We report a quasidifferential upper limit on the extremely-high-energy (EHE) neutrino flux above $5\times {10}^6\mathrm{GeV}$ based on an analysis of nine years of IceCube data. The astrophysical neutrino flux measured by IceCube extends to PeV energies, and it is a background flux when searching for an independent signal flux at higher energies, such as the cosmogenic neutrino signal. We have developed a new method to place robust limits on the EHE neutrino flux in the presence of an astrophysical background, whose spectrum has yet to be understood with high precision at PeV energies. A distinct event with a deposited energy above $10^6\mathrm{GeV}$ was found in the new two-year sample, in addition to the one event previously found in the seven-year EHE neutrino search. These two events represent a neutrino flux that is incompatible with predictions for a cosmogenic neutrino flux and are considered to be an astrophysical background in the current study. The obtained limit is the most stringent to date in the energy range between $5\times {10}^6$ and $2\times {10}^{10}\mathrm{GeV}$. This result constrains neutrino models predicting a three-flavor neutrino flux of $E_{\nu }^2{\varphi }_{{\nu }_e+{\nu }_{\mu }+{\nu }_{\tau }}\simeq 2\times {10}^{-8}\mathrm{GeV}/{\mathrm{cm}}^2\sec \mathrm{sr}$ at $10^9\mathrm{GeV}$. A significant part of the parameter space for EHE neutrino production scenarios assuming a proton-dominated composition of ultra-high-energy cosmic rays is disfavored independently of uncertain models of the extragalactic background light which previous IceCube constraints partially relied on.

We estimate the target-normal single-spin asymmetry at near forward angles in elastic electron-nucleon scattering. In the leading-order approximation, this asymmetry is proportional to the imaginary part of the two-photon exchange (TPE) amplitude, which can be expressed as an integral over the doubly virtual Compton scattering (VVCS) tensor. We develop a model that parametrizes the VVCS tensor for the case of near forward scattering angles. Our parametrization ensures a proper normalization of the imaginary part of the TPE amplitude on the well-known forward limit expression, which is given in terms of nucleon structure functions measurable in inelastic electron-nucleon scattering experiments. We discuss applicability limits of our theory and provide target-normal single-spin asymmetry predictions for both elastic electron-proton and electron-neutron scattering.

We make use of recently released parton density functions (PDFs) with threshold-resummation improvement to consistently calculate theoretical predictions for neutralino and chargino pair production at next-to-leading order and next-to-leading logarithmic accuracy. The updated cross sections have been computed for experimentally relevant Higgsino and gaugino search channels at the ongoing Run II of the LHC. A factorization method is applied to exploit the smaller PDF uncertainty of the global PDF sets and to avoid complications arising in the refitting of threshold-resummation improved PDF replicas in Mellin space. The reduction of the scale uncertainty due to the resummation is, however, explicitly taken into account. As expected, the resummation contributions in the PDF fits partially compensate the cross section enhancements induced by those in the partonic matrix elements.

We analyze the sensitivity of low-energy fundamental symmetry tests to interactions mediated by doubly-charged scalars that arise in type II seesaw models of neutrino mass and their left-right symmetric extensions. We focus on the next generation measurement of the parity-violating asymmetry in Møller scattering planned by the MOLLER collaboration at Jefferson Laboratory. We compare the MOLLER sensitivity to that of searches for charged lepton flavor violation (CLFV) and neutrinoless double beta-decay ($0\nu \beta \beta$-decay) as well as present and possible future high-energy collider probes. We show that for the simplest type-II seesaw scenario, CLFV searches have the greatest sensitivity. However, in a left-right symmetric extension where the scale of parity-breaking is decoupled from the $SU(2{)}_R$-breaking scale, the MOLLER experiment will provide a unique probe of scalar triplet interactions in the right-handed sector for a doubly-charged scalar mass up to $\sim 10\mathrm{TeV}$ and help elucidate the mechanism of $0\nu \beta \beta$-decay.

