Analytical Behaviour of Positronium Decay Amplitudes

Positronium annihilation amplitudes that are computed by assuming a factorization approximation with on-shell intermediate leptons do not exhibit good analytical behaviour. Using dispersion techniques, we find new contributions that interfere with the known results to restore analytical properties. Those new amplitudes which cannot be obtained using standard factorized amplitude formalism, contribute at order alpha^2. Therefore they have to be evaluated before any theoretical conclusion can be drawn upon the orthopositronium lifetime puzzle.Comment: LaTeX, 22 pages, 3 eps figure

Parapositronium Decay and Dispersion Relations

Positronium decay rates are computed at the one-loop level, using convolution-type factorized amplitudes. The dynamics of this factorization is probed with dispersion relations, showing that unallowed approximations are usually made, and some ordre alpha^2 corrections missed. Further, we discuss the relevance of the Schrodinger wavefunction as the basis for perturbative calculations. Finally, we apply our formalism to the parapositronium two-photon decay.Comment: LaTeX, 13 pages, 1 eps figur

Condensation transition in DNA-polyaminoamide dendrimer fibers studied using optical tweezers

When mixed together, DNA and polyaminoamide (PAMAM) dendrimers form fibers that condense into a compact structure. We use optical tweezers to pull condensed fibers and investigate the decondensation transition by measuring force-extension curves (FECs). A characteristic plateau force (around 10 pN) and hysteresis between the pulling and relaxation cycles are observed for different dendrimer sizes, indicating the existence of a first-order transition between two phases (condensed and extended) of the fiber. The fact that we can reproduce the same FECs in the absence of additional dendrimers in the buffer medium indicates that dendrimers remain irreversibly bound to the DNA backbone. Upon salt variation FECs change noticeably confirming that electrostatic forces drive the condensation transition. Finally, we propose a simple model for the decondensing transition that qualitatively reproduces the FECs and which is confirmed by AFM images.Comment: Latex version, 4 pages+3 color figure

Infrared to Ultraviolet Wavelength-Dependent Variations Within the Pulse Profile Peaks of the Crab Nebula Pulsar

We present evidence of wavelength-dependent variations within the infrared, optical, and ultraviolet pulse profile peaks of the Crab Nebula pulsar. The leading and trailing edge half-width half-maxima of the peaks display clear differences in their wavelength dependences. In addition, phase-resolved infrared-to-ultraviolet color spectra show significant variations from the leading to trailing edges of the peaks. The color variations between the leading and trailing edges remain significant over phase differences smaller than 0.0054, corresponding to timescales of $<180 \mu$s. These results are not predicted by any current models of the pulsar emission mechanism and offer new challenges for the development of such models.Comment: 12 pages, 4 figure

Tuning edge state localization in graphene nanoribbons by in-plane bending

The electronic properties of graphene are influenced by both geometric confinement and strain. We study the electronic structure of in-plane bent graphene nanoribbons, systems where confinement and strain are combined. To understand its electronic properties, we develop a tight-binding model that has a small computational cost and is based on exponentially decaying hopping and overlap parameters. Using this model, we show that the edge states in zigzag graphene nanoribbons are sensitive to bending and develop an effective dispersion that can be described by a one-dimensional atomic chain model. Because the velocity of the electrons at the edge is proportional to the slope of the dispersion, the edge states become gradually delocalized upon increasing the strength of bending.Comment: 11 pages, 8 figure

The effects of man-marking on work intensity in small-sided soccer games

The aim of this study was to examine the effect of manipulating defensive rules: with and without man-marking (MM and NMM) on exercise intensity in 3 vs. 3 small-sided games (SSGs). Twelve adolescent soccer players (age: 16.2 ± 0.7 years; body mass: 55.7 ± 6.4 kg; body height: 1.70 ± 0.07 m) participated in this repeated measures study. Each participant performed in four different SSGs formats: 3 vs. 3 MM with and without goals and 3 vs. 3 NMM with and without goals. Each SSG lasted 3 x 4 minutes interspersed with 4 minutes passive recovery. The percentage heart rate reserve (%HRreserve) was recorded continuously during SSG and session-rating of perceived exertion (session-RPE) after the SSG. MANOVA showed that defensive rule had significant effects on intensity (F = 5.37, p < 0.01). Specifically, MM during SSG induced significantly higher %HRreserve compared to NMM (Goal: 80.5 vs. 75.7%; No goal: 80.5 vs. 76.1%; p < 0.05, effect size = 0.91-1.06), irrespective of the presence or absence of goals. However, only MM with the presence of goals induced significant higher session-RPE compared to NMM (7.1 vs. 6.0; p < 0.05, effect size = 1.36), whereas no difference in session-RPE was observed between MM and NMM (7.4 vs. 6.9; p > 0.05, effect size = 0.63) when no goals were used. Higher intra-class reliability and lower coefficient of variation values were also reported in MM as compared to NMM. This study in youth soccer players shows there is ~4.5% increase in heart rate response by using the man-marking in 3 vs. 3 SSG thus the intensity of SSG can be significantly increased when using man-marking tactics

p-Wave stabilization of three-dimensional Bose-Fermi solitons

We explore bright soliton solutions of ultracold Bose-Fermi gases, showing that the presence of p-wave interactions can remove the usual collapse instability and support stable soliton solutions that are global energy minima. A variational model that incorporates the relevant s- and p-wave interactions in the system is established analytically and solved numerically to probe the dependencies of the solitons on key experimental parameters. Under attractive s-wave interactions, bright solitons exist only as meta-stable states susceptible to collapse. Remarkably, the presence of repulsive p-wave interactions alleviates this collapse instability. This dramatically widens the range of experimentally-achievable soliton solutions and indicates greatly enhanced robustness. While we focus specifically on the boson-fermion pairing of 87Rb and 40K, the stabilization inferred by repulsive p-wave interactions should apply to the wider remit of ultracold Bose-Fermi mixtures.Comment: 9 pages, 6 figure

Jet Modification in a Brick of QGP Matter

We have implemented the LPM effect into a microscopic transport model with partonic degrees of freedom by following the algorithm of Zapp & Wiedemann. The Landau-Pomeranchuk-Migdal (LPM) effect is a quantum interference process that modifies the emission of radiation in the presence of a dense medium. In QCD this results in a quadratic length dependence for radiative energy loss. This is an important effect for the modification of jets by their passage through the QGP. We verify the leading parton energy loss in the model against the leading order Baier-Dokshitzer-Mueller-Peigne-Schiff-Zakharov (BDMPS-Z) result. We apply our model to the recent observations of the modification of di-jets at the LHC.Comment: Presented at Panic 1