12,769 research outputs found
Searches for Majorana Neutrinos and Direct Searches for Exotics at LHCb
These proceedings present the LHCb results on Majorana neutrino searches and
direct production of exotic particles using the data collected during Run I of
LHC. For the former, Majorana neutrinos are searched for both on-shell and
off-shell in and decays to final states with two same-sign muons. For
the latter, different types of new particles are studied profiting the unique
coverage of LHCb with respect to other detectors.Comment: 9 pages, 27 figures. To be published in the LISHEP 2015 proceeding
Muon Identification in the LHCb experiment
A short summary of the LHCb muon identification procedure is given in this
article. First, the muon system of LHCb is presented, together with some
examples of physics measurements of the experiment where the muon
identification is crucial. Then, the muon identification algorithm is
introduced in three single steps. With this, the efficiency vs.
misidentification rate is shown for MC simulated data. The way this method will
be calibrated with real data is also seen. Finally, some preliminary muon
identification results with proton-proton collisions at sqrt(s) = 900 GeV are
presented.Comment: Proceedings for the Moriond 2010 E
Emergent Fermions and Anyons in the Kitaev Model
We study the gapped phase of the Kitaev model on the honeycomb lattice using
perturbative continuous unitary transformations. The effective low-energy
Hamiltonian is found to be an extended toric code with interacting anyons.
High-energy excitations are emerging free fermions which are composed of
hardcore bosons with an attached string of spin operators. The excitation
spectrum is mapped onto that of a single particle hopping on a square lattice
in a magnetic field. We also illustrate how to compute correlation functions in
this framework. The present approach yields analytical perturbative results in
the thermodynamical limit without using the Majorana or the Jordan-Wigner
fermionization initially proposed to solve this problem.Comment: 4 pages, 5 figures, published versio
Concurrence in collective models
We review the entanglement properties in collective models and their
relationship with quantum phase transitions. Focusing on the concurrence which
characterizes the two-spin entanglement, we show that for first-order
transition, this quantity is singular but continuous at the transition point,
contrary to the common belief. We also propose a conjecture for the concurrence
of arbitrary symmetric states which connects it with a recently proposed
criterion for bipartite entanglement.Comment: 8 pages, 2 figures, published versio
Mixture of multiple copies of maximally entangled states is quasi-pure
Employing the general BXOR operation and local state discrimination, the
mixed state of the form
\rho^{(k)}_{d}=\frac{1}{d^{2}}\sum_{m,n=0}^{d-1}(|\phi_{mn}><\phi_{mn}|)^{\otim
es k} is proved to be quasi-pure, where is the canonical set
of mutually orthogonal maximally entangled states in . Therefore
irreversibility does not occur in the process of distillation for this family
of states. Also, the distillable entanglement is calculated explicitly.Comment: 6 pages, 1 figure. The paper is subtantially revised and the general
proof is give
Interactions in Quasicrystals
Although the effects of interactions in solid state systems still remains a
widely open subject, some limiting cases such as the three dimensional Fermi
liquid or the one-dimensional Luttinger liquid are by now well understood when
one is dealing with interacting electrons in {\it periodic} crystalline
structures. This problem is much more fascinating when periodicity is lacking
as it is the case in {\it quasicrystalline} structures. Here, we discuss the
influence of the interactions in quasicrystals and show, on a controlled
one-dimensional model, that they lead to anomalous transport properties,
intermediate between those of an interacting electron gas in a periodic and in
a disordered potential.Comment: Proceedings of the Many Body X conference (Seattle, Sept. 99); 9
pages; uses epsfi
Continuous unitary transformations and finite-size scaling exponents in the Lipkin-Meshkov-Glick model
We analyze the finite-size scaling exponents in the Lipkin-Meshkov-Glick
model by means of the Holstein-Primakoff representation of the spin operators
and the continuous unitary transformations method. This combination allows us
to compute analytically leading corrections to the ground state energy, the
gap, the magnetization, and the two-spin correlation functions. We also present
numerical calculations for large system size which confirm the validity of this
