Study of the rare decays of B 0 s and B 0 mesons into muon pairs using data collected during 2015 and 2016 with the ATLAS detector The ATLAS Collaboration A study of the decays B 0 s → µ + µ − and B 0 → µ + µ − has been performed using 26.3 fb −1 of 13 TeV LHC proton-proton collision data collected with the ATLAS detector in 2015 and 2016. Since the detector resolution in µ + µ − invariant mass is comparable to the B 0 s -B 0 mass difference, a single fit determines the signal yields for both decay modes. This results in a measurement of the branching fraction B(B 0 s → µ + µ − ) = 3.2 +1.1 −1.0 × 10 −9 and an upper limit B(B 0 → µ + µ − ) < 4.3 × 10 −10 at 95% confidence level. The result is combined with the Run 1 ATLAS result, yielding B(B 0 s → µ + µ − ) = 2.8 +0.8 −0.7 ×10 −9 and B(B 0 → µ + µ − ) < 2.1×10 −10 at 95% confidence level. The combined result is consistent with the Standard Model prediction within 2.4 standard deviations in the B(B 0 → µ + µ − )-B(B 0 s → µ + µ − ) plane.

We describe a new technique, using thin lines of triplet-state He * 2 molecular tracers created by femtosecond-laser field-ionization of helium atoms, for visualizing the flow of the normal fluid in superfluid 4 He, together with its application to thermal counterflow in a channel. We show that, at relatively small velocities, where the superfluid is already turbulent, the flow of the normal fluid remains laminar, but with a distorted velocity profile, while at a higher velocity there is a transition to turbulence. The form of the structure function in this turbulent state differs significantly from that found in types of conventional turbulence. This visualization technique also promises to be applicable to other fluid dynamical problems involving cryogenic helium.

Metastable helium molecules generated in a discharge near a sharp tungsten tip immersed in superfluid 4He are imaged using a laser-induced-fluorescence technique. By pulsing the tip, a small cloud of He(2*) molecules is produced. We can determine the normal-fluid velocity in a heat-induced counterflow by tracing the position of a single molecule cloud. As we run the tip in continuous field-emission mode, a normal-fluid jet from the tip is generated and molecules are entrained in the jet. A focused 910 nm pump laser pulse is used to drive a small group of molecules to the first excited vibrational level of the triplet ground state. Subsequent imaging of the tagged molecules with an expanded 925 nm probe laser pulse allows us to measure the flow velocity of the jet. The techniques we developed provide new tools in quantitatively studying the normal fluid flow in superfluid helium.

Cryogenic flow visualization techniques have been proved in recent years to be a very powerful experimental method to study superfluid turbulence. Micron-sized solid particles and metastable helium molecules are specifically being used to investigate in detail the dynamics of quantum flows. These studies belong to a well-established, interdisciplinary line of inquiry that focuses on the deeper understanding of turbulence, one of the open problem of modern physics, relevant to many research fields, ranging from fluid mechanics to cosmology. Progress made to date is discussed, to highlight its relevance to a wider scientific community, and future directions are outlined. The latter include, e.g., detailed studies of normal-fluid turbulence, dissipative mechanisms, and unsteady/oscillatory flows. He is used as a coolant for superconducting magnets and infrared detectors (2), to astrophysics, where superfluidity is invoked to explain glitches in the rotation of neutron stars (3, 4) and the formation of cosmic strings (5, 6). More recently, superfluidity has been used to describe the collective behavior of birds (7) and a cosmological model has been used to obtain results relevant to superfluid turbulence (8). The latter form of turbulence, occurring in quantum fluids, is indeed an especially interesting topic because of its quantum peculiarities and its similarity to classical turbulence. Superfluids, in which turbulence can be directly visualized and studied, include superfluid 4 He and atomic Bose-Einstein condensates (9). Due to the limit of small sample volumes, the experimental study of turbulence in Bose-Einstein condensates has hardly begun. The development of visualization techniques applicable to superfluid 4 He is thus essential, if our understanding of quantum turbulence is to make significant progress in the near future.Superfluid 4 He is viewed as consisting of two interpenetrating fluids. The gas of thermal excitations forms the normal component, which can be considered as a viscous fluid. The superfluid component is inviscid and its rotational motion is possible only in the presence of topological defects, in the form of quantized vortex filaments. Turbulence in the superfluid component therefore takes the form of a tangle of quantized vortex lines. Turbulence in the normal fluid is more conventional, although the interaction between the normal fluid and the vortices leads to the nonclassical force of mutual friction between the two fluids. Turbulence in such a system can exhibit a behavior that is similar to that found in a classical fluid; but it may take forms that are unknown in classical fluid mechanics: for example, forms relevant to a fluid in which there is no viscous dissipation, and those that depend on the coexistence of the two fluids. Study of quantum turbulence can therefore enrich our knowledge of turbulence in general, as well as being interesting in its own right. Visualization TechniquesFlow visualization techniques have been developed to a high degree of precision and speed for clas...