22,562 research outputs found
On the automorphisms of a graph product of abelian groups
We study the automorphisms of a graph product of finitely-generated abelian
groups W. More precisely, we study a natural subgroup Aut* W of Aut W, with
Aut* W = Aut W whenever vertex groups are finite and in a number of other
cases. We prove a number of structure results, including a semi-direct product
decomposition of Aut* W in which one of the factors is Inn W. We also give a
number of applications, some of which are geometric in nature.Comment: 38 pages, 4 figure
Quark mass effects in the soft-collinear effective theory and B -> X_s + photon in the endpoint region
We consider the effects of a light quark mass in the soft-collinear effective
theory (SCET) and we apply them to B -> X_s gamma in the endpoint region. We
find that the reparameterization invariance can be extended by including the
collinear quark mass in the SCET Lagrangian. This symmetry constrains the
theory with the quark mass terms, and we present explicit results at one loop.
It also relates the Wilson coefficients of some mass operators to those of the
leading operators, which are useful in organizing the subleading effects due to
the quark mass in B -> X_s gamma. We present strange quark mass corrections to
B -> X_s gamma in the endpoint region as an application. The forward scattering
amplitude from the mass corrections is factorized, and it can be expressed as a
convolution of the m_s^2/p_X^2-suppressed jet function and the leading-order
shape function of the B meson. This contribution should be added to the
existing subleading contributions from the B meson shape functions to obtain
complete subleading corrections.Comment: 30 pages, 6 figures; some errors are corrected and numerical
estimates are adde
The automorphism group of the free group of rank two is a CAT(0) group
We prove that the automorphism group of the braid group on four strands acts
faithfully and geometrically on a CAT(0) 2-complex. This implies that the
automorphism group of the free group of rank two acts faithfully and
geometrically on a CAT(0) 2-complex, in contrast to the situation for rank
three and above.Comment: 7 pages, 2 figures. The manuscript has been modified in minor ways in
accordance with a referee's recommendations, and a misattribution of the
result "Aut F_2 is biautomatic" has been correcte
Probing electroweak physics using B -> XM decays in the endpoint region
Using soft-collinear effective theory we describe at leading order in 1/m_b
all the semi-inclusive hadronic B -> XM decays near the endpoint, where an
energetic light meson M recoils against an inclusive jet X. We also include the
decays involving eta, eta' mesons that receive additional contributions from
gluonic operators. The predicted branching ratios and CP asymmetries depend on
fewer hadronic parameters than the corresponding two-body B decays. This makes
semi-inclusive hadronic B -> XM decays a powerful probe of the potential
nonperturbative nature of charming penguins as well as a useful probe of new
physics effects in electroweak flavor changing transitions. A comparison with B
-> KX data from BaBar points to an enhanced charming penguin, albeit with large
experimental errors.Comment: 27 pages, 6 figure
Interactional aerodynamics and acoustics of a propeller-augmented compound coaxial helicopter
The aerodynamic and acoustic characteristics of a generic hingeless coaxial helicopter with a tail-mounted propulsor and stabiliser have been simulated using Brown's Vorticity Transport Model. This has been done to investigate the ability of models of this type to capture the aerodynamic interactions that are generated between the various components of realistic, complex helicopter configurations. Simulations reveal the aerodynamic environment of the coaxial main rotor of the configuration to be dominated by internal interactions that lead to high vibration and noise. The wake of the main rotor is predicted to interact strongly with the tailplane, particularly at low forward speed, to produce a strong nose-up pitching moment that must be countered by significant longitudinal cyclic input to the main rotor. The wake from the main rotor is ingested directly into the tail propulsor over a broad range of forward speeds, where it produces significant vibratory excitation of the system as well as broadband noise. The numerical calculations also suggest the possibility that poor scheduling of the partition of the propulsive force between the main rotor and propulsor as a function of forward speed may yield a situation where the propulsor produces little thrust but high vibration as a result of this interaction. Although many of the predicted effects might be ameliorated or eliminated entirely by more careful or considered design, the model captures many of the aerodynamic interactions, and the resultant effects on the loading on the system, that might be expected to characterise the dynamics of such a vehicle. It is suggested that the use of such numerical techniques might eventually allow the various aeromechanical problems that often beset new designs to be circumvented - hopefully well before they manifest on the prototype or production aircraft
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