Quantum Yang--Mills Dark Energy

In this short review, I discuss basic qualitative characteristics of quantum non-Abelian gauge dynamics in the non-stationary background of the expanding Universe in the framework of the standard Einstein--Yang--Mills formulation. A brief outlook of existing studies of cosmological Yang--Mills fields and their properties will be given. Quantum effects have a profound impact on the gauge field-driven cosmological evolution. In particular, a dynamical formation of the spatially-homogeneous and isotropic gauge field condensate may be responsible for both early and late-time acceleration, as well as for dynamical compensation of non-perturbative quantum vacua contributions to the ground state of the Universe. The main properties of such a condensate in the effective QCD theory at the flat Friedmann--Lema\'itre--Robertson--Walker (FLRW) background will be discussed within and beyond perturbation theory. Finally, a phenomenologically consistent dark energy can be induced dynamically as a remnant of the QCD vacua compensation arising from leading-order graviton-mediated corrections to the QCD ground state.Comment: 24 pages; invited review for the Special Issue "Modified Gravity Cosmology: From Inflation to Dark Energy", in Universe 2 (2016)

Quasi-classical Gravity effect on neutrino oscillations in a gravitational field of an heavy astrophysical object

In the framework of quantum field theory, a graviton interacts locally with a quantum state having definite mass, i.e. the gravitational mass eigenstate, while a weak boson interacts with a state having definite flavor, i.e. the flavor eigenstate. An interaction of a neutrino with an energetic graviton may trigger the collapse of the neutrino to a definite mass eigenstate with probability expressed in terms of PMNS mixing matrix elements. Thus, gravitons would induce quantum decoherence of a coherent neutrino flavor state similarly to how weak bosons induce quantum decoherence of a neutrino in a definite mass state. We demonstrate that such an essentially quantum gravity effect may have strong consequences for neutrino oscillation phenomena in astrophysics due to relatively large scattering cross sections of relativistic neutrinos undergoing large-angle radiation of energetic gravitons in gravitational field of a classical massive source (i.e. the quasi-classical case of gravitational Bethe-Heitler scattering). This graviton-induced {\it decoherence} is compared to {\it decoherence} due to propagation in the presence of the Earth matter effect. Based on this study, we propose a new technique for the indirect detection of energetic gravitons by measuring the flavor composition of astrophysical neutrinos.Comment: 25 pages, 4 figures, minor revision with clarifications, main conclusions are unchange

Phenomenological Review on Quark-Gluon Plasma: Concepts vs. Observations

In this review, we present an up-to-date phenomenological summary of research developments in the physics of the Quark--Gluon Plasma (QGP). A short historical perspective and theoretical motivation for this rapidly developing field of contemporary particle physics is provided. In addition, we introduce and discuss the role of the quantum chromodynamics (QCD) ground state, non-perturbative and lattice QCD results on the QGP properties, as well as the transport models used to make a connection between theory and experiment. The experimental part presents the selected results on bulk observables, hard and penetrating probes obtained in the ultra-relativistic heavy-ion experiments carried out at the Brookhaven National Laboratory Relativistic Heavy Ion Collider (BNL RHIC) and CERN Super Proton Synchrotron (SPS) and Large Hadron Collider (LHC) accelerators. We also give a brief overview of new developments related to the ongoing searches of the QCD critical point and to the collectivity in small ($p+p$ and $p+A$) systems.Comment: 64 pages, 29 figures; a new subsection 4.4.2 and a few references have been added; minor changes; published versio

Light meson gas in the QCD vacuum and oscillating Universe

We have developed a phenomenological effective quantum-field theoretical model describing the "hadron gas" of the lightest pseudoscalar mesons, scalar {\sigma}-meson and {\sigma}-vacuum, i.e. the expectation value of the {\sigma}-field, at finite temperatures. The corresponding thermodynamic approach was formulated in terms of the generating functional derived from the effective Lagrangian providing the basic thermodynamic information about the "meson plasma + QCD condensate" system. This formalism enables us to study the QCD transition from the hadron phase with direct implications for cosmological evolution. Using the hypothesis about a positively-definite QCD vacuum contribution stochastically produced in early universe, we show that the universe could undergo a series of oscillations during the QCD epoch before resuming unbounded expansion.Comment: 30 pages, 17 figure

Nucleon spin structure at low momentum transfers

The generalized Gerasimov-Drell-Hearn (GDH) sum rule is known to be very sensitive to QCD radiative and power corrections. We improve the previously developed QCD-inspired model for the $Q^2$-dependence of the GDH sum rule. We take into account higher order radiative and higher twist power corrections extracted from precise Jefferson Lab data on the lowest moment of the spin-dependent proton structure function $\Gamma_1^{p}(Q^2)$ and on the Bjorken sum rule $\Gamma_1^{p-n}(Q^2)$. By using the singularity-free analytic perturbation theory we demonstrate that the matching point between chiral-like positive-$Q^2$ expansion and QCD operator product $1/Q^2$-expansion for the nucleon spin sum rules can be shifted down to rather low $Q\simeq\Lambda_{QCD}$ leading to a good description of recent proton, neutron, deuteron and Bjorken sum rule data at all accessible $Q^2$.Comment: 12 pages, 6 figure

Diffractive Bremsstrahlung in Hadronic Collisions

Production of heavy photons (Drell-Yan), gauge bosons, Higgs bosons, heavy flavors, which is treated within the QCD parton model as a result of hard parton-parton collision, can be considered as a bremsstrahlung process in the target rest frame. In this review, we discuss the basic features of the diffractive channels of these processes in the framework of color dipole approach. The main observation is a dramatic breakdown of diffractive QCD factorisation due to the interplay between soft and hard interactions, which dominates these processes. This observation is crucial for phenomenological studies of diffractive reactions in high-energy hadronic collisions.Comment: 29 pages, 16 figures; typos corrected; references added; minor correction

Search for technipions in exclusive production of diphotons with large invariant masses at the LHC

We focus on exclusive production of neutral technipion $\tilde \pi^0$ in $pp$ collisions at the LHC, i.e. on $p p \to p p \tilde \pi^0$ reaction. The dependence of the cross section on parameters of recently proposed vector-like Technicolor model is studied. Characteristic features of the differential distributions are discussed. For not too large technipion masses the diphoton decay channel has the dominant branching fraction. This is also the main reason for an enhanced production of neutral technipions in $\gamma\gamma$-fusion reaction. We discuss potential backgrounds of the QCD and QED origin to the $p p \to p p (\tilde{\pi}^0 \to \gamma \gamma)$ process at large invariant $\gamma\gamma$ masses. We conclude that compared to inclusive case the signal-to-background ratio in the considered exclusive reaction is vary favorable which thereby could serve as a good probe for Technicolor dynamics searches at the LHC.Comment: 20 pages, 12 figures, 1 tabl