We present a complete discussion of the boundary term in the action functional of general relativity when the boundary includes null segments in addition to the more usual timelike and spacelike segments. We confirm that ambiguities appear in the contribution from a null segment, because it depends on an arbitrary choice of parametrization for the generators. We also show that similar ambiguities appear in the contribution from a codimension-two surface at which a null segment is joined to another (spacelike, timelike, or null) segment. The parametrization ambiguity can be tamed by insisting that the null generators be affinely parametrized; this forces each null contribution to the boundary action to vanish, but leaves intact the fredom to rescale the affine parameter by a constant factor on each generator. Once a choice of parametrization is made, the ambiguity in the joint contributions can be eliminated by formulating well-motivated rules that ensure the additivity of the gravitational action. Enforcing these rules, we calculate the time rate of change of the action when it is evaluated for a so-called “Wheeler-DeWitt patch” of a black hole in asymptotically anti de Sitter space. We recover a number of results cited in the literature, obtained with a less complete analysis.

We calculate the ${\pi }^0\to {\gamma }^{*}{\gamma }^{*}$ transition form factor ${\mathcal{F}}_{{\pi }^0{\gamma }^{*}{\gamma }^{*}}(q_1^2,q_2^2)$ in lattice QCD with two flavors of quarks. Our main motivation is to provide the input to calculate the ${\pi }^0$-pole contribution to hadronic light-by-light scattering in the muon ($g-2$), $a_{\mu }^{\mathrm{HLbL};{\pi }^0}$. We therefore focus on the region where both photons are spacelike up to virtualities of about $1.5{\mathrm{GeV}}^2$, which has so far not been experimentally accessible. Results are obtained in the continuum at the physical pion mass by a combined extrapolation. We reproduce the prediction of the chiral anomaly for real photons with an accuracy of about 8–9%. We also compare to various recently proposed models and find reasonable agreement for the parameters of some of these models with their phenomenological values. Finally, we use the parametrization of our lattice data by these models to calculate $a_{\mu }^{\mathrm{HLbL};{\pi }^0}$.

Motivated by a recent experiment of J. Steinhauer, we reconsider the spectrum and the correlations of the phonons spontaneously emitted in stationary transonic flows. The latter are described by “waterfall” configurations which form a one-parameter family of stable flows. For parameters close to their experimental values, in spite of high gradients near the sonic horizon, the spectrum is accurately Planckian in the relevant frequency domain, where the temperature differs from the relativistic prediction by less than 10%. We then study the density correlations across the horizon and the nonseparable character of the final state. We show that the relativistic expressions provide accurate approximations when the initial temperature is not too high. We also show that the phases of the scattering coefficients introduce a finite shift of the location of the correlations which has so far been overlooked. This shift is due to the asymmetry of the flow across the horizon, and persists in the dispersionless regime. Finally we show how the formation of the sonic horizon modifies both local and nonlocal density correlations.

We present results from a search for neutrinoless double-$\beta$ ($0\nu \beta \beta$) decay using 36.6 g of the isotope ${}^{150}\mathrm{Nd}$ with data corresponding to a live time of 5.25 y recorded with the NEMO-3 detector. We construct a complete background model for this isotope, including a measurement of the two-neutrino double-$\beta$ decay half-life of $T_{1/2}^{2\nu }=[9.34\pm 0.22(\text{stat}){}_{-0.60}^{+0.62}(\text{syst})]\times {10}^{18}\mathrm{y}$ for the ground state transition, which represents the most precise result to date for this isotope. We perform a multivariate analysis to search for $0\nu \beta \beta$ decays in order to improve the sensitivity and, in the case of observation, disentangle the possible underlying decay mechanisms. As no evidence for $0\nu \beta \beta$ decay is observed, we derive lower limits on half-lives for several mechanisms involving physics beyond the standard model. The observed lower limit, assuming light Majorana neutrino exchange mediates the decay, is $T_{1/2}^{0\nu }>2.0\times 10^{22}\mathrm{y}$ at the 90% C.L., corresponding to an upper limit on the effective neutrino mass of $⟨m_{\nu }⟩<1.6–5.3\mathrm{eV}$.

