Ultra-high Energy Neutrino Detectors Research Articles

Deep-learning-based reconstruction of the neutrino direction and energy for in-ice radio detectors

Ultra-high-energy (UHE) neutrinos (>1016eV) can be measured cost-effectively using in-ice radio detection, which has been explored successfully in pilot arrays. A large radio detector is currently being constructed in Greenland with the potential to measure the first UHE neutrino, and an order-of-magnitude more sensitive detector is being planned with IceCube-Gen2. For such shallow radio detector stations, we present an end-to-end reconstruction of the neutrino energy and direction using deep neural networks (DNNs) developed and tested on simulated data. The DNN determines the energy with a standard deviation of a factor of two around the true energy (σ≈ 0.3 in log10(E)), which meets the science requirements of UHE neutrino detectors. For the first time, we are able to predict the neutrino direction precisely for all event topologies including the complicated electron neutrino charged-current (νe-CC) interactions. The obtained angular resolution shows a narrow peak at O(1°) with extended tails that push the 68% quantile for non-νe-CC (resp. νe-CC interactions) to 4°(5°). This highlights the advantages of DNNs for modeling the complex correlations in radio detector data, thereby enabling measurement of neutrino energy and direction.

Indirect dark matter searches at ultrahigh energy neutrino detectors

High to ultrahigh energy neutrino detectors can uniquely probe the properties of dark matter $\chi$ by searching for the secondary products produced through annihilation and/or decay processes. We evaluate the sensitivities to dark matter thermally averaged annihilation cross section $\langle\sigma v\rangle$ and partial decay width into neutrinos $\Gamma_{\chi\rightarrow\nu\bar{\nu}}$ (in the mass scale $10^7 \leq m_\chi/{\rm GeV} \leq 10^{15}$) for next generation observatories like POEMMA and GRAND. We show that in the range $ 10^7 \leq m_\chi/{\rm GeV} \leq 10^{11}$, space-based Cherenkov detectors like POEMMA have the advantage of full-sky coverage and rapid slewing, enabling an optimized dark matter observation strategy focusing on the Galactic center. We also show that ground-based radio detectors such as GRAND can achieve high sensitivities and high duty cycles in radio quiet areas. We compare the sensitivities of next generation neutrino experiments with existing constraints from IceCube and updated 90\% C.L. upper limits on $\langle\sigma v\rangle$ and $\Gamma_{\chi\rightarrow\nu\bar{\nu}}$ using results from the Pierre Auger Collaboration and ANITA. We show that in the range $ 10^7 \leq m_\chi/{\rm GeV} \leq 10^{11}$ POEMMA and GRAND10k will improve the neutrino sensitivity to particle dark matter by factors of 2 to 10 over existing limits, whereas GRAND200k will improve this sensitivity by two orders of magnitude. In the range $10^{11} \leq m_\chi/{\rm GeV} \leq 10^{15}$, POEMMA's fluorescence observation mode will achieve an unprecedented sensitivity to dark matter properties. Finally, we highlight the importance of the uncertainties related to the dark matter distribution in the Galactic halo, using the latest fit and estimates of the Galactic parameters.

Charged Higgs effects in IceCube: PeV events and NSIs

Extensions of the Standard Model with charged Higgs, having a non-negligible coupling with neutrinos, can have interesting implications vis-à-vis neutrino experiments. Such models can leave their footprints not only in the ultra-high energy neutrino detectors like IceCube but can also give rise to the neutrino non-standard interactions (NSIs). We consider a model based on the neutrinophilic two-Higgs doublets and study its imprints in the excess neutrino events in the 1–3 PeV energy bins at the IceCube. This is facilitated by the existence of a charged scalar in the model which can result in a Glashow-like resonance. The same charged scalar can be responsible for sizeable NSIs. We perform a combined study of the latest IceCube data along with various other constraints arising from different neutrino experiments together with the limits set by the LEP experiment, and explore the parameter space which can lead to a sizeable NSI.

