6,672 research outputs found
Soft and Hard Pomeron in the Structure Function of the Proton at Low x and Low Q^2
We study inclusive electroproduction on the proton at low and low
using a soft and a hard Pomeron. The contribution of the soft Pomeron is based
on the Stochastic Vacuum Model, in which a nonperturbative dipole-dipole cross
section can be calculated by means of a gauge invariant gluon field strength
correlator. To model the hard Pomeron exchange we phenomenologically extend the
leading order evolution of a power-behaved structure function, , proposed by L\'opez and Yndur\'ain. This extension allows to
consider both the case and the region of higher on the basis of
the same parametrization. A good simultaneous fit to the data on and on
the cross section of real photoproduction is obtained for
. With four parameters we achieve a for 222 data points. In addition, we use our model of the inclusive
interaction to compute the longitudinal structure function
.Comment: 18 pages, Latex, 6 PS-figures, Regge-exchange neglected, more details
concerning the soft Pomeron included, section on the longitudinal structure
function added, all conclusions unchanged, final version to appear in Eur.
Phys. J.
Estimate of the Collins function in a chiral invariant approach
We estimate the Collins function at a low energy scale by calculating the
fragmentation of a quark into a pion at the one-loop level in the chiral
invariant model of Manohar and Georgi. We give a useful parametrization of our
results and we briefly discuss different spin and/or azimuthal asymmetries
containing the Collins function and measurable in semi-inclusive DIS and e+ e-
annihilationComment: 5 pages, 4 figures, to appear in Proceedings of 10th International
Workshop on Deep Inelastic Scattering (DIS 2002), Cracow, Poland, 30 Apr-4
May 200
Putting It All Together: Guiding Principles for Quality After-School Programs Serving Preteens
Successfully navigating early adolescence depends, in large part, on the availability of safe and engaging activities and supportive relationships with adults, yet many preteens have limited access to positive supports and opportunities -- such as high-quality after-school programs -- that could put them on a path to success. Funders, policymakers and practitioners share the common goal of supporting strategies that will have the most long-lasting positive effects on young people.Recognizing this, the Lucile Packard Foundation for Children's Health commissioned P/PV to identify the characteristics of quality after-school programs that are linked to positive outcomes for preteens. Based on the latest research and experience in the field, P/PV developed the publication, Putting It All Together: Guiding Principles for Quality After-School Programs Serving Preteens, along with a companion Resource Guide (http://www.lpfch.org/afterschool/resourceguide.html) that includes links to research and tools to strengthen programs.Putting It All Together focuses on six after-school program components associated with positive outcomes for preteens:Focused and Intentional Strategy: Programs have a clear set of goals, target specific skills, and deliberately plan all aspects of the program with a youth development framework in mind.Exposure: Programs are designed to: a) provide preteens with a sufficient number of hours per week over an extended period of time, that matches program outcome goals; and b) allow preteens to attend a variety of activities.Supportive Relationships: Programs emphasize positive adult-youth relationships regardless of the curriculum.Family Engagement: Programs strive to include families through various strategies, such as clear communication and a welcoming environment.Cultural Competence: Programs have diverse staff whose backgrounds are reflective of participants and who create practices and policies that: a) make services available to and inclusive of a variety of populations; and b) help participants understand and value a broad range of cultures.Continuous Program Improvement: Programs strengthen quality through an ongoing and integrated process of targeted staff training, coaching and monitoring, and data collection and analysis.While a host of factors, including organizational capacity, the needs of the youth served and the resources available, all play a role in determining a program's ability to achieve its goals, research suggests that these guiding principles are essential for program quality. That quality, in turn, is the foundation for positive results for youth.NOTE: This version of Putting It All Together contains a full list of endnotes and references, which we chose to omit from hard copies of the report, in the interest of brevity
Dihadron fragmentation functions for large invariant mass
Using perturbative Quantum Chromodynamics, we compute dihadron fragmentation
functions for a large invariant mass of the dihadron pair. The main focus is on
the interference fragmentation function H_1^{\open}, which plays an important
role in spin physics of the nucleon. Our calculation also reveals that
H_1^{\open} and the Collins fragmentation function have a closely related
underlying dynamics. By considering semi-inclusive deep-inelastic scattering,
we further show that collinear factorization in terms of dihadron fragmentation
functions, and collinear factorization in terms of single hadron fragmentation
functions provide the same result in the region of intermediate invariant mass.Comment: 4 pages; issue with layout fixe
State injury profile for Kentucky
A widespread tenet is that evolution of pathogens maximises their basic reproduction ratio, R0. The breakdown of this principle is typically discussed as exception. Here, we argue that a radically different stance is needed, based on evolutionarily stable strategy (ESS) arguments that take account of the ‘dimension of the environmental feedback loop’. The R0 maximisation paradigm requires this feedback loop to be one-dimensional, which notably excludes pathogen diversification. By contrast, almost all realistic ecological ingredients of host–pathogen interactions (density-dependent mortality, multiple infections, limited cross-immunity, multiple transmission routes, host heterogeneity, and spatial structure) will lead to multidimensional feedbacks
Intrinsic transverse parton momenta in deeply inelastic reactions
Intrinsic transverse parton momenta pT play an important role in the
understanding of azimuthal/spin asymmetries in semi-inclusive deep-inelastic
scattering (SIDIS) and the Drell-Yan process (DY). We review and update what is
presently known about pT from these processes. In particular, we address the
question to which extent data support the popular Gauss model for the
pT-distributions. We find that the Gauss model works very well, and observe
that the intrinsic transverse momenta in SIDIS and DY are compatible, which is
a support for the factorization approach. As a byproduct we recover a simple
but practical way of taking into account the energy dependence of
pT-distributions.Comment: 19 pages, 11 figure
Partonic Pole Matrix Elements for Fragmentation
A model-independent analysis of collinear three-parton correlation functions
for fragmentation is performed. By investigating their support properties it is
shown, in particular, that the so-called partonic pole matrix elements vanish.
This sheds new light on the understanding of transverse single spin asymmetries
in various hard semi-inclusive reactions. Moreover, it gives additional strong
evidence for the universality of transverse-momentum-dependent fragmentation
functions.Comment: 4 pages, 2 figures; minor changes, matches journal versio
Population growth in discrete time: a renewal equation oriented survey
Traditionally, population models distinguish individuals on the basis of
their current state. Given a distribution, a discrete time model then specifies
(precisely in deterministic models, probabilistically in stochastic models) the
population distribution at the next time point. The renewal equation
alternative concentrates on newborn individuals and the model specifies the
production of offspring as a function of age. This has two advantages: (i) as a
rule, there are far fewer birth states than individual states in general, so
the dimension is often low; (ii) it relates seamlessly to the next-generation
matrix and the basic reproduction number. Here we start from the renewal
equation for the births and use results of Feller and Thieme to characterise
the asymptotic large time behaviour. Next we explicitly elaborate the
relationship between the two bookkeeping schemes. This allows us to transfer
the characterisation of the large time behaviour to traditional
structured-population models
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