Although $f(R)$ modification of late-time cosmology is successful in explaining the present cosmic acceleration, it is difficult to simultaneously satisfy the fifth-force constraint. Even when the fifth-force constraint is satisfied, the effective scalar degree of freedom may move to a point, close to its potential minimum, in the field space where the Ricci scalar diverges. We elucidate this point further with a specific example of $f(R)$ gravity that incorporates several viable $f(R)$ gravity models from the literature. In particular, we show that the nonlinear evolution of the scalar field in pressureless contracting dust can easily lead to the curvature singularity, making this theory unviable.

The leptonic bound states positronium and muonium are used to constrain Galileon contributions to the Lamb shift of muonic hydrogen. Through the application of a variety of bounds on lepton compositeness, it is shown that either the assumption of equating the charge radius of a particle with its Galileon scale radius is incompatible with experiments, or the scale of Galileons must be $M>1.33\mathrm{GeV}$, too large to solve the muon problem. The possibility of stronger constraints in the future from true muonium is discussed.

We discuss inclusive production of open charm mesons in proton-proton scattering at the BNL RHIC. The calculation is performed in the framework of $k_t$-factorization approach which effectively includes higher-order pQCD corrections. Different models of unintegrated gluon distributions (UGDF) from the literature are used. We focus on UGDF models favored by the LHC data and on a new up-to-date parametrizations based on the HERA collider deep-inelastic scattering high-precision data. Results of the $k_t$-factorization approach are compared to next-to-leading order collinear predictions. The hadronization of heavy quarks is done by means of fragmentation function technique. The theoretical transverse momentum distributions of charmed mesons are compared with recent experimental data of the STAR collaboration at $\sqrt{s}=200$ and 500 GeV. Theoretical uncertainties related to the choice of renormalization and factorization scales as well as due to the quark mass are discussed. A very good description of the measured integrated cross sections and differential distributions is obtained for the Jung setB0 CCFM UGDF. Revised charm and bottom theoretical cross sections corresponding to those measured recently by the STAR and PHENIX collaborations for semileptonic decays of $D$ and $B$ mesons are presented. Significant improvement in theoretical description of the nonphotonic electrons measurements is clearly obtained with respect to the previous studies within the $k_t$-factorization.

Mass spectra of strange baryons are calculated in the framework of the relativistic quark model based on the quasipotential approach. Baryons are treated as relativistic quark-diquark bound systems. It is assumed that two quarks with equal constituent masses form a diquark. The diquark excitations and its internal structure are consistently taken into account. Calculations are performed up to rather high orbital and radial excitations of strange baryons. On this basis the Regge trajectories are constructed. The obtained results are compared with available experimental data and previous predictions. It is found that all masses of the 4- and 3-star states of strange baryons with established quantum numbers, as well as most of the 2- and 1-star states, are well reproduced. The developed relativistic quark-diquark model predicts less excited states than three-quark models of strange baryons.

A new theoretical insight into the pattern of neutrino mixing and leptonic $CP$ violation is presented. It leads naturally and uniquely to a specific dark sector of three real neutral scalar singlets, with the radiative implementation of the inverse seesaw mechanism for neutrino mass. The new simple but crucial enabling idea is that a familiar $A_4$ transformation turns any orthogonal $3\times 3$ matrix into one which predicts ${\theta }_{23}=\pi /4$ and ${\delta }_{CP}=\pm \pi /2$ for the neutrino mixing matrix, in good agreement with present data.

In this paper, we present a formal solution to the nonlinear field equations of ten-dimensional super Yang-Mills theory. It is assembled from products of linearized superfields which have been introduced as multiparticle superfields in the context of superstring perturbation theory. Their explicit form follows recursively from the conformal field theory description of the gluon multiplet in the pure spinor superstring. Furthermore, superfields of higher-mass dimensions are defined and their equations of motion are spelled out.

Recent results in the construction of anomaly-free models of loop quantum gravity have shown obstacles when local physical degrees of freedom are present. Here, a set of no-go properties is derived in polarized Gowdy models, raising the question of whether these systems can be covariant beyond a background treatment. As a side product, it is shown that normal deformations in classical polarized Gowdy models can be Abelianized.

Theories of modified gravity, in both the linear and fully nonlinear regimes, are often studied under the assumption that the evolution of the new (often scalar) degree of freedom present in the theory is quasistatic. This approximation significantly simplifies the study of the theory, and one often has good reason to believe that it should hold. Nevertheless it is a crucial assumption that should be explicitly checked whenever possible. In this paper we do so for the Vainshtein mechanism in a cosmological setting. By solving for the full spatial and time evolution of the Dvali–Gabadadze–Porrati and the cubic Galileon models, in an expanding spherical symmetric spacetime, we are able to demonstrate that the Vainshtein solution is a stable attractor and forms no matter what initial conditions we take for the scalar field. Furthermore,the quasistatic approximation is also found to be a very good approximation whenever it exists. For the best-fit cubic Galileon model, however, we find that for deep voids at late times the numerical solution blows up at the same time as the quasistatic solution ceases to exist. We argue that this phenomenon is a true instability of the model.

