We interpret the recent hints for lepton flavor universality violation in rare $B$ meson decays. Based on a model-independent effective Hamiltonian approach, we determine regions of new physics parameter space that give a good description of the experimental data on $R_K$ and $R_{K^{*}}$, which is in tension with Standard Model predictions. We suggest further measurements that can help narrowing down viable new physics explanations. We stress that the measured values of $R_K$ and $R_{K^{*}}$ are fully compatible with new physics explanations of other anomalies in rare $B$ meson decays based on the $b\to s\mu \mu$ transition. If the hints for lepton flavor universality violation are the first signs of new physics, perturbative unitarity implies new phenomena below a scale of $\sim 100\mathrm{TeV}$.

We perform Monte Carlo simulations of the $C{P}^{N-1}$ model on the square lattice for $N=10$, 21, and 41. Our focus is on the severe slowing down related to instantons. To fight this problem we employ open boundary conditions as proposed by Lüscher and Schaefer for lattice QCD. Furthermore we test the efficiency of parallel tempering in a line defect. Our results for open boundary conditions are consistent with the expectation that topological freezing is avoided, while autocorrelation times are still large. The results obtained with parallel tempering are encouraging.

The prospective Higgs factories, exemplified by ILC, FCC-ee and CEPC, plan to conduct precision Higgs measurements at the $e^{+}{e}^{-}$ center-of-mass energy around 250 GeV. The cross sections for the dominant Higgs production channel, the Higgsstrahlung process, can be measured to a (sub)percent accuracy. Merely incorporating the well-known next-to-leading-order (NLO) electroweak corrections appears to be far from sufficient to match the unprecedented experimental precision. In this work, we make an important advancement toward this direction by investigating the mixed electroweak-QCD corrections to $e^{+}{e}^{-}\to HZ$ at next-to-next-to-leading order (NNLO) for both unpolarized and polarized $Z$ bosons. The corrections turn out to reach the 1% level of the Born order results, and thereby must be incorporated in future confrontations with the data.

We evaluate one-loop amplitudes of $\mathcal{N}=4$ supergravity in $D$ dimensions using the double-copy procedure that expresses gravity integrands in terms of corresponding ones in Yang–Mills theory. We organize the calculation in terms of a set of gauge-invariant tensors, allowing us to identify evanescent contributions. Among the latter, we find the matrix elements of supersymmetric completions of curvature-squared operators. In addition, we find that such evanescent terms and the U(1)-anomalous contributions to one-loop $\mathcal{N}=4$ amplitudes are tightly intertwined. The appearance of evanescent operators in $\mathcal{N}=4$ supergravity and their relation to anomalies raises the question of their effect on the known four-loop divergence in this theory. We provide bases of gauge-invariant tensors and corresponding projectors useful for Yang–Mills theories as a by-product of our analysis.

We investigate the possibility of overcharging spherically symmetric black holes in spacetime dimensions $D>4$ by the capture of a charged particle. We generalize Wald’s classic result that extremal black holes cannot be overcharged. For nearly extremal black holes, we also generalize Hubeny’s scenario by showing that overcharging is possible in a small region of parameter space. We check how $D$ affects the overcharging parameter space and find that this appears to shrink in the large-$D$ limit, which suggests that overcharging becomes increasingly difficult in higher dimensions.

In the present work we study the branon dark matter particles impact on compact objects, and we provide the first constraints of the parameter space using white dwarf stars. The branon dark matter model is characterized by two free parameters, namely the branon mass particle $M$ and the brane tension factor $f$. The latter determines the strength of the interaction of branon dark matter particles with baryons. By considering a typical white dwarf star we were able to obtain constraints on branon dark matter competitive with current limits obtained by direct detection and collider searches. In particular, our results show that (i) for heavy branons with a mass $M>10\mathrm{GeV}$ white dwarfs fail to provide us with bounds better than current limits from dark matter direct detection searches, and (ii) for light branons in the mass range $2\mathrm{keV}<M<1\mathrm{GeV}$, which cannot be probed either with current dark matter experiments or with the next generation of detectors, the dark matter abundance constraint determines $f$ as a function of $M$ in the range $0.1<M<1\mathrm{GeV}$ for the branon mass and $1<f<5\mathrm{GeV}$ for the brane tension factor. Furthermore, our findings indicate that the limits from white dwarfs are not stronger than the dark matter abundance constraint.

