Near-threshold <Emphasis Type="Italic">J</Emphasis>/<Emphasis Type="Italic">ψ</Emphasis>-meson photoproduction on nuclei

On the basis of the first-collision model that relies on the nuclear spectral function and which includes incoherent processes involving charmonium production in proton–nucleon collisions, the photoproduction of J/ψ mesons on nuclei is considered at energies close to the threshold for their production on a nucleon. The absorption of final J/ψ mesons, their formation length, and the binding and Fermi motion of target nucleons are taken into account in this model along with the effect of the nuclear potential on these processes. The A dependences of the absolute and relative charmonium yields are calculated together with absolute and relative excitation functions under various assumptions on the magnitude of the cross section for J/ψN absorption, the J/ψ-meson formation length, and their inmedium modification. It is shown that, at energies above the threshold, these features are virtually independent of the formation length and the change in the J/ψ-meson mass in nuclear matter but are rather highly sensitive to the cross section for J/ψN interaction. The calculations performed in the present study can be used to determine the unknown cross section for J/ψ-meson absorption in nuclei from a comparison of their results with data expected from experiments in the Hall C of the CEBAF (USA) facility upgraded to the energy of 12 GeV. It is also shown that the absolute and relative excitation functions for J/ψ mesons in photon–nucleus reactions at subthreshold energies are sensitive to the change in the meson mass and, hence, carry information about the properties of charmonium in nuclear matter.     

Transverse-momentum dependence of <Emphasis Type="Italic">J</Emphasis>/<Emphasis Type="Italic">ψ</Emphasis> shadowing effects

We present a new approach to estimate the effect of gluon shadowing in nucleus + nucleus collisions and its consequences on the J/ψ production yield. Using kinematical information available from the measured J/ψ production in proton + proton collisions at  GeV, we build a Glauber Monte Carlo code which takes into account shadowing in two alternative ways: multiple-scattering corrections or Q ² evolution of parton densities. We exploit the dependence of these different parameterizations to the J/ψ transverse momentum and we give the first predictions on the resulting p _T dependence of the nuclear modification factor in deuteron + gold collisions at the same energy.     

Anisotropic quark stars in <Emphasis Type="Italic">f</Emphasis>(<Emphasis Type="Italic">R</Emphasis>, <Emphasis Type="Italic">T</Emphasis>) gravity

The aim of this paper is to analyze the nature of anisotropic spherically symmetric relativistic star models in the framework of f(R, T) gravity. To discuss the features of compact stars, we consider that in the interior of the stellar system, the fluid distribution is influenced by MIT bag model equation of state. We construct the field equations by employing Krori–Barua solutions and obtain the values of unknown constants with the help of observational data of Her X-1, SAX J 1808.4-3658, RXJ 1856-37 and 4U1820-30 star models. For a viable f(R, T) model, we study the behavior of energy density, transverse as well as radial pressure and anisotropic factor in the interior of these stars for a specific value of the bag constant. We check the physical viability of our proposed model and stability of stellar structure through energy conditions, causality condition and adiabatic index. It is concluded that our model satisfies the stability criteria as well as other physical requirements, and the value of bag constant is in well agreement with the experimental value which highlights the viability of our considered model.     

Cosmology in modified <Emphasis Type="Italic">f</Emphasis>(<Emphasis Type="Italic">R</Emphasis>, <Emphasis Type="Italic">T</Emphasis>)-gravity

In the present paper we propose a further modification of f(R, T)-gravity (where T is trace of the energy-momentum tensor) by introducing higher derivatives matter fields. We discuss stability conditions in the proposed theory and find restrictions for the parameters to prevent appearance of main type of instabilities, such as ghost-like and tachyon-like instabilities. We derive cosmological equations for a few representations of the theory and discuss main differences with conventional f(R, T)-gravity without higher derivatives. It is demonstrated that in the theory presented inflationary scenarios appear quite naturally even in the dust-filled Universe without any additional matter sources. Finally, we construct an inflationary model in one of the simplest representation of the theory, calculate the main inflationary parameters and find that it may be in quite good agreement with observations.     

Final state interactions in the decays <Emphasis Type="Italic">J</Emphasis>/<Emphasis Type="Italic">ψ</Emphasis>→<Emphasis Type="Italic">VPP</Emphasis>

We investigate the interplay between crossed channel final state interactions and the constraints from two-particle unitarity for the reactions J/ψ→V π π and VK[`(K)]VK\bar{K} , where V is either ω or φ. Using a model where the parameters are largely constrained by other sources, we find that, although small, crossed channel final state interaction can influence the amplitudes considerably, in special areas of phase space. These results cast doubt on the inapplicability of unitarity constraints on production amplitudes as recently claimed in the literature.     

