A modified gravitational model whose action is given by an arbitrary function of the Ricci scalar, the matter Lagrangian density, a scalar field, and its kinetic term is investigated as an extension of the gravitational sector including an additional dynamical degree of freedom. Within this framework, the causal structure of rotating cosmological solutions is analyzed by considering Gödel and Gödel-type spacetimes as background geometries used as theoretical probes of the model consistency. Different matter sources are examined, including a perfect fluid and scalar-field configurations. It is found that the standard Gödel metric is not compatible with the scalar sector of the theory unless the model reduces to the General Relativity limit. In contrast, Gödel-type geometries admit a wider class of solutions whose causal properties depend on the model parameters and on the matter content. In particular, perfect-fluid sources may lead to either causal or noncausal configurations, whereas scalar-field configurations constrain the geometry to the causal limit, preventing the formation of closed timelike curves, highlighting that the scalar field plays a nontrivial dynamical role distinct from that of a cosmological constant.

Brazil

L. Evangelista

M. Silva

J. Moretti

A. Santos

Causality and its violation in $$f(R,\mathcal {L}_m,\phi ,g^{\mu \nu }\nabla _\mu \phi \nabla _\nu \phi )$$ gravity

The novel proposal to invoke the split of the Ricci scalar into bulk and boundary terms in the gravitational action, opens up a new avenue of investigation into stellar dynamics. The Lagrangian contains functional forms of the bulk term while the boundary term do not contribute to the dynamics. The advantage of the proposition is that the stellar structure equations are up to order two, thus the theory is not haunted by ghosts. We obtain explicitly the defining equations for the thermodynamical variables and the geometry for the pure quadratic case, since the linear case amounts to general relativity. In trying to establish the vacuum geometry associated with the theory it turns out that two possible metrics emerge through the vanishing of the energy-momentum tensor. Next, we analyse the isotropy equation and make the observation that it is autonomous. It is rare that this happens in astrophysical modelling. This behaviour prompted the use of dynamical systems to understand the stability properties of fixed points or invariant submanifolds. It was necessary to choose a gauge in order to split the autonomous equation into a system from which we could plot a phase portrait and deduce the stability of solution trajectories. We find that the invariant submanifolds were generally stable with nearby paths approaching them.

South Africa

Sudan Hansraj

United Kingdom

Christian Boehmer

Ndumiso Buthelezi

Dynamical systems approach to stellar modelling in $\textit{f}$($\textit{G}$, $\textit{B}$) gravity

We study finite-volume (FV) corrections to determinations of <math xmlns="http://www.w3.org/1998/Math/MathML"><msub><mi>g</mi><mi>A</mi></msub></math> via lattice quantum chromodynamics (QCD) using analytic results and numerical analysis. We observe that <math xmlns="http://www.w3.org/1998/Math/MathML"><mrow><mi>S</mi><mi>U</mi><mo>(</mo><mn>2</mn><mo>)</mo></mrow></math> heavy Baryon chiral perturbation theory does not provide an unambiguous prediction for the sign of the FV correction, which is not surprising when one also considers large-<math xmlns="http://www.w3.org/1998/Math/MathML"><msub><mi>N</mi><mi>c</mi></msub></math> constraints on the axial couplings. We further show that nonmonotonic FV corrections are naturally allowed when one considers either including explicit <math xmlns="http://www.w3.org/1998/Math/MathML"><mi mathvariant="normal">Δ</mi></math>-resonance degrees of freedom or one works to higher orders in the chiral expansion. We investigate the potential impact of these FV corrections with a precision study of <math xmlns="http://www.w3.org/1998/Math/MathML"><msub><mi>g</mi><mi>A</mi></msub></math> using models of FV corrections that are monotonic and nonmonotonic. Using lattice QCD data that is approximately at the 1% level of precision, we do not see significant evidence of nonmonotonic corrections. Looking forward to the next phase of lattice QCD calculations, we estimate that calculations that are between the 0.1% and 1% level of precision may be sensitive to these FV artifacts. Finally, we present an update of the CalLat prediction of <math xmlns="http://www.w3.org/1998/Math/MathML"><msub><mi>g</mi><mi>A</mi></msub></math> in the isospin limit with subpercent precision, <math xmlns="http://www.w3.org/1998/Math/MathML"><mrow><msubsup><mi>g</mi><mi>A</mi><mi>QCD</mi></msubsup><mo>=</mo><mn>1.2674</mn><mrow><mo>(</mo><mn>96</mn><mo>)</mo></mrow></mrow></math>.

