arXiv subjects

Ariel R. Zhitnitsky

Publications and source records attributed to Ariel R. Zhitnitsky.

At least 55 records · Page 3Linked to original sources

arXiv2009

The cosmological constant as a manifestation of the conformal anomaly?

We propose that the solution to the cosmological vacuum energy puzzle may come from the infrared sector of the effective theory of gravity, where the impact of the trace anomaly is of upmost relevance. We proceed by introducing two auxiliary fields, which are capable of describing a diversity of quantum states via specification of their macroscopic (IR) boundary conditions, in contrast to ultraviolet quantum effects. Our investigation aims at finding a realistic cosmological solution which interprets the observed cosmological constant as a well defined deficit in the zero point energy density of the Universe. The energy density arises from a phase transition, which alters the properties of the quantum ground state. We explicitly formulate low energy gravity as an effective field theory with a precise definition of the "point of normalization" as the point at which the "renormalized cosmological constant" is set to zero in the Minkowski vacuum, in which the Einstein equations are automatically satisfied as the Ricci tensor identically vanishes. With this definition the effective QFT of gravity has a predictive power. In particular, it must predict the evolution of the system in any nontrivial geometry, including the vacuum energy behaviour as a function of infrared, rather than ultraviolet, input parameters.

gr-qc↗

arXiv2009

Bremsstrahlung emission from quark stars

We calculate numerically the emissivity and surface flux of electron-electron bremsstrahlung radiation from the surface of a bare quark star. The restricted electronic phase space due to the presence of an effective photon mass results in a strong suppression. The emissivity and surface flux are found to be substantially smaller than those found in previous work, to the point where electron-positron pair production would remain the dominant mechanism at all temperatures in the relativistic regime. As a consequence, pair production remains a dominant process even at low surface temperatures as originally suggested by Usov in arXiv:astro-ph/9712304.

astro-ph.HE↗

arXiv2008

Diffuse x-rays: Directly Observing Dark Matter?

Several independent observations of the Galactic core suggest hitherto unexplained sources of energy. We suggest that dark matter in the form of dense antimatter nuggets could provide a natural site for electron and proton annihilation, providing 511 keV photons, gamma-rays, and diffuse keV x-ray radiation. We show that identifying dark matter as antimatter nuggets is consistent with the observed emissions, and we make definite predictions about their spectrum and morphology. If correct, our proposal not only identifies dark matter and explains baryogenesis, but allows x-ray observations to directly probe the matter distribution in our Galaxy.

astro-ph↗

arXiv2008

WMAP Haze: Directly Observing Dark Matter?

In this paper we show that dark matter in the form of dense matter/antimatter nuggets could provide a natural and unified explanation for several distinct bands of diffuse radiation from the core of the Galaxy spanning over 12 orders of magnitude in frequency. We fix all of the phenomenological properties of this model by matching to x-ray observations in the keV band, and then calculate the unambiguously predicted thermal emission in the microwave band, at frequencies smaller by 10 orders of magnitude. Remarkably, the intensity and spectrum of the emitted thermal radiation are consistent with--and could entirely explain--the so-called "WMAP haze": a diffuse microwave excess observed from the core of our Galaxy by the Wilkinson Microwave Anisotropy Probe (WMAP). This provides another strong constraint of our proposal, and a remarkable nontrivial validation. If correct, our proposal identifies the nature of the dark matter, explains baryogenesis, and provides a means to directly probe the matter distribution in our Galaxy by analyzing several different types of diffuse emissions.

astro-ph↗

arXiv2008

Confinement- Deconfinement Phase Transition in Hot and Dense QCD at Large N

We conjecture that the confinement- deconfinement phase transition in QCD at large number of colors N and N_f\ll N at T\neq 0 and μ\neq 0 is triggered by the drastic change in θbehavior. The conjecture is motivated by the holographic model of QCD where confinement -deconfinement phase transition indeed happens precisely at the value of temperature T=T_c where θdependence experiences a sudden change in behavior[1]. The conjecture is also supported by quantum field theory arguments when the instanton calculations (which trigger the θdependence) are under complete theoretical control for T>T_c, suddenly break down immediately below T<T_c with sharp changes in the θdependence. Finally, the conjecture is supported by a number of numerical lattice results. We employ this conjecture to study confinement -deconfinement phase transition of dense QCD at large μin large N limit by analyzing the θdependence. We find that the confinement- deconfinement phase transition at N_f\ll N happens at very large quark chemical potential μ_c\sim \sqrt{N}Λ_{QCD}. This result agrees with recent findings by McLerran and Pisarski[2]. We also speculate on case when N_f\sim N.

hep-ph↗

arXiv2007

Diffuse cosmic gamma-rays at 1-20 MeV: A trace of the dark matter?

