A vectorlike lepton could make up a tiny fraction of the dark matter. Its large $Z$-boson mediated direct detection cross section can compensate for the small relic abundance, giving rise to an interesting signal at future experiments—perhaps even the first one detected. We discuss how such a scenario might arise in the context of a simple nonthermal cosmology and investigate bounds from direct detection experiments and whether this scenario might be probed at the LHC. Searches for disappearing tracks appear promising.

I present a calculation of next-to-next-to-next-to-leading-order (NNNLO) soft-gluon corrections for top-antitop pair production in hadronic collisions. Approximate NNNLO (aNNNLO) results are obtained by adding the NNNLO soft-gluon corrections to the complete next-to-next-to-leading-order (NNLO) cross section. Theoretical predictions are shown for the total aNNNLO $t\overline{t}$ cross section at LHC and Tevatron energies. The aNNNLO cross sections are larger but have smaller theoretical uncertainties than at NNLO.

The Brownian motion of a test particle interacting with a quantum scalar field in the presence of a perfectly reflecting boundary is studied in ($1+1$)-dimensional flat spacetime. Particularly, the expressions for dispersions in velocity and position of the particle are explicitly derived and their behaviors examined. The results are similar to those corresponding to an electric charge interacting with a quantum electromagnetic field near a reflecting plane boundary, mainly regarding the divergent behavior of the dispersions at the origin (where the boundary is placed), and at the time interval corresponding to a round trip of a light pulse between the particle and the boundary. We close by addressing some effects of allowing the position of the particle to fluctuate.

We prove that the functional renormalization group flow equation admits a perturbative solution and show explicitly the scheme transformation that relates it to the standard schemes of perturbation theory. We then define a universal scheme within the functional renormalization group.

We found new signatures of the dynamical Casimir effect (DCE) in the context of superconducting circuits. We show that if the recent experiment made by Wilson et al., which brought the DCE into reality for the first time, is repeated with slight modifications (for instance, different values for the capacitance of the superconducting quantum interference device), three remarkable results will show up, namely, (i) a quite different spectral distribution for the created particles, deviating from the typical parabolic shape, (ii) an enhancement by a factor of approximately $5\times 10^3$ in the number of created particles with half driving frequency of the effective moving mirror, and (iii) an enhancement by a factor of $3\times 10^2$ in the particle creation rate. These results may guide the experimentalists in their search for alternative routes to observe the DCE in future experiments.

We derive the exact form of the bosonized Hamiltonian for a many-body fermion system in one spatial dimension with arbitrary dispersion relations, using the droplet bosonization method. For a single-particle Hamiltonian polynomial in the momentum, the bosonized Hamiltonian is a polynomial of one degree higher in the bosonic “boundary” field and includes subleading lower-order and derivative terms. This generalizes the known results for massless relativistic and nonrelativistic fermions (quadratic and cubic bosonic Hamiltonians, respectively). We also consider two-body interactions and demonstrate that they lead to interesting collective behavior and phase transitions in the Fermi sea.

Within the left-right model contributions to the neutrino dipole magnetic moments coming from the charged gauge bosons $W_{1,2}^{\pm }$ and the singly charged Higgs bosons ${\tilde{\delta }}^{(\pm )}$ are considered. Calculations show that the Higgs sector contributions to the dipole magnetic moments could exceed the contributions caused by the charged gauge bosons. The resonance transitions in the light left-handed neutrino beam moving in a matter and a magnetic field are investigated in two flavor approximations. Analysis leads to the conclusion that the structure of the heavy neutrino sector admits only three possibilities: (i) the light-heavy neutrino mixing angles ${\theta }_{ii}$ ($i=1,2$) are arbitrary but equal each other whereas the heavy neutrino masses are quasidegenerate; (ii) the heavy neutrino masses are hierarchical ($m_{N_1}<m_{N_2}$) while the angles ${\theta }_{ii}$ are equal to zero; (iii) the light-heavy mixing angles ${\theta }_{ii}$ are equal to each other and the heavy-heavy neutrino mixing is maximal.

We revisit the gravitational collapse of spherically symmetric Lemaître-Tolman-Bondi dust models. A sufficient condition for global visibility of singularity is given. This condition also allows us to extend the condition of local visibility to mass functions which are not Taylor expandable near the center.

