Advanced LIGO detectors at Hanford and Livingston made two confirmed and one marginal detection of binary black holes during their first observing run. The first event, GW150914, was from the merger of two black holes much heavier that those whose masses have been estimated so far, indicating a formation scenario that might differ from “ordinary” stellar evolution. One possibility is that these heavy black holes resulted from a previous merger. When the progenitors of a black hole binary merger result from previous mergers, they should (on average) merge later, be more massive, and have spin magnitudes clustered around a dimensionless spin $\sim 0.7$. Here we ask the following question: can gravitational-wave observations determine whether merging black holes were born from the collapse of massive stars (“first generation”), rather than being the end product of earlier mergers (“second generation”)? We construct simple, observationally motivated populations of black hole binaries, and we use Bayesian model selection to show that measurements of the masses, luminosity distance (or redshift), and “effective spin” of black hole binaries can indeed distinguish between these different formation scenarios.

In the top-down holographic model of QCD based on D4/D8-branes in type IIA string theory and some of the bottom-up models, the low energy effective theory of mesons is described by a five-dimensional Yang-Mills-Chern-Simons theory in a certain curved background with two boundaries. The five-dimensional Chern-Simons term plays a crucial role in reproducing the correct chiral anomaly in four-dimensional massless QCD. However, there are some subtle ambiguities in the definition of the Chern-Simons term for the cases with topologically nontrivial gauge bundles, which include the configurations with baryons. In particular, for the cases with three flavors, it was pointed out by Hata and Murata that the naive Chern-Simons term does not lead to an important constraint on the baryon spectrum, which is needed to pick out the correct baryon spectrum observed in nature. In this paper, we propose a formulation of a well-defined Chern-Simons term which can be used for the cases with baryons, and show that it recovers the correct baryon constraint as well as the chiral anomaly in QCD.

Familiar factorized descriptions of classic QCD processes such as deeply inelastic scattering (DIS) apply in the limit of very large hard scales, much larger than nonperturbative mass scales and other nonperturbative physical properties like intrinsic transverse momentum. Since many interesting DIS studies occur at kinematic regions where the hard scale, $Q\sim 1–2\mathrm{GeV}$, is not very much greater than the hadron masses involved, and the Bjorken scaling variable $x_{\mathrm{bj}}$ is large, $x_{\mathrm{bj}}\gtrsim 0.5$, it is important to examine the boundaries of the most basic factorization assumptions and assess whether improved starting points are needed. Using an idealized field-theoretic model that contains most of the essential elements that a factorization derivation must confront, we retrace the steps of factorization approximations and compare with calculations that keep all kinematics exact. We examine the relative importance of such quantities as the target mass, light quark masses, and intrinsic parton transverse momentum, and argue that a careful accounting of parton virtuality is essential for treating power corrections to collinear factorization. We use our observations to motivate searches for new or enhanced factorization theorems specifically designed to deal with moderately low-$Q$ and large-$x_{\mathrm{bj}}$ physics.

We investigate the gravitational property of the quantum vacuum by treating its large energy density predicted by quantum field theory seriously and assuming that it does gravitate to obey the equivalence principle of general relativity. We find that the quantum vacuum would gravitate differently from what people previously thought. The consequence of this difference is an accelerating universe with a small Hubble expansion rate $H\propto \mathrm{\Lambda }{e}^{-\beta \sqrt{G}\mathrm{\Lambda }}\to 0$ instead of the previous prediction $H=\sqrt{8\pi G{\rho }^{\mathrm{vac}}/3}\propto \sqrt{G}{\mathrm{\Lambda }}^2\to \infty$ which was unbounded, as the high energy cutoff $\mathrm{\Lambda }$ is taken to infinity. In this sense, at least the “old” cosmological constant problem would be resolved. Moreover, it gives the observed slow rate of the accelerating expansion as $\mathrm{\Lambda }$ is taken to be some large value of the order of Planck energy or higher. This result suggests that there is no necessity to introduce the cosmological constant, which is required to be fine tuned to an accuracy of ${10}^{-120}$, or other forms of dark energy, which are required to have peculiar negative pressure, to explain the observed accelerating expansion of the Universe.

