We consider parton showers based on radiation from QCD dipoles or “antennas”. These showers are built from $2\to 3$ parton splitting processes. The question then arises of what functions replace the Altarelli-Parisi splitting functions in this approach. We give a detailed answer to this question, applicable to antenna showers in which partons carry definite helicity, and to both initial- and final-state emissions.

Scalar-isoscalar states ($J^{\mathrm{PC}}=0^{++}$) are investigated within the large-$N_c$ Regge approach. We elaborate on the consequences of including the lightest $f_0(600)$ scalar-isoscalar state into such an analysis, where the position of $f_0(600)$ fits very well into the pattern of the radial Regge trajectory. Furthermore, we point out that the pion and nucleon spin-0 gravitational form factors, recently measured on the lattice, provide valuable information on the low-mass spectrum of the scalar-isoscalar states on the basis of the scalar-meson dominance in the spin-0 channel. Through the fits to these data we find $m_{\sigma }=450–600\mathrm{MeV}$. We compare the predictions of various fits and methods. An analysis of the QCD condensates in the two-point correlators provides further constraints on the parameters of the scalar-isoscalar sector. We find that a simple two-state model suggests a meson nature of $f_0(600)$, and a glueball nature of $f_0(980)$, which naturally explains the ratios of various coupling constants. Finally, we note that the fine-tuned condition of the vanishing dimension-two condensate in the Regge approach with infinitely many scalar-isoscalar states yields a reasonable value for the mass of the lightest glueball state.

We study the spin-dependent quark distributions at large transverse momentum in the collinear factorization approach. The naive-time-reversal-even and $k_{\perp }$-odd quark distributions are calculated in terms of a class of twist-three quark-gluon correlation functions. We further calculate the angular distribution of the Drell-Yan lepton pair production in the polarized nucleon-nucleon collisions. In the intermediate transverse momentum region, we find that the two approaches, the collinear factorization and the transverse momentum dependent factorization, are consistent in the description of the lepton pair angular distributions.

We study the dihadron azimuthal correlation produced nearly back-to-back in unpolarized hadron collisions, arising from the product of two Collins fragmentation functions. Using the latest Collins fragmentation functions extracted from the global analysis of available experimental data, we make predictions for the azimuthal correlation of two-pion production in $pp$ collisions at Relativistic Heavy Ion Collider energies. We find that the correlation is sizable in the midrapidity region for moderate jet transverse momentum.

On the basis of perturbative QCD and the relativistic quark model we calculate relativistic and bound state corrections in the pair production of S-wave and P-wave charmonium states. Relativistic factors in the production amplitude connected with the relative motion of heavy quarks and the transformation law of the bound state wave function to the reference frame of the moving S- and P-wave mesons are taken into account. For the gluon and quark propagators entering the production vertex function we use a truncated expansion in the ratio of the relative quark momenta to the center-of-mass energy $\sqrt{s}$ up to the second order. The relativistic treatment of the wave functions makes all such second order terms convergent, thus allowing the reliable calculation of their contributions to the production cross section. Relativistic corrections to the quark bound state wave functions in the rest frame are considered by means of the QCD generalization of the standard Breit potential. It turns out that the examined effects change essentially the nonrelativistic results of the cross section for the reaction $e^{+}+e^{-}\to J/\Psi ({\eta }_c)+{\chi }_{cJ}(h_c)$ at the center-of-mass energy $\sqrt{s}=10.6\mathrm{GeV}$.

Using a sample of $1.06\times {10}^8$ ${\psi }^{\prime }$ decays collected by the BESIII detector, ${\chi }_{c0}$ and ${\chi }_{c2}$ decays into ${\pi }^0{\pi }^0$ and $\eta \eta$ are studied. The branching fraction results are $\mathrm{Br}({\chi }_{c0}\to {\pi }^0{\pi }^0)=(3.23\pm 0.03\pm 0.23\pm 0.14)\times {10}^{-3}$, $\mathrm{Br}({\chi }_{c2}\to {\pi }^0{\pi }^0)=(8.8\pm 0.2\pm 0.6\pm 0.4)\times {10}^{-4}$, $\mathrm{Br}({\chi }_{c0}\to \eta \eta )=(3.44\pm 0.10\pm 0.24\pm 0.2)\times {10}^{-3}$, and $\mathrm{Br}({\chi }_{c2}\to \eta \eta )=(6.5\pm 0.4\pm 0.5\pm 0.3)\times {10}^{-4}$, where the uncertainties are statistical, systematic due to this measurement, and systematic due to the branching fractions of ${\psi }^{\prime }\to \gamma {\chi }_{cJ}$. The results provide information on the decay mechanism of ${\chi }_c$ states into pseudoscalars.

