The Cheshire Cat is a relatively poor group of galaxies dominated by two luminous elliptical galaxies surrounded by at least four arcs from gravitationally lensed background galaxies that give the system a humorous appearance. Our combined optical/X-
ray study of this system reveals that it is experiencing a line of sight merger between two groups with a roughly equal mass ratio with a relative velocity of ~1350 km/s. One group was most likely a low-mass fossil group, while the other group would have almost fit the classical definition of a fossil group. The collision manifests itself in a bimodal galaxy velocity distribution, an elevated central X-ray temperature and luminosity indicative of a shock, and gravitational arc centers that do not coincide with either large elliptical galaxy. One of the luminous elliptical galaxies has a double nucleus embedded off-center in the stellar halo. The luminous ellipticals should merge in less than a Gyr, after which observers will see a massive 1.2-1.5 x 10^14 solar mass fossil group with an M_r = -24.0 brightest group galaxy at its center. Thus, the Cheshire Cat offers us the first opportunity to study a fossil group progenitor. We discuss the limitations of the classical definition of a fossil group in terms of magnitude gaps between the member galaxies. We also suggest that if the merging of fossil (or near-fossil) groups is a common avenue for creating present-day fossil groups, the time lag between the final galactic merging of the system and the onset of cooling in the shock-heated core could account for the observed lack of well-developed cool cores in some fossil groups.
A Josephson junction may be driven through a transition where the superconducting condensate favors an odd over an even number of electrons. At this switch in the ground-state fermion parity, an Andreev bound state crosses through the Fermi level, pr
oducing a zero-mode that can be probed by a point contact to a grounded metal. We calculate the time-dependent charge transfer between superconductor and metal for a linear sweep through the transition. One single quasiparticle is exchanged with charge $Q$ depending on the coupling energies $gamma_1,gamma_2$ of the metal to the Majorana operators of the zero-mode. For a single-channel point contact, $Q$ equals the electron charge $e$ in the adiabatic limit of slow driving, while in the opposite quenched limit $Q=2esqrt{gamma_1gamma_2}/(gamma_1+gamma_2)$ varies between $0$ and $e$. This provides a method to produce single charge-neutral quasiparticles on demand.
The geomagnetic field (Bgeo) sets a lower cutoff rigidity (Rc) to the entry of cosmic particles to Earth which depends on the geomagnetic activity. From numerical simulations of the trajectory of a proton using different models for Bgeo (performed wi
th the MAGCOS code), we use backtracking to analyze particles arriving at the location of two nodes of the net LAGO (Large Aperture Gamma ray burst Observatory) that will be built in the near future: Buenos Aires and Marambio (Antarctica), Argentina. We determine the asymptotic trajectories and the values of Rc for different incidence directions, for each node. Simulations were done using several models for Bgeo that emulate different geomagnetic conditions. The presented results will help to make analysis of future observations of the flux of cosmic rays done at these two LAGO nodes.
We present results for the spectrum of excited mesons obtained from temporal correlations of spatially-extended single-hadron and multi-hadron operators computed in lattice QCD. The stochastic LapH algorithm is implemented on anisotropic, dynamical l
attices for isovectors for pions of mass $390$ MeV. A large correlation matrix with single-particle and two-particle probe operators is diagonalized to identify resonances. The masses of excited states in the $I=1, S=0, T_{1u}^+$ channel as well as the mixing of single and multi-particle probe operators are presented.
Progress in computing the hadron spectrum in lattice QCD using stochastic LapH quark propaga- tors is described. The stochastic LapH algorithm is a particular quark smearing algorithm that also allows the computation of all-to-all quark propagators.
All-to-all quark propagators are required in our approach of using a large set of spatially extended hadron operators and explicit multi- particle operators to access excited states. We report on the progress made in the various isospin channels on 2+1 dynamical, anisotropic lattices generated by the Hadron Spectrum Collaboration.
Tree-based models have proven to be an effective solution for web ranking as well as other problems in diverse domains. This paper focuses on optimizing the runtime performance of applying such models to make predictions, given an already-trained mod
el. Although exceedingly simple conceptually, most implementations of tree-based models do not efficiently utilize modern superscalar processor architectures. By laying out data structures in memory in a more cache-conscious fashion, removing branches from the execution flow using a technique called predication, and micro-batching predictions using a technique called vectorization, we are able to better exploit modern processor architectures and significantly improve the speed of tree-based models over hard-coded if-else blocks. Our work contributes to the exploration of architecture-conscious runtime implementations of machine learning algorithms.
In recent years, there has been a substantial amount of work on large-scale data analytics using Hadoop-based platforms running on large clusters of commodity machines. A less-explored topic is how those data, dominated by application logs, are colle
cted and structured to begin with. In this paper, we present Twitters production logging infrastructure and its evolution from application-specific logging to a unified client events log format, where messages are captured in common, well-formatted, flexible Thrift messages. Since most analytics tasks consider the user session as the basic unit of analysis, we pre-materialize session sequences, which are compact summaries that can answer a large class of common queries quickly. The development of this infrastructure has streamlined log collection and data analysis, thereby improving our ability to rapidly experiment and iterate on various aspects of the service.
We report on our progress in computing the excitation spectrum in Lattice QCD. We focus on the isospin 0, 1 and 2 channels using the stochastic LapH algorithm for the quark propagators. For the isospin-0 channel, a new glueball operator constructed f
rom the stochastic LapH operator is included in the variational basis along with the isoscalar meson and pi-pi operators. A representative signal for each channel is presented and the feasibility of extending the calculations to larger lattices is discussed.
The pion-pion scattering phase shift is computed using LapH propagators. The LapH method for computing quark propagators is used to form two-particle correlation functions with a number of different operators. Excited state energies of two-particle s
tates on 2+1 dynamical, anisotropic lattices (Mpi=390 MeV) are computed to determine the phase shift in the isospin-2 channel. The signal for t-to-t diagrams for the isospin-0 channel are also presented to demonstrate the efficacy of the stochastic LapH method which combines LapH with diluted Z4 noise sources.
Correlation functions of the simplest multi-particle state will be presented using distilled quark propagators. The I=2 pi-pi state can be simulated without computing disconnected diagrams and thus is the simplest two-particle state that can be studi
ed with quark sources placed on a single time-slice. We study the quality of the signals of this pi-pi correlation function using the quark-smearing guided distillation method. Results will be presented for pi-pi correlation functions computed on dynamical, anisotropic lattices.
