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N. O. Agasian

Publications and source records attributed to N. O. Agasian.

At least 19 recordsLinked to original sources
arXiv2024

Colour-electric and colour-magnetic confinement

The basic properties of the confinement mechanism in QCD -- the temperature dependence of the spatial and temporal string tensions ($σ_s(T)$ and $σ_E(T)$) -- are studied in the framework of the Field Correlator Method (FCM). It is shown that both functions are connected respectively to the spatial and temporal parts of the vacuum gluon energy $ε_s$ and $ε_E$ which define their equal values at $T=0$. However at $T>0$ the spatial part is growing with $T$ while the temporal part is destroyed by the hadronic pressure at $T=T_c$ (the deconfinement). Both properties are derived within the same method and are in a good agreement with the corresponding lattice data.

hep-ph↗
arXiv2023

The spatial string tension from the Field Correlator Method

The phenomenon of the almost linear growth of the square root of spatial string tension $\sqrt{σ_s(T)}= c_σ g^2 T$ was found both in lattice and in theory, based on the Field Correlator Method (FCM). In the latter the string tension (both spatial and colorelectric) is expressed as an integral of the two gluon Green's function calculated with the same string tension: $σ= \int G^{(2g)}_σ$. This relation allows to check the selfconsistency of the theory. However at nonzero temperature $T$ in the two-gluon Green's function in the space-like region appear terms which create in $σ_s$ quadratic in T behavior. We calculate below in the paper the $σ_s$ numerically in the whole temperature region $T_c< T < 5 T_c$ using the FCM method and compare the results with lattice data finding a good agreement. This justifies the use of the FCM in the space-like region and in high T thermodynamics without extra parameters.

hep-lat↗
arXiv2017

Nonperturbative SU(3) thermodynamics and the phase transition

The $SU(3)$ equation of state ($P(T),\,s(T),\,I(T)$) are calculated within the Field Correlator Method both in the confined and the deconfined phases. The basic dynamics in our approach is contained in the vacuum correlators, both of the colorelectric (CE) and colormagnetic (CM) types, which ensure CE and CM confinement below $T_c$ and CM confinement and Polyakov loops above $T_c$. The resulting values of $T_c$ and $P(T),\,I(T)$, $s(T)$ are in good agreement with lattice measurements.

hep-ph↗
arXiv2016

Dynamical role of Polyakov loops in the QCD thermodynamics

Polyakov loops $L_a(T), a=3,8,...$ are shown to give the most important nonperturbative contribution to the thermodynamic potentials. Derived from the gluonic field correlators they enter as factors into free energy. It is shown in the $SU(3)$ case that $L_a (T)$ define to a large extent the behavior of the free energy and the trace anomaly $I(T)$, most sensitive to nonperturbative effects.

hep-lat↗
arXiv2016

Low-energy relation for the trace of the energy-momentum tensor in QCD and the gluon condensate in a magnetic field

A treatment is given of the nonperturbative QCD vacuum in a magnetic field. The low-energy equation for the trace of the energy-momentum tensor in a magnetic field is derived. It is shown that the derivatives with respect to a magnetic field of the quark and gluon contributions to the trace of the energy-momentum tensor are equal. The dependence of the gluon condensate on the magnetic field strength is derived both for strong and weak fields.

hep-ph↗
arXiv2015

Hydrogen recombination in the early Universe in the presence of a magnetic field

Hydrogen recombination in the early Universe in the presence of a magnetic field is studied. An equation for the temperature of recombination in the presence of a magnetic field is derived. Limiting cases of weak and strong fields are considered. It is demonstrated that there exists a critical magnetic field, above which the system stays in the phase of atomic hydrogen for all temperatures. The relative shift of the temperature of recombination in the presence of a magnetic field is estimated and it is demonstrated that this shift is small.

hep-ph↗
arXiv2012

Low-energy theorems of QCD and bulk viscosity at finite temperature and baryon density in a magnetic field

The nonperturbative QCD vacuum at finite temperature and a finite baryon density in an external magnetic field is studied. Equations relating nonperturbative condensates to the thermodynamic pressure for $T\neq 0$, $μ_q \neq 0$ and $H\neq 0$ are obtained, and low-energy theorems are derived. A bulk viscosity $ζ(T, μ, H)$ is expressed in terms of basic thermodynamical quantities describing the quark-gluon matter at $T\neq 0$, $μ_q \neq 0$, and $H\neq 0$. Various limiting cases are also considered.

