This study presents the first observation of ultra-long-range two-particle azimuthal correlations with pseudorapidity separation of ($|\Delta \eta| > 5.0$) in proton-proton (pp) and ($|\Delta \eta| > 6.5$) in proton-lead (p-Pb) collisions at the LHC, down to and below the minimum-bias multiplicity. Two-particle correlation coefficients (${V}_{2\Delta}$) are measured after removing non-flow (jets and resonance decays) contributions using the template-fit method across various multiplicity classes, providing novel insights into the origin of long-range correlations in small systems. Comparisons with the 3D-Glauber + MUSIC + UrQMD hydrodynamic model reveal significant discrepancies at low multiplicities, indicating possible dynamics beyond typical hydrodynamic behavior. Initial-state models based on the Color Glass Condensate framework generate only short-range correlations, while PYTHIA simulations implemented with the string-shoving mechanism also fail to describe these ultra-long-range correlations. The results challenge existing paradigms and question the underlying mechanisms in low-multiplicity pp and p-Pb collisions. The findings impose significant constraints on models describing collective phenomena in small collision systems and advance the understanding of origin of long-range correlations at Large Hadron Collider (LHC) energies.
Submitted to: PRL
e-Print: arXiv:2504.02359 | PDF | inSPIRE
CERN-EP-2025-080
Figure group
Figure 3
Left: The Nch dependence of the second-order two-particle correlation coefficient V2Δ(2PC), estimated using the template-fit method, in pp (|Δη|>5.0) collisions at √s=13 TeV, and compared with predictions from the 3D-Glauber + MUSIC + UrQMD model , as well as with PYTHIA8 variations with string-shoving and ropes implementations. Right: The $N_{\rm ch}$ dependence of the V2Δ(2PC), estimated using template-fit method, in p--Pb ($|\Delta \eta| > 5.0$ and $|\Delta \eta| > 6.5$) collisions at $\snn=5.02$ TeV and its comparison with the 3D-Glauber + MUSIC + UrQMD predictions. |   |