Citation Networks in High Energy Physics

The citation network constituted by the SPIRES data base is investigated empirically. The probability that a given paper in the SPIRES data base has $k$ citations is well described by simple power laws, $P(k) \propto k^{-\alpha}$, with $\alpha \approx 1.2$ for $k$ less than 50 citations and $\alpha \approx 2.3$ for 50 or more citations. Two models are presented that both represent the data well, one which generates power laws and one which generates a stretched exponential. It is not possible to discriminate between these models on the present empirical basis. A consideration of citation distribution by subfield shows that the citation patterns of high energy physics form a remarkably homogeneous network. Further, we utilize the knowledge of the citation distributions to demonstrate the extreme improbability that the citation records of selected individuals and institutions have been obtained by a random draw on the resulting distribution.Comment: 9 pages, 6 figures, 2 table

Managing Risk on the Final Frontier

The National Aeronautics and Space Administration (NASA). Exploration Systems Mission Directorate (ESMD) has combined the Continuous Risk Management (CRM) discipline with innovative knowledge management (KM) practices to more effectively enable the accomplishment of work. CRM enables proactive problem identification and problem solving in the complex world of rocket science. while KM is used to improve this process

Assessment of the LC-2 Prelaunch Fatigue Spectra of the CM-to-SM Flange Weld

The pad stay and rollout components of the Ares I-X life cycle can generate cyclic stress oscillations to the vehicle that could initiate and grow fatigue cracks from weld defects. The Ares I-X Project requested that a study be performed to determine if stabilization of the vehicle is required to reduce the stresses that could initiate and grow fatigue cracks at the flange-to-skin weld of the Command Module (CM) and Service Module (SM) interface. A fatigue crack growth analysis was conducted that used loads (LC-2) and stress analyses developed by the Ares I-X Project and utilized material data and analysis methods developed by a critical initial flaw size (CIFS) analysis conducted by NASA Engineering and Safety Center (NESC) for the Upper Stage Simulator (USS) of the Ares I-X vehicle. A full CIFS analysis for the CM-to-SM flange-to-skin weld was not performed because the full flight spectrum was not provided and was not necessary to answer the question posed by the Ares I-X Project. Instead, an approach was developed to determine if the crack growth due to the pad stay and rollout components of the flight spectrum would adversely influence the CIFS. The approach taken used a number of conservative assumptions that eliminated the need for high-fidelity analyses and additional material testing, but still provided a bounding solution for the uncertainties of the problem. The results from this analysis indicate that the LC-2 pad stay and rollout spectrum components would not produce significant fatigue crack growth on the CM-to-SM flange-to-skin weld. Thus, from a fatigue crack growth standpoint, no stabilization is required to reduce the LC-2 pad stay and rollout cyclic stresses on the CM-to-SM flange-to-skin weld

The Density Profiles of Massive, Relaxed Galaxy Clusters. I. The Total Density Over Three Decades in Radius

Clusters of galaxies are excellent locations to probe the distribution of baryons and dark matter (DM) over a wide range of scales. We study a sample of seven massive, relaxed galaxy clusters with centrally-located brightest cluster galaxies (BCGs) at z=0.2-0.3. Using the observational tools of strong and weak gravitational lensing, combined with resolved stellar kinematics within the BCG, we measure the total radial density profile, comprising both dark and baryonic matter, over scales of ~3-3000 kpc. Lensing-derived mass profiles typically agree with independent X-ray estimates within ~15%, suggesting that departures from hydrostatic equilibrium are small and that the clusters in our sample (except A383) are not strongly elongated along the line of sight. The inner logarithmic slope gamma_tot of the total density profile measured over r/r200=0.003-0.03, where rho_tot ~ r^(-gamma_tot), is found to be nearly universal, with a mean = 1.16 +- 0.05 (random) +0.05-0.07 (systematic) and an intrinsic scatter of < 0.13 (68% confidence). This is further supported by the very homogeneous shape of the observed velocity dispersion profiles, obtained via Keck spectroscopy, which are mutually consistent after a simple scaling. Remarkably, this slope agrees closely with numerical simulations that contain only dark matter, despite the significant contribution of stellar mass on the scales we probe. The Navarro-Frenk-White profile characteristic of collisionless cold dark matter is a better description of the total mass density at radii >~ 5-10 kpc than that of dark matter alone. Hydrodynamical simulations that include baryons, cooling, and feedback currently provide a poorer match. We discuss the significance of our findings for understanding the assembly of BCGs and cluster cores, particularly the influence of baryons on the inner DM halo. [abridged]Comment: Updated to matched the published version in Ap

