With the recognition that fast flavor instabilities likely affect supernova and neutron-star-merger neutrinos, using simulation data to pin down when and where the instabilities occur has become a high priority. The effort faces an interesting problem. Fast instabilities are related to neutrino angular crossings, but simulations often employ moment methods, sacrificing momentum-space angular resolution in order to allocate resources elsewhere. How can limited angular information be used most productively? The main aims here are to sharpen this question and examine some of the available answers. A recently proposed method of searching for angular crossings is scrutinized, the limitations of moment closures are highlighted, and two ways of reconstructing angular distributions solely from the flux factors (based, respectively, on maximum-entropy and sharp-decoupling assumptions) are compared. In (semi)transparent regions, the standard closure prescriptions likely miss some crossings that should be there and introduce others that should not.

There have been a number of recent proposals to enhance the performance of machine learning strategies for collider physics by combining many distinct events into a single ensemble feature. To evaluate the efficacy of these proposals, we study the connection between single-event classifiers and multievent classifiers under the assumption that collider events are independent and identically distributed. We show how one can build optimal multievent classifiers from single-event classifiers, and we also show how to construct multievent classifiers such that they produce optimal single-event classifiers. This is illustrated for a Gaussian example as well as for classification tasks relevant for searches and measurements at the Large Hadron Collider. We extend our discussion to regression tasks by showing how they can be phrased in terms of parametrized classifiers. Empirically, we find that training a single-event (per-instance) classifier is more effective than training a multievent (per-ensemble) classifier, as least for the cases we studied, and we relate this fact to properties of the loss function gradient in the two cases. While we did not identify a clear benefit from using multievent classifiers in the collider context, we speculate on the potential value of these methods in cases involving only approximate independence, as relevant for jet substructure studies.

There is a surprising connection between the top quark and Higgs alignment in Gildener-Weinberg multi-Higgs doublet models. Were it not for the top quark and its large mass, the coupling of the 125 GeV Higgs boson $H$ to gauge bosons and fermions would be indistinguishable from those of the Standard Model Higgs. The top quark’s coupling to a single Higgs doublet breaks this perfect alignment in higher orders of the Coleman-Weinberg loop expansion of the effective potential. But the effect is still small, $\lesssim \mathcal{O}(1\%)$, and probably experimentally inaccessible.

There has been a longstanding discrepancy between the experimental measurements of the electron and muon anomalous magnetic moments and their predicted values in the Standard Model. This is particularly relevant in the case of the muon $g-2$, which has attracted a remarkable interest in the community after the long-awaited announcement of the first results by the Muon $g-2$ collaboration at Fermilab, which confirms a previous measurement by the E821 experiment at Brookhaven and enlarges the statistical significance of the discrepancy, now at $4.2\sigma$. In this paper we consider an extension of the inverse type-III seesaw with a pair of vectorlike leptons that induces masses for neutrinos at the electroweak scale and show that one can accommodate the electron and muon anomalous magnetic moments, while being compatible with all relevant experimental constraints.

A vector-like top partner plays a central role in many new physics models that attempt to address the hierarchy problem. The top partner is conventionally assumed to decay to a quark and a Standard Model (SM) boson. However, it is also possible that the top partner decays to a non-SM scalar and a top quark. Such an exotic decay channel can be the main decay channel of top partners, and thus provides new windows to search for top partners at the LHC. In this paper, with the classical machine learning method of boosted decision trees, we perform a model-independent study of the discovery potential of this new signature at the LHC. In order to suppress the main QCD background, we consider the subdominant but clean decay channel $a\to \gamma \gamma$. For completeness, the single production process and pair production process of top partners are taken into account. We find that, for both single and pair production, the future high-luminosity LHC can exclude a top partner mass up to the TeV scale through the channel $T\to ta$ ($a\to \gamma \gamma$), even if $\mathrm{BR}(a\to \gamma \gamma )$ is as small as $\mathcal{O}(0.1\%)$. Besides, our result shows that single production can overmatch pair production at the 14 TeV LHC, provided that the top partner is heavier than 800–900 GeV.

