Earth-skimming Tau Neutrinos Research Articles

TAROGE-M: radio antenna array on antarctic high mountain for detecting near-horizontal ultra-high energy air showers

The TAROGE-M radio observatory is a self-triggered antenna array on top of the ∼2700 m high Mt. Melbourne in Antarctica, designed to detect impulsive geomagnetic emission from extensive air showers induced by ultra-high energy (UHE) particles beyond 1017 eV, including cosmic rays, Earth-skimming tau neutrinos, and particularly, the “ANITA anomalous events” (AAE) from near and below the horizon. The six AAE discovered by the ANITA experiment have signal features similar to tau neutrinos but that hypothesis is in tension either with the interaction length predicted by Standard Model or with the flux limits set by other experiments. Their origin remains uncertain, requiring more experimental inputs for clarification.The detection concept of TAROGE-M takes advantage of a high altitude with synoptic view toward the horizon as an efficient signal collector, and the radio quietness as well as strong and near vertical geomagnetic field in Antarctica, enhancing the relative radio signal strength. This approach has a low energy threshold, high duty cycle, and is easy to extend for quickly enlarging statistics. Here we report experimental results from the first TAROGE-M station deployed in January 2020, corresponding to approximately one month of livetime. The station consists of six receiving antennas operating at 180–450 MHz, and can reconstruct source directions of impulsive events with an angular resolution of ∼0.3°, calibrated in situ with a drone-borne pulser system. To demonstrate TAROGE-M's ability to detect UHE air showers, a search for cosmic ray signals in 25.3-days of data together with the detection simulation were conducted, resulting in seven identified candidates. The detected events have a mean reconstructed energy of 0.95-0.31 +0.46 EeV and zenith angles ranging from 25° to 82°, with both distributions agreeing with the simulations, indicating an energy threshold at about 0.3 EeV. The estimated cosmic ray flux at that energy is 1.2-0.9 +0.7 × 10-16 eV-1 km-2 yr-1 sr-1, also consistent with results of other experiments. The TAROGE-M sensitivity to AAEs is approximated by the tau neutrino exposure with simulations, which suggests comparable sensitivity as ANITA's at around 1 EeV energy with a few station-years of operation. These first results verified the station design and performance in a polar and high-altitude environment, and are promising for further discovery of tau neutrinos and AAEs after an extension in the near future.

Prospects for high-elevation radio detection of >100 PeV tau neutrinos

Tau neutrinos are expected to comprise roughly one third of both the astrophysical and cosmogenic neutrino flux, but currently the flavor ratio is poorly constrained and the expected flux at energies above 1017 eV is low. We present a detector concept aimed at measuring the diffuse flux of tau neutrinos in this energy range via a high-elevation mountaintop detector using the radio technique. The detector searches for radio signals from upgoing air showers generated by Earth-skimming tau neutrinos. Signals from several antennas in a compact array are coherently summed at the trigger level, permitting not only directional masking of anthropogenic backgrounds, but also a low trigger threshold. This design takes advantage of both the large viewing area available at high-elevation sites and the nearly full duty cycle available to radio instruments. We present trade studies that consider the station elevation, frequency band, number of antennas in the array, and the trigger threshold to develop a highly efficient station design. Such a mountaintop detector can achieve a factor of ten improvement in acceptance over existing instruments with 100 independent stations. With 1000 stations and three years of observation, it can achieve a sensitivity to an integrated ℰ−2 flux of <10−9 GeV cm−2 sr−1 s−1, in the range of the expected flux of all-flavor cosmogenic neutrinos assuming a pure iron cosmic-ray composition.

A New Concept for High-Elevation Radio Detection of Tau Neutrinos

Cosmic neutrinos are expected to include a significant flux of tau neutrinos due to flavor mixing over astronomical length scales. However, the tau-neutrino content of astrophysical neutrinos is poorly constrained and a significant flux of cosmogenic tau neutrinos awaits discovery. Earth-skimming tau neutrinos undergo charged-current interactions that result in a tau lepton exiting the Earth. The tau lepton decay generates anextensive air shower and geomagnetic radio emission. To target the tau neutrinos, we present a new tau neutrino detector concept that uses phased antenna arrays placed on high elevation mountains. Simulation studies indicate that a modest array size and small number of stations can achieve competitive sensitivity, provided the receivers are at highs enough elevation.

LIMIT ON UHE NEUTRINO FLUXES FROM THE PIERRE AUGER OBSERVATORY

The surface detector of the Pierre Auger Observatory is sensitive to Ultra High Energy (UHE) neutrinos. Neutrinos of all flavors can interact in the atmosphere producing inclined showers near the ground. Moreover, ultra high energy Earth-skimming tau neutrinos can be observed through the detection of showers induced by the decay of tau leptons created by interactions in the Earth’s crust. In both cases, neutrino showers can be identified through the time structure of the signals in the surface detector stations. Two sets of identification criteria have been designed to search for down-going and up-going neutrinos in the recorded data, with no candidates found. We will discuss the identification criteria used, and we will present the corresponding limits on the diffuse and point source neutrino fluxes.

