A search for heavy Majorana neutrinos produced in the B- → π+ μ- μ- decay mode is performed using 3  fb(-1) of integrated luminosity collected with the LHCb detector in pp collisions at center-of-mass energies of 7 and 8 TeV at the LHC. Neutrinos with masses in the range 250 to 5000 MeV and lifetimes from zero to 1000 ps are probed. In the absence of a signal, upper limits are set on the branching fraction B(B- → π+ μ- μ-) as functions of neutrino mass and lifetime. These limits are on the order of 10(-9) for short neutrino lifetimes of 1 ps or less. Limits are also set on the coupling between the muon and a possible fourth-generation neutrino.

No abstract

An important concept in bone mechanics is that osteons influence mechanical properties in several ways, including contributing to toughness and fatigue strength by debonding from the interstitial matrix so as to "bridge" developing cracks. Observations of "pulled out" osteons on fracture surfaces are thought to be indicative of such behavior. We tested the hypothesis that osteon pullout varies with mode of loading (fatigue vs. monotonic), cortical region, elastic modulus, and fatigue life. Mid-diaphseal beams from the dorsal, medial, and lateral regions of the equine third metacarpal bone were fractured in four point bending by monotonic loading to failure under deflection control, with or without lo5 cycles of previous fatigue loading producing 5000 microstrain (15-20'%) of the expected failure strain) on the first cycle; or sinusoidal fatigue loading to failure, under load or deflection control, with the initial cycle producing 10,000 microstrain (3040% of the expected failure strain). Using scanning electron microscopy, percent fracture surface area exhibiting osteon pullout (%OP.Ar) was measured. Monotonically loaded specimens and the compression side of fatigue fracture surfaces exhibited no osteon pullout. In load-controlled fatigue, pullout was present on the tension side of fracture surfaces, was regionally dependent (occurring to a greater amount dorsally), and was correlated negatively with elastic modulus and positively with fatigue life. Regional variation in '%OP.Ar was also significant for the pooled (load and deflection controlled) fatigue specimens. '%)OP.Ar was nearly significantly greater in deflection controlled fatigue specimens than in load-controlled specimens (p = 0.059). The data suggest that tensile fatigue loading of cortical bone eventually introduces damage that results in osteonal debonding and pullout, which is also associated with increased fatigue life via mechanisms that are not yet clear.

In studying the flexural fatigue properties of the equine third metacarpal (cannon) bone, we previously found that the dorsal region was weaker monotonically, but more fatigue resistant, than the lateral region. Fatigue resistance was associated with fracture surfaces which demonstrated that secondary osteons had "pulled out" of the surrounding matrix; this never happened in lateral specimens. We therefore became interested in the osteonal structure of this bone, and began to study its birefringence patterns in circularly polarized light. We found that the predominant type of secondary osteon was one in which only the outermost few lamellae were circumferential, with the inner lamellae being longitudinally oriented. This "hoop" pattern had not been described in Ascenzi's classic papers. Using basic fuchsin-stained, undecalcified cross-sections from the dorsal, medial, and lateral midshaft regions of 12 pairs of cannon bones, we classified 360 secondary osteons according to their birefringence patterns, and measured their inner and outer diameters. We found that variants of the hoop category comprised 60% of all osteons, but were significantly less common in the dorsal region, where the predominant types were Ascenzi's "longitudinal" or "alternating" patterns. The dorsal region also had smaller osteons (OD = 156 +/- 19 microns) than the medial (179 +/- 13 microns, p = 0.0004) and lateral (182 +/- 13 microns, p = 0.0001) regions. We postulate that these regional variations in osteonal size and structure, which are obviously produced by regional variations in remodeling, have important mechanical implications.