Observations of the Askaryan Effect in Ice

Abstract

We report on the first observations of the Askaryan effect in ice: coherent impulsive radio Cherenkov radiation from the charge asymmetry in an electromagnetic (EM) shower. Such radiation has been observed in silica sand and rock salt, but this is the first direct observation from an EM shower in ice. These measurements are important since the majority of experiments to date that rely on the effect for ultra-high energy neutrino detection are being performed using ice as the target medium. As part of the complete validation process for the Antarctic Impulsive Transient Antenna (ANITA) experiment, we performed an experiment at the Stanford Linear Accelerator Center (SLAC) in June 2006 using a 7.5 metric ton ice target, yielding results fully consistent with theoretical expectations.Very large scale optical Cherenkov detectors such as the Antarctic Muon and Neutrino Detector Array (AMANDA) and its successor IceCube have demonstrated the excellent utility of Cherenkov radiation in detection of neutrino interactions at >TeV energies [1, 2] with ice as a target medium. However, at neutrino energies above 100 PeV, the cubic-km scale of such detectors is inadequate to detect more than a handful of events from the predicted cosmogenic neutrino fluxes [3] which represent the most compelling models at these energies. The relevant detector volume for convincing detection and characterization of these neutrinos is in the range of hundreds to thousands of cubic km of water equivalent mass, and the economic constraints of scaling up the optical Cherenkov technique almost certainly preclude extending it much beyond the size of the current IceCube detector, which will be completed early in the next decade.Given the need for an alternative technique with a more tractable economy of scale to reach into the EeV (=1000 PeV) energy regime, a new method which we denote the radio Cherenkov technique, has emerged within the last decade. This method relies on properties of electromagnetic cascades in a dielectric medium. It was first hypothesized by Askaryan [4] and confirmed in 2001 at SLAC [5]. High energy processes such as Compton, Bhabha, and Moller scattering, along with positron annihilation rapidly lead to a ∼ 20% negative charge asymmetry in the electron-photon part of a cascade. In dense media the shower charge bunch is compact, largely contained within a several cm radius. At wavelengths of 10 cm or more, much larger than the characteristic shower bunch size, the relativistic shower bunch appears as a single charge moving through the dielectric over a distance of several meters or more. As an example, a typical shower with mean Bjorken inelasticity y = 0.2, initiated by a E ν = 100 PeV neutrino will create a total number of charged particles at shower maximum of order n e+ +n e− = y E ν /1 GeV ∼ 2 × 10 7 . The net charge is thus n e+ − n e− − ∼ 4 × 10 6 e. Since the radiated power for Cherenkov emission grows quadratically with the charge of the emitter, the coherent power in the cm-to-m wavelength regime is ∼ 10 13 times gre...

SLAC T-510: A beam-line experiment for radio emission from particle cascades in the presence of a magnetic field

“…In the absence of the magnetic field in the target, SLAC T-510 experiment was able to observe the Askaryan signal confirming observations by previous experiments [9,10,11]. Askaryan emission is inherently radially polarized, and it appeared as a vertically polarized signal on top of theČerenkov cone where the measurements were done.…”

“…SLAC T-510 confirmed the Askaryan radiation for particle cascades in dense media that was reported by previous experiments [9,10,11] and also confirmed that the emission scales linearly with the beam charge. We observed the expected emission component due to transverse electric currents induced in the target by cascade particle deflection in an applied magnetic field and found that this magnetic component scales linearly with field strength in agreement with models.…”

“…Askaryan radiation forms from a charge excess built up in the shower due to pair production, positron absorption and Compton scattering, creating a time-varying current along the shower axis [8]. Such emission has been already measured in accelerator experiments [9,10,11]. The Lorentz force also acts on charges in the shower, generating a time-variable transverse current, the Hall current, and subsequent radiation.…”

First results from two deep Askaryan Radio Array stations

“…This radiation becomes coherent for wavelengths comparable to the lateral cascade dimensions, hence, in the radio regime. The above effect has been verified in beam tests for various materials including ice [6]. In radio-transparent media like South Pole ice, radio waves have an attenuation length of more than 800 m which allows for the survey of a large volume with a relatively small number of sensors and thus for the cost effective construction of a neutrino detector [7].…”

Site Characterization and Detector Development for the Greenland Neutrino Observatory

“…The radio frequency (RF) emission arises from a slight charge imbalance built up as a particle shower develops in a dense dielectric, the Askaryan effect [2,3]. Such emission has been measured and characterized at beam-line experiments [4,5,6]. This technique has been exploited by several experiments in Antarctica, including ANITA [7], ARA [8], and ARIANNA [9].…”