Emergence of long-range angular correlations in low-multiplicity proton-proton collisions

This Letter presents the measurement of near-side associated per-trigger yields, denoted ridge yields, from the analysis of angular correlations of charged hadrons in proton-proton collisions at $\sqrt{s}$ = 13 TeV. Long-range ridge yields are extracted for pairs of charged particles with a pseudorapidity difference of $1.4 <~ |\Delta\eta| <~ 1.8$ and a transverse momentum of $1 <~ p_{\rm T} <~ 2$ GeV/$c$, as a function of the charged-particle multiplicity measured at midrapidity. This study extends the measurements of the ridge yield to the low multiplicity region, where in hadronic collisions it is typically conjectured that a strongly-interacting medium is unlikely to be formed. The precision of the new low multiplicity results allows for the first direct quantitative comparison with the results obtained in $\mathrm {e^{+}e^{-}}$ collisions at $\sqrt{s}$ = 91 GeV and $\sqrt{s}$ = 183$-$209 GeV, where initial-state effects such as pre-equilibrium dynamics and collision geometry are not expected to play a role. In the multiplicity range $8\lesssim\langle N_\mathrm{ch}\rangle\lesssim 24$ where the $\mathrm {e^{+}e^{-}}$ results have good precision, the measured ridge yields in pp collisions are substantially larger than the limits set in $\mathrm {e^{+}e^{-}}$ annihilations. Consequently, the findings presented in this Letter suggest that the processes involved in $\mathrm {e^{+}e^{-}}$ annihilations do not contribute significantly to the emergence of long-range correlations in pp collisions.


Phys. Rev. Lett. 132 (2024) 172302
HEP Data
e-Print: arXiv:2311.14357 | PDF | inSPIRE
Figure group

Figure 1

Two-particle per-trigger yield measured for charged track pairs with $1< p_{\rm T,trig} <2$ GeV/$c$ and $1< p_{\rm T,assoc} <2$ GeV/$c$ within the multiplicity range $32 < N_\mathrm{ch} \leq 37$. The jet fragmentation peak has been truncated to ensure a better visibility of the long-range structure. The right panel shows the zero-suppressed projection to $\Delta\varphi$ overlaid with $F(\Delta\varphi)$ (red line) and the area in which the ridge yield is extracted (shaded area). The blue and purple lines represent the second and third harmonic terms of $F(\Delta\varphi)$.

Figure 2

Ridge yield as a function of multiplicity. The black points correspond to the measurement presented in this Letter, while data from CMS  are drawn as green and blue markers. Vertical bars denote statistical uncertainties while systematic uncertainty is shown as shaded area. For both results, at low multiplicity where the lower uncertainty reaches zero, an upper limit is reported, which is drawn as a bar and down arrow. Such points are given at 95% CL for the results from this Letter and at 67% for the results from CMS. The ``MB'' arrow at $\langle N_\mathrm{ch}\rangle=11.3$ indicates the multiplicity averaged over the entire studied multiplicity range. Ridge yield as a function of multiplicity, compared to the upper limits on the ridge yield in \ee collisions. Vertical bars denote statistical uncertainties while systematic uncertainty is shown as the shaded areas. The orange limits represent the measurement in the thrust-axis reference frame with ALEPH . The horizontal bars in the ALEPH points represent the uncertainty related to the multiplicity conversion from the ALEPH to the ALICE acceptance (see text). All limits are given at 95% CL.

Figure 3

Ridge yield as a function of multiplicity compared to the predictions of PYTHIA 8.3  with Monash tune  (green) and string shoving  (orange) as well as EPOS LHC simulations  (blue). Because of a larger jet fragmentation peak width in the simulations than in data, the yield is extracted within $2<|\Delta\eta|<4$ for the model calculations. A 95% CL is indicated for model calculations when the lower limit of statistical uncertainty is below zero. Some points are slightly displaced along the $x$-axis for better visualization. The band indicates the statistical uncertainty from the event generation and the extraction procedure.