Long-baseline horizontal radio-frequency transmission through polar ice

We report on analysis of englacial radio-frequency (RF) pulser data received over horizontal baselines of 1–5 km, based on broadcasts from two sets of transmitters deployed to depths of up to 1500 meters at the South Pole. First, we analyze data collected using two RF bicone transmitters 1400 meters below the ice surface, and frozen into boreholes drilled for the IceCube experiment in 2011. Additionally, in Dec., 2018, a fat-dipole antenna, fed by one of three high-voltage (\U0001d4aa(1 kV)), fast (\U0001d4aa(1-5 ns risetime)) signal generators was lowered into the 1700-m deep icehole drilled for the South Pole Ice Core Experiment (SPICE), approximately 3 km from the geographic South Pole. Signals from transmitters were recorded on the five englacial multi-receiver ARA stations, with receiver depths between 60–200 m. From analysis of deep transmitter data, we estimate: i) the range of refractive index profiles of Antarctic ice with depth allowed by our measurements, ii) due to birefringence, a time difference between arrival times for vertically polarized vs. horizontally polarized signals (per km) for horizontally propagating signal, and iii) for the first time, the attenuation length for electromagnetic signals in the radio-frequency regime broadcast horizontally (rather than reflected vertically from bedrock). We additionally present data suggesting anomalous ice propagation effects, and contrary to expectations for a transport medium with a smoothly varying refractive index profile. Our results imply negligible uncertainty in overall neutrino detection volume due to refractive index uncertainties. Our birefringence time-difference measurements are fit to the functional form δt(H−V) [ns/km]=acosθ+b, with H/V the signal arrival times for the horizontally/vertically polarized EM signal components, and θ the opening angle in the horizontal plane between the signal propagation direction and the local ice flow direction, extracting a=8.3±1.3 ns/km, and b=-8.6±0.9 ns/km (errors combined statistical and systematic), allowing a ∼15% range estimate for future measurements of in-ice neutrino interactions. Finally, we find attenuation length values clustering around 1.5 km, with measurements from the bicone transmitters yielding Latten=1.43±0.25±0.37 km. Taken together, these measurements support cold polar ice as a near-optimal platform for ultra-high energy neutrino detectors.

NuRadioMC: simulating the radio emission of neutrinos from interaction to detector

NuRadioMC is a Monte Carlo framework designed to simulate ultra-high energy neutrino detectors that rely on the radio detection method. This method exploits the radio emission generated in the electromagnetic component of a particle shower following a neutrino interaction. NuRadioMC simulates everything from the neutrino interaction in a medium, the subsequent Askaryan radio emission, the propagation of the radio signal to the detector and finally the detector response. NuRadioMC is designed as a modern, modular Python-based framework, combining flexibility in detector design with user-friendliness. It includes a state-of-the-art event generator, an improved modelling of the radio emission, a revisited approach to signal propagation and increased flexibility and precision in the detector simulation. This paper focuses on the implemented physics processes and their implications for detector design. A variety of models and parameterizations for the radio emission of neutrino-induced showers are compared and reviewed. Comprehensive examples are used to discuss the capabilities of the code and different aspects of instrumental design decisions.

Identifying ultrahigh-energy cosmic-ray accelerators with future ultrahigh-energy neutrino detectors

The detection of ultrahigh-energy (UHE) neutrino sources would contribute significantly to solving the decades-old mystery of the origin of the highest-energy cosmic rays. We investigate the ability of a future UHE neutrino detector to identify the brightest neutrino point sources, by exploring the parameter space of the total number of observed events and the angular resolution of the detector. The favored parameter region can be translated to requirements for the effective area, sky coverage and angular resolution of future detectors, for a given source number density and evolution history. Moreover, by studying the typical distance to sources that are expected to emit more than one event for a given diffuse neutrino flux, we find that a significant fraction of the identifiable UHE neutrino sources may be located in the nearby Universe if the source number density is above ∼10−6 Mpc−3. If sources are powerful and rare enough, as predicted in blazar scenarios, they can first be detected at distant locations. Our result also suggests that if UHE cosmic-ray accelerators are neither beamed nor transients, it will be possible to associate the detected UHE neutrino sources with nearby UHE cosmic-ray and gamma-ray sources, and that they may also be observed using other messengers, including ones with limited horizons such as TeV gamma rays, UHE gamma rays and cosmic rays. We find that for a ≳5σ detection of UHE neutrino sources with a uniform density, ns∼10−7−10−5 Mpc−3, at least ∼100−1000 events and sub-degree angular resolution are needed, and the results depend on the source evolution model.

Simulation chain for acoustic ultra-high energy neutrino detectors

Acoustic neutrino detection is a promising approach for large-scale ultra-high energy neutrino detectors in water. In this paper, a Monte Carlo simulation chain for acoustic neutrino detection devices in water is presented. It is designed within the SeaTray/IceTray software framework. Its modular architecture is highly flexible and makes it easy to adapt to different environmental conditions, detector geometries, and hardware. The simulation chain covers the generation of the acoustic pulse produced by a neutrino interaction and the propagation to the sensors within the detector. In this phase of the development, ambient and transient noise models for the Mediterranean Sea and simulations of the data acquisition hardware, similar to the one used in ANTARES/AMADEUS, are implemented. A pre-selection scheme for neutrino-like signals based on matched filtering is employed, as it can be used for on-line filtering. To simulate the whole processing chain for experimental data, signal classification and acoustic source reconstruction algorithms are integrated. In this contribution, an overview of the design and capabilities of the simulation chain will be given, and some applications and preliminary studies will be presented.

