Accelerator measurements of the Askaryan effect in rock salt: A roadmap toward teraton underground neutrino detectors

Abstract

We report on further SLAC measurements of the Askaryan effect: coherent radio emission from charge asymmetry in electromagnetic cascades. We used synthetic rock salt as the dielectric medium, with cascades produced by GeV bremsstrahlung photons at the Final Focus Test Beam. We extend our prior discovery measurements to a wider range of parameter space and explore the effect in a dielectric medium of great potential interest to large scale ultra-high energy neutrino detectors: rock salt (halite), which occurs naturally in high purity formations containing in many cases hundreds of km 3 of water-equivalent mass. We observed strong coherent pulsed radio emission over a frequency band from 0.2-15 GHz. A grid of embedded dual-polarization antennas was used to confirm the high degree of linear polarization and track the change of direction of the electric-field vector with azimuth around the shower. Coherence was observed over 4 orders of magnitude of shower energy. The frequency dependence of the radiation was tested over two orders of magnitude of UHF and microwave frequencies. We have also made the first observations of coherent transition radiation from the Askaryan charge excess, and the result agrees well with theoretical predictions. Based on these results we have performed a detailed and conservative simulation of a realistic GZK neutrino telescope array within a salt-dome, and we find it capable of detecting 10 or more contained events per year from even the most conservative GZK neutrino models.

Observations of the Askaryan Effect in Ice

“…MonteCarlo simulations of the showers indicate that about 90% of the shower was contained in the target; the remainder was dumped into a pair of downstream concrete blocks. In contrast to previous experiments [5,12], we did not convert the electrons to photons via a bremsstrahlung radiator. Such methods were used in earlier Askaryan discovery experiments to avoid any initial excess charge in the shower development.…”

“…Prior to the first laboratory tests of the Askaryan effect in 1999-2000 [5,6], and subsequent measurements in 2002 [12], it had been largely ignored since initial putative measurements of the effect in air showers were found instead to be due to a process related to synchrotron emission [13,14]. In the mid-to-late 1980's, proposals to observe Askaryan impulses from neutrino interactions in Antarctic ice [15,16,17] and the Lunar regolith [18] created a renewed interest in Askaryan's work.…”

Thinned simulations of extremely energetic showers in dense media for radio applications

“…Coherent Cherenkov radiation produced by a beam of particles was first observed in tests performed at the Argonne Wakefield Accelerator [11]. The experimental confirmation of the Askaryan effect came later in a series of experiments at SLAC, first in silica sand [12,13] and then in rock salt [14] and ice [15], with results in good agreement with the theoretical calculations [16]. However, to achieve EeV-energy showers in a laboratory requires bunches of a large number of relatively low energy particles, and the resulting cascade of secondaries has neither the same spatial structure as a shower arising from a single ultra-high energy (UHE) particle nor the same expected spectrum and angular distribution of the emitted Cherenkov radiation.…”

The modern FPGA as discriminator, TDC and ADC

“…An example of such a detector is the proposed Salt-dome Shower Array (SalSA) [10]. A schematic view of such a detector may be seen in Fig.…”