Future cosmic microwave background polarization experiments will search for evidence of primordial tensor modes at large angular scales, in the multipole range $4\le \ell \le 50$. Because in that range there is some mild evidence of departures from scale invariance in the power spectrum of primordial curvature perturbations, one may wonder about the possibility of similar deviations appearing in the primordial power spectrum of tensor modes. Here we address this issue and analyze the possible presence of features in the tensor spectrum resulting from the dynamics of primordial fluctuations during inflation. We derive a general, model-independent relation linking features in the spectra of curvature and tensor perturbations. We conclude that even with large deviations from scale invariance in the curvature power spectrum, the tensor spectrum remains scale invariant for all observational purposes.

A precise measurement of the Higgs $h\to Z\gamma$ decay is very challenging at the LHC, due to the very low branching fraction and the shortage of kinematic handles to suppress the large SM $Z\gamma$ background. We show how such a measurement would be significantly improved by considering Higgs production in association with a hard jet. We compare the prospective HL-LHC sensitivity in this channel with other Higgs production modes where $h$ is fairly boosted, e.g. weak boson fusion, and also to the potential $h\to Z\gamma$ measurement achievable with a future $e^{+}{e}^{-}$ circular collider (fcc-ee). Finally, we discuss new physics implications of a precision measurement of $h\to Z\gamma$.

We introduce an improved model that links the frequency shift of the ${}^{133}\mathrm{Cs}$ hyperfine Zeeman transitions $|F=3,m_F⟩\leftrightarrow |F=4,m_F⟩$ to the Lorentz-violating Standard Model extension (SME) coefficients of the proton and neutron. The new model uses Lorentz transformations developed to second order in boost and additionally takes the nuclear structure into account, beyond the simple Schmidt model used previously in Standard Model extension analyses, thereby providing access to both proton and neutron SME coefficients including the isotropic coefficient ${\tilde{c}}_{TT}$. Using this new model in a second analysis of the data delivered by the FO2 dual Cs/Rb fountain at Paris Observatory and previously analyzed in [1], we improve by up to 13 orders of magnitude the present maximum sensitivities for laboratory tests [2] on the ${\tilde{c}}_Q$, ${\tilde{c}}_{TJ}$, and ${\tilde{c}}_{TT}$ coefficients for the neutron and on the ${\tilde{c}}_Q$ coefficient for the proton, reaching respectively ${10}^{-20}$, ${10}^{-17}$, ${10}^{-13}$, and ${10}^{-15}\mathrm{GeV}$.

We study the emergence of anisotropic (Bianchi IX) inflationary universes with no-boundary conditions in the path integral approach to quantum gravity. In contrast to previous work, we find no evidence for any limit to how large the anisotropies can become, although for increasing anisotropies the shape of the instantons becomes significantly different from Hawking’s original no-boundary instanton. In all cases an inflationary phase is reached, with the anisotropies decaying away. Larger anisotropies are associated with a much larger imaginary part of the action, implying that the highly anisotropic branches of the wavefunction are heavily suppressed. Interestingly, the presence of anisotropies causes the wavefunction to become classical much more slowly than for isotropic inflationary universes. We derive the associated scaling of the WKB classicality conditions both numerically and analytically.

The tidal Love numbers (TLNs) encode the deformability of a self-gravitating object immersed in a tidal environment and depend significantly both on the object’s internal structure and on the dynamics of the gravitational field. An intriguing result in classical general relativity is the vanishing of the TLNs of black holes. We extend this result in three ways, aiming at testing the nature of compact objects: (i) we compute the TLNs of exotic compact objects, including different families of boson stars, gravastars, wormholes, and other toy models for quantum corrections at the horizon scale. In the black-hole limit, we find a universal logarithmic dependence of the TLNs on the location of the surface. (ii) We compute the TLNs of black holes beyond vacuum general relativity, including Einstein-Maxwell, Brans-Dicke, and Chern-Simons gravity. (iii) We assess the ability of present and future gravitational-wave detectors to measure the TLNs of these objects, including the first analysis of TLNs with LISA. Both LIGO, ET, and LISA can impose interesting constraints on boson stars, while LISA is able to probe even extremely compact objects. We argue that the TLNs provide a smoking gun of new physics at the horizon scale and that future gravitational-wave measurements of the TLNs in a binary inspiral provide a novel way to test black holes and general relativity in the strong-field regime.

