1,950 research outputs found
Aspects on Effective Theories and the QCD transition
We review recent advances in the understanding of the Quantum Chromodynamics
(QCD) transition and its nature, paying special attention to the analysis of
chiral symmetry restoration within different approaches based on effective
theories. After presenting some of the main aspects of the current knowledge of
the phase diagram from the theoretical, experimental and lattice sides, we
discuss some recent problems where approaches relying on effective theories
have been particularly useful. In particular, the combination of ideas such as
Chiral Perturbation Theory, unitarity and Ward Identities allows us to describe
successfully several observables of interest. This is particularly relevant for
quantities expected to be dominated by the light meson components of the hadron
gas such as the scalar and topological susceptibilities. In addition, ward
identities and effective Lagrangians provide systematic results regarding
chiral and partner degeneration properties which are of great
importance for the interplay between those two transitions and the nature of
chiral symmetry restoration. Special attention is paid to the connection of
this theoretical framework with lattice simulations.Comment: Invited review paper submitted to Symmetry Journal, special issue
"Effective Field Theories - Chiral Perturbation Theory and Non-relativistic
QFT". 28 pages, 15 figures. Version accepted for publication. Various
comments and one reference adde
Transport properties of a meson gas
We present recent results on a systematic method to calculate transport
coefficients for a meson gas (in particular, we analyze a pion gas) at low
temperatures in the context of Chiral Perturbation Theory. Our method is based
on the study of Feynman diagrams with a power counting which takes into account
collisions in the plasma by means of a non-zero particle width. In this way, we
obtain results compatible with analysis of Kinetic Theory with just the leading
order diagram. We show the behavior with temperature of electrical and thermal
conductivities and shear and bulk viscosities, and we discuss the fundamental
role played by unitarity. We obtain that bulk viscosity is negligible against
shear viscosity near the chiral phase transition. Relations between the
different transport coefficients and bounds on them based on different
theoretical approximations are also discussed. We also comment on some
applications to heavy-ion collisions.Comment: 4 pages, 4 figures, IJMPE style. Contribution to the International
Workshop X Hadron Physics (2007), Florianopolis, Brazil. Accepted for
publication in IJMPE; 1 typo correcte
Bulk viscosity and the conformal anomaly in the pion gas
We calculate the bulk viscosity of the massive pion gas within Unitarized
Chiral Perturbation Theory. We obtain a low temperature peak arising from
explicit conformal breaking due to the pion mass and another peak near the
critical temperature, dominated by the conformal anomaly through gluon
condensate terms. The correlation between bulk viscosity and conformal breaking
supports a recent QCD proposal. We discuss the role of resonances, heavier
states and large- counting.Comment: Revised version accepted in Phys.Rev.Lett. 4 pages, 3 figure
Light quarks at finite temperature: chiral restoration and the fate of the symmetry
We review recent results on the role of light quark states within the QCD
phase diagram. In particular, we will discuss how the combined use of
theoretical techniques such as Effective Theories, Unitarization and Ward
Identities helps to shed light on several important issues regarding chiral
symmetry restoration, building bridges with recent lattice analyses. Special
attention will be paid to the role of chiral and partners in the
interplay between those symmetries, which is crucial to properly understand the
transition pattern. Light scalar mesons at finite temperature will be shown to
be responsible for the description of susceptibilities encoding chiral and
restoration properties.Comment: Invited review paper submitted to European Physical Journal Special
Topics. 18 pages, 11 figures. Version accepted for publicatio
The Inverse Amplitude Method and Adler Zeros
The Inverse Amplitude Method is a powerful unitarization technique to enlarge
the energy applicability region of Effective Lagrangians. It has been widely
used to describe resonances from Chiral Perturbation Theory as well as for the
Strongly Interacting Symmetry Breaking Sector. In this work we show how it can
be slightly modified to account also for the sub-threshold region,
incorporating correctly the Adler zeros required by chiral symmetry and
eliminating spurious poles. These improvements produce negligible effects on
the physical region.Comment: 17 pages, 4 figure
One loop calculations on the Wess-Zumino-Witten anomalous functional at finite temperature
We analyze the finite temperature (T) extension of the Wess-Zumino- Witten
functional, discussed in a previous work, to one loop in chiral perturbation
theory. As a phenomenological application, we calculate finite temperature
corrections to the amplitude of the decay into two photons. This
calculation is performed in three limits : i), ii)the chiral
limit at finite T and iii) ( being the pion mass). The
-corrections tend to vanish in the chiral limit, where only the kaon
contribution remains (although it is exponentially suppressed).Comment: Latex, 13 pages and 3 figures avalaible upon reques
Pion scattering poles and chiral symmetry restoration
Using unitarized Chiral Perturbation Theory methods, we perform a detailed
analysis of the scattering poles and behaviour
when medium effects such as temperature or density drive the system towards
Chiral Symmetry Restoration. In the analysis of real poles below threshold, we
show that it is crucial to extend properly the unitarized amplitudes so that
they match the perturbative Adler zeros. Our results do not show threshold
enhancement effects at finite temperature in the channel, which
remains as a pole of broad nature. We also implement T=0 finite density effects
related to chiral symmetry restoration, by varying the pole position with the
pion decay constant. Although this approach takes into account only a limited
class of contributions, we reproduce the expected finite density restoration
behaviour, which drives the poles towards the real axis, producing threshold
enhancement and bound states. We compare our results with several
model approaches and discuss the experimental consequences, both in
Relativistic Heavy Ion Collisions and in and
reactions in nuclei.Comment: 17 pages, 9 figures, final version to appear in Phys.Rev.D, added
comments and reference
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