Several independent observations of the galactic core suggest hitherto unexplained sources of energy. The most well known case is the 511 keV line which has proven very difficult to explain with conventional astrophysical positron sources. A similar, but less well known mystery is the excess of gamma-ray photons detected by COMPTEL across a broad energy range 1-20 MeV. Such photons are found to be very difficult to produce via known astrophysical sources. We show in this work that dark matter in the form of dense antimatter droplets provides a natural explanations for the observed flux of gamma-rays in the 1-20 MeV range. We argue thatsuch photons must always accompany the 511 keV line as they produced by the same mechanism within our framework. We calculate the spectrum and intensity of the 1-20 MeV gamma-rays, and find it to be consistent with the COMPTEL data.

astro-ph↗

arXiv2006

Confinement- Deconfinement Phase Transition and Fractional Instanton Quarks in Dense Matter

We present arguments suggesting that large size overlapping instantons are the driving mechanism of the confinement-deconfinement phase transition at nonzero chemical potential mu. The arguments are based on the picture that instantons at very large chemical potential in the weak coupling regime are localized configurations with finite size ρ\simμ^{-1}. At the same time, the same instantons at smaller chemical potential in the strong coupling regime are well represented by the so-called instanton-quarks with fractional topological charge 1/N_c. We estimate the critical chemical potential mu_c(T) where transition between these two regimes takes place. We identify this transition with confinement- deconfinement phase transition. We also argue that the instanton quarks carry magnetic charges. As a consequence of it, there is a relation between our picture and the standard t'Hooft and Mandelstam picture of the confinement. We also comment on possible relations of instanton-quarks with "periodic instantons", " center vortices", and "fractional instantons" in the brane construction. We also argue that the variation of the external parameter mu, which plays the role of the vacuum expectation value of a "Higgs" field at mu >> Λ_{QCD}, allows to study the transition from a "Higgs -like" gauge theory (weak coupling regime, mu>> Λ_{QCD}) to ordinary QCD (strong coupling regime, mu<< Λ_{QCD}). We also comment on some recent lattice results on topological charge density distribution which support our picture.

hep-ph↗

arXiv2005

$θ$-dependence of QCD at Finite Isospin Density

We probe the $θ$-dependence of QCD at finite isospin chemical potential $μ_I$ using the effective chiral Lagrangian approach. The phase diagram in the $θ$, $μ_I$ plane is constructed and described in detail in terms of chiral and pion condensates. The physics at $θ~ π$ is investigated in both the normal and superfluid phase. Finally, the behaviour of the gluon condensate at finite $μ_I$ is computed.

hep-ph↗

arXiv2005

$θ-$ Parameter in 2 Color QCD at Finite Baryon and Isospin Density

We use 2-color QCD as a model to study the effects of simultaneous presence of the so-called $θ$ parameter, chemical potentials for baryon number, $μ_B$ and for isospin charge, $μ_I$. We pay special attention to $θ$, $μ_B$, $μ_I$ dependence of different vacuum condensates, including chiral and diquark condensates, as well as the gluon condensate, $ $, and the topological susceptibility. We find that two phase transitions of the second order will occur when $θ$ relaxes from $θ=2π$ to $θ=0$, if $μ$ is of order of the pion mass. We demonstrate that the transition to the superfluid phase at $θ= π$ occurs at a much lower chemical potential than at $θ= 0$. We also show that the strong $θ$ dependence present near $θ= π$ in vacuum (Dashen's phenomenon), becomes smoothed out in the superfluid phase. Finally, we comment on the relevance of this study for the real world with N_c=3.

hep-ph↗

arXiv2005

Anomalous Axion Interactions and Topological Currents in Dense Matter

Recently an effective Lagrangian for the interactions of photons, Nambu-Goldstone bosons and superfluid phonons in dense quark matter has been derived using anomaly matching arguments. In this paper we illuminate the nature of certain anomalous terms in this Lagrangian by an explicit microscopic calculation. We also generalize the corresponding construction to introduce the axion field. We derive an anomalous axion effective Lagrangian describing the interactions of axions with photons and superfluid phonons in the dense matter background. This effective Lagrangian, among other things, implies that an axion current will be induced in the presence of magnetic field. We speculate that this current may be responsible for the explanation of neutron star kicks.

hep-ph↗

arXiv2005

Confinement-Deconfinement Phase Transition at Nonzero Chemical Potential

We present arguments suggesting that large size overlapping instantons are the driving mechanism of the confinement-deconfinement phase transition at nonzero chemical potential mu. The arguments are based on the picture that instantons at very large chemical potential in the weak coupling regime are localized configurations with finite size rho~1/mu. At the same time, the same instantons at smaller chemical potential in the strong coupling regime are well represented by the so-called instanton-quarks with fractional topological charge 1/Nc. We estimate the critical chemical potential mu_c(T) where this phase transition takes place as a function of temperature in the domain where our approach is justified. In this picture, the long standing problem of the "accidental" coincidence of the chiral and deconfinement phase transitions at nonzero temperature (observed in lattice simulations) is naturally resolved. We also derive results at nonzero isospin chemical potential mu_I where direct lattice calculations are possible, and our predictions can be explicitly tested.