The gravitational wave observations GW150914 and GW151226 by Advanced LIGO provide the first opportunity to learn about physics in the extreme gravity environment of coalescing binary black holes. The LIGO Scientific Collaboration and the Virgo Collaboration have verified that this observation is consistent with Einstein’s theory of general relativity, constraining the presence of certain parametric anomalies in the signal. This paper expands their analysis to a larger class of anomalies, highlighting the inferences that can be drawn on nonstandard theoretical physics mechanisms that could otherwise have affected the observed signals. We find that these gravitational wave events constrain a plethora of mechanisms associated with the generation and propagation of gravitational waves, including the activation of scalar fields, gravitational leakage into large extra dimensions, the variability of Newton’s constant, the speed of gravity, a modified dispersion relation, gravitational Lorentz violation and the strong equivalence principle. Though other observations limit many of these mechanisms already, GW150914 and GW151226 are unique in that they are direct probes of dynamical strong-field gravity and of gravitational wave propagation. We also show that GW150914 constrains inferred properties of exotic compact object alternatives to Kerr black holes. We argue, however, that the true potential for GW150914 to both rule out exotic objects and constrain physics beyond general relativity is severely limited by the lack of understanding of the coalescence regime in almost all relevant modified gravity theories. This event thus significantly raises the bar that these theories have to pass, both in terms of having a sound theoretical underpinning and reaching the minimal level of being able to solve the equations of motion for binary merger events. We conclude with a discussion of the additional inferences that can be drawn if the lower-confidence observation of an electromagnetic counterpart to GW150914 holds true, or such a coincidence is observed with future events; this would provide dramatic constraints on the speed of gravity and gravitational Lorentz violation.

We present a search for ultrarelativistic magnetic monopoles with the Pierre Auger observatory. Such particles, possibly a relic of phase transitions in the early Universe, would deposit a large amount of energy along their path through the atmosphere, comparable to that of ultrahigh-energy cosmic rays (UHECRs). The air-shower profile of a magnetic monopole can be effectively distinguished by the fluorescence detector from that of standard UHECRs. No candidate was found in the data collected between 2004 and 2012, with an expected background of less than 0.1 event from UHECRs. The corresponding 90% confidence level (C.L.) upper limits on the flux of ultrarelativistic magnetic monopoles range from ${10}^{-19}({\mathrm{cm}}^2\mathrm{sr}\mathrm{s}{)}^{-1}$ for a Lorentz factor $\gamma =10^9$ to $2.5\times {10}^{-21}({\mathrm{cm}}^2\mathrm{sr}\mathrm{s}{)}^{-1}$ for $\gamma =10^{12}$. These results—the first obtained with a UHECR detector—improve previously published limits by up to an order of magnitude.

Upgraded electronics, improved water system dynamics, better calibration and analysis techniques allowed Super-Kamiokande-IV to clearly observe very low-energy ${}^8\mathrm{B}$ solar neutrino interactions, with recoil electron kinetic energies as low as $\sim 3.5\mathrm{MeV}$. Super-Kamiokande-IV data-taking began in September of 2008; this paper includes data until February 2014, a total livetime of 1664 days. The measured solar neutrino flux is $(2.308\pm 0.020{(\mathrm{stat})}_{-0.040}^{+0.039}(\mathrm{syst}))\times 10^6/({\mathrm{cm}}^2\sec )$ assuming no oscillations. The observed recoil electron energy spectrum is consistent with no distortions due to neutrino oscillations. An extended maximum likelihood fit to the amplitude of the expected solar zenith angle variation of the neutrino-electron elastic scattering rate in SK-IV results in a day/night asymmetry of $(-3.6\pm 1.6(\mathrm{stat})\pm 0.6(\mathrm{syst}))\%$. The SK-IV solar neutrino data determine the solar mixing angle as ${\sin }^2{\theta }_{12}={0.327}_{-0.031}^{+0.026}$, all SK solar data (SK-I, SK-II, SK III and SK-IV) measures this angle to be ${\sin }^2{\theta }_{12}={0.334}_{-0.023}^{+0.027}$, the determined mass-squared splitting is $\mathrm{\Delta }{m}_{21}^2={4.8}_{-0.8}^{+1.5}\times {10}^{-5}{\mathrm{eV}}^2$.

The upcoming operation of the extra low energy antiprotons ring at CERN, the upgrade of the antiproton decelerator (AD), and the installation in the AD hall of an intense slow positron beam with an expected flux of $10^8{\mathrm{e}}^{+}/\mathrm{s}$ will open the possibility for new experiments with antihydrogen ($\overline{\mathrm{H}}$). Here we propose a scheme to measure the Lamb shift of $\overline{\mathrm{H}}$. For four months of data taking, we anticipate an uncertainty of 100 ppm. This will provide a test of $CPT$ and the first determination of the antiproton charge radius at the level of 10%.

