We study the cosmological effects of two-body dark matter decays in which the products of the decay include a massless and a massive particle. We show that if the massive daughter particle is slightly warm it is possible to relieve the tension between distance ladder measurements of the present-day Hubble parameter with measurements from the cosmic microwave background.

Perturbative expansions, such as the well-known gradient series and the recently proposed slow-roll expansion, have been recently used to investigate the emergence of hydrodynamic behavior in systems undergoing Bjorken flow. In this paper we determine for the first time the large order behavior of these perturbative expansions in relativistic hydrodynamics in the case of Gubser flow. While both series diverge, the slow-roll series can provide a much better overall description of the system’s dynamics than the gradient expansion when both series are truncated at low orders. The truncated slow-roll series can also describe the attractor solution of Gubser flow as long as the system is sufficiently close to equilibrium near the origin (i.e., $\rho =0$) in $d{S}_3\otimes \mathbb{R}$. Differently than the case of Bjorken flow, here we show that the Gubser flow attractor solution is not solely a function of the effective Knudsen number ${\tau }_R\sqrt{{\sigma }_{\mu \nu }{\sigma }^{\mu \nu }}\sim {\tau }_R\tanh \rho$. Our results give further support to the idea that new resummed constitutive relations between dissipative currents and the gradients of conserved quantities can emerge in systems far from equilibrium that are beyond the regime of validity of the usual gradient expansion.

We present constraints on extensions of the minimal cosmological models dominated by dark matter and dark energy, $\mathrm{\Lambda }\mathrm{CDM}$ and $w\mathrm{CDM}$, by using a combined analysis of galaxy clustering and weak gravitational lensing from the first-year data of the Dark Energy Survey (DES Y1) in combination with external data. We consider four extensions of the minimal dark energy-dominated scenarios: (1) nonzero curvature ${\mathrm{\Omega }}_k$, (2) number of relativistic species $N_{\mathrm{eff}}$ different from the standard value of 3.046, (3) time-varying equation-of-state of dark energy described by the parameters $w_0$ and $w_a$ (alternatively quoted by the values at the pivot redshift, $w_p$, and $w_a$), and (4) modified gravity described by the parameters ${\mu }_0$ and ${\mathrm{\Sigma }}_0$ that modify the metric potentials. We also consider external information from Planck cosmic microwave background measurements; baryon acoustic oscillation measurements from SDSS, 6dF, and BOSS; redshift-space distortion measurements from BOSS; and type Ia supernova information from the Pantheon compilation of datasets. Constraints on curvature and the number of relativistic species are dominated by the external data; when these are combined with DES Y1, we find ${\mathrm{\Omega }}_k=0.0020_{-0.0032}^{+0.0037}$ at the 68% confidence level, and the upper limit $N_{\mathrm{eff}}<3.28(3.55)$ at 68% (95%) confidence, assuming a hard prior $N_{\mathrm{eff}}>3.0$. For the time-varying equation-of-state, we find the pivot value $(w_p,w_a)=(-0.91_{-0.23}^{+0.19},-0.57_{-1.11}^{+0.93})$ at pivot redshift $z_p=0.27$ from DES alone, and $(w_p,w_a)=(-1.01_{-0.04}^{+0.04},-0.28_{-0.48}^{+0.37})$ at $z_p=0.20$ from DES Y1 combined with external data; in either case we find no evidence for the temporal variation of the equation of state. For modified gravity, we find the present-day value of the relevant parameters to be ${\mathrm{\Sigma }}_0=0.43_{-0.29}^{+0.28}$ from DES Y1 alone, and $({\mathrm{\Sigma }}_0,{\mu }_0)=(0.06_{-0.07}^{+0.08},-0.11_{-0.46}^{+0.42})$ from DES Y1 combined with external data. These modified-gravity constraints are consistent with predictions from general relativity.

