We discuss the eight independent spin observables for the $W$ boson in terms of its vector and tensor polarizations and identify the angular distributions and asymmetries able to separate them in collider experiments. The results are applied to the study of polarized top quark decays and diboson resonances. These novel observables are of great value for disentangling new physics mechanisms in $W$ boson production.

Two quantum field theories whose Hilbert spaces do not overlap cannot transmit a signal to one another. From this simple principle, we deduce some highly nontrivial consequences for holographic quantum gravity. These include: (i) certain cosmological bounces are forbidden, (ii) generic singularities inside black holes cannot be resolved, and (iii) traversable wormholes do not exist. At the classical level, this principle rules out certain types of naked singularities and suggests that new singularity theorems should exist.

We calculate fermionic response in domain wall backgrounds of four-dimensional gauged supergravity interpolating between distinct stable anti–de Sitter (AdS) vacua, holographically dual to zero-temperature states of ABJM theory at finite density for monopole charge. The backgrounds were found by Bobev et al. and are similar to zero-temperature limits of holographic superconductors, but with a symmetry-breaking source as well. The condensed scalar mixes charged and neutral fields dual to composite fermionic operators in the top-down Dirac equations. Both gapped and gapless bands of stable quasiparticles are found.

The observed IR and the spectator UV particles of a regulated, cutoff quantum field theory are entangled by their interactions; hence, the IR sector can be described by the help of the density matrix only. The tree-level renormalized trajectory is obtained for a self-interacting scalar field theory, containing the mixed state contributions. One needs a sharp cutoff in the momentum space as a regulator to realize the true loss of information caused by massive particles.

A cosmic axion, via the electromagnetic anomaly, induces an oscillating electric dipole for the electron of frequency $m_a$ and strength $\sim (\mathrm{few})\times {10}^{-32}\mathrm{e}\text{−}\mathrm{cm}$, two orders of magnitude above the nucleon, and within a few orders of magnitude of the present standard model constant limit. We give a detailed study of this phenomenon via the interaction of the cosmic axion, through the electromagnetic anomaly, with particular emphasis on the decoupling limit of the axion, ${\partial }_{t}a(t)\propto m_a\to 0$. The analysis is subtle, and we find the general form of the action involves a local contact interaction and a nonlocal contribution, analogous to the “transverse current” in QED, that enforces the decoupling limit. We carefully derive the effective action in the Pauli-Schrödinger nonrelativistic formalism, and in Georgi’s heavy quark formalism adapted to the “heavy electron” ($m_e\gg m_a$). We compute the electric dipole radiation emitted by free electrons, magnets and currents, immersed in the cosmic axion field, and discuss experimental configurations that may yield a detectable signal.

Detailed knowledge about equilibration processes is of interest for various fields of physics, including heavy ion collision experiments and quantum quenched condensed matter systems. We study the approach to equilibrium at late times within two types of strongly coupled thermal systems in $3+1$ dimensions: systems in the presence of (i) a nonzero charge density, or (ii) a magnetic field at vanishing charge density. Utilizing the gauge/gravity correspondence, we map the aforementioned problem to the computation of quasinormal frequencies around two particular classes of black branes within the Einstein-Maxwell theory. We compute (i) the tensor and vector quasinormal modes of Reissner-Nordström black branes and (ii) the scalar, as well as tensor quasinormal modes of magnetic black branes. Some of these quasinormal modes correspond to the late-time relaxation of the above systems after starting with initial pressure anisotropy. We provide benchmarks which need to be matched at late-times by all holographic thermalization codes with the appropriate symmetries.

We investigate the effect of the cosmological expansion on the bending of light due to an isolated pointlike mass. We adopt the McVittie metric as the model for the geometry of the lens. Assuming a constant Hubble factor, we find an analytic expression involving the bending angle, which turns out to be increased by a contribution $1+z_L$, where $z_L$ is the redshift of the lens. Employing the lens equation in the thin lens approximation, we find that for Einstein ring systems, the lens mass estimation gains a correction $(1+z_S)/(1+z_L)$ with respect to the one based on the usual formula.