We study the $s$-wave kaon-nucleon bound state with the strangeness $S=-1$ in the Bethe-Salpeter formalism in the ladder and instantaneous approximations. We solve the Bethe-Salpeter equation of the bound state and obtain the Bethe-Salpeter amplitude. It is shown that the $K^{-}p$ bound state exists in this formalism. We also study the decay width of the bound state based on the Bethe-Salpeter techniques. The mass of this bound state is 1422 MeV, and its decay width is obviously smaller than that of $\mathrm{\Lambda }(1405)$. These results indicate that there may be some other structures in the observed resonance.

We introduce a novel program of fixed-target searches for thermal-origin dark matter (DM), which couples inelastically to the Standard Model. Since the DM only interacts by transitioning to a heavier state, freeze-out proceeds via coannihilation and the unstable heavier state is depleted at later times. For sufficiently large mass splittings, direct detection is kinematically forbidden and indirect detection is impossible, so this scenario can only be tested with accelerators. Here we propose new searches at proton- and electron-beam fixed-target experiments to probe sub-GeV coannihilation, exploiting the distinctive signals of up- and downscattering as well as decay of the excited state inside the detector volume. We focus on a representative model in which DM is a pseudo-Dirac fermion coupled to a hidden gauge field (dark photon), which kinetically mixes with the visible photon. We define theoretical targets in this framework and determine the existing bounds by reanalyzing results from previous experiments. We find that LSND, E137, and BABAR data already place strong constraints on the parameter space consistent with a thermal freeze-out origin, and that future searches at Belle II and MiniBooNE, as well as recently proposed fixed-target experiments such as LDMX and BDX, can cover nearly all remaining gaps. We also briefly comment on the discovery potential for proposed beam dump and neutrino experiments which operate at much higher beam energies.

We analyze the cross section of $e^{+}{e}^{-}\to {\pi }^{+}{\pi }^{-}\psi (3686)$ measured at the Belle, BABAR, and BESIII experiments. Besides the well-known $Y(4360)$ and $Y(4660)$ resonances, evidence is found for another resonant structure in the low mass region. By fitting the structure with a Breit-Wigner function, the mass and width for the resonance are found to be $M=(4209.1\pm 6.8\pm 7.0)\mathrm{MeV}$ and $\mathrm{\Gamma }=(76.6\pm 14.2\pm 2.4)\mathrm{MeV}$, where the first uncertainties are statistical and the second systematic. The parameters of the structure are consistent with those of the $Y(4220)$ found in $e^{+}{e}^{-}\to \omega {\chi }_{c0}$, ${\pi }^{+}{\pi }^{-}{h}_c$, ${\pi }^{+}{\pi }^{-}J/\psi$, and ${\pi }^{+}{D}^0{D}^{*-}$.

We present a discussion of radiative decays ${\chi }_{cJ}\to \gamma \rho (\omega ,\varphi )$. The decay amplitudes are computed within the QCD factorization framework. NRQCD has been used in the heavy meson sector as well as a collinear expansion in order to describe the overlap with light mesons in the final state. The color-singlet contributions to all helicity amplitudes have been computed using the light-cone distribution amplitudes of twist-2 and twist-3. All obtained expressions are well defined at least in the leading-order approximation. The color-octet contributions have also been studied in the Coulomb limit in order to exhibit their scaling behavior. In order to understand the relevance of the different contributions we perform numerical estimates using the color-singlet contributions. For the ${\chi }_{c1}\to \gamma V_{\perp }$ decays, to vector mesons with transverse polarization, we find that the color-singlet contribution potentially allows for a reliable description. On the other hand, for the ${\chi }_{c1}\to \gamma V_{\parallel }$ decays, to vector mesons with longitudinal polarization, our findings indicate that the color-octet mechanism may be important for a good description. We expect that more accurate measurements of the decay ${\chi }_{c2}\to \gamma V_{\perp ,\parallel }$ can help to better understand the mechanism of radiative decays.

