249 research outputs found
Colored Spin Systems, BKP Evolution and finite N_c effects
Even within the framework of the leading logarithmic approximation the
eigenvalues of the BKP kernel for states of more than three reggeized gluons
are unknown in general, contrary to the planar limit case where the problem
becomes integrable. We consider a 4-gluon kernel for a finite number of colors
and define some simple toy models for the configuration space dynamics, which
are directly solvable with group theoretical methods. Then we study the
dependence of the spectrum of these models with respect to the number of colors
and make comparisons with the large limit case.Comment: 17 pages, 4 figures, references update, to appear on EPJ
The Semiclassical Gluon Distribution at Next-to-Leading Order
The interaction of the partonic fluctuation of the virtual photon in deep
inelastic scattering with soft color fields describing the hadron is treated in
an eikonal approximation. It is known that, in this approach, the small-x limit
of the leading-order gluon distribution xg(x,Q^2) is a constant characterizing
the averaged local field strength in the target. Matching the next-to-leading
order calculation in this semiclassical framework with the one-loop parton
model result, we obtain the next-to-leading order contribution to xg(x,Q^2). It
shows a ln(1/x) enhancement at small x and is sensitive to the large distance
structure of the target. The final expression is a simple integral over
non-Abelian eikonal factors measuring the target color field. We derive a
quantitative relation between the short-distance cutoff of this integral and
the scale of the gluon distribution function in the MS-bar scheme. Our
calculation demonstrates that higher order contributions can be systematically
included in the semiclassical approach.Comment: 19 pages LaTeX, 8 figures include
Interaction of Reggeized Gluons in the Baxter-Sklyanin Representation
We investigate the Baxter equation for the Heisenberg spin model
corresponding to a generalized BFKL equation describing composite states of n
Reggeized gluons in the multi-color limit of QCD. The Sklyanin approach is used
to find an unitary transformation from the impact parameter representation to
the representation in which the wave function factorizes as a product of Baxter
functions and a pseudo-vacuum state. We show that the solution of the Baxter
equation is a meromorphic function with poles (lambda - i r)^{-(n-1)} (r= 0,
1,...) and that the intercept for the composite Reggeon states is expressed
through the behavior of the Baxter function around the pole at lambda = i . The
absence of pole singularities in the two complex dimensional lambda-plane for
the bilinear combination of holomorphic and anti-holomorphic Baxter functions
leads to the quantization of the integrals of motion because the holomorphic
energy should be the same for all independent Baxter functions.Comment: LaTex, 48 pages, 1 .ps figure, to appear in Phys. Rev.
Exact resolution of the Baxter equation for reggeized gluon interactions
The interaction of reggeized gluons in multi-colour QCD is considered in the
Baxter-Sklyanin representation, where the wave function is expressed as a
product of Baxter functions Q(lambda) and a pseudo-vacuum state. We find n
solutions of the Baxter equation for a composite state of n gluons with poles
of rank r in the upper lambda semi-plane and of rank n-1-r in the lower lambda
semi-plane (0 leq r leq n-1). These solutions are related by n-2 linear
equations with coefficients depending on coth (pi lambda). The poles cancel in
the wave function, bilinear combination of holomorphic and anti-holomorphic
Baxter functions, guaranteeing its normalizability. The quantization of the
intercepts of the corresponding Regge singularities appears as a result of the
physical requirements that the holomorphic energies for all solutions of the
Baxter equation are the same and the total energies, calculated around two
singularities lambda, lambda^* --> + i or -i, coincide. It results in simple
properties of the zeroes of the Baxter functions. For illustration we calculate
the parameters of the reggeon states constructed from three and four gluons.
For the Odderon the ground state has conformal spin |m -m | = 1 and its
intercept equals unity. The ground state of four reggeized gluons possesses
conformal spin 2 and its intercept turns out to be higher than that for the
BFKL Pomeron. We calculate the anomalous dimensions of the corresponding
operators for arbitrary alpha_s/omega.Comment: LaTex, 42 pages, 8 .ps figures. Expanded and improved versio
mixing and the next-to-leading-order power correction
The next-to-leading-order power correction for and
form factors are evaluated and employed to explore the
mixing. The parameters of the two mixing angle scheme are
extracted from the data for form factors, two photon decay widths and radiative
decays. The analysis gives the result:
, where
and are the decay constants and the mixing
angles for the singlet (octet) state. In addition, we arrive at a stringent
range for MeV MeV.Comment: 23 pages, 9 figures, To be publshied in Phys. Rev.
