5,333 research outputs found
On the dynamical breaking of chiral symmetry: a new mechanism
We consider a U(1) gauge theory, minimally coupled to a massless Dirac field,
where a higher-derivative term is added to the pure gauge sector, as in the
Lee-Wick models. We find that this term can trigger chiral symmetry breaking at
low energy in the weak coupling regime. Then, the fermion field acquires a mass
that turns out to be a function of both the energy scale associated to the
higher-derivative term and the gauge coupling. The dependence of the fermion
mass on the gauge coupling is non-perturbative. Extensions to SU(N) gauge
theories and fermion-scalar interactions are also analyzed, as well as to
theories with massive gauge fields. A few implications of these results in the
framework of quark-mass generation are discussed.Comment: 15 pages 2 figures, a few comments and 4 references added. To appear
in Physical Review
Comparison of lunar rocks and meteorites: Implications to histories of the moon and parent meteorite bodies
A number of similarities between lunar and meteoritic rocks are reported and suggest that the comparison is essential for a clear understanding of meteorites as probes of the early history of the solar systems: (1) Monomict and polymict breccias occur in lunar rocks, as well as in achondritic and chondritic meteorites, having resulted from complex and repeated impact processes. (2) Chondrules are present in lunar, as well as in a few achondritic and most chondritic meteorites. It is pointed out that because chondrules may form in several different ways and in different environments, a distinction between the different modes of origin and an estimate of their relative abundance is important if their significance as sources of information on the early history of the solar system is to be clearly understood. (3) Lithic fragments are very useful in attempts to understand the pre- and post-impact history of lunar and meteoritic breccias. They vary from little modified (relative to the apparent original texture), to partly or completely melted and recrystallized lithic fragments
Spontaneous symmetry breaking in a two-doublet lattice Higgs model
An SU(2) lattice gauge theory with two doublets of complex scalar fields is
considered. All continuous symmetries are identified and, using the
nonperturbative methods of lattice field theory, the phase diagram is mapped
out by direct numerical simulation. Two-doublet models contain phase
transitions that separate qualitatively distinct regions of the parameter
space. In some regions global symmetries are spontaneously broken. For some
special choices of the model parameters, the symmetry-breaking order parameter
is calculated. The pattern of symmetry breaking is verified further through
observation of Goldstone bosons.Comment: 24 pages, 13 figures, references added, published versio
QCD matter within a quasi-particle model and the critical end point
We compare our quasi-particle model with recent lattice QCD results for the
equation of state at finite temperature and baryo-chemical potential. The
inclusion of the QCD critical end point into models is discussed. We propose a
family of equations of state to be employed in hydrodynamical calculations of
particle spectra at RHIC energies and compare with the differential azimuthal
anisotropy of strange and charm hadrons.Comment: talk at Quark Matter 2005, August 4 - 9, 2005, Budapest, Hungar
Two flavor color superconductivity in nonlocal chiral quark models
We study the competence between chiral symmetry restoration and two flavor
color superconductivity (2SC) using a relativistic quark model with covariant
nonlocal interactions. We consider two different nonlocal regulators: a
Gaussian regulator and a Lorentzian regulator. We find that although the phase
diagrams are qualitative similar to those obtained using models with local
interactions, in our case the superconducting gaps at medium values of the
chemical potential are larger. Consequently, we obtain that in that region the
critical temperatures for the disappearance of the 2SC phase might be of the
order of 100-120 MeV. We also find that for ratios of the quark-quark and
quark-antiquark couplings somewhat above the standard value 3/4, the end point
and triple point in the phase diagram meet and a phase where both the
chiral and diquark condensates are non-negligible appears.Comment: 15 pages incl. 5 Postscript figure
Speed Sensorless mixed sensitivity linear parameter variant H_inf control of the induction motor
