22,658 research outputs found
Electroproduction of Soft Pions at Large Momentum Transfers
We consider pion electroproduction on a proton target close to threshold for
Q^2 in the region 1-10 GeV^2. The momentum transfer dependence of the S-wave
multipoles at threshold, E_{0+} and L_{0+}, is calculated using light-cone sum
rules.Comment: 8 pages, 3 figures; Invited talk at the workshop on Exclusive
Reactions at High Momentum Transfer, 21-24 May 2007, Newport News, Virginia,
U.S.A. and International Conference on hadron Physics TROIA'07, 30 Aug. - 3
Sept. 2007, Canakkale, Turke
Systematization of tensor mesons and the determination of the glueball
It is shown that new data on the -resonances in the mass
range MeV support the linearity of the -trajectories,
where is the radial quantum number of quark--antiquark state. In this way
all vacancies for the isoscalar tensor -mesons in the range up to 2450
MeV are filled in. This allows one to fix the broad -state with
MeV and MeV as the lowest tensor glueball. PACS
numbers: 14.40.-n, 12.38.-t, 12.39.-MkComment: 10 pages, 1 figur
Generalized (m,k)-Zipf law for fractional Brownian motion-like time series with or without effect of an additional linear trend
We have translated fractional Brownian motion (FBM) signals into a text based
on two ''letters'', as if the signal fluctuations correspond to a constant
stepsize random walk. We have applied the Zipf method to extract the
exponent relating the word frequency and its rank on a log-log plot. We have
studied the variation of the Zipf exponent(s) giving the relationship between
the frequency of occurrence of words of length made of such two letters:
is varying as a power law in terms of . We have also searched how
the exponent of the Zipf law is influenced by a linear trend and the
resulting effect of its slope. We can distinguish finite size effects, and
results depending whether the starting FBM is persistent or not, i.e. depending
on the FBM Hurst exponent . It seems then numerically proven that the Zipf
exponent of a persistent signal is more influenced by the trend than that of an
antipersistent signal. It appears that the conjectured law
only holds near . We have also introduced considerations based on the
notion of a {\it time dependent Zipf law} along the signal.Comment: 24 pages, 12 figures; to appear in Int. J. Modern Phys
Threshold Pion Production at Large Momentum Transfers
We consider threshold pion electroproduction on a proton target for photon
virtualities in the region 1-10 GeV. The S-wave multipoles at threshold,
and , are calculated using light-cone sum rules.Comment: 3 pages, talkpresented at PANIC 200
Quark-gluonium content of the scalar-isoscalar states f_0(980), f_0(1300), f_0(1500), f_0(1750), f_0(1420 ^{+150}_{- 70}) from hadronic decays
On the basis of the decay couplings f_0 -> \pi\pi, K\bar K, \eta\eta,
\eta\eta', which had been found before, in the study of analytical
(IJ^{PC}=00^{++})-amplitude in the mass range 450-1900 MeV, we analyse the
quark-gluonium content of resonances f_0(980), f_0(1300), f_0(1500), f_0(1750)
and the broad state f_0(1420 ^{+ 150}_{-70}). The K-matrix technique used in
the analysis makes it possible to evaluate the quark-gluonium content both for
the states with switched-off decay channels (bare states, f^{bare}_0) and the
real resonances. We observe significant change of the quark-gluonium
composition in the evolution from bare states to real resonances, that is due
to the mixing of states in the transitions f_0(m_1)-> real mesons-> f_0(m_2)
responsible for the decay processes as well. For the f_0(980), the analysis
confirmed the dominance of q\bar q component thus proving the n\bar n/s\bar s
composition found in the study of the radiative decays. For the mesons
f_0(1300), f_0(1500) and f_0(1750), the hadronic decays do not allow one to
determine uniquely the n\bar n, s\bar s and gluonium components providing
relative pecentage only. The analysis shows that the broad state f_0(1420 ^{+
150}_{-70}) can mix with the flavour singlet q\bar q component only, that is
consistent with gluonium origin of the broad resonance.Comment: 20 pages, LaTeX, 10 PostScript figures, epsfig.st
Superfast front propagation in reactive systems with anomalous diffusion
We study a reaction diffusion system where we consider a non-gaussian process
instead of a standard diffusion. If the process increments follow a probability
distribution with tails approaching to zero faster than a power law, the usual
qualitative behaviours of the standard reaction diffusion system, i.e.,
exponential tails for the reacting field and a constant front speed, are
recovered. On the contrary if the process has power law tails, also the
reacting field shows power law tail and the front speed increases exponentially
with time. The comparison with other reaction-transport systems which exhibit
anomalous diffusion shows that, not only the presence of anomalous diffusion,
but also the detailed mechanism, is relevant for the front propagation.Comment: 4 pages and 4 figure
Stretched exponential relaxation in the mode-coupling theory for the Kardar-Parisi-Zhang equation
