Cosmic Neutrinos


Neutrinos come in three different flavors: electron-neutrino, muon-neutrino, and tauon-neutrino. Because of their extremely weak interaction and being charge-neutral, they are the least understood among all elementary particles. For example, only until the past couple decades did we recognize that neutrinos actually have non-zero rest masses and they change their flavors back-and-forth. The universe is filled with neutrinos. They contribute to 0.3% of the total energy density of the universe, more than radiation (photons).

Cosmic neutrinos can be subdivided into three categories according to their origin and energy. These are the cosmic neutrino background (CNB, or CnB), stellar neutrinos, and cosmogenic neutrinos. Similar to the cosmic microwave background (CMB), induced at around 380,000 years after Big Bang, the CnB was induced at around 2 seconds after the Big Bang when the rate of their interactions with other particles became slower than the rate of expansion of the universe at that time. It is estimated that by now CnB has been cooled to ~1.9 °K temperature, or about 10-4 eV in energy, which makes them non-relativistic. The CnB has never been detected.

All stars shine as a result of nuclear fusion reaction in the stellar core. One necessary product of nuclear fusion is electron-neutrinos. Being weakly interacting, these neutrinos escape from the star. For example, the neutrino flux from the Sun at the Earth is about 1011 particles/cm2/sec. Solar neutrinos were first detected by Raymond Davis, who discovered neutrino flavor oscillations at the same time. Also when a star uses up its nuclear fuel and explodes as a supernova, it releases a huge number of neutrinos; about 99% of the exploding energy is carried by neutrinos. Masatoshi Koshiba led the effort in the first detection of supernova neutrinos of SN1987A in 1987. These marked the beginning of neutrino astronomy. Davis and Koshiba were jointly awarded the 2002 Nobel Prize in Physics for their contributions.

Finally, there also exist the extremely high-energy cosmogenic neutrinos. These are the neutrinos produced by the collisions between ultra high energy cosmic rays (UHECR) and CMB photons, also referred to as Greisen-Zatsepin-Kuzmin (GZK) neutrinos. The fact that UHECRs (mostly protons) with energies up to 1020 eV and CMB have been observed on Earth implies that GZK neutrinos with energies around 1017-1019 eV must exist with a sufficient flux based on known, standard model particle physics. However, so far these neutrinos have not been observed yet. The world’s largest cosmic neutrino observatory IceCube at South Pole have recently observed a pair of

cosmic neutrinos at energies around 1015 eV, which were nicknamed by the IceCube Collaboration as “Bert” and “Ernie”, after characters from the Sesame Street TV show. Later in 2013 an even more energetic neutrino was found, and it was nicknamed “Big Bird”.

To go beyond this energy range to search for GZK neutrinos, several international projects have been launched, including the balloon-borne ANITA observatory and the ground based Askaryan Radio Array (ARA) Observatory at South Pole. NTU LeCosPA has been an active member in these two projects. (link to the relevant page of our website). Most recently (2014), LeCosPA has developed a new cosmic neutrino and UHECR observatory by the name TAROGE (Taiwan Astroparticle Radiowave Observatory based on Geo-synchrotron Emissions), or 太魯閣, after an aborigines tribe on the east coast of Taiwan where the antenna station is located. It stands on top of the steep coastal mountain ridge overlooking the Pacific to search for the signals.