In three dimensions, we consider a particular truncation of the Horndeski action that reduces to the Einstein-Hilbert Lagrangian with a cosmological constant $\mathrm{\Lambda }$ and a scalar field whose dynamics is governed by its usual kinetic term together with a nonminimal kinetic coupling. Requiring the radial component of the conserved current to vanish, the solution turns out to be the BTZ black hole geometry with a radial scalar field well defined at the horizon. This means in particular that the stress tensor associated to the matter source behaves on shell as an effective cosmological constant term. We construct a Euclidean action whose field equations are consistent with the original ones and such that the constraint on the radial component of the conserved current also appears as a field equation. With the help of this Euclidean action, we derive the mass and the entropy of the solution, and find that they are proportional to the thermodynamical quantities of the BTZ solution by an overall factor that depends on the cosmological constant. The reality condition and the positivity of the mass impose the cosmological constant to be bounded from above as $\mathrm{\Lambda }\le -\frac{1}{l^2}$ where the limiting case $\mathrm{\Lambda }=-\frac{1}{l^2}$ reduces to the BTZ solution with a vanishing scalar field. Exploiting a scaling symmetry of the reduced action, we also obtain the usual three-dimensional Smarr formula. In the last section, we extend all these results in higher dimensions where the metric turns out to be the Schwarzschild–anti–de Sitter spacetime with planar horizon.

A finite-energy static classical solution is obtained for standard Einstein gravity coupled to an $SO(3)\times SO(3)$ chiral model of scalars (a Skyrme model). This nonsingular localized solution has nontrivial topology for both the spacetime manifold and the $SO(3)$ matter fields. The solution corresponds to a single spacetime defect embedded in flat Minkowski spacetime.

We discuss the semiclassical limit of quantum reduced loop gravity, a recently proposed model to address the quantum dynamics of the early Universe. We apply loop quantum gravity (LQG) techniques in order to define the semiclassical states in the kinematical Hilbert space and we demonstrate that the expectation value of the euclidean scalar constraint coincides with the classical expression, i.e., one of the local Bianchi I dynamics. The result holds as a leading order expansion in the scale factors of the Universe and opens the way to study the subleading corrections to the semiclassical dynamics. We outline how by retaining a suitable finite coordinate length for holonomies that our effective Hamiltonian at the leading order coincides with the one expected from loop quantum cosmology (LQC). This result is an important step in fixing the correspondence between LQG and LQC.

We study gravity with torsion in extra dimensions and derive an effective four-dimensional theory containing four-fermion contact operators at the fundamental scale of quantum gravity in the TeV range. These operators may have an impact on the low-energy observables and can manifest themselves or can be constrained in precision measurements. We calculate possible contributions of these operators to some observables at the one-loop level. We show that the existing precision data on the lepton decay mode of $Z$ boson set limits on the fundamental scale of the gravity and compactification radius, which are more stringent than the limits previously derived in the literature.

The canonical structure of theories whose Lagrangian contains higher powers of time derivatives is often obscured by the nonlinear relationship between the velocities and momenta. We use the Dirac formalism and define a generalized Legendre transform to overcome some of the difficulties associated with inverting the relation between velocities and momenta. We are then able to define a standard single valued symplectic structure on phase space and a compatible single valued Hamiltonian. We demonstrate the application of our formalism in several examples.

The cascading gravity model was proposed to eliminate instabilities of the original Dvali, Gabadadze, Porrati model by embedding our $4D$ (where $D$ refers to the number of space-time dimensions) Universe into a $5D$ brane, which is itself embedded in a $6D$ bulk. Thus gravity cascades from $6D$ down to $4D$ as we decrease the length scales. We show that it is possible to extend this setup to lower dimensions as well, i.e. there is a self-consistent embedding of a $3D$ brane into a $4D$ brane, which is itself embedded in a $5D$ bulk and so on. This extension fits well into the “vanishing dimensions" framework in which dimensions open up as we increase the length scales.

We study a new screening mechanism which is present in Dirac-Born-Infeld (DBI)-like theories. A scalar field with a DBI-like Lagrangian is minimally coupled to matter. In the vicinity of sufficiently dense sources, nonlinearities in the scalar dominate and result in an approximately constant acceleration on a test particle, thereby suppressing the scalar force relative to gravity. Unlike generic $P(X)$ or chameleon theories, screening happens within the regime of validity of the effective field theory thanks to the DBI symmetry. We derive an exact form for the field profile around multiple sources and determine the constraints on the theory parameters from tests of gravity. Perturbations around the spherically-symmetric background propagate superluminally, but we argue for a chronology protection analogous to Galileons. This is the first example of a screening mechanism for which quantum corrections to the theory are under control and exact solutions to cosmological $N$-body problems can be found.