We have studied two approaches to predict quark and lepton mixing patterns from the same flavor symmetry group in combination with $CP$ symmetry. The first approach is based on the residual symmetry $Z_2$ in the charged lepton sector and $Z_2\times CP$ in the neutrino sector. All lepton mixing angles and $CP$ violation phases depend on three real parameters ${\theta }_l$, ${\delta }_l$ and ${\theta }_{\nu }$. This approach is extended to the quark sector, the up and down quark mass matrices are assumed to be invariant under a $Z_2$ subgroup and $Z_2\times CP$. The necessary and sufficient conditions for the equivalence of two mixing patterns are derived. The second approach has an Abelian subgroup and a single $CP$ transformation as residual symmetries of the charged lepton and neutrino sectors, respectively. The lepton mixing would be determined up to a real orthogonal matrix multiplied from the right hand side. Analogously we assume that a single $CP$ transformation is preserved by the down (or up) quark mass matrix, and the residual symmetry of the up (or down) quark sector is be an Abelian subgroup. As an example, we analyze the possible mixing patterns which can be obtained from the breaking of $\mathrm{\Delta }(6n^2)$ and $CP$ symmetries. We find $\mathrm{\Delta }(294)$ combined with $CP$ is the smallest flavor group which can give a good fit to the experimental data of quark and lepton mixing in both approaches.

In quantum chromodynamics, axial charge is known to be nonconserved due to chiral anomaly and nonvanishing quark mass. In this paper, we explore the role of quark mass in axial charge fluctuation and dissipation. We present two separate calculations of the axial charge correlator, which describe dynamics of axial charge. The first is free quarks at a finite temperature. We find that axial charge can be generated through effective quantum fluctuations in free theory. However, the fluctuation does not follow a random walk behavior. Because of the presence of an axial symmetry breaking mass term, the axial charge also does not settle asymptotically to the thermodynamic limit given by susceptibility. The second calculation is in a weakly coupled quark gluon plasma. We find in the hard thermal loop (HTL) approximation, the quark-gluon interaction leads to random walk growth of the axial charge, but dissipation is not visible. We estimate the relaxation timescale for the axial charge, finding it lies beyond the HTL regime.

An instanton is known to exist in Euclidean spacetime only. Their role in real time dynamics is usually understood as a tunneling effect by Wick rotation. We illustrate other effects of instanton in holography by investigating five-dimensional effective gravity theory of the black D3-brane-D-instanton system. The supergravity description of the D3-brane-D-instanton system is dual to the super-Yang-Mills theory with topological excitations of the vacuum. We obtain Euclidean correlators in the presence of instantons by analyzing the fluctuations of the bulk fields in the five-dimensional effective theory. Furthermore, analytic continuation of Euclidean correlators leads to retarded correlators, which characterize real time dynamics. We find interestingly real time fluctuations of topological charge can destroy instantons and the lifetime of instanton is set by temperature. This implies instanton contribution to “real time dynamics” is suppressed at high temperature, which is analogous to classic field theory results that the instanton contribution to “thermodynamics” is suppressed at high temperature.

A cosmological model with two global internal times shows that time reparametrization invariance, and therefore covariance, is not guaranteed by deparametrization. In particular, it is impossible to derive unambiguous proper-time effective equations from a single deparametrized model if quantum corrections from fluctuations and higher moments are included. The framework of effective constraints shows how different choices of internal time can be compared, and it suggests a new definition of coordinate time in quantum cosmology which is amenable to a notion of time reparametrization invariance. In addition to corrections from moments such as quantum fluctuations, factor ordering corrections may appear also. The latter generically lead to inequivalent internal-time formulations owing to quantization ambiguities.

We study the vacuum fluctuations of a massless scalar field $\widehat{\mathrm{\Psi }}$ on the background of a global monopole. Due to the nontrivial topology of the global monopole spacetime characterized by a solid deficit angle parametrized by ${\eta }^2$, we expect that ${⟨{\widehat{\mathrm{\Psi }}}^2⟩}_{\mathrm{ren}}$ and ${⟨{\widehat{T}}_{\mu \nu }⟩}_{\mathrm{ren}}$ are nonzero and proportional to ${\eta }^2$, so that they annul in the Minkowski limit $\eta \to 0$. However, due to the naked singularity at the monopole core, the evolution of the scalar field is not unique. In fact, they are in one-to-one correspondence with the boundary conditions which turn into self-adjoint the spatial part of the wave operator. We show that only the Dirichlet boundary condition corresponds to our expectations and gives zero contribution to the vacuum fluctuations in the Minkowski limit. All other boundary conditions give nonzero contributions in this limit due to the nontrivial interaction between the field and the singularity.