We discuss some implications of the recently suggested swampland conjecture $\frac{|\nabla V|}{V}\gtrsim c\sim 1$, together with a previous one $\mathrm{\Delta }\varphi \lesssim 1$. We list some implications for particle phenomenology and the early Universe. The most intriguing implication of the conjecture could be a significant shift in allowed inflationary models, if not ruling out slow-roll (single field) inflation altogether. The tension of inflation and the conjecture does not only regard the amplitude of the tensor spectrum, but also its tilt, as $c\gtrsim 1$ implies both a yet unobserved tensor to scalar ratio, and an enhancement of the observed scalar power spectrum on large scales in discord with current data that favors a suppression on these scales. Scalar fields are abundant in theories of quantum gravity. Considering a second scalar field, its dynamics are dictated by the relation between its mass, $m$, and the Hubble parameter, $H$, at different epochs in the history of the Universe. This scalar field, a drainon, fulfills the conjecture draining up the swampland. For inflation, this drainon requires a modest hierarchy compared to the inflaton. For the rest of the thermal history of the Universe, the drainon can be a coherently oscillating scalar field strengthening the case of dark matter candidates of that sort.

Force-free electrodynamics (FFE) is a closed set of equations for the electromagnetic field of a magnetically dominated plasma. There are strong arguments for the existence of force-free plasmas near pulsars and active black holes, but FFE alone cannot account for the observational signatures, such as coherent radio emission and relativistic jets and winds. We reformulate FFE as the effective field theory of a cold string fluid and initiate a systematic study of corrections in a derivative expansion. At leading order the effective theory is equivalent to (generalized) FFE, with the strings comprised by magnetic field line world sheets. Higher-order corrections generically give rise to nonzero accelerating electric fields ($\mathbf{E}·\mathbf{B}\ne 0$). We discuss potential observable consequences and comment on an intriguing numerical coincidence.

We show that adjoint QCD features very strong Bose-Fermi cancellations in the large-$N$ limit, despite the fact that it is manifestly nonsupersymmetric. The difference between the bosonic and fermionic densities of states in large-$N$ adjoint QCD turns out to have a “two-dimensional” scaling $\sim \exp (\sqrt{\ell E})$ for large energies $E$ in finite spatial volume, where $\ell$ is a length scale associated with the curvature of the spatial manifold. In particular, all Hagedorn growth cancels, and so does the growth $\exp (V^{1/4}{E}^{3/4})$ expected in a standard local four-dimensional theory in spatial volume $V$. In these ways, large-$N$ adjoint QCD, a manifestly nonsupersymmetric theory, acts similarly to supersymmetric theories. We also show that at large-$N$ the vacuum energy of multiflavor adjoint QCD is non-negative and exponentially small compared to the UV cutoff with several natural regulators.

The LHCb Collaboration announced the observation of three narrow structures consistent with hidden-charm pentaquark states. They are candidates of hadronic molecules formed of a pair of a charmed baryon and an anticharmed meson. Among them, the $P_c(4457)$ mass is consistent with earlier predictions of a ${\mathrm{\Sigma }}_c{\overline{D}}^{*}$ molecule with $I=1/2$. We point out that if such a picture were true, one would have $\mathcal{B}(P_c(4457)\to J/\psi {\mathrm{\Delta }}^{+})/\mathcal{B}(P_c(4457)\to J/\psi p)$ at the level ranging from a few percent to about 30%. Such a large isospin breaking decay ratio is two to three orders of magnitude larger than that for normal hadron resonances. It is a unique feature of the ${\mathrm{\Sigma }}_c{\overline{D}}^{*}$ molecular model, and can be checked by LHCb.

We perform a search for light sterile neutrinos using the data from the T2K far detector at a baseline of 295 km, with an exposure of $14.7(7.6)\times {10}^{20}$ protons on target in neutrino (antineutrino) mode. A selection of neutral-current interaction samples is also used to enhance the sensitivity to sterile mixing. No evidence of sterile neutrino mixing in the $3+1$ model was found from a simultaneous fit to the charged-current muon, electron and neutral-current neutrino samples. We set the most stringent limit on the sterile oscillation amplitude ${\sin }^2{\theta }_{24}$ for the sterile neutrino mass splitting $\mathrm{\Delta }{m}_{41}^2<3\times {10}^{-3}{\mathrm{eV}}^2/c^4$.

We present a new method to expose the dead-cone effect at colliders using iterative declustering techniques. Iterative declustering allows one to unwind the jet clustering and to access the subjets or branches at different depths of the jet tree. Our method consists of declustering the heavy flavor-tagged jet using the Cambridge-Achen algorithm following the branch containing the heavy flavor at each step and registering the kinematics of the complementary untagged prong. The kinematics of the complementary untagged prong fill a Lund map representing the gluon radiation off the heavy-flavor quark at each step of the vacuum shower. Using Pythia8 Monte Carlo, we show that a simple cut on the Lund plane introduced by $\ln (k_{\mathrm{T}})>0$ suppresses hadronization effects and the angular separation between the jet prongs becomes very sensitive to flavor effects. A clear suppression for heavy-flavor jets relative to inclusive jets in the region of splitting angles delimited by the relation $\theta <m_Q/E$ is observed, where $m_Q$ is the mass of the heavy quark and $E$ is the energy of the radiator or splitting prong.

