arXiv subjects

Daniel Boyanovsky

Publications and source records attributed to Daniel Boyanovsky.

67 records · Page 4Linked to original sources

arXiv2007

Free streaming in mixed dark matter

Free streaming in a \emph{mixture} of collisionless non-relativistic dark matter (DM) particles is studied by implementing methods from the theory of multicomponent plasmas. The mixture includes Fermionic, condensed and non condensed Bosonic particles decoupling in equilibrium while relativistic, heavy non-relativistic thermal relics (WIMPs), and sterile neutrinos that decouple \emph{out of equilibrium} when they are relativistic. The free-streaming length $λ_{fs}$ is obtained from the marginal zero of the gravitational polarization function, which separates short wavelength Landau-damped from long wavelength Jeans-unstable \emph{collective} modes. At redshift $z$ we find $ \frac{1}{λ^2_{fs}(z)}= \frac{1}{(1+z)} \big[\frac{0.071}{\textrm{kpc}} \big]^2 \sum_{a}ν_a g^{2/3}_{d,a}({m_a}/{\mathrm{keV}})^2 I_a $,where $0\leq ν_a \leq 1$ are the \emph{fractions} of the respective DM components of mass $m_a$ that decouple when the effective number of ultrarelativistic degrees of freedom is $g_{d,a}$, and $I_a$ only depend on the distribution functions at decoupling, given explicitly in all cases. If sterile neutrinos produced either resonantly or non-resonantly that decouple near the QCD scale are the \emph{only} DM component,we find $λ_{fs}(0) \simeq 7 \mathrm{kpc} (\mathrm{keV}/m)$ (non-resonant), $λ_{fs}(0) \simeq 1.73 \mathrm{kpc} (\mathrm{keV}/m)$ (resonant).If WIMPs with $m_{wimp} \gtrsim 100 \mathrm{GeV}$ decoupling at $T_d \gtrsim 10 \mathrm{MeV}$ are present in the mixture with $ν_{wimp} \gg 10^{-12}$,$λ_{fs}(0) \lesssim 6.5 \times 10^{-3} \mathrm{pc}$ is \emph{dominated} by CDM. If a Bose Einstein condensate is a DM component its free streaming length is consistent with CDM because of the infrared enhancement of the distribution function.

astro-ph↗

arXiv2004

Photon production from a thermalized quark gluon plasma: quantum kinetics and nonperturbative aspects

We study the production of photons from a quark gluon plasma in local thermal equilibrium by introducing a non-perturbative formulation of the real time evolution of the density matrix. The main ingredient is the real time effective action for the electromagnetic field to $\mathcal{O}(α_{em})$ and to all orders in $α_s$. The real time evolution is completely determined by the solution of a \emph{classical stochastic} non-local Langevin equation which provides a Dyson-like resummation of the perturbative expansion. The Langevin equation is solved in closed form by Laplace transform in terms of the thermal photon polarization. A quantum kinetic description emerges directly from this formulation. We find that photons with $k \lesssim 200 ~{Mev}$ \emph{thermalize} as plasmon quasiparticles in the plasma on time scales $t \sim 10-20 ~{fm}/c$ which is of the order of the lifetime of the QGP expected at RHIC and LHC. We then obtain the direct photon yield to lowest order in $α_{em}$ and to leading logarithmic order in $α_s$ in a \emph{uniform} expansion valid at all time. The yield during a QGP lifetime $t \sim 10 ~{fm}/c$ is systematically larger than that obtained with the equilibrium formulation and the spectrum features a distinct flattening for $k \gtrsim 2.5 ~{Gev}$. We discuss the window of reliability of our results, the theoretical uncertainties in \emph{any} treatment of photon emission from a QGP in LTE and the shortcomings of the customary S-matrix approach.

hep-ph↗

arXiv2004

Graviton-induced Bremsstrahlung at e^+e^- colliders

We consider graviton-induced Bremsstrahlung at future e^+e^- colliders in both the ADD and RS models, with emphasis on the photon perpendicular momentum and angular distribution. The photon spectrum is shown to be harder than in the Standard Model, and there is an enhancement for photons making large angles with respect to the beam. In the ADD scenario, the excess at large photon perpendicular momenta should be measurable for values of the cut-off up to about twice times the c.m. energy. In the RS scenario, radiative return to graviton resonances below the c.m. energy can lead to large enhancements of the cross section.

hep-ph↗

arXiv2002

Nonequilibrium relaxation in neutral BCS superconductors: Ginzburg-Landau approach with Landau damping in real time

