22,741 research outputs found
Enhancing with the -penguin
Lepton flavor violation (LFV) has been observed in neutrino oscillations. For
charged lepton FV decays only upper limits are known, but sizable branching
ratios are expected in many neutrino mass models. High scale models, such as
the classical supersymmetric seesaw, usually predict that decays are roughly a factor maller than the corresponding decays . Here we demonstrate that the -penguin diagram can give an
enhancement for decays in many extensions of the MSSM. We first
discuss why the -penguin is not dominant in the MSSM with seesaw and show
that much larger contributions from the -penguin are expected in general.
We then demonstrate the effect numerically in two example models, namely, the
supersymmetric inverse seesaw and R-parity violating supersymmetry.Comment: 5 pages; v2: minor corrections, final version to appear in PR
A Modern Union for the Modern Economy
Membership in traditional unions has steeply declined over the past two decades. As the White House and Congress are now completely Republican controlled, there promises to be no reversal of this trend in the near future. In the face of this rejection of traditional bargaining efforts, several attempts have been made to create alternative “quasi-union” or “alt-labor” relationships between workers and employers. These arrangements represent a creative approach by workers to have their voices heard in a collective manner, though still falling far short of the traditional protections afforded by employment and labor law statutes. This Article critiques one such high-profile, quasi-union effort in the technology sector—the Uber Guild. While the Guild does not provide any of the traditional bargaining protections found in the National Labor Relations Act (NLRA), it offers Uber drivers some input over the terms and conditions under which they work. Falling somewhere between employment-at-will and unionization protected under the NLRA, the Uber Guild is a creative attempt to help both workers and the company to better understand how they can improve the working relationship. This Article navigates the Uber Guild and other nontraditional efforts that promise a collective voice for workers in the face of a precipitous decline in union membership. This Article further explores how workers in the technology sector face unique challenges under workplace laws. We argue that these workers are particularly well situated to benefit from a nontraditional union model and explain what that model should look like. While a traditional union protected by the NLRA is the optimal bargaining arrangement, we must consider the enormous challenges workers in the technology sector face in obtaining these protections
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.
R-parity Conserving Supersymmetry, Neutrino Mass and Neutrinoless Double Beta Decay
We consider contributions of R-parity conserving softly broken supersymmetry
(SUSY) to neutrinoless double beta (\znbb) decay via the (B-L)-violating
sneutrino mass term. The latter is a generic ingredient of any weak-scale SUSY
model with a Majorana neutrino mass. The new R-parity conserving SUSY
contributions to \znbb are realized at the level of box diagrams. We derive
the effective Lagrangian describing the SUSY-box mechanism of \znbb-decay and
the corresponding nuclear matrix elements. The 1-loop sneutrino contribution to
the Majorana neutrino mass is also derived.
Given the data on the \znbb-decay half-life of Ge and the neutrino
mass we obtain constraints on the (B-L)-violating sneutrino mass. These
constraints leave room for accelerator searches for certain manifestations of
the 2nd and 3rd generation (B-L)-violating sneutrino mass term, but are most
probably too tight for first generation (B-L)-violating sneutrino masses to be
searched for directly.Comment: LATEX, 29 pages + 4 (uuencoded) figures appende
Superconductivity from Undressing
Photoemission experiments in high cuprates indicate that quasiparticles
are heavily 'dressed' in the normal state, particularly in the low doping
regime. Furthermore these experiments show that a gradual undressing occurs
both in the normal state as the system is doped and the carrier concentration
increases, as well as at fixed carrier concentration as the temperature is
lowered and the system becomes superconducting. A similar picture can be
inferred from optical experiments. It is argued that these experiments can be
simply understood with the single assumption that the quasiparticle dressing is
a function of the local carrier concentration. Microscopic Hamiltonians
describing this physics are discussed. The undressing process manifests itself
in both the one-particle and two-particle Green's functions, hence leads to
observable consequences in photoemission and optical experiments respectively.
An essential consequence of this phenomenology is that the microscopic
Hamiltonians describing it break electron-hole symmetry: these Hamiltonians
predict that superconductivity will only occur for carriers with hole-like
character, as proposed in the theory of hole superconductivity
Towards an understanding of hole superconductivity
From the very beginning K. Alex M\"uller emphasized that the materials he and
George Bednorz discovered in 1986 were superconductors. Here I would
like to share with him and others what I believe to be key reason for why
high cuprates as well as all other superconductors are hole
superconductors, which I only came to understand a few months ago. This paper
is dedicated to Alex M\"uller on the occasion of his 90th birthday.Comment: Dedicated to Alex M\"uller on the Occasion of his 90th Birthday.
arXiv admin note: text overlap with arXiv:1703.0977
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
Determining R-parity violating parameters from neutrino and LHC data
In supersymmetric models neutrino data can be explained by R-parity violating
operators which violate lepton number by one unit. The so called bilinear model
can account for the observed neutrino data and predicts at the same time
several decay properties of the lightest supersymmetric particle. In this paper
we discuss the expected precision to determine these parameters by combining
neutrino and LHC data and discuss the most important observables. We show that
one can expect a rather accurate determination of the underlying R-parity
parameters assuming mSUGRA relations between the R-parity conserving ones and
discuss briefly also the general MSSM as well as the expected accuracies in
case of a prospective e+ e- linear collider. An important observation is that
several parameters can only be determined up to relative signs or more
generally relative phases.Comment: 13 pages, 13 figure
New Leptoquark Mechanism of Neutrinoless Double Beta Decay
A new mechanism for neutrinoless double beta (\znbb) decay based on
leptoquark exchange is discussed. Due to the specific helicity structure of the
effective four-fermion interaction this contribution is strongly enhanced
compared to the well-known mass mechanism of \znbb decay. As a result the
corresponding leptoquark parameters are severely constrained from
non-observation of \znbb-decay. These constraints are more stringent than
those derived from other experiments.Comment: LaTeX, 6 pages, 1 figur
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