Measurement of the Bottom-Strange Meson Mixing Phase in the Full CDF Data Set

We report a measurement of the bottom-strange meson mixing phase \beta_s using the time evolution of B0_s -> J/\psi (->\mu+\mu-) \phi (-> K+ K-) decays in which the quark-flavor content of the bottom-strange meson is identified at production. This measurement uses the full data set of proton-antiproton collisions at sqrt(s)= 1.96 TeV collected by the Collider Detector experiment at the Fermilab Tevatron, corresponding to 9.6 fb-1 of integrated luminosity. We report confidence regions in the two-dimensional space of \beta_s and the B0_s decay-width difference \Delta\Gamma_s, and measure \beta_s in [-\pi/2, -1.51] U [-0.06, 0.30] U [1.26, \pi/2] at the 68% confidence level, in agreement with the standard model expectation. Assuming the standard model value of \beta_s, we also determine \Delta\Gamma_s = 0.068 +- 0.026 (stat) +- 0.009 (syst) ps-1 and the mean B0_s lifetime, \tau_s = 1.528 +- 0.019 (stat) +- 0.009 (syst) ps, which are consistent and competitive with determinations by other experiments.Comment: 8 pages, 2 figures, Phys. Rev. Lett 109, 171802 (2012

Multi-messenger observations of a binary neutron star merger

On 2017 August 17 a binary neutron star coalescence candidate (later designated GW170817) with merger time 12:41:04 UTC was observed through gravitational waves by the Advanced LIGO and Advanced Virgo detectors. The Fermi Gamma-ray Burst Monitor independently detected a gamma-ray burst (GRB 170817A) with a time delay of ~1.7 s with respect to the merger time. From the gravitational-wave signal, the source was initially localized to a sky region of 31 deg2 at a luminosity distance of 40+8-8 Mpc and with component masses consistent with neutron stars. The component masses were later measured to be in the range 0.86 to 2.26 Mo. An extensive observing campaign was launched across the electromagnetic spectrum leading to the discovery of a bright optical transient (SSS17a, now with the IAU identification of AT 2017gfo) in NGC 4993 (at ~40 Mpc) less than 11 hours after the merger by the One- Meter, Two Hemisphere (1M2H) team using the 1 m Swope Telescope. The optical transient was independently detected by multiple teams within an hour. Subsequent observations targeted the object and its environment. Early ultraviolet observations revealed a blue transient that faded within 48 hours. Optical and infrared observations showed a redward evolution over ~10 days. Following early non-detections, X-ray and radio emission were discovered at the transient’s position ~9 and ~16 days, respectively, after the merger. Both the X-ray and radio emission likely arise from a physical process that is distinct from the one that generates the UV/optical/near-infrared emission. No ultra-high-energy gamma-rays and no neutrino candidates consistent with the source were found in follow-up searches. These observations support the hypothesis that GW170817 was produced by the merger of two neutron stars in NGC4993 followed by a short gamma-ray burst (GRB 170817A) and a kilonova/macronova powered by the radioactive decay of r-process nuclei synthesized in the ejecta

Energetics and dynamics of SNAREpin folding across lipid bilayers

International audienceMembrane fusion occurs when SNAREpins fold up between lipid bilayers. How much energy is generated during SNAREpin folding and how this energy is coupled to the fusion of apposing membranes is unknown. We have used a surface forces apparatus to determine the energetics and dynamics of SNAREpin formation and characterize the different intermediate structures sampled by cognate SNAREs in the course of their assembly. The interaction energy-versus-distance profiles of assembling SNAREpins reveal that SNARE motifs begin to interact when the membranes are 8 nm apart. Even after very close approach of the bilayers (B2-4 nm), the SNAREpins remain partly unstructured in their membrane-proximal region. The energy stabilizing a single SNAREpin in this configuration (35 k B T) corresponds closely with the energy needed to fuse outer but not inner leaflets (hemifusion) of pure lipid bilayers (40-50 k B T). Intercellular communication and intracellular protein transport rely upon the fusion of cargo-containing vesicles with target membranes. As lipid bilayers are inherently stable, such fusion events are energetically costly and require specialized fusion proteins that harvest the energy made available during their own binding and folding to drive membrane disruption and merging 1-5. In neuronal synapses, the core of the fusion machinery consists of three proteins from the SNARE family: the synaptic vesicle (v)-SNARE protein VAMP-2 and the two target plasma membrane (t)-SNARE proteins syntaxin-1A and SNAP-25 (refs. 6-8). When separately reconstituted into synthetic liposomes or ectopically expressed on the surfaces of cells, neuronal v-and t-SNARE proteins are sufficient to drive membrane fusion through their assembly in the form of SNAREpins 2,9. The interacting domains of SNARE proteins (SNARE motifs) contain 60-70 amino acid residues; they are mostly unstruc-tured as monomers 10-12 and assemble in solution into a highly stable heterotrimer consisting of four a-helices aligned in parallel, with VAMP-2 and syntaxin-1A each contributing one helix and SNAP-25 contributing two helices 13,14. In the context of lipid bilayers, the assembly of SNAREs starts at their membrane-distal N termini and proceeds toward their membrane-proximal C termini (zipper model), a process that also includes passage through a stable intermediate binding state 15-21. This zipper-like assembly progressively brings the membranes into close apposition and creates a tight bridge between them that triggers lipid bilayer fusion. Progressive assembly of SNAREs may culminate in a release of energy sufficient to drive membrane merging. Alternatively, the assembling SNAREs may pass through a series of intermediates, each of which contributes enough energy for advancement through the successive stages of membrane fusion. Characterization of these inter-mediates requires the capacity to measure the interactions between membrane-associated proteins at nanometer distance resolutions. Thermodynamic and atomic force microscopy (AFM) measurements have successfully described the kinetics of SNARE assembly and disassembly in solution 22 and the rupture forces of SNARE complexes affixed to solid supports 23,24. However, none of these studies has been able to offer information about the dynamics and energetics of SNAREpin folding, including conformational changes and distance-energy correlations during SNARE assembly. Furthermore, the previous experiments were not performed in the context of lipid bilayers, preventing any investigation of the interplay of lipids and SNARE proteins in membrane interaction and fusion. Here, we have investigated these questions using the surface forces apparatus (SFA), which directly measures the interaction energy between two facing functionalized membranes as a function of their separation distance and makes it possible to identify molecular rearrangements of interacting species during their association 25. Direct measurements of force versus distance between membrane-embedded neuronal SNARE proteins (derived from mouse and rat) allow us to explore in real time the molecular details of SNAREpin formation across two lipid bilayers, including conformational changes, kinetics of association, binding energy and extent of assembly. RESULTS Interactions between SNAREs in apposing bilayers Force was measured between two mica-supported lipid bilayers reconstituted with the neuronal cognate t-and v-SNARE proteins (Fig. 1). The surfaces were approached toward each other and the