The gravitational wave memory effect is characterized by the permanent relative displacement of a pair of initially comoving test particles that is caused by the passage of a burst of gravitational waves. Recent research on this effect has clarified the physical origin and the interpretation of this gravitational phenomenon in terms of conserved charges at null infinity and “soft theorems.” In this paper, we describe a more general class of effects than the gravitational wave memory that are not necessarily associated with these charges and soft theorems, but that are, in principle, measurable. We shall refer to these effects as persistent gravitational wave observables. These observables vanish in nonradiative regions of a spacetime, and their effects “persist” after a region of spacetime which is radiating. We give three examples of such persistent observables, as well as general techniques to calculate them. These examples, for simplicity, restrict the class of nonradiative regions to those which are exactly flat. Our first example is a generalization of geodesic deviation that allows for arbitrary acceleration. The second example is a holonomy observable, which is defined in terms of a closed loop. It contains the usual “displacement” gravitational wave memory; three previously identified, though less well known memory effects (the proper time, velocity, and rotation memories); and additional new observables. Finally, the third example we give is an explicit procedure by which an observer could measure a persistent effect using a spinning test particle. We briefly discuss the ability of gravitational wave detectors (such as LIGO and Virgo) to measure these observables.

By introducing an auxiliary parameter, we find a new representation for Feynman integrals, which defines a Feynman integral by analytical continuation of a series containing only vacuum integrals. The new representation therefore conceptually translates the problem of computing Feynman integrals to the problem of performing analytical continuations. As an application of the new representation, we use it to construct a novel reduction method for multiloop Feynman integrals, which is expected to be more efficient than the known integration-by-parts reduction method. Using the new method, we successfully reduced all complicated two-loop integrals in the $gg\to HH$ process and $gg\to ggg$ process.

We forecast the constraints on the neutrino mass sum ($\mathrm{\Sigma }{m}_{\nu }$) from the one-point probability distribution function (PDF) and power spectrum of weak lensing measurements for an Large Synoptic Survey Telescope-like survey, using MassiveNuS simulations. The PDF provides access to non-Gaussian information beyond the power spectrum. It is particularly sensitive to nonlinear growth on small scales, where massive neutrinos also have the largest effect. We find that tomography helps improve the constraint on $\mathrm{\Sigma }{m}_{\nu }$ by 14% and 32% for the power spectrum and the PDF, respectively, compared to a single redshift bin. The PDF alone outperforms the power spectrum in constraining $\mathrm{\Sigma }{m}_{\nu }$. When the two statistics are combined, the constraint is further tightened by 35%, with respect to the power spectrum alone. We conclude that the weak lensing PDF is complementary to the power spectrum and has the potential to become a powerful tool for constraining neutrino mass. In this work, we do not include systematics such as baryon effects, intrinsic alignments, $\mathrm{photo}\text{−}z$ errors, and stress that they need to be carefully studied in future work.