We show that a metastable dark matter candidate arises naturally from the conformal transformation between the Einstein metric, where gravitons are normalized states, and the Jordan metric dictating the coupling between gravity and matter. Despite being secluded from the Standard Model by a large scale above which the Jordan metric shows modifications to the Einstein frame metric, dark matter couples to the energy momentum tensor of the Higgs field in the primordial plasma primarily. This allows for the production of dark matter in a sufficient amount, which complies with observations. The seclusion of dark matter makes it long-lived for masses $\lesssim 1\mathrm{MeV}$, with a lifetime much above the age of the Universe and the present experimental limits. Such a dark matter scenario has clear monochromatic signatures generated by the decay of the dark matter candidate into neutrino and/or $\gamma$ rays.

Normalizing flows have recently been applied to the problem of accelerating Markov chains in lattice field theory. We propose a generalization of normalizing flows that allows them to applied to theories with a sign problem. These complex normalizing flows are closely related to contour deformations (i.e., the generalized Lefschetz thimble method), which been applied to sign problems in the past. We discuss the question of the existence of normalizing flows; they do not exist in the most general case, but we argue that exact normalizing flows are likely to exist for many physically interesting problems, including cases where the Lefschetz thimble decomposition has an intractable sign problem. Finally, normalizing flows can be constructed in perturbation theory. We give numerical results on their effectiveness across a range of couplings for the Schwinger-Keldysh sign problem associated to a real scalar field in $0+1$ dimensions.

We extend HPQCD’s earlier $n_f=2+1+1$ lattice-QCD analysis of the ratio of $\overline{\mathrm{MS}}$ masses of the $b$ and $c$ quark to include results from finer lattices (down to 0.03 fm) and a new calculation of QED contributions to the mass ratio. We find that ${\overline{m}}_b(\mu )/{\overline{m}}_c(\mu )=4.586(12)$ at renormalization scale $\mu =3\mathrm{GeV}$. This result is nonperturbative. Combining it with HPQCD’s recent lattice $\mathrm{QCD}+\mathrm{QED}$ determination of ${\overline{m}}_c(3\mathrm{GeV})$ gives a new value for the $b$-quark mass: ${\overline{m}}_b(3\mathrm{GeV})=4.513(26)\mathrm{GeV}$. The $b$-mass corresponds to ${\overline{m}}_b({\overline{m}}_b,n_f=5)=4.202(21)\mathrm{GeV}$. These results are the first based on simulations that include QED.

We perform a new Monte Carlo QCD analysis of pion parton distribution functions, including, for the first time, transverse momentum dependent pion-nucleus Drell-Yan cross sections together with $p_{\mathrm{T}}$-integrated Drell-Yan and leading neutron electroproduction data from HERA. We assess the sensitivity of the Monte Carlo fits to kinematic cuts, factorization scale, and parametrization choice, and we discuss the impact of the various datasets on the pion’s quark and gluon distributions. This study provides the necessary step towards the simultaneous analysis of collinear and transverse momentum dependent pion distributions and the determination of the pion’s three-dimensional structure.

A limiting temperature of a species can cause the Universe to asymptote to it yielding a de-Sitter (dS) phase due to macroscopic emergent behavior. The limiting temperature is generic for theories slightly shifted from their conformal point. We demonstrate such behavior in the example of unparticles/Banks-Zaks theory. The unparticles behave like radiation at high energies reducing the Hubble tension and a cosmological constant at low energies yielding a model that follows closely $\mathrm{\Lambda }$ cold dark matter model but due to collective phenomenon. It is technically natural and avoids the no-dS conjecture. The model is free of the coincidence and initial conditions problems, scalar fields, and modified gravity.

We analyze the post-Newtonian orbit of stars around a deformed Kerr black hole. The deformation we consider is a class of disformal transformations of a nontrivial Kerr solution in scalar-tensor theory which are labeled via the disformal parameter $D$. We study different limits of the disformal parameter, and compare the trajectories of stars orbiting a black hole to the case of the Kerr spacetime in general relativity, up to 2PN order. Our findings show that for generic nonzero $D$, the no-hair theorem of general relativity is violated, in the sense that the black hole’s quadrupole $Q$ is not determined by its mass $M$ and angular momentum $J$ through the relation $Q=-J^2/M$. Limiting values of $D$ provide examples of simple and exact noncircular metric solutions, whereas in a particular limit, where $1+D$ is small but finite, we obtain a leading correction to the Schwarzschild precession due to disformality. In this case, the disformal parameter is constrained using the recent measurement of the pericenter precession of the star S2 by the GRAVITY Collaboration.

