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.
