452 research outputs found
Negative heat capacity in the critical region of nuclear fragmentation: an experimental evidence of the liquid-gas phase transition
An experimental indication of negative heat capacity in excited nuclear
systems is inferred from the event by event study of energy fluctuations in
quasi-projectile sources formed in collisions at 35 A.MeV. The
excited source configuration is reconstructed through a calorimetric analysis
of its de-excitation products. Fragment partitions show signs of a critical
behavior at about 5 A.MeV excitation energy. In the same energy range the heat
capacity shows a negative branch providing a direct evidence of a first order
liquid gas phase transition.Comment: 4 Postscript figures, submitted to Phys. Rev. Lett. on 14-apr-199
On the reliability of negative heat capacity measurements
A global protocol for the thermostatistical analysis of hot nuclear sources
is discussed. Within our method of minimization of variances we show that the
abnormal kinetic energy fluctuation signal recently reported in different
experimental data (M.D'Agostino et al.-Phys. Lett. B 473 (2000) 219, N. Le
Neindre et al.- contr. to the XXXVIII Bormio Winter Meeting on Nucl. Phys.
(2001) 404) is a genuine signal of a first order phase transition in a finite
system.Comment: 15 Postscript figures, submitted to NUCL. Phys. A on 24-apr-200
Contemporary presence of dynamical and statistical production of intermediate mass fragments in midperipheral Ni+Ni collisions at 30 MeV/nucleon
The reaction at 30 MeV/nucleon has been experimentally
investigated at the Superconducting Cyclotron of the INFN Laboratori Nazionali
del Sud. In midperipheral collisions the production of massive fragments
(4Z12), consistent with the statistical fragmentation of the
projectile-like residue and the dynamical formation of a neck, joining
projectile-like and target-like residues, has been observed. The fragments
coming from these different processes differ both in charge distribution and
isotopic composition. In particular it is shown that these mechanisms leading
to fragment production act contemporarily inside the same event.Comment: 9 pages, minor correction
Size and asymmetry of the reaction entrance channel: influence on the probability of neck production
The results of experiments performed to investigate the Ni+Al, Ni+Ni, Ni+Ag
reactions at 30 MeV/nucleon are presented. From the study of dissipative
midperipheral collisions, it has been possible to detect events in which
Intermediate Mass Fragments (IMF) production takes place. The decay of a
quasi-projectile has been identified; its excitation energy leads to a
multifragmentation totally described in terms of a statistical disassembly of a
thermalized system (T4 MeV, E4 MeV/nucleon). Moreover, for
the systems Ni+Ni, Ni+Ag, in the same nuclear reaction, a source with velocity
intermediate between that of the quasi-projectile and that of the quasi-target,
emitting IMF, is observed. The fragments produced by this source are more
neutron rich than the average matter of the overall system, and have a charge
distribution different, with respect to those statistically emitted from the
quasi-projectile. The above features can be considered as a signature of the
dynamical origin of the midvelocity emission. The results of this analysis show
that IMF can be produced via different mechanisms simultaneously present within
the same collision. Moreover, once fixed the characteristics of the
quasi-projectile in the three considered reactions (in size, excitation energy
and temperature), one observes that the probability of a partner IMF production
via dynamical mechanism has a threshold (not present in the Ni+Al case) and
increases with the size of the target nucleus.Comment: 16 pages, 7 figures, accepted for publication on Nuclear Physics
Statistical Multifragmentation in Central Au+Au Collisions at 35 MeV/u
Multifragment disintegrations, measured for central Au + Au collisions at E/A
= 35 MeV, are analyzed with the Statistical Multifragmentation Model. Charge
distributions, mean fragment energies, and two-fragment correlation functions
are well reproduced by the statistical breakup of a large, diluted and
thermalized system slightly above the multifragmentation threshold.Comment: Latex file, 8 pages + 4 postscript figures available upon request
from [email protected]
Multifragment production in Au+Au at 35 MeV/u
Multifragment disintegration has been measured with a high efficiency
detection system for the reaction at . From the event
shape analysis and the comparison with the predictions of a many-body
trajectories calculation the data, for central collisions, are compatible with
a fast emission from a unique fragment source.Comment: 9 pages, LaTex file, 4 postscript figures available upon request from
[email protected]. - to appear in Phys. Lett.
New data on OZI rule violation in bar{p}p annihilation at rest
The results of a measurement of the ratio R = Y(phi pi+ pi-) / Y(omega pi+
pi-) for antiproton annihilation at rest in a gaseous and in a liquid hydrogen
target are presented. It was found that the value of this ratio increases with
the decreasing of the dipion mass, which demonstrates the difference in the phi
and omega production mechanisms. An indication on the momentum transfer
dependence of the apparent OZI rule violation for phi production from the 3S1
initial state was found.Comment: 11 pages, 3 PostScript figures, submitted to Physics Letter
Anisotropic flow of charged hadrons, pions and (anti-)protons measured at high transverse momentum in Pb-Pb collisions at TeV
The elliptic, , triangular, , and quadrangular, , azimuthal
anisotropic flow coefficients are measured for unidentified charged particles,
pions and (anti-)protons in Pb-Pb collisions at TeV
with the ALICE detector at the Large Hadron Collider. Results obtained with the
event plane and four-particle cumulant methods are reported for the
pseudo-rapidity range at different collision centralities and as a
function of transverse momentum, , out to GeV/.
The observed non-zero elliptic and triangular flow depends only weakly on
transverse momentum for GeV/. The small dependence
of the difference between elliptic flow results obtained from the event plane
and four-particle cumulant methods suggests a common origin of flow
fluctuations up to GeV/. The magnitude of the (anti-)proton
elliptic and triangular flow is larger than that of pions out to at least
GeV/ indicating that the particle type dependence persists out
to high .Comment: 16 pages, 5 captioned figures, authors from page 11, published
version, figures at http://aliceinfo.cern.ch/ArtSubmission/node/186
Centrality dependence of charged particle production at large transverse momentum in Pb-Pb collisions at TeV
The inclusive transverse momentum () distributions of primary
charged particles are measured in the pseudo-rapidity range as a
function of event centrality in Pb-Pb collisions at
TeV with ALICE at the LHC. The data are presented in the range
GeV/ for nine centrality intervals from 70-80% to 0-5%.
The Pb-Pb spectra are presented in terms of the nuclear modification factor
using a pp reference spectrum measured at the same collision
energy. We observe that the suppression of high- particles strongly
depends on event centrality. In central collisions (0-5%) the yield is most
suppressed with at -7 GeV/. Above
GeV/, there is a significant rise in the nuclear modification
factor, which reaches for GeV/. In
peripheral collisions (70-80%), the suppression is weaker with almost independently of . The measured nuclear
modification factors are compared to other measurements and model calculations.Comment: 17 pages, 4 captioned figures, 2 tables, authors from page 12,
published version, figures at
http://aliceinfo.cern.ch/ArtSubmission/node/284
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