A variational approach to the low energy properties of even-legged d-dimensional quantum spin systems

We develop a variational formalism in order to study the structure of low energy spectra of frustrated quantum spin systems. It is first applied to trial wavefunctions of ladders with one spin-1/2 on each site. We determine energy minima of these states. The variational ground state shows a finite energy gap with respect to the energies of states which span the Hilbert space and are orthogonal to it. This is the case for any size of the system. Under some justifiable approximations the argument can be extended to even-legged ladder systems in 2d and higher dimensional spaces. The Hamiltonian can contain spin-spin coupling interactions of any range. For specific values of the coupling strengths level degeneracies can occur.Comment: 28 pages, 3 figure

A Renormalisation Approach to Effective Interactions in Hilbert Space

The low-lying bound states of a microscopic quantum many-body system of $n$ particles and the related physical observables can be worked out in a truncated $n$--particle Hilbert space. We present here a non-perturbative analysis of this problem which relies on a renormalisation concept and work out the link with perturbative approaches.Comment: 5 pages, no figures, LateX fil

ALICE light-flavor results at intermediate and high pT in p-Pb and Pb-Pb collisions

The ALICE experiment has unique capabilities for particle identification at mid rapidity over a wide range of transverse momenta, making it an ideal tool for comprehensive measurements of hadrons such as charged pions, kaons, and protons as well as lambdas, K0s, and phi. The transverse momentum distributions and nuclear modification factors, R_pPb and R_PbPb, of these hadrons measured in p-Pb and Pb-Pb collisions are presented. Baryon-to-meson ratios exhibit a multiplicity-dependent enhancement at intermediate transverse momenta for both p-Pb and Pb-Pb collisions, while no significant dynamics is observed in the ratios at larger transverse momenta. Finally, measurements of identified particle ratios in association with high-pT particles as well as within reconstructed jets are presented.Comment: Proceedings of Winter Workshop in Nuclear Dynamics (WWND) conference, 26-31 January, 201

Is binary sequential decay compatible with the fragmentation of nuclei at high energy?

We use a binary sequential decay model in order to describe the fragmentation of a nucleus induced by the high energy collisions of protons with Au nuclei. Overall agreement between measured and calculated physical observables is obtained. We evaluate and analyse the decay times obtained with two different parametrisations of the decay rates and discuss the applicability of the model to high energy fragmentation.Comment: 6 pages, 4 eps figures. Small changes at the end of the text. More arguments are given in the discussion of the time scale of the proces

Robust distributed linear programming

This paper presents a robust, distributed algorithm to solve general linear programs. The algorithm design builds on the characterization of the solutions of the linear program as saddle points of a modified Lagrangian function. We show that the resulting continuous-time saddle-point algorithm is provably correct but, in general, not distributed because of a global parameter associated with the nonsmooth exact penalty function employed to encode the inequality constraints of the linear program. This motivates the design of a discontinuous saddle-point dynamics that, while enjoying the same convergence guarantees, is fully distributed and scalable with the dimension of the solution vector. We also characterize the robustness against disturbances and link failures of the proposed dynamics. Specifically, we show that it is integral-input-to-state stable but not input-to-state stable. The latter fact is a consequence of a more general result, that we also establish, which states that no algorithmic solution for linear programming is input-to-state stable when uncertainty in the problem data affects the dynamics as a disturbance. Our results allow us to establish the resilience of the proposed distributed dynamics to disturbances of finite variation and recurrently disconnected communication among the agents. Simulations in an optimal control application illustrate the results