Revised Spherically Symmetric Solutions of <Emphasis Type="Italic">R</Emphasis>+<Emphasis Type="Italic">ε</Emphasis>/<Emphasis Type="Italic">R</Emphasis> Gravity

We study spherically symmetric static empty space solutions in R+ε/R model of f(R) gravity. We show that the Schwarzschild metric is an exact solution of the resulted field equations and consequently there are general solutions which are perturbed Schwarzschild metric and viable for solar system. Our results for large scale contains a logarithmic term with a coefficient producing a repulsive gravity force which is in agreement with the positive acceleration of the universe.     

Contribution of final-state interaction to the branching ratio of <Emphasis Type="Italic">B</Emphasis>→<Emphasis Type="Italic">J</Emphasis>/<Emphasis Type="Italic">ψ</Emphasis><Emphasis Type="Italic">D</Emphasis>

To test the validity of perturbative QCD (pQCD) and investigate its range of application, one should look for a suitable process. B→J/ψ D is a promising candidate. The linear momentum of the products is relatively small, so that there may exist a region where exchanged gluons are soft and the perturbative treatment may fail, so that the non-perturbative effect would be significant. We attribute such non-perturbative QCD effects to the long-distance final-state interaction (FSI) which is estimated in this work. We find that the contribution from the FSI to the branching ratio is indeed sizable and may span the rather wide range of 10⁻⁶∼10⁻⁵ and cover a region where the pQCD prediction is of the same order. A more accurate measurement of its branching ratio may provide important information about the application region of pQCD and help to clarify the picture of inelastic rescattering (i.e. FSI), which is generally believed to play an important role in B decays.     

Late-time cosmological approach in mimetic <Emphasis Type="Italic">f</Emphasis>(<Emphasis Type="Italic">R</Emphasis>, <Emphasis Type="Italic">T</Emphasis>) gravity

In this paper, we investigate the late-time cosmic acceleration in mimetic f(R, T) gravity with the Lagrange multiplier and potential in a Universe containing, besides radiation and dark energy, a self-interacting (collisional) matter. We obtain through the modified Friedmann equations the main equation that can describe the cosmological evolution. Then, with several models from \(\mathcal {Q}(z)\) and the well-known particular model f(R, T), we perform an analysis of the late-time evolution. We examine the behavior of the Hubble parameter, the dark energy equation of state and the total effective equation of state and in each case we compare the resulting picture with the non-collisional matter (assumed as dust) and also with the collisional matter in mimetic f(R, T) gravity. The results obtained are in good agreement with the observational data and show that in the presence of the collisional matter the dark energy oscillations in mimetic f(R, T) gravity can be damped.     

Exact solutions and conserved quantities in <Emphasis Type="Italic">f</Emphasis>(<Emphasis Type="Italic">R</Emphasis>, <Emphasis Type="Italic">T</Emphasis>) Gravity

This paper explores Noether and Noether gauge symmetries of anisotropic universe model in f(R, T) gravity. We consider two particular models of this gravity and evaluate their symmetry generators as well as associated conserved quantities. We also find exact solution by using cyclic variable and investigate its behavior via cosmological parameters. The behavior of cosmological parameters turns out to be consistent with recent observations which indicates accelerated expansion of the universe. Next we study Noether gauge symmetry and corresponding conserved quantities for both isotropic and anisotropic universe models. We conclude that symmetry generators and the associated conserved quantities appear in all cases.     

Plane Symmetric Vacuum Bianchi Type <Emphasis Type="Italic">III</Emphasis> Cosmology in <Emphasis Type="Italic">f</Emphasis>(<Emphasis Type="Italic">R</Emphasis>) Gravity

The main purpose of this paper is to study the exact solution of Bianchi type III spacetime in the context of metric f(R) gravity. The field equations are solved by taking expansion scalar θ proportional to shear scalar σ which gives C=A ⁿ , where A and C are the metric coefficients. The physical behavior of the solution has been discussed using some physical quantities. Also, the function of the Ricci scalar is evaluated.     

The anisotropic cosmological models in <Emphasis Type="Italic">f</Emphasis>(<Emphasis Type="Italic">R</Emphasis>, <Emphasis Type="Italic">T</Emphasis>) gravity with <Emphasis Type="Italic">Λ(T)</Emphasis>

The rare decay \(B\rightarrow {K^{*}}\ell ^+\ell ^-\) is a very significant mode to search for physics beyond the Standard Model (SM). The mode provides a very rich spectrum of observables obtained from the angular distribution of its decay products. The recent LHCb measured values of these observables are used to conclude an evidence of right-handed currents at the kinematic endpoint of this decay mode. As the conclusion is drawn at the maximum dilepton invariant mass square (\(q^2\)) kinematic endpoint, it relies only on heavy quark symmetries where it is valid without significant corrections.     

