Bosonic Clouds Research Articles

Electromagnetic structure of the nucleon in the perturbative chiral quark model

We apply the perturbative chiral quark model (PCQM) at one loop to analyze the electromagnetic structure of nucleons. This model is based on an effective Lagrangian, where baryons are described by relativistic valence quarks and a perturbative cloud of Goldstone bosons. Including the electromagnetic interaction we first develop the formalism up to one loop in the Goldstone boson fluctuation relying on renormalization by use of counterterms. Local gauge invariance is satisfied both on the Lagrangian level and also for the relevant baryon matrix elements in the Breit frame. We apply the formalism to obtain analytical expressions for the nucleon charge and magnetic form factors, which are expressed in terms of fundamental parameters of low-energy pion-nucleon physics (weak pion decay constant, axial nucleon coupling, strong pion-nucleon form factor) and of only one model parameter (radius of the nucleonic three-quark core). A detailed numerical analysis of the nucleon magnetic moments, charge and magnetic radii, and also the momentum dependence of form factors is presented.

Dynamical symmetry breaking in the sea of the nucleon.

We derive the nonanalytic chiral behavior of the flavor asymmetry d - u. Such behavior is a unique characteristic of Goldstone boson loops in chiral theories, including QCD, and establishes the unambiguous role played by the Goldstone boson cloud in the sea of the proton. Generalizing the results to the SU(3) sector, we show that strange chiral loops require that the s - s distribution be nonzero.

Criterion for Bose-Einstein condensation for particles in traps

We consider the criterion for Bose condensation for particles in a harmonic trap. For a fixed angular momentum, the lowest energy state for a cloud of bosons with attractive interactions is the ground state of the cloud with all the angular momentum in the center-of-mass motion, and the one-particle reduced density matrix generally does not have a single large eigenvalue, but a number of them, suggesting that the state is an example of a fragmented condensate (Wilkin, Gunn, and Smith, Phys. Rev. Lett. 80, 2265 (1998)). We show that a convenient way to describe correlations in the system is by defining an internal one-particle reduced density matrix, in which the center-of-mass motion is eliminated, and that this has a single eigenvalue equal to the number of particles for the problem considered here. Our considerations indicate that care is necessary in formulating a criterion for Bose-Einstein condensation.

Structure and stability of bosonic clouds: Alkali-metal atoms with negative scattering length

We investigate the form and stability of a cloud of atoms confined in a harmonic trap when the scattering length is negative. We find that, besides the known low density metastable solution, a new branch of Bose condensate appears at higher density when non locality effects in the attractive part are taken into account. The transition between the two classes of solutions as a function of the number $N$ of atoms can be either sharp or smooth according to the strength and range of the attractive interaction. Use of tight traps is favorable for investigating the evolution of the system as the strength of the effective interaction increases with $N$.

Bosonic Clouds with Attractive Interaction Beyond the Local Interaction Approximation

We study the properties of a Bose–Einstein condensed cloud of atoms with negative scattering length confined in a harmonic trap. When a realistic non local (finite range) effective interaction is taken into account, we find that, besides the known low density metastable solution, a new branch of Bose condensate appears at higher density. This state is self–bound but its density can be quite low if the number N of atoms is not too big. The transition between the two classes of solutions as a function of N can be either sharp or smooth according to the ratio between the range of the attractive interaction and the length of the trap. A tight trap leads to a smooth transition. In addition to the energy and the shape of the cloud we study also the dynamics of the system. In particular, we study the frequencies of collective oscillation of the Bose condensate as a function of the number of atoms both in the local and in the non local case. Moreover, we consider the dynamics of the cloud when the external trap is switched off.

JEANS MASS OF NEWTONIAN BOSE CLOUDS

The gravitational instability of free and dilute Newtonian boson clouds has been examined. For collisionless Newtonian boson clouds with a uniform unperturbated Bose gas density, it is deduced that the gravitational instability will never give rise to the formation of boson stars, but only black holes; however if the unperturbated density, in the neighbourhood of the perturbation, has a scale length equal to or smaller than the perturbation wavelength, the formation of free Newtonian boson stars could also be possible. For dilute Newtonian boson clouds it is shown that the gravitational instability can lead the cloud to condense into boson stars of different masses but identical radii, in agreement with studies about equilibrium configurations of dilute Newtonian boson stars. The galactic distribution of these Newtonian boson stars, and hence, the average distance among them is also discussed.

Low-energy collective electronic excitations in a polaron gas.

A dense gas of electrons whose effective mass is significantly enhanced at low frequencies is studied with the help of a simple classical analogy. The electromagnetic response is analyzed, and characteristic features are found in infrared-absorption and electron-energy-loss spectra. These spectral features arise from an additional light-absorbing excitation in midinfrared, and from a collective excitation (oscillation of the density of boson clouds) located in the far infrared.

