In the present paper we demonstrate that there exists a new type of neutron stars in the tensor-multi-scalar theories of gravity. We call this new type of neutron stars topological neutron stars. In addition to the standard characteristics of the usual neutron stars the topological neutron stars are also characterized by a topological charge. We numerically construct explicit examples of topological neutron stars in tensor-multi-scalar theories whose target space is ${\mathbb{S}}^3$. Besides the topological charge the topological neutron stars also exhibit other attractive features which can place them among the realistic compact objects that could exist in nature. For example they possess zero scalar charge and thus evade the strong binary pulsar constraints on the dipole scalar radiation.

We carry out a systematic study on the motion of test particles in the region inner to the horizon of a hyperbolically symmetric black hole. The geodesic equations are written and analyzed in detail. The obtained results are contrasted with the corresponding results obtained for the spherically symmetric case. It is found that test particles experience a repulsive force within the horizon, which prevents them to reach the center. These results are obtained for radially moving particles as well as for particles moving in the $\theta -R$ subspace. To complement our study we calculate the precession of a gyroscope moving along a circular path (nongeodesic) within the horizon. We obtain that the precession of the gyroscope is retrograde in the rotating frame, unlike the precession close to the horizon ($R=2m+\epsilon$) in the Schwarzschild spacetime, which is forward.

In this work the neutral meson properties have been investigated in the presence of thermo-magnetic background using a two-flavor Nambu–Jona-Lasinio model. Mass, spectral function, and dispersion relations are obtained in the scalar ($\sigma$) and pseudoscalar (${\pi }^0$) channels as well as in the vector (${\rho }^0$) and axial vector ($a_1^0$) channels. The general Lorentz structures for the vector and axial-vector meson polarization functions have been considered in detail. The ultraviolet divergences appearing in this work have been regularized using a mixed regularization technique where the gamma functions arising in dimensional regularization are replaced with incomplete gamma functions as is usually done in the proper time regularization procedure. The meson spectral functions obtained in the presence of a magnetic field possess nontrivial oscillatory structure. Similar to the scalar and pseudoscalar channel, the spectral functions for each of the modes of ${\rho }^0$ are observed to overlap with the corresponding modes of its chiral partner $a_1^0$ mesons in the chiral symmetry restored phase. We observe discontinuities in the masses of all the mesonic excitations for a nonzero external magnetic field.

As a toy model for QED in strong background fields, we consider the impact of backreaction and loop effects on scattering processes in quantum optics. We show that neglecting backreaction misses qualitative and quantitative features of strong-field physics. We are able to study an analog of the Narozhny-Ritus conjecture on the scaling of higher loop diagrams with intensity: we prove that there is no corresponding behavior in our model. Implications for QED are identified and discussed.

We derive constraints on all possible general neutrino-electron interactions (scalar, vector, pseudo-scalar, axial-vector and tensor) using the recent real time Borexino event rate measurements of $pp$, $pep$ and ${}^7\mathrm{Be}$ solar neutrinos. Some of the limits improve from TEXONO and CHARM-II for incoming electron and muon neutrinos while the rest remains weaker for Borexino and those for the tau flavor are the first ones. Future improvements by next-generation solar neutrino experiments are also studied. The limits extend the physics reach of solar neutrino measurements to TeV-scale physics. Finally, the different properties of the new interactions for Dirac and Majorana neutrinos are discussed.

We study the spontaneous $CP$ violation through the stabilization of the modulus $\tau$ in modular invariant flavor models. The $CP$-invariant potential has the minimum only at $\mathrm{Re}[\tau ]=0$ or $1/2(\mathrm{mod}1)$. From this result, we study $CP$ violation in modular invariant flavor models. The physical $CP$ phase is vanishing. The important point for the $CP$ conservation is the $T$ transformation in the modular symmetry. One needs the violation of $T$ symmetry to realize the $CP$ violation.

Even if the LHC observations are consistent with the Standard model (SM), current LHC results are not precise enough to rule out the presence of new physics. Taking a contrarian view of the SM Higgs fandom, we look out for a more suitable candidate for the 125 GeV boson observed at the LHC. At the same time, a recent result from CMS hints toward an excess near 95 GeV in the diphoton ($\gamma \gamma$) channel. Given these aspects, we revisit the Higgs-radion mixing model to explore the viability of the radion mixed Higgs to be the 125 GeV boson along with the presence of a light radion (to be precise, Higgs mixed radion) that can show up in future experiments in the $\gamma \gamma$ channel. We find that the mixed radion-Higgs scenario gives a better fit than the SM, with the radion mixed Higgs as a more suitable 125 GeV scalar candidate. It also gives rise to a diphoton excess from the light radion, consistent with the LHC observations.

