arXiv subjects

M. Loewe

Publications and source records attributed to M. Loewe.

At least 55 records · Page 3Linked to original sources

arXiv2014

Weinberg sum rules at finite temperature

The saturation of the two Weinberg sum rules is studied at finite temperature, using recent independent QCD sum rule results for the thermal behaviour of hadronic parameters in the vector and axial-vector channels. Both sum rules are very well satisfied from $T=0$ up to $T/T_c \simeq 0.7-0.8$. At higher temperatures close to $T_c$ a hadronic, pion-loop contribution in the space-like region proportional to $T^2$, present at leading order in the vector but not in the axial-vector channel, induces an asymmetry leading to a small deviation. In this region, though, QCD sum rules for the hadronic parameters begin to have no solutions, as the hadronic widths of the $ρ$ and the $a_1$ mesons diverge signalling deconfinement. Close to, and at $T=T_c$ there are no pions left in the medium and chiral symmetry is restored, so that the sum rules are trivially satisfied.

hep-ph↗

arXiv2014

Scalar form factor of the pion in the Kroll-Lee-Zumino field theory

The renormalizable Kroll-Lee-Zumino field theory of pions and a neutral rho-meson is used to determine the scalar form factor of the pion in the space-like region at next-to-leading order. Perturbative calculations in this framework are parameter free, as the masses and the rho-pion-pion coupling are known from experiment. Results compare favourably with lattice QCD calculations.

hep-ph↗

arXiv2013

Vacuum instability by a chromoelectric field in 2 + 1-dimensions

The problem of vacuum instability and creation of pairs by external fields SU(2) is studied. The effective mass operator has complex eigenvalues implying decay and pair creation. We consider a constant external chromoelectric field and a region in the parameter space where the vaccum becomes unstable and decays in pairs. Although the calculation is done in SU(2) color group it is also valid for SU(3).

hep-ph↗

arXiv2013

The Linear Sigma Model and the formation of a charged pion condensate in the presence of an external magnetic field

We discuss the charged pion condensation phenomenon in the linear sigma model, in the presence of an external uniform magnetic field. The critical temperature is obtained as a function of the external magnetic field, assuming the transition is of second order, by considering a dilute gas at low temperature. As a result we found magnetic anti-catalysis in the Bose-Einstein condensation for lower values of the external magnetic field, and catalysis for higher values of the external magnetic field. This behavior confirms previous results with a single charged scalar field.

hep-ph↗

arXiv2013

Dimuon production from in-medium rho decays from QCD sum rules

We compute the dimuon-excess invariant mass distribution at the rho-meson peak in the context of relativistic heavy-ion collisions at SPS energies. The temperature, T, and chemical potential, mu, dependent parameters describing the rho-meson --its width, mass and leptonic decay constant-- are determined from finite energy QCD sum rules. Results show that the rho-meson width increases whereas its mass and leptonic decay constant decrease near the (chemical potential-dependent) critical temperature T_c(mu) for chiral symmetry restoration/quark-gluon deconfinement. As a consequence, starting from T_c(mu), for a short lived cooling the main effect is a broadening of the dimuon distribution. However, when the evolution brings the system to a lower freeze-out temperature, with the rho-meson parameters approaching their vacuum values, the dimuon distribution shows a broad peak centered at the vacuum rho-meson mass. For even lower freeze-out temperatures the peak becomes less prominent since the thermal phase space factor suppresses the distribution for larger values of the invariant dimuon mass, given that the average temperature is smaller. The dimuon distribution exhibits a non-trivial behavior with mu. For small mu values the distribution broadens with increasing mu becoming a bit steeper at low invariant masses for larger values of mu. Our results are in very good agreement with data from the NA60 Collaboration.

hep-ph↗

arXiv2013

The nNJL model with a fractional Lorentzian regulator in the real time formalism

In this article we study the finite temperature and chemical potential effects in a nonlocal Nambu-Jona-Lasinio (nNJL) model in the real time formalism. We make the usual Wick rotation to get from imaginary to real time formalism. In doing so, we need to define our regulator in the complex plane q^2. This deffinition will be crucial in our later analysis. We study the poles in the propagator of this model and conclude that only some of them are of interst to us. Once we have a well defined model in real time formalism, we look at the chiral condensate to find the temperature at which chiral symmetry restoration will occur. We find a second order phase transition that turns to a first order one for high enough values of the chemical potential.