A measurement of electron antineutrino oscillation by the Daya Bay Reactor Neutrino Experiment is described in detail. Six 2.9-GWth nuclear power reactors of the Daya Bay and Ling Ao nuclear power facilities served as intense sources of ${\overline{\nu }}_e$’s. Comparison of the ${\overline{\nu }}_e$ rate and energy spectrum measured by antineutrino detectors far from the nuclear reactors ($\sim 1500–1950\mathrm{m}$) relative to detectors near the reactors ($\sim 350–600\mathrm{m}$) allowed a precise measurement of ${\overline{\nu }}_e$ disappearance. More than 2.5 million ${\overline{\nu }}_e$ inverse beta-decay interactions were observed, based on the combination of 217 days of operation of six antineutrino detectors (December, 2011–July, 2012) with a subsequent 1013 days using the complete configuration of eight detectors (October, 2012–July, 2015). The ${\overline{\nu }}_e$ rate observed at the far detectors relative to the near detectors showed a significant deficit, $R=0.949\pm 0.002(\text{stat})\pm 0.002(\text{syst})$. The energy dependence of ${\overline{\nu }}_e$ disappearance showed the distinct variation predicted by neutrino oscillation. Analysis using an approximation for the three-flavor oscillation probability yielded the flavor-mixing angle ${\sin }^{2}2{\theta }_{13}=0.0841\pm 0.0027(\text{stat})\pm 0.0019(\text{syst})$ and the effective neutrino mass-squared difference of $|\mathrm{\Delta }{m}_{\mathrm{ee}}^2|=(2.50\pm 0.06(\text{stat})\pm 0.06(\text{syst}))\times {10}^{-3}{\mathrm{eV}}^2$. Analysis using the exact three-flavor probability found $\mathrm{\Delta }{m}_{32}^2=(2.45\pm 0.06(\text{stat})\pm 0.06(\text{syst}))\times {10}^{-3}{\mathrm{eV}}^2$ assuming the normal neutrino mass hierarchy and $\mathrm{\Delta }{m}_{32}^2=(-2.56\pm 0.06(\text{stat})\pm 0.06(\text{syst}))\times {10}^{-3}{\mathrm{eV}}^2$ for the inverted hierarchy.

We report the discovery of a statistically significant hardening in the Fermi-LAT $\gamma$-ray spectrum of Centaurus A’s (Cen A) core, with the spectral index hardening from ${\mathrm{\Gamma }}_1=2.73\pm 0.02$ to ${\mathrm{\Gamma }}_1=2.29\pm 0.07$ at a break energy of $(2.6\pm 0.3)\mathrm{GeV}$. Using a likelihood analysis, we find no evidence for flux variability in Cen A’s core light curve above or below the spectral break when considering the entire 8 year period. Interestingly, however, the first $\sim 3.5\mathrm{years}$ of the low energy light curve shows evidence of flux variability at the $\sim 3.5\sigma$ confidence level. To understand the origin of this spectral break, we assume that the low energy component below the break feature originates from leptons in Centaurus A’s radio jet, and we investigate the possibility that the high energy component above the spectral break is due to an additional source of very high energy particles near the core of Cen A. We show for the first time that the observed $\gamma$-ray spectrum of an active galactic nucleus is compatible with either a very large localized enhancement (referred to as a spike) in the dark matter halo profile or a population of millisecond pulsars. Our work constitutes the first robust indication that new $\gamma$-ray production mechanisms can explain the emission from active galaxies and could provide tantalizing first evidence for the clustering of heavy dark matter particles around black holes.

