Fermionic Background Research Articles

Creation of Dirac neutrinos in a dense medium with a time-dependent effective potential

We consider Dirac neutrinos interacting with background fermions in the frame of the standard model. We demonstrate that a time-dependent effective potential is quite possible in a protoneutron star (PNS) at certain stages of its evolution. For the first time, we formulate a nonperturbative treatment of neutrino processes in a matter with arbitrary time-dependent effective potential. Using linearly growing effective potential, we study the typical case of a slowly varying matter interaction potential. We calculate differential mean numbers of $\nu \bar{\nu}$ pairs created from the vacuum by this potential and find that they crucially depend on the magnitude of masses of the lightest neutrino eigenstate. These distributions uniformly span up to $\sim 10$ eV energies for muon and tau neutrinos created in PNS core due to the compression just before the hydrodynamic bounce and up to $\sim 0.1$ eV energies for all three active neutrino flavors created in the neutronization. Considering different stages of the PNS evolution, we derive constraints on neutrino masses, $m_{\nu}\lesssim (10^{-8}-10^{-7})$ eV corresponding to the nonvanishing $\nu \bar{\nu}$ pairs flux produced by this mechanism. We show that one can distinguish such coherent flux from chaotic fluxes of any other origin. Part of these neutrinos, depending on the flavor and helicity, are bounded in the PNS, while antineutrinos of any flavor escape the PNS. If the created pairs are $\nu_{e}\bar{\nu}_{e}$, then a part of the corresponding neutrinos also escape the PNS. The detection of $\nu $ and $\bar{\nu}$ with such low energies is beyond current experimental techniques.

The photino sector and a confining potential in a supersymmetric Lorentz-symmetry-violating model

We study the spectrum of the minimal supersymmetric extension of the Carroll-Field-Jackiw model for Electrodynamics with a topological Chern-Simons-like Lorentz-symmetry violating term. We identify a number of independent background fermion condensates, work out the gaugino dispersion relation and propose a photonic effective action to consider aspects of confinement induced by the SUSY background fermion condensates, which also appear to signal Lorentz-symmetry violation in the photino sector of the action. Our calculations of the static potential are carried out within the framework of the gauge-invariant but path-dependent variables formalism which are alternative to the Wilson loop approach. Our results show that the interaction energy contains a linear term leading to the confinement of static probe charges.

Equations of motion for a classical color particle in external non-Abelian fermionic and bosonic fields

On the basis of principles of gauge and reparametrization invariance of a real-valued Lagrangian, construction of the action describing the dynamics of a classical color-charged particle interacting with background non-Abelian gauge and fermion fields is considered. The cases of the linear and quadratic dependence of a Lagrangian on the background fermion field are discussed. It is shown that, in both cases, there exist an infinite number of interaction terms. From an iteration scheme, examples of construction of the first few currents and sources induced by a moving particle with non-Abelian charge are given. It is shown that these quantities by a suitable choice of parameters exactly reproduce additional currents and sources obtained previously in [1] on the basis of heuristic considerations.

Resonant superfluidity in an optical lattice

We have studied a system of ultracold fermionic potassium (40K) atoms in a three-dimensional (3D) optical lattice in the vicinity of an s-wave Feshbach resonance. Close to resonance, the system is described by a multi-band Bose–Fermi Hubbard Hamiltonian. We derive an effective lowest-band Hamiltonian in which the effect of higher bands is incorporated by a self-consistent mean-field approximation. The resulting model is solved by means of generalized dynamical mean-field theory. In addition to the BEC–BCS crossover, we find a phase transition to a fermionic Mott insulator at half-filling, induced by the repulsive fermionic background scattering length. We also calculate the critical temperature of the BEC/BCS state and find it to be minimal at resonance.

Neutrino–neutrino interactions and flavour mixing in dense matter

An algebraic approach to the neutrino propagation in dense media is presented. The Hamiltonian describing a gas of neutrinos interacting with each other and with background fermions is written in terms of the appropriate SU(N) operators, where N is the number of neutrino flavours. The evolution of the resulting many-body problem is formulated as a coherent-state path integral. Some commonly used approximations are shown to represent the saddle-point solution of the path integral for the full many-body system.

NEUTRINO SPIN RELAXATION IN MEDIUM WITH STOCHASTIC CHARACTERISTICS

The helicity evolution of a neutrino propagating in randomly moving and polarized matter is studied. The type of the neutrino interaction with background fermions is arbitrary. We derive the equation for the description of the averaged neutrino helicity evolution. In the particular case of a τ-neutrino interacting with ultrarelativistic electron–positron plasma, we obtain the expression for the neutrino helicity relaxation rate in the explicit form. We study the neutrino spin relaxation in the relativistic primordial plasma. Supposing that the conversion of left-handed neutrinos into right-handed ones is suppressed at the early stages of the Universe evolution, we get the upper limit on the τ-neutrino mass.

