9 research outputs found
Isospin Fluctuations from a Thermally Equilibrated Hadron Gas
Partition functions, multiplicity distributions, and isospin fluctuations are
calculated for canonical ensembles in which additive quantum numbers as well as
total isospin are strictly conserved. When properly accounting for
Bose-Einstein symmetrization, the multiplicity distributions of neutral pions
in a pion gas are significantly broader as compared to the non-degenerate case.
Inclusion of resonances compensates for this broadening effect. Recursion
relations are derived which allow calculation of exact results with modest
computer time.Comment: 10 pages, 5 figure
Forming Disoriented Chiral Condensates through Fluctuations
Using the influence functional formalism, classical equations of motion for
the O(N) model are derived in the presence of a heat bath, in both the
symmetric phase as well as the phase of spontaneously broken symmetry. The heat
bath leads to dissipation and fluctuation terms in the classical equations of
motion, which are explicitly computed to lowest order in perturbation theory.
In the broken phase these terms are found to be large for the sigma field, even
at zero temperature, due to the decay process sigma -> pi pi, while they are
small for the pi fields at temperatures below T_c = 160 MeV. It is shown that
in large volumes the presence of dissipation and fluctuations suppresses the
formation of disoriented chiral condensates (DCC's). In small volumes, however,
fluctuations become sufficiently large to induce the formation of DCC's even if
chiral symmetry has not been restored in the initial stage of the system's
evolution.Comment: 34 pages, 11 figures, ReVTeX, eps-, aps-, psfig-style files require
Nonequilibrium Quantum Dynamics Of Disoriented Chiral Condensates
The nonequilibrium dynamics of the chiral phase transition expected during
the expansion of the quark-qluon plasma produced in a high energy hadron or
heavy ion collision is studied in the O(4) linear sigma model to leading order
in a large expansion. Starting from an approximate equilibrium
configuration at an initial proper time in the disordered phase we study
the transition to the ordered broken symmetry phase as the system expands and
cools. We give results for the proper time evolution of the effective pion
mass, the order parameter as well as for the pion two point
correlation function expressed in terms of a time dependent phase space number
density and pair correlation density. We determine the phase space of initial
conditions that lead to instabilities (exponentially growing long wave length
modes) as the system evolves in time. These instabilities are what eventually
lead to disoriented chiral condensates. In our simulations,we found that
instabilities that are formed during the initial phases of the expansion exist
for proper times that are at most and lead to condensate regions that
do not contain large numbers of particles. The damping of instabilities is a
consequence of strong coupling.Comment: 49 pages, figures available by reques
Astrobiology and the possibility of life on Earth and elsewhere…
Astrobiology is an interdisciplinary scientific field not only focused on the search of extraterrestrial life, but also on deciphering the key environmental parameters that have enabled the emergence of life on Earth. Understanding these physical and chemical parameters is fundamental knowledge necessary not only for discovering life or signs of life on other planets, but also for understanding our own terrestrial environment. Therefore, astrobiology pushes us to combine different perspectives such as the conditions on the primitive Earth, the physicochemical limits of life, exploration of habitable environments in the Solar System, and the search for signatures of life in exoplanets. Chemists, biologists, geologists, planetologists and astrophysicists are contributing extensively to this interdisciplinary research field. From 2011 to 2014, the European Space Agency (ESA) had the initiative to gather a Topical Team of interdisciplinary scientists focused on astrobiology to review the profound transformations in the field that have occurred since the beginning of the new century. The present paper is an interdisciplinary review of current research in astrobiology, covering the major advances and main outlooks in the field. The following subjects will be reviewed and most recent discoveries will be highlighted: the new understanding of planetary system formation including the specificity of the Earth among the diversity of planets, the origin of water on Earth and its unique combined properties among solvents for the emergence of life, the idea that the Earth could have been habitable during the Hadean Era, the inventory of endogenous and exogenous sources of organic matter and new concepts about how chemistry could evolve towards biological molecules and biological systems. In addition, many new findings show the remarkable potential life has for adaptation and survival in extreme environments. All those results from different fields of science are guiding our perspectives and strategies to look for life in other Solar System objects as well as beyond, in extrasolar worlds