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| CMS-PAS-HIN-21-012 | ||
| Correlations between multiparticle cumulants and mean transverse momentum in small collision systems with the CMS detector | ||
| CMS Collaboration | ||
| April 2022 | ||
| Abstract: Correlations between multiparticle cumulants and mean transverse momentum in proton-proton (pp), proton-lead (pPb) and peripheral lead-lead (PbPb) collisions are presented as a function of charged-particle multiplicity. The data, corresponding to integrated luminosities of 28.6 pb$^{-1}$ for pp at $\sqrt{s}= $ 13 TeV, 186 nb$^{-1}$ for pPb at $\sqrt{s_{_{\mathrm{NN}}}}= $ 8.16 TeV, and 0.58 nb$^{-1}$ for PbPb at $\sqrt{s_{_{\mathrm{NN}}}}= $ 5.02 TeV, were collected using the CMS detector at the LHC. Modified Pearson correlation coefficients of charged particles are measured to characterize these events. The two- and four-particle cumulants for the second- and third-order Fourier harmonics are correlated with mean transverse momentum on an event-by-event basis. Sign changes are observed as a function of multiplicity when using two-particle cumulants in pp and pPb systems. The sign changes disappear as pseudorapidity gaps between the two subevents increase. Predictions based on color-glass condensate and hydrodynamic models are compared to the experimental results. | ||
| Links: CDS record (PDF) ; CADI line (restricted) ; | ||
| Figures | |
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Figure 1:
The covariances of cumulants from two- and four- particle correlations and ${[p_{\mathrm {T}}]}$ as a function of ${N_\text {ch}}$ in 13 TeV pp (left), 8.16 TeV pPb (middle), and 5.02 TeV PbPb (right). The top (bottom) panels are for harmonic $n=$ 2 ($n=$ 3). The error bars correspond to statistical uncertainties, while the shaded areas denote the systematic uncertainties. |
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Figure 2:
The correlator using two-particle cumulant from $|\eta | > $ 0.75 and $|\eta | > $ 1.0 as a function of ${N_\text {ch}}$ in 13 TeV pp (left), 8.16 TeV pPb (middle), and 5.02 TeV PbPb (right). The top (bottom) panels are for harmonic $n=$ 2 ($n=$ 3). The error bars correspond to statistical uncertainties, while the shaded areas denote the systematic uncertainties. Calculations from PYTHIA8 (left red and black lines), IP-Glasma+MUSIC+UrQMD (middle blue lines) [40], and hydrodynamic (middle red lines) [61] models are compared to the data. The magenta lines represent the ATLAS results [58] in pPb and PbPb collisions at 5.02 TeV using $|\eta | > $ 0.75 with the error bars denoting the statistical and systematic uncertainties added in quadrature. |
| Tables | |
png pdf |
Table 1:
Average multiplicity of reconstructed tracks per ${N_\mathrm {ch}^\mathrm {rec}}$ bin for ${N_\text {ch}}$ and ${N_\mathrm {trk}^\mathrm {offline}}$ in pp, pPb, and PbPb collisions. Uncertainties for the tracking efficiency corrected ${N_\text {ch}}$ are included. |
| Summary |
| In summary, correlations between mean transverse momentum and multiparticle cumulants from two- and four-particle correlations for Fourier harmonics $n=$ 2 and $n=$ 3 are presented in proton-proton (pp) collisions at $\sqrt{s}=$ 13 TeV, proton-lead (pPb) collisions at ${\sqrt {\smash [b]{s_{_{\mathrm {NN}}}}}}=$ 8.16 TeV and peripheral lead-lead (PbPb) collisions at ${\sqrt {\smash [b]{s_{_{\mathrm {NN}}}}}}=$ 5.02 TeV. Apparent sign changes in the modified Pearson correlators (Eq. xxxxx) are observed as a function of charged-particle multiplicity when using two-particle cumulants with $|\eta|>$ 0.75, making the minimum $\eta$ gap of 1.5, in pp and pPb systems. The sign changes disappear when the nonflow is suppressed using $|\eta|>$ 1.0, making the minimum $\eta$ gap of 2.0. Nonflow studies using the color-glass condensate model are needed to shed more light on the origin of azimuthal anisotropy in small systems. To further reduce nonflow contribution, four-particle cumulants $c_2\{4\}$ are correlated with mean $p_{\mathrm{T}}$ which shows no sign change, similar to the two-particle correlation results with a larger $\eta$ gap. The correlations between $c_3\{2\}$ and mean $p_{\mathrm{T}}$ are found to be negative in all the three collision systems. Comparisons with hydrodynamic predictions in pPb show that the data is better described by a smaller initial fireball with average transverse radius of 0.9 fm instead of 1.5 fm. These high-precision data and the observables employing multiparticle correlators shown here provide new insight into the origin of azimuthal anisotropy in small collision systems. |
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