Neutrinos are expected to form an asymmetric distribution around a dark matter halo due to Chandrasekhar dynamical friction effect. These are called neutrino wakes. NAOC Graduate Student Hong-Ming Zhu, Prof. Xuelei Chen and others proposed that this effect can be used to probe neutrino hierarchy and chirality. This work has been published in the Physical Review Letters (Phys, Rev. Lett. 116, 141301 (2016)).
The neutrinos decoupled from the primordial plasama as the Universe expended, forming the cosmic neutrino background. There are about 112 neutrinos per cm^3. In 1998, neutrinos were found to oscillate, which indicated their masses are non-zero. Thus, neutrinos became non-relativistic at low redshifts, influencing the formation of the large-scale structure. By observing the imprints on the large-scale structure from neutrinos, we can constrain the neutrino masses.
Zhu et al found that as halos move through the cosmic neutrino background sea they leave an over density in the neutrino density, analogous to ships ploughing through a lake. This is called the wake effect. The neutrino wakes can be observed through the galaxy-galaxy lensing, and used to constrain neutrino masses. Future 21 cm lensing surveys could determine the neutrino hierarchy by this method. If neutrinos are Dirac particles instead of Majorana particles, there would be right-handed neutrinos in addition to the left-handed ones. Due to the different interaction mechanism, right-handed neutrinos decoupled at a different time. If so, there would be right-handed wakes in addition to the left-handed ones, which can also be observed through galaxy-galaxy lensing. This can be probed by the future 21cm lensing surveys. If these effects can be observed, many new properties of neutrinos would be discovered. The wake effect opens up a new window to probe neutrinos.