We present a field-theoretical method to obtain consistently the equations of motion for small amplitude fluctuations of the order parameter directly in real time for a homogeneous, neutral BCS superconductor. This method allows to study the nonequilibrium relaxation of the order parameter as an initial value problem. We obtain the Ward identities and the effective actions for small phase the amplitude fluctuations to one-loop order. Focusing on the long-wavelength, low-frequency limit near the critical point, we obtain the time-dependent Ginzburg-Landau effective action to one-loop order, which is nonlocal as a consequence of Landau damping. The nonequilibrium relaxation of the phase and amplitude fluctuations is studied directly in real time. The long-wavelength phase fluctuation (Bogoliubov-Anderson-Goldstone mode) is overdamped by Landau damping and the relaxation time scale diverges at the critical point, revealing critical slowing down.

cond-mat.supr-con↗

arXiv2001

Direct photons: a nonequilibrium signal of the expanding quark-gluon plasma

Direct photon production from a longitudinally expanding quark-gluon plasma (QGP) at Relativistic Heavy Ion Collider (RHIC) and Large Hadron Collider (LHC) energies is studied with a real-time kinetic description that is consistently incorporated with hydrodynamics. Within Bjorken's hydrodynamical model, energy nonconserving (anti)quark bremsstrahlung q(\bar{q})\to q(\bar{q})γand quark-antiquark annihilation q\bar{q}\to γare shown to be the dominant nonequilibrium effects during the transient lifetime of the QGP. For central collisions we find a significant excess of direct photons in the range of transverse momentum 1-2 \lesssim p_T \lesssim 5 GeV/c as compared to equilibrium results. The photon rapidity distribution exhibits a central plateau. The transverse momentum distribution at midrapidity falls off with a {\em power law} p^{-ν}_T with 2.5 \lesssim ν\lesssim 3 as a consequence of these energy nonconserving processes, providing a distinct experimental {\em nonequilibrium signature}. The power law exponent νincreases with the initial temperature of the QGP and hence with the total multiplicity rapidity distribution dN_π/dy.

hep-ph↗

arXiv2001

Non-Equilibrium Large N Yukawa Dynamics: marching through the Landau pole

The non-equilibrium dynamics of a Yukawa theory with N fermions coupled to a scalar field is studied in the large N limit with the goal of comparing the dynamics predicted from the renormalization group improved effective potential to that obtained including the fermionic backreaction. The effective potential is of the Coleman-Weinberg type. Its renormalization group improvement is unbounded from below and features a Landau pole. When viewed self-consistently, the initial time singularity does not arise. The different regimes of the dynamics of the fully renormalized theory are studied both analytically and numerically. Despite the existence of a Landau pole in the model, the dynamics of the mean field is smooth as it passes the location of the pole. This is a consequence of a remarkable cancellation between the effective potential and the dynamical chiral condensate. The asymptotic evolution is effectively described by a quartic upright effective potential. In all regimes, profuse particle production results in the formation of a dense fermionic plasma with occupation numbers nearly saturated up to a scale of the order of the mean field. This can be interpreted as a chemical potential. We discuss the implications of these results for cosmological preheating.

hep-ph↗

arXiv2001

The Specific Heat of Normal, Degenerate Quark Matter: Non-Fermi Liquid Corrections

In normal degenerate quark matter, the exchange of dynamically screened transverse gluons introduces infrared divergences in the quark self-energies that lead to the breakdown of the Fermi liquid description. If the core of neutron stars are composed of quark matter with a normal component, cooling by direct quark Urca processes may be modified by non-Fermi liquid corrections. We find that while the quasiparticle density of states is finite and non-zero at the Fermi surface, its frequency derivative diverges and results in non-Fermi liquid corrections to the specific heat of the normal, degenerate component of quark matter. We study these non-perturbative non-Fermi liquid corrections to the specific heat and the temperature dependence of the chemical potential and show that these lead to a reduction of the specific heat.

hep-ph↗

arXiv2000

Enhanced photon production from quark-gluon plasma: Finite-lifetime effect

Photon production from a thermalized quark-gluon plasma of finite lifetime is studied directly in real time with a nonequilibrium formulation that includes off-shell (energy nonconserving) effects. To lowest order we find that production of direct photons form a quark-gluon plasma of temperature T ~ 200 MeV and lifetime t ~ 10-20 fm/c is strongly enhanced by off-shell (anti)quark bremsstrahlung q(\bar{q})->q(\bar{q})γ. The yield from this nonequilibrium finite-lifetime effect dominates over those obtained from higher order equilibrium rate calculations in the range of energy E > 2 GeV and falls off with a power law for E >> T.

