799 research outputs found

    Electric charge fluctuations in central Pb + Pb collisions at 20, 30, 40, 80 and 158 AGeV

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    Results are presented on event-by-event electric charge fluctuations in central Pb+Pb collisions at 20, 30, 40, 80 and 158 AGeV. The observed fluctuations are close to those expected for a gas of pions correlated by global charge conservation only. These fluctuations are considerably larger than those calculated for an ideal gas of deconfined quarks and gluons. The present measurements do not necessarily exclude reduced fluctuations from a quark-gluon plasma because these might be masked by contributions from resonance decays

    Upper limit of D 0 production in central Pb-Pb collisions at 158 A GeV

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    Results are presented from a search for the decays D0 -> K min pi plus and D0 bar -> K plus pi min in a sample of 3.8x10^6 central Pb-Pb events collected with a beam energy of 158A GeV by NA49 at the CERN SPS. No signal is observed. An upper limit on D0 production is derived and compared to predictions from several models

    Directed and elliptic flow of charged pions and protons in Pb + Pb collisions at 40 and 158 A GeV

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    Directed and elliptic flow measurements for charged pions and protons are reported as a function of transverse momentum, rapidity, and centrality for 40 and 158A GeV Pb + Pb collisions as recorded by the NA49 detector. Both the standard method of correlating particles with an event plane, and the cumulant method of studying multiparticle correlations are used. In the standard method the directed flow is corrected for conservation of momentum. In the cumulant method elliptic flow is reconstructed from genuine 4, 6, and 8-particle correlations, showing the first unequivocal evidence for collective motion in A+A collisions at SPS energies

    Observation of an exotic S = -2, Q = -2 baryon resonance in proton-proton collisions at the CERN SPS

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    Results of resonance searches in the Xi- pi-, Xi- pi+, antiXi+ pi- and antiXi+ pi+ invariant mass spectra in proton-proton collisions at sqrt s=17.2 GeV are presented. Evidence is shown for the existence of a narrow Xi- pi- baryon resonance with mass of 1.862+/-0.002 GeV/c^2 and width below the detector resolution of about 0.018 GeV/c^2. The significance is estimated to be 4.0 sigma. This state is a candidate for the hypothetical exotic Xi_(3/2)^-- baryon with S = -2, I = 3/2 and a quark content of (d s d s ubar). At the same mass a peak is observed in the Xi- pi+ spectrum which is a candidate for the Xi_(3/2)^0 member of this isospin quartet with a quark content of (d s u s dbar). The corresponding antibaryon spectra also show enhancements at the same invariant mass

    An Airlifted Tidal Mesocosm for Oil Degradation Studies

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    Mesocosms were constructed to allow scientists to isolate variables in a microtidal marsh environment, mimicking the natural conditions found in Terrebonne Bay. The control offered by these mesocosms is given by a process control system that takes user inputs and automates water movement. Twelve mesocosms were constructed, each holding 4.02 m3 (142 ft3) of marsh soil and 3.88 m3 (137 ft3) of water to give 3 experimental levels and a control, all in triplicate. Each mesocosms that was constructed contained marsh plants in soil plugs 9 feet in diameter and 3 feet deep. These marsh plugs are held in fiberglass tanks that allow for tides to vary up to 11 inches above and below the marsh surface, with the low tide dropping along an exposed side of the marsh. The backbone of the mesocosms’ design is the airlift, which allows for centralized air blowers to move water to mimic tides, keeping each system isolated and mechanical parts shielded from corrosion. The system exchanges experimental water with water from the nearby bayou using a system of ball valves and pumps and returns used water to the bayou after going through a treatment system. The operator of the system uses a GUI (graphical user interface) to input commands to a computer program that then operates the various components of the system. The central program was coded using the Processing© program with control over various valves and sensors in the field controlled with the Arduino Uno. The two programs communicate wirelessly using the XBee software and hardware. The results of the process control system show that the system closely matches what was input by the user, giving the operators of the system the ability to control the system without needing a deep understanding of computer code

    Symmetry and structure of carbon-nitrogen complexes in gallium arsenide from infrared spectroscopy and first-principles calculations