We identify a new contribution to the chiral magnetic conductivity at finite frequencies—the magnetization current. This allows us to quantitatively reproduce the known field-theoretic time-dependent (AC) chiral magnetic response in terms of kinetic theory. We evaluate the corresponding AC chiral magnetic conductivity in two-flavor QCD plasma at weak coupling. The magnetization current results from the spin response of chiral quasiparticles to magnetic field, and is thus proportional to the quasiparticle’s $g$-factor. In condensed matter systems, where the chiral quasiparticles are emergent and the $g$-factor can significantly differ from 2, this opens up the possibility of tuning the AC chiral magnetic response.

We examine in depth a recent proposal to utilize superfluid helium for direct detection of sub-MeV mass dark matter. For sub-keV recoil energies, nuclear scattering events in liquid helium primarily deposit energy into long-lived phonon and roton quasiparticle excitations. If the energy thresholds of the detector can be reduced to the meV scale, then dark matter as light as $\sim \mathrm{MeV}$ can be reached with ordinary nuclear recoils. If, on the other hand, two or more quasiparticle excitations are directly produced in the dark matter interaction, the kinematics of the scattering allows sensitivity to dark matter as light as $\sim \mathrm{keV}$ at the same energy resolution. We present in detail the theoretical framework for describing excitations in superfluid helium, using it to calculate the rate for the leading dark matter scattering interaction, where an off-shell phonon splits into two or more higher-momentum excitations. We validate our analytic results against the measured and simulated dynamic response of superfluid helium. Finally, we apply this formalism to the case of a kinetically mixed hidden photon in the superfluid, both with and without an external electric field to catalyze the processes.

We show there are analogs to the Unruh temperature that can be defined for any quantum field theory and region of the space. These local temperatures are defined using relative entropy with localized excitations. We show that important restrictions arise from relative entropy inequalities and causal propagation between Cauchy surfaces. These suggest a large amount of universality for local temperatures, especially the ones affecting null directions. For regions with any number of intervals in two spacetime dimensions, the local temperatures might arise from a term in the modular Hamiltonian proportional to the stress tensor. We argue this term might be universal, with a coefficient that is the same for any theory, and check analytically and numerically that this is the case for free massive scalar and Dirac fields. In dimensions $d\ge 3$, the local terms in the modular Hamiltonian producing these local temperatures cannot be formed exclusively from the stress tensor. For a free scalar field, we classify the structure of the local terms.

The aim of this work is to provide the details of a calculation summarized in the recent paper by Maltz and Susskind which conjectured a potentially rigorous framework where the status of de Sitter space is the same as that of a resonance in a scattering process. The conjecture is that transition amplitudes between certain states with asymptotically supersymmetric flat vacua contain resonant poles characteristic metastable intermediate states. A calculation employing constrained instantons is presented that illustrates this idea.

Within models containing a very light scalar particle coupled to the 125 GeV Higgs boson, we present the first detailed study of Higgs decays into three of these light scalars. We determine model-independent conditions which the scalar sector after electroweak symmetry breaking has to satisfy in order for the three-body channel to become relevant. Using a specific model—the real scalar singlet-extension of the Standard Model (SM)—we then identify scenarios, where the rates of scalar three-body Higgs decays are comparable to or even exceed those of the well-studied two-body channel. All those scenarios are shown to be compatible with current experimental and theoretical constraints. We finally argue that scalar three-body Higgs decays lead to exciting new collider signatures with six SM fermions in the final state. Calculating the corresponding event rates, we find that e.g. six-muon or six-tau final states may be in reach of dedicated searches at the LHC or ILC experiments.