LIMIT ON UHE NEUTRINO FLUXES FROM THE PIERRE AUGER OBSERVATORY

The surface detector of the Pierre Auger Observatory is sensitive to Ultra High Energy (UHE) neutrinos. Neutrinos of all flavors can interact in the atmosphere producing inclined showers near the ground. Moreover, ultra high energy Earth-skimming tau neutrinos can be observed through the detection of showers induced by the decay of tau leptons created by interactions in the Earth’s crust. In both cases, neutrino showers can be identified through the time structure of the signals in the surface detector stations. Two sets of identification criteria have been designed to search for down-going and up-going neutrinos in the recorded data, with no candidates found. We will discuss the identification criteria used, and we will present the corresponding limits on the diffuse and point source neutrino fluxes.

Using cosmic neutrinos to search for nonperturbative physics at the Pierre Auger Observatory

The Pierre Auger (cosmic ray) Observatory provides a laboratory for studying fundamental physics at energies far beyond those available at colliders. The Observatory is sensitive not only to hadrons and photons but can in principle detect ultrahigh energy neutrinos in the cosmic radiation. Interestingly, it may be possible to uncover new physics by analyzing characteristics of the neutrino flux at the Earth. By comparing the rate for quasihorizontal, deeply penetrating air showers triggered by all types of neutrinos with the rate for slightly up-going showers generated by Earth-skimming tau neutrinos, we determine the ratio of events which would need to be detected in order to signal the existence of new nonperturbative interactions beyond the TeV scale in which the final state energy is dominated by the hadronic component. We use detailed Monte Carlo simulations to calculate the effects of interactions in the Earth and in the atmosphere. We find that observation of 1 Earth skimming and 10 quasihorizontal events would exclude the standard model at the 99% confidence level. If new nonperturbative physics exists, a decade or so would be required to find it in the most optimistic case of a neutrino flux at the Waxman-Bahcall level and a neutrino-nucleon cross section an order of magnitude above the standard model prediction.

The propagation of ultra-high energy tau leptons and neutrinos while skimming the Earth

The detection of Earth-skimming tau neutrinos has turned into a very promising strategy for the observation of UHE cosmic neutrinos. The sensitivity of this channel crucially depends on the parameters of the propagation of the tau neutrino (and the tau lepton) through the terrestrial crust, which governs the flux of emerging tau leptons that can be detected. This propagation problem is usually treated in a simplified framework where several effects are neglected, e.g. the possibility of multiple regenerations of the tau neutrino, the weak interactions of the tau lepton, as well as the stochastic nature of its energy losses. We discuss here the validity of such approximations by studying the propagation in standard rock of tau leptons and neutrinos with both mono-energetic and power-law spectra. We also investigate the impact of such simplifications in non-standard scenarios for the neutrino-nucleon interactions as well as for the tau energy losses.

Ultrahigh energy tau neutrino flux regeneration while skimming the Earth

The detection of Earth-skimming tau neutrinos has turned into a very promising strategy for the observation of ultra-high-energy cosmic neutrinos. The sensitivity of this channel crucially depends on the parameters of the propagation of the tau neutrinos through the terrestrial crust, which governs the flux of emerging tau leptons that can be detected. One of the characteristics of this propagation is the possibility of regeneration through multiple {nu}{sub {tau}}{r_reversible}{tau} conversions, which are often neglected in the standard picture. In this paper, we solve the transport equations governing the {nu}{sub {tau}} propagation and compare the flux of emerging tau leptons obtained allowing regeneration or not. We discuss the validity of the approximation of neglecting the {nu}{sub {tau}} regeneration using different scenarios for the neutrino-nucleon cross sections and the tau energy losses.

Upper Limit on the Diffuse Flux of Ultrahigh Energy Tau Neutrinos from the Pierre Auger Observatory

The surface detector array of the Pierre Auger Observatory is sensitive to Earth-skimming tau neutrinos that interact in Earth's crust. Tau leptons from nu(tau) charged-current interactions can emerge and decay in the atmosphere to produce a nearly horizontal shower with a significant electromagnetic component. The data collected between 1 January 2004 and 31 August 2007 are used to place an upper limit on the diffuse flux of nu(tau) at EeV energies. Assuming an E(nu)(-2) differential energy spectrum the limit set at 90% C.L. is E(nu)(2)dN(nu)(tau)/dE(nu)<1.3 x 10(-7) GeV cm(-2) s(-1) sr(-1) in the energy range 2 x 10(17) eV< E(nu)< 2 x 10(19) eV.

