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SLAC T-510 experiment for radio emission from particle showers: Detailed simulation study and interpretation

K. Bechtol et al.
Phys. Rev. D 105, 063025 – Published 28 March 2022
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  • EXPERIMENTAL SET-UP
  • MICROSCOPIC MODELING
  • MODELING OF THE RADIO EMISSION
  • APPLICATION OF THE SIMULATIONS TO THE…
  • COMPARISON OF THE SIMULATION RESULTS TO…
  • CONCLUSION AND OUTLOOK
  • ACKNOWLEDGMENTS
    • References

    Abstract

    Over the last several decades, radio detection of air showers has been widely used to detect ultrahigh-energy cosmic rays. We developed an experiment under controlled laboratory conditions at SLAC with which we measured the radio-frequency radiation from a charged particle shower produced by bunches of electrons as primaries with known energy. The shower took place in a target made of high density polyethylene located in a strong magnetic field. The experiment was designed so that Askaryan and magnetically-induced components of the radio emission could be measured independently. At the same time, we performed a detailed simulation of this experiment to predict the radio signal using two microscopic formalisms, endpoint and ZHS. In this paper, we present the simulation scheme and make a comparison with data characteristics such as linearity with magnetic field and amplitude. The simulations agree with the measurements within uncertainties and present a good description of the data. In particular, reflections within the target that accounted for the largest systematic uncertainties are addressed. The prediction of the amplitude of Askaryan emission agrees with measurements to within 5% for the endpoint formalism and 11% for the ZHS formalism. The amplitudes of magnetically-induced emission agree to within 5% for the endpoint formalism and less than 1% for the ZHS formalism. The agreement of the absolute scale of emission gives confidence in state-of-the-art air shower simulations which are based on the applied formalisms.

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    • Received 20 October 2021
    • Accepted 2 March 2022

    DOI:https://doi.org/10.1103/PhysRevD.105.063025

    © 2022 American Physical Society

    Physics Subject Headings (PhySH)

    1. Research Areas
    Cosmic ray & astroparticle detectorsRadio frequency calculations
    Accelerators & BeamsGravitation, Cosmology & Astrophysics

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    Vol. 105, Iss. 6 — 15 March 2022

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    Images

    • Figure 1
      Figure 1

      Schematic view of the experimental setup including the antenna tower position and the geometry for the signal propagation, not to scale.

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    • Figure 2
      Figure 2

      Top: picture of the staggered coils which were used to produce a strong and uniform magnetic field in the vertical direction. Middle: measured three-dimensional magnetic field map for a current of 2400 A (in a 5  cm×5  cm grid) which is included in simulations. The dashed gray lines mark the target area, the green lines the position of the beam [13] (modified). Bottom: measurement of the linear dependence of the magnetic field strength on the applied current.

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    • Figure 3
      Figure 3

      Positioning of the antenna tower at the Cherenkov ring (red) so that contributions of the charge-excess (green) and magnetic effect (blue) are separated into the horizontal and vertical channels of the antenna.

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    • Figure 4
      Figure 4

      Top left: sketch of the experiment geometry for the calculation of the point of refraction at the upper slanted target surface. Right: enlarged view of the sketch after rotation to define the parameters for the calculation of the point of refraction. Bottom left: values of the Fresnel transmission coefficient for the electric field components parallel and perpendicular to the plane of incidence in dependency on the emergent angle. For further details see Ref. [23].

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    • Figure 5
      Figure 5

      Testing Cherenkov-like effects using the endpoint formalism: Horizontally (top) and vertically polarized (bottom) components of the electric field produced by a “sliced” particle shower in a magnetic field with a strength of B=−970  G for an antenna on the Cherenkov cone, using the endpoint formalism and filtered from 300–1200 MHz.

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    • Figure 6
      Figure 6

      Peak amplitude of the electric field for a 2D antenna array using the endpoint formalism for a magnetic field of a maximum strength of 970 G: Top: horizontally polarized component. Bottom: vertically polarized component [23].

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    • Figure 7
      Figure 7

      Relative deviation of the peak amplitude in the time domain defined as EZHS−EendpointEZHS for a magnetic field strength of B=970  G: Top: horizontally polarized component. Antenna positions with negligible signals have been excluded in the comparison (white area). Bottom: vertically polarized component [23].

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    • Figure 8
      Figure 8

      Contribution of transition radiation to the peak amplitude in the time domain. Top: horizontally polarized component. Bottom: vertically polarized component. The relative contribution by the transition radiation to the radio signal strength is about 1% for both polarizations near the Cherenkov cone.

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    • Figure 9
      Figure 9

      Ray tracing of the main signal and internal reflections that reach the same antenna.

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    • Figure 10
      Figure 10

      The CP-510 experiment designed to measure the reflection coefficient of the rf blanket and plywood used in the original T-510 experiment. The primary difference between the two runs used to measure the reflection coefficient is that the rf blanket was removed between the runs.

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    • Figure 11
      Figure 11

      Waveforms measured during the follow-up reflection experiment, CP-510, from the co-polar feed (Hpol) on the horn antenna. The pulses are shown for cases both with (green) and without the rf blanket (blue) on top of a steel reflector. The pulses are nearly identical indicating that the blanket acts primarily as a reflector rather than an absorber at these frequencies.

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    • Figure 12
      Figure 12

      Measured reflection coefficient of the rf blanket. We use this as the reflection coefficient for both polarizations. The shaded area reflects the uncertainty, which is ±15% with an upper limit on the allowed reflection coefficient of 1.

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    • Figure 13
      Figure 13

      Illustration of the separation of the electric field into a horizontally polarized component (EH) and a vertically polarized component (EV). It is also shown that the horizontally polarized component experiences a phase shift of π at the boundary of the target to the rf absorbing blanket.

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    • Figure 14
      Figure 14

      Horizontal (left) and vertical (right) polarization for an antenna position at 6.5 m height: Simulated time traces (top) and frequency spectra (bottom) are shown for data (in black) and for endpoint and ZHS simulations including reflections, in blue and red respectively. Simulations without reflections are shown as solid lines and simulations with reflections are shown as dashed lines.

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    • Figure 15
      Figure 15

      Top: the ratio of horizontally to vertically polarized signal is shown for an antenna 652 cm above beam height. Data is represented with black points. The error bars represent the systematic uncertainty on the measurement. Simulations with and without reflections added are also shown, with the solid line indicating no reflections, and the simulations with reflections added are show as dashed lines. Bottom: The distribution of residuals (data simulation) is shown. The solid points in blue and red represent (data simulation without reflections added) for ZHS and endpoint formalisms respectively, and the crosses represent (data simulation with reflections added) for ZHS and endpoint formalisms.

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    • Figure 16
      Figure 16

      Maximum signal strength of the horizontally (left) and vertically (right) polarized signal along the vertical axis with a 970 G magnetic field. Data is shown as black dots and the error bars indicate systematic uncertainties. Simulations with reflections included are indicated with the dashed lines, and without reflections by the solid lines. ZHS simulations are shown in blue and endpoint in red. The light blue shaded region represents the envelope of systematic uncertainties of both endpoint and ZHS simulations.

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