hep-ph↗

arXiv2013

Bottonium in QCD at finite temperature

Vector ($Υ$) and pseudoscalar ($η_b$) bottonium ground states are studied at finite temperature in the framework of thermal Hilbert moment QCD sum rules. The mass, the onset of perturbative QCD in the complex squared energy plane, $s_0$, the leptonic decay constant, and the total width are determined as a function of the temperature. Results in both channels show very little temperature dependence of the mass and of $s_0$, in line with expectations. However, the width and the leptonic decay constant exhibit a very strong $T$-dependence. The former increases with increasing temperature, as in the case of light- and heavy-light-quark systems, but close to the critical temperature, $T_c$, and for $T/T_c \simeq 0.9$ it drops dramatically approaching its value at T=0, as obtained recently in this framework for charmonium states. The leptonic decay constant is basically a monotonically increasing function of the temperature, also as obtained in the charmonium channel . These results are interpreted as the survival of these bottonium states above $T_c$, in line with lattice QCD results.

hep-ph↗

arXiv2012

Particle-antiparticle asymmetry from magnetogenesis through the Landau mechanism

Motivated by string theory an extension of the Landau problem to quantum field theory is considered. We show that the commutator between momenta of the fields violates Lorentz and CPT invariance leading to an alternative method of understanding the question of particle-antiparticle asymmetry. The presence of magnetic field at very early moments of the universe would then suggest that the particle-antiparticle asymmetry can be understood as a consequence of magnetogenesis.

hep-th↗

arXiv2012

The $a_1(1260)$ meson and chiral symmetry restoration and deconfinment at finite temperature QCD

We consider the light-quark axial-vector current correlator in the framework of thermal QCD sum rules to: (a) find a relation between chiral-symmetry restoration and deconfinement, and (b) determine the temperature behaviour of the $a_1(1260)$ width and coupling. Our results show that deconfinement takes place at a slightly lower temperature than chiral-symmetry restoration.This difference is not significant given the accuracy of the method. The behaviour of the $a_1(1260)$ parameters is consistent with quark-gluon deconfinement, since the width grows and the coupling decreases with increasing temperature.

hep-ph↗

arXiv2012

Rho-meson resonance broadening in QCD at finite temperature

Thermal Finite Energy QCD sum rules for the vector current correlator are used to study quark-gluon deconfinement. Assuming $ρ$-meson saturation of the correlator in the hadronic sector, and the Operator Product Expansion in QCD, we obtain the temperature behavior of the resonance parameters (coupling, mass, and width), and of the leading vacuum condensates, as well as the perturbative QCD threshold in the complex squared energy plane. The results are consistent with quark-gluon deconfinement at a critical temperature $T_c \simeq 197 MeV$. The temperature dependence of the $ρ$-meson width is of importance for current experiments on dimuon production in nuclear collisions.

hep-ph↗

arXiv2012

The cylindrical δ-potential and the Dirac equation

In this article we discuss the Dirac equation in the presence of an attractive cylindrical δ-shell potential V(ρ)=-aδ(ρ-ρ_0), where ρis the radial coordinate and a>0. We present a detailed discussion on the boundary conditions the wave function has to satisfy when crossing the support of the potential, proceeding then to explore the dependence of the ground state on the parameter a, analyzing the occurrence of supercritical effects. We also apply the Foldy-Wouthuysen transformation, discussing the non-relativistic limit of this problem.

hep-ph↗

arXiv2012

Noncommutativity in (2+1)-dimensions and the Lorentz group

In this article we considered models of particles living in a three-dimensional space-time with a nonstandard noncommutativity induced by shifting canonical coordinates and momenta with generators of a unitary irreducible representation of the Lorentz group. The Hilbert space gets the structure of a direct product with the representation space, where we are able to construct operators which realize the algebra of Lorentz transformations. We study the modified Landau problem for both Schrödinger and Dirac particles, whose Hamiltonians are obtained through a kind of non-Abelian Bopp's shift of the dynamical variables from the ones of the usual problem in the normal space. The spectrum of these models are considered in perturbation theory, both for small and large noncommutativity parameters. We find no constraint between the parameters referring to no-commutativity in coordinates and momenta but they rather play similar roles. Since the representation space of the unitary irreducible representations SL(2,R) can be realized in terms of spaces of square-integrable functions, we conclude that these models are equivalent to quantum mechanical models of particles living in a space with an additional compact dimension.

