CMS-PAS-HIN-25-016

CMS-PAS-HIN-25-016
Transverse-momentum differential radial flow fluctuations across collision systems with CMS
CMS Collaboration
2026-07-07

Abstract: Event-by-event fluctuations of the mean transverse momentum carry direct information about collective radial expansion. The first measurement of transverse-momentum-dependent radial flow fluctuations, characterized by the observable $ v_{0}(p_{\mathrm{T}}) $, is presented for small collision systems. The measurement is performed using data from pp collisions at $ \sqrt{s}= $ 13 TeV, pPb collisions at $ \sqrt{s_{\mathrm{NN}}}= $ 8.16 TeV, and OO and NeNe collisions at $ \sqrt{s_{\mathrm{NN}}}= $ 5.36 TeV, all recorded by the CMS experiment at the CERN LHC. A two-particle correlation technique with a pseudorapidity gap is used to suppress short-range nonflow contributions. After normalization by the integrated radial flow magnitude, $ v_{0} $, the shape of $ v_{0}(p_{\mathrm{T}})/v_{0} $ as a function of $ p_{\mathrm{T}}/\langle p_{\mathrm{T}}\rangle $ is found to be approximately independent of multiplicity in all intervals with $ N_{\mathrm{trk}}^{\mathrm{offline}} \geq $ 20 in pp, pPb, OO, and NeNe collisions. The lowest multiplicity interval, 10 $ \leq N_{\mathrm{trk}}^{\mathrm{offline}} < $ 20, deviates from this common shape in all four systems. The measured $ v_{0}(p_{\mathrm{T}})/v_{0} $ is compared with hydrodynamic predictions based on TRENTO+MUSIC initial conditions for pp, pPb, OO, and NeNe collisions. Qualitative agreement is observed across all four systems up to $ p_{\mathrm{T}}/\langle p_{\mathrm{T}}\rangle \approx $ 2.5. The results are also compared with predictions from nonthermal event generators, PYTHIA and HIJING, which underpredict the magnitude of the correlation observed in data.
Links: CDS record (PDF) ; CADI line (restricted) ;

Figures	Summary	References	CMS Publications

Figures
png pdf	Figure 1: The observable $ v_{0}(p_{\mathrm{T}}) $ for pp, $ \mathrm{p}\mathrm{Pb} $, OO, and NeNe collisions in different $ N_\text{trk}^\text{offline} $ intervals. The vertical bars represent the statistical uncertainties, and the boxes represent the systematic uncertainties.
png pdf	Figure 2: The observable $ v_{0}(p_{\mathrm{T}}) $ in pp collisions for different values of $ \eta_\text{gap} $ and in different $ N_\text{trk}^\text{offline} $ intervals.
png pdf	Figure 3: The observable $ v_{0}(p_{\mathrm{T}}) $ in pp collisions for three different reference $ p_{\mathrm{T}} $ ranges and in different $ N_\text{trk}^\text{offline} $ intervals.
png pdf	Figure 4: The normalized observable $ v_{0}(p_{\mathrm{T}})/v_{0} $ as a function of $ p_{\mathrm{T}}/\langle p_{\mathrm{T}}\rangle $ in different $ N_\text{trk}^\text{offline} $ intervals for pp, $ \mathrm{p}\mathrm{Pb} $, OO, and NeNe collisions. The vertical bars represent the statistical uncertainties, and the boxes represent the systematic uncertainties. The larger systematic uncertainties relative to those in Fig. 1 arise primarily from the $ v_0 $ normalization term, which has systematic uncertainties of up to 10% and 6% in pp and $ \mathrm{p}\mathrm{Pb} $ collisions, respectively.
png pdf	Figure 5: The normalized observable $ v_{0}(p_{\mathrm{T}})/v_{0} $ as a function of $ p_{\mathrm{T}}/\langle p_{\mathrm{T}}\rangle $ in the highest multiplicity interval, compared with hydrodynamic predictions based on TRENTO+MUSIC [40]. Results are shown for OO (upper left), NeNe (upper right), $ \mathrm{p}\mathrm{Pb} $ (lower left), and pp (lower right) collisions. The vertical bars and open boxes represent the statistical and systematic uncertainties, respectively. The shaded band represents the model uncertainty. Comparisons with HIJING and PYTHIA are also shown for $ \mathrm{p}\mathrm{Pb} $ and pp collisions, respectively.
png pdf	Figure 6: The correlation strength $ v_{0}(p_{\mathrm{T}})\cdot v_{0} $ in data, compared with predictions from PYTHIA for pp collisions (left) and from HIJING for $ \mathrm{p}\mathrm{Pb} $ collisions (right).

Summary

The first measurement of the $ p_{\mathrm{T}} $-dependent radial flow fluctuation observable $ v_{0}(p_{\mathrm{T}}) $, defined through the correlation between the single-particle spectrum and the event-by-event mean transverse momentum, is presented for pp collisions at $ \sqrt{s}= $ 13 TeV, $ \mathrm{p}\mathrm{Pb} $ collisions at $ \sqrt{\smash[b]{s_{_{\mathrm{NN}}}}}= $ 8.16 TeV, and OO and NeNe collisions at $ \sqrt{\smash[b]{s_{_{\mathrm{NN}}}}}= $ 5.36 TeV. A two-subevent method with a pseudorapidity gap is used to suppress short-range nonflow contributions. The observable $ v_{0}(p_{\mathrm{T}}) $ is found to be approximately independent of the pseudorapidity gap between the two subevents at low $ p_{\mathrm{T}} $, demonstrating the long-range nature of the correlations. The magnitude of $ v_{0}(p_{\mathrm{T}}) $ increases with decreasing multiplicity in all collision systems studied. After normalization by the integrated radial flow magnitude, $ v_{0} $, the shape of $ v_{0}(p_{\mathrm{T}})/v_{0} $ at low $ p_{\mathrm{T}} $ is approximately independent of multiplicity for $ N_\text{trk}^\text{offline} \geq $ 20 across all four collision systems. The lowest multiplicity interval, 10 $ \leq N_\text{trk}^\text{offline} < $ 20, deviates from this common shape in all four collision systems. This deviation may reflect an enhanced relative contribution from nonflow correlations, reduced collective dynamics, or initial-state effects not captured by the hydrodynamic framework in the lowest-multiplicity events. A hydrodynamic calculation based on TRENTO+MUSIC provides a qualitative framework in which the observed universal shape emerges from collective radial expansion in all four collision systems, pp, $ \mathrm{p}\mathrm{Pb} $, OO, and NeNe, up to $ p_{\mathrm{T}}/\langle p_{\mathrm{T}}\rangle \approx $ 3. Comparisons with the nonthermal event generators PYTHIA and HIJING show that they qualitatively underpredict the correlation strength and deviate from the universal shape in the higher multiplicity intervals. This result suggests that additional collective dynamics may contribute to the observed correlations. These measurements provide a new empirical baseline for the collective response of small and light-ion collision systems and complement existing measurements of anisotropic flow.

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