Cerenkov angle and charge reconstruction with the RICH detector of the AMS experiment

The Alpha Magnetic Spectrometer (AMS) experiment to be installed on the International Space Station (ISS) will be equipped with a proximity focusing Ring Imaging Cerenkov (RICH) detector, for measurements of particle electric charge and velocity. In this note, two possible methods for reconstructing the Cerenkov angle and the electric charge with the RICH, are discussed. A Likelihood method for the Cerenkov angle reconstruction was applied leading to a velocity determination for protons with a resolution of around 0.1%. The existence of a large fraction of background photons which can vary from event to event, implied a charge reconstruction method based on an overall efficiency estimation on an event-by-event basis.Comment: Proceedings submitted to RICH 2002 (Pylos-Greece

Antiresonance and interaction-induced localization in spin and qubit chains with defects

We study a spin chain with an anisotropic XXZ coupling in an external field. Such a chain models several proposed types of a quantum computer. The chain contains a defect with a different on-site energy. The interaction between excitations is shown to lead to two-excitation states localized next to the defect. In a resonant situation scattering of excitations on each other might cause decay of an excitation localized on the defect. We find that destructive quantum interference suppresses this decay. Numerical results confirm the analytical predictions.Comment: Updated versio

Quantum interference-induced stability of repulsively bound pairs of excitations

We study the dynamics of two types of pairs of excitations which are bound despite their strong repulsive interaction. One corresponds to doubly occupied sites in one-dimensional Bose-Hubbard systems, the so-called doublons. The other is pairs of neighboring excited spins in anisotropic Heisenberg spin-1/2 chains. We investigate the possibility of decay of the bound pairs due to resonant scattering by a defect or due to collisions of the pairs. We find that the amplitudes of the corresponding transitions are very small. This is a result of destructive quantum interference and explains the stability of the bound pairs.Comment: 12 pages, 3 figure

Entanglement of excited states in critical spin chians

Renyi and von Neumann entropies quantifying the amount of entanglement in ground states of critical spin chains are known to satisfy a universal law which is given by the Conformal Field Theory (CFT) describing their scaling regime. This law can be generalized to excitations described by primary fields in CFT, as was done in reference (Alcaraz et. al., Phys. Rev. Lett. 106, 201601 (2011)), of which this work is a completion. An alternative derivation is presented, together with numerical verifications of our results in different models belonging to the c=1,1/2 universality classes. Oscillations of the Renyi entropy in excited states and descendant fields are also discussed.Comment: 23 pages, 13 figure

Integrability of a disordered Heisenberg spin-1/2 chain

We investigate how the transition from integrability to nonintegrability occurs by changing the parameters of the Hamiltonian of a Heisenberg spin-1/2 chain with defects. Randomly distributed defects may lead to quantum chaos. A similar behavior is obtained in the presence of a single defect out of the edges of the chain, suggesting that randomness is not the cause of chaos in these systems, but the mere presence of a defect.Comment: 4 pages, 4 figure

On the Distribution of a Second Class Particle in the Asymmetric Simple Exclusion Process

We give an exact expression for the distribution of the position X(t) of a single second class particle in the asymmetric simple exclusion process (ASEP) where initially the second class particle is located at the origin and the first class particles occupy the sites {1,2,...}