United States

Zack Hall

Dimitra A. Pefkou

Aaron S. Meyer

Thomas R. Richardson

Raúl A. Briceño

M.A. Clark

Switzerland

Martin Hoferichter

Emanuele Mereghetti

Henry Monge-Camacho

Colin Morningstar

Amy Nicholson

Pavlos Vranas

André Walker-Loud

Signs of nonmonotonic finite-volume corrections to <math xmlns="http://www.w3.org/1998/Math/MathML"><msub><mi>g</mi><mi>A</mi></msub></math>

The Standard Model prediction of the muon $$g-2$$ increasingly depends on lattice QCD computations of the hadronic vacuum polarization (HVP), where the isospin-breaking (IB) effects remain a significant source of uncertainty. To complement the lattice QCD evaluations, the data-driven approach to HVP has been used to assess some of the electromagnetic IB effects, in particular from the channels with a photon in the final state, e.g., $$e^+e^-\rightarrow \pi ^0 \gamma $$ . Here we argue that such contributions are largely canceled by virtual electromagnetic corrections to the purely hadronic channels: $$\pi ^+ \pi ^-$$ , $$\pi ^+ \pi ^- \pi ^0$$ , etc. We identify these leading corrections by performing a field-theoretic calculation in a vector-meson dominance model, thereby reconciling the timelike and spacelike approaches to electromagnetic effects. Although these virtual corrections are more difficult to extract in a systematic manner, addressing them is essential for the data-driven method to consistently complement the lattice QCD program.

Germany

France

Volodymyr Biloshytskyi

Dominik Erb

Harvey Meyer

Julian Parrino

Vladimir Pascalutsa

Field-theoretic versus data-driven evaluations of electromagnetic corrections to hadronic vacuum polarization in $$(g-2)_\mu $$

In this work, we investigate the internal structure and physical features of compact stars in the background of modified gravity, specifically within <italic>f</italic>( <italic>R, G</italic>) gravity, where <italic>R</italic> represents the Ricci scalar and <italic>G</italic> denotes the Gauss&#8211;Bonnet invariant. The interior spacetime of the stellar configuration is described by the Kohler&#8211;Chao metric, which provides a suitable geometric representation for dense compact objects. Our analysis considers ten well-known compact star candidates, namely Her X-1, SAX J1808.4-3658, SMC X-1, LMC X-4, Cen X-3, 4U 1820-30, PSR J1903+327, 4U 1608-52, Vela X-1, and PSR J1614-2230. To incorporate higher-curvature effects, the functional form of <italic>f</italic>( <italic>R, G</italic>) is decomposed into two independent contributions, <italic>f</italic> <inf>1</inf>( <italic>R</italic>) and <italic>f</italic> <inf>2</inf>( <italic>G</italic>), representing the Ricci scalar and Gauss&#8211;Bonnet sectors, respectively. For the <italic>R</italic>-dependent part, we adopt the Hu&#8211;Sawicki type model, while three different models, namely logarithmic and power-law models, are considered for the Gauss&#8211;Bonnet contribution. This reconstruction gives three distinct modified gravity models used to study the different features of compact stellar configurations. Using these models, we study the physical viability of the stellar structures by examining anisotropic pressure content, equilibrium scenario through the Tolman&#8211;Oppenheimer&#8211;Volkoff equation, energy conditions, and stability conditions. The obtained results demonstrate that the energy density, radial pressure, tangential pressure, and anisotropy remain well-behaved throughout the stellar interior for all considered stars. The analysis indicates that the proposed <italic>f</italic>( <italic>R, G</italic>) models together with the Kohler&#8211;Chao interior solution provide physically consistent and stable configurations for compact stars within the modified gravity framework.

Saudi Arabia

Ahdab K. Althukair

Azerbaijan

Pakistan

M. Ilyas

Greece

Russia

Euaggelos E. Zotos

Kohler-Chao interior solutions for compact stellar objects in <italic>f</italic>( <italic>R, G</italic>) gravity

Gauss–Bonnet inflation typically affects the dynamics over an extended portion of the trajectory, making it difficult to isolate a controlled imprint at CMB scales. We consider a trajectory-selective coupling $$\mu (\phi ,X)$$ that gates the Gauss–Bonnet sector in phase space, enabling the higher-curvature contribution to be localized within a finite e-fold window while remaining negligible elsewhere. We identify stable inflationary solutions consistent with this localization and enforce standard ghost and gradient stability conditions for both scalar and tensor perturbations. For these viable backgrounds we compute pivot-scale observables and examine their dependence on the overall Gauss–Bonnet strength and on the kinetic gating. The framework offers a controlled route for realizing localized higher-curvature effects with predictable consequences for CMB-scale measurements.

