogenic calorimeters. Similar to other low-threshold experiments, NUCLEUS observes a sharp rise in the event rate below a few hundred eV, referred to as the low energy excess (LEE), whose origin remains yet unidentified. Building on results from the NUCLEUS testing and commissioning at the Technical University of Munich and from previous characterization campaigns, we present a comprehensive study of the background rate measured with a sapphire detector equipped with two transition-edge sensors under various experimental conditions. We find no evidence for a dependence of the LEE rate on the particle background level, whereas the results indicate that slower cooling-down procedures lead to lower initial LEE rates. The behavior of the LEE rate during the same cooldown is comparable across the measurements and is best described by a power law with a common exponent across datasets of $$(-0.59\pm 0.06)$$ , when time is expressed from the moment the detector reaches the 4 K temperature. These findings provide valuable guidance for future LEE mitigation strategies in the NUCLEUS experiment.

Austria

H. Abele

Germany

G. Angloher

B. Arnold

M. Atzori Corona

A. Bento

France

E. Bossio

F. Buchsteiner

J. Burkhart

F. Cappella

M. Cappelli

N. Casali

R. Cerulli

A. Cruciani

G. Del Castello

M. del Gallo Roccagiovine

S. Dorer

A. Erhart

M. Friedl

S. Fichtinger

V. Ghete

M. Giammei

C. Goupy

J. Hakenmüller

D. Hauff

F. Jeanneau

E. Jericha

M. Kaznacheeva

H. Kluck

A. Langenkämper

T. Lasserre

D. Lhuillier

M. Mancuso

R. Martin

B. Mauri

A. Mazzolari

L. McCallin

H. Neyrial

C. Nones

L. Oberauer

L. Peters

F. Petricca

W. Potzel

F. Pröbst

F. Pucci

F. Reindl

M. Romagnoni

J. Rothe

N. Schermer

J. Schieck

S. Schönert

C. Schwertner

L. Scola

G. Soum-Sidikov

L. Stodolsky

A. Schröder

R. Strauss

R. Thalmeier

C. Tomei

L. Valla

M. Vignati

M. Vivier

A. Wallach

P. Wasser

A. Wex

L. Wienke

Characterization of the low energy excess using a NUCLEUS Al $$_2$$ O $$_3$$ detector

In this research paper, we have discussed the thermodynamic behavior of the viscous variable Chaplygin gas (VVCG) model. The equation of state $$p=-\frac{B}{\rho }$$ has been used, where $$B=B_{0}V^{-\frac{n}{3}}$$ and $$B_{0}$$ is a positive universal constant. Our study also includes the bulk viscosity coefficient, $$\xi =\xi _{0}{\rho }^{\frac{1}{2}}$$ . As long as the values of $\textit{n}$ remain negative, we arrive at the intriguing conclusion that when a fluid satisfies the established stability conditions, then for $$\xi _{0}<0$$ , $$(\frac{\partial p_{eff}}{\partial V})_{S}<0$$ and $$(\frac{\partial p_{eff}}{\partial V})_{T}<0$$ throughout the entire evolution. Furthermore, it can be confirmed that both the positivity of the thermal heat capacity ( $$c_V$$ ) and the third law of thermodynamics are applicable in this situation. Additionally, this model has been compared graphically with the VCG model, and we have discussed how all the thermal variables are functions of volume or temperature. A new general equation of state has been obtained for the VVCG model as a function of volume and temperature during a thermal process, which also shows that the universe is thermodynamically stable. Furthermore, when $$T\rightarrow 0$$ , the parameters of the thermal and caloric equations of state become identical for large volumes.

