Modern Physics Letters A

We review some recent works by Carone, Erlich and Vaman on composite gravitons in metric-independent quantum field theories, with the aim of clarifying a number of basic issues. Focusing on a theory of scalar fields presented previously in the literature, we clarify the meaning of the tunings required to obtain a massless graviton. We argue that this formulation can be interpreted as the massless limit of a theory of massive composite gravitons in which the graviton mass term is not of Pauli–Fierz form. We then suggest closely related theories that can be defined without such a limiting procedure (and hence without worry about possible ghosts). Finally, we comment on the importance of finding a compelling ultraviolet completion for models of this type, and discuss some possibilities.

We suggest a renormalizable standard model (SM) extension based on $D_5$ symmetry which accommodates leptonic mass and mixing parameters with nonzero ${𝜃}_{13}$ and Dirac CP violating phase. Both normal and inverted neutrino mass ordering as well as the smallness of the active neutrino masses are generated at leading order through type-I seesaw mechanism in which the obtained physical parameters are well consistent with the global fit of neutrino oscillation data [P. F. de Salas et al., Phys. Lett. B 782, 633 (2018)], while the quark masses are in good agreement with the recent experimental data [Particle Data Group (M. Tanabashi et al.), Phys. Rev. D 98, 030001 (2018)]. The model also predicts an effective Majorana neutrino mass parameter of $m_{\beta \beta }=8.81123\times 10^{-2}\mathrm{\text{eV}}$ for normal hierarchy and $m_{\beta \beta }=4.90862\times 10^{-2}\mathrm{\text{eV}}$ for inverted hierarchy which are all well below the most current upper limit given [P. F. de Salas et al., Front. Astron. Space Sci. 5, 36 (2018); CUORE Collab. (C. Alduino et al.), Phys. Rev. Lett. 120, 132501 (2018)] and beyond the reach of the present $0\nu \beta \beta$ decay experiments.

Lepton flavor violation decays are channels which may lead to fundamental discoveries in the forthcoming years and this makes it an exciting research field for beyond the Standard Model (SM) searches. In this work, we present an analysis of the lepton flavor violation decays $l_{\alpha }\to 3l_{\beta }$ in Minimal R-symmetric Supersymmetric Standard Model. The prediction for $\mathrm{\text{BR}}(l_{\alpha }\to 3l_{\beta })$ depends on the off-diagonal entries of the slepton mass matrix. The contributions to Wilson coefficients can be classified into Higgs penguins, photon penguins, Z penguins and box diagrams. It shows the contribution from Z penguins dominates the predictions for $\mathrm{\text{BR}}(l_{\alpha }\to 3l_{\beta })$, and the contributions from Higgs penguins and box diagrams play different roles in different decay channels. The theoretical predictions for $\mathrm{\text{BR}}(l_{\alpha }\to 3l_{\beta })$ can reach the future experimental limits, and the channels are very promising to be observed in near future experiment.

Analysis of the fusion reactions of halo nuclei is one of the important subjects of nuclear physics. In addition, temperature-dependent analysis of fusion cross-sections of these nuclei is a deficient topic in the literature. In order to overcome this deficiency, the fusion cross-sections of ⁶He, ⁸He and ${}^{11}$Li which are the most important halo nuclei are analyzed by using both temperature-independent potential and temperature-dependent potential. All the theoretical results are compared with each other as well as the experimental data. It is seen that the results of temperature-independent potential are in good agreement with the data while the temperature-dependent potential has a significant impact on the fusion cross-sections. Finally, the changes with the temperature of both real and nuclear potentials of all the reactions are investigated.

