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Near-field effects of Cherenkov radiation induced by ultra high energy cosmic neutrinos |
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| Authors: | Chia-Yu Hu Chih-Ching Chen Pisin Chen |
| Institution: | a Graduate Institute of Astrophysics, National Taiwan University, Taipei 10617, Taiwan b Leung Center for Cosmology and Particle Astrophysics (LeCosPA), National Taiwan University, Taipei 10617, Taiwan c Department of Physics, National Taiwan University, Taipei 10617, Taiwan d Kavli Institute for Particle Astrophysics and Cosmology, SLAC National Accelerator Laboratory, Menlo Park, CA 94025, USA |
| Abstract: | The radio approach based on the Askaryan effect for detecting the ultra-high energy cosmic neutrinos has become a mature experimental technique. So far the existing calculations of the Cherenkov radiation associated with the Askaryan effect has been mostly based on the far-field approximation, whose validity maybe challenged when the detector is close to the event. In this paper we present an alternative approach to calculate the Cherenkov pulse by a numerical code based on the finite difference time-domain (FDTD) method. This approach has the advantage of providing the solution everywhere in space, contrary to other methods that rely on the far-field approximation. We also present a one-dimensional theoretical model for the shower with analytical solution, which helps to elucidate our nonzero-width simulation results. We show that for a shower with symmetric longitudinal development, the resulting near-field waveform would be asymmetric in time. In addition, we demonstrate that for a shower elongated by the LPM (Landau-Pomeranchuk-Migdal) effect and thus with a multi-peak structure, a bipolar, asymmetric waveform is still preserved in the near-field regime irrespective of the specific variations of the multi-peak structure, which makes it a generic, distinctive feature. This should provide an important characteristic signature for the identification of ultra-high energy cosmogenic neutrinos. |
| Keywords: | Cosmogenic neutrino Askaryan effect Cherenkov radiation Near-field effect |
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