430 research outputs found
New Results in the Analysis of the O+Si Elastic Scattering by Modifying the Optical Potential
The elastic scattering of the O+Si system has been analyzed
with a modified potential within the framework of the optical model over a wide
energy range in the laboratory system from 29.0 to 142.5 MeV. This system has
been extensively studied over the years and a number of serious problems has
remained unsolved: The explanation of the anomalous large angle scattering
data; the out-of-phase problem between theoretical predictions and experimental
data; the reproduction of the oscillatory structure near the Coulomb barrier;
the consistent description of angular distributions together with the
excitation functions data are just some of these problems. We propose the use
of a modified potential method to explain these problems over this wide energy
range. This new method consistently improves the agreement with the
experimental data and achieves a major improvement on all the previous Optical
model calculations for this system.Comment: 19 pages with 8 figure
From GHz to mHz: A Multiwavelength Study of the Acoustically Active 14 August 2004 M7.4 Solar Flare
We carried out an electromagnetic acoustic analysis of the solar flare of 14
August 2004 in active region AR10656 from the radio to the hard X-ray spectrum.
The flare was a GOES soft X-ray class M7.4 and produced a detectable sun quake,
confirming earlier inferences that relatively low-energy flares may be able to
generate sun quakes. We introduce the hypothesis that the seismicity of the
active region is closely related to the heights of coronal magnetic loops that
conduct high-energy particles from the flare. In the case of relatively short
magnetic loops, chromospheric evaporation populates the loop interior with
ionized gas relatively rapidly, expediting the scattering of remaining trapped
high-energy electrons into the magnetic loss cone and their rapid precipitation
into the chromosphere. This increases both the intensity and suddenness of the
chromospheric heating, satisfying the basic conditions for an acoustic emission
that penetrates into the solar interior.Comment: Accepted in Solar Physic
Chiral phase boundary of QCD at finite temperature
We analyze the approach to chiral symmetry breaking in QCD at finite
temperature, using the functional renormalization group. We compute the running
gauge coupling in QCD for all temperatures and scales within a simple truncated
renormalization flow. At finite temperature, the coupling is governed by a
fixed point of the 3-dimensional theory for scales smaller than the
corresponding temperature. Chiral symmetry breaking is approached if the
running coupling drives the quark sector to criticality. We quantitatively
determine the phase boundary in the plane of temperature and number of flavors
and find good agreement with lattice results. As a generic and testable
prediction, we observe that our underlying IR fixed-point scenario leaves its
imprint in the shape of the phase boundary near the critical flavor number:
here, the scaling of the critical temperature is determined by the
zero-temperature IR critical exponent of the running coupling.Comment: 39 pages, 8 figure
Non-perturbative thermal flows and resummations
We construct a functional renormalisation group for thermal fluctuations.
Thermal resummations are naturally built in, and the infrared problem of
thermal fluctuations is well under control. The viability of the approach is
exemplified for thermal scalar field theories. In gauge theories the present
setting allows for the construction of a gauge-invariant thermal
renormalisation group.Comment: 16 pages, eq (38) added to match published versio
Fluctuations as probe of the QCD phase transition and freeze-out in heavy ion collisions at LHC and RHIC
We discuss the relevance of higher order moments of net baryon number
fluctuations for the analysis of freeze-out and critical conditions in heavy
ion collisions at LHC and RHIC. Using properties of O(4) scaling functions, we
discuss the generic structure of these higher moments at vanishing baryon
chemical potential and apply chiral model calculations to explore their
properties at non-zero baryon chemical potential. We show that the ratios of
the sixth to second and eighth to second order moments of the net baryon number
fluctuations change rapidly in the transition region of the QCD phase diagram.