The unexpected energy spectrum of the positron/electron ratio is interpreted astrophysically, with a possible exception of the 100–300 GeV range. The data indicate that this ratio, after a decline between 0.5 and 8 GeV, rises steadily with a trend towards saturation at 200–400 GeV. These observations (except for the trend) appear to be in conflict with the diffusive shock acceleration (DSA) mechanism, operating in a single supernova remnant (SNR) shock. We argue that $e^{+}/e^{-}$ ratio can still be explained by the diffusive shock acceleration if positrons are accelerated in a subset of SNR shocks which (i) propagate in clumpy gas media and (ii) are modified by accelerated cosmic ray protons. The protons penetrate into the dense gas clumps upstream to produce positrons and charge the clumps positively. The induced electric field expels positrons into the upstream plasma where they are shock accelerated. Since the shock is modified, these positrons develop a harder spectrum than that of the cosmic ray electrons accelerated in other SNRs. Mixing these populations explains the increase in the $e^{+}/e^{-}$ ratio at $E>8\mathrm{GeV}$. It decreases at $E<8\mathrm{GeV}$ because of a subshock weakening which also results from the shock modification. Contrary to the expelled positrons, most of the antiprotons, electrons, and heavier nuclei, are left unaccelerated inside the clumps. Scenarios for the 100–300 GeV AMS-02 fraction exceeding the model prediction, including, but not limited to, possible dark matter contribution, are also discussed.

We compute the transverse single spin asymmetry in light hadron production $p^{\uparrow }p\to hX$ and $p^{\uparrow }A\to hX$ including the gluon saturation effect in the unpolarized nucleon/nucleus. In the forward (large-$x_F$) region, the dominant contribution comes from the so-called derivative term associated with the soft gluonic pole. This leads to the cancellation of nuclear effects in $A_N$ which can be tested at the RHIC. We also show that the soft fermionic pole disappears in the saturation environment.

We derive the collision term relevant for neutrino quantum kinetic equations in the early universe and compact astrophysical objects, displaying its full matrix structure in both flavor and spin degrees of freedom. We include in our analysis neutrino-neutrino processes, scattering and annihilation with electrons and positrons, and neutrino scattering off nucleons (the latter in the low-density limit). After presenting the general structure of the collision terms, we take two instructive limiting cases. The one-flavor limit highlights the structure in helicity space and allows for a straightforward interpretation of the off-diagonal entries in terms of the product of scattering amplitudes of the two helicity states. The isotropic limit is relevant for studies of the early universe: in this case the terms involving spin coherence vanish and the collision term can be expressed in terms of two-dimensional integrals, suitable for computational implementation.

The rapidly improving precision of measurements of gravitational lensing of the cosmic microwave background (CMB) also requires a corresponding increase in the precision of theoretical modeling. A commonly made approximation is to model the CMB deflection angle or lensing potential as a Gaussian random field. In this paper, however, we analytically quantify the influence of the non-Gaussianity of large-scale structure (LSS) lenses, arising from nonlinear structure formation, on CMB lensing measurements. In particular, evaluating the impact of the nonzero bispectrum of large-scale structure on the relevant CMB four-point correlation functions, we find that there is a bias to estimates of the CMB lensing power spectrum. For temperature-based lensing reconstruction with CMB stage III and stage IV experiments, we find that this lensing power spectrum bias is negative and is of order 1% of the signal. This corresponds to a shift of multiple standard deviations for these upcoming experiments. We caution, however, that our numerical calculation only evaluates two of the largest bias terms and, thus, only provides an approximate estimate of the full bias. We conclude that further investigation into lensing biases from nonlinear structure formation is required and that these biases should be accounted for in future lensing analyses.

The bulk properties of spherically symmetric stars in general relativity can be obtained by integrating the Tolman-Oppenheimer-Volkoff (TOV) equations. In previous work [K. Glampedakis, G. Pappas, H. O. Silva, and E. Berti, Phys. Rev. D 92, 024056 (2015)], we developed a “post-TOV” formalism—inspired by parametrized post-Newtonian theory—which allows us to classify in a parametrized, phenomenological form all possible perturbative deviations from the structure of compact stars in general relativity that may be induced by modified gravity at second post-Newtonian order. In this paper we extend the formalism to deal with the stellar exterior, and we compute several potential astrophysical observables within the post-TOV formalism: the surface redshift $z_s$, the apparent radius $R_{\text{app}}$, the Eddington luminosity at infinity $L_{\mathrm{E}}^{\infty }$ and the orbital frequencies. We show that, at leading order, all of these quantities depend on just two post-TOV parameters ${\mu }_1$ and $\chi$, and we discuss the possibility to measure (or set upper bounds on) these parameters.

We prove that the polynomial form of the scattering equations is a Macaulay $H$-basis. We demonstrate that this $H$-basis facilitates integrand reduction and global residue computations in a way very similar to using a Gröbner basis, but circumvents the heavy computation of the latter. As an example, we apply the $H$-basis to prove the conjecture that the dual basis of the polynomial scattering equations must contain one constant term.

