We consider scenarios of warped extra dimensions with all matter fields in the bulk and in which both the hierarchy and the flavor puzzles of the Standard Model are addressed. Inspired by the puzzling excess of diphoton events at 750 GeV reported in the early LHC Run II data (since then understood as a statistical excess), we consider here the general question as to whether the simplest extra-dimensional extension of the Standard Model Higgs sector, i.e., a five-dimensional bulk Higgs doublet, can lead to an intermediate mass resonance (between 500 GeV and 1.5 TeV) of which the first signature would be the presence of diphoton events. This surprising phenomenology can happen if the resonance is the lightest $CP$-odd state coming from the Higgs sector. No new matter content is required, the only new ingredient being the presence of (positive) brane localized kinetic terms associated to the five-dimensional bulk Higgs (which reduce the mass of the $CP$-odd states). Production and decay of this resonance can naturally give rise to observable diphoton signals, keeping dijet production under control, with very low ZZ and WW signals and with a highly reduced top pair production in an important region of parameter space. We use the 750 GeV excess as an example case scenario.

We study the gravitational collapse of a homogeneous time-dependent scalar field that, besides its coupling to curvature, is also kinematically coupled to the Einstein tensor. This coupling is a part of the Horndeski theory and we investigate its effect on the collapsing process. We find that the time required for the scalar field to collapse depends on the value of the derivative coupling and the singularity is protected by a horizon. Matching the internal solution with an external Schwarzschild-anti–de Sitter metric we show that a black hole is formed, while the weak energy condition is satisfied during the collapsing process. The scalar field takes on a finite value at the singularity.

The sum of squared epicyclic frequencies of nearly circular motion (${\omega }_r^2+{\omega }_{\theta }^2$) in axially symmetric configurations of Newtonian gravity is known to depend both on the matter density and on the angular velocity profile of circular orbits. It was recently found that this sum goes to zero at the photon orbits of Schwarzschild and Kerr spacetimes. However, these are the only relativistic configurations for which such a result exists in the literature. Here, we extend the above formalism in order to describe the analogous relation for geodesic motion in arbitrary static, axially symmetric, asymptotically flat solutions of general relativity. The sum of squared epicyclic frequencies is found to vanish at photon radii of vacuum solutions. In the presence of matter, we obtain that ${\omega }_r^2+{\omega }_{\theta }^2>0$ for perturbed timelike circular geodesics on the equatorial plane if the strong energy condition holds for the matter-energy fluid of spacetime; in vacuum, the allowed region for timelike circular geodesic motion is characterized by the inequality above. The results presented here may be of use to shed light on general issues concerning the stability of circular orbits once they approach photon radii, mainly the ones corresponding to stable photon motion.

Scalar fields, ${\varphi }_i$, can be coupled nonminimally to curvature and satisfy the general criteria: (i) the theory has no mass input parameters, including $M_P=0$; (ii) the ${\varphi }_i$ have arbitrary values and gradients, but undergo a general expansion and relaxation to constant values that satisfy a nontrivial constraint, $K({\varphi }_i)=\text{constant}$; (iii) this constraint breaks scale symmetry spontaneously, and the Planck mass is dynamically generated; (iv) there can be adequate inflation associated with slow roll in a scale-invariant potential subject to the constraint; (v) the final vacuum can have a small to vanishing cosmological constant; (vi) large hierarchies in vacuum expectation values can naturally form; (vii) there is a harmless dilaton which naturally eludes the usual constraints on massless scalars. These models are governed by a global Weyl scale symmetry and its conserved current, $K_{\mu }$. At the quantum level the Weyl scale symmetry can be maintained by an invariant specification of renormalized quantities.

We numerically solve the evolution equations of neutrino three-flavor density matrices, and show that, even if neutrino oscillations mix neutrino flavors, large lepton number asymmetries are still allowed in certain limits by big bang nucleosynthesis.