hep-ph↗
arXiv2010

Dilaton and gluon condensate in a nucleon medium

The gluon condensate as a function of temperature and baryon density in a nucleon medium is obtained from an effective dilaton Lagrangian. It is shown that, at a normal nuclear density of nucleons, n_0 = 0.17 fm^{-3} the gluon condensate decreases by about 10%.

nucl-th↗
arXiv2008

Quark-hadron phase transition in a magnetic field

Quark-hadron phase transition in QCD in the presence of magnetic field is studied. It is shown that both the temperature of a phase transition and latent heat decrease compared to the case of zero magnetic field. The phase diagram in the plane temperature--magnetic field is presented. Critical point, T_\ast=104 MeV, \sqrt{eH_\ast}=600 MeV, for which the latent heat goes to zero, is found.

hep-ph↗
arXiv2007

Debye screening in the hot non-Abelian plasma

The Debye mass m_D is computed nonperturbatively in the deconfined phase of QCD, where chromomagnetic confinement is known to be present. The latter defines m_D to be m_D=c_D\sqrt{σ_s}, where c_D \approx 2 and σ_s=σ_s(T) is the spatial string tension. The resulting magnitude of m_D(T) and temperature dependence are in good agreement with lattice calculations.

hep-ph↗
arXiv2006

New nonperturbative approach to the Debye mass in hot QCD

The Debye mass m_D is computed nonperturbatively in the deconfined phase of QCD, where chromomagnetic confinement is known to be present. The latter defines m_D to be m_D=c_D\sqrt{σ_s}, where c_D \cong 2.06 and σ_s=σ_s(T) is the spatial string tension. The resulting magnitude of m_D(T) and temperature dependence are in good agreement with lattice calculations. Background perturbation theory expansion for m_D(T) is discussed in comparison to standard perturbative results and recent gauge-invariant definitions.

hep-ph↗
arXiv2004

Non-Abelian field strength bilocal correlator at finite temperature in the model of dilute instanton gas

Bilocal correlator in gluodynamics is evaluated at finite temperature in the framework of instanton gas model. It is demonstrated that vacuum correlation length decreases with a growing temperature. Obtained results are compared with lattice data for the bilocal correlator at finite temperature. Density of instantons and possible structures of nonperturbative vacuum are discussed.

hep-ph↗
arXiv2002

Instanton infra-red stabilization in the nonperturbative QCD vacuum

The influence of nonperturbative fields on instantons in quantum chromodynamics is studied. Nonperturbative vacuum is described in terms of nonlocal gauge invariant vacuum averages of gluon field strength. Effective action for instanton is derived in bilocal approximation and it is demonstrated that stochastic background gluon fields are responsible for infra-red (IR)stabilization of instantons. Comparison of obtained instanton size distribution with lattice data is made.

hep-ph↗
arXiv2001

Instanton IR stabilization in the nonperturbative confining vacuum

The influence of nonperturbative fields on instantons in quantum chromodynamics is studied. Nonperturbative vacuum is described in terms of nonlocal gauge invariant vacuum averages of gluon field strength. Effective action for instanton is derived in bilocal approximation and it is demonstrated that stochastic background gluon fields are responsible for infra-red (IR) stabilization of instantons. Dependence of characteristic instanton size on gluon condensate and correlation length in nonperturbative vacuum is found. It is shown that instanton size in QCD is of order of 0.25 fm. Comparison of obtained instanton size distribution with lattice data is made.

hep-ph↗
arXiv2001

Instanton in the nonperturbative QCD vacuum

The influence of nonperturbative fields on instantons in quantum chromodynamics is studied. Effective action for instanton is derived in bilocal approximation and it is demonstrated that stochastic background gluon fields are responsible for IR stabilization of instantons. It is shown that instanton size in QCD is of order of 0.25 fm. Comparison of obtained instanton size distribution with lattice data is made.

hep-ph↗
arXiv2001

Nonperturbative vacuum and condensates in QCD below thermal phase transition

Thermodynamic properties of the QCD nonperturbative vacuum with two light quarks are studied. It is shown that at low temperatures T<M_πrelativistic massive pions can be treated within the dilute gas approximation. Analytic temperature dependence of the quark condensate is found in perfect agreement with the numerical calculations obtained at the three-loop level of the chiral perturbation theory with non-zero quark mass. The gluon condensate slightly varies with the increase of the temperature. It is shown that the temperature derivatives of the anomalous and normal (quark massive term) contributions to the trace of the energy-momentum tensor in QCD are equal to each other in the low temperature region.

hep-ph↗