22,936 research outputs found
Quantum Monte Carlo and exact diagonalization study of a dynamic Hubbard model
A one-dimensional model of electrons locally coupled to spin-1/2 degrees of
freedom is studied by numerical techniques. The model is one in the class of
that describe the relaxation of an atomic orbital
upon double electron occupancy due to electron-electron interactions. We study
the parameter regime where pairing occurs in this model by exact
diagonalization of small clusters. World line quantum Monte Carlo simulations
support the results of exact diagonalization for larger systems and show that
kinetic energy is lowered when pairing occurs. The qualitative physics of this
model and others in its class, obtained through approximate analytic
calculations, is that superconductivity occurs through hole undressing even in
parameter regimes where the effective on-site interaction is strongly
repulsive. Our numerical results confirm the expected qualitative behavior, and
show that pairing will occur in a substantially larger parameter regime than
predicted by the approximate low energy effective Hamiltonian.Comment: Some changes made in response to referees comments. To be published
in Phys.Rev.
Proton decay and light sterile neutrinos
Within the standard model, non-renormalizable operators at dimension six
() violate baryon and lepton number by one unit and thus lead to proton
decay. Here, we point out that the proton decay mode with a charged pion and
missing energy can be a characteristic signature of operators containing
a light sterile neutrino, if it is not accompanied by the standard
final state. We discuss this effect first at the level of effective operators
and then provide a concrete model with new physics at the TeV scale, in which
the lightness of the active neutrinos and the stability of the proton are
related.Comment: 7 pages, 2 figures, published versio
Superconductivity from Undressing. II. Single Particle Green's Function and Photoemission in Cuprates
Experimental evidence indicates that the superconducting transition in high
cuprates is an 'undressing' transition. Microscopic mechanisms giving
rise to this physics were discussed in the first paper of this series. Here we
discuss the calculation of the single particle Green's function and spectral
function for Hamiltonians describing undressing transitions in the normal and
superconducting states. A single parameter, , describes the strength
of the undressing process and drives the transition to superconductivity. In
the normal state, the spectral function evolves from predominantly incoherent
to partly coherent as the hole concentration increases. In the superconducting
state, the 'normal' Green's function acquires a contribution from the anomalous
Green's function when is non-zero; the resulting contribution to
the spectral function is for hole extraction and for hole
injection. It is proposed that these results explain the observation of sharp
quasiparticle states in the superconducting state of cuprates along the
direction and their absence along the direction.Comment: figures have been condensed in fewer pages for easier readin
Compressible Flows in Fluidic Oscillators
We present qualitative observations on the internal flow characteristics of
fluidic oscillator geometries commonly referred to as sweeping jets in active
flow control applications. This is part of the fluid dynamics videos.Comment: Videos include
Lepton number violation phenomenology of d=7 neutrino mass models
We study the phenomenology of d=7 1-loop neutrino mass models. All models in
this particular class require the existence of several new
multiplets, both scalar and fermionic, and thus predict a rich phenomenology at
the LHC. The observed neutrino masses and mixings can easily be fitted in these
models. Interestingly, despite the smallness of the observed neutrino masses,
some particular lepton number violating (LNV) final states can arise with
observable branching ratios. These LNV final states consists of leptons and
gauge bosons with high multiplicities, such as 4l+4W, 6l+2W, etc. We study
current constraints on these models from upper bounds on charged lepton flavour
violating decays, existing lepton number conserving searches at the LHC and
discuss possible future LNV searches
Invisible Higgs Boson Decays in Spontaneously Broken R-Parity
The Higgs boson may decay mainly to an invisible mode characterized by
missing energy, instead of the Standard Model channels. This is a generic
feature of many models where neutrino masses arise from the spontaneous
breaking of ungauged lepton number at relatively low scales, such as
spontaneously broken R-parity models. Taking these models as framework, we
reanalyze this striking suggestion in view of the recent data on neutrino
oscillations that indicate non-zero neutrino masses. We show that, despite the
smallness of neutrino masses, the Higgs boson can decay mainly to the invisible
Goldstone boson associated to the spontaneous breaking of lepton number. This
requires a gauge singlet superfield coupling to the electroweak doublet
Higgses, as in the Next to Minimal Supersymmetric Standard Model (NMSSM)
scenario for solving the -problem. The search for invisibly decaying Higgs
bosons should be taken into account in the planning of future accelerators,
such as the Large Hadron Collider and the Next Linear Collider.Comment: 24 pages, 10 figures; typos corrected, published versio
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