2,628 research outputs found
Defining the Force between Separated Sources on a Light Front
The Newtonian character of gauge theories on a light front requires that the
longitudinal momentum P^+, which plays the role of Newtonian mass, be
conserved. This requirement conflicts with the standard definition of the force
between two sources in terms of the minimal energy of quantum gauge fields in
the presence of a quark and anti-quark pinned to points separated by a distance
R. We propose that, on a light front, the force be defined by minimizing the
energy of gauge fields in the presence of a quark and an anti-quark pinned to
lines (1-branes) oriented in the longitudinal direction singled out by the
light front and separated by a transverse distance R. Such sources will have a
limited 1+1 dimensional dynamics. We study this proposal for weak coupling
gauge theories by showing how it leads to the Coulomb force law. For QCD we
also show how asymptotic freedom emerges by evaluating the S-matrix through one
loop for the scattering of a particle in the N_c representation of color
SU(N_c) on a 1-brane by a particle in the \bar N_c representation of color on a
parallel 1-brane separated from the first by a distance R<<1/Lambda_{QCD}.
Potential applications to the problem of confinement on a light front are
discussed.Comment: LaTeX, 15 pages, 12 figures; minor typos corrected; numerical
correction in equation 3.
The Outburst of the Blazar AO 0235+164 in 2006 December: Shock-in-Jet Interpretation
We present the results of polarimetric ( band) and multicolor photometric
() observations of the blazar AO 0235+16 during an outburst in 2006
December. The data reveal a short timescale of variability (several hours),
which increases from optical to near-IR wavelengths; even shorter variations
are detected in polarization. The flux density correlates with the degree of
polarization, and at maximum degree of polarization the electric vector tends
to align with the parsec-scale jet direction. We find that a variable component
with a steady power-law spectral energy distribution and very high optical
polarization (30-50%) is responsible for the variability. We interpret these
properties of the blazar withina model of a transverse shock propagating down
the jet. In this case a small change in the viewing angle of the jet, by
, and a decrease in the shocked plasma compression by a factor of
1.5 are sufficient to account for the variability.Comment: 22 pages, 8 figures, accepted for Ap
Color Variability of the Blazar AO 0235+16
Multicolor (UBVRIJHK) observations of the blazar AO 0235+16 are analyzed. The
light curves were compiled at the Turin Observatory from literature data and
the results of observations obtained in the framework of the WEBT program
(http://www.to.astro/blazars/webt/). The color variability of the blazar was
studied in eight time intervals with a sufficient number of multicolor optical
observations; JHK data are available for only one of these. The spectral energy
distribution (SED) of the variable component remained constant within each
interval, but varied strongly from one interval to another. After correction
for dust absorption, the SED can be represented by a power law in all cases,
providing evidence for a synchrotron nature of the variable component. We show
that the variability at both optical and IR wavelengths is associated with the
same variable source.Comment: 11 pages, 9 figures, 4 tables, accepted for publication in Astronomy
Report
Spontaneous Symmetry Breaking at Infinite Momentum without P+ Zero-Modes
The nonrelativistic interpretation of quantum field theory achieved by
quantization in an infinite momentum frame is spoiled by the inclusion of a
mode of the field carrying p+=0. We therefore explore the viability of doing
without such a mode in the context of spontaneous symmetry breaking (SSB),
where its presence would seem to be most needed. We show that the physics of
SSB in scalar quantum field theory in 1+1 space-time dimensions is accurately
described without a zero-mode.Comment: LaTeX, 8 pages, 3 eps figure
String Bit Models for Superstring
We extend the model of string as a polymer of string bits to the case of
superstring. We mainly concentrate on type II-B superstring, with some
discussion of the obstacles presented by not II-B superstring, together with
possible strategies for surmounting them. As with previous work on bosonic
string we work within the light-cone gauge. The bit model possesses a good deal
less symmetry than the continuous string theory. For one thing, the bit model
is formulated as a Galilei invariant theory in dimensional
space-time. This means that Poincar\'e invariance is reduced to the Galilei
subgroup in space dimensions. Naturally the supersymmetry present in the
bit model is likewise dramatically reduced. Continuous string can arise in the
bit models with the formation of infinitely long polymers of string bits. Under
the right circumstances (at the critical dimension) these polymers can behave
as string moving in dimensional space-time enjoying the full
Poincar\'e supersymmetric dynamics of type II-B superstring.Comment: 43 pages, phyzzx require
Functional Stability of Transfemoral Amputee Gait Using the 3R80 and Total Knee 2000 Prosthetic Knee Units
Prosthetic knee designs have become extremely sophisticated compared to early constant friction single axis mechanisms. Today, prosthetists and physicians have a wide variety of components to choose from for their transfemoral amputee patients. Unfortunately, there are little quantitative data comparing the performance of different types of prosthetic components, and prosthetic prescription is often based on empirical knowledge and experience, rather than objective comparison of prosthetic designs. This study used gait analysis to objectively evaluate the stability characteristics of two types of prosthetic knee designs, the polycentric Total Knee 2000 (Ossur, Aliso Viejo, CA) and the single axis 3R80 stance control knee (Otto Bock Healthcare, Minneapolis, MN). Five healthy, active transfemoral amputees participated in this study by completing two gait analysis sessions, one with the Total Knee 2000 and one with the 3R80. At the end of each session, subjects were asked to complete a questionnaire for comparison of their subjective preferences with the objective biomechanical measures from the gait analysis. Kinematic and kinetic gait data