The European Physical Journal C最新文献

In current studies for testing Bell inequalities at colliders, the reconstruction of spin correlations from scattering cross-sections relies on the bilinear form of the spin correlations, but not all local hidden variable models (LHVMs) have such a property. To demonstrate that a general LHVM cannot be rule out via scattering cross-section data, we propose a specific LHVM, which can exactly duplicate the same scattering cross-section for particle production and decay as the standard quantum theory, making it indistinguishable at colliders in principle. Despite of this, we find that reconstructing spin correlations through scattering cross-sections can still exclude a broad class of LHVMs, e.g., those models employing classical spin correlations as a surrogate for quantum spin correlations.

This research delves into the potential existence of traversable wormholes (WHs) within the framework of modified, curvature based gravity. The modification includes linear perturbations of the matter Lagrangian and the trace of the energy-momentum tensor with specific coupling strengths (alpha ) and (beta ) and can thus be viewed as a special case of linear f(R, T)-gravity with a variable matter coupling or as the simplest additively separable (f(R,{mathcal {L}}_m,T))-model. A thorough examination of static WH solutions is undertaken using a constant redshift function; therefore, our work can be regarded as the first-order approximation of WH theories in (f(R,{mathcal {L}}_m,T)). The analysis involves deriving WH shape functions based on non-commutative geometry, with a particular focus on Gaussian and Lorentzian matter distributions (rho ). Constraints on the coupling parameters are developed so that the shape function satisfies both the flaring-out and asymptotic flatness conditions. Moreover, for positive coupling parameters, violating the null energy condition (NEC) at the WH throat (r_0) demands the presence of exotic matter. For negative couplings, however, we find that exotic matter can be avoided by establishing the upper bound (beta +alpha /2<-frac{1}{rho r_0^2}-8pi ). Additionally, the effects of gravitational lensing are explored, revealing the repulsive force of our modified gravity for large negative couplings. Lastly, the stability of the derived WH solutions is verified using the Tolman–Oppenheimer–Volkoff (TOV) formalism.

We formulate a new class of modified gravity models, that is, generalized four-dimensional Chern–Pontryagin models, whose action is characterized by an arbitrary function of the Ricci scalar R and the Chern–Pontryagin topological term ( ^*RR), i.e., (f(R, ^*RR)). Within this framework, we derive the gravitational field equations and solve them for the particular models, (f(R, ^*RR)=R+beta ( ^*RR)^2) and (f(R, ^*RR)=R+alpha R^2+beta ( ^*RR)^2), considering two ansatzes: the slowly rotating Schwarzschild metric and first-order perturbations of Gödel-type metrics. For the former, we find a first-order correction to the frame-dragging effect boosted by the parameter L, which characterizes the departures from general relativity results. For the latter, Gödel-type metrics hold unperturbed, for specific sort of perturbed metric functions. We conclude this paper by displaying that generalized four-dimensional Chern–Pontryagin models admit a scalar-tensor representation, whose explicit form presents two scalar fields: (Phi ), a dynamical degree of freedom, while the second, (vartheta ), a non-dynamical degree of freedom. In particular, the scalar field (vartheta ) emerges coupled with the Chern–Pontryagin topological term ( ^*RR), i.e., (vartheta ^*RR), which is nothing more than Chern–Simons term.

We consider gedanken experiments to destroy Kerr black holes, by means of absorbing matter with sufficient energy and angular momentum. It is shown that extremal and near-extremal Kerr black holes cannot be destroyed in a process that includes a second order contribution to its final mass, and matter sources satisfy the null energy condition. Such contribution is calculated using hypersurface integration on the event horizon, and it traces similarities with terms related to matter-black hole interactions and a rotational self-energy lower bound suggested in previous works.

Matching conditions are universal ingredients that describe how fragmentation functions change when heavy-flavour thresholds are crossed during the factorisation scale evolution. They are the last missing piece for a consistent description of observables with identified final-state hadrons at next-to-next-to leading order accuracy in quantum chromodynamics. We present an analytical form of the matching condition for light-flavour to hadron fragmentation function at next-to-next-to leading order. The derivation is performed by extending the formalism employed in the extraction of the next-to leading order matching conditions to the subsequent order, making use of (e^+e^-) annihilation cross sections. We obtain the first non-trivial heavy-quark effect in the light-quark fragmentation functions and provide results in Mellin space.