Scalar-tensor theories are well-studied extensions of general relativity that offer deviations which are yet within observational boundaries. We present the time evolution equations governing the perturbations of a nonrotating scalarized neutron star, including a dynamic spacetime as well as scalar field within the framework of such scalar-tensor theories. We employ a theory that allows for a massive scalar field or a self-interaction term and we study the impact of those parameters on the nonaxisymmetric $f$-mode. The time evolution approach allows for a comparatively simple implementation of the boundary conditions. We find that the $f$-mode frequency is no longer a simple function of the star’s average density when a scalar field is present. We also evaluate the accuracy of different variants of the Cowling approximation commonly used in previous studies of neutron star oscillation modes in alternative theories of gravity and demonstrate that it can give us not only qualitatively correct results, but in some cases also good quantitative estimates of the oscillations frequencies.

We provide evidence that gravitational radiation in a 4D radiation dominated universe, with equation of state $w=1/3$, consists of two components: helicity-2 gravitons and massless scalar acoustic waves. On physical grounds, we would expect the homogeneous solution of the Weyl tensor components to be a good approximation to its inhomogeneous counterparts, whenever the observer is located well in the far zone of an isolated astrophysical source of cosmological gravitational radiation. We show explicitly that these homogeneous and inhomogeneous solutions both receive contributions from the gauge-invariant tensor and the two Bardeen (acoustic) scalars. Comparison of these Weyl tensor computations thus allows us to not only identify, in the high frequency limit, the corresponding gravitational tensor and scalar radiation; but also their oscillatory polarization patterns.

One of the primary sources for the future space-based gravitational wave detector, the Laser Interferometer Space Antenna, are the inspirals of small compact objects into massive black holes in the centers of galaxies. The gravitational waveforms from such Extreme Mass Ratio Inspiral (EMRI) systems will provide measurements of their parameters with unprecedented precision, but only if the waveforms are accurately modeled. Here we explore the impact of transient orbital resonances which occur when the ratio of radial and polar frequencies is a rational number. We introduce a new effective resonance model, which is an extension of the numerical kludge EMRI waveform approximation to include the effect of resonances, and use it to explore the impact of resonances on EMRI parameter estimation. For one-year inspirals, we find that the few cycle dephasings induced by $3:2$ resonances can lead to systematic errors in parameter estimates, that are up to several times the typical measurement precision of the parameters. The bias is greatest for $3:2$ resonances that occur closer to the central black hole. By regarding them as unknown model parameters, we further show that observations will be able to constrain the size of the changes in the orbital parameters across the resonance to a relative precision of 10% for a typical one-year EMRI observation with signal-to-noise ratio of 20. Such measurements can be regarded as tests of fundamental physics, by comparing the measured changes to those predicted in general relativity.

We extend the notion of the Nieh-Yan invariant to generic metric-affine geometries, where both torsion and nonmetricity are taken into account. Notably, we show that the properties of projective invariance and topologicity can be independently accommodated by a suitable choice of the parameters featuring this new Nieh-Yan term. We then consider a special class of modified theories of gravity able to promote the Immirzi parameter to a dynamical scalar field coupled to the Nieh-Yan form, and we discuss in more detail the dynamics of the effective scalar tensor theory stemming from such a revised theoretical framework. We focus, in particular, on cosmological Bianchi I models and we derive classical solutions where the initial singularity is safely removed in favor of a big bounce, which is ultimately driven by the nonminimal coupling with the Immirzi field. These solutions, moreover, turn out to be characterized by finite time singularities, but we show that such critical points do not spoil the geodesic completeness and wave regularity of these spacetimes.

We report three manifestly Lorentz-invariant Hamiltonian formulations of minimally and nonminimally coupled fermion fields to the Holst action. These formulations are achieved by making a suitable parametrization of both the tetrad and the Lorentz connection, which allows us to integrate out some auxiliary fields without spoiling the local Lorentz symmetry. They have the peculiarity that their noncanonical symplectic structures as well as the phase-space variables for the gravitational sector are real. Moreover, two of these Hamiltonian formulations involve half-densitized fermion fields. We also impose the time gauge on these formulations, which leads to real connections for the gravitational configuration variables. Finally, we perform a symplectomorphism in one of the manifestly Lorentz-invariant Hamiltonian formulations and analyze the resulting formulation, which becomes the Hamiltonian formulation of fermion fields minimally coupled to the Palatini action for particular values of the coupling parameters.