Since quantum field theories do not possess proper position observables, Unruh-DeWitt detector models serve as a key theoretical tool for extracting localized spatiotemporal information from quantum fields. Most studies have been limited, however, to Unruh-DeWitt (UDW) detectors that are coupled linearly to a scalar bosonic field. Here, we investigate UDW detector models that probe fermionic as well as bosonic fields through both linear and quadratic couplings. In particular, we present a renormalization method that cures persistent divergencies of prior models. We then show how perturbative calculations with UDW detectors can be streamlined through the use of extended Feynman rules that include localized detector-field interactions. Our findings pave the way for the extension of previous studies of the Unruh and Hawking effects with UDW detectors, and provide new tools for studies in relativistic quantum information, for example, regarding relativistic quantum communication and studies of the entanglement structure of the fermionic vacuum.

We prove the quantum null energy condition (QNEC), a lower bound on the stress tensor in terms of the second variation in a null direction of the entropy of a region. The QNEC arose previously as a consequence of the quantum focusing conjecture, a proposal about quantum gravity. The QNEC itself does not involve gravity, so a proof within quantum field theory is possible. Our proof is somewhat nontrivial, suggesting that there may be alternative formulations of quantum field theory that make the QNEC more manifest. Our proof applies to free and super-renormalizable bosonic field theories, and to any points that lie on stationary null surfaces. An example is Minkowski space, where any point $p$ and null vector $k^a$ define a null plane $N$ (a Rindler horizon). Given any codimension-2 surface $\mathrm{\Sigma }$ that contains $p$ and lies on $N$, one can consider the von Neumann entropy $S_{\mathrm{out}}$ of the quantum state restricted to one side of $\mathrm{\Sigma }$. A second variation $S_{\mathrm{out}}^{\prime \prime }$ can be defined by deforming $\mathrm{\Sigma }$ along $N$, in a small neighborhood of $p$ with area $\mathcal{A}$. The QNEC states that $⟨T_{kk}(p)⟩\ge \frac{\hslash }{2\pi }\underset{\mathcal{A}\to 0}{\lim }{S}_{\mathrm{out}}^{\prime \prime }/\mathcal{A}$.

Double-null coordinates are highly useful in numerical simulations of dynamical spherically symmetric black holes (BHs). However, they become problematic in long-time simulations: Along the event horizon, the truncation error grows exponentially in the outgoing Eddington null coordinate—which we denote $v_e$—and runs out of control for a sufficiently long interval of $v_e$. This problem, if not properly addressed, would destroy the numerics both inside and outside the black hole at late times (i.e. large $v_e$). In this paper we explore the origin of this problem, and propose a resolution based on adaptive gauge for the ingoing null coordinate $u$. This resolves the problem outside the BH—and also inside the BH, if the latter is uncharged. However, in the case of a charged BH, an analogous large-$v_e$ numerical problem occurs at the inner horizon. We thus generalize our adaptive gauge method in order to overcome the inner-horizon problem as well. This improved adaptive gauge, to which we refer as the maximal-$\sigma$ gauge, allows long-$v$ double-null numerical simulation across both the event horizon and the (outgoing) inner horizon, and up to the vicinity of the spacelike $r=0$ singularity. We conclude by presenting a few numerical results deep inside a perturbed charged BH, in the vicinity of the contracting Cauchy horizon.

The autocorrelation between two members of a galaxy population is symmetric under the interchange of the two galaxies being correlated. The cross-correlation between two different types of galaxies, separated by a vector $\mathbf{r}$, is not necessarily the same as that for a pair separated by $-\mathbf{r}$. Local anisotropies in the two-point cross-correlation function may thus indicate a specific direction which when mapped as a function of position trace out a vector field. This vector field can then be decomposed into longitudinal and transverse components, and those transverse components written as positive- and negative-helicity components. A locally asymmetric cross-correlation of the longitudinal type arises naturally in halo clustering, even with Gaussian initial conditions, and could be enhanced with local-type non-Gaussianity. Early-Universe scenarios that introduce a vector field may also give rise to such effects. These antisymmetric cross-correlations also provide a new possibility to seek a preferred cosmic direction correlated with the hemispherical power asymmetry in the cosmic microwave background and to seek a preferred location associated with the cosmic microwave background cold spot. New ways to seek cosmic parity breaking are also possible.