On a Ströbel-Inspired <Emphasis Type="Italic">k</Emphasis>(<Emphasis Type="Italic">t</Emphasis>) FLRW Ansatz in a Class of Metric <Emphasis Type="Italic">F</Emphasis>(<Emphasis Type="Italic">R</Emphasis>) Models

Assuming a D≥4 dimensional FLRW (Friedmann–Lemaître–Robertson–Walker) inspired ansatz with spatial curvature being a non-trivial function of time k(t) in a class of metric and Jordan frame F(R) gravity models, non-existence theorems for several types of sources are derived in a simple manner (using specific form of the modified gravity Einstein tensor components).     

Global analysis of <Emphasis Type="Italic">J</Emphasis>/<Emphasis Type="Italic">ψ</Emphasis> suppression in cold nuclear matter

Interpreting the J/ψ suppression reported in nucleus–nucleus collisions at SPS and RHIC requires a quantitative understanding of cold nuclear-matter effects, such as the inelastic rescattering of J/ψ states in nuclei or the nuclear modification of parton densities. With respect to our former Glauber analysis, we include in the present work the new PHENIX d–Au measurements, and we analyze as well all existing data using the EPS08 nuclear parton densities recently released. The largest suppression reported in the new PHENIX analysis leads in turn to an increase of σ _J/ψN from 3.5±0.3 to 5.4±2.5 mb using the PDF of the proton. The stronger x-dependence of the G ^A /G ^p ratio in EPS08 as compared to e.g. EKS98 shifts the cross section towards larger values at fixed-target energies (x ₂∼0.1), while decreasing somehow the value extracted at RHIC (x ₂∼10⁻²).     

Cosmological analysis of scalar field models in <Emphasis Type="Italic">f</Emphasis>(<Emphasis Type="Italic">R</Emphasis>, <Emphasis Type="Italic">T</Emphasis>) gravity

This paper determines the existence of Noether symmetry in non-minimally coupled f(R, T) gravity admitting minimal coupling with scalar field models. We consider a generalized spacetime which corresponds to different anisotropic and homogeneous universe models. We formulate symmetry generators along with conserved quantities through Noether symmetry technique for direct and indirect curvature–matter coupling. For dust and perfect fluids, we evaluate exact solutions and construct their cosmological analysis through some cosmological parameters. We conclude that decelerated expansion is obtained for the quintessence model with a dust distribution, while a perfect fluid with dominating potential energy over kinetic energy leads to the current cosmic expansion for both phantom as well as quintessence models.     

Inclusive <Emphasis Type="Italic">J</Emphasis>/<Emphasis Type="Italic">ψ</Emphasis> photoproduction and polarization at HERA in the <Emphasis Type="Italic">k</Emphasis><Subscript><Emphasis Type="Italic">T</Emphasis></Subscript>-factorization approach

We investigate the inclusive photoproduction of J/ψ mesons at HERA within the framework of the k _T -factorization QCD approach. Our study is based on the color singlet model supplemented with the relevant off-shell matrix elements and the CCFM and KMR unintegrated gluon densities in a proton and in a photon. Both the direct and resolved photon contributions are taken into account. Our predictions are compared with the recent experimental data taken by the H1 and ZEUS collaborations. Special attention is put on the J/ψ polarization parameters λ and ν, which are sensitive to the production dynamics.     

Effects of charge on dynamical instability of spherical collapse in <Emphasis Type="Italic">f</Emphasis>(<Emphasis Type="Italic">R</Emphasis>, <Emphasis Type="Italic">T</Emphasis>) gravity

The effects of charge on stable structure of spherically symmetric collapsing model comprising anisotropic matter distribution are studied in f(R, T) gravity, where R and T correspond to scalar curvature and trace of the energy-momentum tensor, respectively. We construct the field equations, Maxwell equations and dynamical equations in this scenario. We employ linear perturbation scheme on physical variables, metric functions as well as modified terms to establish the evolution or collapse equation for a consistent functional form of f(R, T) gravity. We investigate the limit of instability in Newtonian as well as post Newtonian regimes. It is found that charge plays a fundamental role to slow down the collapse and form a more stable system.     

Static spherically symmetric solution of (<Emphasis Type="Italic">R</Emphasis> ± <Emphasis Type="Italic">μ</Emphasis><Superscript>4</Superscript>/<Emphasis Type="Italic">R</Emphasis>) gravity

The static spherically symmetric solution for R ± μ ⁴/R model of f(R) gravity is investigated. We obtain the metric for space-time in the solar system that reduces to the Schwarzschild metric, when μ tends to zero. For the obtained metric, the deviation from Einstein gravity is very small. This result is different from the other results have been obtained by equivalence between f(R) gravity and scalar tensor theory. Also it is shown that the vacuum solution in the solar system depends on the shape of matter distribution which differ from the Einstein’s gravity.