We update the search for features, due to transient reductions in inflaton’s speed of sound, in the cosmic microwave background (CMB) angular power spectrum using Planck 2018 temperature, polarization and lensing data. We develop a new methodology to test more flexible templates to reconstruct the reduction of the speed of sound based on Gaussian processes. We formally derive a dynamical prior for the shape of the reduction using a maximum-entropy approach to ensure the physical conditions of the model are satisfied. The posterior allows for one or more consecutive reductions, fitting apparent features in the CMB power spectra in multipoles from a few tens to $\ell \simeq 2000$. As expected, these fits are not statistically favored with respect to the standard cosmological $\mathrm{\Lambda }\mathrm{CDM}$ model. The methodology derived here allows for the inclusion of additional data sets (in particular, Large Scale Structure data), which in principle will increase the statistical significance of the reconstruction of the inflaton’s speed of sound.

The possibility of a nongravitational interaction between dark matter and dark energy is reconstructed using some recent data sets. The crucial aspect is that the interaction is not parametrized at the outset, but rather reconstructed directly from the data in a nonparametric way. The cosmic chronometer Hubble data, the Pantheon supernova compilation of CANDELS and CLASH Multi-Cycle Treasury programs obtained by the Hubble Space Telescope, and the baryon acoustic oscillation Hubble data are considered in this work. The widely accepted Gaussian process is used for the reconstruction. The results clearly indicate that a no-interaction scenario is definitely a possibility. Also, the interaction (if any) is not really significant at the present epoch. The direction of the flow of energy is clearly from dark energy to dark matter, which is consistent with the thermodynamic requirement.

Adopting various unified equations of state (EOSs), we examine the quasinormal modes of gravitational waves from cold neutron stars. We focus on the fundamental ($f$-), 1st pressure ($p_1$-), and 1st spacetime ($w_1$-) modes, and derive the empirical formulas for the frequencies and damping rate of those modes. With the resultant empirical formulas, we find that the value of $\eta$, which is a specific combination of the nuclear saturation parameters, can be estimated within $\sim 30\%$ accuracy, if the $f$-mode frequency from the neutron star whose mass is known would be observed or if the $f$- and $p_1$-mode frequencies would be simultaneously observed, even though this estimation is applicable only for the low-mass neutron stars. Additionally, we find that the mass and radius of canonical neutron stars can be estimated within a few per cent accuracy via the simultaneous observations of the $f$- and $w_1$-mode frequencies. We also find that, if the $f$-, $p_1$-, and $w_1$-mode frequencies would be simultaneously observed, the mass of canonical neutron stars can be estimated within 2% accuracy, while the radius can be estimated within 1% for the neutron star with $M\ge 1.6M_{\odot }$ or within 0.6% for the neutron star with $M\ge 1.4M_{\odot }$ constructed with the EOS constrained via the GW170817 event. Furthermore, we find the strong correlation between the maximum $f$-mode frequency and the neutron star radius with the maximum mass, between the minimum $w_1$-mode frequency and the maximum mass, and between the minimum damping rate of the $w_1$-mode and the stellar compactness for the neutron star with the maximum mass. With these correlations, one may constrain the upper limit of the neutron star radius with the maximum mass if a larger $f$-mode frequency would be observed, the lower limit of the maximum mass if a smaller $w_1$-mode frequency would be observed, or the lower limit of the stellar compactness for the neutron star with the maximum mass if a smaller value of the $w_1$-mode damping rate would be observed.

Real scalar fields with attractive self-interaction may form self-bound states, called oscillons. These dense objects are ubiquitous in leading theories of dark matter and inflation; of particular interest are long-lived oscillons which survive past 14 Gyr, offering dramatic astrophysical signatures into the present day. We introduce a new formalism for computing the properties of oscillons with improved accuracy, which we apply to study the internal structure of oscillons and to identify the physical mechanisms responsible for oscillon longevity. In particular, we show how imposing realistic boundary conditions naturally selects a near-minimally radiating solution and how oscillon longevity arises from its geometry. Furthermore, we introduce a natural vocabulary for the issue of oscillon stability, which we use to predict new features in oscillon evolution. This framework allows for new efficient algorithms, which we use to address questions of whether and to what extent long-lived oscillons are fine-tuned. Finally, we construct a family of potentials supporting ultra-long-lived oscillons, with lifetimes in excess of ${10}^{17}\mathrm{yr}$.