approach. Finally, we use these results to discuss the entanglement properties
of the ground state focusing on the (rescaled) concurrence that we compute in
the thermodynamical limit.Comment: 20 pages, 9 figures, published versio
Exploring corner transfer matrices and corner tensors for the classical simulation of quantum lattice systems
In this paper we explore the practical use of the corner transfer matrix and
its higher-dimensional generalization, the corner tensor, to develop tensor
network algorithms for the classical simulation of quantum lattice systems of
infinite size. This exploration is done mainly in one and two spatial
dimensions (1d and 2d). We describe a number of numerical algorithms based on
corner matri- ces and tensors to approximate different ground state properties
of these systems. The proposed methods make also use of matrix product
operators and projected entangled pair operators, and naturally preserve
spatial symmetries of the system such as translation invariance. In order to
assess the validity of our algorithms, we provide preliminary benchmarking
calculations for the spin-1/2 quantum Ising model in a transverse field in both
1d and 2d. Our methods are a plausible alternative to other well-established
tensor network approaches such as iDMRG and iTEBD in 1d, and iPEPS and TERG in
2d. The computational complexity of the proposed algorithms is also considered
and, in 2d, important differences are found depending on the chosen simulation
scheme. We also discuss further possibilities, such as 3d quantum lattice
systems, periodic boundary conditions, and real time evolution. This discussion
leads us to reinterpret the standard iTEBD and iPEPS algorithms in terms of
corner transfer matrices and corner tensors. Our paper also offers a
perspective on many properties of the corner transfer matrix and its
higher-dimensional generalizations in the light of novel tensor network
methods.Comment: 25 pages, 32 figures, 2 tables. Revised version. Technical details on
some of the algorithms have been moved to appendices. To appear in PR
Pathological element-based active device models and their application to symbolic analysis
This paper proposes new pathological element-based active device models which can be used in analysis tasks of linear(ized) analog circuits. Nullators and norators along with the voltage mirror-current mirror (VM-CM) pair (collectively known as pathological elements) are used to model the behavior of active devices in voltage-, current-, and mixed-mode, also considering parasitic elements. Since analog circuits are transformed to nullor-based equivalent circuits or VM-CM pairs or as a combination of both, standard nodal analysis can be used to formulate the admittance matrix. We present a formulation method in order to build the nodal admittance (NA) matrix of nullor-equivalent circuits, where the order of the matrix is given by the number of nodes minus the number of nullors. Since pathological elements are used to model the behavior of active devices, we introduce a more efficient formulation method in order to compute small-signal characteristics of pathological element-based equivalent circuits, where the order of the NA matrix is given by the number of nodes minus the number of pathological elements. Examples are discussed in order to illustrate the potential of the proposed pathological element-based active device models and the new formulation method in performing symbolic analysis of analog circuits. The improved formulation method is compared with traditional formulation methods, showing that the NA matrix is more compact and the generation of nonzero coefficients is reduced. As a consequence, the proposed formulation method is the most efficient one reported so far, since the CPU time and memory consumption is reduced when recursive determinant-expansion techniques are used to solve the NA matrix.Promep-Mexico UATLX-PTC-088Junta de Andalucía TIC-2532Ministerio de Educación y Ciencia TEC2007-67247, TEC2010-14825UC-MEXUS-CONACyT CN-09-31
Hahn echo and criticality in spin-chain systems
We establish a relation between Hahn spin-echo of a spin-
particle and quantum phase transition in a spin-chain, which couples to the
particle. The Hahn echo is calculated and discussed at zero as well as at
finite temperatures. On the example of XY model, we show that the critical
points of the chain are marked by the extremal values in the Hahn echo, and
influence the Hahn echo in surprising high temperature. An explanation for the
relation between the echo and criticality is also presented.Comment: 5 pages, 6 figure
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