It has been known for a while that there is spontaneous breaking of Lorentz symmetry in the nonzero charged sectors of quantum electrodynamics due to the infrared problem of soft photons. More recently, it has also been suggested that similar results hold for color transformations in a non-Abelian gauge theory. Here, we show that an action where a diffeomorphism has been carried out for the part describing hard gauge particles and matter fields can be used to analyze these issues. In addition to rederiving old results in this formalism, we also show that color transformations cannot be unitarily implemented on perturbative gluon states if gluon fields of arbitrarily low energy are allowed. Implications for confinement and mass gap are briefly commented upon.

We treat an Unruh-DeWitt detector as an open quantum system and evaluate the response of a uniformly accelerated detector: (i) interacting locally with the derivatives of a massless scalar field and (ii) linearly coupled to an electromagnetic field. We find that the early-time transition rate of the detector strongly depends on the type of the interaction between the detector and the quantum field, and may not follow a Planck distribution. In contrast, the late time asymptotic state is always thermal at the Unruh temperature and thus provides a more fundamental and persistent characterization of the acceleration temperature: A uniformly accelerated detector experiences the field vacuum as a genuine thermal bath at the Unruh temperature and eventually settles at a thermal state, regardless of their intermediate dynamics or the type of interaction.

In this paper, we investigate the continual gravitational collapse of a spherically symmetric radiation shell in five-dimensional Einstein-Gauss-Bonnet gravity. We show that the final fate of such a collapse is an extended and weak curvature naked conical singularity at the centre, which then subsequently becomes covered by an apparent horizon. This process is entirely different from the five dimensional general relativity counterpart, where a strong curvature singularity develops at the centre. Since the singularity in the case of Einstein-Gauss-Bonnet gravity is sufficiently weak, we argue that the spacetime can be extended through it, which gives us an elegant way of constructing regular black holes in higher dimensions without violating any energy conditions. We also extend our study to spacetimes with null and string fluids, which are the counterpart of generalized Vaidya spacetimes in general relativity. We show that similar end states are also possible in those cases. Higher-dimensional spacetimes are then considered.

We numerically study the gravitational collapse of a massless scalar field with spherical symmetry in Einstein-æther theory, and show that apparent, spin-0, and dynamical universal horizons (dUHs) can be all formed. The spacetime and the æther field are well-behaved and regular, including regions nearby these horizons (but away from the center of spherical symmetry). The spacetime outside the apparent and spin-0 horizons settles down to a static configuration, and some of such resulting static black holes were already found numerically in the literature. On the other hand, the proper distance of the outermost dUH from the apparent (or spin-0) horizon keeps increasing on æther-orthogonal time slices. This indicates that the outermost dUH is evolving into the causal boundary, even for excitations with large speeds of propagation.

A crucial property of Weyl gravity is its conformal invariance. It is shown how this gauge symmetry is exactly reflected by the two constraints in the Hamiltonian framework. Since the spatial 3-metric is one of the configuration variables, the phase space of Weyl gravity can be extended to include internal gauge freedom by the triad formalism. Moreover, by canonical transformations, we obtain two new Hamiltonian formulations of Weyl gravity with an SU(2) connection as one of its configuration variables. The connection-dynamical formalisms lay the foundation to quantize Weyl gravity nonperturbatively by applying the method of loop quantum gravity. In one of the formulations, the so-called Immirzi parameter ambiguity in loop quantum gravity is avoided by the conformal invariance.

We investigate the effects of massive gravitons on the rotation curves of the Milky Way, spiral galaxies, and low surface brightness (LSB) galaxies. Using a simple de Rham, Gabadadze, and Tolley (dRGT) massive gravity model, we find a static spherically symmetric metric and a modified Tolman-Oppenheimer-Volkoff (TOV) equation. The dRGT nonlinear graviton interactions generate density and pressures, which behave like dark energy that can mimic the gravitational effects of a dark matter halo. We find that rotation curves of most galaxies can be fitted well by a single constant-gravity parameter $\gamma \sim m_g^{2}C\sim {10}^{-28}{\mathrm{m}}^{-1}$ corresponding to the graviton mass in the range $m_g\sim {10}^{-21}-{10}^{-30}\mathrm{eV}$ depending on the choice of the fiducial metric parameter $C\sim 1-{10}^{18}\mathrm{m}$. Fitting the rotation curve of the Milky Way puts a strong constraint on the Yukawa-type coupling of the massive graviton exchange as a result of the shell effects.

