Landau damping of partially incoherent Langmuir waves

It is shown that partial incoherence, in the form of stochastic phase noise, of a Langmuir wave in an unmagnetized plasma gives rise to a Landau-type damping. Starting from the Zakharov equations, which describe the nonlinear interaction between Langmuir and ion-acoustic waves, a kinetic equation is derived for the plasmons by introducing the Wigner-Moyal transform of the complex Langmuir wave field. This equation is then used to analyze the stability properties of small perturbations on a stationary solution consisting of a constant amplitude wave with stochastic phase noise. The concomitant dispersion relation exhibits the phenomenon of Landau-like damping. However, this damping differs from the classical Landau damping in which a Langmuir wave, interacting with the plasma electrons, loses energy. In the present process, the damping is non-dissipative and is caused by the resonant interaction between an instantaneously-produced disturbance, due to the parametric interactions, and a partially incoherent Langmuir wave, which can be considered as a quasi-particle composed of an ensemble of partially incoherent plasmons.Comment: 12 page

CsI(Tl) for WIMP dark matter searches

We report a study of CsI(Tl) scintillator to assess its applicability in experiments to search for dark matter particles. Measurements of the mean scintillation pulse shapes due to nuclear and electron recoils have been performed. We find that, as with NaI(Tl), pulse shape analysis can be used to discriminate between electron and nuclear recoils down to 4 keV. However, the discrimination factor is typically (10-15)% better than in NaI(Tl) above 4 keV. The quenching factor for caesium and iodine recoils was measured and found to increase from 11% to ~17% with decreasing recoil energy from 60 to 12 keV. Based on these results, the potential sensitivity of CsI(Tl) to dark matter particles in the form of neutralinos was calculated. We find an improvement over NaI(Tl) for the spin independent WIMP-nucleon interactions up to a factor of 5 assuming comparable electron background levels in the two scintillators.Comment: 16 pages, 8 figures, to be published in Nucl. Instrum. and Meth. in Phys. Res.

Simulations of neutron background in a time projection chamber relevant to dark matter searches

Presented here are results of simulations of neutron background performed for a time projection chamber acting as a particle dark matter detector in an underground laboratory. The investigated background includes neutrons from rock and detector components, generated via spontaneous fission and (alpha, n) reactions, as well as those due to cosmic-ray muons. Neutrons were propagated to the sensitive volume of the detector and the nuclear recoil spectra were calculated. Methods of neutron background suppression were also examined and limitations to the sensitivity of a gaseous dark matter detector are discussed. Results indicate that neutrons should not limit sensitivity to WIMP-nucleon interactions down to a level of (1 - 3) x 10^{-8} pb in a 10 kg detector.Comment: 27 pages (total, including 3 tables and 11 figures). Accepted for publication in Nuclear Instruments and Methods in Physics Research - Section

Measurements of Scintillation Efficiency and Pulse-Shape for Low Energy Recoils in Liquid Xenon

Results of observations of low energy nuclear and electron recoil events in liquid xenon scintillator detectors are given. The relative scintillation efficiency for nuclear recoils is 0.22 +/- 0.01 in the recoil energy range 40 keV - 70 keV. Under the assumption of a single dominant decay component to the scintillation pulse-shape the log-normal mean parameter T0 of the maximum likelihood estimator of the decay time constant for 6 keV < Eee < 30 keV nuclear recoil events is equal to 21.0 ns +/- 0.5 ns. It is observed that for electron recoils T0 rises slowly with energy, having a value ~ 30 ns at Eee ~ 15 keV. Electron and nuclear recoil pulse-shapes are found to be well fitted by single exponential functions although some evidence is found for a double exponential form for the nuclear recoil pulse-shape.Comment: 11 pages, including 5 encapsulated postscript figure

Neutron background in large-scale xenon detectors for dark matter searches

Simulations of the neutron background for future large-scale particle dark matter detectors are presented. Neutrons were generated in rock and detector elements via spontaneous fission and (alpha,n) reactions, and by cosmic-ray muons. The simulation techniques and results are discussed in the context of the expected sensitivity of a generic liquid xenon dark matter detector. Methods of neutron background suppression are investigated. A sensitivity of $10^{-9}-10^{-10}$ pb to WIMP-nucleon interactions can be achieved by a tonne-scale detector.Comment: 35 pages, 13 figures, 2 tables, accepted for publication in Astroparticle Physic

4f spin density in the reentrant ferromagnet SmMn2Ge2

The spin contribution to the magnetic moment in SmMn2Ge2 has been measured by magnetic Compton scattering in both the low and high temperature ferromagnetic phases. At low temperature, the Sm site is shown to possess a large 4f spin moment of 3.4 +/- 0.1 Bohr magnetons, aligned antiparallel to the total magnetic moment. At high temperature, the data show conclusively that ordered magnetic moments are present on the samarium site.Comment: 5 pages, 2 figures, transferred from PRL to PRB (Rapid Comm.

Negative Energy Density in Calabi-Yau Compactifications

We show that a large class of supersymmetric compactifications, including all simply connected Calabi-Yau and G_2 manifolds, have classical configurations with negative energy density as seen from four dimensions. In fact, the energy density can be arbitrarily negative -- it is unbounded from below. Nevertheless, positive energy theorems show that the total ADM energy remains positive. Physical consequences of the negative energy density include new thermal instabilities, and possible violations of cosmic censorship.Comment: 25 pages, v2: few clarifying comments and reference adde

The ZEPLIN II dark matter detector: data acquisition system and data reduction

ZEPLIN-II is a two-phase (liquid/gas) xenon dark matter detector searching for WIMP-nucleon interactions. In this paper we describe the data acquisition system used to record the data from ZEPLIN-II and the reduction procedures which parameterise the data for subsequent analysis.Comment: 11 pages, 10 figure

The ZEPLIN II dark matter detector: data acquisition system and data reduction

ZEPLIN-II is a two-phase (liquid/gas) xenon dark matter detector searching for WIMP-nucleon interactions. In this paper we describe the data acquisition system used to record the data from ZEPLIN-II and the reduction procedures which parameterise the data for subsequent analysis.Comment: 11 pages, 10 figure

Answering a Basic Objection to Bang/Crunch Holography

The current cosmic acceleration does not imply that our Universe is basically de Sitter-like: in the first part of this work we argue that, by introducing matter into *anti-de Sitter* spacetime in a natural way, one may be able to account for the acceleration just as well. However, this leads to a Big Crunch, and the Euclidean versions of Bang/Crunch cosmologies have [apparently] disconnected conformal boundaries. As Maldacena and Maoz have recently stressed, this seems to contradict the holographic principle. In the second part we argue that this "double boundary problem" is a matter not of geometry but rather of how one chooses a conformal compactification: if one chooses to compactify in an unorthodox way, then the appearance of disconnectedness can be regarded as a *coordinate effect*. With the kind of matter we have introduced here, namely a Euclidean axion, the underlying compact Euclidean manifold has an unexpectedly non-trivial topology: it is in fact one of the 75 possible underlying manifolds of flat compact four-dimensional Euclidean spaces.Comment: 29 pages, 3 figures, added references and comparison with "cyclic" cosmology, JHEP versio