"The investigators focused on neutrinos whose energy levels suggest they are linked with gamma-ray bursts. The fireballs that give rise to the gamma rays seen in gamma-ray bursts were thought to potentially hurl particles at very high energies, generating both cosmic rays and energetic neutrinos. After analyzing data on 307 gamma-ray bursts in 2008 and 2009, the scientists discovered the levels of these neutrinos were at least 3.7 times lower than expected. This suggests gamma-ray bursts are probably not the sources of the most powerful cosmic rays."
The cosmic rays in question are notable because their energies can be beyond the Greisen–Zatsepin–Kuzmin limit, our current theoretical upper limit for the cosmic ray energies.