Since the duration of inflation is finite, imposing the initial condition in infinite past, i.e., the Bunch-Davies vacuum, is inherently ambiguous. In this paper, we resort to the mixed states as the initial condition, which are called the $\alpha$-vacua, and then we introduce a physical momentum cutoff $\mathrm{\Lambda }$ [1], in which the evolution of perturbations begins. We show that the initial time $t_i$, when the initial condition is imposed, depends on the wave number of fluctuation, as it is for the time of horizon crossing, $t_q$. Then, we calculate the corrections to the scalar and tensor power spectra and their corresponding spectral indices. Throughout this work, the calculation is done up to the first order in slow-roll parameters. We indicate that the leading order corrections to the spectral indices have a $q$-dependent amplitude, $2{\epsilon }_f[2{\epsilon }_f(\frac{q}{q_f}{)}^{4\epsilon +4\eta }-{\eta }_f(\frac{q}{q_f}{)}^{3\epsilon +\xi }]$, times a $q$-dependent oscillatory part, $\cos (\frac{2\mathrm{\Lambda }(q/q_f{)}^{\epsilon }}{H_f})$, where $H$, $\epsilon$, $\eta$, and $\xi$ are the Hubble and slow-roll parameters, respectively, and the subscript $f$ denotes that these quantities are evaluated at the time when the first scale, $q_f$, satisfies the initial condition, i.e., $q=a(t_i)\mathrm{\Lambda }$.

We perform a detailed confrontation of various oscillating dark-energy parametrizations with the latest sets of observational data. In particular, we use data from the joint light curve analysis (JLA) sample from supernoave type Ia, baryon acoustic oscillations (BAO) distance measurements, cosmic microwave background (CMB) observations, redshift space distortion, weak gravitational lensing, Hubble parameter measurements from cosmic chronometers, and we impose constraints on four oscillating models. From the analyses, we find that the best-fit characters of almost all models are bent towards the phantom region; nevertheless, in all of them, the quintessential regime is also allowed within $1\sigma$ confidence level. Furthermore, the deviations from $\mathrm{\Lambda }\mathrm{CDM}$ cosmology are not significant; however, for two of the models they could be visible at large scales, through the impact on the temperature anisotropy of the CMB spectra and on the matter power spectra. Finally, we perform the Bayesian analysis, which shows that the current observational data support the $\mathrm{\Lambda }\mathrm{CDM}$ paradigm over this set of oscillating dark-energy parametrizations.

The cosmological constant problem arises because the magnitude of vacuum energy density predicted by quantum mechanics is about 120 orders of magnitude larger than the value implied by cosmological observations of accelerating cosmic expansion. Recently, some of the current authors proposed that the stochastic nature of the quantum vacuum can resolve this tension [Q. Wang, Z. Zhu, and W. G. Unruh, Phys. Rev. D 95, 103504 (2017)]. By treating the fluctuations in the vacuum seriously and allowing fluctuations up to some high-energy cutoff at which Quantum Field Theory is believed to break down, a parametric resonance effect arises that leads to a slow expansion and acceleration. In this work, we thoroughly examine the implications of this proposal by investigating the resulting dynamics. First, we improve upon numerical calculations in the original work and show that convergence issues had overshadowed some important effects. Correct calculations reverse some of the conclusions in [Q. Wang, Z. Zhu, and W. G. Unruh, Phys. Rev. D 95, 103504 (2017)], however the premise that parametric resonance can explain a very slowly accelerating expansion appears to remain sound. After improving the resolution and efficiency of the numerical tests, we explore a wider range of cutoff energies, and examine the effects of multiple particle fields. We introduce a simple model using the Mathieu equation (a prototypical example of parametric resonance), and find that it closely matches numerical results in regimes where its assumptions are valid. Using this model, we extrapolate to find that in a universe with 28 bosonic fields and a high-energy cutoff 40 times higher than the Planck energy, the acceleration would be comparable to what is observed.

We derive a generalized luminosity distance versus redshift relation for a linearly perturbed Friedmann-Lemaitre-Robertson-Walker (FLRW) metric with two scalar mode excitations. We use two equivalent approaches, based on the Jacobi map and the van Vleck determinant, respectively. We apply the resultant formula to two simple models—an exact FLRW universe and an approximate FLRW universe perturbed by a single scalar mode sinusoidally varying with time. For both models, we derive a cosmographic expansion for $d_L$ in terms of $z$. We comment on the interpretation of our results and their possible application to more realistic cosmological models.