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Catch me if you can: radio waves, a cascade of particles and ice to capture neutrinos



β€œ-Do you see the gopher?” -No. β€œAnd I don’t see, but he is.” - with this quote you can quite clearly describe the situation with neutrinos. For many years, scientists from around the world have been trying to understand the nature of these mysterious subatomic particles, explain their behavior and describe their characteristics. However, this is far from the easiest task, because in order to learn something, you must first β€œcatch” it. Scientists from the University of Ohio (USA) have proposed their own method of capturing and, as a result, studying neutrinos, one of the main roles in which Antarctic ice plays. What physical phenomena were involved in the capture of neutrinos, why did ice help in this process, and what new could be found out about one of the most mysterious particles? The answers to these questions await us in the report of the research group. Go.

Study basis


Neutrinos are neutral (they have no electric charge) particles with a half-integer spin. These particles literally pass through everything that stands in their way. There are both low-energy neutrinos that interact very weakly with matter, and high-energy neutrinos, whose interaction with matter can be fixed.

In this study, scientists focused on high-energy neutrinos (οΉ₯ 10 16 eV). The potential of these particles lies in the possible possibility of using them to study cosmic rays * , which are recorded up to ～ 10 20 eV.
Cosmic rays * - elementary particles and nuclei of atoms moving with high energies in outer space.
Unlike cosmic rays, which are scattered against the background of cosmic microwave radiation and also deflect in magnetic fields, the detected neutrinos can directly point to their sources.

When high-energy neutrinos interact in a substance, they produce a relativistic cascade of particles, as well as a chain of non-relativistic electrons and nuclei resulting from the loss of energy of relativistic particles * .
Relativistic particle * - a particle moving with a speed comparable to the speed of light.
The profile of this cascade is an ellipsoid with a length of 10 m and a radius of about 0.1 m. Almost all the energy of the primary interaction goes to the ionization of the medium.

Separate cascade electrons and positrons * emit incoherent Cherenkov optical radiation * , which can be detected using TeV – PeV detectors (for example, the IceCube neutrino observatory).
Positron * is an antiparticle of an electron.

The antiparticle * is the double of a certain particle with the same mass and spin, but with opposite interaction characteristics (electric charge, color charge, etc.).
* ( ) β€” , , .
IceCube* β€” 1450 2450 , «» ( 60 ). , - . .
The existing project of the updated observatory (IceCube-Gen2) has its drawbacks - the optical component is not powerful enough to detect high-energy neutrinos due to a sharp drop in the neutrino spectrum.

Therefore, it is necessary to look for more suitable methods for capturing high-energy neutrinos. Some methods are based on coherent radio-frequency Cherenkov radiation from the total asymmetry of the charge in the cascade. Others study leptons that can be produced by interacting neutrinos.

There is also the option of detecting cascades due to radar reflections from the ionization trail left in the path of the cascade. This method promises to be extremely accurate, which provided him with special attention from the research team.

In the work we are considering today, scientists have applied the above method to successfully observe the radar echo from a cascade of particles.

Preparing for the experiment




Image No. 1: experimental setup.

The experimental setup was prepared and installed in the SLAC national accelerator laboratory.

High-density polyethylene (HDPE) was used as the target of the installation where the electron beam was directed. A continuous signal was transmitted to the target in the frequency range 1 ... 2.1 GHz using a single signal generator, a power amplifier (50 W) and a transmitting antenna (TX). Receiving antennas (RX) have also been aimed at this goal for measuring radar reflection.

Two types of antennas were used in the experiments: Vivaldi ultra-wideband antenna (0.6–6 GHz) with a measured transmission coefficient of +12 dBi (isotropic decibel) at a frequency of 2 GHz; Log-periodic dipole antenna (LPDA) 0.9–4 GHz, made specifically for this study.

LPDA was used in conjunction with a parabolic reflector with a measured gain of +18 dBi at a frequency of 2 GHz.

Around the beam exit was an integrating toroid current (ICT), which allows accurate measurement of the charge in each bunch.

Data collection was divided into routines consisting of 100–500 events. Between subprocesses, some parameters (TX frequency, TX amplitude, TX position, and RX position) varied. Experiments in which data were taken for analysis are called signal runs. Other subprocesses have been reserved for collecting background data. The duration of one experiment was 8 days.