A new gauge singlet scalar field can undergo a strongly first-order phase transition (PT) leading to gravitational waves (GW) potentially observable at aLIGO and stabilizes the electroweak vacuum at the same time by ensuring that the Higgs quartic coupling remains positive up to at least the grand unification (GUT) scale. aLIGO (O5) is potentially sensitive to cosmological PTs at $10^7–10^8\mathrm{GeV}$, which coincides with the requirement that the singlet scale is less than the standard model (SM) vacuum instability scale, which is between $10^8\mathrm{GeV}$ and $10^{14}\mathrm{GeV}$. After sampling its parameter space, we identify three benchmark points with a PT at about $T\approx 10^7\mathrm{GeV}$ in a gauge singlet extension of the SM. We calculate the nucleation temperature, order parameter, characteristic time scale, and peak amplitude and frequency of GW from bubble collisions during the PT for the benchmarks and find that, in an optimistic scenario, GW from such a PT may be in reach of aLIGO (O5). We confirm that the singlet stabilizes the electroweak vacuum while remaining consistent with zero-temperature phenomenology as well. Thus, this scenario presents an intriguing possibility that aLIGO may detect traces of fundamental physics motivated by vacuum stability at an energy scale that is well above the reach of any other experiment.

In the next few years, Advanced LIGO (aLIGO) may see gravitational waves (GWs) from thousands of black hole (BH) mergers. This marks the beginning of a new precision tool for physics. Here we show how to search for new physics beyond the standard model using this tool, in particular the QCD axion in the mass range ${\mu }_a\sim {10}^{-14}$ to ${10}^{-10}\mathrm{eV}$. Axions (or any bosons) in this mass range cause rapidly rotating BHs to shed their spin into a large cloud of axions in atomic Bohr orbits around the BH, through the effect of superradiance (SR). This results in a gap in the mass vs spin distribution of BHs when the BH size is comparable to the axion’s Compton wavelength. By measuring the spin and mass of the merging objects observed at LIGO, we could verify the presence and shape of the gap in the BH distribution produced by the axion. The axion cloud can also be discovered through the GWs it radiates via axion annihilations or level transitions. A blind monochromatic GW search may reveal up to $10^5$ BHs radiating through axion annihilations, at distinct frequencies within $\sim 3\%$ of $2{\mu }_a$. Axion transitions probe heavier axions and may be observable in future GW observatories. The merger events are perfect candidates for a targeted GW search. If the final BH has high spin, a SR cloud may grow and emit monochromatic GWs from axion annihilations. We may observe the SR evolution in real time.

Quantum entanglement is the characteristic quantum correlation. Here, we use this concept to analyze the quantum entanglement generated by Schwinger production of particle-antiparticle pairs in an electric field, as well as the change of entanglement as a consequence of the electric field effect on a pre-existing entangled pair of particles. The system is partitioned by using momentum modes. Various kinds of pairwise mode entanglement are calculated as functions of the electric field. Both constant and pulsed electric fields are considered. The use of entanglement exposes information beyond that in particle number distributions.

We augment the chiral Lagrangian by an isosinglet scalar and compute the one-loop radiative corrections to the pion mass and decay constant, as well as the scalar mass. The calculations are carried out for different patterns of chiral symmetry breaking of immediate relevance for phenomenology and lattice investigations. By construction our results encompass several interesting limits, ranging from the dilaton to the linear sigma model.