Astrophysical tests of the stability of fundamental couplings, such as the fine-structure constant $\alpha$, are a powerful probe of new physics. Recently these measurements, combined with local atomic clock tests and Type Ia supernova and Hubble parameter data, were used to constrain the simplest class of dynamical dark energy models where the same degree of freedom is assumed to provide both the dark energy and (through a dimensionless coupling, $\zeta$, to the electromagnetic sector) the $\alpha$ variation. One caveat of these analyses was that it was based on fiducial models where the dark energy equation of state was described by a single parameter (effectively its present day value, $w_0$). Here we relax this assumption and study broader dark energy model classes, including the Chevallier-Polarski-Linder and early dark energy parametrizations. Even in these extended cases we find that the current data constrains the coupling $\zeta$ at the $10^{-6}$ level and $w_0$ to a few percent (marginalizing over other parameters), thus confirming the robustness of earlier analyses. On the other hand, the additional parameters are typically not well constrained. We also highlight the implications of our results for constraints on violations of the weak equivalence principle and improvements to be expected from forthcoming measurements with high-resolution ultrastable spectrographs.

We investigate the quantumness of primordial cosmological fluctuations and its detectability. The quantum discord of inflationary perturbations is calculated for an arbitrary splitting of the system, and shown to be very large on super-Hubble scales. This entails the presence of large quantum correlations, due to the entangled production of particles with opposite momentums during inflation. To determine how this is reflected at the observational level, we study whether quantum correlators can be reproduced by a nondiscordant state, i.e. a state with vanishing discord that contains classical correlations only. We demonstrate that this can be done for the power spectrum, the price to pay being twofold: first, large errors in other two-point correlation functions that cannot however be detected since they are hidden in the decaying mode; second, the presence of intrinsic non-Gaussianity, the detectability of which remains to be determined but which could possibly rule out a nondiscordant description of the cosmic microwave background. If one abandons the idea that perturbations should be modeled by quantum mechanics and wants to use a classical stochastic formalism instead, we show that any two-point correlators on super-Hubble scales can be exactly reproduced regardless of the squeezing of the system. The latter becomes important only for higher order correlation functions that can be accurately reproduced only in the strong squeezing regime.

We develop methods to calculate the curvature power spectrum in models where features in the inflaton potential nonlinearly excite modes and generate high frequency features in the spectrum. The first nontrivial effect of excitations generating further excitations arises at third order in deviations from slow roll. If these further excitations are contemporaneous, the series can be resummed, showing the exponential sensitivity of the curvature spectrum to potential features. More generally, this exponential approximation provides a power spectrum template which nonlinearly obeys relations between excitation coefficients and whose parameters may be appropriately adjusted. For a large sharp step in the potential, it greatly improves the analytic power spectrum template and its dependence on potential parameters. For axionic oscillations in the potential, it corrects the mapping between the potential and the amplitude, phase and zero point of the curvature oscillations, which might otherwise cause erroneous inferences in for example the tensor-scalar ratio, formally even when that amplitude is ${10}^3$ times larger than the slow-roll power spectrum. It also estimates when terms that produce double frequency oscillations that are usually omitted when analyzing data should be included. These techniques should allow future studies of high frequency features in the cosmic microwave background and large-scale structure to extend to higher amplitude and/or higher precision.

We test the models of vacuum energy interacting with cold dark matter and try to probe the possible deviation from the $\mathrm{\Lambda }\mathrm{CDM}$ model using current observations. We focus on two specific models, $Q=3\beta H{\rho }_{\mathrm{\Lambda }}$ and $Q=3\beta H{\rho }_c$. The data combinations come from the Planck 2013 data, the baryon acoustic oscillations measurements, the type-Ia supernovae data, the Hubble constant measurement, the redshift space distortions data and the galaxy weak lensing data. For the $Q=3\beta H{\rho }_c$ model, we find that it can be tightly constrained by all the data combinations, while for the $Q=3\beta H{\rho }_{\mathrm{\Lambda }}$ model, there still exist significant degeneracies between parameters. The tightest constraints for the coupling constant are $\beta ={-0.026}_{-0.053}^{+0.036}$ (for $Q=3\beta H{\rho }_{\mathrm{\Lambda }}$) and $\beta =-0.00045\pm 0.00069$ (for $Q=3\beta H{\rho }_c$) at the $1\sigma$ level. For all the fit results, we find that the null interaction $\beta =0$ is always consistent with data. Our work completes the discussion on the interacting dark energy model in the recent Planck 2015 papers. Considering this work together with the Planck 2015 results, it is believed that there is no evidence for the models beyond the standard $\mathrm{\Lambda }\mathrm{CDM}$ model from the point of view of possible interaction.

