912 research outputs found
Abelian Z-theory: NLSM amplitudes and alpha'-corrections from the open string
In this paper we derive the tree-level S-matrix of the effective theory of
Goldstone bosons known as the non-linear sigma model (NLSM) from string theory.
This novel connection relies on a recent realization of tree-level
open-superstring S-matrix predictions as a double copy of super-Yang-Mills
theory with Z-theory --- the collection of putative scalar effective field
theories encoding all the alpha'-dependence of the open superstring. Here we
identify the color-ordered amplitudes of the NLSM as the low-energy limit of
abelian Z-theory. This realization also provides natural higher-derivative
corrections to the NLSM amplitudes arising from higher powers of alpha' in the
abelian Z-theory amplitudes, and through double copy also to Born-Infeld and
Volkov-Akulov theories. The Kleiss-Kuijf and Bern-Carrasco-Johansson relations
obeyed by Z-theory amplitudes thereby apply to all alpha'-corrections of the
NLSM. As such we naturally obtain a cubic-graph parameterization for the
abelian Z-theory predictions whose kinematic numerators obey the duality
between color and kinematics to all orders in alpha'.Comment: 37 pages; v2: references, explanations and arguments for
factorization added; published versio
Towards Guidelines for Preventing Critical Requirements Engineering Problems
Context] Problems in Requirements Engineering (RE) can lead to serious
consequences during the software development lifecycle. [Goal] The goal of this
paper is to propose empirically-based guidelines that can be used by different
types of organisations according to their size (small, medium or large) and
process model (agile or plan-driven) to help them in preventing such problems.
[Method] We analysed data from a survey on RE problems answered by 228
organisations in 10 different countries. [Results] We identified the most
critical RE problems, their causes and mitigation actions, organizing this
information by clusters of size and process model. Finally, we analysed the
causes and mitigation actions of the critical problems of each cluster to get
further insights into how to prevent them. [Conclusions] Based on our results,
we suggest preliminary guidelines for preventing critical RE problems in
response to context characteristics of the companies.Comment: Proceedings of the 42th Euromicro Conference on Software Engineering
and Advanced Applications, 201
The One-loop Open Superstring Massless Five-point Amplitude with the Non-Minimal Pure Spinor Formalism
We compute the massless five-point amplitude of open superstrings using the
non-minimal pure spinor formalism and obtain a simple kinematic factor in pure
spinor superspace, which can be viewed as the natural extension of the
kinematic factor of the massless four-point amplitude. It encodes bosonic and
fermionic external states in supersymmetric form and reduces to existing
bosonic amplitudes when expanded in components, therefore proving their
equivalence. We also show how to compute the kinematic structures involving
fermionic states.Comment: 38 pages, harvmac TeX, v2: fix typo in (4.2) and add referenc
Explicit BCJ Numerators from Pure Spinors
We derive local kinematic numerators for gauge theory tree amplitudes which
manifestly satisfy Jacobi identities analogous to color factors. They naturally
emerge from the low energy limit of superstring amplitudes computed with the
pure spinor formalism. The manifestation of the color--kinematics duality is a
consequence of the superstring computation involving no more than (n-2)!
kinematic factors for the full color dressed n-point amplitude. The bosonic
part of these results describe gluon scattering independent on the number of
supersymmetries and captures any N^kMHV helicity configuration after
dimensional reduction to D=4 dimensions.Comment: 32 pages, harvma
Phonon self-energy corrections to non-zero wavevector phonon modes in single-layer graphene
Phonon self-energy corrections have mostly been studied theoretically and
experimentally for phonon modes with zone-center (q = 0) wave-vectors. Here,
gate-modulated Raman scattering is used to study phonons of a single layer of
graphene (1LG) in the frequency range from 2350 to 2750 cm-1, which shows the
G* and the G'-band features originating from a double-resonant Raman process
with q \not= 0. The observed phonon renormalization effects are different from
what is observed for the zone-center q = 0 case. To explain our experimental
findings, we explored the phonon self-energy for the phonons with non-zero
wave-vectors (q \not= 0) in 1LG in which the frequencies and decay widths are
expected to behave oppositely to the behavior observed in the corresponding
zone-center q = 0 processes. Within this framework, we resolve the
identification of the phonon modes contributing to the G* Raman feature at 2450
cm-1 to include the iTO+LA combination modes with q \not= 0 and the 2iTO
overtone modes with q = 0, showing both to be associated with wave-vectors near
the high symmetry point K in the Brillouin zone
Os desafios para o controle da mosca sul americana.
bitstream/item/82809/1/MAFRA-NETO-Agapomi-n228p10-2013.pd
Observation of the Kohn anomaly near the K point of bilayer graphene
The dispersion of electrons and phonons near the K point of bilayer graphene
was investigated in a resonant Raman study using different laser excitation
energies in the near infrared and visible range. The electronic structure was
analyzed within the tight-binding approximation, and the
Slonczewski-Weiss-McClure (SWM) parameters were obtained from the analysis of
the dispersive behavior of the Raman features. A softening of the phonon
branches was observed near the K point, and results evidence the Kohn anomaly
and the importance of considering electron-phonon and electron-electron
interactions to correctly describe the phonon dispersion in graphene systems.Comment: 4 pages, 4 figure
Group theory analysis of electrons and phonons in N-layer graphene systems
In this work we study the symmetry properties of electrons and phonons in
graphene systems as function of the number of layers. We derive the selection
rules for the electron-radiation and for the electron-phonon interactions at
all points in the Brillouin zone. By considering these selection rules, we
address the double resonance Raman scattering process. The monolayer and
bilayer graphene in the presence of an applied electric field are also
discussed.Comment: 8 pages, 6 figure
The Structure of n-Point One-Loop Open Superstring Amplitudes
In this article we present the worldsheet integrand for one-loop amplitudes
in maximally supersymmetric superstring theory involving any number n of
massless open string states. The polarization dependence is organized into the
same BRST invariant kinematic combinations which also govern the leading string
correction to tree level amplitudes. The dimensions of the bases for both the
kinematics and the associated worldsheet integrals is found to be the unsigned
Stirling number S_3^{n-1} of first kind. We explain why the same combinatorial
structures govern on the one hand finite one-loop amplitudes of equal helicity
states in pure Yang Mills theory and on the other hand the color tensors at
quadratic alpha prime order of the color dressed tree amplitude.Comment: 75 pp, 8 figs, harvmac TeX, v2: published versio
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