We study the critical behavior at the threshold of black hole formation in a model consisting of a scalar field incident to a reflector barrier enclosing a Schwarzschild black hole. Weak incident scalar field waves disturb slightly the black hole spacetime and are completely radiated by the reflector, like water waves striking against the wall of a dam. Strong incident waves produce the formation of an apparent horizon outside the barrier. In this case, a fraction of scalar field crosses the horizon together with the barrier, whereas another fraction escapes to infinity. We have integrated the field equations using a Galerkin collocation code that allowed the necessary accuracy to investigate the behavior of the black hole masses for a broad range of scalar field initial amplitude. We have shown that a scaling law describes the black hole masses for amplitudes very close to the critical value. In the limit of very strong scalar fields, the black hole masses either scale linearly with the initial amplitude or saturate depending on the existence of the initial monopole moment.

Excess emission over expected diffuse astrophysical backgrounds in the direction of the Galactic center region has been claimed at various wavelengths, from radio to gamma rays. Among particle models advocated to explain such observations, a few invoke interactions between dark matter particles and ordinary matter, such as cosmic rays, interstellar gas or free electrons. Depending on the specific interstellar matter particles’ species and energy, such models predict distinct morphological features. In this study we make detailed, model-independent predictions for the morphology of models where the relevant electromagnetic emission is proportional to the product of the dark matter density profile and the density of interstellar matter or cosmic rays. We compare the predicted latitudinal and longitudinal distributions with observations, and provide the associated set of relevant spatial templates.

Production rates of dileptons and photons from the quark-gluon (QGP) phase are calculated taking into account effects of confinement and spontaneous chiral symmetry breaking ($\chi \mathrm{SB}$) not far from the transition temperature $T_c$. We find that the production rates of dileptons with large momenta and of photons originating from the QGP around $T_c$ are suppressed by the $\chi \mathrm{SB}$ effect. We also discuss to what extent information about details of the chiral transition, such as its characteristic temperature range and the steepness of the crossover, are reflected in these quantities.

The inert scalar doublet model of dark matter can be valid up to the Planck scale. We briefly review the bounds on the model in such a scenario and identify parameter spaces that lead to absolute stability and metastability of the electroweak vacuum.

Recently it was shown that in QCD-like theories with $N_f>N$, where $N_f$ is the number of light flavors and $N$ is the number of colors, there are correlation functions that vanish in perturbation theory and at short distances receive dominant, calculable contributions from small instantons. Here we extend the set of such objects to theories with $N_f=N$, which includes real QCD, and discuss their application as a calibration of lattice computations at small quark mass. We revisit the related issue of the $u$ quark mass and its additive renormalization by small instantons, and discuss an alternative test of $m_u=0$ on the lattice.

We compare the effects of timelike, lightlike and spacelike one-dimensional inhomogeneities on the probability of nonperturbative pair production in strong fields. Using interpolating coordinates we give a unifying picture in which the effect of the inhomogeneity is encoded in branch cuts and poles circulated by complex worldline instantons. For spacelike inhomogeneities the length of the cut is related to the existence of critical points, while for lightlike inhomogeneities the cut contracts to a pole and the instantons become contractible to points, leading to simplifications particular to the lightlike case. We calculate the effective action in fields with up to three nonzero components, and investigate its behavior under changes in field dependence.

There is a forbidden region in the parameter space of quasinormal modes of black holes in general relativity. Using both inspiral and ringdown phases of gravitational waves from binary black holes, we propose two methods to test general relativity. We also evaluate how our methods will work when we apply them to Pop III black-hole binaries with typical masses. Adopting the simple mean of the estimated range of the event rate, we have the expected rate of $500{\mathrm{yr}}^{-1}$. Then, the rates of events with signal-to-noise ratios greater than 20 and greater than 50 are $32{\mathrm{yr}}^{-1}$ and $2{\mathrm{yr}}^{-1}$, respectively. Therefore, there is a good chance to confirm (or refute) the Einstein theory in the strong gravity region by observing the expected quasinormal modes.

We generalize the Buchdahl–Bondi limit for the case of static, spherically symmetric, relativistic compact stars immersed in Schwarzschild vacuum in the $f(R)$-theory of gravity, subject to very generic regularity, thermodynamic stability, and matching conditions. Similar to the case of general relativity, our result is model independent and remains true for any physically realistic equation of state of standard stellar matter. We show that an extramassive stable star can exist in these theories, with a surface redshift larger than 2, which is forbidden in general relativity. This result gives a novel and interesting observational test for validity or otherwise of general relativity and also provides a possible solution to the dark matter problem.