Fermionic backgrounds and condensation of supergravity fields in the type IIB matrix model

In a previous paper [1] we constructed wave functions of a D-instanton and vertex operators in type IIB matrix model by expanding supersymmetric Wilson line operators. They describe couplings of a D-instanton and type IIB matrix model to the massless closed string fields, respectively, and form a multiplet of $D=10$ $\mathcal{N}=2$ supersymmetries. In this paper we consider fermionic backgrounds and condensation of supergravity fields in IIB matrix model by using these wave functions. We start from the type IIB matrix model in a flat background whose matrix size is $(N+1)\ifmmode\times\else\texttimes\fi{}(N+1)$, or equivalently the effective action for $(N+1)$ D-instantons. We then calculate an effective action for $N$ D-instantons by integrating out 1 D-instanton (which we call a mean-field D-instanton) with an appropriate wave function and show that various terms can be induced corresponding to the choice of the wave functions. In particular, a Chern-Simons-like term is induced when the mean-field D-instanton has a wave function of the antisymmetric tensor field. A fuzzy sphere becomes a classical solution to the equation of motion for the effective action. We also give an interpretation of the above wave functions in the superstring theory side as overlaps of the D-instanton boundary state with the closed string massless states in the Green-Schwarz formalism.

High energy neutrino spin light

The quantum theory of spin light (electromagnetic radiation emitted by a Dirac massive neutrino propagating in dense matter due to the weak interaction of a neutrino with background fermions) is developed. In contrast to the Cherenkov radiation, this effect does not disappear even if the medium refractive index is assumed to be equal to unity. The formulas for the transition rate and the total radiation power are obtained. It is found out that radiation of photons is possible only when the sign of the particle helicity is opposite to that of the effective potential describing the interaction of a neutrino (antineutrino) with the background medium. Due to the radiative self-polarization the radiating particle can change its helicity. As a result, the active left-handed polarized neutrino (right-handed polarized antineutrino) converting to the state with inverse helicity can become practically “sterile”. Since the sign of the effective potential depends on the neutrino flavor and the matter structure, the spin light can change a ratio of active neutrinos of different flavors. In the ultra relativistic approach, the radiated photons averaged energy is equal to one third of the initial neutrino energy, and two thirds of the energy are carried out by the final “sterile” neutrinos.

Radiative transitions of high-energy neutrinos in a dense medium

The quantum theory of the ``spin light'' (electromagnetic radiation emitted by a massive neutrino propagating in dense matter due to the weak interaction of a neutrino with background fermions) is developed. In contrast to the Cherenkov radiation, this effect does not disappear even if the medium refractive index is assumed to be equal to unity. The formulas for the transition rate and the total radiation power are obtained. It is found out that radiation of photons is possible only when the sign of the particle helicity is opposite to that of the effective potential describing the interaction of a neutrino (antineutrino) with the background medium. Due to the radiative self-polarization the radiating particle can change its helicity. As a result, the active left-handed polarized neutrino (right-handed polarized antineutrino) converting to the state with inverse helicity can become practically ``sterile''. Since the sign of the effective potential depends on the neutrino flavor and the matter structure, the ``spin light'' can change a ratio of active neutrinos of different flavors. In the ultra relativistic approach, the radiated photons averaged energy is equal to one third of the initial neutrino energy, and two thirds of the energy are carried out by the final ``sterile'' neutrinos. This fact can be important for the understanding of the ``dark matter'' formation mechanism on the early stages of evolution of the Universe.

Instanton Calculations forN=1/ 2 super Yang-Mills Theory

We study (anti-) instantons in super Yang-Mills theories defined on a non anticommutative superspace. The instanton solution that we consider is the same as in ordinary SU(2) N=1 super Yang-Mills, but the anti-instanton receives corrections to the U(1) part of the connection which depend quadratically on fermionic coordinates, and linearly on the deformation parameter C. By substituting the exact solution into the classical Lagrangian the topological charge density receives a new contribution which is quadratic in C and quartic in the fermionic zero-modes. The topological charge turns out to be zero. We perform an expansion around the exact classical solution in presence of a fermionic background and calculate the full superdeterminant contributing to the one-loop partition function. We find that the one-loop partition function is not modified with respect to the usual N=1 super Yang-Mills.

Penrose limits and Green–Schwarz strings

We discuss the Green–Schwarz action for type IIB strings in general plane-wave backgrounds obtained as Penrose limits from any IIB supergravity solutions with vanishing background fermions. Using the normal-coordinate expansion in superspace, we prove that the light-cone action is necessarily quadratic in the fermionic coordinates. This proof is valid for more general pp-wave backgrounds under certain conditions. We also write down the complete quadratic action for general bosonic on-shell backgrounds in a form in which its geometrical meaning is manifest both in the Einstein and string frames. When the dilaton and 1-form field strength are vanishing, and the other field strengths are constant, our string-frame action reduces, up to conventions, to the one which has been written down using the supercovariant derivative.

Casimir interaction among objects immersed in a fermionic environment.

Using ensembles of two, three, and four spheres immersed in a fermionic background we evaluate the (integrated) density of states and the Casimir energy. We thus infer that for sufficiently smooth objects, whose various geometric characteristic lengths are larger then the Fermi wave length one can use the simplest semiclassical approximation (the contribution due shortest periodic orbits only) to evaluate the Casimir energy. We also show that the Casimir energy for several objects can be represented fairly accurately as a sum of pairwise Casimir interactions between pairs of objects.