We study the mode-coupling theory for the Kardar-Parisi-Zhang equation in the
strong-coupling regime, focusing on the long time properties. By a saddle point
analysis of the mode-coupling equations, we derive exact results for the
correlation function in the long time limit - a limit which is hard to study
using simulations. The correlation function at wavevector k in dimension d is
found to behave asymptotically at time t as C(k,t)\simeq 1/k^{d+4-2z}
(Btk^z)^{\gamma/z} e^{-(Btk^z)^{1/z}}, with \gamma=(d-1)/2, A a determined
constant and B a scale factor.Comment: RevTex, 4 pages, 1 figur
Low Temperature Anomaly in Mesoscopic Kondo Wires
We report the observation of an anomalous magnetoresistance in extremely
dilute quasi-one-dimensional AuFe wires at low temperatures, along with a
hysteretic background at low fields. The Kondo resistivity does not show the
unitarity limit down to the lowest temperature, implying uncompensated spin
states. We suggest that the anomalous magnetoresistance may be understood as
the interference correction from the accumulation of geometric phase in the
conduction electron wave function around the localized impurity spin.Comment: Four pages, five figure
Mass and Angular Momentum Transfer in the Massive Algol Binary RY Persei
We present an investigation of H-alpha emission line variations observed in
the massive Algol binary, RY Per. We give new radial velocity data for the
secondary based upon our optical spectra and for the primary based upon high
dispersion UV spectra. We present revised orbital elements and an estimate of
the primary's projected rotational velocity (which indicates that the primary
is rotating 7 times faster than synchronous). We use a Doppler tomography
algorithm to reconstruct the individual primary and secondary spectra in the
region of H-alpha, and we subtract the latter from each of our observations to
obtain profiles of the primary and its disk alone. Our H-alpha observations of
RY Per show that the mass gaining primary is surrounded by a persistent but
time variable accretion disk. The profile that is observed outside-of-eclipse
has weak, double-peaked emission flanking a deep central absorption, and we
find that these properties can be reproduced by a disk model that includes the
absorption of photospheric light by the band of the disk seen in projection
against the face of the star. We developed a new method to reconstruct the disk
surface density distribution from the ensemble of H-alpha profiles observed
around the orbit, and this method accounts for the effects of disk occultation
by the stellar components, the obscuration of the primary by the disk, and flux
contributions from optically thick disk elements. The resulting surface density
distribution is elongated along the axis joining the stars, in the same way as
seen in hydrodynamical simulations of gas flows that strike the mass gainer
near trailing edge of the star. This type of gas stream configuration is
optimal for the transfer of angular momentum, and we show that rapid rotation
is found in other Algols that have passed through a similar stage.Comment: 39 pages, 12 figures, ApJ in press, 2004 June 20 issu
Model of Low-pass Filtering of Local Field Potentials in Brain Tissue
Local field potentials (LFPs) are routinely measured experimentally in brain
tissue, and exhibit strong low-pass frequency filtering properties, with high
frequencies (such as action potentials) being visible only at very short
distances (10~) from the recording electrode. Understanding
this filtering is crucial to relate LFP signals with neuronal activity, but not
much is known about the exact mechanisms underlying this low-pass filtering. In
this paper, we investigate a possible biophysical mechanism for the low-pass
filtering properties of LFPs. We investigate the propagation of electric fields
and its frequency dependence close to the current source, i.e. at length scales
in the order of average interneuronal distance. We take into account the
presence of a high density of cellular membranes around current sources, such
as glial cells. By considering them as passive cells, we show that under the
influence of the electric source field, they respond by polarisation, i.e.,
creation of an induced field. Because of the finite velocity of ionic charge
movement, this polarization will not be instantaneous. Consequently, the
induced electric field will be frequency-dependent, and much reduced for high
frequencies. Our model establishes that with respect to frequency attenuation
properties, this situation is analogous to an equivalent RC-circuit, or better
a system of coupled RC-circuits. We present a number of numerical simulations
of induced electric field for biologically realistic values of parameters, and
show this frequency filtering effect as well as the attenuation of
extracellular potentials with distance. We suggest that induced electric fields
in passive cells surrounding neurons is the physical origin of frequency
filtering properties of LFPs.Comment: 10 figs, revised tex file and revised fig
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