Source of ultra high energy (UHE) cosmic rays remains undiscovered since Hess found the cosmic ray in 1912. The shock wave acceleration mechanism is established for cosmic rays up to 1 PeV (10¹⁵ eV), but there is no successful model to explain how particles are accelerated to higher energies.

Locating the sources must be an effective approach to solve this problem, however the clusters of galaxies and the galaxies themselves are filled with magnetic field that severally deflected the charged cosmic ray particles. Unless the energy is very high, e.g. above 50 EeV (1EeV=10¹⁸ eV), the cosmic ray can not be used for locating its source. However, they are very rare (about one event per 100 km² per year) at such high energies. Photons are traditionally used to study the features of their sources, including the acceleration mechanism of their charged parent particles. We lose those messengers rapidly at higher energies since they interact with the 3K cosmological radiation background photons before reach to the earth. Eventually the universe becomes opaque for photon energy above 100 TeV (1TeV=10¹² eV). Neutrino is another type of messenger and is unique in the UHE region since it is not interecting with phoyons. The difficulty is that the neutrino has so little interaction with matter that a huge volume of matter as a converter is essential to detect them. Most of neutrino experiments, such as the IceCube at the South Pole and the Antares in the Mediterranean, choose natural ice and water as the converter and the detector material. Mountains could be used as the converter as well. The CRTNT project is based on this idea and is building shower detectors behind the mountain to catch air showers induced by the neutrinos. So far, no neutrino is measured from outside of our galaxy, only about 15 neutrinos are measured from outside of the solar system, the supernova 1987A. The new experiments will open a new window for astronomy in the UHE range.

Studying the propagation of the cosmic rays is the other approach. In general, the energy spectrum and composition of cosmic rays are basic tools for this purpose. Specifically, any significant deviation from the general E^-3 spectrum and a constant composition, such as the "knee" around 1 PeV and the "second knee" around the 0.1 EeV, where the cosmic ray flux undergoes a sudden decrease and composition has a significant transition from heavy to light nuclei. Those transitions indicate that the acceleration and propagation of cosmic rays undergo changes in their mechanisms, e.g. from sourced inside our galaxy to the outside. The CRTNT project will focus on the second knee region. The energy spectrum and composition will be precisely measured by co-site observation with the TALE and the TA experiments in order to carry out a cross calibration between detectors.

^{1. Search for UHE tau neutrinos}
In a converter with a huge volume, neutrinos interact with nucleons and produce detectable corresponding charged leptons, electron, muon or tauon, depending on the incident neutrinos, i.e. electron neutrino, muon neutrino or tauon neutrino, respectively. Existing or under constructing neutrino detectors are using ice or water as the converter and the detector material in which the Cerenkov light is generated by the leptons. The detector must be as large as the converter and the photo tubes are burried inside. Using mountain as the converter and the air behind the mountain as the detector material, the detector can be much more compact, therefore inexpensive to maintain a compatible acceptance of the Ice/water detectors. Fluorescence/Cerenkov light detector is shown to be successful in the Dice, HiRes and Auger experiments. This type of detector has an aperture of thousands of square kilometers stereo radian. The centralized telescope array is installed in a building of thousands of square meters. Since the detector is designed to measure the air shower induced by the leptons, it is only sensitive to the tau neutrinos. Electrons from electron neutrinos will shower inside mountain and muons from muon neutrinos will not shower at all. UHE tau lepton has a range more than tens of kilometers that allows it escaping from the mountain body and 60% of its decay products are hadrons and 20% are electrons that will shower in the air. However, all astrophysics models show that there is no tau neutrino generated in any source. Since sources are such far away that the mixture will be 1:1:1 among the three flavors as they reach to the earth as long as the neutrino oscillation is correct. A successful observation based on this mechanism, like the CRTNT project, will confirm the neutrino oscillation as well.

A convolution of the neutrino-nuclei interaction cross section and the tau decay probability gives the probability of a tau neutrino inducing a detectable air shower via an escaped tau decay. The probability is optimized as the thickness T of the mountain is between 20 km and 30 km as shown in the Fig. 1. ,

Fig. 1. CPD changes with the thickness of the mountain. where CPD refers to neutrino-to-shower conversion probability density and r is the distance from the escape point to the location where the shower starts. Most of showers starts inside the mountain if it is too thick, e.g. skimming the earth crest in front of the mountain (left plot in Fig. 1.), while the neutrino would not interact if the mountain is too thin. 20~30 km is the best for almost all energies below 10 EeV. The CPD is larger for higher energy neutrinos.

The spectra of the neutrinos, discussed in literatures, depending on the sources of the neutrinos, usually are power laws and some of them follows by cutoffs. Fig. 2. shows the most of the theoretic estimates of the neutrino fluxes.

