Determination of the spin and parity of all-charm tetraquarks

The traditional quark model^1,2 accounts for the existence of baryons, such as protons and neutrons, which consist of three quarks, as well as mesons, composed of a quark–antiquark pair. Only recently has substantial evidence started to accumulate for exotic states composed of four or five quarks and antiquarks³. The exact nature of their internal structure remains uncertain^{4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22, 23, 24, 25, 26, 27, 28–29}. Here we report the first measurement of quantum numbers of the recently discovered family of three all-charm tetraquarks^{30, 31–32}, using data collected by the CMS experiment at the Large Hadron Collider from 2016 to 2018 (refs. ^33,34). The angular analysis techniques developed for the discovery and characterization of the Higgs boson^{35, 36–37} have been applied to the new exotic states. Here we show that the quantum numbers for parity P and charge conjugation C symmetries are found to be +1. The spin J of these exotic states is determined to be consistent with 2ħ, while 0ħ and 1ħ are excluded at 95% and 99% confidence levels, respectively. The J^PC = 2⁺⁺ assignment implies particular configurations of constituent spins and orbital angular momenta, which constrain the possible internal structure of these tetraquarks.