Polarization observables of the gamma d --> PiNN reaction in the Delta(1232)-resonance region

Polarization observables of the three charge states of the pion for the $\gamma d\to\pi NN$ reaction with polarized photon beam and/or oriented deuteron target are evaluated over the whole $\Delta$(1232)-resonance region adopting a nonrelativistic model based on time-ordered perturbation theory. Results for the $\pi$-meson spectra, linear photon asymmetry, vector and tensor target asymmetries are presented. Particular attention is given, for the first time, to double polarization asymmetries for which we present results for $T_{20}^{\ell}$ and $T_{2\pm 2}^{\ell}$. We found that all other double polarization asymmetries of photon and deuteron target are vanished.Comment: 17 Pages, 8 Figures, accepted for publication in Int. J. Mod. Phys.

Threshold Electrodisintegration of ^3He

Cross sections were measured for the near-threshold electrodisintegration of ^3He at momentum transfer values of q=2.4, 4.4, and 4.7 fm^{-1}. From these and prior measurements the transverse and longitudinal response functions R_T and R_L were deduced. Comparisons are made against previously published and new non-relativistic A=3 calculations using the best available NN potentials. In general, for q<2 fm^{-1} these calculations accurately predict the threshold electrodisintegration of ^3He. Agreement at increasing q demands consideration of two-body terms, but discrepancies still appear at the highest momentum transfers probed, perhaps due to the neglect of relativistic dynamics, or to the underestimation of high-momentum wave-function components.Comment: 9 pages, 7 figures, 1 table, REVTEX4, submitted to Physical Review

Photo- and Electro-Disintegration of 3He at Threshold and pd Radiative Capture

The present work reports results for: pd radiative capture observables measured at center-of-mass (c.m.) energies in the range 0--100 keV and at 2 MeV by the TUNL and Wisconsin groups, respectively; contributions to the Gerasimov-Drell-Hearn (GDH) integral in 3He from the two- up to the three-body breakup thresholds, compared to experimental determinations by the TUNL group in this threshold region; longitudinal, transverse, and interference response functions measured in inclusive polarized electron scattering off polarized 3He at excitation energies below the threshold for breakup into ppn, compared to unpolarized longitudinal and transverse data from the Saskatoon group. The calculations are based on a realistic Hamiltonian with two- and three-nucleon interactions and a realistic current operator, including one- and two-body components. The theoretical predictions obtained by including only one-body currents are in violent disagreement with data. These differences between theory and experiment are, to a large extent, removed when two-body currents are taken into account, although some rather large discrepancies remain in the c.m. energy range 0--100 keV, particularly for the pd differential cross section and tensor analyzing power at small angles, and contributions to the GDH integral. A rather detailed analysis indicates that these discrepancies have, in large part, a common origin, and can be traced back to an excess strength obtained in the theoretical calculation of the E1 reduced matrix element associated with the pd channel having L,S,J=1,1/2,3/2. It is suggested that this lack of E1 strength observed experimentally might have implications for the nuclear interaction at very low energies. Finally, the validity of the long-wavelength approximation for electric dipole transitions is discussed.Comment: 47 pages RevTex file, 10 PostScript figures, submitted to Phys. Rev.

Modern Ab Initio Approaches and Applications in Few-Nucleon Physics with A \ge 4

We present an overview of the evolution of ab initio methods for few-nucleon systems with A \ge 4, tracing the progress made that today allows precision calculations for these systems. First a succinct description of the diverse approaches is given. In order to identify analogies and differences the methods are grouped according to different formulations of the quantum mechanical many-body problem. Various significant applications from the past and present are described. We discuss the results with emphasis on the developments following the original implementations of the approaches. In particular we highlight benchmark results which represent important milestones towards setting an ever growing standard for theoretical calculations. This is relevant for meaningful comparisons with experimental data. Such comparisons may reveal whether a specific force model is appropriate for the description of nuclear dynamics.Comment: extension of the previous version from 70 to 78 pages, 24 figures, 17 tables, in press: Progress in Particle and Nuclear Physic

Introduction to Panel IV

Saskia Köbschall, Introduction to Panel IV of the conference Wir sind alle Berliner Part II: 1884-2014. Programme in Commemoration of the Berlin Conference, ICI Berlin, 27–28 February 2015, video recording, mp4, 08:02 <https://doi.org/10.25620/e150227_01