Transverse frames for Petrov type I spacetimes: a general algebraic procedure

We develop an algebraic procedure to rotate a general Newman-Penrose tetrad in a Petrov type I spacetime into a frame with Weyl scalars $\Psi_{1}$ and $\Psi_{3}$ equal to zero, assuming that initially all the Weyl scalars are non vanishing. The new frame highlights the physical properties of the spacetime. In particular, in a Petrov Type I spacetime, setting $\Psi_{1}$ and $\Psi_{3}$ to zero makes apparent the superposition of a Coulomb-type effect $\Psi_{2}$ with transverse degrees of freedom $\Psi_{0}$ and $\Psi_{4}$.Comment: 10 pages, submitted to Classical Quantum Gravit

On the Origin of [OII] Emission in Red Sequence and Post-starburst Galaxies

We investigate the emission-line properties of galaxies with red rest-frame colors using spectra from SDSS DR4. Emission lines are detected in more than half of the red galaxies. We focus on the relationship between two emission lines commonly used as star formation rate indicators: Ha 6563 and [OII] 3727. There is a strong bimodality in [OII]/Ha ratio in the full SDSS sample which closely corresponds to the bimodality in rest-frame color. Nearly all of the line-emitting red galaxies have line ratios typical of various types of AGN -- most commonly LINERs, a small fraction of transition objects and, more rarely, Seyferts. Only ~6% of red galaxies display star-forming line ratios. A straight line in the [OII]-Ha equivalent width plane separates LINER-like galaxies from other categories. Quiescent galaxies with no detectable emission lines and LINER-like galaxies combine to form a single, tight red sequence in color-magnitude-concentration space. [OII] EWs in LINER- and AGN-like galaxies can be as large as in star-forming galaxies. Thus, unless objects with AGN/LINER-like line ratios are excluded, [OII] emission cannot be used directly as a proxy for star formation rate. Lack of [OII] emission is generally used to indicate lack of star formation when post-starburst galaxies are selected at high redshift. Our results imply, however, that these samples have been cut on AGN properties as well as star formation, and therefore may provide seriously incomplete sets of post-starburst galaxies. Furthermore, post-starburst galaxies identifed in SDSS by requiring minimal Ha EW generally exhibit weak but nonzero line emission with ratios typical of AGNs; few of them show residual star formation. This suggests that most post-starbursts may harbor AGNs/LINERs.Comment: 21 pages, 15 figures. v2: Added 4 new figures and updated many; extended text. No conclusions change. v3: minor modifications and figure updates to match version accepted by Ap

Glassy behaviour in an exactly solved spin system with a ferromagnetic transition

We show that applying simple dynamical rules to Baxter's eight-vertex model leads to a system which resembles a glass-forming liquid. There are analogies with liquid, supercooled liquid, glassy and crystalline states. The disordered phases exhibit strong dynamical heterogeneity at low temperatures, which may be described in terms of an emergent mobility field. Their dynamics are well-described by a simple model with trivial thermodynamics, but an emergent kinetic constraint. We show that the (second order) thermodynamic transition to the ordered phase may be interpreted in terms of confinement of the excitations in the mobility field. We also describe the aging of disordered states towards the ordered phase, in terms of simple rate equations.Comment: 11 page

Link-space formalism for network analysis

We introduce the link-space formalism for analyzing network models with degree-degree correlations. The formalism is based on a statistical description of the fraction of links l_{i,j} connecting nodes of degrees i and j. To demonstrate its use, we apply the framework to some pedagogical network models, namely, random-attachment, Barabasi-Albert preferential attachment and the classical Erdos and Renyi random graph. For these three models the link-space matrix can be solved analytically. We apply the formalism to a simple one-parameter growing network model whose numerical solution exemplifies the effect of degree-degree correlations for the resulting degree distribution. We also employ the formalism to derive the degree distributions of two very simple network decay models, more specifically, that of random link deletion and random node deletion. The formalism allows detailed analysis of the correlations within networks and we also employ it to derive the form of a perfectly non-assortative network for arbitrary degree distribution.Comment: This updated version has been expanded to include a number of new results. 19 pages, 11 figures. Minor Typos correcte