Coherent radio pulses from showers in different media: A unified parametrization

Abstract

We study the frequency and angular dependences of Cherenkov radio pulses originated by the excess of electrons in electromagnetic showers in different dense media. We develop a simple model to relate the main characteristics of the electric field spectrum to the properties of the shower such as longitudinal and lateral development. This model allows us to establish the scaling of the electric field spectrum with the properties of the medium such as density, radiation length, Molière radius, critical energy and refraction index. We normalize the predictions of the scaling relations to the numerical results obtained in our own developed GEANT4-based Monte Carlo simulation, and we give a unified parameterization of the frequency spectrum and angular distribution of the electric field in ice, salt, and the lunar regolith, in terms of the relevant properties of the media. Our parameterizations are valid for electromagnetic showers below the energy at which the Landau-Pomeranchuk-Migdal effect starts to be relevant in these media. They also provide an approximate estimate of radio emission in hadronic showers induced by high energy cosmic rays or neutrinos. PACS numbers: 95.85.Bh, 95.85.Ry,

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.…”

“…For our purposes it suffices to choose tracks between significant discrete interactions, defined here as the bremsstrahlung emission of a E 100 keV photon and pair production, Moeller, Bhabha and Compton scatterings. This method is accurate up to ∼10 GHz, above the frequency at which the coherent behaviour of the emitted signal is expected to break down in all of the media considered in this work [16]. Shorter tracks will lead to a significant increase in computing time but at no improvement in accuracy.…”

“…[25] and references contained therein). The scaling relationships of the electric field with medium parameters obtained in previous works [16] are tested by comparison of our results in the lunar regolith and megaregolith, which have the same composition but different densities and refractive indices, both of which change smoothly with depth. This is also the case for Antarctic ice.…”

“…A parameterisation of the emitted radiation in ice at EeV energies was performed in the past exploiting the analogy with diffraction by a single slit by Fourier-transforming an approximate longitudinal and lateral profile of the shower [18][19][20]. Results for other 2 media were obtained using simple scaling relationships, which at TeV energies and for the regolith differ by ∼30% from the results obtained in full Monte Carlo simulations [16].…”

“…In the second section, we use the ZHS-thinned code to develop new parameterisations of coherent Cherenkov radiation from electromagnetic showers at and above EeV energies for a wide range of media, which extends the work of [16] to energies above those at which the Landau-Pomeranchuk-Migdal (LPM [24]) effect becomes important. This includes a first parameterisation for the deep regolith ("megaregolith") of the lunar highlands, which is relevant for lunar observations utilising low frequencies which could probe the sub-regolith layers (see Ref.…”