Astronomy has developed with observations using photons of various wavelength, from radio togamma rays, which is called 'multi-wavelength astronomy'. However, in the beginning of the 21th century, itfaces on a turning point toward 'multi-particle astronomy', with particles other than photon, such as neutrinosand gravitational waves. They can be strong tools to reveal the origin of high-energy universe. In particular,neutrinos can serve as a key to the mechanism of the particle acceleration, which is an open question sincedecades ago.

The Ashra (All-sky Survey High Resolution Air-shower detector) Experiment is a project that aimsto catch signals from transient objects with air light, to open the door to the multi-particle astronomy. It candetect with trigger, very-high-energy gamma rays, ultra-high-energy cosmic rays, and very-high-energyneutrinos, as well as UV light without trigger. The detector is a type with fixed optics, which has a wide fieldof view of 42 degrees in diameter and a high angular resolution of several arc-minutes. It is most suitable forthe monitoring observation for transient objects.

We have designed an observation at the site on Mauna Loa on Hawaii Island, to study neutrinoemissions from transient objects, and to confirm the performance of the trigger system of the Ashra detector.The principle of the trigger system had been tested in an observation for gamma rays, carried out in 2004 onHaleakala on Maui Island. We have planned to use the trigger system to the observation in 2008, while theother components are the same as the final design of the Ashra detector. For this observation we haveadopted the earth-skimming tau neutrino detection technique, with which the Earth's rock is used as a targetfor neutrino interaction while the atmosphere is used as a detector. We have simulated neutrino signalsdetected as Cherenkov radiation from air-showers generated by tau leptons emerging from Mauna Kea, amountain close to our site. The detector was calibrated with various ways in advance.

During the observation time, conditions must be monitored to keep stable observation and dataquality. We have achieved high observation efficiency with operation schemes and Slow Control Systemneatly designed for monitoring surroundings. Also, as another condition for observation, background light ofthe sky and Mauna Kea have been observed in advance. As a result, during the observation from Septemberto December in 2008, we observed a gamma-ray burst (GRB), GRB081203A, in a period before and after itsoccurrence. Also we observed 13 GRBs after their occurrence, and 32 active galactic nuclei (AGN).

The obtained data were analyzed. In order to estimate the sensitivity we calculated the physicsfunction for signal and noise, and detector function for both. The physics function for signal is sourcemodels. We focuses on neutrino emissions from GRBs and AGN. The physics function for noise isbackground fake events. In principle, the earth-skimming neutrino detection technique is free frombackground cosmic rays, thanks to the screening effect of the Earth's rock. We have studied the screeningeffect of Mauna Kea and make sure the absence of the background events. As a consequence, the largestbackground source is Cherenkov radiations from muons that interact with the detector itself. We havesimulated and removed the fake events carefully.

We have not discovered any neutrino emission above 1015eV (PeV) up to about 1018eV (EeV), andset upper limits to the fluence and flux of source models, related to the mechanism for the particleacceleration in transient objects. For GRB precursor, we set a limit EΦ(E) < 8.3・10-8 [cm-2], assuming aspectrum extended up to 5 EeV. For GRB late prompt, the obtained limit is E2Φ(E) < 6.2 [erg cm-2]. ForGRB afterglow, the limit leads E2Φ(E) < 26 [erg cm-2]. Furthermore, neutrino emission from AGN is studied.The limit for diffuse flux is about 2 orders of magnitude larger than that by other experiments. In addition,we set an upper limit, E1.4Φ(E) < 6.5・10-13[cm-2 s-1 PeV0.4], based on a source model assuming protonsynchrotron emission. Though this limit corresponds to a diffuse flux rejected by experiments, it is stillmeaningful as a limit for point source, which is not affected by the source distribution in the universe. All theabove limits are derived with 90% confidence level, including systematic errors.

This is one of the first experiments that searched for astrophysical neutrinos by the earth-skimmingtau neutrino detection technique. These results demonstrate the strength of our detector. For future neutrinoexperiments, pointing accuracy in PeV region is important to localize transient objects, as well as it is ofimportance to extend sensitive region of air monitoring toward lower energies. The results in this thesisreveals the strength of our detector, and hence we expect the Ashra detector as a candidate to realize 'multiparticleastronomy', which is necessary for us to understand the particle acceleration in the universe.

本論文は8章からなる。第1章は、イントロダクションであり、本研究の背景が述べられている。第2章と第3章は本研究を行った装置の説明と観測方法が述べられている。第4章は観測に関する詳細が述べられ、第5章で観測データの解析、すなわち本論文のテーマである宇宙の突発天体からの超高エネルギーニュートリノ探索が述べられている。第6章は観測結果を基にしたニュートリノのフラックスの上限の導出について述べられ、第7章ではこの結果をもとに粒子加速に関する議論をおこなっている。第8章は本論文のまとめである。

超高エネルギー宇宙ニュートリノ、それも特にタウニュートリノについては、山などをニュートリノのターゲットとして用い、そこで生成されたタウレプトンが山から出てきて大気中で崩壊して空気シャワーを生成する原理を用いて、この空気シャワーを観測することでバックグラウンドのほとんどない観測が可能ではないかと指摘されていた。本研究はこの観測原理に基づいた専用の装置をハワイ島のマウナロア山に設置して、このようなニュートリノの観測を試みた。この装置の集光や信号の読み出しは世界的に見てもユニークであり、これを製作して観測を行ったことは、評価に値する。

今回の観測では、特に別の天体観測で得られたガンマ線バーストなどの突発天体からのニュートリノの観測に焦点を当てた。残念ながら有意なニュートリノ信号は観測できず、ニュートリノの流量に関しての上限を与え、いくつかの突発天体のモデルに制限をつける結果となった。それとともに、上記の観測原理に基づいて特化した測定器を製作して観測を行った意義は世界的に見ても十分大きいと判断し、この研究は博士の学位を授与するに十分に値すると判断する。

なお、本論文の研究内容はAshra共同実験の観測結果に基づく結果であるが、論文提出者が主体となって観測機器のデザイン、建設から解析までにおよび研究を遂行したもので、論文提出者の寄与が十分大きいと判断する。

従って、博士(理学)の学位を授与できると認める。