In a recent work [S. Kumar, R. C. Nunes and S. K. Yadav, Phys. Rev. D 98, 043521 (2018)], we have investigated a dark matter (DM)-photon coupling model in which the DM decays into photons in the presence of dark energy (DE) with constant equation of state (EoS) parameter. Here, we study an extension of the DM-photon coupling model by considering a time-varying EoS of DE via Chevalier–Polarski–Linder (CPL) parametrization. We derive observational constraints on the model parameters by using the data from cosmic microwave background (CMB), baryonic acoustic oscillations (BAO), the local value of Hubble constant from Hubble Space Telescope (HST), and large-scale structure (LSS) information from the abundance of galaxy clusters, in four different combinations. We find that in this DM-photon coupling scenario the mean values of $w_{\mathrm{\text{de}}0}$ are in quintessence region $(w_{\mathrm{\text{de}}0}>-1)$ whereas they were in the phantom region $(w_{\mathrm{\text{de}}0}<-1)$ in our previous study with all data combinations. The constraints on the DM-photon coupling parameter do not reflect any significant deviation from the previous results. Due to the decay of DM into photons, we obtain higher values of $H_0$, consistent with the local measurements, similar to our previous study. But, the time-varying DE leads to lower values of ${\sigma }_8$ in the DM-photon coupling model with all data combinations, in comparison to the results in our previous study. Thus, allowing time-varying DE in the DM-photon coupling scenario is useful to alleviate the $H_0$ and ${\sigma }_8$ tensions.

In this paper, we have presented a model of the Friedmann–Lemaitre–Robertson–Walker (FLRW) universe filled with matter and dark energy (DE) fluids by assuming an ansatz that deceleration parameter (DP) is a linear function of the Hubble constant. This results in a time-dependent DP having decelerating–accelerating transition phase of the universe. This is a quintessence model ${\omega }_{(de)}\ge -1$. The quintessence phase remains for the period $(0\le z\le 0.5806)$. The model is shown to satisfy current observational constraints. Various cosmological parameters relating to the history of the universe have been investigated.

We discuss the application of point transformations in cosmological studies and the application of Lie symmetries as a selection rule for the constraint of the free parameters of the theories. We show that recent published results in Brans–Dicke theory are not correct while the main motivation of that applied is not mathematically inadequate.

In this paper, we have studied the Klein’s paradox in the presence of both scalar and vector potential barriers. From the corresponding Dirac equation, we have calculated the transmission and reflection coefficients. It is shown that the presence of a scalar barrier the scalar potential widens the gap between positive and negative energies and so the forbidden region. Accordingly, the Klein’s paradox disappears when the scalar barrier exceeds a critical value. Considering the problem within the framework of quantum field theory, we have calculated the related pair creation probability, the mean number of created particles and the probability of a vacuum to remain a vacuum. Then it is shown that the scalar potential cuts down the Klein range and minimizes the creation of particles; the particle creation decreases as the scalar potential increases and ceases definitely when the scalar potential reaches the critical value.

We have investigated two-fluid cosmological models in $F(T)$ theory of gravity. $F(T)$ theory is an extension of teleparallel theory of gravity, where $T$ is the torsion scalar. In this paper, we formulated new idea about $F(T)$ theory of gravity with the help of two fluids: one fluid is matter field modeling material content of the Universe and another fluid is radiation field modeling the cosmic microwave background. Some geometric aspects of the $F(T)$ model are discussed with the help of MATLAB.

Thermodynamics of the evolving Lorentzian wormhole at the apparent horizon is investigated in $f(R)$ gravity. Redefining the energy density and the pressure, the continuity equation is satisfied and the field equations in $f(R)$ gravity reduce to the ones in general relativity. However, the energy–momentum tensor includes all the corrections from $f(R)$ gravity. Therefore, one can apply the standard entropy-area relation within $f(R)$ gravity. It is shown that there may be an equivalency between the field equations and the first law of thermodynamics. It seems that an equilibrium thermodynamics may be held on the apparent horizon. The validity of the generalized second law of thermodynamics (GSL) is also investigated in the wormholes.

In this paper, we searched two new exact solutions of Einstein’s field equations for modeling of compact cold stars using embedded class one spacetime continuum. We find out the expressions for pressure, density, anisotropy, redshift, metric potentials and other physical variables in terms of algebraic and trigonometric expressions and observe that all variables show well-behaved trends inside the compact stellar configurations. The causality condition is well maintained by our stellar models, i.e. the radial velocity and transverse velocity are less than l. The stability of our models is assessed via different stability criteria. The Buchdahl condition holds good for our solution. Herrera’s cracking method is applied to check the stability of stellar models. We generate anisotropic compact star models, whose masses and radii are in close agreement with the observed values for compact stars 4U 1538-52, LMCX-4, PSRJ1614-2230. A comparative analysis of the proposed models is carried out based on two different solutions reported in the paper. The appropriate graphical analysis is provided to authenticate the viability of the models.