On the possibility of Bose-star formation

The relaxation time of a gravitationally bounded cloud of bosons is estimated for the case of a large phase-space density of particles. The axionic mass and self-coupling relaxation time are compatible to the age of the Universe if the escape velocity and the mass in the core region satisfy the relation M < 10 8v e 5 2 φ 10 − 1 4 M ⊙ . In the central part of such a cloud an axionic cosmic matter with luminosity ∼ 10 −2v r 5 2 φ 10 − 1 4 M ⊙/ yr will form. An upper bound on the self-coupling is obtained provided the mass distribution in galaxies is dominated by this kind of dark matter.

Superconducting transition temperatures of strongly coupled electron-boson systems: A self-consistent method for fluctuation from the Bardeen-Cooper-Schrieffer limit to the bipolaronic limit.

Superconducting transition temperatures ${T}_{c}$ of a many-electron system coupling strongly with bosonic excitations are studied in the context of polaron theory. The boson clouds that move with electrons are taken into account within the variational method. The effect of fluctuation of the pairing order is also taken into account by the coherent-potential approximation. The resultant theory covers the whole region of the three basic parameters characterizing this system: the intersite transfer energy of the electron, T, the site-diagonal electron-boson coupling energy, S, and the energy of the boson, \ensuremath{\omega}. ${T}_{c}$ is found to become maximum in the transition region S\ensuremath{\approxeq}T, which lies between the weak (Bardeen-Cooper-Schrieffer) region S\ensuremath{\ll}T and the strong (bipolaronic) region S\ensuremath{\gg}T. The width of this region is narrow in the adiabatic case \ensuremath{\omega}\ensuremath{\ll}T, but wide in the inverse-adiabatic case \ensuremath{\omega}\ensuremath{\gg}T. This region is characterized by the large ratio of the energy gap to ${k}_{B}$${T}_{c}$. These results are applied to the Cu-O--type new ceramics, and the breathing mode of oxygen is shown to be able to give a high ${T}_{c}$ of about 100 K. The effects of other quasibosons with high frequencies, such as plasmons and excitons, are also studied.

Dynamics of vacuum excitations and quark confinement in quantum field theories

Abstract A unified approach to interacting vacuum excitations and quark confinement is formulated in quantum field theories with symmetry breakdown. Vacuum excitations are shown to be coherent clouds of Goldstone bosons or gauge bosons and are interpreted as new asymptotic extended particle states. They correspond to all dynamically possible space-time dependent Bose condensations of the Goldstone bosons in a given theory. Different configurations of vacuum excitations are connected to one another by a family of invariant boson transformations. As an example, the Nambu theory of interacting vortex strings is derived from a Nambu-Heisenberg quark-gluon field theory. The quarks can be completely confined to the strings while the gluons cluster in quantized magnetic flux bundles of penetration width m v −1 and provide a short range interaction force.

Electromagnetic Structure of the Nucleon with Many-Particle Structure

The electric and magnetic form factors of nucleons are treated on the basis of the model in which the nucleon has a “central part” with spin one half, which is composed of three urbaryons (like quarks) and whose extent is of the order of magnitude of the nucleon Compton wave length. These quarks need not have a “particle-nature” in all respects, but may be some sort of quasi-particles. In the case where the quarks are fermions or parafermions, the general forms of the nonrelativistic wave functions of the three-body systems are given and the Fourier representations of the completely symmetric or anti-symmetric space wave functions are obtained in order to get the electric and magnetic form factors coming from the “central part”. The effects of the boson cloud are taken into account as the contribution from boson resonances. It has become clear that the electric form factor of proton evaluated only from the boson resonances does not agree with the recent experimental data. It is shown that if the contribution of the “central part” is taken into account, the agreement of the theoretical form factors with experiment is much improved. Although this does not necessarily establish the existence of the “central part”, it indicates at least that our model is consistent with the experimental data. The existence of the “central part” not only gives the electric and magnetic form factors by itself, but also, as the source of bosons, explains the effective masses of the boson resonances, which are smaller than the real masses required by the experimental data. If the radius of the “central” part is assumed to be much smaller than the order of magnitude of the nucleon Compton wave length, for example one-tenth of it, it is hard to get agreement with the present experimental data. In addition to this, if the absorption effect is taken into account, in the case of peripheral approximation, the assumed order of magnitude of the radius of the “central” part seems to be consistent with experiment. The existence of the “central part” suppresses the processes with impact parameter smaller than the nucleon Compton wave length. Further, since the many-particle exchange processes at relatively low energies are suppressed more severely as the number of exchanged particles increases, as a result of the existence of the “central part”, this gives one of the reasons for the apparent partial success of the peripheral approximation.