Angel Abusleme, Thomas Adam, Kai Adamowicz, D. Adey, S. Ahmad, Rizwan Ahmed, Timo Ahola, S. Aiello, Fengpeng An, Guangpeng An, C. Andreopoulos, Giuseppe Andronico, J. P. A. M. de André, Н. Анфимов, V. Antonelli, Tatiana Antoshkina, Burin Asavapibhop, Didier Auguste, M. Buizza Avanzini, Andréj Babič, Jingzhi Bai, Weidong Bai, Nikita Balashov, Roberto Barbera, Andrea Barresi, D. Basilico, E. Baussan, Beatrice Bellantonio, M. Bellato, J.L. Béney, Marco Beretta, Antonio Bergnoli, E. Bernieri, Nikita Bessonov, David Biaré, D. Bick, Lukas Bieger, S. Biktemerova, Thilo Birkenfeld, David Blum, S.C. Blyth, Sara Boarin, Manuel Boehles, Anastasia Bolshakova, M. Bongrand, A. Bonhomme, Clément Bordereau, M. Borghesi, A. Brigatti, Timothée Goubault de Brugière, R. Brugnera, Riccardo Bruno, Jonas Buchholz, A. Budano, Max Buesken, Mario Buscemi, Severino Bussino, José Busto, Ilya Butorov, Marcel Büchner, Anatael Cabrera, Barbara Caccianiga, Boshuai Cai, Hao Cai, Xiao Cai, Yanke Cai, Yi-Zhou Cai, Zhiyan Cai, S. Callier, S. Calvez, Antonio Cammi, Agustín Campeny, Dechang Cai, Chuanya Cao, Dewen Cao, Guofu 国富 Cao 曹, Jun Cao, Yaoqi 尧齐 Cao 曹, R. Caruso, Aurelio Caslini, C. Cerna, Vanessa Cerrone, Daniele Cesini, Chi Kuen Chan, J. F. Chang, Yun Sil Chang, Milo Charavet, Tim Charissé, Auttakit Chatrabhuti, Chao Chen, Guo-Ming Chen, Haitao Chen, Haotian Chen, J. L. Chen, Jian Chen, Jing Chen, Junyou Chen, Lihao Chen, Mali Chen, Mingming Chen
Abstract The Jiangmen Underground Neutrino Observatory (JUNO) started physics data taking on 26 August 2025. JUNO consists of a 20-kton liquid scintillator central detector, surrounded by a 35 kton water pool serving as a Cherenkov veto, and almost 1000 m 2 of plastic scintillator veto on top. The detector is located in a shallow underground laboratory with an overburden of 1800 m.w.e. This paper presents the performance results of the detector, extensively studied during the commissioning of the water phase, the subsequent liquid scintillator filling phase, and the first physics runs. The liquid scintillator achieved an attenuation length of 20.6 m at 430 nm, while the high coverage PMT system and scintillator together yielded about 1785 photoelectrons per MeV of energy deposit at the detector centre, measured using the 2.223 MeV γ from neutron captures on hydrogen with an Am-C calibration source. The reconstructed energy resolution is 3.4% for two 0.511 MeV γ at the detector centre and 2.9% for the 0.93 MeV quenched 214 Po alpha decays from natural radioactive sources. The energy non-linearity is calibrated to better than 1%. Intrinsic contaminations of 238 U and 232 Th in the liquid scintillator are below 10 -16 g/g, assuming secular equilibrium. The water Cherenkov detector achieves a muon detection efficiency better than 99.9% for muons traversing the liquid scintillator volume. During the initial science runs, the data acquisition duty cycle exceeded 97.8%, demonstrating the excellent stability and readiness of JUNO for high-precision neutrino physics.