3,348 research outputs found
Fluctuations of fragment observables
This contribution presents a review of our present theoretical as well as
experimental knowledge of different fluctuation observables relevant to nuclear
multifragmentation. The possible connection between the presence of a
fluctuation peak and the occurrence of a phase transition or a critical
phenomenon is critically analyzed. Many different phenomena can lead both to
the creation and to the suppression of a fluctuation peak. In particular, the
role of constraints due to conservation laws and to data sorting is shown to be
essential. From the experimental point of view, a comparison of the available
fragmentation data reveals that there is a good agreement between different
data sets of basic fluctuation observables, if the fragmenting source is of
comparable size. This compatibility suggests that the fragmentation process is
largely independent of the reaction mechanism (central versus peripheral
collisions, symmetric versus asymmetric systems, light ions versus heavy ion
induced reactions). Configurational energy fluctuations, that may give
important information on the heat capacity of the fragmenting system at the
freeze out stage, are not fully compatible among different data sets and
require further analysis to properly account for Coulomb effects and secondary
decays. Some basic theoretical questions, concerning the interplay between the
dynamics of the collision and the fragmentation process, and the cluster
definition in dense and hot media, are still open and are addressed at the end
of the paper. A comparison with realistic models and/or a quantitative analysis
of the fluctuation properties will be needed to clarify in the next future the
nature of the transition observed from compound nucleus evaporation to
multi-fragment production.Comment: Contribution to WCI (World Consensus Initiative) Book " "Dynamics and
Thermodynamics with Nuclear Degrees of Freedom", to appear on Euorpean
Physics Journal A as part of the Topical Volume. 9 pages, 12 figure
Tracking energy fluctuations from fragment partitions in the Lattice Gas model
Partial energy fluctuations are known tools to reconstruct microcanonical
heat capacities. For experimental applications, approximations have been
developed to infer fluctuations at freeze out from the observed fragment
partitions. The accuracy of this procedure as well as the underlying
independent fragment approximation is under debate already at the level of
equilibrated systems. Using a well controlled computer experiment, the Lattice
Gas model, we critically discuss the thermodynamic conditions under which
fragment partitions can be used to reconstruct the thermodynamics of an
equilibrated system.Comment: version accepted for publication in Phys.Rev.
Comment on "Partial energies fluctuations and negative heat capacities" by X. Campi et al
Studying the energy partioning published in nucl-th/0406056v2 we show that
the presented results do not fulfill the sum rule due to energy conservation.
The observed fluctuations of the energy conservation test point to a numerical
problem. Moreover, analysis of the binding energies show that the fragment
recognition algorithm adopted by Campi et al. leads with a sizeable probability
to fragments containing up to the total mass even for excitation energies as
large as 3/4 of the total binding. This surprising result points to another
problem since the published inter-fragment energy is not zero while a unique
fragment is present. This problem may be due to either the fragment recognition
algorithm or to the definition of the inter and intra-fragment energy. These
numerical inconsistencies should be settled before any conclusion on the
physics can be drawn
Experimental Signals of Phase Transition
The connection between the thermodynamics of charged finite nuclear systems
and the asymptotically measured partitions is presented. Some open questions,
concerning in particular equilibrium partitions are discussed. We show a
detailed comparison of the decay patterns in Au+ C,Cu,Au central collisions and
in Au quasi-projectile events. Observation of abnormally large fluctuations in
carefully selected samples of data is reported as an indication of a first
order phase transition (negative heat capacity) in the nuclear equation of
state.Comment: 8 pages, 8th International Conference on Nucleus-Nucleus Collisions,
Moscow 200
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