Models of modified gravity, whereby local tests of gravity are evaded thanks to a screening mechanism of the chameleon or Damour-Polyakov types, lead to a spatial variation of the particle masses and the fine structure constant. This is triggered by the environmental dependence of the value of the scalar field whose presence modifies gravity. In dense media, the field settles at a density dependent value while in sparse environments it takes the background cosmological value. We estimate that the maximal deviation of constants from their present values is constrained by local tests of gravity, and must be less than $10^{-6}$.

In this work we study models of anisotropic inflation with the generalized nonvacuum initial states for the inflaton field and the gauge field. The effects of the non-Bunch-Davies initial condition on the anisotropic power spectrum and bispectrum are calculated. We show that the non-Bunch-Davies initial state can help to reduce the fine-tuning on the anisotropic power spectrum while reducing the level of anisotropic bispectrum.

We present improved constraints on an interacting vacuum model using updated astronomical observations including the first data release from Planck. We consider a model with one dimensionless parameter, $\alpha$, describing the interaction between dark matter and vacuum energy (with fixed equation of state $w=-1$). The background dynamics correspond to a generalized Chaplygin gas cosmology, but the perturbations have a zero sound speed. The tension between the value of the Hubble constant, $H_0$, determined by Planck data plus WMAP polarization ($\mathrm{Planck}+\mathrm{WP}$) and that determined by the Hubble Space Telescope (HST) can be alleviated by energy transfer from dark matter to vacuum ($\alpha >0$). A positive $\alpha$ increases the allowed values of $H_0$ due to parameter degeneracy within the model using only cosmic microwave background data. Combining with additional data sets of including supernova type Ia (SN Ia) and baryon acoustic oscillation (BAO), we can significantly tighten the bounds on $\alpha$. Redshift-space distortions (RSD), which constrain the linear growth of structure, provide the tightest constraints on vacuum interaction when combined with Planck+WP, and prefer energy transfer from vacuum to dark matter ($\alpha <0$) which suppresses the growth of structure. Using the combined data sets of $\mathrm{Planck}+\mathrm{WP}+\mathrm{Union}2.1+\mathrm{BAO}+\mathrm{RSD}$, we obtain the constraint on $\alpha$ to be $-0.083<\alpha <-0.006$ (95% C.L.), allowing low $H_0$ consistent with the measurement from 6dF Galaxy survey. This interacting vacuum model can alleviate the tension between RSD and $\mathrm{Planck}+\mathrm{WP}$ in the $\mathrm{\Lambda }\mathrm{CDM}$ model for $\alpha <0$, or between HST measurements of $H_0$ and Planck+WP for $\alpha >0$, but not both at the same time.

We study cosmological perturbations by solving the governing Boltzmann and Einstein field equations up to second order and calculate the corresponding CMB bispectrum during recombination. We include all the second-order Liouville and collision terms, truncating the multipole hierarchy at $l=10$, consistently including all $m\ne 0$ terms when calculating the bispectrum in the flat-sky limit. At this stage, we focus on contributions at and around recombination, and we neglect second-order vector and tensor perturbations, lensing effects, and late-time nonlinear ISW effects. We find that the signal-to-noise for the bispectrum is 0.69 for $l_{\max }=2000$, yielding an overall signal $F_{\mathrm{NL}}=3.19$ (normalized relative to the local model). We find that the effective $f_{\mathrm{NL}}$’s of the equilateral and local type are 5.11 and 0.88, respectively. This recombination signal will have to be taken into account in a quantitative analysis of the Planck data.

Lensing of the cosmic microwave background by modes that are larger than the size of the survey dilates intrinsic scales in the temperature and polarization fields and coherently shifts their observed power spectra with respect to the ensemble or all-sky mean. The effect can be simply encapsulated as a contribution to the power spectrum covariance matrix in accordance with the lensing trispectrum or as an additional parameter, the mean convergence in the field, for parameter estimation. It should be included for upcoming surveys that precisely measure acoustic polarization features deep into the damping tail at multipoles of $\ell \gtrsim 1500$ with less than 10% of the sky. Its omission may lead to seemingly conflicting values for the angular scale of the sound horizon, which may then provide erroneous cosmological parameters when compared to baryon acoustic oscillation measurements.