Interest in the idea that primordial black holes (PBHs) might comprise some or all of the dark matter has recently been rekindled following LIGO’s first direct detection of a binary-black-hole merger. Here we revisit the effect of accreting PBHs on the cosmic microwave background (CMB) frequency spectrum and the angular temperature and polarization power spectra. We compute the accretion rate and luminosity of PBHs, accounting for their suppression by Compton drag and Compton cooling by CMB photons. We estimate the gas temperature near the Schwarzschild radius and, hence, the free-free luminosity, accounting for the cooling resulting from collisional ionization when the background gas is mostly neutral. We account approximately for the velocities of PBHs with respect to the background gas. We provide a simple analytic estimate of the efficiency of energy deposition in the plasma. We find that the spectral distortions generated by accreting PBHs are too small to be detected by FIRAS, as well as by future experiments now being considered. We analyze Planck CMB temperature and polarization data and find, under our most conservative hypotheses, and at the order-of-magnitude level, that they rule out PBHs with masses $\gtrsim 10^2{M}_{\odot }$ as the dominant component of dark matter.

Quantum systems in extreme conditions can exhibit universal behavior far from equilibrium associated to nonthermal fixed points with a wide range of topical applications from early-Universe inflaton dynamics and heavy-ion collisions to strong quenches in ultracold quantum gases. So far, most studies have relied on a mapping of the quantum dynamics onto a classical-statistical theory that can be simulated on a computer. However, the mapping is based on a weak-coupling limit, while phenomenological applications often require moderate interaction strengths. We report on the observation of nonthermal fixed points directly in quantum field theory beyond the weak-coupling limit. For the example of a relativistic scalar $\mathrm{O}(N)$-symmetric quantum field theory, we numerically solve the nonequilibrium dynamics employing a $1/N$ expansion to next-to-leading order, which does not rely on a small coupling parameter. Starting from two different sets of overoccupied and of strong-field initial conditions, we find that nonthermal fixed points are not restricted to parameter ranges suitable for classical-statistical simulations but extend also to couplings of order 1. While the infrared behavior is found to be insensitive to the differences in the initial conditions, we demonstrate that transport phenomena to higher momenta depend on the presence or absence of a symmetry-breaking field expectation value.

Despite their use as cosmological distance indicators and their importance in the chemical evolution of galaxies, the unequivocal identification of the progenitor systems and explosion mechanism of normal type Ia supernovae (SNe Ia) remains elusive. The leading hypothesis is that such a supernova is a thermonuclear explosion of a carbon-oxygen white dwarf, but the exact explosion mechanism is still a matter of debate. Observation of a galactic SN Ia would be of immense value in answering the many open questions related to these events. One potentially useful source of information about the explosion mechanism and progenitor is the neutrino signal because the neutrinos from the different mechanisms possess distinct spectra as a function of time and energy. In this paper, we compute the expected neutrino signal from a gravitationally confined detonation (GCD) explosion scenario for a SN Ia and show how the flux at Earth contains features in time and energy unique to this scenario. We then calculate the expected event rates in the Super-K, Hyper-K, JUNO, DUNE, and IceCube detectors and find both Hyper-K and IceCube will see a few events for a GCD supernova at 1 kpc or closer, while Super-K, JUNO, and DUNE will see events if the supernova is closer than $\sim 0.3\mathrm{kpc}$. The distance and detector criteria needed to resolve the time and spectral features arising from the explosion mechanism, neutrino production, and neutrino oscillation processes are also discussed. The neutrino signal from the GCD is then compared with the signal from a deflagration-to-detonation transition (DDT) explosion model computed previously. We find the overall event rate is the most discriminating feature between the two scenarios followed by the event rate time structure. Using the event rate in the Hyper-K detector alone, the DDT can be distinguished from the GCD at $2\sigma$ if the distance to the supernova is less than 2.3 kpc for a normal mass ordering and 3.6 kpc for an inverted ordering.

In this work, we study linearized gravitational fields on the entire Minkowski spacetime including spacelike infinity. The generalized conformal field equations linearized about a Minkowski background are utilized for this purpose. In principle, this conformal representation of Einstein’s equations can be used to carry out global simulations of Minkowski spacetime. We thoroughly investigate this possibility.