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 $x$ and low $Q^2$ 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, $F_2 \propto x^{- \lambda}$, proposed by L\'opez and Yndur\'ain. This extension allows to consider both the case $Q^2 = 0$ and the region of higher $Q^2$ on the basis of the same parametrization. A good simultaneous fit to the data on $F_2$ and on the cross section $\sigma_{\gamma p}$ of real photoproduction is obtained for $\lambda=0.37$. With four parameters we achieve a $\chi^2/\textrm{d.o.f.} = 0.98$ for 222 data points. In addition, we use our model of the inclusive $\gamma^{\ast} p$ interaction to compute the longitudinal structure function $F_L$.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

Instability of frozen-in states in synchronous Hebbian neural networks

The full dynamics of a synchronous recurrent neural network model with Ising binary units and a Hebbian learning rule with a finite self-interaction is studied in order to determine the stability to synaptic and stochastic noise of frozen-in states that appear in the absence of both kinds of noise. Both, the numerical simulation procedure of Eissfeller and Opper and a new alternative procedure that allows to follow the dynamics over larger time scales have been used in this work. It is shown that synaptic noise destabilizes the frozen-in states and yields either retrieval or paramagnetic states for not too large stochastic noise. The indications are that the same results may follow in the absence of synaptic noise, for low stochastic noise.Comment: 14 pages and 4 figures; accepted for publication in J. Phys. A: Math. Ge

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

Universality of soft and collinear factors in hard-scattering factorization

Universality in QCD factorization of parton densities, fragmentation functions, and soft factors is endangered by the process dependence of the directions of Wilson lines in their definitions. We find a choice of directions that is consistent with factorization and that gives universality between e^+e^- annihilation, semi-inclusive deep-inelastic scattering, and the Drell-Yan process. Universality is only modified by a time-reversal transformation of the soft function and parton densities between Drell-Yan and the other processes, whose only effect is the known reversal of sign for T-odd parton densities like the Sivers function. The modifications of the definitions needed to remove rapidity divergences with light-like Wilson lines do not affect the results.Comment: 4 pages. Extra references. Text and references as in published versio