hep-ph↗

arXiv2005

511 KeV Photons From Color Superconducting Dark Matter

We discuss the possibility that the recent detection of 511 keV gamma rays from the galactic bulge, as observed by INTEGRAL, can be naturally explained by the supermassive very dense droplets (strangelets) of dark matter. These droplets are assumed to be made of ordinary light quarks (or antiquarks) condensed in non-hadronic color superconducting phase. The droplets can carry electrons (or positrons) in the bulk or/and on the surface. The e^+e^- annihilation events take place due to the collisions of electrons from the visible matter with positrons from dark matter droplets which may result in the bright 511 KeV gamma-ray line from the bulge of the Galaxy.

hep-ph↗

arXiv2005

Topological Defects in QCD at large Baryon Density and/or large $N_c$

The main leitmotiv of the present talk is analysis of different topological objects such as strings and domain walls in QCD. As it is well known, the standard model in general (and QCD in particular) does not support any kind of such objects due to some simple topological arguments. However, the situation cardinally changes when one considers QCD in large $N_c$ limit or in the limit of large baryon density when the unique light $η'$ field emerges. I discuss $η'$- domain walls and $η'$ global strings which occur in both systems: in high density QCD and in QCD at large $N_c $. I also discuss the effects of quantum anomalies in the presence of topological objects in high-density QCD. The anomaly induced interactions lead to a number of interesting phenomena (such as the induced currents along the strings) which may have phenomenological consequences observable in neutron stars.

hep-ph↗

arXiv2004

Neutron Stars as Type-I Superconductors

In a recent paper by Link, it was pointed out that the standard picture of the neutron star core composed of a mixture of a neutron superfluid and a proton type-II superconductor is inconsistent with observations of a long period precession in isolated pulsars. In the following we will show that an appropriate treatment of the interacting two-component superfluid (made of neutron and proton Cooper pairs), when the structure of proton vortices is strongly modified, may dramatically change the standard picture, resulting in a type-I superconductor. In this case the magnetic field is expelled from the superconducting regions of the neutron star leading to the formation of the intermediate state when alternating domains of superconducting matter and normal matter coexist.

astro-ph↗

arXiv2004

Vortices and type-I superconductivity in neutron stars

In a recent paper by Link, it was pointed out that the the standard picture of the neutron star core, composed of a mixture of a neutron superfluid and a proton type-II superconductor, is inconsistent with observations of long period precession in isolated pulsars. In the following we will show that intervortex force between the magnetic flux tubes may be attractive resulting in a type-I (rather than type-II) superconductor. In this case the magnetic field cannot exist in the form of magnetic flux tubes, supporting Link's observation. This behavior of the system is due to the strong interaction between the proton-neutron Cooper pairs, which was previously ignored. We also calculate the critical magnetic fields H_c and H_c2 for type-I/II superconductors. These results also support our claim of type-I superconductivity in the cores of neutron stars.

hep-ph↗

arXiv2003

Drum vortons in high density QCD

Recently it was shown that high density QCD supports of number of topological defects. In particular, there are U(1)_Y strings that arise due to K^0 condensation that occurs when the strange quark mass is relatively large. The unique feature of these strings is that they possess a nonzero K^+ condensate that is trapped on the core. In the following we will show that these strings (with nontrivial core structure) can form closed loops with conserved charge and currents trapped on the string worldsheet. The presence of conserved charges allows these topological defects, called vortons, to carry angular momentum, which makes them classically stable objects. We also give arguments demonstrating that vortons carry angular momentum very efficiently (in terms of energy per unit angular momentum) such that they might be the important degrees of freedom in the cores of neutron stars.

hep-ph↗

arXiv2003

"Nonbaryonic" Dark Matter as Baryonic Color Superconductor

We discuss a novel cold dark matter candidate which is formed from the ordinary quarks during the QCD phase transition when the axion domain wall undergoes an unchecked collapse due to the tension in the wall. If a large number of quarks is trapped inside the bulk of a closed axion domain wall, the collapse stops due to the internal Fermi pressure. In this case the system in the bulk, may reach the critical density when it undergoes a phase transition to a color superconducting phase with the ground state being the quark condensate, similar to the Cooper pairs in BCS theory. If this happens, the new state of matter representing the diquark condensate with a large baryon number $B \sim 10^{32}$ becomes a stable soliton-like configuration. Consequently, it may serve as a novel cold dark matter candidate.

hep-ph↗

arXiv2003

Vortex Rings in two Component Bose-Einstein Condensates

We study the structure of the vortex core in two-component Bose-Einstein condensates. We demonstrate that the order parameter may not vanish and the symmetry may not be restored in the core of the vortex. In this case such vortices can form vortex rings known as vortons in particle physics literature. In contrast with well-studied superfluid $^4He$, where similar vortex rings can be stable due to Magnus force only if they move, the vortex rings in two-component BECs can be stable even if they are at rest. This beautiful effect was first discussed by Witten in the cosmic string context, where it was shown that the stabilization occurs due to condensation of the second component of the field in the vortex core. This second condensate trapped in the core may carry a current along the vortex ring counteracting the effect of string tension that causes the loop to shrink. We speculate that such vortons may have been already observed in the laboratory. We also speculate that the experimental study of topological structures in BECs can provide a unique opportunity to study cosmology and astrophysics by doing laboratory experiments.

cond-mat↗