Probing vacuum structures deformed by high intense fields is of great interest in general. In the context of quantum electrodynamics (QED), the vacuum exposed by a linearly polarized high-intensity laser field is expected to show birefringence. We consider the combination of a 10 PW laser system to pump the vacuum and 1 GeV photons to probe the birefringent effect. The vacuum birefringence can be measured via the polarization flip of the probe $\gamma$-rays which can also be interpreted as phase retardation of probe photons. We provide theoretically how to extract phase retardation of GeV probe photons via pairwise topology of the Bethe-Heitler process in a polarimeter and then evaluate the measurability of the vacuum birefringence via phase retardation given a concrete polarimeter design with a realistic set of laser parameters and achievable pulse statistics.

The Bayesian discovery probability of future experiments searching for neutrinoless double-$\beta$ decay is evaluated under the popular assumption that neutrinos are their own antiparticles. A Bayesian global fit is performed to construct a probability distribution for the effective Majorana mass, the observable of interest for these experiments. This probability distribution is then combined with the sensitivity of each experiment derived from a heuristic counting analysis. The discovery probability is found to be higher than previously considered, but strongly depends on whether the neutrino mass ordering is normal or inverted. For the inverted ordering, next-generation experiments are likely to observe a signal already during their first operational stages. Even for the normal ordering, in the absence of neutrino mass mechanisms that drive the lightest state or the effective Majorana mass to zero, the probability of discovering neutrinoless double-$\beta$ decay can reach $\sim 50\%$ or more in the most promising experiments.

Assuming that high-energy effects may alter the standard dispersion relations governing quantized fields, the influence of such modifications on various phenomena has been studied extensively in the literature. In different contexts, it has generally been found that, while superluminal dispersion relations hardly affect the standard results, subluminal relations can lead to (even substantial) modifications to the conventional results. A polymer quantized scalar field is characterized by a series of modified dispersion relations along with suitable changes to the standard measure of the density of modes. Amongst the modified dispersion relations, one finds that the lowest in the series can behave subluminally over a small domain in wave numbers. In this work, we study the response of a uniformly rotating Unruh-DeWitt detector that is coupled to a polymer quantized scalar field. While certain subluminal dispersion relations can alter the response of the rotating detector considerably, in the case of polymer quantization, due to the specific nature of the dispersion relations, the modification to the transition probability rate of the detector does not prove to be substantial. We discuss the wider implications of the result.

We show that the spontaneous scalarization scenario in scalar-tensor theories is a specific case of a more general phenomenon. The key fact is that the instability causing the spontaneous growth in scalars is due to the nonminimal coupling in the theory, and not related to the nature of the scalar. Another field with the same form of coupling undergoes spontaneous growth as well. We explicitly demonstrate this idea for vectors, naming it “spontaneous vectorization”, and study spherically symmetric neutron stars in such a theory. We also comment on other tensor fields the idea can be applied, naming the general mechanism “spontaneous tensorization.”

We study quasinormal modes of static Einstein-Gauss-Bonnet-dilaton black holes. Both axial and polar perturbations are considered and studied from $l=0$ to $l=3$. We emphasize the difference in the spectrum between the Schwarzschild solutions and dilatonic black holes. At large Gauss-Bonnet coupling constant, a small secondary branch of black holes is present, when the dilaton coupling is sufficiently strong. The modes of the primary branch can differ from the Schwarzschild modes up to 10%. The secondary branch is unstable and possesses long-lived modes. We address the possible effects of these modes on future observations of gravitational waves emitted during the ringdown phase of astrophysical black holes.