Prompted by plans for a free neutron oscillation experiment at the European Spallation Source (ESS), we consider issues associated with the magnetic fields that must be present. To this end, we introduce a stochastic model of the residual magnetic field within the propagation region which draws on features of magnetic profiles measured during the last free oscillation experiment at the Institut Laue-Langevin (ILL). A perturbative analysis, which relates the antineutron probability to the power spectral density of the magnetic field sampled, suggests that deviations from the quasifree result will increase quadratically with the length $l$ of the propagation region. However, with inclusion of averaging over representative spectra of neutron speeds, departures from the quasifree result are found to be approximately linear in $l$. As regards the large spikes in the magnetic field at, for example, joints in the magnetic shielding of the propagation region (despite compensating currents and magnetic idealization of the shield), we demonstrate that their effect scales as $l/D^{3/2}$, where $D$ is the diameter of the cylindrical magnetic shielding, and identify conditions under which they can be neglected. We also establish that any large magnetic field encountered after the propagation region is exited will not diminish the probability for antineutron detection. For the range of values of $l$ of most interest to the ESS experiment, it should suffice to improve on the level of magnetic suppression achieved at the ILL by a factor of 2.

Motivated by the fact that, so far, the whole body of evidence for dark matter is of gravitational origin, we study the decays of dark matter into Standard Model particles mediated by gravity portals, i.e., through nonminimal gravitational interactions of dark matter. We investigate the decays in several widely studied frameworks of scalar and fermionic dark matter where the dark matter is stabilized in flat spacetime via global symmetries. We find that the constraints on the scalar singlet dark matter candidate are remarkably strong and exclude large regions of the parameter space, suggesting that an additional stabilizing symmetry should be in place. In contrast, the scalar doublet and the fermionic singlet candidates are naturally protected against too-fast decays by gauge and Lorentz symmetry, respectively. For a nonminimal coupling parameter $\xi \sim \mathcal{O}(1)$, decays through the gravity portal are consistent with observations if the dark matter mass is smaller than $\sim {10}^5\mathrm{GeV}$, for the scalar doublet, and $\sim 10^6\mathrm{GeV}$, for the fermionic singlet.

Inspired by the recent CMS $h\to \mu \tau$ excess, we calculate the lepton flavor violating Higgs decay $h\to \mu \tau$ in the littlest Higgs model with $T$ parity (LHT). Under the constraints of ${\ell }_i\to {\ell }_j\gamma$, $Z\to {\ell }_i{\overline{\ell }}_j$ and Higgs data, we find that the branching ratio of $h\to \mu \tau$ can maximally reach $\mathcal{O}({10}^{-4})$. We also investigate the correlation between $h\to \mu \tau$, $\tau \to \mu \gamma$, and $Z\to \mu \tau$, which can be used to test the LHT model at future $e^{+}{e}^{-}$ colliders.

Observation of Higgs pair production is an important long term objective of the LHC physics program as it will shed light on the scalar potential of the Higgs field and the nature of electroweak symmetry breaking. While numerous studies have examined the impact of new physics on di-Higgs production, little attention has been given to the well-motivated possibility of exotic Higgs decays in this channel. Here we investigate the consequences of exotic invisible Higgs decays in di-Higgs production. We outline a search sensitive to such invisible decays in the $b\overline{b}+{\cancel{E}}_T$ channel. We demonstrate that probing invisible branching ratios of order 10% during the LHC’s high-luminosity run will be challenging, but in resonance enhanced di-Higgs production, this final state can become crucial to establish the existence of physics beyond the Standard Model at collider energies. We also briefly discuss the outlook for other exotic Higgs decay modes and the potential to observe such exotic decays in the di-Higgs channel.

A model of spin-$3/2$ vectorlike leptons is introduced, which serve as an alternative probe of possible lepton compositeness. Possible signatures at the Large Hadron Collider are explored, and current data are used to constrain the masses of the hypothetical new leptons.