hep-ph↗

arXiv1999

Anomalous Pseudoscalar-Photon Vertex In and Out of Equilibrium

The anomalous pseudoscalar-photon vertex is studied in real time in and out of equilibrium in a constituent quark model. The goal is to understand the in-medium modifications of this vertex, exploring the possibility of enhanced isospin breaking by electromagnetic effects as well as the formation of neutral pion condensates in a rapid chiral phase transition in peripheral, ultrarelativistic heavy-ion collisions. In equilibrium the effective vertex is afflicted by infrared and collinear singularities that require hard thermal loop (HTL) and width corrections of the quark propagator. The resummed effective equilibrium vertex vanishes near the chiral transition in the chiral limit. In a strongly out of equilibrium chiral phase transition we find that the chiral condensate drastically modifies the quark propagators and the effective vertex. The ensuing dynamics for the neutral pion results in a potential enhancement of isospin breaking and the formation of $π^0$ condensates. While the anomaly equation and the axial Ward identity are not modified by the medium in or out of equilibrium, the effective real-time pseudoscalar-photon vertex is sensitive to low energy physics.

hep-ph↗

arXiv1998

Real-time Relaxation and Kinetics in Hot Scalar QED: Landau Damping

The real time evolution of field condensates with soft length scales k^{-1}>(eT)^{-1} is solved in hot scalar electrodynamics, with a view towards understanding relaxational phenomena in the QGP and the electroweak plasma. We find that transverse gauge invariant non-equilibrium expectation values of fields relax via {\em power laws} to asymptotic amplitudes that are determined by the quasiparticle poles. The long time relaxational dynamics and relevant time scales are determined by the behaviour of the retarded self-energy not at the small frequencies, but at the Landau damping thresholds. This explains the presence of power laws and not of exponential decay. Furthermore, we derive the influence functional, the Langevin equation and the fluctuation-dissipation theorem for the soft modes, identifying the correlation functions that emerge in the classical limit. We show that a Markovian approximation fails to describe the dynamics {\em both} at short and long times. We also introduce a novel kinetic approach that goes beyond the standard Boltzmann equation and incorporates off-shell processes and find that the distribution function for soft quasiparticles relaxes with a power law through Landau damping. We also find an unusual dressing dynamics of bare particles and anomalous (logarithmic) relaxation of hard quasiparticles.

hep-ph↗

arXiv1997

Non-equilibrium evolution of a `Tsunami': Dynamical Symmetry Breaking

We propose to study the non-equilibrium features of heavy-ion collisions by following the evolution of an initial state with a large number of quanta with a distribution around a momentum |\vec k_0| corresponding to a thin spherical shell in momentum space, a `tsunami'. An O(N); ({\vec Φ}^2)^2 model field theory in the large N limit is used as a framework to study the non-perturbative aspects of the non-equilibrium dynamics including a resummation of the effects of the medium (the initial particle distribution). In a theory where the symmetry is spontaneously broken in the absence of the medium, when the initial number of particles per correlation volume is chosen to be larger than a critical value the medium effects can restore the symmetry of the initial state. We show that if one begins with such a symmetry-restored, non-thermal, initial state, non-perturbative effects automatically induce spinodal instabilities leading to a dynamical breaking of the symmetry. As a result there is explosive particle production and a redistribution of the particles towards low momentum due to the nonlinearity of the dynamics. The asymptotic behavior displays the onset of Bose condensation of pions and the equation of state at long times is that of an ultrarelativistic gas although the momentum distribution is non-thermal.

hep-ph↗

arXiv1993

Dynamics of Phase Transitions Induced by a Heat Bath

We study the non-equilibrium dynamics of a symmetry restoring phase transition in a scalar field theory, the ``system'', linearly coupled to another scalar field taken as a ``heat bath''. The ``system'' is initially in an ordered low temperature phase, and the heat bath is at a temperature close to the critical temperature for the system. We estimate the time at which the phase transition to the disordered (symmetric) phase takes place. We derive, and integrate the one-loop effective equations of motion for the order parameter that include the effects of the heat bath. A semiclassical Langevin equation is derived and it is found that it contains a non-dissipative, non-Markovian kernel, the noise term is colored and correlated on time scales determined by the temperature of the heat bath. The range of validity of the Langevin equation and a consistent procedure to incorporate corrections are discussed.

hep-ph↗

arXiv1993

Quantum Spinodal Decomposition

We study the process of spinodal decomposition in a scalar quantum field theory that is quenched from an equilibrium disordered initial state at $T_i > T_f$ to a final state at $T_f \approx 0$. The process of formation and growth of correlated domains is studied in a Hartree approximation. We find an approximate scaling law for the size of the domains $ξ_D(t) \approx \sqrt{t ξ_0}$ at long times for weakly coupled theories, with $ξ_0$ the zero temperature correlation length.

hep-th↗