arXiv subjects

M. Loewe

Publications and source records attributed to M. Loewe.

At least 91 records · Page 5Linked to original sources

arXiv2005

Skyrme model and Isospin Chemical Potential

We discuss the stability of the Skyrmion solution in the presence of a finite isospin chemical potential $μ$. Solving numerically the mass of the Skyrmion as function of $μ$, we find a critical value $μ_c=222.8$ MeV where the Skyrmion mass vanishes. We compare the exact numerical treatment with an analytical discussion based on a special shape for the profile of the Skyrmion due to Atiyah and Manton. The extension of this ansatz for finite $μ$ works quite well for $μ<121$ MeV. Then, for small values of $μ$, where the analytical approach is valid, we consider the possibility of having an angular deformation for the Skyrmionic profile, which is possible for finite values of $μ$. This is however, a small effect. Finally we introduce finite temperature corrections, which strength the instability induced by the chemical potential, finding the dependence of the critical temperature on $μ$.

hep-ph↗

arXiv2005

A new determination of the electromagnetic nucleon form factors from QCD Sum Rules

We obtain the electromagnetic form factors of the nucleon, in the space-like region, using three-point function Finite Energy QCD Sum Rules. The QCD calculation is performed to leading order in perturbation theory in the chiral limit, and also to leading order in the non-perturbative power corrections. For the Dirac form factor, $F_1(q^2)$, we get a very good agreement with the data for both the proton and the neutron, in the currently accessible experimental region of momentum transfers. Unfortunately this is not the case, though, for the Pauli form factor $F_2(q^2)$, which has a soft $q^2$-dependence proportional to the quark condensate $<0|\bar{q}q|0>$.

hep-ph↗

arXiv2005

Two-flavor condensates in chiral dynamics: temperature and isospin density effects

Isospin density and thermal corrections for several condensates are discussed, at the one-loop level, in the frame of chiral dynamics with pionic degrees of freedom. The evolution of such objects give an additional insight into the condensed-pion phase transition, that occurs basically when $|\mui|>m_π$, being $|\mui|$ the isospin chemical potential. Calculations are done in both phases, showing a good agreement with lattice results for such condensates.

hep-ph↗

arXiv2005

Weinberg-Salam model at finite temperature and density

We present a new gauge fixing condition for the Weinberg-Salam electro-weak theory at finite temperature and density. After spontaneous symmetry breaking occurs, every unphysical term in the Lagrangian is eliminated with our gauge fixing condition. A new and simple Lagrangian can be obtained where we can identify the propagators and vertices. Some consequences are discussed, as the new gauge dependent masses of the gauge fields and the new Faddeev-Popov Lagrangian. After obtaining the quadratic terms, we calculate exactly the 1-loop effective potential identifying the contribution of every particular field.

hep-ph↗

arXiv2005

Electromagnetic nucleon form factors from QCD sum rules

The electromagnetic form factors of the nucleon, in the space-like region, are determined from three-point function Finite Energy QCD Sum Rules. The QCD calculation is performed to leading order in perturbation theory in the chiral limit, and to leading order in the non-perturbative power corrections. The results for the Dirac form factor, $F_1(q^2)$, are in very good agreement with data for both the proton and the neutron, in the currently accessible experimental region of momentum transfers. This is not the case, though, for the Pauli form factor $F_2(q^2)$, which has a soft $q^2$-dependence proportional to the quark condensate $<0|\bar{q}q|0>$.

hep-ph↗

arXiv2004

Topological Field Configurations in the Presence of Isospin Chemical Potential

We analyze the stability of different topological solutions in Quantum Field Theory when an isospin chemical potential $μ$ is included. We work in the limit when temperature vanishes. We find that static vortex solutions in $2+1D$ do exist. However, the 't Hooft-Polyakov monopole in $3+1D$ is no longer stable, as soon as the chemical potential acquires a finite value. In the case of the Skyrmion, this topological solution still exists for finite $μ$, up to a certain critical value.