A search for depletion of the combined flux of active neutrino species over a 735 km baseline is reported using neutral-current interaction data recorded by the MINOS detectors in the NuMI neutrino beam. Such a depletion is not expected according to conventional interpretations of neutrino oscillation data involving the three known neutrino flavors. A depletion would be a signature of oscillations or decay to postulated noninteracting sterile neutrinos, scenarios not ruled out by existing data. From an exposure of $3.18\times {10}^{20}$ protons on target in which neutrinos of energies between $\sim 500\mathrm{MeV}$ and 120 GeV are produced predominantly as ${\nu }_{\mu }$, the visible energy spectrum of candidate neutral-current reactions in the MINOS far detector is reconstructed. Comparison of this spectrum to that inferred from a similarly selected near-detector sample shows that of the portion of the ${\nu }_{\mu }$ flux observed to disappear in charged-current interaction data, the fraction that could be converting to a sterile state is less than 52% at 90% confidence level (C.L.). The hypothesis that active neutrinos mix with a single sterile neutrino via oscillations is tested by fitting the data to various models. In the particular four-neutrino models considered, the mixing angles ${\theta }_{24}$ and ${\theta }_{34}$ are constrained to be less than 11° and 56° at 90% C.L., respectively. The possibility that active neutrinos may decay to sterile neutrinos is also investigated. Pure neutrino decay without oscillations is ruled out at 5.4 standard deviations. For the scenario in which active neutrinos decay into sterile states concurrently with neutrino oscillations, a lower limit is established for the neutrino decay lifetime ${\tau }_3/m_3>2.1\times {10}^{-12}\mathrm{s}/\mathrm{eV}$ at 90% C.L.

We update the study of the total $\psi$ and ${\rm Y}$ production cross section in proton-proton collisions at RHIC energies using the QCD-based color-singlet model, including next-to-leading order partonic matrix elements. We also include charm-quark initiated processes which appear at leading order in ${\alpha }_s$, so far overlooked in such studies. Contrary to earlier claims, we show that the color-singlet yield is consistent with measurements over a broad range of $J/\psi$ rapidities. We find that intrinsic plus sealike charm-initiated processes contribute more than $20\%$ of the direct $J/\psi$ yield. The key signature for such processes is the observation of a charm-quark jet opposite in azimuthal angle $\varphi$ to the detected $J/\psi$. Our results have impact on the proper interpretation of heavy-quarkonium production in heavy-ion collisions and its use as a probe for the quark-gluon plasma.

Microgauss magnetic fields are observed in all galaxies at low and high redshifts. The origin of these intense magnetic fields is a challenging question in astrophysics. We show here that the natural plasma fluctuations in the primordial Universe (assumed to be random), predicted by the fluctuation -dissipation theorem, predicts $\sim 0.034\mu \mathrm{G}$ fields over $\sim 0.3\mathrm{kpc}$ regions in galaxies. If the dipole magnetic fields predicted by the fluctuation-dissipation theorem are not completely random, microgauss fields over regions $\gtrsim 0.34\mathrm{kpc}$ are easily obtained. The model is thus a strong candidate for resolving the problem of the origin of magnetic fields in $\lesssim {10}^9$ years in high redshift galaxies.

We trace the origin of the black hole entropy $S$, replacing a black hole by a quasiblack hole. Let the boundary of a static body approach its own gravitational radius, in such a way that a quasihorizon forms. We show that if the body is thermal with the temperature taking the Hawking value at the quasihorizon limit, it follows, in the nonextremal case, from the first law of thermodynamics that the entropy approaches the Bekenstein-Hawking value $S=A/4$. In this setup, the key role is played by the surface stresses on the quasihorizon and one finds that the entropy comes from the quasihorizon surface. Any distribution of matter inside the surface leads to the same universal value for the entropy in the quasihorizon limit. This can be of some help in the understanding of black hole entropy. Other similarities between black holes and quasiblack holes such as the mass formulas for both objects had been found previously. We also discuss the entropy for extremal quasiblack holes, a more subtle issue.

The mass varying neutrino scenario is a model that successfully explains the origin of dark energy while at the same time solves the coincidence problem. The model is, however, heavily constrained by its stability towards the formation of neutrino bound states when the neutrinos become nonrelativistic. We discuss these constraints and find that natural, adiabatic, stable models with the right amount of dark energy today do not exist. Second, we explain why using the lightest neutrino, which is still relativistic, as an explanation for dark energy does not work because of a feedback mechanism from the heavier neutrinos.

