Observation of Radar Echoes from High-Energy Particle Cascades

We report the observation of radar echoes from the ionization trails of high-energy particle cascades. Data were taken at the SLAC National Accelerator Laboratory, where the full electron beam ($\sim {10}^9{e}^{-}$ at $\sim 10\mathrm{GeV}/e^{-}$) was directed into a plastic target to simulate an ultrahigh-energy neutrino interaction. The target was interrogated with radio waves, and coherent radio reflections from the cascades were detected with properties consistent with theoretical expectations. This is the first definitive observation of radar echoes from high-energy particle cascades, which may lead to a viable neutrino detection technology for energies $\gtrsim {10}^{16}\mathrm{eV}$.

Figure 1

Cartoon of the radar method and T576, with range of experimental dimensions indicated (antennas were placed in various configurations). (a) A transmitter (TX) illuminates a target. (b) The beam exits the beam pipe and creates a cascade inside the target, leaving behind an ionization cloud. The transmitted signal is reflected to a monitoring receiver (RX). Not to scale.

Figure 2

Left: time-versus-frequency (spectrogram) representation of the observed signal in data. This is the average of 200 events in a single signal run. Right: the same representation for the associated null data set for this sub-run. In both plots, the cross-hairs indicate the signal and sideband regions used to calculate the significance as described in the main text.

Figure 3

An example time-domain average of the highest-SNR reflections from a signal run (solid black), compared to the output of an FDTD simulation for the same signal run (dashed blue), and a RadioScatter simulation for the same signal run (dashed red). The plasma lifetime for the simulation is 3 ns.

Figure 4

The effective scattering cross section, ${\sigma }_{\mathrm{eff}}$, as a function of transmitter output power, for various receiving antennas ($\mathrm{LPDA}+\mathrm{dish}$ and Vivaldi) and two different frequencies. Errors are statistical and systematic, and dominated by the latter. Each set of four points has been clustered around the true X-axis value, for clarity, with the offset indicated in the figure legend. The solid bands are ${\sigma }_{\mathrm{eff}}$ from RadioScatter, including statistical and systematic errors.