Time-Domain Measurement of Broadband Coherent Cherenkov Radiation

Abstract

We report on further analysis of coherent microwave Cherenkov impulses emitted via the Askaryan mechanism from high-energy electromagnetic showers produced at the Stanford Linear Accelerator Center (SLAC). In this report, the time-domain based analysis of the measurements made with a broadband (nominally 1-18 GHz) log periodic dipole array antenna is described. The theory of a transmit-receive antenna system based on time-dependent effective height operator is summarized and applied to fully characterize the measurement antenna system and to reconstruct the electric field induced via the Askaryan process. The observed radiation intensity and phase as functions of frequency were found to agree with expectations from 0.75-11.5 GHz within experimental errors on the normalized electric field magnitude and the relative phase; σ R|E| = 0.039 µV/MHz/TeV and σ φ = 17• . This is the first time this agreement has been observed over such a broad bandwidth, and the first measurement of the relative phase variation of an Askaryan pulse. The importance of validation of the Askaryan mechanism is significant since it is viewed as the most promising way to detect cosmogenic neutrino fluxes at Eν > ∼ 10 15 eV.

Askaryan radiation from neutrino-induced showers in ice

“…Askaryan radiation is known to be directly related to the time-variation of the net charge of the shower [29,30], inducing a complicated bipolar electric pulse with a frequency spectrum which is angular dependent. It lasts a few nanosecond in the Cherenkov direction in a dense medium, and has been shown to be in good agreement with experimental observations [28]. These findings have motivated a variety of experiments to search for these pulses using Antarctic ice [15-17, 31, 32] and the surface of the Moon [33][34][35][36][37][38][39][40] as targets.…”

The sensitivity of past and near-future lunar radio experiments to ultra-high-energy cosmic rays and neutrinos

“…If this phase is zero, then the time-domain function E(t) has its power concentrated at a single point in time with peak amplitude |E|∆ν, as implicitly assumed in the above discussion. However, an Askaryan pulse has a phase close to the worst-case value of π/2 [8], for which it takes on a bipolar profile with the power split between the poles, causing the peak amplitude to be reduced by a factor ∼ √ 2. If this pulse is recorded directly without correcting the phase, this gives α ∼ 0.71.…”

A lunar radio experiment with the Parkes radio telescope for the LUNASKA project

“…9: that the signal has a spectrum S(ν) with zero phase, so in the time domain it combines coherently in a single direction. Experimental tests of the Askaryan effect show that this is not the case: the phase is closer to −90 • , and it manifests in the time domain as a bipolar pulse [34], consistent with more recent simulations [35]. In theoretical terms, the two poles of the pulse can be understood as originating from the beginning and end of the particle cascade; the order in which they are observed depends on whether the observer is on the inside or outside of the Cherenkov cone.…”