Iran

Ali Seidabadi

Sara Saghafi

Kourosh Nozari

Beyond $$f(\phi ){\mathcal {G}}$$ : Gauss–Bonnet inflation with $$\mu (\phi ,X)$$

<list id='lst0001'><list-item id='lstitem0001'><label>&#8226;</label><para id='p0001'><italic>f</italic>(<italic>G</italic>) Gravity.</para></list-item><list-item id='lstitem0002'><label>&#8226;</label><para id='p0002'>Bulk viscous cosmological model.</para></list-item><list-item id='lstitem0003'><label>&#8226;</label><para id='p0003'>Bianchi type III cosmological model.</para></list-item></list>

India

Nihir Basumatary

Mukunda Dewri

Derphungshar Basumatary

Investigating accelerating bulk viscous cosmological model universe in <italic>f</italic>( <italic>G</italic>) gravity

We compute the <math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><mi>q</mi><mover accent="true"><mi>q</mi><mo stretchy="false">¯</mo></mover><mi>g</mi></math> contribution to the diffractive structure functions in high-energy deep inelastic scattering. The obtained result corresponds to a finite part of the next-to-leading-order contribution to the diffractive cross section. Previous phenomenological applications have included this contribution only in the high-<math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><msup><mi>Q</mi><mn>2</mn></msup></math> or high-<math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><msubsup><mi>M</mi><mi>X</mi><mn>2</mn></msubsup></math> limits in the case of a soft gluon, and we numerically demonstrate that these existing estimates do not provide a good approximation for the full <math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><mi>q</mi><mover accent="true"><mi>q</mi><mo stretchy="false">¯</mo></mover><mi>g</mi></math> contribution. Furthermore, we demonstrate that in addition to the soft gluon contribution, there is an equally important soft quark contribution to the diffractive structure functions at high <math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><msup><mi>Q</mi><mn>2</mn></msup></math>.

Finland

Abhiram Kaushik

Heikki Mäntysaari

Jani Penttala

Diffractive deep inelastic scattering in the dipole picture: The <math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><mi>q</mi><mover accent="true"><mi>q</mi><mo stretchy="false">¯</mo></mover><mi>g</mi></math> contribution in exact kinematics

The reduced cross section of the semiexclusive <math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><mo stretchy="false">(</mo><mi>l</mi><mo>,</mo><msup><mi>l</mi><mo>′</mo></msup><mi>p</mi><mo stretchy="false">)</mo></math> lepton scattering process irrespective of the type of interaction is determined mainly by the bound nucleon momentum distribution in the target and nucleon final state interaction with the residual nucleus. These cross sections can be identified with distorted nuclear spectral functions and therefore, they are similar up to Coulomb corrections for neutrino and electron scattering on nuclei. In this article, we exploit this similarity and use data with precise kinematics and large statistics for semiexclusive electron scattering on an oxygen target to test models employed in the genie neutrino event generator. We find that these models cannot reproduce well the measured reduced cross sections in all allowed kinematic regions and the genie event generator needs to describe better both the nuclear ground states and nucleon final state interaction. The approach presented in this paper provides a great opportunity to better test the accuracy of nuclear models of quasielastic neutrino-nucleus scattering, employed in neutrino event generators.

A.V. Butkevich

S.V. Luchuk

Test of the <math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><mrow><mi mathsize="small">G</mi><mi mathsize="small">E</mi><mi mathsize="small">N</mi><mi mathsize="small">I</mi><mi mathsize="small">E</mi></mrow></math> neutrino event generator against reduced cross sections extracted from <math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><mrow><mrow><mmultiscripts><mrow><mi mathvariant="normal">O</mi></mrow><mprescripts></mprescripts><none></none><mrow><mn>16</mn></mrow></mmultiscripts></mrow><mo stretchy="false">(</mo><mi>e</mi><mo>,</mo><msup><mrow><mi>e</mi></mrow><mrow><mo>′</mo></mrow></msup><mi>p</mi><mo stretchy="false">)</mo></mrow></math> data

In the present work, we incorporate redshift space distortion measurement to investigate the growth of large scale structure within the framework of multi-fluid cosmology in the context of $\textit{f}$($\textit{G}$) gravity. Using 3 different $\textit{f}$($\textit{G}$) gravity models, where $\textit{f}$($\textit{G}$) is the function of Gauss–Bonnet invariant, we compare the predictions of $\textit{f}$($\textit{G}$) gravity expansion history—through the Friedmann equation with Hubble data, BAO data sets and constrain models parameters such as $$\Omega _{m}$$ and $$H_{0}$$ . Within the context of multi-fluid cosmology in $\textit{f}$($\textit{G}$) gravity, we obtain the structure growth equation. This equation is then combined with $$\sigma _{8}$$ to get $$f\sigma _{8}$$ predictions—which is compared with redshift space distortion data to constrain models parameters to obtain best-fit values including $$\sigma _{8}$$ . This involves performing a Markov Chain Monte Carlo (MCMC) analysis for these specific forms of $\textit{f}$($\textit{G}$) models. we also perform a statistical analysis incorporating the Akaike Information Criterion (AIC), the Bayesian Information Criterion (BIC), to assess the goodess-of-fit of the considered models.

Praveen Dhankar

Rwanda

Albert Munyeshyaka

Aritra Sanyal

Safiqul Islam

Farook Rahaman

Constraints on multi-fluid cosmology in $\textit{f}$($\textit{G}$) gravity with different observational data sets