107,389 research outputs found
The light-cone gauge without prescriptions
Feynman integrals in the physical light-cone gauge are harder to solve than
their covariant counterparts. The difficulty is associated with the presence of
unphysical singularities due to the inherent residual gauge freedom in the
intermediate boson propagators constrained within this gauge choice. In order
to circumvent these non-physical singularities, the headlong approach has
always been to call for mathematical devices --- prescriptions --- some
successful ones and others not so much so. A more elegant approach is to
consider the propagator from its physical point of view, that is, an object
obeying basic principles such as causality. Once this fact is realized and
carefully taken into account, the crutch of prescriptions can be avoided
altogether. An alternative third approach, which for practical computations
could dispense with prescriptions as well as prescinding the necessity of
careful stepwise watching out of causality would be of great advantage. And
this third option is realizable within the context of negative dimensions, or
as it has been coined, negative dimensional integration method, NDIM for short.Comment: 9 pages, PTPTeX (included
Feynman integrals with tensorial structure in the negative dimensional integration scheme
Negative dimensional integration method (NDIM) is revealing itself as a very
useful technique for computing Feynman integrals, massless and/or massive,
covariant and non-covariant alike. Up to now, however, the illustrative
calculations done using such method are mostly covariant scalar integrals,
without numerator factors. Here we show how those integrals with tensorial
structures can also be handled with easiness and in a straightforward manner.
However, contrary to the absence of significant features in the usual approach,
here the NDIM also allows us to come across surprising unsuspected bonuses. In
this line, we present two alternative ways of working out the integrals and
illustrate them by taking the easiest Feynman integrals in this category that
emerges in the computation of a standard one-loop self-energy diagram. One of
the novel and as yet unsuspected bonus is that there are degeneracies in the
way one can express the final result for the referred Feynman integral.Comment: 9 pages, revtex, no figure
Negative dimensional approach for scalar two-loop three-point and three-loop two-point integrals
The well-known -dimensional Feynman integrals were shown, by Halliday and
Ricotta, to be capable of undergoing analytic continuation into the domain of
negative values for the dimension of space-time. Furthermore, this could be
identified with Grassmannian integration in positive dimensions. From this
possibility follows the concept of negative dimensional integration for loop
integrals in field theories. Using this technique, we evaluate three two-loop
three-point scalar integrals, with five and six massless propagators, with
specific external kinematic configurations (two legs on-shell), and four
three-loop two-point scalar integrals. These results are given for arbitrary
exponents of propagators and dimension, in Euclidean space, and the particular
cases compared to results published in the literature.Comment: 6 pages, 7 figures, Revte
Non-Gaussian fluctuations near the QCD critical point
We study the effect of the QCD critical point on non-Gaussian moments
(cumulants) of fluctuations of experimental observables in heavy-ion
collisions. We find that these moments are very sensitive to the proximity of
the critical point, as measured by the magnitude of the correlation length xi.
For example, the cubic central moment of multiplicity ~ xi^4.5 and the quartic
cumulant ~ xi^7. We estimate the magnitude of critical point contributions to
non-Gaussian fluctuations of pion and proton multiplicities.Comment: 4 pages, 3 figure
Service Learning in Undergraduate Nursing Education: Strategies to Facilitate Meaningful Reflection
Service learning is recognized as a valuable pedagogy involving experiential learning, reflection, and reciprocal learning. Students develop critical thinking and social awareness by using the crucial activity of reflecting upon their experiential learning with community partners. The purpose of this paper is to demystify the process of reflection by identifying best practices to enhance reflection and offering suggestions for grading. By understanding “the what” and “the how” of reflection, educators can implement service learning experiences designed to include the essential component of reflection. Strategies for facilitating meaningful reflection are described including descriptions of what students should reflect upon and how to initiate reflection through writing, reading, doing, and telling. Grading rubrics are suggested to facilitate evaluation of student reflection. When properly implemented, service learning encourages students to be good citizens of the world. By using best practices associated with reflection, students can be challenged to think critically about the world and how their service can achieve community goals
Electron Beam Ion Sources
Electron beam ion sources (EBISs) are ion sources that work based on the
principle of electron impact ionization, allowing the production of very highly
charged ions. The ions produced can be extracted as a DC ion beam as well as
ion pulses of different time structures. In comparison to most of the other
known ion sources, EBISs feature ion beams with very good beam emittances and a
low energy spread. Furthermore, EBISs are excellent sources of photons (X-rays,
ultraviolet, extreme ultraviolet, visible light) from highly charged ions. This
chapter gives an overview of EBIS physics, the principle of operation, and the
known technical solutions. Using examples, the performance of EBISs as well as
their applications in various fields of basic research, technology and medicine
are discussed.Comment: 37 pages, contribution to the CAS-CERN Accelerator School: Ion
Sources, Senec, Slovakia, 29 May - 8 June 2012, edited by R. Baile
Context unification is in PSPACE
Contexts are terms with one `hole', i.e. a place in which we can substitute
an argument. In context unification we are given an equation over terms with
variables representing contexts and ask about the satisfiability of this
equation. Context unification is a natural subvariant of second-order
unification, which is undecidable, and a generalization of word equations,
which are decidable, at the same time. It is the unique problem between those
two whose decidability is uncertain (for already almost two decades). In this
paper we show that the context unification is in PSPACE. The result holds under
a (usual) assumption that the first-order signature is finite.
This result is obtained by an extension of the recompression technique,
recently developed by the author and used in particular to obtain a new PSPACE
algorithm for satisfiability of word equations, to context unification. The
recompression is based on performing simple compression rules (replacing pairs
of neighbouring function symbols), which are (conceptually) applied on the
solution of the context equation and modifying the equation in a way so that
such compression steps can be in fact performed directly on the equation,
without the knowledge of the actual solution.Comment: 27 pages, submitted, small notation changes and small improvements
over the previous tex
Massless and massive one-loop three-point functions in negative dimensional approach
In this article we present the complete massless and massive one-loop
triangle diagram results using the negative dimensional integration method
(NDIM). We consider the following cases: massless internal fields; one massive,
two massive with the same mass m and three equal masses for the virtual
particles. Our results are given in terms of hypergeometric and
hypergeometric-type functions of external momenta (and masses for the massive
cases) where the propagators in the Feynman integrals are raised to arbitrary
exponents and the dimension of the space-time D. Our approach reproduces the
known results as well as other solutions as yet unknown in the literature.
These new solutions occur naturally in the context of NDIM revealing a
promising technique to solve Feynman integrals in quantum field theories
- …