Oscillating scalar fields are useful to model a variety of matter components in the Universe. One or more scalar fields participate in the reheating process after inflation, while at much lower energies scalar fields are robust dark matter candidates. Pertaining to structure formation in these models, it is well known that inhomogeneities of the Klein-Gordon field are unstable above the characteristic de Broglie wavelength. In this paper we show that such instability implies the existence of a threshold amplitude for the collapse of primordial fluctuations. We use this threshold to correctly predict the cutoff scale of the matter power spectrum in the scalar field dark matter model. Furthermore, for a Klein-Gordon field during reheating we show that this same threshold allows for abundant production of structure (oscillons but not necessarily black holes). Looking at the production of primordial black holes (PBHs) in this scenario we note that the sphericity condition yields a much lower probability of PBH formation at the end of inflation. Remarkably, even after meeting a such stringent condition, we find that PBHs may be overproduced during reheating. We finally constrain the epochs at which an oscillating Klein-Gordon field could dominate the early universe.

For primordial black holes (PBH) to be the dark matter in single-field inflation, the slow-roll approximation must be violated by at least $\mathcal{O}(1)$ in order to enhance the curvature power spectrum within the required number of $e$-folds between cosmic microwave background scales and PBH mass scales. Power spectrum predictions which rely on the inflaton remaining on the slow-roll attractor can fail dramatically leading to qualitatively incorrect conclusions in models like an inflection potential and misestimate the mass scale in a running mass model. We show that an optimized temporal evaluation of the Hubble slow-roll parameters to second order remains a good description for a wide range of PBH formation models where up to a $10^7$ amplification of power occurs in 10 $e$-folds or more.

Analyzing quantum cosmological scenarios containing one scalar field with exponential potential, we have obtained a universe model which realizes a classical dust contraction from very large scales, the initial repeller of the model, and moves to a stiff matter contraction near the singularity, which is avoided due to a quantum bounce. The universe is then launched in a stiff matter expanding phase, which then moves to a dark energy era, finally returning to the dust expanding phase, the final attractor of the model. Hence, one has obtained a nonsingular cosmological model where a single scalar field can describe both the matter contracting phase of a bouncing model, necessary to give an almost scale invariant spectrum of scalar cosmological perturbations, and a transient expanding dark energy phase. As the universe is necessarily dust dominated in the far past, usual adiabatic vacuum initial conditions can be easily imposed in this era, avoiding the usual issues appearing when dark energy is considered in bouncing models.

We present a general and systematic study of how a Bell experiment on the cosmic microwave background could be carried out. We introduce different classes of pseudo-spin operators and show that, if the system is placed in a two-mode squeezed state as inflation predicts, they all lead to a violation of the Bell inequality. However, we also discuss the obstacles that one faces in order to realize this program in practice and show that they are probably insurmountable. We suggest alternative methods that could reveal the quantum origin of cosmological structures without relying on Bell experiments.

Dark matter may be discovered through its capture in stars and subsequent annihilation. It is usually assumed that dark matter is captured after a single scattering event in the star; however this assumption breaks down for heavy dark matter, which requires multiple collisions with the star to lose enough kinetic energy to become captured. We analytically compute how multiple scatters alter the capture rate of dark matter and identify the parameter space where the effect is largest. Using these results, we then show how multiscatter capture of dark matter on compact stars can be used to probe heavy $m_X\gg \mathrm{TeV}$ dark matter with remarkably small dark matter–nucleon scattering cross sections. As one example, it is demonstrated how measuring the temperature of old neutron stars in the Milky Way’s center provides sensitivity to high mass dark matter with dark matter–nucleon scattering cross sections smaller than the xenon direct detection neutrino floor.