Tibet's Ali: A New Window to Detect the CMB Polarization

The Cosmic Microwave Background (CMB) Polarization plays an important role in current cosmological studies. CMB B-mode polarization is the most effective probe to primordial gravitational waves (PGWs) and a test of the inflation as well as other theories of the early universe such as bouncing and cyclic universe. So far, major ground-based CMB polarization experiments are located in the southern hemisphere.Recently, China has launched the Ali CMB Polarization Telescope (AliCPT) in Tibetan Plateau to measure CMB B mode polarization and detect the PGWs in northern hemisphere. AliCPT include two stages, the first one is to build a telescope at the 5250m site (AliCPT-1) and the second one is to have a more sensitive telescope at a higher altitude of about 6000m (AliCPT-2). In this paper, we report the atmospherical conditions, sky coverage and the current infrastructure associated with AliCPT. We analyzed the reanalysis data from MERRA-2 together with radiosonde data from the Ali Meteorological Service and found that the amount of water vapor has a heavy seasonal variation and October to March is the suitable observation time. We also found 95/150 GHz to be feasible for AliCPT-1 and higher frequencies to be possible for AliCPT-2. Then we analyzed the observable sky and the target fields, and showed that Ali provides us a unique opportunity to observe CMB with less foreground contamination in the northern hemisphere and is complementary to the existed southern CMB experiments. Together with the developed infrastructure, we point out that Ali opens a new window for CMB observation and will be one of the major sites in the world along with Antarctic and Atacama.