1,134 research outputs found
Monte Carlo simulation of events with Drell-Yan lepton pairs from antiproton-proton collisions: the fully polarized case
In this paper, we extend the study of Drell-Yan processes with antiproton
beams already presented in a previous work. We consider the fully polarized
process, because this is the
simplest scenario for extracting the transverse spin distribution of quarks, or
transversity, which is the missing piece to complete the knowledge of the
nucleon spin structure at leading twist. We perform Monte Carlo simulations for
transversely polarized antiproton and proton beams colliding at a
center-of-mass energy of interest for the future HESR at GSI. The goal is to
possibly establish feasibility conditions for an unambiguous extraction of the
transversity from data on double spin asymmetries.Comment: Produced in RevTeX 4, 10 figures in .eps forma
First extraction of Interference Fragmentation Functions from e+e- data
We report on the first extraction of interference fragmentation functions
from the semi-inclusive production of two hadron pairs in back-to-back jets in
e+e- annihilation. A nonzero asymmetry in the correlation of azimuthal
orientations of opposite \pi+\pi- pairs is related to the transverse
polarization of fragmenting quarks through a significant polarized dihadron
fragmentation function. Extraction of the latter requires the knowledge of its
unpolarized counterpart, the probability density for a quark to fragment in a
\pi+\pi- pair. Since data for the unpolarized cross section are missing, we
extract the unpolarized dihadron fragmentation function from a Monte Carlo
simulation of the cross section.Comment: 17 pages, 7 (multiple) figures, 15 tables, RevTeX format; refined
version of the fit, conclusions unchanged; added referenc
Experience versus Talent Shapes the Structure of the Web
We use sequential large-scale crawl data to empirically investigate and
validate the dynamics that underlie the evolution of the structure of the web.
We find that the overall structure of the web is defined by an intricate
interplay between experience or entitlement of the pages (as measured by the
number of inbound hyperlinks a page already has), inherent talent or fitness of
the pages (as measured by the likelihood that someone visiting the page would
give a hyperlink to it), and the continual high rates of birth and death of
pages on the web. We find that the web is conservative in judging talent and
the overall fitness distribution is exponential, showing low variability. The
small variance in talent, however, is enough to lead to experience
distributions with high variance: The preferential attachment mechanism
amplifies these small biases and leads to heavy-tailed power-law (PL) inbound
degree distributions over all pages, as well as over pages that are of the same
age. The balancing act between experience and talent on the web allows newly
introduced pages with novel and interesting content to grow quickly and surpass
older pages. In this regard, it is much like what we observe in high-mobility
and meritocratic societies: People with entitlement continue to have access to
the best resources, but there is just enough screening for fitness that allows
for talented winners to emerge and join the ranks of the leaders. Finally, we
show that the fitness estimates have potential practical applications in
ranking query results
Enhancement of Tc in the Superconductor-Insulator Phase Transition on Scale-Free Networks
A road map to understand the relation between the onset of the
superconducting state with the particular optimum heterogeneity in granular
superconductors is to study a Random Tranverse Ising Model on complex networks
with a scale-free degree distribution regularized by and exponential cutoff
p(k) \propto k^{-\gamma}\exp[-k/\xi]. In this paper we characterize in detail
the phase diagram of this model and its critical indices both on annealed and
quenched networks. To uncover the phase diagram of the model we use the tools
of heterogeneous mean-field calculations for the annealed networks and the most
advanced techniques of quantum cavity methods for the quenched networks. The
phase diagram of the dynamical process depends on the temperature T, the
coupling constant J and on the value of the branching ratio / where
k is the degree of the nodes in the network. For fixed value of the coupling
the critical temperature increases linearly with the branching ration which
diverges with the increasing cutoff value \xi or value of the \gamma exponent
\gamma< 3. This result suggests that the fractal disorder of the
superconducting material can be responsible for an enhancement of the
superconducting critical temperature. At low temperature and low couplings T<<1
and J<<1, instead, we observe a different behavior for annealed and quenched
networks. In the annealed networks there is no phase transition at zero
temperature while on quenched network we observe a Griffith phase dominated by
extremely rare events and a phase transition at zero temperature. The Griffiths
critical region, nevertheless, is decreasing in size with increasing value of
the cutoff \xi of the degree distribution for values of the \gamma exponents
\gamma< 3.Comment: (17 pages, 3 figures
Potencializando o papel dos morcegos frugÃvoros na recuperação de áreas degradadas.
Os morcegos frugÃvoros estão entre os principais dispersores de sementes, portanto, técnicas que potencializem esse papel ecológico podem ter grande aplicação na recuperação de ambientes florestais. Nesse sentido, testes com óleos essenciais isolados de frutos quiropterocóricos maduros mostraram que eles podem atrair morcegos frugÃvoros no interior de remanescentes florestais e em áreas abertas. Além disso, os resultados revelaram que os indivÃduos atraÃdos já haviam se alimentado, visto que a maioria de suas amostras fecais continham sementes. Sendo assim, o uso de óleos essenciais de frutos consumidos por morcegos frugÃvoros potencialmente incrementa a chuva de sementes em locais especÃficos, como por exemplo ao longo de rios onde seja necessário recuperar florestas ciliares ou em uma área de propriedade rural onde seja necessário implantar ou recuperar uma Reserva Legal. Em função da ampla aplicação da técnica, novos estudos estão sendo conduzidos no sentido de mensurar a chuva de sementes e a regeneração da vegetação provocadas pelo seu uso, bem como para identificar os compostos responsáveis pela atração e sintetizá-los.Nota técnica
Two-bands superconductivity with intra- and interband pairing for synthetic superlattices
We consider a model for superconductivity in a two-band superconductor,
having an anisotropic electronic structure made of two partially overlapping
bands with a first hole-like and a second electron-like fermi surface. In this
pairing scenario, driven by the interplay between interband and
intraband pairing terms, we have solved the two gap equations at the
critical temperature and calculate and the chemical potential
as a function of the number of carriers for various values of pairing
interactions, , , and . The results show the
complexity of the physics of condensates with multiple order parameters with
the chemical potential near band edges.Comment: 6 pages, 2 figure
Quantum statistics in Network Geometry with Fractional Flavor
(23 pages, 6 figures)(23 pages, 6 figures
Theory of the Fermi Arcs, the Pseudogap, and the Anisotropy in k-space of Cuprate Superconductors
The appearance of the Fermi arcs or gapless regions at the nodes of the Fermi
surface just above the critical temperature is described through
self-consistent calculations in an electronic disordered medium. We develop a
model for cuprate superconductors based on an array of Josephson junctions
formed by grains of inhomogeneous electronic density derived from a phase
separation transition. This approach provides physical insights to the most
important properties of these materials like the pseudogap phase as forming by
the onset of local (intragrain) superconducting amplitudes and the zero
resistivity critical temperature due to phase coherence activated by
Josephson coupling. The formation of the Fermi arcs and the dichotomy in
k-space follows from the direction dependence of the junctions tunneling
current on the d-wave symmetry on the planes. We show that this
semi-phenomenological approach reproduces also the main future of the cuprates
phase diagram.Comment: 5 pages 7 fig
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