We find model-independent upper limits on rates of dark matter annihilation in galactic halos. The Born approximation generally fails, while exotic threshold enhancements akin to “Sommerfeld factors” also turn out to be baseless. The most efficient annihilation mechanism involves perturbatively small decay widths that have largely been ignored. Widths that are very small compared to TeV mass scales suffice to cause large enhancements in the velocity averaged cross sections. Bound state formation in weakly coupled theories produces small effects due to wave function normalizations. Unitarity shows the Sommerfeld factor cannot produce large enhancements of cross sections, and serves to identify where those approximations break down.

A left-right symmetric model is discussed with new mirror fermions and a Higgs sector with two doublets and neutral scalar singlets. The seesaw mechanism is generalized, including not only neutrino masses but also charged fermion masses. The spectrum of heavy neutrinos presents a second seesaw mass matrix and has neutrinos masses naturally in the TeV region. The model has very clear signatures for the new neutral vector gauge bosons. Two classes of models are discussed. New mirror neutrinos can be very light and a new $Z^{\prime }$ can be discriminated from other models by a very high invisible branching fraction. The other possibility is that mirror neutrinos can have masses naturally in the TeV region and can be produced through $Z^{\prime }$ decays into heavy neutrino pairs. Signatures and production processes for the model at the LHC energy are also presented.

Experimental results for the radiation emission from ultrarelativistic electrons in targets of 0.03%–5% radiation length is presented. For the thinnest targets, the radiation emission is in accordance with the Bethe-Heitler formulation of bremsstrahlung, the target acting as a single scatterer. In this regime, the radiation intensity is proportional to the thickness. As the thickness increases, the distorted Coulomb field of the electron that is the result of the first scattering events, leads to a suppressed radiation emission per interaction, upon subsequent scattering events. In that case, the radiation intensity becomes proportional to a logarithmic function of the thickness, due to the suppression. Eventually, once the target becomes sufficiently thick, the entire radiation process becomes influenced by multiple scattering and the radiation intensity is again proportional to the thickness, but with a different constant of proportionality. The observed logarithmic thickness dependence of radiation intensity at intermediate values of the thickness can be directly interpreted as a manifestation of the distortion of the electron Coulomb field resulting from a scattering event. The Landau-Pomeranchuk-Migdal effect is explored with high primary energy using materials with low nuclear charge ($Z$). Also, targets that should give rise to the claimed interference effect in high-energy radiation emission from a structured target of thin foils are investigated.

We extend our two-component Pomeron, Gotsman-Levin-Maor-Miller (GLMM) model, for soft high-energy scattering to single inclusive cross sections. To this end we present a suitable formulation which also includes the semienhanced Pomeron-particle vertex corrections. The available data on single inclusive density $(1/{\sigma }_{\mathrm{in}})d\sigma /dy$ in the c.m. energy range of 200–1800 GeV are well reproduced by our model. The just-published ALICE Collaboration point at 900 GeV is in excellent agreement with the calculations of our model. We also present predictions covering the complete LHC energy range which can be readily tested in the early low-luminosity LHC runs. The results presented in this paper provide additional support to our Pomeron model approach.

We propose that there is no analogue of the Breitenlohner-Freedman stability bound on the mass of a scalar field in the context of the nonrelativistic AdS/CFT correspondence. Our treatment is based on an equivalence between the field equation of a complex scalar in the AdS/CFT correspondence and the one-dimensional Schrödinger equation with an inverse square potential. We compute the two-point boundary correlation function for $m^2<m_{\mathrm{BF}}^2$ and discuss its relation to renormalization group limit cycles and the Efimov effect in quantum mechanics. The equivalence also helps to elucidate holographic renormalization group flows and calculations in the global coordinates for Schrödinger spacetime.

We calculate the one loop beta functions of nonlinear sigma models in four dimensions containing general two- and four-derivative terms. In the $O(N)$ model there are four such terms and nontrivial fixed points exist for all $N\ge 4$. In the chiral $SU(N)$ models there are in general six couplings, but only five for $N=3$ and four for $N=2$; we find fixed points only for $N=2$, 3. In the approximation considered, the four-derivative couplings are asymptotically free but the coupling in the two-derivative term has a nonzero limit. These results support the hypothesis that certain sigma models may be asymptotically safe.

The perturbation equation in a Kerr background is written as a coupled system of one dimensional equations for the different modes in the time domain. Numerical simulations show that the dominant mode in the gravitational response is the one corresponding to the mode of the initial perturbation, allowing us to conjecture that the coupling among the modes has a weak influence in our system of equations. We conclude that by neglecting the coupling terms it can be obtained as a one dimensional harmonic equation which indeed describes with good accuracy the gravitational response from the Kerr black hole with low spin, while only a few couplings are necessary to describe a high-spin one. This result may help us to understand the structure of test fields in a Kerr background and even to generate accurate waveforms for various cases in an efficient manner.