GeneralizedSUq(1|2) Gaudin model

In this paper we propose the Hamiltonians of the generalized SUq(1|2) Gaudin model corresponding to the periodic generalized t-J model. With the help of the well defined graded quantum determinant, we obtain the eigenstate and eigenvalues of the generating function and the Hamiltonians of the Gaudin model in the fermionic background in the framework of the graded quantum inverse scattering method. The Bethe ansatz equations are also obtained.

Supercurrent interactions in noncommutative Yang–Mills and IIB matrix model

It is known that noncommutative Yang-Mills is equivalent to IIB matrix model with a noncommutative background, which is interpreted as a twisted reduced model. In noncommutative Yang-Mills, long range interactions can be seen in nonplanar diagrams after integrating high momentum modes. These interactions can be understood as block-block interactions in the matrix model. Using this relation, we consider long range interactions in noncommutative Yang-Mills associated with fermionic backgrounds. Exchanges of gravitinos, which couple to a supersymmetry current, are examined.

Fermionic dispersion relations at finite temperature and non-vanishing chemical potentials in the minimal standard model

We calculate the fermionic dispersion relations in the minimal standard model at finite temperature in presence of non-vanishing chemical potentials due to the CP-asymmetric fermionic background. The dispersion relations are calculated for a vacuum expectation value of the Higgs field equal to zero (unbroken electroweak symmetry). The calculation is performed in the real time formalism of the thermal field theory at one-loop order in a general $\xi$ gauge. The fermionic self-energy is calculated at leading order in temperature and chemical potential and this fact permits us to obtain gauge invariant analytical expressions for the dispersion relations.

Derivative expansion and soliton masses

We present a simple algorithm to implement the generalized derivative expansion introduced previously by L-H. Chan, and apply it to the calculation of the one-loop mass correction to the classical soliton mass in the 1+1 dimensional Jacobi model. We then show how this derivative expansion approach implies that the total (bosonic plus fermionic) mass correction in an N=1 supersymmetric soliton model is determined solely by the asymptotic values (and derivatives) of the fermionic background potential. For a static soliton the total mass correction is − m/(2 π), in agreement with recent analyses using phase-shift methods.

Supergravity currents and linearized interactions for matrix theory configurations with fermionic backgrounds

The leading terms in the long-range interaction potential between an arbitrary pair of matrix theory objects are calculated at one-loop order. This result generalizes previous calculations by including arbitrary fermionic background field configurations. The interaction potential at orders 1/r^7 and 1/r^8 is shown to correspond precisely with the leading terms expected from linearized supergravity interactions between arbitrary objects in M-theory. General expressions for the stress tensor, membrane current and 5-brane current of an arbitrary matrix configuration are derived, including fermionic contributions. Supergravity effects which are correctly reproduced include membrane/5-brane interactions, 0-brane/6-brane interactions, supercurrent/supercurrent interactions and the spin contributions to moments of the supergravity currents. The matrix theory description of the supergravity stress tensor, membrane current and 5-brane current are used to propose an explicit formulation of matrix theory in an arbitrary background metric and 3-form field.

Higher order spin-dependent terms in D0-brane scattering from the Matrix model

The potential describing long-range interactions between D0-branes contains spin-dependent terms. In the matrix model, these should be reproduced by the one-loop effective action computed in the presence of a non-trivial fermionic background ψ. The ν 3 ψ 2/ r 8 term in the effective action has been computed by Kraus and shown to correspond to a spin-orbit interaction between D0-branes, and the ψ 8/ r 11 term in the static potential has been obtained by Barrio et al. In this paper, the ν 2 ψ 4/ r 9 term is computing in the matrix model and compared with the corresponding results of Morales et al. obtained using string theoretic methods. The technique employed is adapted to the underlying supersymmetry of the matrix model, and should be useful in the calculation of spin-dependent effects in more general D p-brane scatterings.

Chiral and parity anomalies at finite temperature and density

Two closely related topological phenomena are studied at finite density and temperature. These are the chiral anomaly and the Chern-Simons term. Using different methods it is shown that μ 2 = m 2 is the crucial point for Chern-Simons at zero temperature. So when μ 2 < m 2 the μ influence is absent and we obtain the usual Chern-Simons term. On the other hand, when μ 2 m 2 the Chern-Simons term vanishes because of the non-zero density of the background fermions. The chiral anomaly does not depend on density and temperature. The connection between parity anomalous Chern-Simons and the chiral anomaly is generalized at finite density. These results hold in any dimension in abelian and in non-abelian cases.

Chern-Simons term at finite density

The Chern-Simons topological term coefficient is derived at arbitrary finite density. As it happens μ 2 = m 2 is the crucial point for Chern-Simons. So when μ 2 < m 2 the μ-influence disappears and we get the usual Chern-Simons term. On the other hand when μ 2 > m 2 the Chern-Simons term vanishes because of the non-zero density of background fermions. In particular for the massless case the parity anomaly is absent at any finite density. This result holds in any odd dimension both in the abelian and in the nonabelian case.