13,406 research outputs found
Mesonic states in the generalised Nambu-Jona-Lasinio theories
For any Nambu-Jona-Lasinio model of QCD with arbitrary nonlocal,
instantaneous, quark current-current confining kernels, we use a generalised
Bogoliubov technique to go beyond BCS level (in the large-Nc limit) so as to
explicitly build quark-antiquark compound operators for creating/annihilating
mesons. In the Hamiltonian approach, the mesonic bound-state equations appear
(from the generalised Bogoliubov transformation) as mass-gap-like equations
which, in turn, ensure the absence, in the Hamiltonian, of mesonic Bogoliubov
anomalous terms. We go further to demonstrate the one-to-one correspondence
between Hamiltonian and Bethe-Salpeter approaches to non-local NJL-type models
for QCD and give the corresponding "dictionary" necessary to "translate" the
amplitudes built using the graphical Feynman rules to the terms of the
Hamiltonian, and vice versa. We comment on the problem of multiple vacua
existence in such type of models and argue that mesonic states in the theory
should be prescribed to have an extra index - the index of the replica in which
they are created. Then the completely diagonalised Hamiltonian should contain a
sum over this new index. The method is proved to be general and valid for any
instantaneous quark kernel.Comment: LaTeX2e, uses aipproc class, Talk given at the conference "Quark
Confinement and the Hadron Spectrum VI", 21-25 September 2004, Sardinia,
Italy, to appear in Proceeding
Quantum field theory approach to the vacuum replica in QCD
Quantum field theory is used to describe the contribution of possible new QCD
vacuum replica to hadronic processes. This sigma-like new state has been
recently shown to be likely to appear for any realistic four-quark interaction
kernel as a consequence of chiral symmetry. The local operator creating the
replica vacuum state is constructed explicitly. Applications to physical
processes are outlined.Comment: LaTeX2e, 2 EPS figures, uses ws-procs9x6 (included) and epsfig
classes, Talk given at the conference "Quark Confinement and the Hadron
Spectrum V", 10-14 September 2002, Gargnano, Italy, to appear in Proceeding
Parity doublers in chiral potential quark models
The effect of spontaneous breaking of chiral symmetry over the spectrum of
highly excited hadrons is addressed in the framework of a microscopic chiral
potential quark model (Generalised Nambu-Jona-Lasinio model) with a vectorial
instantaneous quark kernel of a generic form. A heavy-light quark-antiquark
bound system is considered, as an example, and the Lorentz nature of the
effective light-quark potential is identified to be a pure Lorentz-scalar, for
low-lying states in the spectrum, and to become a pure spatial Lorentz vector,
for highly excited states. Consequently, the splitting between the partners in
chiral doublets is demonstrated to decrease fast in the upper part of the
spectrum so that neighboring states of an opposite parity become almost
degenerate. A detailed microscopic picture of such a "chiral symmetry
restoration" in the spectrum of highly excited hadrons is drawn and the
corresponding scale of restoration is estimated.Comment: LaTeX2e, 4 pages, uses aipproc class, Talk given by A.Nefediev at the
conference "Quark Confinement and the Hadron Spectrum VII", 2-7 September
2006, Acores, Portugal, to appear in Proceeding
Confinement and parity doubling in heavy-light mesons
In this paper, we study the chiral symmetry restoration in the hadronic
spectrum in the framework of generalised Nambu-Jona-Lasinio quark models with
instantaneous confining quark kernels. We investigate a heavy-light quarkonium
and derive its bound-state equation in the form of a Schroedingerlike equation
and, after the exact inverse Foldy-Wouthuysen transformation, in the form of a
Diraclike quation. We discuss the Lorentz nature of confinement for such a
system and demonstrate explicitly the effective chiral symmetry restoration for
highly excited states in the mesonic spectrum. We give an estimate for the
scale of this restoration.Comment: RevTeX4, 21 pages, 6 Postscript figures, uses epsfig.sty, to appear
in Phys.Rev.
Castanheira (Bertholletia excelsa) em sistemas agroflorestais implantadas em áreas de pastagens degradadas na Amazônia Ocidental.
O presente trabalho objetivou avaliar comportamento da castanheira em sistemas agroflorestais implantados em pastagens degradáveis em diferentes histórico e uso e graus de degradação
Symmetry, incommensurate magnetism and ferroelectricity: the case of the rare-earth manganites RMnO3
The complete irreducible co-representations of the paramagnetic space group
provide a simple and direct path to explore the symmetry restrictions of
magnetically driven ferroelectricity. The method consists of a straightforward
generalization of the method commonly used in the case of displacive modulated
systems and allows us to determine, in a simple manner, the full magnetic
symmetry of a given phase originated from a given magnetic order parameter. The
potential ferroic and magneto-electric properties of that phase can then be
established and the exact Landau free energy expansions can be derived from
general symmetry considerations. In this work, this method is applied to the
case of the orthorhombic rare-earth manganites RMnO3. This example will allow
us to stress some specific points, such as the differences between commensurate
or incommensurate magnetic phases regarding the ferroic and magnetoelectric
properties, the possible stabilization of ferroelectricity by a single
irreducible order parameter or the possible onset of a polarization oriented
parallel to the magnetic modulation. The specific example of TbMnO3 will be
considered in more detail, in order to characterize the role played by the
magneto-electric effect in the mechanism for the polarization rotation induced
by an external magnetic field.Comment: Conference: Aperiodic`0
Vacuum fluctuations of a scalar field near a reflecting boundary and their effects on the motion of a test particle
The contribution from quantum vacuum fluctuations of a real massless scalar
field to the motion of a test particle that interacts with the field in the
presence of a perfectly reflecting flat boundary is here investigated. There is
no quantum induced dispersions on the motion of the particle when it is alone
in the empty space. However, when a reflecting wall is introduced, dispersions
occur with magnitude dependent on how fast the system evolves between the two
scenarios. A possible way of implementing this process would be by means of an
idealized sudden switching, for which the transition occurs instantaneously.
