Particle tracking in the CALET experiment

The Calorimetric Electron Telescope (CALET) is a space mission installed on the Exposed Fa- cility of the Japanese Experiment Module (JEM-EF) of the International Space Station (ISS) in August 2015 and collecting data since October 2015. In addition to high precision measure- ments of the electron spectrum up to TeV scale, CALET will also investigate the mechanism of cosmic-ray (CR) acceleration and propagation in the Galaxy, by performing direct measurements of the energy spectra and elemental composition of CR nuclei from H to Fe, and the abundance of trans-iron elements up to about Z=40. The instrument consists of two layers of segmented plas- tic scintillators to identify the particle charge, a thin (3 radiation lengths) tungsten-scintillating fiber calorimeter providing accurate particle tracking, and a thick (27 radiation lengths) calorime- ter made of lead-tungstate crystal logs. In this paper we will describe an original reconstruction method of the primary particle direction based on a combinatorial Kalman filter algorithm. This method exploits the fine granularity and imaging capability of the IMC and provides robust track finding and fitting, allowing to identify the incident CR track in a large amount of shower par- ticle tracks backscattered from the calorimeter. The track fitting algorithm has been extensively validated and tuned with simulated data. Its performance (angular resolution, impact point res- olution, tracking efficiency) for electrons and nuclei will be discussed and comparisons between flight data and simulations will be shown

Pair-breaking quantum phase transition in superconducting nanowires

A quantum phase transition (QPT) between distinct ground states of matter is a wide-spread phenomenon in nature, yet there are only a few experimentally accessible systems where the microscopic mechanism of the transition can be tested and understood. These cases are unique and form the experimentally established foundation for our understanding of quantum critical phenomena. Here we report the discovery that a magnetic-field-driven QPT in superconducting nanowires - a prototypical 1d-system - can be fully explained by the critical theory of pair-breaking transitions characterized by a correlation length exponent $\nu \approx 1$ and dynamic critical exponent $z \approx 2$. We find that in the quantum critical regime, the electrical conductivity is in agreement with a theoretically predicted scaling function and, moreover, that the theory quantitatively describes the dependence of conductivity on the critical temperature, field magnitude and orientation, nanowire cross sectional area, and microscopic parameters of the nanowire material. At the critical field, the conductivity follows a $T^{(d-2)/z}$ dependence predicted by phenomenological scaling theories and more recently obtained within a holographic framework. Our work uncovers the microscopic processes governing the transition: The pair-breaking effect of the magnetic field on interacting Cooper pairs overdamped by their coupling to electronic degrees of freedom. It also reveals the universal character of continuous quantum phase transitions.Comment: 22 pages, 5 figure

Arresting bubble coarsening: A two-bubble experiment to investigate grain growth in presence of surface elasticity

Many two-phase materials suffer from grain-growth due to the energy cost which is associated with the interface that separates both phases. While our understanding of the driving forces and the dynamics of grain growth in different materials is well advanced by now, current research efforts address the question of how this process may be slowed down, or, ideally, arrested. We use a model system of two bubbles to explore how the presence of a finite surface elasticity may interfere with the coarsening process and the final grain size distribution. Combining experiments and modelling in the analysis of the evolution of two bubbles, we show that clear relationships can be predicted between the surface tension, the surface elasticity and the initial/final size ratio of the bubbles. We rationalise these relationships by the introduction of a modified Gibbs criterion. Besides their general interest, the present results have direct implications for our understanding of foam stability