We investigate the standard model with the extension of a charged scalar having fractional electromagnetic charge of $-1/3$ unit and with lepton and baryon number-violating couplings at tree level. Without directly taking part in the electroweak (EW) symmetry breaking, this scalar can affect stability of the EW vacuum via loop effects. The impact of such a scalar, i.e., a leptoquark, on the perturbativity of standard model dimensionless couplings as well as on new physics couplings has been studied at two-loop order. The vacuum stability of the Higgs potential is checked using the one-loop renormalization group-improved effective potential approach with a two-loop beta function for all the couplings. From the stability analysis, various bounds are drawn on parameter space by identifying the region corresponding to the metastability and stability of the EW vacuum. Later, we also address the Higgs mass fine-tuning issue via the Veltman condition, and the presence of such a scalar increases the scale up to which the theory can be considered as reasonably fine-tuned. All these constraints give a very predictive parameter space for leptoquark couplings which can be tested at present and future colliders. Especially, a leptoquark with mass $\mathcal{O}(\mathrm{TeV})$ can give rise to lepton-quark flavor-violating signatures via decaying into the $t\tau$ channel at tree level, which can be tested at the LHC or future colliders.

We present next-to-leading-order (NLO) QCD predictions to $W^{+}{W}^{-}$ production in association with up to three jets at hadron colliders. We include contributions from couplings of the $W$ bosons to light quarks as well as trilinear vector couplings. These processes are used in vector-boson coupling measurements, are background to Higgs signals and are needed to constrain many new physics scenarios. For the first time NLO QCD predictions are shown for electroweak di-vector boson production with three jets at a hadron collider. We show total and differential cross sections for the LHC with proton center-of-mass energies of 8 and 13 TeV. To perform the calculation we employ on-shell and unitarity methods implemented in the blackhat library along with the sherpa package. We have produced event files that can be accessed for future dedicated studies.

We show that Bose-Einstein correlations of identical particles in hadron and nucleus high-energy collisions, lead to long-range rapidity correlations in the azimuthal angle. These correlations are inherent features of the CGC/saturation approach, however, their origin is more general than this approach. In framework of the proposed technique both even and odd $v_n$ occur naturally, independent of the type of target and projectile. We are of the opinion that it is premature to conclude that the appearance of azimuthal correlations are due to the hydrodynamical behavior of the quark-gluon plasma.

We perform calculations showing that a source producing $K^{*}{\overline{K}}^{*}$ in $J=2$ and $L=0$ gives rise to a triangle singularity at 1810 MeV with a width of about 200 MeV from the mechanism $K^{*}\to \pi K$ and then $K{\overline{K}}^{*}$ merging into the $a_1(1260)$ resonance. We suggest that this is the origin of the present $f_2(1810)$ resonance and propose to look at the $\pi a_1(1260)$ mode in several reactions to clarify the issue.

A search is presented for anomalous quartic gauge boson couplings in vector-boson scattering. The data for the analysis correspond to $20.2{\mathrm{fb}}^{-1}$ of $\sqrt{s}=8\mathrm{TeV}$ $pp$ collisions and were collected in 2012 by the ATLAS experiment at the Large Hadron Collider. The search looks for the production of $WW$ or $WZ$ boson pairs accompanied by a high-mass dijet system, with one $W$ decaying leptonically and a $W$ or $Z$ decaying hadronically. The hadronically decaying $W/Z$ is reconstructed as either two small-radius jets or one large-radius jet using jet substructure techniques. Constraints on the anomalous quartic gauge boson coupling parameters ${\alpha }_4$ and ${\alpha }_5$ are set by fitting the transverse mass of the diboson system, and the resulting 95% confidence intervals are $-0.024<{\alpha }_4<0.030$ and $-0.028<{\alpha }_5<0.033$.

We present the first measurement of the $CP$ violating charge asymmetry in $B^{\pm }\to {\mu }^{\pm }{\nu }_{\mu }{D}^0$ decays using the full Run II integrated luminosity of $10.4{\mathrm{fb}}^{-1}$ in proton-antiproton collisions collected with the D0 detector at the Fermilab Tevatron Collider. We measure a difference in the yield of $B^{-}$ and $B^{+}$ mesons in these decays by fitting the reconstructed invariant mass distributions. This results in an asymmetry of $A^{\mu D^0}=[-0.14\pm 0.20]\%$, which is consistent with standard model predictions.