The IceCube Neutrino Observatory has observed a diffuse flux of TeV-PeV astrophysical neutrinos at $5.7\sigma$ significance from an all-flavor search. The direct detection of tau neutrinos in this flux has yet to occur. Tau neutrinos become distinguishable from other flavors in IceCube at energies above a few hundred TeV, when the cascade from the tau neutrino charged current interaction becomes resolvable from the cascade from the tau lepton decay. This paper presents results from the first dedicated search for tau neutrinos with energies between 214 TeV and 72 PeV in the full IceCube detector. The analysis searches for IceCube optical sensors that observe two separate pulses in a single event—one from the tau neutrino interaction and a second from the tau decay. No candidate events were observed in three years of IceCube data. For the first time, a differential upper limit on astrophysical tau neutrinos is derived around the PeV energy region, which is nearly 3 orders of magnitude lower in energy than previous limits from dedicated tau neutrino searches.

The resonant excesses around 2 TeV reported by the ATLAS Collaboration can be explained in the left-right model, and the tight constraints from lepton plus missing energy searches can be evaded if the $SU(2{)}_R$ gauge symmetry is leptophobic. We, for the first time, propose an anomaly- free leptophobic left-right model with gauge symmetry $SU(3{)}_C\times SU(2{)}_L\times SU(2{)}_R\times U(1{)}_X$, where the SM leptons are singlets under $SU(2{)}_R$. The gauge anomalies are cancelled by introducing extra vectorlike quarks. The mass of the $Z^{\prime }$ gauge boson, which cannot be leptophobic, is assumed to be around or above 2.5 TeV so that the constraint on the dilepton final state can be avoided. Moreover, we find that the $W^{\prime }\to WZ$ channel cannot explain the ATLAS diboson excess due to the tension with the constraint on the $W^{\prime }\to jj$ decay mode. We solve this problem by considering the mixings between the SM quarks and vectorlike quarks. We show explicitly that the ATLAS diboson excess can be explained in the viable parameter space of our model, which is consistent with all the current experimental constraints.

We show that a recently proposed framework that provides a simple connection between Majorana neutrinos and an invisible axion in minimal scalar extensions of the standard electroweak model can be naturally embedded in a classically scale-invariant setup. The explicit breaking of the scale invariance à la Coleman-Weinberg generates the Peccei-Quinn and electroweak scales. The spontaneous breaking of the chiral $U(1{)}_{\mathrm{PQ}}$ triggers the generation of neutrino masses via Type-II seesaw and, at the same time, provides a dynamical solution to the strong $CP$ problem as well as the axion as a dark matter candidate. The electroweak and neutrino mass scales are obtained via a technically natural ultraweak limit of the singlet scalar interactions. Accordingly, a realistic and perturbatively stable scalar spectrum, possibly in the reach of the LHC, is naturally obtained. A very light pseudodilaton characterizes such a setting. The vacuum stability of the extended setup is discussed.

We consider a supersymmetric version of the inert Higgs doublet model, whose motivation is to explain smallness of neutrino masses and existence of dark matter. In this supersymmetric model, due to the presence of discrete symmetries, neutrinos acquire masses at loop level. After computing these neutrino masses, in order to fit the neutrino oscillation data, we show that by tuning some supersymmetry-breaking soft parameters of the model, neutrino Yukawa couplings can be unsuppressed. In the above-mentioned parameter space, we compute the branching ratio of the decay $\mu \to e\gamma$. To be consistent with the current experimental upper bound on $\mathrm{Br}(\mu \to e\gamma )$, we obtain constraints on the right-handed neutrino mass of this model.