It has long been recognized that the neutrinos detected from the next core-collapse supernova in the Galaxy have the potential to reveal important information about the dynamics of the explosion and the nucleosynthesis conditions as well as allowing us to probe the properties of the neutrino itself. The neutrinos emitted from thermonuclear—type Ia—supernovae also possess the same potential, although these supernovae are dimmer neutrino sources. For the first time, we calculate the time, energy, line of sight, and neutrino-flavor-dependent features of the neutrino signal expected from a three-dimensional delayed-detonation explosion simulation, where a deflagration-to-detonation transition triggers the complete disruption of a near-Chandrasekhar mass carbon-oxygen white dwarf. We also calculate the neutrino flavor evolution along eight lines of sight through the simulation as a function of time and energy using an exact three-flavor transformation code. We identify a characteristic spectral peak at $\sim 10\mathrm{MeV}$ as a signature of electron captures on copper. This peak is a potentially distinguishing feature of explosion models since it reflects the nucleosynthesis conditions early in the explosion. We simulate the event rates in the Super-K, Hyper-K, JUNO, and DUNE neutrino detectors with the SNOwGLoBES event rate calculation software and also compute the IceCube signal. Hyper-K will be able to detect neutrinos from our model out to a distance of $\sim 10\mathrm{kpc}$. At 1 kpc, JUNO, Super-K, and DUNE would register a few events while IceCube and Hyper-K would register several tens of events.

Superconducting targets have recently been proposed for the direct detection of dark matter as light as a keV, via elastic scattering off conduction electrons in Cooper pairs. Detecting such light dark matter requires sensitivity to energies as small as the superconducting gap of $\mathcal{O}(\mathrm{meV})$. Here we show that these same superconducting devices can detect much lighter DM, of meV to eV mass, via dark matter absorption on a conduction electron, followed by emission of an athermal phonon. We demonstrate the power of this setup for relic kinetically mixed hidden photons, pseudoscalars, and scalars, showing that the reach can exceed current astrophysical and terrestrial constraints with only a moderate exposure.

We present a simple numerical scheme for perturbation theory (PT) calculations of large-scale structure. Solving the evolution equations for perturbations numerically, we construct the PT kernels as building blocks of statistical calculations, from which the power spectrum and/or correlation function can be systematically computed. The scheme is especially applicable to the generalized structure formation including modified gravity, in which the analytic construction of PT kernels is intractable. As an illustration, we show several examples for power spectrum calculations in $f(R)$ gravity and $\mathrm{\Lambda }\mathrm{CDM}$ models.

Jet substructure techniques such as subjet $p_T$-asymmetry, mass-drop, and grooming have become powerful and widely used tools in experimental searches at the LHC. While these tools provide much-desired handles to separate signal from background, they can introduce unexpected mass scales into the analysis. These scales may be misinterpreted as excesses if these are not correctly incorporated into background modeling. As an example, we study the ATLAS hadronic di-$W/Z$ resonance search. There, we find that the substructure analysis—in particular the combination of a subjet asymmetry cut with the requirement on the number of tracks within a jet—induces a mass scale where the dominant partonic subprocess in the background changes from $pp\to g+q/\overline{q}$ to $pp\to q\overline{q}$. In light of this scale, modeling the QCD background using a simple smooth function with monotonically decreasing slope appears insufficient.

We present cosmological-scale numerical simulations of an evolving universe in full general relativity and introduce a new numerical tool, cosmograph, which employs the Baumgarte-Shapiro-Shibata-Nakamura formalism on a three-dimensional grid. Using cosmograph, we calculate the effect of an inhomogeneous matter distribution on the evolution of a spacetime. We also present the results of a set of standard stability tests to demonstrate the robustness of our simulations.

We present the high-energy-neutrino follow-up observations of the first gravitational wave transient GW150914 observed by the Advanced LIGO detectors on September 14, 2015. We search for coincident neutrino candidates within the data recorded by the IceCube and Antares neutrino detectors. A possible joint detection could be used in targeted electromagnetic follow-up observations, given the significantly better angular resolution of neutrino events compared to gravitational waves. We find no neutrino candidates in both temporal and spatial coincidence with the gravitational wave event. Within $\pm 500\mathrm{s}$ of the gravitational wave event, the number of neutrino candidates detected by IceCube and Antares were three and zero, respectively. This is consistent with the expected atmospheric background, and none of the neutrino candidates were directionally coincident with GW150914. We use this nondetection to constrain neutrino emission from the gravitational-wave event.