were collected. Stride and temporal parameters (i.e., velocity, cadence, stride length, step length, stance and swing phase durations, and single and double support durations), joint angles and moments were computed from the Vicon data. A within subjects statistical analysis was then conducted, using repeated measures ANOVA, to determine if there were significant differences in performance between the two knee designs. The stride and temporal parameters computed in this study indicated that the Total Knee 2000 demonstrated a high degree of overall stability compared to the 3R80. The differences in inherent mechanical stability in the Total Knee 2000 and the 3R80 resulted in different calculated hip moments. The corresponding required hip moments indicated that the relative stability of the two knee designs changed throughout the gait cycle due to the polycentric nature of the Total Knee 2000. Specifically, the Total Knee 2000 was found to be less stable than the 3R80 in early stance and more stable than the 3R80 in mid- and late-stance. In addition to the biomechanical evidence of increased stability with the Total Knee 2000, the responses to the subjective questionnaires indicated that most subjects felt more stable and more confident bearing weight on the Total Knee 2000 than the 3R80
A Longitudinal Study of Conversations with Parents about Sex and Dating During College
Emerging adulthood is a time of sexual and romantic relationship development as well as change in the parent-child relationship. This study provides a longitudinal analysis of 30 young adultsâ (17 women, 13 men) sexual experiences, attitudes about sexuality and dating, and reported conversations with parents about sexuality and dating from the first and fourth years of college. Self-report questionnaires revealed increases in general closeness with parents, increases in sexual and dating experiences, and both more sexually permissive as well as more gender stereotyped attitudes. Qualitative analyses of individual interviews indicated a movement from unilateral and restrictive, sex-based topics to more reciprocal and relationship-focused conversations over time. Gender analyses revealed that young women reported more restrictive sex messages and young men more positive-sex messages. Participants also described increased openness and comfort in talking about sexual topics with both mothers and fathers from the first to fourth year of college. Overall, the results suggest that prior findings of increased mutuality with parents during the college years extend to the traditionally taboo topic of sexuality
Calculating the Rest Tension for a Polymer of String Bits
We explore the application of approximation schemes from many body physics,
including the Hartree-Fock method and random phase approximation (RPA), to the
problem of analyzing the low energy excitations of a polymer chain made up of
bosonic string bits. We accordingly obtain an expression for the rest tension
of the bosonic relativistic string in terms of the parameters
characterizing the microscopic string bit dynamics. We first derive an exact
connection between the string tension and a certain correlation function of the
many-body string bit system. This connection is made for an arbitrary
interaction potential between string bits and relies on an exact dipole sum
rule. We then review an earlier calculation by Goldstone of the low energy
excitations of a polymer chain using RPA. We assess the accuracy of the RPA by
calculating the first order corrections. For this purpose we specialize to the
unique scale invariant potential, namely an attractive delta function potential
in two (transverse) dimensions. We find that the corrections are large, and
discuss a method for summing the large terms. The corrections to this improved
RPA are roughly 15\%.Comment: 44 pages, phyzzx, psfig required, Univ. of Florida preprint,
UFIFT-HEP-94
Quantum Newtonian Dynamics on a Light Front
We recall the special features of quantum dynamics on a light-front (in an
infinite momentum frame) in string and field theory. The reason this approach
is more effective for string than for fields is stressed: the light-front
dynamics for string is that of a true Newtonian many particle system, since a
string bit has a fixed Newtonian mass. In contrast, each particle of a field
theory has a variable Newtonian mass P^+, so the Newtonian analogy actually
requires an infinite number of species of elementary Newtonian particles. This
complication substantially weakens the value of the Newtonian analogy in
applying light-front dynamics to nonperturbative problems. Motivated by the
fact that conventional field theories can be obtained as infinite tension
limits of string theories, we propose a way to recast field theory as a
standard Newtonian system. We devise and analyze some simple quantum mechanical
systems that display the essence of the proposal, and we discuss prospects for
applying these ideas to large N_c QCD.Comment: 13 pages, 3 figures, LaTex, psfig, references added, APS copyrigh
Electron delocalization in planar metallacycles : HĂŒckel or Möbius aromatic?
In this work the relationship between the formal number of Ï-electrons, d-orbital conjugation topology, Ï-electron delocalization and aromaticity in d-block metallacycles is investigated in the context of recent findings concerning the correlation of Ï-HOMO topology and the magnetic aromaticity indices in these species. It is demonstrated that for Ï-electron rich d-metallacycles the direct link between aromaticity, the number of Ï-electrons and the frontier Ï-orbital topology does not strictly hold and for such systems it is very difficult to unambiguously associate their aromaticity with the '4n+2' (HĂŒckel) and '4n' (Möbius) rules. It is also shown that the recently proposed electron density of delocalized bonds (EDDB) method can successfully be used not only to quantify and visualize aromaticity in such difficult cases, but also - in contrast to magnetic aromaticity descriptors - to provide a great deal of information on the real role of d-orbitals in metallacycles without the ambiguity of bookkeeping of electrons in the Ï-subsystem of the molecular ring. Interestingly, some of the metallacycles studied cannot be classified exclusively as HĂŒckel or Möbius because they have a hybrid HĂŒckel-Möbius or even quasi-aromatic natur
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