We perform a study of the nonlinear clustering of matter in the late-forming dark matter (LFDM) scenario in which dark matter results from the transition of a nonminimally coupled scalar field from radiation to collisionless matter. A distinct feature of this model is the presence of a damped oscillatory cutoff in the linear matter power spectrum at small scales. We use a suite of high-resolution N-body simulations to study the imprints of LFDM on the nonlinear matter power spectrum, the halo mass and velocity functions and the halo density profiles. The model largely satisfies high-redshift matter power spectrum constraints from Lyman-$\alpha$ forest measurements, while it predicts suppressed abundance of low-mass halos ($\sim {10}^9–{10}^{10}{\mathrm{h}}^{-1}{\mathrm{M}}_{\odot }$) at all redshifts compared to a vanilla $\mathrm{\Lambda }\mathrm{CDM}$ model. The analysis of the LFDM halo velocity function shows a better agreement than the $\mathrm{\Lambda }\mathrm{CDM}$ prediction with the observed abundance of low-velocity galaxies in the local volume. Halos with mass $M\gtrsim {10}^{11}{\mathrm{h}}^{-1}{\mathrm{M}}_{\odot }$ show minor departures of the density profiles from $\mathrm{\Lambda }\mathrm{CDM}$ expectations, while smaller-mass halos are less dense, consistent with the fact that they form later than their $\mathrm{\Lambda }\mathrm{CDM}$ counterparts.

The purpose of this mock data and science challenge is to prepare the data analysis and science interpretation for the second generation of gravitational-wave experiments Advanced LIGO-Virgo in the search for a stochastic gravitational-wave background signal of astrophysical origin. Here we present a series of signal and data challenges, with increasing complexity, whose aim is to test the ability of current data analysis pipelines at detecting an astrophysically produced gravitational-wave background, test parameter estimation methods and interpret the results. We introduce the production of these mock data sets that includes a realistic observing scenario data set where we account for different sensitivities of the advanced detectors as they are continuously upgraded toward their design sensitivity. After analyzing these with the standard isotropic cross-correlation pipeline we find that we are able to recover the injected gravitational-wave background energy density to within $2\sigma$ for all of the data sets and present the results from the parameter estimation. The results from this mock data and science challenge show that advanced LIGO and Virgo will be ready and able to make a detection of an astrophysical gravitational-wave background within a few years of operations of the advanced detectors, given a high enough rate of compact binary coalescing events.

The observed spectrum of Galactic cosmic rays has several exciting features such as the rise in the positron fraction above $\sim 10\mathrm{GeV}$ of energy and the spectral hardening of protons and helium at $\gtrsim 300\mathrm{GeV}/\mathrm{n}\mathrm{ucleon}$ of energy. The ATIC-2 experiment has recently reported an unexpected spectral upturn in the elemental ratios involving iron, such as the $\mathrm{C}/\mathrm{Fe}$ or $\mathrm{O}/\mathrm{Fe}$ ratios, at energy $\gtrsim 50\mathrm{GeV}$ per nucleon. It is recognized that the observed positron excess can be explained by pion production processes during diffusive shock acceleration of cosmic-ray hadrons in nearby sources. Recently, it was suggested that a scenario with nearby source dominating the GeV-TeV spectrum may be connected with the change of slope observed in protons and nuclei, which would be interpreted as a flux transition between the local component and the large-scale distribution of Galactic sources. Here I show that, under a two-component scenario with nearby source, the shape of the spectral transition is expected to be slightly different for heavy nuclei, such as iron, because their propagation range is spatially limited by inelastic collisions with the interstellar matter. This enables a prediction for the primary/primary ratios between light and heavy nuclei. From this effect, a spectral upturn is predicted in the $\mathrm{C}/\mathrm{Fe}$ and $\mathrm{O}/\mathrm{Fe}$ ratios in good accordance with the ATIC-2 data.

A weighted average weak mixing angle ${\theta }_W$ derived from relatively low $Q^2$ experiments is compared with the standard model prediction obtained from precision measurements. The approximate 1.8 sigma discrepancy is fit with an intermediate mass ($\sim 10–35\mathrm{GeV}$) “dark” $Z$ boson $Z_d$, corresponding to a $U(1{)}_d$ gauge symmetry of hidden dark matter, which couples to our world via kinetic and $Z–Z_d$ mass mixing. Constraints on such a scenario are obtained from precision electroweak bounds and searches for the rare Higgs decays $H\to Z{Z}_d\to 4$ charged leptons at the LHC. The sensitivity of future anticipated low $Q^2$ measurements of ${\sin }^2{\theta }_W(Q^2)$ to intermediate mass $Z_d$ is also illustrated. This dark $Z$ scenario can provide interesting concomitant signals in low energy parity violating measurements and rare Higgs decays at the LHC over the next few years.