31,618 research outputs found
On the massless contributions to the vacuum polarization of heavy quarks
Recently Groote and Pivovarov have given notice of a possible fault in the
use of sum rules involving two-point correlation functions to extract
information on heavy quark parameters, due to the presence of massless
contributions that invalidate the construction of moments of the spectral
densities. Here we show how to circumvent this problem through a new definition
of the moments, providing an infrared safe and consistent procedure.Comment: 1+9 pages, 3 figures. Discussion on QCD sum rules applications added.
Conclusions unchanged. Version to be published in Journal of Physics
Compressibility and structural stability of ultra-incompressible bimetallic interstitial carbides and nitrides
We have investigated by means of high-pressure x-ray diffraction the
structural stability of Pd2Mo3N, Ni2Mo3C0.52N0.48, Co3Mo3C0.62N0.38, and
Fe3Mo3C. We have found that they remain stable in their ambient-pressure cubic
phase at least up to 48 GPa. All of them have a bulk modulus larger than 330
GPa, being the least compressible material Fe3Mo3C, B0 = 374(3) GPa. In
addition, apparently a reduction of compressibility is detected as the carbon
content increased. The equation of state for each material is determined. A
comparison with other refractory materials indicates that interstitial nitrides
and carbides behave as ultra-incompressible materials.Comment: 14 pages, 3 figures, 1 tabl
Microcanonical finite-size scaling in specific heat diverging 2nd order phase transitions
A Microcanonical Finite Site Ansatz in terms of quantities measurable in a
Finite Lattice allows to extend phenomenological renormalization (the so called
quotients method) to the microcanonical ensemble. The Ansatz is tested
numerically in two models where the canonical specific-heat diverges at
criticality, thus implying Fisher-renormalization of the critical exponents:
the 3D ferromagnetic Ising model and the 2D four-states Potts model (where
large logarithmic corrections are known to occur in the canonical ensemble). A
recently proposed microcanonical cluster method allows to simulate systems as
large as L=1024 (Potts) or L=128 (Ising). The quotients method provides
extremely accurate determinations of the anomalous dimension and of the
(Fisher-renormalized) thermal exponent. While in the Ising model the
numerical agreement with our theoretical expectations is impressive, in the
Potts case we need to carefully incorporate logarithmic corrections to the
microcanonical Ansatz in order to rationalize our data.Comment: 13 pages, 8 figure
High-pressure study of substrate material ScAlMgO4
We report on the structural properties of ScAlMgO4 studied under
quasi-hydrostatic pressure using synchrotron high-pressure x-ray diffraction up
to 40 GPa. We also report on single-crystal studies of ScAlMgO4 performed at
300 K and 100 K. We found that the low-pressure phase remains stable up to 24
GPa. At 28 GPa, we detected a reversible phase transformation. The
high-pressure phase is assigned to a monoclinic distortion of the low-pressure
phase. No additional phase transition is observed up to 40 GPa. In addition,
the equation of state, compressibility tensor, and thermal expansion
coefficients of ScAlMgO4 are determined. The bulk modulus of ScAlMgO4 is found
to be 143(8) GPa, with a strong compressibility anisotropy. For the trigonal
low-pressure phase, the compressibility along the c-axis is twice than
perpendicular one. A perfect lattice match with ZnO is retained under pressure
in the pressure range of stability of wurtzite ZnO.Comment: 22 pages, 5 figures, 4 tables, 24 reference
The frozen nucleon approximation in two-particle two-hole response functions
We present a fast and efficient method to compute the inclusive two-particle
two-hole (2p-2h) electroweak responses in the neutrino and electron
quasielastic inclusive cross sections. The method is based on two
approximations. The first neglects the motion of the two initial nucleons below
the Fermi momentum, which are considered to be at rest. This approximation,
which is reasonable for high values of the momentum transfer, turns out also to
be quite good for moderate values of the momentum transfer . The
second approximation involves using in the "frozen" meson-exchange currents
(MEC) an effective -propagator averaged over the Fermi sea. Within the
resulting "frozen nucleon approximation", the inclusive 2p-2h responses are
accurately calculated with only a one-dimensional integral over the emission
angle of one of the final nucleons, thus drastically simplifying the
calculation and reducing the computational time. The latter makes this method
especially well-suited for implementation in Monte Carlo neutrino event
generators.Comment: 8 pages, 5 figures and 1 tabl
Conceptual design study for heat exhaust management in the ARC fusion pilot plant
The ARC pilot plant conceptual design study has been extended beyond its
initial scope [B. N. Sorbom et al., FED 100 (2015) 378] to explore options for
managing ~525 MW of fusion power generated in a compact, high field (B_0 = 9.2
T) tokamak that is approximately the size of JET (R_0 = 3.3 m). Taking
advantage of ARC's novel design - demountable high temperature superconductor
toroidal field (TF) magnets, poloidal magnetic field coils located inside the
TF, and vacuum vessel (VV) immersed in molten salt FLiBe blanket - this
follow-on study has identified innovative and potentially robust power exhaust
management solutions.Comment: Accepted by Fusion Engineering and Desig
Two-nucleon emission in neutrino and electron scattering from nuclei: the modified convolution approximation
The theoretical formalism of inclusive lepton-nucleus scattering in the
two-nucleon emission channel is discussed in the context of a simplified
approach, the modified convolution approximation. This allows one to write the
2p2h responses of the relativistic Fermi gas as a folding integral of two 1p1h
responses with the energies and momenta transferred to each nucleon. The idea
behind this method is to introduce different average momenta for the two
initial nucleons in the matrix elements of the two-body current, with the
innovation that they depend on the transferred energies and momenta. This
method treats exactly the two-body phase space kinematics, and reduces the
formulae of the response functions from seven-dimensional integrals over
momenta to much simpler three-dimensional ones. The applicability of the method
is checked by comparing with the full results within a model of electroweak
meson-exchange currents. The predictions are accurate enough, especially in the
low-energy threshold region where the average momentum approximation works the
best.Comment: 35 pages, 13 figure
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