Introduction

YBJ-ARGO is a collaborative project undertaken by the Chinese Academy of Sciences (CAS) and Instituto Nazionale di Fisca Nucleare (INFN) on the construction of a RPC “carpet” at Yangbajing Cosmic Ray Observatory. YBJ is the abbreviation for Yangbajing whereas ARGO is a colossal monster in a Greek fairy tale. It has 100 eyes always with 50 open and 50 closed alternately. The collaboration named after YBJ-ARGO indicates that once the “carpet” is completed, cosmic ray and astrophysics can be studied under all weather conditions.

YBJ Cosmic Ray Observatory is located in a small basin of about 70 km long by 7-15 km wide at the foot of Tanggula Mountain, close to the cross point of the Qinghai-Tibet Highway and the China-Nepal Highway 90 kilometers away northwest of Lhasa. The small basin is 4,300 meters above sea level, with a unique climate. People can work and live there normally even in the severe cold winter. This experimental station on cosmic ray located at the highest elevation on the Northern Hemisphere has become renown in the world.

Since 1990, the extended air shower array built by the China/Japan collaboration at Yangbajing has been operated and expanded year after year, with many important physics results achieved, such as the obtainment of the precise energy spectrum of primary cosmic ray at the knee region, the first attainment of the 10 TeV cosmic ray sun shadow, the discovery of the change of the sun shadow with the solar activities and the interplanetary magnetic fields, the search for the possible existence of high energy gamma-ray burst, the confirmation of the stable emittance of 3 TeV gamma-ray from Crab Nebula and the observation of a 3 TeV gamma-ray burst signal from the extragalactic MrK 501 during its high activity in 1997. All these results were achieved by using the EAS array technique, which have aroused extensive interest from scientists working in the same line in the world.

The YBJ-ARGO collaborative project is to build a carpet of RPCs covering an area of 10,000 square meters and develop the traditional AS particle sampling array into full coverage so as to give full play to the geographical advantage of YBJ. With the adoption of the technique of array with wide angle and round-the-clock operation in the energy region of about 100 GeV, the astronomical observation of gamma from the ground can be extended to the red shift Z～1.0 of the depth in the Universe. With the method to distinguish the primary components of cosmic ray by studying each event with the detailed information of the AS particle time-space distribution, the development of ultrahigh energy gamma ray physics at the knee region can be promoted. All these will facilitate the construction of a comprehensive international cosmic ray observatory at Yangbajing for the extensive study of cosmic ray and astrophysics and secure its leading position with the unique vitality in the world in the 21st century.

In December 1998, CAS and INFN signed an agreement on YBJ-ARGO collaboration. Under this agreement, both sides invested altogether 9 million US dollars in the collaborative project. Following 5 years’ preparations, the program of the cosmic ray experiment was initiated in the end. Now, CAS has supported the YBJ-ARGO collaboration as a knowledge innovation project. And the National Natural Science Foundation of China and the Ministry of Science and Technology have all allocated fund for this project. The construction of the site of the YBJ cosmic ray observatory started in the summer of 2000. The assembly of the first RPCs began in the winter of 2000. If the amount of 34.6 million Chinese yuan needed by the Chinese side could be allocated in time, hopefully the 50,000 square meters’ “carpet” will be completed and the early stage data acquisition and data analysis be started.

Collaborators 

Chinese Collaborators 

The Institute of High Energy Physics, the Chinese Academy of Sciences

Department of Mathematics and Physics, Tibet University

High Energy Physics Laboratory, Shandong University

Department of Physics, Hong Kong University

The Institute of Modern Physics, Southwest Communications University

Department of Physics & Cosmic Ray Institute, Yunnan University

Department of Physics, Zhengzhou University

Italian collaborators 

INFN and Dipartimento di Fisica dell’Universita di Napoli

INFN and Dipartimento di Fisica dell’Universita di Roma Tre

INFN and Dipartimento di Fisica dell’Universita “Tor Vergata” di Roma

INFN and Dipartimento di Fisica dell’Universita di Lecce

INFN and Dipartimento di Fisica dell’Universita di Padova

Istituto di Cosmogeofisica del CNR and INFN, Torino

Basilicata University