Showing papers by "Gareth Hughes published in 2007"

Status and Prospect of Telescope Array (TA) Experiment

Abstract: Telescope Array (TA) is an air shower experiment composed of an ar r y of ground particle detectors and 3 sets of fluorescence telescopes installed in Utah, USA. It aims at drawing a conclusion on the (non-)existence of the GZK cutoff reported controversially by AGAS A and HiRes experiments. An anisotropy of the UHECR arrival directions will be studied in the northern h misphere where the galactic disturbances are small. The plastic scintillator is useful for the determination of the air shower energy independent of the hadronic interaction model and the primary compositio n. Various calibration methods will be applied for the accurate determination of event energy scale. A tota l acceptance of the ground array and the telescope will be more than 20 times larger than that of AGASA . n operation of partial detector has started in spring 2007. The status of the experiment is repor ted and prospects for the physics are given. Search for the GZK Cutoff A cutoff structure is expected in the energy spectrum of extremely high energy cosmic rays (EHECRs) at ∼10 eV. It originates from the interaction of cosmic ray protons with the cosmic microwave background and was predicted by Greisen, Zatsepin and Kuzmin (GZK) in 1966 [1]. Since that prediction, the search for the GZK cutoff has been a central theme in the study of EHECRs. Major efforts were made by the Akeno Giant Air Shower Array (AGASA) in Japan and the High Resolution Fly’s Eye (HiRes) in the USA. The AGASA experiment published an energy spectrum which does not exhibit the GZK cutoff in 1998 [2]. The spectrum above 10 19 eV is well described byE distribution, and a total of 6 events was observed above 10 eV with an exposure of 0.83× 10 km sr yr. It is updated to 11 events in 2003 with an exposure of 1.62 × 10 km sr yr [3]. The HiRes published a result of monocular measurement in 2004 [4] and asserted that the energy spectrum is consistent with the existence of the GZK cutoff In the monocular HiRes data set, the number of events above 10 eV is 2 with an exposure of 2.4× 10 km sr yr. It is apparent that the discrepancy in the Flux( E) ×E 3 plots is largely due to the systematic difference of energy measurement between the two experiments. It is known that the spectra of AGASA and HiRes agree well below∼10 eV if either the overall energy scale of AGASA is lowered by ∼20% or the energy scale of HiRes is increased by the same amount. The AGASA claims its uncertainty in energy determination is 18% [3] and the corresponding number of HiRes is 17% [4]. Even after the energy rescaling, however, the number of events above10 eV seems to show a disagreement between AGASA and HiRes. For AGASA, the number of events with E > 10 eV becomes 5 with -20% energy rescaling. Normalizing the exposure to that of AGASA but keeping its energy scale, the corresponding number is 1.4 events for HiRes. Each experiment stays unchanged on the conclusion of the GZK cutoff but with less statistical significance. This disagreement is originating from the physics of UHECRs if it is not explained by the systematics inherent to two experimental methods; AGASA is the ground particle array and HiRes is the fluorescence telescope. In order to establish the energy spectrum for cosmic rays in the GZK cutoff region, it is urgent to understand the reason of this difference by the hybrid experiment. New Generation Detectors AGASA completed 13 years of data collection in January, 2004. The HiRes stopped taking data in April, 2006. Two new experiments, the Pierre Auger Observatory (hereafter called “Auger”) and