Search for Solar Flare Neutrinos with the KamLAND detector

We report the result of a search for neutrinos in coincidence with solar flares from the GOES flare database. The search was performed on a 10.8 kton-year exposure of KamLAND collected from 2002 to 2019. We found no statistical excess of neutrinos and established 90% confidence level upper limits of $8.4 times 10^7$,cm$^{-2}$ ($3.0 times 10^{9}$,cm$^{-2}$) on electron anti-neutrino (electron neutrino) fluence at 20,MeV normalized to the X12 flare, assuming that the neutrino fluence is proportional to the X-ray intensity. The 90% C.L. upper limits from this work exclude the entire region of parameter space associated with the Homestake event excess for the large solar flare in 1991.