Transport Processes in Snow and Ice Crystal Icing

Icing caused by snow and ice crystals poses a serious threat in aviation. Ice crystals ingested into jet engines can partially melt, which facilitates the adhesion of the ice particles to engine components. The accumulated ice deteriorates engine performance and shedding of the accretion can extinguish the combustion or cause severe damage. Helicopters are particularly vulnerable to wet snow, which can clog the engine intake, reducing efficiency and potentially causing a flameout when shed into the engine. The associated physical phenomena are diverse and not yet completely understood. Therefore, the objective of this thesis is to provide insight into several key processes involved in snow and ice crystal icing. In the first part of this thesis, transport processes of airborne snowflakes are studied. Numerical tools to predict icing require accurate models to determine the trajectory and liquid fraction of snowflakes, which is challenging due to their highly complex shape. To improve the estimation of the drag coefficient, an experimental study is performed using artificial snowflakes. Additionally, a theoretical model, based on the convex hull of the particle is developed. This model enables the estimation of three-dimensional descriptors from two-dimensional particle projections, which drastically reduces the required information to predict snowflake drag. Experiments are conducted on the melting of laboratory-generated snowflakes. A theoretical model is proposed and validated by comparing the predicted size evolution and melting duration against experimental results. The novel model requires less empiricism than previous models and is able to account for different particle morphologies. The second part of this thesis considers the water transport in porous ice layers. First, imbibition into melting granular ice layers is investigated. A capacitive sensor is developed and utilized to characterize the space and time resolved liquid distribution during imbibition. Pore saturation is found to increase with decreasing porosity. Decreasing grain size and increasing volume flux due to melting result in a sharper decline in saturation. Finally, a capacitive measurement instrument for the application in icing wind tunnels is developed. This novel instrument uncovers the liquid distribution in ice accretions, which was previously inaccessible in experimental studies. The conducted wind tunnel experiments reveal the relation between liquid fraction and growth rate of an ice layer. The gained insights presented in the present thesis enable an enhanced prediction of snow and ice crystal icing and can thereby improve safety and efficiency in aviation

Melting of fractal snowflakes: Experiments and modeling

Numerical simulations of snow accretion on aircraft or weather phenomena require an accurate prediction of the melting of snowflakes. In the present study, an experimental setup for the investigation of snowflake melting is presented, which utilizes an acoustic levitator holding a snowflake in a warm airflow. Based on the experimental findings, a theoretical model for the melting process is developed. This theoretical model takes into account that the generated liquid water is driven into the porous ice structure by capillary forces and does not accumulate at the outer surface of the snowflake. This enables the use of a mass–size relationship in form of a power law, which is common in atmospheric sciences. The model prediction of the evolution of the maximum dimension during melting agrees well with the experiments. Moreover, the snowflake melting durations are also estimated well, if an accurate value is available for the particle mass fractal dimension, i.e., the exponent in the mass–size power law

Geometric descriptors for the prediction of snowflake drag

Experimental data on the drag measurement of artificial snowflakes and a data set of artificial snowflakes and the geometric properties of their projections

Wir sind alle Berliner Part I:1884-2014. Programme in Commemoration of the Berlin Conference

The discursive programme WIR SIND ALLE BERLINER: 1884-2014 commemorates 130 years of the Berlin Conference and proposes a space for deliberation on the repercussions of this crucial event, offering thereby an occasion to analyse the ideological, economic, political, and humanitarian justifications that underlay colonialism and still frame the asymmetric relations between the West and the non-West today. Keynote Lectures by Simon Njami and Ann L. Stoler 18:00 Simon Njami: In the Heart of the Lights Simon Njami &#8211; curator of the exhibition WIR SIND ALLE BERLINER: 1884-2014 at SAVVY Contemporary &#8211; will deliberate on the concept of the exhibition and the role and importance of artistic and cultural interventions in the context of shifting historical discourses and investigating politics of memory. Simon Njami is a writer and an independent curator, lecturer, art critic and essayist. He curated many international exhibitions, being among the first ones to think and show African contemporary artists’ work on international stages. He curated Africa Remix (2004-07 in Düsseldorf, London, Paris, Tokyo, Johannesburg) and co-curated the first African Pavilion at the 52nd Venice Biennale. His exhibition The Divine Comedy – Heaven, Hell, Purgatory by Contemporary African Artists is currently touring after stations in Frankfurt a.M. and Savannah (USA) to Washington D.C and Lisbon. Njami is the co-founder of Revue Noire, a journal of contemporary African and extra-occidental art. His latest book publication is a biography of Léopold Sédar Senghor. 19:00 Ann L. Stoler: Imperial Debris and Why it Matters Now Ann Stoler’s keynote lecture will set the framework of the discursive programme by redirecting the attention of critical engagement with colonial aftermaths towards the “less dramatic durabilities of duress” , the less visible and perceptible repercussions of imperial dispositions and the complex ways in which they shape not only the material but also the psychic space in which we live today. Stoler’s shift of focus from “left over” relics (ruins) as evidence of the past to what we are “left with” &#8211; the ongoing process of ruination through which imperial power occupies the present &#8211; allows for an account of those subtle durabilities. She challenges established assumptions about the way colonial pasts and colonial presents relate to each other, about the remnants of empire that do not only persist, but also continue to be reanimated. Stoler’s lecture will take place against the backdrop of a Germany &#8211; and a Europe &#8211; that is experiencing protests and attacks against foreigners of an unforeseen magnitude and a sharpening of public anti-foreigner rhetoric. It will provide a crucial starting point to reflect upon the complexity of colonial presents and a basis to rethink contemporary socio-political developments in order to shift them. Ann L. Stoler is the Willy Brandt Distinguished University Professor of Anthropology and Historical Studies at the New School for Social Research.Wir sind alle Berliner Part I: 1884-2014. Programme in Commemoration of the Berlin Conference, conference, ICI Berlin, 17 February 2015 <https://doi.org/10.25620/e150217

Experimental results of snowflake melting

Experimental data on the melting of snowflakes in forced convection. The data set contains the shape and size evolution during melting and the duration of melting of laboratory generated aggregates of fragments of dendritic ice crystals