The existence of dark matter particles that carry phenomenologically relevant self-interaction cross sections mediated by light dark sector states is considered to be severely constrained through a combination of experimental and observational data. The conclusion is based on the assumption of specific dark matter production mechanisms such as thermal freeze-out together with an extrapolation of a standard cosmological history beyond the epoch of primordial nucleosynthesis. In this work, we drop these assumptions and examine the scenario from the perspective of the current firm knowledge we have results from direct and indirect dark matter searches and cosmological and astrophysical observations, without additional assumptions on dark matter genesis or the thermal state of the very early universe. We show that even in the minimal setup, where dark matter particles self-interact via a kinetically mixed vector mediator, a significant amount of parameter space remains allowed. Interestingly, however, these parameter regions imply a metastable, light mediator, which in turn calls for modified search strategies.

The $tthh$ production at colliders contains rich information on the nature of the Higgs boson. In this article, we systematically studied its physics at the high-luminosity Large Hadron Collider (HL-LHC), using exclusive channels with multiple ($\ge 5$) $b$-jets and one lepton ($5b1\ell$), multiple ($\ge 5$) $b$-jets and opposite-sign dilepton ($5b2\ell$), same-sign dilepton ($\mathrm{SS}2\ell$), multiple leptons (multi-$\ell$), and ditau resonance ($\tau \tau$). The scenarios analyzed include: (1) the $tthh$ production in Standard Model; (2) the $tthh$ production mediated by anomalous cubic Higgs self-coupling and $tthh$ contact interaction; (3) heavy Higgs ($H$) production with $ttH\to tthh$; and (4) pair production of fermionic top partners ($T$) with $TT\to tthh$. To address the complication of event topologies and the mess of combinatorial backgrounds, a tool of boosted-decision-tree was applied in the analyses. The $5b1\ell$ and $\mathrm{SS}2\ell$ analyses define the two most promising channels. For the nonresonant $tthh$ production, a combination of these exclusive analyses allows for its measurement in the SM with a statistical significance $\sim 0.9\sigma$ (with $S/B>1\%$), and may partially break the sensitivity degeneracy with respect to a varying cubic Higgs self-coupling, a difficulty usually thought to exist in gluon fusion di-Higgs analysis at HL-LHC. These sensitivities were also projected to future hadron colliders at 27 TeV and 100 TeV. For the resonant $tthh$ productions, the heavy Higgs boson in type II two-Higgs-doublet-model could be efficiently searched for between the mass thresholds $2m_h<m_H<2m_t$ and even beyond that, for relatively small $\tan \beta$ (vacuum alignment), while the fermionic top partners in composite Higgs models could be probed up to $\sim 1.5\mathrm{TeV}$ and $\sim 1.7\mathrm{TeV}$, for $\mathrm{Br}(T\to th)=25\%$ and 50%, respectively.

We explore a simple model which naturally explains the observed baryon asymmetry of the Universe. In this model the strong coupling is promoted to a dynamical quantity, which evolves through the vacuum expectation value of a singlet scalar field that mixes with the Higgs field. In the resulting cosmic history, QCD confinement and electroweak symmetry breaking initially occur simultaneously close to the weak scale. The early confinement triggers a chemical potential between baryons and antibaryons through the interactions of the ${\eta }^{\prime }$ meson, resulting in spontaneous baryogenesis. The electroweak sphalerons are sharply switched off after confinement and the baryon asymmetry is frozen in. Subsequently, evolution of the Higgs vacuum expectation value (which is modified in the confined phase) triggers a relaxation to a Standard Model–like vacuum. We identify viable regions of parameter space and describe various experimental probes, including current and future collider constraints and gravitational wave phenomenology.

We prove that the values of the electroweak mixing angle ${\sin }^2{\theta }_W$ and the intermediate mass scale $M_I$ have vanishing contributions due to one-loop, two-loop, and gravitational corrections in grand unified theories which accommodate an intermediate trinification symmetry ($G_{333D}$) invoked with spontaneous D-parity mechanism operative at mass scale greater than $M_I$. The proof of the theorem is robust and we verify the results numerically using supersymmetric as well as nonsupersymmetric versions of $E_6$-GUT.

In present work, we show how the threshold expansion formula of $N$ identical bosons in finite volume may be derived by iterations of Faddeev-type coupled dynamical equations. The energy shift of $N$-boson system near threshold is dominated by zero momenta mode of $N$-body amplitudes with all particles nearly static. The dominant zero momenta mode and subleading nonzero momenta mode contributions are connected through finite volume Faddeev-type coupled dynamical equations. Eliminating nonzero momenta modes by iterations ultimately yields an analytic expression that can be solved by threshold expansion.

We apply the $k_T$-factorization approach to the production of $\varphi$ mesons in deep inelastic scattering. The helicity-conserving ${\gamma }^{*}(T,L)\to \varphi$ impact factor is calculated for longitudinal and transverse photon polarization using $\varphi$ meson distribution amplitudes. Different unintegrated gluon distributions are used in the calculations. The formalism for massless quarks/antiquarks gives too large transverse and longitudinal cross sections for photon virtualities below $Q^2\sim 8{\mathrm{GeV}}^2$. We suggest how to improve the description of the HERA data by introducing effective strange-quark masses into the formalism. We derive the corresponding impact factor for a finite quark mass by comparing to the light-cone wave function representation used in previous $k_T$-factorization calculations and the color-dipole approaches. As a byproduct we present expressions for higher twist amplitudes as weighted integrals over the light-cone wave function. The quark mass $m_q\approx 0.5\mathrm{GeV}$ allows to improve the description of both longitudinal and transverse cross sections down to $Q^2\sim 4{\mathrm{GeV}}^2$ but this depends slightly on the renormalization scale used in the calculation. We also present the polarized cross section ratio ${\sigma }_L/{\sigma }_T$ and the behavior of the total cross section ${\sigma }_{\text{tot}}={\sigma }_L+{\sigma }_T$ as a function of photon virtuality.

