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 1
The ultra-long-range (\(|\Delta \eta| > 5.0\)) per-trigger yield, measured as a function of \(\Delta \eta\) and \(\Delta \varphi\) in p--Pb collisions at \(\sqrt{s_{\text{NN}}} = 5.02\) TeV for ($7 < N_{\rm ch} < 40$), shows a double-ridge structure after non-flow removal using the template-fit method. |  |
Figure 2
The \(N_{\text{ch}}\) dependence of the second-order two-particle correlation coefficient \( V_{2\Delta} (\text{2PC}) \), estimated using the template-fit method, in pp (\(|\Delta \eta| > 5.0\)) collisions at \(\sqrt{s} = 13\) TeV and p--Pb (\(|\Delta \eta| > 5.0\) and \(|\Delta \eta| > 6.5\)) collisions at \(\sqrt{s_{\text{NN}}} = 5.02\) TeV. |  |
Figure 3
Left: The \( N_{\rm ch} \) dependence of the second-order two-particle correlation coefficient \( V_{2\Delta} (\rm{2PC}) \), estimated using the template-fit method, in pp (\( |\Delta \eta| > 5.0 \)) collisions at \( \sqrt{s} = 13 \) 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 \( V_{2\Delta} (\rm{2PC}) \), 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. |   |