India

Purna Barman

Abhinath Barman

Thermodynamic behaviour of a variable Chaplygin gas in a flat universe with bulk viscosity

A nontopological soliton of the Q-ball type in a Chern–Simons–Higgs gauge model is studied using both analytical and numerical methods. The general non-self-dual case is considered. It is shown that the soliton solution is an extremum of the energy functional at a fixed Noether charge. A differential relation between the energy, Noether charge, and the boundary value of the gauge potential of the soliton is derived. A linear relation between the components of the soliton energy is obtained. The parametric domain of existence of the soliton solution is determined. It is established that the soliton properties depend significantly on the form of the self-interaction potential of the scalar field. In particular, the energy and charge of the soliton can take arbitrarily large values only if the self-interaction potential has two degenerate zero minima.

Russia

A. Loginov

Non-self-dual nontopological soliton in a pure Chern–Simons gauge model

This paper presents a detailed description of a model-independent method for the direct measurement of the Υ(4$\textit{S}$) → $\textit{B}$ $\textit{B}$ and Υ(4$\textit{S}$) → $$ {B}^0{\overline{B}}^0 $$ production fractions at $\textit{B}$ factories. The method is based on counting single- and double-inclusive charmed meson production at the Υ(4$\textit{S}$) resonance, providing statistical tagging of $$ {B}^0{\overline{B}}^0 $$ and $\textit{B}$ $\textit{B}$ events. A feasibility study indicates that a precision comparable to the current world average can be achieved without making any underlying assumptions, as the calculations rely solely on event yields.

M. Yasaveev

P. Pakhlov

N. Peters

A. Mufazalova

A model independent method for measurement of $\textit{B}$ ± and $\textit{B}$ 0 meson production fractions at Υ(4$\textit{S}$)

A sharp signature of the black hole singularity in holography is a divergence in the boundary thermal two-point function at a specific point in the complex time plane. This divergence arises from a null geodesic that bounces off the black hole singularity. At finite ’t Hooft coupling, stringy corrections to the bulk dynamics cannot be neglected, and the fate of the bouncing geodesic is an open question. We propose a simple scenario in which the singularity in the two-point function is shifted slightly into the complex plane, thereby smoothing it out into a finite-size bump. We demonstrate this smoothing explicitly in a microscopic example, namely the Sachdev-Ye-Kitaev model at infinite temperature, where the correlator is under analytic control. Our result suggests a bulk description of planar theories at finite coupling as stringy black holes.

United States

Matthew Dodelson

Switzerland

Cristoforo Iossa

United Kingdom

Robin Karlsson

Bouncing off a stringy singularity

We present the first comprehensive study of the rare Dalitz decay $$K^*(892) \rightarrow K \ell ^+ \ell ^- (\ell = e, \mu )$$ , providing a prediction for the branching fraction and the dilepton mass spectrum. This decay involves the emission of a virtual photon which converts into a lepton pair, offering a probe of the transition form factor $$F_{K^*K}(q^2)$$ and underlying meson structure. Using a single pole approximation for the form factor, we present the calculation of the branching fraction for this rare decay channel for the first time. Furthermore, we also investigate the potential to search for a light $$A'$$ boson (dark photon) appearing as a narrow resonance in the dilepton spectrum, and discuss the experimental sensitivity and new physics opportunities at the dedicated BESIII experiment. Our results establish $$K^*(892) \rightarrow K \ell ^+ \ell ^- (\ell = e, \mu )$$ as a new laboratory for hadronic structure and dark-sector searches.

Benhou Xiang

Wanling Chang

Shuangshi Fang

Jingqing Zhang

Dalitz decay of $$K^*(892) \rightarrow K \ell ^+\ell ^-$$ : a new probe for hadronic structure and dark photon searches

A study of retardation models for the hadronic quark interaction is performed starting from a coordinate-space calculation that elaborates a classical electrodynamics procedure. The possibility of constructing a corresponding quantum operator is critically analyzed also performing some numerical matrix element calculations. A comparison of the developed model with the Feynman diagram interaction at tree-level is studied, showing a substantial physical correspondence of the two models. Possible applications and generalizations are discussed. Abstract Published by the Jagiellonian University 2026 authors

Colombia

M. De Sanctis

A study of Retardation Models for Phenomenological Interactions in Hadronic Physics