In the case of using a transmitter and a receiver in this experiment, the spectral content of the reflected signal is both a function of Ο„ and the geometry of the cascade. For a compact cascade, as in the case of the experiment, any lifetime exceeding 1 ns would lead to a significant radar reflection at the transmitted frequency. The transmission was carried out at a peak power of 50 W without amplification of the receiver. With this setting, a radar signal was expected with a level of several mV and a duration of several nanoseconds at the transmitter frequency.

Experiment Results


After filtering, the data set was further processed using the method developed in the analysis of the first experiment. In order to investigate both the temporal and spectral content of the signal, a frequency-time spectrogram was generated for each filtered event in a series of signals, and these spectrograms were averaged.


Image No. 2

Image No. 2 shows the result of this process. Here you can clearly see the excess in real data, and not in zero data, at a transmitter frequency of 2.1 GHz with a duration of several nanoseconds.

A similar excess was observed at many transmission frequencies, antenna positions and at different antennas, but no excess was observed at the same time and at a frequency point in zero data.

The signal with the highest amplitude was received during experiments with a horizontally polarized antenna with a high transmission coefficient at a specular angle, where the resulting signal (with SVD filtering, SVD - singular decomposition) was large enough to be extracted in the time domain by careful alignment and averaging. Alignment was performed so that events could be shifted by no more than a fraction of the transmission period.


Image No. 3

The image above shows the resulting average in the time domain. It also shows a comparison of experimental results with those obtained during FDTD modeling (FDTD - the method of finite differences in the time domain) of the same signal, as well as during the simulation of RadioScatter (software for modeling radar echoes from cascades of particles).

Several checks were also carried out, which made it possible to verify that the observed signal has properties corresponding to radar scattering. One of these supporting criteria is the fact that the signal is scaled with the output power of the transmitter (image No. 4).


Image No. 4

Scientists note that since the signal is so small with respect to the burst of the beam, andthe null hypothesis * relies on a linear combination of background components; the nonlinearity of the entire system is an obvious problem.
The null hypothesis * is the default assumption that there is no connection between the two observed events.
Several experimental runs of the system were carried out in which a continuous signal at the same frequency and amplitude was amplified and transmitted through one Vivaldi antenna, and the second, connected to the oscilloscope, was configured as a receiver. A high voltage pulse with a spectral content similar to a ray burst was transmitted simultaneously.

To establish the exact value, N = 107 sets of 100 zero events were created using the initial bootstrap (Monte Carlo pseudo-sample generation method based on the available sample). Next, an average spectrogram was compiled for each set and an estimate of the statistical criterion subtracted from the sideband of excess power in the signal region was performed.

For null data, the statistical criterion was TS null = 2.20+6.56 -6.20 , and for the measured data TS data = 61.2 +7.40 -6.58 .

Thus, the experiment made it possible to observe radar reflection from a cascade of particles in a dense material (in ice). The recorded signal is in excellent agreement with theoretical expectations, and the probability that these are only background vibrations is extremely small.

For a more detailed acquaintance with the nuances of the study, I recommend that you look into the report of scientists .

Epilogue


Neutrinos are extremely difficult to study, because they behave like flies: only you waved the bundle of the newspaper, as it had already disappeared from sight. However, everything is not so hopeless, since there are a number of techniques that allow us to study these particles. In this work, a new method was considered, based on a radar echo from a cascade of particles generated by neutrinos interacting in a dense substance, which in this case was ice.

Previously, neutrinos were already recorded in the ice of Antarctica, but these were low-energy neutrinos. With high energy neutrinos, things are a little more complicated. In this study, scientists conducted an experiment in which the role of ice was played by a plastic target 4 meters long. They aimed the target at the target and bombarded it with electrons packed in a tiny bunch simulating neutrinos. If the calculations were correct, then the total energy of such a bunch should be equal to the total energy of a high-energy neutrino. Then, radio waves were sent to the target, which recorded a cascade of particles.

The study of neutrinos is of great importance, since these are the only particles that move along a constant straight path. Therefore, you can track their source, which will allow you to learn much more about the processes taking place in the Universe than we know at the moment.

The next step in this prospective study is to conduct experiments not in a laboratory with plastic, but directly in Antarctica with real ice. This will allow you to find out how effective the method of radio waves in the field is, so to speak.

Thank you for your attention, remain curious and have a good working week, guys. :)

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