The $B_c^{+}$ family of $(c\overline{b})$ mesons with beauty and charm is of special interest among heavy quarkonium systems. The $B_c^{+}$ mesons are intermediate between $(c\overline{c})$ and $(b\overline{b})$ states both in mass and size, so many features of the $(c\overline{b})$ spectrum can be inferred from what we know of the charmonium and bottomonium systems. The unequal quark masses mean that the dynamics may be richer than a simple interpolation would imply, in part because the charmed quark moves faster in $B_c$ than in the $J/\psi$. Close examination of the $B_c^{+}$ spectrum can test our understanding of the interactions between heavy quarks and antiquarks and may reveal where approximations break down. Whereas the $J/\psi$ and $\mathrm{\Upsilon }$ levels that lie below the flavor threshold are metastable with respect to strong decays, the $B_c$ ground state is absolutely stable against strong or electromagnetic decays. Its dominant weak decays arise from $\overline{b}\to \overline{c}{W}^{\star +}$, $c\to s{W}^{\star +}$, and $c\overline{b}\to W^{\star +}$ transitions, where $W^{\star }$ designates a virtual weak boson. Prominent examples of the first category are quarkonium transmutations such as $B_c^{+}\to J/\psi {\pi }^{+}$ and $B_c^{+}\to J/\psi {\ell }^{+}{\nu }_{\ell }$, where $J/\psi$ designates the $(c\overline{c})$ $1S$ level. The high data rates and extraordinarily capable detectors at the Large Hadron Collider give renewed impetus to the study of mesons with beauty and charm. Motivated by the recent experimental searches for the radially excited $B_c$ states, we update the expectations for the low-lying spectrum of the $B_c$ system. We make use of lattice QCD results, a novel treatment of spin splittings, and an improved quarkonium potential to obtain detailed predictions for masses and decays. We suggest promising modes in which to observe excited states at the LHC. The $3P$ and $3S$ states, which lie close to or just above the threshold for strong decays, may provide new insights into the mixing between quarkonium bound states and nearby two-body open-flavor channels. Searches in the $B^{(*)}{D}^{(*)}$ final states could well reveal narrow resonances in the $J^P=0^{-}$, $1^{-}$, and $2^{+}$ channels and possibly in the $J^P=0^{+}$ and $1^{+}$ channels at threshold. Looking further ahead, the prospect of very-high-luminosity $e^{+}{e}^{-}$ colliders capable of producing tera-$Z$ samples raises the possibility of investigating $B_c$ spectroscopy and rare decays in a controlled environment.

We consider the standard problem of observational astronomy, i.e., the observations of light emission from a distant region of spacetime in general relativity. The goal is to describe the changes between the measurements of the light performed by a sample of observers slightly displaced with respect to each other and moving with different 4-velocities and 4-accelerations. In our approach, all results of observations can be expressed as functions of the kinematic variables, describing the motions of the observers and the emitting bodies with respect to their local inertial frames, and four linear bilocal geodesic operators, describing the influence of the spacetime geometry on light propagation. The operators are functionals of the curvature tensor along the line of sight. The results are based on the assumption that the regions of emissions and observations are sufficiently small so that the spacetime curvature effects are negligible within each of them, although they are significant for the light propagation between them. The new formulation provides a uniform approach to optical phenomena in curved spacetimes and, as an application, we discuss the problem of a fully relativistic definition of the parallax and position drifts (or proper motions). We then use the results to construct combinations of observables which are completely insensitive to the motion of both the observer and the emitter. These combinations by construction probe the spacetime geometry between the observation and emission regions and in our formalism we may express them as functionals of the Riemann tensor along the line of sight. For short distances one of these combinations depends only on the matter content along the line of sight. This opens up the possibility to measure the matter content of a spacetime in a tomographylike manner irrespective of the motions of the emitter and the observer.

We present nuclear structure factors that describe the generalized spin-independent coupling of weakly interacting massive particles (WIMPs) to nuclei. Our results are based on state-of-the-art nuclear structure calculations using the large-scale nuclear shell model. Starting from quark- and gluon-level operators, we consider all possible coherently enhanced couplings of spin-$1/2$ and spin-0 WIMPs to one and two nucleons up to third order in chiral effective field theory. This includes a comprehensive discussion of the structure factors corresponding to the leading two-nucleon currents covering, for the first time, the contribution of spin-2 operators. We provide results for the most relevant nuclear targets considered in present and planned dark matter direct detection experiments: fluorine, silicon, argon, and germanium, complementing our previous work on xenon. All results are also publicly available in a Python notebook.

We study thermalization, hydrodynamization, and chemical equilibration in out-of-equilibrium quark-gluon plasma starting from various initial conditions using QCD effective kinetic theory, valid at weak coupling. In nonexpanding systems gauge bosons rapidly lose information of the initial state and achieve kinetic equilibrium among themselves, while fermions approach the equilibrium distribution only at a later time. In systems undergoing rapid longitudinal expansion, both gluons and quarks are kept away from equilibrium by the expansion, but the evolution is well described by fluid dynamics even before local thermal equilibrium is reached. For realistic couplings we determine the ordering between the separate hydrodynamization, chemical equilibration and thermalization time scales to be ${\tau }_{\text{hydro}}<{\tau }_{\mathrm{chem}}<{\tau }_{\text{therm}}$.