hep-th↗

arXiv2012

Magnetic catalysis of a charged Bose-Einstein condensate

We study the condensation phenomenon for a system of charged bosons in the presence of an external magnetic field. We show that condensation happens for a definite critical temperature instead of through a diffuse phase transition. The essential ingredient, overlooked in previous analyses and accounted for in this work, is the treatment of the plasma screening effects by means of resummation. We compute the critical temperature, for the case in which the condensate is made of charged pions and for typical densities found in compact astrophysical objects, for small and large values of the magnetic field. We show that the magnetic field catalyzes the onset of condensation at very small and at large values of the magnetic field, and that for intermediate values, the critical temperature for condensation is lower than for the zero magnetic field case.

hep-ph↗

arXiv2012

Chiral symmetry restoration and deconfinement in QCD at finite temperature

The light-quark correlator in the axial-vector channel is used, in conjunction with finite energy QCD sum rules at finite temperature, in order to (a) establish a relation between chiral-symmetry restoration and deconfinement, and (b) determine the temperature behavior of the $a_1(1260)$ width and coupling. Results indicate that deconfinement takes place at a slightly lower temperature than chiral-symmetry restoration, although this difference is not significant given the accuracy of the method. The behaviour of the $a_1(1260)$ parameters is consistent with quark-gluon deconfinement, as the width grows and the coupling decreases with increasing temperature.

hep-ph↗

arXiv2012

Graphene and non-Abelian quantization

In this article we employ a simple nonrelativistic model to describe the low energy excitation of graphene. The model is based on a deformation of the Heisenberg algebra which makes the commutator of momenta proportional to the pseudo-spin. We solve the Landau problem for the resulting Hamiltonian which reduces, in the large mass limit while keeping fixed the Fermi velocity, to the usual linear one employed to describe these excitations as massless Dirac fermions. This model, extended to negative mass, allows to reproduce the leading terms in the low energy expansion of the dispersion relation for both nearest and next-to-nearest neighbor interactions. Taking into account the contributions of both Dirac points, the resulting Hall conductivity, evaluated with a $ζ$-function approach, is consistent with the anomalous integer quantum Hall effect found in graphene. Moreover, when considered in first order perturbation theory, it is shown that the next-to-leading term in the interaction between nearest neighbor produces no modifications in the spectrum of the model while an electric field perpendicular to the magnetic field produces just a rigid shift of this spectrum. PACS: 03.65.-w, 81.05.ue, 73.43.-f

math-ph↗

arXiv2011

Spin Non-commutativity and the Three-Dimensional Harmonic Oscillator

A three-dimensional harmonic oscillator with spin non-commutativity in the phase space is considered. The system has a regular symplectic structure and by using supersymmetric quantum mechanics techniques, the ground state is calculated exactly. We find that this state is infinitely degenerate and it has explicit spontaneous broken symmetry. Analyzing the Heisenberg equations, we show that the total angular momentum is conserved.

hep-th↗

arXiv2011

Pion stability in a hot dense media

Pions may remain stable under certain conditions in a dense media at zero temperature in the normal phase (non pion superfluid state). The stability condition is achieved when the in-media pion width vanishes. However, thermal fluctuations will change this stable regime. For low temperature pions will remain in a metastable state. Here we discuss the different possible scenarios for leptonic pion decays at finite temperature, taking into account all the different chemical potentials involved. The neutrino emission due to pions in a hot-dense media is calculated, as well as the coolig rate of a pion-lepton gas.

hep-ph↗

arXiv2011

QCD phase diagram from finite energy sum rules

We study the QCD phase diagram at finite temperature and baryon chemical potential by relating the behavior of the light-quark condensate to the threshold energy for the onset of perturbative QCD. These parameters are connected to the chiral symmetry restoration and the deconfinement phase transition, respectively. This relation is obtained in the framework of finite energy QCD sum rules at finite temperature and density, with input from Schwinger-Dyson methods to determine the light-quark condensate. Results indicate that both critical temperatures are basically the same within some 3% accuracy. We also obtain bounds for the position of the critical end point, mu_{B c} >~ 300 MeV and T_c <~ 185 MeV.

hep-ph↗