hep-ph↗

arXiv2004

Thermal pion masses in the second phase: $|μ_{I}| >m_π$

Density and thermal corrections to the mass of the pions are studied in the framework of the SU(2) low energy effective chiral lagrangian, in terms of the isospin chemical potential $μ_\tin{I}$. We concentrate the discussion in the region where the isospin chemical potential (absolute value) becomes bigger than the pion mass at zero temperature and density, i.e. in the phase where the condensed $π^-$ phase appears (for negative chemical potential). We are able to calculate the thermal and density evolution of masses in the limits where $|μ_{I}|\gg m$ and where $|μ_{I}| \gtrsim m$. We also identified the phase transition curve.

hep-ph↗

arXiv2004

Quantum Theory of Tensionless Noncommutative p-Branes

The quantum theory involving noncommutative tensionless p-branes is studied following path integral methods. Our procedure allow a simple treatment for generally covariant noncommutative extended systems and it contains, as a particular case, the thermodynamics and the quantum tensionless string theory. The effect induced by noncommutativity in the field space is to produce a confinement among pairing of null p-branes.

hep-th↗

arXiv2004

Central Charges and Effective Action at Finite Temperature and Density

The current algebra for gauge theories like QCD at finite temperature and density is studied. We start considering, the massless Thirring model at finite temperature and density, finding an explicit expression for the current algebra. The central charge only depends on the coupling constant and there are not new effects due to temperature and density. From this calculation, we argue how to compute the central charge for $QCD_4$ and we argue why the central charge in four dimensions could be modified by finite temperature and density.

hep-th↗

arXiv2003

Relativistic Violation Invariance, Multiverses and Quantum Field Theory

The possibility of interaction among multiverses is studied assuming that in the first instants of the big-bang, many disjoint regions were created producing many independent universes (multiverses). Many of these mini-universes were unstable and they decayed, but other remained as topological remnant (like domain walls or baby universes) or possibly as mini-black-holes. In this paper, we study the quantum statistical mechanics of multiverses assuming that in the first instants of the big-bang, the relativistic symmetry was only an approximate symmetry and the interaction among multiverses was produced by non-local communication. The breaking of the relativistic symmetry induces on each multiverse a tiny harmonic interaction. The oscillation frequency for each multiverse is proportional to 1/B, where B is the noncommutativity parameter. We argue that B can identified as the primordial magnetic field, {\it i.e.} $\sim 10^{-16} {GeV}^2$. This tiny frequency could suggest that the relativistics invariance --from the cosmological point of view-- is almost exact and the multiverses could be not detected using the presently astronomical observations.

hep-th↗

arXiv2003

A new treatment of the in-medium chiral condensates

A new formalism to calculate the in-medium chiral condensate is presented. At lower densities, this approach leads to a linear expression. If we demand a compatibility with the famous model-independent result, then the pion-nucleon sigma term should be six times the average current mass of light quarks. QCD-like interactions may slow the decreasing behavior of the condensate with increasing densities, compared with the linear extrapolation, if densities are lower than twice the nuclear saturation density. At higher densities, the condensate vanishes inevitably.

hep-ph↗

arXiv2003

Chiral Condensates in Quark and nuclear Matter

We present a novel treatment for calculating the in-medium quark condensates. The advantage of this approach is that one does not need to make further assumptions on the derivatives of model parameters with respect to the quark current mass. The normally accepted model-independent result in nuclear matter is naturally reproduced. The change of the quark condensate induced by interactions depends on the incompressibility of nuclear matter. When it is greater than 260 MeV, the density at which the condensate vanishes is higher than that from the linear extrapolation. For the chiral condensate in quark matter, a similar model-independent linear behavior is found at lower densities, which means that the decreasing speed of the condensate in quark matter is merely half of that in nuclear matter if the pion-nucleon sigma commutator is six times the average current mass of u and d quarks. The modification due to QCD-like interactions is found to slow the decreasing speed of the condensate, compared with the linear extrapolation.