Coherent Phonon Dynamics in Short-Period InAs/GaSb Superlattices
We have performed ultrafast pump-probe spectroscopy studies on a series of
InAs/GaSb-based short-period superlattice (SL) samples with periods ranging
from 46 \AA to 71 \AA. We observe two types of oscillations in the differential
reflectivity with fast ( 1- 2 ps) and slow ( 24 ps) periods. The
period of the fast oscillations changes with the SL period and can be explained
as coherent acoustic phonons generated from carriers photoexcited within the
SL. This mode provides an accurate method for determining the SL period and
assessing interface quality. The period of the slow mode depends on the
wavelength of the probe pulse and can be understood as a propagating coherent
phonon wavepacket modulating the reflectivity of the probe pulse as it travels
from the surface into the sample.Comment: 6 pages, 4 figure
Sex Differences in Sum Scores May Be Hard to Interpret: The Importance of Measurement Invariance
In most assessment instruments, distinct items are designed to measure a trait, and the sum score of these items serves as an approximation of an individualâs trait score. In interpreting group differences with respect to sum scores, the instrument should measure the same underlying trait across groups (e.g., male/female, young/old). Differences with respect to the sum score should accurately reflect differences in the latent trait of interest. A necessary condition for this is that the instrument is measurement invariant. In the current study, the authors illustrate a stepwise approach for testing measurement invariance with respect to sex in a four-item instrument designed to assess disordered eating behavior in a large epidemiological sample (1,195 men and 1,507 women). This approach can be applied to other phenotypes for which group differences are expected. Any analysis of such variables may be subject to measurement bias if a lack of measurement invariance between grouping variables goes undetected
Breast cancer screening: evidence for false reassurance?
Tumour stage distribution at repeated mammography screening is, unexpectedly, often not more favourable than stage distribution at first screenings. False reassurance, i.e., delayed symptom presentation due to having participated in earlier screening rounds, might be associated with this, and unfavourably affect prognosis. To assess the role of false reassurance in mammography screening, a consecutive group of 155 breast cancer patients visiting a breast clinic in Rotterdam (The Netherlands) completed a questionnaire on screening history and self-observed breast abnormalities. The length of time between the initial discovery of breast abnormalities and first consultation of a general practitioner ("symptom-GP period") was compared between patients with ("screening group") and without a previous screening history ("control group"), using Kaplan-Meier survival curves and log-rank testing. Of the 155 patients, 84 (54%) had participated in the Dutch screening programme at least once before tumour detection; 32 (38%) of whom had noticed symptoms. They did not significantly differ from control patients (n = 42) in symptom-GP period (symptom-GP period > or = 30 days: 31.2% in the symptomatic screened group, 31.0% in the control group; p = 0.9). Only 2 out of 53 patients (3.8%) with screen-detected cancer had noticed symptoms prior to screening, reporting symptom-GP periods of 2.5 and 4 years. The median period between the first GP- and breast clinic visit was 7.0 days (95% C.I. 5.9-
S-Matrix Unitarity, Impact Parameter Profiles, Gluon Saturation and High-Energy Scattering
A model combining perturbative and non-perturbative QCD is developed to
compute high-energy reactions of hadrons and photons and to investigate
saturation effects that manifest the S-matrix unitarity. Following a functional
integral approach, the S-matrix factorizes into light-cone wave functions and
the universal amplitude for the scattering of two color-dipoles which are
represented by Wegner-Wilson loops. In the framework of the non-perturbative
stochastic vacuum model of QCD supplemented by perturbative gluon exchange, the
loop-loop correlation is calculated and related to lattice QCD investigations.
With a universal energy dependence motivated by the two-pomeron (soft + hard)
picture that respects the unitarity condition in impact parameter space, a
unified description of pp, pip, Kp, gamma* p, and gamma gamma reactions is
achieved in good agreement with experimental data for cross sections, slope
parameters, and structure functions. Impact parameter profiles for pp and
longitudinal gamma* p reactions and the gluon distribution of the proton
xG(x,Q^2,b) are calculated and found to saturate in accordance with S-matrix
unitarity. The c.m. energies and Bjorken x at which saturation sets in are
determined.Comment: 65 pages with 13 figures, Introduction, Sec. 3, and Conclusion
extende
Critical Correlations of Wilson Lines in SU(3) and the High Energy Cross Section
We discuss deep inelastic scattering at high energies as a critical
phenomenon in 2+1 space - time dimensions. In the limit of Bjorken ,
SU(3) with quark fields becomes a critical theory with a diverging
correlation length where the
exponent is obtained from the center group Z(3) of SU(3). We
conjecture that the dipole wave function of the virtual photon for transverse
sizes obeys correlation scaling before exponentially decaying for distances larger than
the correlation length. Using this behavior combined with different
-independent dipole proton cross sections we calculate the proton structure
function and compare with the experimental data. We take the good agreement
with the measured proton structure function F as an indication that
at high energies dimensional reduction to an effective three dimensional theory
with a critical point occurs.Comment: 21 pages, 3 figure
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