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    Molecular-like carbon-nitrogen complexes in GaAs are investigated both experimentally and theoretically. Two characteristic high-frequency stretching modes at \num{1973} and \SI{2060}{cm^{-1}}, detected by Fourier transform infrared absorption (FTIR) spectroscopy, appear in carbon- and nitrogen-implanted and annealed layers. From isotopic substitution it is deduced that the chemical composition of the underlying complexes is CN2_2 and C2_2N, respectively. Piezospectroscopic FTIR measurements reveal that both centers have tetragonal symmetry. For density functional theory (DFT) calculations linear entities are substituted for the As anion, with the axis oriented along the \hkl direction, in accordance with the experimentally ascertained symmetry. The DFT calculations support the stability of linear N-C-N and C-C-N complexes in the GaAs host crystal in the charge states ranging from +3+3 to −3-3. The valence bonds of the complexes are analyzed using molecular-like orbitals from DFT. It turns out that internal bonds and bonds to the lattice are essentially independent of the charge state. The calculated vibrational mode frequencies are close to the experimental values and reproduce precisely the isotopic mass splitting from FTIR experiments. Finally, the formation energies show that under thermodynamic equilibrium CN2_2 is more stable than C2_2N

    ZentralitÀtsabhÀngigkeit von Bose-Einstein-Korrelationen in Blei-Blei-Kollisionen bei 40 und 158 GeV pro Nucleon

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    In der vorliegenden Arbeit wurde die AbhĂ€ngigkeit der HBT-Radien im Rahmen des NA49-Experimentes bei einer Strahlenergie von 40 und 158 AGeV untersucht. Es zeigte sich, dass die Radien Rside, Rout und Rlong monoton mit der ZentralitĂ€t von 2-3 fm bis 5-6 fm anwachsen, jedoch nur eine sehr geringe EnergieabhĂ€ngigkeit aufweisen. Dabei ist die EnergieabhĂ€ngigkeit bei Rside am schwĂ€chsten und bei Rlong am stĂ€rksten ausgeprĂ€gt. Bei Rout zeigte sich, dass die Werte bei 40 AGeV steiler mit der ZentralitĂ€t ansteigen als die entsprechenden Werte bei 158 AGeV, was zur Folge hat, dass fĂŒr zentrale Ereignisse Rout bei 40 AGeV um etwa 0.5 fm grĂ¶ĂŸer ist, als bei 158 AGeV. Die Signifikanz dieses Befundes ist wegen der statistischen (maximal 0.3 fm) und systematischen Fehler (maximal 1 fm) jedoch sehr gering. Allerdings wurde auch bei der Analyse zentraler Blei-Blei-Kollisionen[28] beobachtet, dass die Werte fĂŒr Rout bei 40 AGeV grĂ¶ĂŸer sind als bei 158 AGeV. Die Radien beider Energien lassen sich als eine lineare Funktion der dritten Wurzel der Anzahl der Partizipanten beschreiben. Letztere sind ein Maß fĂŒr die transversale GrĂ¶ĂŸe des Ausgangszustandes. Aus diesem Verhalten folgt, dass die HBT-Radien aus dem Ausgangszustand der Kollision bestimmt werden. Betrachtet man jedoch die geringe EnergieabhĂ€ngigkeit der HBT-Radien, so liegt der Schluss nahe, dass die HBT-Radien eher durch den Anfangszustand der Kollision bestimmt werden als durch den Endzustand. Dies steht im Widerspruch zu der ĂŒblichen Interpretation der Bose-Einstein-Korrelation in Schwerionenkollisionen. Beim Betrachten des VerhĂ€ltnisses Rout/Rside als Funktion der ZentralitĂ€t stellte sich heraus, dass es grĂ¶ĂŸer als eins ist und nur sehr schwach von der ZentralitĂ€t abhĂ€ngt. Der Wert von Rout/Rside nimmt dabei Werte zwischen 1.2 und 1.5 an. Ermittelt man aus Rout und Rside die Emissionsdauer, so stellt man fest, dass diese GrĂ¶ĂŸe bei beiden Energien nicht signifikant von der ZentralitĂ€t abhĂ€ngt und die Werte zwischen 2 und 4 fm/c liegen

    Strategien zur Vermeidung von Kratzern in intrinsisch weichen Lackschichten

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    In dieser Arbeit wurden grundlegende Forschungen auf dem Gebiet der wĂ€ssrigen Polyurethandispersionen mit dem Ziel der Vermeidung von Mikroverkratzungen geleistet. Es sollte gezeigt werden, dass dieses Themengebiet nicht nur industrielles, sondern wissenschaftliches Interesse weckt. Teile des enormen Potentials und Facettenreichtums werden an dieser Stelle angesprochen. WĂ€ssrige Polyurethandispersionen eignen sich hervorragend als TrĂ€germaterial fĂŒr die verschiedensten anorganischen FĂŒllstoffe oder können durch geschickte Modifikation des PolymergerĂŒsts selbst zum FunktionstrĂ€ger werden. Diese Konzepte werden aufgegriffen, um die Entwicklung innovativer Produkte auf Basis der Polyurethandispersionen voranzutreiben
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