620 research outputs found

    Laser-induced electron emission from a tungsten nanotip: identifying above threshold photoemission using energy-resolved laser power dependencies

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    We present an experiment studying the interaction of a strongly focused 25 fs laser pulse with a tungsten nanotip, investigating the different regimes of laser-induced electron emission. We study the dependence of the electron yield with respect to the static electric field applied to the tip. Photoelectron spectra are recorded using a retarding field spectrometer and peaks separated by the photon energy are observed with a 45 % contrast. They are a clear signature of above threshold photoemission (ATP), and are confirmed by extensive spectrally resolved studies of the laser power dependence. Understanding these mechanisms opens the route to control experiment in the strong-field regime on nanoscale objects.Comment: 9 pages, 6 figure

    A study of atmospheric neutrinos with the IMB detector

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    A sample of 401 contained neutrino interactions collected in the 3300 metric ton fiducial mass IMB detector was used to study neutrino oscillations, geomagnetic modulation of the flux and to search for point sources. The majority of these events are attributed to neutrino interactions. For the most part, these neutrinos are believed to originate as tertiary products of cosmic ray interactions in the atmosphere. The neutrinos are a mixture of v sub e and v sub micron

    Bounds on Dark Matter from the ``Atmospheric Neutrino Anomaly''

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    Bounds are derived on the cross section, flux and energy density of new particles that may be responsible for the atmospheric neutrino anomaly. 4.6×10−45cm2<σ<2.4×10−34cm24.6 \times 10^{-45} cm^2 < \sigma <2.4 \times 10^{-34} cm^2 Decay of primordial homogeneous dark matter can be excluded.Comment: 10 pages, TeX (revtex

    Measuring the ΜΌ\nu_{\mu} to ΜΌˉ\bar{\nu_{\mu}} Ratio in a High Statistics Atmospheric Neutrino Experiment

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    By exploiting differences in muon lifetimes it is possible to distinguish ΜΌ\nu_{\mu} from ΜΌˉ\bar{\nu_{\mu}} charged current interactions in underground neutrino detectors. Such observations would be a useful tool in understanding the source of the atmospheric neutrino anomaly.Comment: 6 pages no figure

    Astrophysical constraints on superlight gravitinos

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    I review the constraints on the mass of gravitinos that follow from considerations on energy loss in stars and from Big Bang Nucleosynthesis arguments.Comment: Invited talk at the 5th Workshop on High Energy Physics Phenomenology(WHEPP-5), Pune, India, 12-26 January 199

    LOTIS Upper Limits and the Prompt OT from GRB 990123

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    GRB 990123 established the existence of prompt optical emission from gamma-ray bursts (GRBs). The Livermore Optical Transient Imaging System (LOTIS) has been conducting a fully automated search for this kind of simultaneous low energy emission from GRBs since October 1996. Although LOTIS has obtained simultaneous, or near simultaneous, coverage of the error boxes obtained with BATSE, IPN, XTE, and BeppoSAX for several GRBs, image analysis resulted in only upper limits. The unique gamma-ray properties of GRB 990123, such as very large fluence (top 0.4%) and hard spectrum, complicate comparisons with more typical bursts. We scale and compare gamma-ray properties, and in some cases afterglow properties, from the best LOTIS events to those of GRB 990123 in an attempt to determine whether the prompt optical emission of this event is representative of all GRBs. Furthermore, using LOTIS upper limits in conjunction with the relativistic blast wave model, we weakly constrain the GRB and afterglow parameters such as density of the circumburster medium and bulk Lorentz factor of the ejecta.Comment: 5 pages, 2 figures, To appear in Proceedings of the 5th Huntsville Gamma-Ray Burst Symposiu

    LOTIS Search for Early Time Optical Afterglows: GRB 971227

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    We report on the very early time search for an optical afterglow from GRB 971227 with the Livermore Optical Transient Imaging System (LOTIS). LOTIS began imaging the `Original' BATSE error box of GRB 971227 approximately 14 s after the onset of gamma-ray emission. Continuous monitoring of the position throughout the evening yielded a total of 499 images (10 s integration). Analysis of these images revealed no steady optical afterglow brighter than R=12.3 +- 0.2 in any single image. Coaddition of different combinations of the LOTIS images also failed to uncover transient optical emission. In particular, assuming a constant early time flux, no optical afterglow brighter than R=14.2 +- 0.2 was present within the first 1200 s and no optical afterglow brighter than R=15.0 +- 0.2 was present in the first 6.0 h. Follow up observations by other groups revealed a likely X-ray afterglow and a possible optical afterglow. Although subsequent deeper observations could not confirm a fading source, we show that these transients are not inconsistent with our present knowledge of the characteristics of GRB afterglows. We also demonstrate that with the upgraded thermoelectrically cooled CCDs, LOTIS is capable of either detecting very early time optical afterglow or placing stringent constraints on the relationship between the gamma-ray emission and the longer wavelength afterglow in relativistic blast wave models.Comment: 17 pages, 3 eps figures, revisions based on reviewers comment