KamLAND Searches for Neutrinos in Gamma Ray Bursts

The KamLAND experiment has released a new paper on the arXiv searching for evidence of electron antineutrinos produced in gamma ray bursts. KamLAND is a liquid scintillator detector located at the Kamioka underground lab that does various studies related to neutrinos at MeV-scale energies. KamLAND has previously made measurements of neutrino oscillations in reactor neutrinos and has more recently been changed to look instead for neutrinoless double beta decay.

Electron antineutrinos are a very convenient signal for such detectors. Charged-current interactions allow for the neutrinos to convert a proton into a neutron. For an electron neutrino, then, the interaction is ν+p→e+n. For a neutrino with an energy in the range of 1-10 MeV, most of the energy will go to the electron, which can be easily measured. The neutron doesn’t get as much energy and is neutral, so it won’t leave a nice ionization or scintillation signal. However, a neutron can’t go very far in a hydrogen-rich target before thermalizing and will eventually be absorbed. Neutron capture is typically associated with gamma emission, so the electron signal will be accompanied by a gamma ray with several MeV of energy. This coincidence signal can help distinguish electron antineutrino interactions from various backgrounds.

This result finds no evidence for neutrinos appearing in coincidence with gamma ray bursts. They mention that there aren’t really good models to rule out yet, so this doesn’t necessarily constrain any fundamental physics at this time. However, they can set an upper limit on how many neutrinos are produced by these kinds of astrophysical events. Gamma ray bursts are still quite mysterious, so any information we can get might be important in the future.