Célia A de Sousa

Phase diagram and critical properties within an effective model of QCD: the Nambu-Jona-Lasinio model coupled to the Polyakov loop

We investigate the phase diagram of the so-called Polyakov–Nambu–Jona-Lasinio model at finite temperature and non-zero chemical potential with three quark flavors. Chiral and deconfinement phase transitions are discussed and the relevant order-like parameters are analyzed. The results are compared with simple thermodynamic expectations and lattice data. We present the phase diagram in the (T, μB) plane, paying special attention to the critical end point: as the strength of the flavor-mixing interaction becomes weaker, the critical end point moves to low temperatures and can even disappear.

Mass redistribution in variable mass systems

We have developed an alternative formulation based on F = Ma rather than F = d P/d t for studying variable mass systems. It is shown that F = Ma can be particularly useful for analysing systems with variable mass, as illustrated by various examples involving chains and ropes. The method implies the division of the whole system into two parts, which are considered separately, allowing us to explore certain aspects as constraint forces.

An Argon Gas Scintillation Counter with Uniform Electric Field

A new type of gas proportional scintillation counter with uniform electric field is described. This counter is the scintillation counter analog of the Frish ionization chamber. Its principle is discussed and the properties experimentally studied using pure argon. Energy resolutions of 1.9% were obtained for 8.1 MeV alpha particles. Some results concerning the interpretation of the mechanism of the secondary scintillation are discussed.

Nonrigid systems: mechanical and thermodynamic aspects

The motion of a uniform chain has been used to exemplify systems with variable mass. The discussion is usually confined to the dynamical aspects of the problem or to other derivations that use energy-conserving methods. Here this type of problem is re-examined by considering the chain as a closed one-dimensional motion, showing that the concept of centre of mass can be particularly useful in this kind of multi-particle system. It is similarly instructive to analyse the work-energy processes from the viewpoint of the first law of thermodynamics.

The Hartree-Fock and Generator Coordinate Methods in field theory. The polaron problem

Abstract Well known results of the strong coupling theory of the polaron are derived by simple variational methods (The Hartree-Fock and the Generator Coordinate Methods) based on coherent-state-wave-functions describing the electron surrounded by a phonon cloud.

Atwood's machine as a tool to introduce variable mass systems

This article discusses an instructional strategy which explores eventual similarities and/or analogies between familiar problems and more sophisticated systems. In this context, the Atwoods machine problem is used to introduce students to more complex problems involving ropes and chains. The methodology proposed helps students to develop the ability needed to apply relevant concepts in situations not previously encountered. The pedagogical advantages are relevant for both secondary and high school students, showing that, through adequate examples, the question of the validity of Newtons second law may even be introduced to introductory level students.

Falling chains as variable-mass systems: theoretical model and experimental analysis

In this paper, we revisit, theoretically and experimentally, the fall of a folded U-chain and of a pile-chain. The model calculation implies the division of the whole system into two subsystems of variable mass, allowing us to explore the role of tensional contact forces at the boundary of the subsystems. This justifies, for instance, that the folded U-chain falls faster than the acceleration due to the gravitational force. This result, which matches quite well with the experimental data independently of the type of chain, implies that the falling chain is well described by energy conservation. We verify that these conclusions are not observed for the pile-chain motion.