Charged lepton flavor violating muon decay ${\mu }^{+}\to e^{+}{X}_H$, where $X_H$ is a massive neutral boson, was sought by searching for extra peaks in the muon decay ${\mu }^{+}\to e^{+}\nu \overline{\nu }$ energy spectrum in the $m_{X_H}$ mass region $47.8–95.1\mathrm{MeV}/c^2$. No signal was found and 90% confidence level upper limits were set on the branching ratio $\mathrm{\Gamma }({\mu }^{+}\to e^{+}{X}_H)/\mathrm{\Gamma }({\mu }^{+}\to e^{+}\nu \overline{\nu })$ at the level of ${10}^{-5}$ for this region.

We elucidate the mismatch between the $A$-anomaly coefficient and the coefficient of the logarithmic term in the entanglement entropy of a Maxwell field. In contrast to the usual assumptions about the protection of renormalization group charges at the infrared, the logarithmic term is different for a free Maxwell field and a Maxwell field interacting with heavy charges. This is possible because of the presence of superselection sectors in the IR theory. However, the correction due to the coupling with charged vacuum fluctuations, that restores the anomaly coefficient, is independent of the precise UV dynamics. The problem is invariant under electromagnetic duality, and the solution requires both the existence of electric charges and magnetic monopoles. We use a real-time operator approach, but we also show how the results for the free and interacting fields are translated into an effective correction to the four-sphere partition function.

We further develop the massive constructive theory of the Standard Model and use it to calculate the amplitude and squared amplitude for all two-body decays, a collection of weak three-body decays, as well as Higgs decay to four neutrinos. We compare our results with those from Feynman diagrams and find complete agreement. We show that in all the cases considered here, the amplitudes of massive constructive theories are significantly simpler than those resulting from Feynman diagrams. In fact, a naive counting of the number of calculations required for a matrix-element generator to compute a phase-space point is orders-of-magnitude smaller for the result coming from the constructive method suggesting that these generators might benefit from this method in the future, even in the case of massive weak amplitudes. We also anticipate that our simpler expressions will produce numerically more stable expressions.

We study the deflection angle of a light ray as it traverses on the equatorial plane of a charged spinning black hole. We provide detailed analysis of the light ray’s trajectory, and derive the closed-form expression of the deflection angle due to the black hole in terms of elliptic integrals. In particular, the geodesic equation of the light ray along the radial direction can be used to define an appropriate “effective potential.” The nonzero charge of the black hole shows stronger repulsive effects to prevent light rays from falling into the black hole as compared with the Kerr case. As a result, the radius of the innermost circular motion of light rays with the critical impact parameter decreases as charge $Q$ of the black hole increases for both direct and retrograde motions. Additionally, the deflection angle decreases when $Q$ increases with the fixed impact parameter. These results will have a direct consequence on constructing the apparent shape of a charged rotating black hole.

Recently, we have shown that dynamically stable ergostar solutions (equilibrium neutron stars that contain an ergoregion) with a compressible and causal equation of state exist [A. Tsokaros, M. Ruiz, L. Sun, S. L. Shapiro, and K. Uryū, Phys. Rev. Lett. 123, 231103 (2019)]. These stars are hypermassive, differentially rotating, and highly compact. In this work, we make a systematic study of equilibrium models in order to locate the position of ergostars in parameter space. We adopt four equations of state that differ in the matching density of a maximally stiff core. By constructing a large number of models both with uniform and differential rotation of different degrees, we identify the parameters for which ergostars appear. We find that the most favorable conditions for the appearance of dynamically stable ergostars are a significant finite density close to the surface of the star (i.e., similar to self-bound quark stars) and a small degree of differential rotation.

We study the cosmological evolution of a scalar field in Horndeski gravity, assuming that the scalar field is subdominant with respect to the cosmic fluid. First, we analyze the most general shift-symmetric action that respects local Lorentz symmetry. We show that the bound on the speed of gravitational waves set by $\mathrm{GW}170817+\mathrm{GRB}170817\mathrm{A}$ imposes a constraint only on the linear coupling between the scalar and the Gauss-Bonnet invariant, and this constraint is rather mild. Then, we consider some interesting examples of theories that break shift symmetry, such as the Damour-Esposito-Farèse model of spontaneous scalarization and a theory with a quadratic coupling to the Gauss-Bonnet invariant that can lead to black hole scalarization. In both cases, tuning of cosmological initial conditions is necessary to keep the scalar field dormant during cosmic evolution.