5,508 research outputs found
The Inclusive Semileptonic Decay Lepton Spectrum from
In this talk, we review the QCD calculation of the lepton spectrum from
inclusive semileptonic decay. We compare this prediction to that of the
ACCMM model. This latter work was done in collaboration with Csaba Csaki.Comment: MIT-CTP-2333, uses LATEX. Invited Talk, Presented at WHEPP-3 Workshop
in Madras,India, January, 199
Are there approximate relations among transverse momentum dependent distribution functions?
Certain exact relations among transverse momentum dependent parton
distribution functions due to QCD equations of motion turn into approximate
ones upon the neglect of pure twist-3 terms. On the basis of available data
from HERMES we test the practical usefulness of one such
``Wandzura-Wilczek-type approximation'', namely of that connecting
h_{1L}^{\perp(1)a}(x) to h_L^a(x), and discuss how it can be further tested by
future CLAS and COMPASS data.Comment: 9 pages, 3 figure
Multi-wavelength constraints on cosmic-ray leptons in the Galaxy
Cosmic rays (CRs) interact with the gas, the radiation field and the magnetic
field in the Milky Way, producing diffuse emission from radio to gamma rays.
Observations of this diffuse emission and comparison with detailed predictions
are powerful tools to unveil the CR properties and to study CR propagation. We
present various GALPROP CR propagation scenarios based on current CR
measurements. The predicted synchrotron emission is compared to radio surveys,
and synchrotron temperature maps from WMAP and Planck, while the predicted
interstellar gamma-ray emission is compared to Fermi-LAT observations. We show
how multi-wavelength observations of the Galactic diffuse emission can be used
to help constrain the CR lepton spectrum and propagation. Finally we discuss
how radio and microwave data could be used in understanding the diffuse
Galactic gamma-ray emission observed with Fermi-LAT, especially at low
energies.Comment: 8 pages, 5 figures; in Proceedings of the 34th International Cosmic
Ray Conference (ICRC 2015), The Hague (The Netherlands); Oral contributio
Casimir forces between arbitrary compact objects
We develop an exact method for computing the Casimir energy between arbitrary
compact objects, either dielectrics or perfect conductors. The energy is
obtained as an interaction between multipoles, generated by quantum current
fluctuations. The objects' shape and composition enter only through their
scattering matrices. The result is exact when all multipoles are included, and
converges rapidly. A low frequency expansion yields the energy as a series in
the ratio of the objects' size to their separation. As an example, we obtain
this series for two dielectric spheres and the full interaction at all
separations for perfectly conducting spheres.Comment: 4 pages, 1 figur
Spin alignment of vector meson in e+e- annihilation at Z0 pole
We calculate the spin density matrix of the vector meson produced in e+e-
annihilation at Z^0 pole. We show that the data imply a significant
polarization for the antiquark which is created in the fragmentation process of
the polarized initial quark and combines with the fragmenting quark to form the
vector meson. The direction of polarization is opposite to that of the
fragmenting quark and the magnitude is of the order of 0.5. A qualitative
explanation of this result based on the LUND string fragmentation model is
given.Comment: 15 pages, 2 fgiures; submitted to Phys. Rev.
High-pressure neutron study of the morphotropic PZT: phase transitions in a two-phase system
In piezoelectric ceramics the changes in the phase stabilities versus stress
and temperature in the vicinity of the phase boundary play a central role. The
present study was dedicated to the classical piezoelectric,
lead-zirconate-titanate (PZT) ceramic with composition
Pb(ZrTi)O at the Zr-rich side of the morphotropic phase
boundary at which both intrinsic and extrinsic contributions to
piezoelectricity are significant. The pressure-induced changes in this
two-phase (rhombohedral +monoclinic at room temperature and
above 1 GPa pressures) system were studied by high-pressure neutron
powder diffraction technique. The experiments show that applying pressure
favors the phase, whereas the phase transforms continuously to the
, which is favored at elevated temperatures due to the competing entropy
term. The phase transformation is discontinuous. The
transformation contributes to the extrinsic piezoelectricity. An important
contribution to the intrinsic piezoelectricity was revealed: a large
displacement of the cations (Zr and Ti) with respect to the oxygen anions
is induced by pressure. Above 600 K a phase transition to a cubic phase took
place. Balance between the competing terms dictates the curvature of the phase
boundary. After high-pressure experiments the amount of rhombohedral phase was
larger than initially, suggesting that on the Zr-rich side of the phase
boundary the monoclinic phase is metastable.Comment: 6 figure
Characterizing Young Brown Dwarfs using Low Resolution Near-IR Spectra
We present near-infrared (1.0-2.4 micron) spectra confirming the youth and
cool effective temperatures of 6 brown dwarfs and low mass stars with
circumstellar disks toward the Chamaeleon II and Ophiuchus star forming
regions. The spectrum of one of our objects indicates that it has a spectral
type of ~L1, making it one of the latest spectral type young brown dwarfs
identified to date. Comparing spectra of young brown dwarfs, field dwarfs, and
giant stars, we define a 1.49-1.56 micron H2O index capable of determining
spectral type to within 1 sub-type, independent of gravity. We have also
defined an index based on the 1.14 micron sodium feature that is sensitive to
gravity, but only weakly dependent on spectral type for field dwarfs. Our 1.14
micron Na index can be used to distinguish young cluster members (t <~ 5 Myr)
from young field dwarfs, both of which may have the triangular H-band continuum
shape which persists for at least tens of Myr. Using effective temperatures
determined from the spectral types of our objects along with luminosities
derived from near and mid-infrared photometry, we place our objects on the H-R
diagram and overlay evolutionary models to estimate the masses and ages of our
young sources. Three of our sources have inferred ages (t ~= 10-30 Myr)
significantly older than the median stellar age of their parent clouds (1-3
Myr). For these three objects, we derive masses ~3 times greater than expected
for 1-3 Myr old brown dwarfs with the bolometric luminosities of our sources.
The large discrepancies in the inferred masses and ages determined using two
separate, yet reasonable methods, emphasize the need for caution when deriving
or exploiting brown dwarf mass and age estimates.Comment: 11 pages, Accepted to Ap
Helicity-Flip Off-Foward Parton Distributions of the Nucleon
We identify quark and gluon helicity-flip distributions defined between
nucleon states of unequal momenta. The evolution of these distributions with
change of renormalization scale is calculated in the leading-logarithmic
approximation. The helicity-flip gluon distributions do not mix with any quark
distribution and are thus a unique signature of gluons in the nucleon. Their
contribution to the generalized virtual Compton process is obtained both in the
form of a factorization theorem and an operator product expansion. In deeply
virtual Compton scattering, they can be probed through distinct angular
dependence of the cross section.Comment: a few corrections made, references change
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