Study of ordered hadron chains in proton-proton, proton-lead and lead-lead collisions with the ATLAS detector

An analysis of momentum differences between charged hadrons in high--energy proton--proton, proton--lead and lead--lead collisions is performed in order to study the dynamics of hadronization. Data were recorded by the ATLAS experiment at a center--of--mass energy of $\sqrt{s}$ = 0.9, 5.02 and 13 TeV for proton--proton collisions, and at $\sqrt{s_{NN}}$ = 5.02 TeV for proton-lead and lead-lead collisions. The difference in the yield of hadron pairs with like-sign and opposite-sign charge is used to extract spectra of pairs adjacent in colour flow. The study of correlations between adjacent hadrons is performed using observables inspired by the properties of string fragmentation. The results highlight differences between the data and the Lund string model which can be attributed to quantum properties of the underlying QCD field. Studies of the charged particle multiplicity dependence reveal a significant contribution to high multiplicity events from low-mass hadronizing sources. In all data samples, the enhanced production of like--sign pions with low momentum difference is associated with the presence of correlated hadron chains. The properties of these chains coincide with the predictions of the model of quantized fragmentation of a helical QCD string. Proton--proton, proton--lead and lead--lead data are used to perform a combined measurement of string parameters from correlations within the measured chains, in a way which minimizes the impact of longitudinal variations of the string shape. The results are consistent with values obtained independently from the quantized mass spectrum of light hadrons. For the first time, a signature of long pion chains is observed in lead-lead data, in the form of distinct quantized patterns.