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

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 ${\mathrm{km}}^3$ 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 2 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.

Figure 1

Longitudinal shower profiles for two media, salt and ice now in use for radio detection of high-energy neutrinos. The solid curves are for the NKG approximation, and the dashed curves are Gaussian approximations to the NKG curves, used for parametrization of the emission (see text).

Figure 2

Čerenkov angular distributions for the two showers given in Fig. 1 above.

Figure 3

Čerenkov polarization diagram. The plane of polarization contains the shower momentum vector ${\mathbf{P}}_{\mathbf{s}\mathbf{h}}$, the radiation Poynting vector $\mathbf{S}$, and the Čerenkov Electric field vector $\mathbf{E}$.

Figure 4

Geometry of the salt-block shower target used in the experiment.

Figure 5

View of the salt stack construction at the antenna layer. Antennas were mounted with their leads passing through drilled salt blocks as seen in several examples on the top of the stack.

Figure 6

Longitudinal distribution of the bremsstrahlung shower, for both total charge and charge excess as simulated by EGS4 for rock salt.

Figure 7

Lateral distributions at various depths. Top: depths less than 70 cm. Bottom: depths greater than 70 cm.

Figure 8

The exit point of the beam in the downstream wall of the salt target, outlined by discoloration of the salt produced by beam and showing in cross section at the exit surface. Even at this location of order 15 radiation lengths along the shower, the shower core is still only several cm across (the salt blocks are 6 by 10 cm here in cross section).

Figure 9

Observed coherence of the 0.3–1.5 GHz radiation as a function of total beam energy per pulse. The curve shows a quadratic relation for power as a function of shower energy, and the points are the measured data.

Figure 10

Top: Summed-in-phase pulse profile from all bowtie antennas, with only cable attenuation amplitude corrections applied. Antenna angular response corrections are not applied here due to unknown phase corrections. The antenna is also in the near-field of the shower, and thus only receives a portion of the total shower radiation. Bottom: Power spectrum of the bowtie data, corrected for cable attenuation as in the time domain case above, and also amplitude-corrected for antenna angular response. No correction is applied here for the near-field effects of the shower. Points are averages over 250 MHz portions of the spectrum, with standard errors from the bin variance.

Figure 11

Top: The intrinsic Askaryan pulse shape derived for the frequency range from 0.8 to 12 GHz, based on deconvolution of the LPDA data. The full-width at half-maximum of the initial pulse is about 60 ps. Bottom: Electric-field amplitude spectrum of the deconvolved pulse shown in the top pane, multiplied by frequency $\nu$ which accounts for first-order near-field effects. Horizontal bars indicate the range of the frequency bins used; vertical bars indicate the standard deviation of the spectral data within a bin. The curve shows a comparison to a standard model for the spectrum [22, 23, 24].

Figure 12

Measured coherence of electric field strength at 4.95, 14.5 with LPDA and 7.4 GHz with horn antennas, respectively, with least-squares fit curve.

Figure 13

Spectral dependence of the measured electric field strength for shower energy $1.9\times {10}^{18}\mathrm{eV}$. The solid curve is a parametrized Monte Carlo simulation scaled for synthetic rock salt. Vertical bars are estimated errors, largely due to systematics in the absolute RF calibration; horizontal bars indicate the bandwidth used.

Figure 14

Top: Measured polarization vector field at the bowtie antenna locations in a plane 34 cm above the shower axis. The antenna feed center locations in that plane give the base of each vector, which is scaled in length as the square root of amplitude for clarity. The angles of the vectors match the observed polarization angles. Bottom: The observed amplitude of the received RF pulses along the center line of the antenna array. Also plotted is a curve of an EGS simulation of these showers; both curves are normalized to unit peak amplitude.

Figure 15

Plane of polarization vs. transverse distance of the antennas, which were all about 35 cm above the axis of the beam.

Figure 16

Top: Transverse spread in the simulated electron shower emerging from the TR radiator, grouped in 1 mm annular bins around the beam axis. Bottom: angular emittance spread in the same electron shower, for the highest energy bin ($0.975<\beta <1$), in annular bins of solid angle, with 10° angular width.

Figure 17

Detection threshold estimates for a Salt dome Shower Array (SalSA). Solid & dash-dot curves show received voltages as a function of distance for a 250 m (solid) and 900 m (dash-dot) attenuation lengths, for the energies marked at left. The $1,2.8\sigma$ noise levels shown are for a 450 K system temperature. The 225 m line is the antenna node spacing value used for the Monte Carlo.

Figure 18

Four views of the same ${10}^{18}\mathrm{eV}$ hadronic shower, with directions in local altitude and azimuth (counter-clockwise from east) shown. The dots mark antenna nodes, and crosses mark triggered nodes, with the incoming neutrino track shown up to its termination at the shower vertex.

Figure 19

Similar to previous, with shower energy $E_{\mathrm{sh}}={10}^{19}\mathrm{eV}$.

Figure 20

Simulation of the reconstructed angular resolution of the SalSA detector simulated here. Top: Examples of two reconstructed event angular distributions for a $8\times {10}^{16}\mathrm{eV}$ hadronic shower. The contained shower occurs near the center of the detector, and the noncontained shower occurs 250 m outside one of the faces, directed parallel to the face. Both showers were from horizontal neutrinos, and the reconstructed directions are for polar angle relative to the incident direction, estimated via the location of the minimum of ${\chi }^2$, for 1000 trials for each shower type. Bottom: Standard deviations of angular distributions vs. shower energy, for the same shower types indicated in the top pane.

Figure 21

Broad-spectrum flux sensitivity for a SalSA for 1 yr of exposure, estimated from a Monte Carlo simulation. A complete set of GZK models is shown, as well as an earlier estimate of SalSA sensitivity [37]. From the highest to lowest the legend indicates fluxes from Kalashev et al. (2002) [34] for the maximal level allowed by EGRET limits, an intermediate model with a 10 ZeV cutoff energy for the UHECR spectrum, and a minimal GZK model. The two models marked “ESS01” are from Engel et al. (2001) [5] for one minimal model and one with strong source evolution. The chosen P&J96 model [35] is for a minimal GZK flux. Three different models are shown from Aramo et al. (2005) [36], a maximal model subject only to observational constraints, an intermediate model which follows a pure $E^{-2}$ spectrum as described by Ref. 39, and another minimal GZK model. Finally, a model based on the unlikely assumption that the UHECR composition is pure iron is shown [38].