Although the sudden process is a simple and mathematically convenient
idealization it brings some divergences to the results, particularly at a time
corresponding to a round trip of a light signal between the particle and the
wall. It is shown that the use of smooth switching functions, besides
regularizing such divergences, enables us to better understand the behavior of
the quantum dispersions induced on the motion of the particle. Furthermore, the
action of modifying the vacuum state of the system leads to a change in the
particle energy that depends on how fast the transition between these states is
implemented. Possible implications of these results to the similar case of an
electric charge near a perfectly conducting wall are discussed.Comment: 17 pages, 8 figure
Shelf-Ocean material exchange influencing the Atlantic chemical composition off NW Iberian margin since the last glaciation
Rivers are the main conduit of sediment to the shelf. The basin geology, the drainage area and the discharge rate are the major factors that determine their sediment load (Milliman and Syvistski, 1992). Besides suspended particles, dissolved components may also give some information on the eroded crust. Sr isotopes in carbonate shells of biological organisms have been used to study, in the geological record, the influence exerted by the chemical weathering of the continental crust on the seawater composition (Macdougall, 1991).
In this work, Sr isotope ratios obtained in tests of foraminifera representing the last 40 ka are presented and discussed in the scope of the palaeogeographical evolution of NW Iberia.
This work aims to present and discuss the results of Sr isotope analyses (performed, by TIMS, in the Isotope Geology Laboratory of the University of Aveiro) of tests of two species of foraminifera, from nine samples taken along the OMEX core KC 024-19 (181 cm; 42°08’98’’N, 10°29´96’’W, and 2765m), collected in the Galicia Bank area, off Galicia. Taking into account that Sr contained in the carbonate tests is usually considered as preserving the signature of the contemporaneous seawater, one planktonic species (Globigerina bulloides) and one benthic species (Cibicides wuellerstorfi) were selected in order to try to detect Sr isotope variations both through time and between two different levels of the water column.
The core age model, which records the last 40 ka, is based on a combination of oxygen isotope stratigraphy, eight AMS 14C datings and the synchronisation of the last four Heinrich Events in the Iberian Margin sedimentary records.
As a whole, the obtained 87Sr/86Sr ratios vary between 0.709209 and 0.709108, with a mean 2σ error of 0.000025. These values lie within the range of modern marine Sr isotope ratios (0.70910-0.70922), as previously defined using analyses of both seawater and marine carbonates (see compilation by Faure and Mensing, 2005). Despite their small variation, the 87Sr/86Sr ratios obtained in G. bulloides seem to indicate that Sr dissolved in seawater at the KC 024-19 core site became slightly less radiogenic after the Last Glacial Maximum (LGM). This decrease is concomitant with diminishing amounts of the detrital components in the sediments (Fig. 1). Therefore, both the composition of dissolved Sr, as revealed by results on tests of planktonic foraminifera, and the proportions of suspended terrigenous particulate material arriving at the KC 024-19 site point to a decreasing importance of the contribution of the erosion of the Iberian Variscan crust since the Last Glacial Maximum and in the Holocene.
The 87Sr/86Sr ratios measured in tests of benthic foraminifera (C. wuellerstorfi) are more erratic and no correlation can be established with palaeogeographical/palaeoclimatic constraints. The difference between the behaviour of Sr compositions in G. bulloides and C. wuellerstorfi may indicate that whilst the planktonic foraminifera should reproduce very closely the seawater composition, the benthic organisms should, in addition to the major role of seawater, also be affected by some sort of interaction with the sediments. As such, planktonic foraminifera are probably more reliable indicators of seawater composition in studies involving very small periods and corresponding very slight variations of the 87Sr/86Sr ratios.
Taking into account that G. bulloides is a common planktonic species (living mostly in the first 50m of the water column), whose tests seem to be in equilibrium with sea water composition, variations in its 87Sr/86Sr ratios can be related with changes in the chemical composition of the water of the Atlantic Ocean off NW Iberian Margin.
The highest 87Sr/86Sr values are contemporaneous with a period of low sea level (about -140 m; Dias et al., 2000) during the LGM. According to Dias et al. (2000) at 18 ka BP the shoreline was close to the shelf break. The summital parts of the Gerês and Estrela mountains were covered by local glaciers and close to the coast freezing occurred frequently. The river catchments, which extended far to the shelf, received more rainfall due to a longer, compared to present day conditions, wet season, which promoted both physical and chemical weathering. Higher pluviosity combined with the effect of spring ice melting maintained high river discharge and consequently caused very important sediment supply to the coastal zone. The extremely narrow shelf was a very energetic environment due to sea bottom inclination and very limited long wave refraction. Therefore, at that time, a long wet season and very competent rivers should have caused important erosion of the Variscan basement in NW Iberia. Additionally, then, the shoreline was much closer to the KC 024-19 site. The combination of all these factors favoured an important deposition of terrigenous sediments and the local slight enrichment in radiogenic Sr of the seawater.
With sea level rise, after the deglaciation and during the Holocene, the river estuaries became progressively far away from the shelf break. Their competence of transport also became progressively reduced and the offshore transport of detrital sediments became progressively lower. Conversely the biogenic carbonate proportion in the sediments increased, due to lower dilution by the terrigenous particles. Simultaneously, the values of 87Sr/86Sr in the seawater at the KC 024-19 site became lower, as a consequence of a complete homogenization with the ocean global composition, which was now more effective with the increasing distance towards shoreline
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