Charged current neutrino cross section and tau energy loss at ultrahigh energies

We evaluate both the tau-lepton energy loss produced by photonuclear interactions and the neutrino charged current cross section at ultrahigh energies, relevant to neutrino bounds with Earth-skimming tau neutrinos, using different theoretical and phenomenological models for nucleon and nucleus structure functions. The theoretical uncertainty is estimated by taking different extrapolations of the structure function ${F}_{2}$ to very low values of $x$, in the low and moderate ${Q}^{2}$ range for the tau-lepton interaction and at high ${Q}^{2}$ for the neutrino-nucleus inelastic cross section. It is at these extremely low values of $x$ where nuclear shadowing and parton saturation effects are unknown and could be stronger than usually considered. For tau and neutrino energies $E={10}^{9}\text{ }\text{ }\mathrm{GeV}$ we find uncertainties of a factor 4 for the tau energy loss and of a factor 2 for the charged current neutrino-nucleus cross section.

Probing low-xQCD with cosmic neutrinos at the Pierre Auger Observatory

The sources of the observed ultra-high energy cosmic rays must also generate ultra-high energy neutrinos. Deep inelastic scattering of these neutrinos with nucleons on Earth probe center-of-mass energies $\sqrt{s} \sim 100$ TeV, well beyond those attainable at terrestrial colliders. By comparing the rates for two classes of observable events, any departure from the benchmark (unscreened perturbative QCD) neutrino-nucleon cross-section can be constrained. Using the projected sensitivity of the Pierre Auger Observatory to quasi-horizontal showers and Earth-skimming tau neutrinos, we show that a `Super-Auger' detector can thus provide an unique probe of strong interaction dynamics.

Exotic neutrino interactions at the Pierre Auger Observatory

The Pierre Auger Observatory for cosmic rays provides a laboratory for studying fundamental interactions at energies well beyond those available at colliders. In addition to hadrons or photons, Auger is sensitive to ultra-high energy neutrinos in the cosmic radiation and models for new physics can be explored by observing neutrino interactions at center-of-mass energies beyond the TeV scale. By comparing the rate for quasi-horizontal, deeply penetrating air showers triggered by all types of neutrinos with the rate for slightly upgoing showers generated by Earth-skimming tau neutrinos, any deviation of the neutrino–nucleon cross-section from the Standard Model expectation can be constrained. We show that this can test models of low-scale quantum gravity (including processes such as Kaluza–Klein graviton exchange, microscopic black hole production and string resonances), as well as non-perturbative electroweak instanton mediated processes. Moreover, the observed ratios of neutrino flavors would severely constrain the possibility of neutrino decay.

Earth-skimming UHE tau neutrinos at the fluorescence detector of Pierre Auger Observatory

Ultra high energy neutrinos are produced by the interaction of hadronic cosmic rays with the cosmic radiation background. More exotic scenarios like topological defects or new hadrons predict even larger fluxes. In particular, Earth-skimming tau neutrinos could be detected by the fluorescence detector (FD) of Pierre Auger Observatory. A detailed evaluation of the expected number of events has been performed for a wide class of neutrino flux models. An updated computation of the neutrino-nucleon cross section and of the tau energy losses has been carried out. For the most optimistic theoretical models, about one Earth-skimming neutrino event is expected in several years at FD.

WHAT DO WE MEAN BY DETECTING EARTH-SKIMMING NEUTRINOS?

In this talk, we elaborate the strategy for detecting the Earth-skimming tau neutrinos. We first show that there are non-negligible astrophysical tau neutrino fluxes due to neutrino flavor oscillations. We then illustrate the idea of detecting Earth-skimming tau neutrinos. In particular, we point out that the tau-lepton flux resulting from neutrino-nucleon scatterings inside the earth is controlled by the tau-lepton range. We demonstrate this observation by showing the tau-lepton flux induced by the GZK tau-neutrino flux. The question on the energy resolutions of tau neutrinos in this detection strategy is briefly discussed.

Cosmic UHE neutrino signatures

Utilizing the unique and reliable ultrasmall- x predictions of the dynamical (radiative) parton model, nominal event rates and their detailed energy dependence caused by a variety of cosmic UHE neutrino fluxes are calculated and analyzed. In addition, maximal Regge-model inspired small- x structure functions are employed for obtaining optimal rates which do not necessarily require ‘new’ physics interpretations. Upward μ ++ μ − event rates are estimated by taking into account total and nadir-angle dependent regeneration effects due to neutral current interactions. For exploring extragalactic neutrino sources at highest energies (≳10 8 GeV) with modern (future) ground-level telescopes, we analyze horizontal air shower event rates and shower events caused by Earth-skimming tau-neutrinos, in particular their detailed shower- and cosmic neutrino-energy dependence. As an illustration of ‘new physics’ implications we estimate the relevant horizontal air shower event rates due to spin-2 Kaluza–Klein ‘graviton’ exchanges in neutral current neutrino-quark and neutrino–gluon interactions at low TeV-scales.