Following a semiclassical eikonal approach—justified at transplanckian energies order by order in the deflection angle ${\mathrm{\Theta }}_s\sim \frac{4G\sqrt{s}}{b}\equiv \frac{2R}{b}$—we investigate the infrared features of gravitational scattering and radiation in four space-time dimensions, and we illustrate the factorization and cancellation of the infinite Coulomb phase for scattering and the eikonal resummation for radiation. As a consequence, both the eikonal phase $2\delta (E,b)$ and the gravitational-wave (GW) spectrum $\frac{\mathrm{d}{E}^{\mathrm{GW}}}{\mathrm{d}\omega }$ are free from infrared problems in a frequency region extending from zero to (and possibly beyond) $\omega =1/R$. The infrared-singular behavior of 4-D gravity leaves a memory in the deep infrared region ($\omega R\ll \omega b<1$) of the spectrum. At $\mathcal{O}(\omega b)$ we confirm the presence of logarithmic enhancements of the form already pointed out by Sen and collaborators on the basis of nonleading corrections to soft-graviton theorems. These, however, do not contribute to the unpolarized and/or azimuthally averaged flux. At $\mathcal{O}({\omega }^2{b}^2)$ we find instead a positive logarithmically enhanced correction to the total flux implying an unexpected maximum of its spectrum at $\omega b\sim 0.5$. At higher orders we find subleading enhanced contributions as well, which can be resummed, and have the interpretation of a finite rescattering Coulomb phase of emitted gravitons.

We investigate whether inflation requires finely tuned initial conditions in order to explain the degree of flatness and homogeneity observed in the Universe. We achieve this by using the Eisenhart lift, which can be used to write any scalar field theory in a purely geometric manner. Using this formalism, we construct a manifold whose points represent all possible initial conditions for an inflationary theory. After equipping this manifold with a natural metric, we show that the total volume of this manifold is finite for a wide class of inflationary potentials. Hence, we identify a natural measure that enables us to distinguish between generic and finely tuned sets of initial conditions without the need for a regulator, in contrast to previous work in the literature. Using this measure, we find that the initial conditions that allow for sufficient inflation are indeed finely tuned. The degree of fine-tuning also depends crucially on the value of the cosmological constant at the time of inflation. Examining some concrete examples, we find that we require percent-level fine tuning if we allow the cosmological constant during inflation to be much larger than it is today. However, if we fix the cosmological constant to its presently observed value, the degree of fine tuning required is of order ${10}^{-54}$.

The radio detection of cosmic rays consists in the estimation of the properties of a primary cosmic ray by observing the electric field emitted by the extensive air shower (EAS) created when the primary cosmic ray enters the atmosphere. This technique is fully operative nowadays and presents a good degree of maturity. In addition, several projects intend to employ this technique for the detection of neutrinos. In order for the technique to be useful, accurate methods for computing the electric field created by a particle shower in the context of a particular experiment must exist. Although current ground-based radio experiments lie on the air-soil interface and some planned experiments on the South Pole envision antennas near the air-ice interface, most of the analytical approaches and Monte Carlo codes used for calculating the electric field either do not take into account the effect of the boundary or calculate the radiation fields only (direct, reflected and transmitted radiation fields). When the particle shower and the antenna are close to the boundary, compared to the observation wavelength, the far-field approximation breaks down, which is the case for the low-frequency EXTASIS experiment, for instance. We present in this work a new formula for calculating the exact field emitted by a particle track in two semi-infinite media separated by a planar boundary. We also explore the validity of the far-field approximation and make some predictions for EAS using a simple shower model.

Sub-GeV dark matter candidates are of increasing interest, because long-favored candidates such as GeV-scale weakly interacting massive particles have not been detected. For low-mass dark matter, model-independent constraints are weak or nonexistent. We show that for such candidates, because the number density is high, cosmic ray propagation can be affected by elastic scattering with dark matter. We call this type of search “reverse direct detection,” because dark matter is the target and standard model particles are the beam. Using a simple propagation model for galactic cosmic rays, we calculate how dark matter affects cosmic ray spectra at Earth, and set new limits on the dark matter-proton and dark matter-electron cross sections. For protons, our limit is competitive with cosmological constraints, but is independent. For electrons, our limit covers masses not yet probed, and improves on cosmological constraints by one to two orders of magnitude. We comment on how future work can significantly improve the sensitivity of cosmic-ray probes of dark matter interactions.