Fig. 2. All fluxes of neutrinos predicated for different source models. There are high energy neutrinos from cosmological strings (the topological deficit model), neutrino-neutrino cascade (z-burst model) and extremely high energy cosmic rays (GZK cutoff mechanism). However, the fluxes are very small so that require a very large aperture of detector to detect them. Neutrinos from AGNs are the most interested because 1. the flux is suitable for most the detectors which are building and under R/D study. The AGN neutrino flux estimation (Sigl) assumes that the AGNs are uniformly distributing in and expanding with the universe; 2. from the point view of locating the cosmic ray sources, AGNs are important candidates, however they all are very far so that emitted UHE photons can not reach to the earth because of the photon-photon interaction. Neutrino is the unique choice for this purpose.

Simulation (Z.Cao et al.) has be done to take the detector acceptance into account. It shows that the AGN neutrino flux can be measured with an event rate of about 8/yr assuming an array of 12 CRTNT telescopes. The spectrum of the measured neutrinos is shown in Fig. 3.

Fig. 3. Simulated AGN neutrino spectrum measured by the CRTNT detector. 12 telescopes are installed in three groups 8 km apart from each other. In this simulation, a mountain as described in the paper (Z.Cao et al.) is assumed. The site search is proposed to NSFC as a Youth Research Fonding by Zhang Yong for support but not approved. This is crucial for the CRTNT project. Further fonding request is under preparation.

The center of our galaxy (GC) is another interesting potential source of the UHE cosmic rays and neutrinos. In order to detect it, the CRTNT telescopes should be installed facing south, therefore the mountain should be in east-west direction. There is about 1000 hours of exposure of the GC to the CRTNT detector in a run of three years.

^{2. Sub-EeV cosmic ray measurements}
Cosmic rays observed in the energy range of 10 PeV to 100 EeV behave that their sources may switch from inside our galaxy to the larger space range. No model of the acceleration and transportation through the space between the sources and the earth perform well enough to explain all structures of the energy spectrum and composition changes, i.e. the "second knee", the "ankle" and the GZK-cutoff. An accurate observation of the spectrum and the composition of the cosmic rays is essential to improve our understanding therefore to model the UHE cosmic ray acceleration correctly. Several measurements have been done by Fly's Eye, Haverah Park, Akeno, Yakutsk, AGASA and prototype HiRese (coincidence with MIA) xperiments, however, there are rather large discrepancies between each others as shown in Fig. 4. This is mainly due to lack of a common calibration for all experiments and to limited dynamic range of a single experiment. Corresponding composition change associated with the second knee are shown in Fig. 5. There are large discepancies between the experiemnts as well.

Fig. 4 & 5. The UHE cosmic ray energy spectrum and composition measurements. There are many discrepancies remains, mainly due to lack of the unique calibration. Co-site measurements with cross calibration may be the best solution. A way to overcome the difficulty is to put several independent experiments together, dedicate each of them to cover a suitable energy range and maintain a sufficient overlap between the experiments. Using the cosmic ray events falling in the overlaps, one can cross-calibrate the detectors and achieve a complete and self-consistent measurement of the cosmic ray energy spectrum and their composition above 0.1 EeV. The CRTNT experiment is designed to cover the energy range from 0.1 EeV to 5 EeV and to co-site with the TALE (1 EeV~50 EeV) and the TA (above 10 EeV) and ultimately achieve the goal.

It is essential to have a statistically significant measurement of the cosmic ray energy spectrum between 0.1 EeV and 5 EeV and accurate measurement of air shower maximum location and other shower development parameters, such as the width of the shower development profile. In order to do so, the CRTNT telescopes will be configured in such way that there should be a main "tower" detector with 12 telescopes to cover elevation angle from 1 degree to 43 degree and azimuth angle from -32 degree to 32 degree. To guarantee the accuracy of shower parameter measurements, a stereoscopic measurement is necessary. Four more telescopes will be installed 5 km apart from the tower detector to cover 41 degree to 55 degree in elevation, two telescopes in a group and 8 km apart from each group, thus to form a triangle and watch over the inner common space.

Simulation shows that the aperture of the CRTNT detector is about 100 km²sr at 5 EeV and 25 km²sr at 0.1 EeV after cuts applied to the simulated events for good quality of reconstruction. The cuts are chosen mainly to avoid those involve the boundary of the field of view. Such a big aperture of the CRTNT detector provides a sufficient room for more strict cuts in order to perform the detector for precise measurement. Constraining events inside a core area of the field of view, the detector provides an almost constant aperture over the range of 0.1 EeV to 1 EeV that guarantees the accuracy of the study associated withe second knee. The large aperture at 5 EeV will provide sufficient statistics of common events with the TALE experiment for cross calibration. According to this simulation, the expected number of events are about 20k reconstructible for the second knee measurement and about 700 high energy events above the threshold of the TALE detector for cross calibration. The aperture and number events with cosmic ray energy are shown in Fig 6. and Fig. 7, respectively.

Fig. 6. The Aperture of the CRTNT detector with 16 telescopes. With different quality cuts, the detector will have an aperture of about 25 km²sr for second knee measurement.
Fig. 7. The corresponding number of events can be collected by the CRTNT detector per year. About 20k reconstructible events are expected above 0.1 EeV and 700 events above 1 EeV for cross calibration.