The paper shows the design of a robust control structure for the speed sensorless vector control of the IM, based on the mixed sensitivity (MS) linear parameter variant (LPV) H8 control theory. The controller makes possible the direct control of the flux and speed of the motor with torque adaptation in noisy environment. The whole control system is tested by intensive simulations and according to the results it shows good dynamic and robust performance. Implementation issues based on a DSP TMS320F243 development platform are also presented
Cognitive Computation sans Representation
The Computational Theory of Mind (CTM) holds that cognitive processes are essentially computational, and hence computation provides the scientific key to explaining mentality. The Representational Theory of Mind (RTM) holds that representational content is the key feature in distinguishing mental from non-mental systems. I argue that there is a deep incompatibility between these two theoretical frameworks, and that the acceptance of CTM provides strong grounds for rejecting RTM. The focal point of the incompatibility is the fact that representational content is extrinsic to formal procedures as such, and the intended interpretation of syntax makes no difference to the execution of an algorithm. So the unique 'content' postulated by RTM is superfluous to the formal procedures of CTM. And once these procedures are implemented in a physical mechanism, it is exclusively the causal properties of the physical mechanism that are responsible for all aspects of the system's behaviour. So once again, postulated content is rendered superfluous. To the extent that semantic content may appear to play a role in behaviour, it must be syntactically encoded within the system, and just as in a standard computational artefact, so too with the human mind/brain - it's pure syntax all the way down to the level of physical implementation. Hence 'content' is at most a convenient meta-level gloss, projected from the outside by human theorists, which itself can play no role in cognitive processing
Thermodynamics of lattice QCD with 3 flavours of colour-sextet quarks
We have been studying QCD with 2 flavours of colour-sextet quarks to
distinguish whether it is QCD-like or conformal. For comparison we are now
studying QCD with 3 flavours of colour-sextet quarks, which is believed to be
conformal in the chiral limit. Here we present the results of simulations of
lattice QCD with 3 colour-sextet quarks at finite temperatures on lattices of
temporal extent and 6, with masses small enough to yield access to the
chiral limit. As for the 2-flavour case, we find well-separated deconfinement
and chiral-symmetry restoration transitions, both of which move to appreciably
weaker couplings as is increased from 4 to 6. If this theory is
conformal, we would expect there to be a bulk chiral transition at a fixed
coupling. For this reason we conclude that for and 6, the chiral and
hence the deconfinement transitions are in the strong-coupling domain where the
theory is essentially quenched. The similarity between the behaviours of the 2
and 3 flavour theories suggested that the and 6 transitions for the
2-flavour theory also lie in the strong-coupling domain. The phase structure of
both theories is very similar.Comment: 17 pages Latex(Revtex), 7 postscript figure
A proper understanding of Millikan
Ruth Millikan’s teleological theory of mental content is complex and often misunderstood. This paper motivates and clarifies some of the complexities of the theory, and shows that paying careful attention to its details yields answers to a number of common objections to teleological theories, in particular, the problem of novel mental states, the problem of functionally false beliefs, and problems about indeterminacy or multiplicity of function
Thermodynamics of Two Flavor QCD to Sixth Order in Quark Chemical Potential
We present results of a simulation of 2-flavor QCD on a 4x16^3 lattice using
p4-improved staggered fermions with bare quark mass m/T=0.4. Derivatives of the
thermodynamic grand canonical partition function Z(V,T,mu_u,mu_d) with respect
to chemical potentials mu_(u,d) for different quark flavors are calculated up
to sixth order, enabling estimates of the pressure and the quark number density
as well as the chiral condensate and various susceptibilities as functions of
mu_q = (mu_u + mu_d)/2 via Taylor series expansion. Furthermore, we analyze
baryon as well as isospin fluctuations and discuss the relation between the
radius of convergence of the Taylor series and the chiral critical point in the
QCD phase diagram. We argue that bulk thermodynamic observables do not, at
present, provide direct evidence for the existence of a chiral critical point
in the QCD phase diagram. Results are compared to high temperature perturbation
theory as well as a hadron resonance gas model.Comment: 38 pages, 30 encapsulated postscript figures, typo corrected, 1
footnote adde
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