The paper studies the possible interplay between matter and geometry in scalar tensor theories of gravitation where the energy–momentum tensor is directly coupled with the Einstein tensor. After obtaining the scalar tensor representation of the (f(R, G_{mu nu }T^{mu nu })) gravity, the analysis continue with an approach based on the thermodynamics of irreversible processes in open systems. To this regard, various thermodynamic properties are directly obtained in this manner, like the matter creation (annihilation) rate and the corresponding creation (annihilation) pressure. In the case of the Roberson–Walker metric several analytic and numerical solutions are found in the asymptotic regime. In the last part of the manuscript a specific parametrization for the Hubble rate is constrained using the Markov Chain Monte Carlo algorithms in the case of cosmic chronometers (CC) and BAO observations, obtaining an approximate numerical solution which can describe the cosmological model. For this model, we have obtained by fine-tuning some numerical solutions which exhibit creation mechanisms in different specific regimes.

The branching ratios of the flavor changing top quark decays in the SM are too small to be detected experimentally. Therefore, any observable signal for these processes at the LHC would serve as compelling evidence for new physics. In the flavor-dependent (U(1)_X) model, a newly introduced Higgs singlet interacts directly with the quark sector and mixes with the SM-like Higgs, influencing the (trightarrow ch) and (trightarrow uh) process. Additionally, the flavor dependence of the (U(1)_X) charge affect the (trightarrow cZ) process. In this work, we investigate these process within the framework of the flavor-dependent (U(1)_X) model. Numerical results indicates that with suitable choices of new physics parameters, the branching ratios of these processes in the flavor-dependent (U(1)_X) model can be significantly enhanced. Specifically, the branching ratios of (trightarrow ch) and (trightarrow uh) can reach the order of (10^{-4}) in some chosen parameter spaces, which is close to the current experimental upper limit. This suggests that these processes have significant opportunities to be observed experimentally, and the parameter space of the model will face constraints from the experimental upper bounds.

We consider the non-commutative (NC) Kerr black hole where the mass of the central object is smeared over a region of linear size (sqrt{b}), b is the strength of the NC character of spacetime. For the spacetime under consideration, we calculate the amplification factor for electromagnetic and gravitational fields and study various properties of a thin accretion disk. The expression for the amplification factor is obtained with the help of the asymptotic matching technique. The amplification factor is then plotted against frequency for various values of the spin a and the NC parameter b. Though the amplification factor increases with a but decreases with b, the cut-off frequency up to which we have amplification increases with a and b. We then study the effect of the spin and the NC nature of spacetime on the energy flux, temperature distribution, emission spectrum, energy conversion efficiency, and the radius of the innermost stable circular orbit of a thin accretion disk around the black hole with the help of the steady-state Novikov–Thorne model. Our study reveals that these quantities increase with the spin and the NC parameter. We also find that the disk around the NC Kerr black is hotter and more luminous than that around the Kerr black hole and the NC Schwarzschild black hole. We can conclusively infer from our investigation that the NC nature of spacetime has a significant impact on the superradiance phenomenon as well as on various properties of thin accretion disks.

In this work we study the thermodynamics formulation for unimodular gravity under the election of two different models for the energy diffusion function. Such function encodes the current for the non-conservation of the energy-momentum tensor and is usually termed as Q(t). In analogy to the cosmological scenario where the cosmic expansion is influenced by Q(t), the thermodynamics implications in this scheme are also determined by the choice of the function Q(t), as we discuss in the work. Specifically, we consider the barotropic and the continuous spontaneous localization models as energy diffusion functions, commonly used in the literature as viable candidates to face the well-known (H_{0}) tension. The consistency conditions demanded for the entropy of the system in terms of the cosmological parameters of the model: positive production ((dS/dt>0)) and convexity condition ((d^{2}S/dt^{2} <0)), are investigated. We show that these conditions strongly constraint the viability of both models. Additionally, we comment about our results and compare with those obtained in recent works where the restriction of the parameters for these two diffusion models was implemented with the use of cosmological data.

We study the hot medium effects in high-multiplicity proton–proton (pp) collisions at (sqrt{s_{NN}}=13) TeV via the charmonium probes. The hot medium is described with the hydrodynamic model, while charmonium evolutions in the medium are studied with a time-dependent Schrödinger equation. The hot medium dissociation on charmonium is considered with the temperature-dependent complex potential parametrized with the results from lattice QCD calculations. The ratio (sigma _{psi (2S)}/sigma _{J/psi }) of (J/psi ) and (psi (2S)) production cross sections are calculated and compared with the LHCb experimental data in pp collisions. Our calculations explain the charmonium relative suppression in different transverse momentum and multiplicity bins. The suppression of this ratio is mainly affected by the effects of the deconfined medium. It is less affected by the initial effects before the generation of the heavy quark pair. We suggest this to be a clear signal of the small QGP droplets generated in high multiplicity pp collisions.