613 research outputs found
A remark on the low mass dilepton yield in heavy ion collisions
The recent data on the enhancement of the low mass dilepton yield in heavy
ion collisions are interpreted as an effect of the "prolonged life" of
resonances in the hadron gas phase. The value of the enhancement factor gives
an upper limit for the duration time of this phase.Comment: Minor changes in text, one reference added. Version to be published
in the XXXVI ISMD proceedings, Brazilian Journal of Physic
Selecting the diffractive events at the LHC energies
The PYTHIA 8 generator is used to estimate the percentage of non-diffractive
and diffractive events at the LHC energies. It is shown that a simple condition
of the absence of charged hadrons in the central pseudorapidity region is
sufficient to remove almost all non-diffractive events. This opens the way to
investigate diffraction without waiting for the future specialized detectors.Comment: 4 page
Has our brain grown too big to think effectively?
A variant of microcephalin, MCPH1 gene, was introgressed about 37,000 years
ago into Homo sapiens genetic pool from an archaic (Homo erectus) lineage and
rose to exceptionally high frequency of around 70 percent worldwide today. It
is involved in regulating neuroblast proliferation and its changes alter the
rate of division and/or differentiation of neuroblasts during the neurogenic
phase of embriogenesis, which could alter the size and structure of the
resulting brain.
At the time of introgression, images had already been painted on the walls of
caves and speech has been in use for over 100,000 years, as had been abstract
thinking. Like today, reasoning and thinking were the primary faculties of
individuals. Homo erectus either did not possess those faculties or was
markedly inferior to Homo sapiens in them. Its brain was smaller and the cortex
was apparently less convoluted. Thus, introgressed microcephalin allele
directed neurogenesis evolutionary back to less complicated brain structure
typical for our evolutionary forefathers, slightly decreasing the level of
complexity already achieved by Homo sapiens 37,000 years ago. Despite that, it
proliferated at a rapid pace.
It yields a supposition: 37,000 years ago the brains of Homo sapiens were too
big and too complicated for the kind of thinking needed for the highest fitness
of individuals. Since adaptation cannot by definition surpass selection
requirements, the volume and complication of the human brain did not originate
under selective pressure to improve effective thinking and they cannot be
explained in terms of such selection.
A proposal to solve this quandary is presented, claiming that Homo sapiens
originated just by chance. Endurance running led to the emergence of Homo
sapiens. The human mind and larynx used for speech are side-effects of more
than a million years of endurance running by pre-human hunters
Tilted plateau in nuclear collisions: data, models and MC-s
The linear dependence of the particle spectra on rapidity is seen in the
central region for asymmetric heavy ion collision in the data and in the Monte
Carlo results, similarly as in the fragmentation region for hadronic and ion
collisions. The origin of such a behaviour is discussed. It is shown that the
color string models produce naturally such a shape if string ends are randomly
distributed in rapidity.Comment: 11 pages, 11 figure
RHIC multiplicity distributions and superposition models
The recent PHENIX mid-rapidity measurements of multiplicity distributions for
centrality bins are analyzed in the framework of superposition models. A simple
superposition of pp events is shown to disagree with the heavy ion data for
dispersion as a function of centrality. However, it is suggested that a model
describing properly the pp data and based in the "wounded quark" idea may be
compatible with the multiplicity data for heavy ion collisions.Comment: 7 pages, 2 figures Fig. 1 corrected, minor changes in text, one
reference adde
On the new HERMES data for the electroproduction on nuclei
We analyze recent data on the electroproduction of hadrons on nuclei using
the Lund model for electroproduction on nucleons and a simple geometrical model
for the absorption effects. We show that the model seems to overestimate the
A-dependence of the absorption effects, although it described the earlier data
of the same HERMES experiment reasonably well. We trace the origin of this
discrepancy to the surprising difference between the data for nitrogen and
neon.Comment: 5 pages, 3 figures, version to be published in the ISMD2007
proceedings in Acta Phys. Pol.
Entropy in cluster analysis of single events in heavy ion collisions
We analyse the cluster structure of the final multihadron states resulting
from heavy ion collisions using the concept of Jaynes - Shannon entropy.
Further evidence for an interesting differentiation of events is provided.Comment: 6 pages, 6 eps figures, 1 tex fil
Event-by-event cluster analysis of final states from heavy ion collisions
We present an event-by-event analysis of the cluster structure of final
multihadron states resulting from heavy ion collisions. A comparison of
experimental data with the states obtained from Monte Carlo generators is
shown. The analysis of the first available experimental events suggests that
the method is suitable for selecting some different types of events.Comment: 7 pages, 4 eps figure
Charge fluctuations in a final state with QGP
Charge fluctuations as a possible signal of quark - gluon plasma (QGP) were
recently suggested. A short summary of comments presented on this subject is
given and supplemented by a discussion of the coexistence of pions produced
"directly" and through a QGP phase. Such a coexistence may obscure the expected
plasma signal similarly to the effects considered in the comments mentioned
above.Comment: 5 page
Parameters in Weight Calculations for the BE Effect
The weight method of implementing the BE effect into Monte Carlo generators
is discussed and presented in some detail. We show how the choice of free
parameters and the definition of "direct" pions influence the results for the
hadronic Z0 decays.Comment: 15 pages, 7 figure
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