hep-ph↗

arXiv2003

Thermal Pions at Finite Isospin Chemical Potential

The density corrections, in terms of the isospin chemical potential $μ_I$, to the mass of the pions are studied in the framework of the SU(2) low energy effective chiral lagrangian. The pion decay constant $f_π(T, μ_{I})$ is also analized. As a function of temperature for $μ_I =0$, the mass remains quite stable, starting to grow for very high values of $T$, confirming previous results. However, there are interesting corrections to the mass when both effects (temperature and chemical potential) are simultaneously present. At zero temperature the $π^{\pm}$ should condensate when $μ_{I} = \mp m_π$. This is not longer valid anymore at finite $T$. The mass of the $π_0$ acquires also a non trivial dependence on $μ_I$ due to the finite temperature.

hep-ph↗

arXiv2002

Thermal Pions ns Isospin Chemical Potential Effects

The density corrections, in terms of the isospin chemical potential $μ_I$, to the mass of the pions are investigated in the framework of the SU(2) low energy effective chiral invariant lagrangian. As a function of temperature and $μ_I =0$, the mass remains quite stable, starting to grow for very high values of $T$, confirming previous results. However, the dependence for a non-vanishing chemical potential turns out to be much more dramatic. In particular, there are interesting corrections to the mass when both effects (temperature and chemical potential) are simultaneously present. At zero temperature the $π^{\pm}$ should condensate when $μ_{I} = \mp m_π$. This is not longer valid anymore at finite $T$. The mass of the $π_0$ acquires also a non trivial dependence on $μ_I$ at finite $T$.

hep-ph↗

arXiv2002

Testing spatial noncommutativiy via the Aharonov-Bohm effect

The possibility of detecting noncommutative space relics is analyzed using the Aharonov-Bohm effect. We show that, if space is noncommutative, the holonomy receives non-trivial kinematical corrections that will produce a diffraction pattern even when the magnetic flux is quantized. The scattering problem is also formulated, and the differential cross section is calculated. Our results can be extrapolated to high energy physics and the bound $θ\sim [ 10 {TeV}]^{-2}$ is found. If this bound holds, then noncommutative effects could be explored in scattering experiments measuring differential cross sections for small angles. The bound state Aharonov- Bohm effect is also discussed.

hep-th↗

arXiv2001

Noncommutativity and the Aharonov-Bohm Effect

The possibility of detecting noncommutive space relics is analyzed by using the Aharonov-Bohm effect. If space is non-commutative, it turns out that the holonomy receives kinematical corrections that tend to diffuse the fringe pattern. This fringe pattern has a non-trivial energy dependence and, therefore, one could observe noncommutative effects by modifying the energy of the incident electrons beam in the Tonomura experimental arrangement

hep-th↗

arXiv2001

Noncommutative Quantum Mechanics: The Two-Dimensional Central Field

Quantum mechanics in a noncommutative plane is considered. For a general two dimensional central field, we find that the theory can be perturbatively solved for large values of the noncommutative parameter ($θ$) and explicit expressions for the eigenstates and eigenvalues are given. The Green function is explicitly obtained and we show that it can be expressed as an infinite series. For polynomial type potentials, we found a smooth limit for small values of $θ$ and for non-polynomial ones this limit is necessarily abrupt. The Landau problem, as a limit case of a noncommutative system, is also considered.

hep-th↗

arXiv2001

Non-Commutative Quantum Mechanics

A general non-commutative quantum mechanical system in a central potential $V=V(r)$ in two dimensions is considered. The spectrum is bounded from below and for large values of the anticommutative parameter $θ$, we find an explicit expression for the eigenvalues. In fact, any quantum mechanical system with these characteristics is equivalent to a commutative one in such a way that the interaction $V(r)$ is replaced by $V = V ({\hat H}_{HO}, {\hat L}_z)$, where ${\hat H}_{HO}$ is the hamiltonian of the two-dimensional harmonic oscillator and ${\hat L}_z$ is z- component of the angular momentum. For other finite values of $θ$ the model can be solved by using perturbation theory.

hep-th↗