Already at vanishing baryon chemical potential they deviate considerably from
the predictions of the hadron resonance gas model which reproduce the second to
fourth order moments of the net proton number fluctuations at RHIC. We point
out that the sixth order moments of baryon number and electric charge
fluctuations remain negative at the chiral transition temperature. Thus, they
offer the possibility to probe the proximity of the thermal freeze-out to the
crossover line.Comment: 24 pages, 12 EPS files, revised version, to appear in EPJ
Properties of high-frequency wave power halos around active regions: an analysis of multi-height data from HMI and AIA onboard SDO
We study properties of waves of frequencies above the photospheric acoustic
cut-off of 5.3 mHz, around four active regions, through spatial maps
of their power estimated using data from Helioseismic and Magnetic Imager (HMI)
and Atmospheric Imaging Assembly (AIA) onboard Solar Dynamics Observatory
(SDO). The wavelength channels 1600 {\AA} and 1700 {\AA} from AIA are now known
to capture clear oscillation signals due to helioseismic p modes as well as
waves propagating up through to the chromosphere. Here we study in detail, in
comparison with HMI Doppler data, properties of the power maps, especially the
so called 'acoustic halos' seen around active regions, as a function of wave
frequencies, inclination and strength of magnetic field (derived from the
vector field observations by HMI) and observation height. We infer possible
signatures of (magneto-)acoustic wave refraction from the observation height
dependent changes, and hence due to changing magnetic strength and geometry, in
the dependences of power maps on the photospheric magnetic quantities. We
discuss the implications for theories of p mode absorption and mode conversions
by the magnetic field.Comment: 22 pages, 12 figures, Accepted by journal Solar Physic
Inter- and Intragranular Effects in Superconducting Compacted Platinum Powders
Compacted platinum powders exhibit a sharp onset of diamagnetic screening at
mK in zero magnetic field in all samples investigated. This
sharp onset is interpreted in terms of the intragranular transition into the
superconducting state. At lower temperatures, the magnetic ac susceptibility
strongly depends on the ac field amplitude and reflects the small intergranular
critical current density . This critical current density shows a strong
dependence on the packing fraction f of the granular samples. Surprisingly,
increases significantly with decreasing f ( A/cm for f = 0.67 and A/cm for f
= 0.50). The temperature dependence of shows strong positive curvature
over a wide temperature range for both samples. The phase diagrams of inter-
and intragranular superconductivity for different samples indicate that the
granular structure might play the key role for an understanding of the origin
of superconductivity in the platinum compacts.Comment: 11 pages including 9 figures. To appear in Phys. Rev. B in Nov. 0
Baryon Tri-local Interpolating Fields
We systematically investigate tri-local (non-local) three-quark baryon fields
with U_L(2)*U_R(2) chiral symmetry, according to their Lorentz and isospin
(flavor) group representations. We note that they can also be called as
"nucleon wave functions" due to this full non-locality. We study their chiral
transformation properties and find all the possible chiral multiplets
consisting J=1/2 and J=3/2 baryon fields. We find that the axial coupling
constant |g_A| = 5/3 is only for nucleon fields belonging to the chiral
representation (1/2,1)+(1,1/2) which contains both nucleon fields and Delta
fields. Moreover, all the nucleon fields belonging to this representation have
|g_A| = 5/3.Comment: 8 pages, 3 tables, accepted by EPJ
Modeling the Subsurface Structure of Sunspots
While sunspots are easily observed at the solar surface, determining their
subsurface structure is not trivial. There are two main hypotheses for the
subsurface structure of sunspots: the monolithic model and the cluster model.
Local helioseismology is the only means by which we can investigate
subphotospheric structure. However, as current linear inversion techniques do
not yet allow helioseismology to probe the internal structure with sufficient
confidence to distinguish between the monolith and cluster models, the
development of physically realistic sunspot models are a priority for
helioseismologists. This is because they are not only important indicators of
the variety of physical effects that may influence helioseismic inferences in
active regions, but they also enable detailed assessments of the validity of
helioseismic interpretations through numerical forward modeling. In this paper,
we provide a critical review of the existing sunspot models and an overview of
numerical methods employed to model wave propagation through model sunspots. We
then carry out an helioseismic analysis of the sunspot in Active Region 9787
and address the serious inconsistencies uncovered by
\citeauthor{gizonetal2009}~(\citeyear{gizonetal2009,gizonetal2009a}). We find
that this sunspot is most probably associated with a shallow, positive
wave-speed perturbation (unlike the traditional two-layer model) and that
travel-time measurements are consistent with a horizontal outflow in the
surrounding moat.Comment: 73 pages, 19 figures, accepted by Solar Physic
Electronic structure of overstretched DNA
Minuscule molecular forces can transform DNA into a structure that is
elongated by more than half its original length. We demonstrate that this
pronounced conformational transition is of relevance to ongoing experimental
and theoretical efforts to characterize the conducting properties of DNA wires.
We present quantum mechanical calculations for acidic, dry, poly(CG).poly(CG)
DNA which has undergone elongation of up to 90 % relative to its natural
length, along with a method for visualizing the effects of stretching on the
electronic eigenstates. We find that overstretching leads to a drastic drop of
the hopping matrix elements between localized occupied electronic states
suggesting a dramatic decrease in the conductivity through holes.Comment: 4 page
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