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)

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

A heuristic description of high-pT hadron production in heavy ion collisions

Using a simplified model for in-medium dipole evolution accounting for color filtering effects we study production of hadrons at large transverse momenta $p_T$ in heavy ion collisions. In the framework of this model, several important sources of the nuclear suppression observed recently at RHIC and LHC have been analysed. A short production length of the leading hadron $l_p$ causes a strong onset of color transparency effects manifested themselves as a steep rise of the nuclear modification factor $R_{AA}(p_T)$ at large hadron $p_T$'s. A dominance of quarks with higher $l_p$ leads to a weaker suppression at RHIC than the one observed at LHC. In the RHIC kinematic region we include an additional suppression factor steeply falling with $p_T$, which is tightly related to the energy conservation constraints. The latter is irrelevant at LHC up to $p_T\lesssim 70$ GeV while it causes a rather flat $p_T$ dependence of the $R_{AA}(p_T)$ factor at RHIC c.m. energy $\sqrt{s} = 200$ GeV and even an increasing suppression with $p_T$ at $\sqrt{s} = 62$ GeV. The calculations contain only a medium density adjustment, and for an initial time scale $t_0$ = 1 fm we found the energy-dependent maximal values of the transport coefficient, $\hat{q}_0 = 0.7, 1.0$ and 1.3 GeV$^2$/fm corresponding to $\sqrt{s} = 62, 200$ GeV and 2.76 TeV, respectively. We present a broad variety of predictions for the nuclear modification factor and the azimuthal asymmetry which are in a good agreement with available data from experiments at RHIC and LHC.Comment: 14 pages, 17 figures; extra clarifications added in Sects. II and III (with additional Figs. 1-6) and in the extended Sect. V B (with additional Fig.11), references added, conclusions unchange

Dark Energy from graviton-mediated interactions in the QCD vacuum

Adopting the hypothesis about the exact cancellation of vacuum condensates contributions to the ground state energy in particle physics to the leading order in graviton-mediated interactions, we argue that the observable cosmological constant can be dynamically induced by an uncompensated quantum gravity correction to them after the QCD phase transition epoch. To start with, we demonstrate a possible cancellation of the quark-gluon condensate contribution to the total vacuum energy density of the Universe at temperatures $T<100$ MeV without taking into account the graviton-mediated effects. In order to incorporate the latter, we then calculate the leading-order quantum correction to the classical Einstein equations due to metric fluctuations induced by the non-perturbative vacuum fluctuations of the gluon and quark fields in the quasiclassical approximation. It has been demonstrated that such a correction to the vacuum energy density has a form $\varepsilon_{\Lambda}\sim G \Lambda_{\rm QCD}^6$, where $G$ is the gravitational constant, and $\Lambda_{\rm QCD}$ is the QCD scale parameter. We analyze capabilities of this approach based on the synthesis between quantum gravity in quasiclassical approximation and theory of non-perturbative QCD vacuum for quantitative explanation of the observed Dark Energy density.Comment: 21 pages, a discussion of cosmological evolution of the \Lambda-term has been added; published versio

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

Spin effects in diffractive charmonia production

We consider exclusive double diffractive production of polarised axial-vector $\chi_c(1^+)$ and tensor $\chi_c(2^+)$ charmonia in proton-(anti)proton collisions at Tevatron energy. The corresponding amplitudes for these processes are derived within the $k_t$-factorisation approach. Contributions from different polarisation states of axial-vector and tensor charmonia are quantified. Corresponding experimental consequences are discussed.Comment: 3 pages, 3 figures, talk given at 11th International Workshop on Meson Production, Properties and Interaction, 10-15 June 2010, Krak\'ow, Polan

Diffractive pQCD mechanism of exclusive production of W+W−W^+ W^- pairs in proton-proton collisions

We present a study of central exclusive production of $W^+W^-$ pairs in proton-proton collisions at the LHC. We compare the contribution of the $\gamma \gamma \to W^+ W^-$ mechanism with a new mechanism of exclusive diffractive production through the $gg \to W^+ W^-$ subprocess with intermediate virtual Higgs boson and quark box diagrams. The amplitude for the latter process is expressed in terms of the off-diagonal unintegrated gluon distribution functions. Several observables related to this process are calculated. The phase space integrated diffractive contribution when separated is only a small fraction of fb compared to 115.4 fb of the $\gamma\gamma$-contribution without absorption. This opens a possibility of efficient searches for anomalous boson $\gamma W^+ W^-$ and $\gamma \gamma W^+ W^-$ couplings due to new physics beyond Standard Model.Comment: 6 pages, 2 figures, Prepared for the Sixth International Conference on Quarks and Nuclear Physics QNP2012, April 16-20, 2012, Ecole Polytechnique, Palaiseau, Franc