Reconstruction of Muon Bundle in the JUNO Central Detector

The Jiangmen Underground Neutrino Observatory (JUNO) is a multi-purpose neutrino experiment. One of the main goals is to determine the neutrino mass ordering by precisely measuring the energy spectrum of reactor antineutrinos. For reactor antineutrino detection, cosmogenic backgrounds such as $^9$Li/$^8$He and fast neutrons induced by cosmic muons should be rejected carefully by applying muon veto cuts, which requires good muon track reconstruction. With a 20~kton liquid scintillator detector, simulation shows the proportion of muon bundles to be around 8\% in the JUNO, while its reconstruction is rarely discussed in previous experiments. According to the charge response of the PMT array, this paper proposes an efficient algorithm for muon bundle track reconstruction. This is the first reconstruction of muon bundles in a large volume liquid scintillator detector. Additionally, the algorithm shows good performance and potential in reconstruction for both single muon and double muons. The spatial resolution of single muon reconstruction is 20~cm and the angular resolution is $0.5^\circ$. As for double muon reconstruction, the spatial resolution and angular resolution could be 30~cm and $1.0^\circ$, respectively. Moreover, this paper has also discussed muon classification and veto strategy.