10,448 research outputs found
Theory of Charmless Inclusive B Decays and the Extraction of V_{ub}
We present ``state-of-the-art'' theoretical expressions for the triple
differential B->X_u l^- nu decay rate and for the B->X_s gamma photon spectrum,
which incorporate all known contributions and smoothly interpolate between the
``shape-function region'' of large hadronic energy and small invariant mass,
and the ``OPE region'' in which all hadronic kinematical variables scale with
M_B. The differential rates are given in a form which has no explicit reference
to the mass of the b quark, avoiding the associated uncertainties. Dependence
on m_b enters indirectly through the properties of the leading shape function,
which can be determined by fitting the B->X_s gamma photon spectrum. This
eliminates the dominant theoretical uncertainties from predictions for B->X_u
l^- nu decay distributions, allowing for a precise determination of |V_{ub}|.
In the shape-function region, short-distance and long-distance contributions
are factorized at next-to-leading order in renormalization-group improved
perturbation theory. Higher-order power corrections include effects from
subleading shape functions where they are known. When integrated over
sufficiently large portions in phase space, our results reduce to standard OPE
expressions up to yet unknown O(alpha_s^2) terms. Predictions are presented for
partial B->X_u l^- nu decay rates with various experimental cuts. An elaborate
error analysis is performed that contains all significant theoretical
uncertainties, including weak annihilation effects. We suggest that the latter
can be eliminated by imposing a cut on high lepton invariant mass.Comment: 45 pages, 4 figures; several minor revisions, more systematic
treatment of subleading shape-function effects, numerical results and tables
updated; version to appear in Physical Review
Comparison of Four Space Propulsion Methods for Reducing Transfer Times of Manned Mars Mission
We assess the possibility of reducing the travel time of a manned mission to
Mars by examining four different propulsion methods, and keeping the mass at
departure under 2,500 tonnes, for a fixed architecture. We evaluated
representative systems of three different state of the art technologies
(chemical, nuclear thermal, and electric), and one advance technology, the
"Pure Electro-Magnetic Thrust" (PEMT) concept (proposed by Rubbia). A mission
architecture mostly based on the Design Reference Architecture 5.0 is assumed
in order to estimate the mass budget, that influences the performance of the
propulsion system. Pareto curves of the duration of the mission and time of
flight versus mass of mission are drawn. We conclude that the ion engine
technology, combined with the classical chemical engine, yields the shortest
mission times for this architecture with the lowest mass, and that chemical
propulsion alone is the best to minimise travel time. The results obtained
using the PEMT suggest that it could be a more suitable solution for farther
destinations than Mars.Comment: Change in title, abstract and presentation so to clarify the main
results. 14 pages, 7 figures and 2 table
The Curious Case of NGC6908
The object NGC6908 was once thought to be simply a surface-brightness
enhancement in the eastern spiral arm of the nearby spiral galaxy NGC6907.
Based on an examination of near-infrared imaging, the object is shown in fact
to be a lenticular S0(6/7) galaxy hidden in the optical glare of the disk and
spiral structure of the larger galaxy. New radial velocities of NGC6908
(3,060+/-16 (emission); 3,113+/-73 km/s (absorption)) have been obtained at the
Baade 6.5m and the duPont 2.5m telescopes at Las Campanas, Chile placing
NGC6908 at the same expansion-velocity distance as NGC6907 (3,190+/-5 km/s),
eliminating the possibility of a purely chance line-of-sight coincidence. The
once-enigmatic asymmetries in the disk and outer spiral structure of NGC6907
are now explained as being due to an advanced merger event. Newly discovered
tails and debris in the outer reaches of this galaxy further support the merger
scenario for this system. This pair of galaxies is a rather striking example of
two objects discovered over 100 years ago, whose true nature was lost until
modern detectors operating at infrared wavelengths gave us a new
(high-contrast) look. Other examples of embedded merger remnants may also
reveal themselves in the growing samples of near-infrared imaging of nearby
galaxies; and a pilot study does reveal several other promising candidates for
follow-up observations.Comment: 17 pages, 8 figures, accepted for publication in A
On the Yamabe Problem concerning the compact locally conformally flat manifolds
AbstractFor all known locally conformally flat compact Riemannian manifolds (Mn, g) (n > 2), with infinite fundamental group, we give the complete proof of Aubin's conjecture on scalar curvature. That solves the Yamabe Problem for these manifolds. There exists a metric g′ conformal to g, such that volg′ = 1 and whose scalar curvature R′ is constant and satisfies R′ < n(n − 1) ωn2n, where ωn is the volume of the sphere Sn with radius 1
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