Design and initial performance of the Askaryan Radio Array prototype EeV neutrino detector at the South Pole

We report on studies of the viability and sensitivity of the Askaryan Radio Array (ARA), a new initiative to develop a Teraton-scale ultra-high energy neutrino detector in deep, radio-transparent ice near Amundsen-Scott station at the South Pole. An initial prototype ARA detector system was installed in January 2011, and has been operating continuously since then. We describe measurements of the background radio noise levels, the radio clarity of the ice, and the estimated sensitivity of the planned ARA array given these results, based on the first five months of operation. Anthropogenic radio interference in the vicinity of the South Pole currently leads to a few-percent loss of data, but no overall effect on the background noise levels, which are dominated by the thermal noise floor of the cold polar ice, and galactic noise at lower frequencies. We have also successfully detected signals originating from a 2.5km deep impulse generator at a distance of over 3 km from our prototype detector, confirming prior estimates of kilometer-scale attenuation lengths for cold polar ice. These are also the first such measurements for propagation over such large slant distances in ice. Based on these data, ARA-37, the ∼200km2 array now in its initial construction phase, will achieve the highest sensitivity of any planned or existing neutrino detector in the 1016–1019eV energy range.

The Antarctic Impulsive Transient Antenna ultra-high energy neutrino detector: Design, performance, and sensitivity for the 2006–2007 balloon flight

We present a comprehensive report on the experimental details of the Antarctic Impulsive Transient Antenna (ANITA) long-duration balloon payload, including the design philosophy and realization, physics simulations, performance of the instrument during its first Antarctic flight completed in January of 2007, and expectations for the limiting neutrino detection sensitivity.

In situ and laboratory studies of radiofrequency propagation through ice and implications for siting a large-scale Antarctic neutrino detector

We report on two studies of radiowave ice response, relevant to the construction of a large-scale, future ultra-high energy neutrino detector in Antarctica. We are specifically interested in the relative merits of South Pole as a detection site. First, using a bistatic radar system on the ice surface, we have studied radiofrequency reflections off internal layers in Antarctic ice at the South Pole. In contrast to nearly all previous measurements, our measurements are conducted exclusively in the time-domain. The total propagation time of ∼ns-duration, vertically broadcast radio signals, as a function of polarization axis in the horizontal plane, provides a direct probe of the geometry-dependence of the ice permittivity to depths of 1–2 km. Previous work has inferred birefringent asymmetries based on the elliptical polarization of signal returns at a fixed frequency as a function of depth. However, such polarization is not an unambiguous signature of birefringence, and could be due to anisotropic reflecting layers and/or Faraday rotation. We do not observe direct evidence for birefringent-induced effects at fractional levels less than 10 - 4 . Second, we have performed a laboratory study of microwave Faraday rotation through ice. Although expected to be small, (to our knowledge) there are no previous measurements of this parameter, which, if substantial, would tend to reduce the neutrino detection estimates for existing and planned experiments. Our results indicate that the signal rotation through 3 km of ice in the South Polar geomagnetic field corresponds to a de-polarization of less than 10% in transmitted power.

The Status of Hawaii Askaryan Salt Radi Array (HASRA) experiment

The exploration of GZK neutrinos through their interactions with matter via produced radio signals requires highly homogeneous material with small attenuation for radio frequencies. Rock salt in some salt dome formations provide dielectric material with great potential to host a large scale (100 km3) water-equivalent ultra-high energy neutrino detector The Hawaii Askaryan in Salt Radio Array (HASRA) detector was built as a testbed for exploration of coherent radio Cherenkov emission in salt from interaction of cosmic ray induced cascades. We report results of 1 year of measurements of Askaryan effect with HASRA detector. Peformance of the detector its sensitivity and analysis of a newest data set will be presented.

Probing very high energy prompt muon and neutrino fluxes and the cosmic ray knee viaunderground muons

We calculate event rates and demonstrate the observational feasibility of veryhigh energy muons (1–1000 TeV) in a large-mass underground detector operatingas a pair-meter. This energy range corresponds to surface muon energies of∼(5–5000 TeV) and primary cosmicray energies of ∼(50 TeV–5 × 104 TeV). Such measurements would significantly assist in an improved understanding of the prompt contributionto νe,νμ and μ fluxes in present and future ultra high energy neutrino detectors. In addition, they wouldshed light on the origin of and possible compositional changes at and around the observed‘knee’ in the cosmic ray spectrum.

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 cubic km 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 two 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 detailedmore » 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.« less

Salt neutrino detector for ultrahigh-energy neutrinos

Rock salt and limestone are studied to determine their suitability for use as a radio-wave transmission medium in an ultrahigh energy (UHE) cosmic neutrino detector. A sensible radio wave would be emitted by the coherent Cherenkov radiation from negative excess charges inside an electromagnetic shower upon interaction of a UHE neutrino in a high-density medium (Askar’yan effect). If the attenuation length for the radio wave in the material is large, a relatively small number of radio-wave sensors could detect the interaction occurring in the massive material. We measured the complex permittivity of the rock salt and limestone by the perturbed cavity resonator method at 9.4 and 1 GHz to good precision. We obtained new results of measurements at the frequency at 1.0 GHz. The measured value of the radio-wave attenuation length of synthetic rock salt samples is 1080 m. The samples from the Hockley salt mine in the United States show attenuation length of 180 m at 1 GHz, and then we estimate it by extrapolation to be as long as 900 m at 200 MHz. The results show that there is a possibility of utilizing natural massive deposits of rock salt for a UHE neutrino detector. A salt neutrino detector with a size of 2×2×2 km would detect 10 UHE neutrino/yr generated through the GZK process.