Two-particle transverse momentum correlations in pp and p-Pb collisions at LHC energies

Two-particle transverse momentum differential correlators, recently measured in Pb-Pb collisions at LHC energies, provide an additional tool to gain insights into particle production mechanisms and infer transport properties, such as the ratio of shear viscosity to entropy density, of the medium created in Pb-Pb collisions. The longitudinal long-range correlations and the large azimuthal anisotropy measured at low transverse momenta in small collision systems, namely pp and p-Pb, at LHC energies resemble manifestations of collective behaviour. This suggests that locally equilibrated matter may be produced in these small collision systems, similar to what is observed in Pb-Pb collisions. In this work, the same two-particle transverse momentum differential correlators are exploited in pp and p-Pb collisions at $\sqrt{s} = 7$ TeV and $\sqrt{s_{\rm NN}} = 5.02$ TeV, respectively, to seek evidence for viscous effects. Specifically, the strength and shape of the correlators are studied as a function of the produced particle multiplicity to identify evidence for longitudinal broadening that might reveal the presence of viscous effects in these smaller systems. The measured correlators and their evolution from pp and p-Pb to Pb-Pb collisions are additionally compared to predictions from Monte Carlo event generators, and the potential presence of viscous effects is discussed.

 

Submitted to: PRC
e-Print: arXiv:2211.08979 | PDF | inSPIRE
CERN-EP-2022-230
Figure group

Figure 1

Two-particle transverse momentum correlations $G_{2}^{\rm CD}$ (top) and $G_{2}^{\rm CI}$ (bottom) for the largest (left), medium (centre) and lowest (right) charged particle multiplicity classes in pp collisions at $\sqrt{s}=7\;\text{TeV}$. The correlator values are not shown in the intervals $|\Delta\eta|<0.1$ and $|\Delta\varphi|<0.09$, which are affected by track merging effects (see text for details).

Figure 2

Two-particle transverse momentum correlations $G_{2}^{\rm CD}$ (top) and $G_{2}^{\rm CI}$ (bottom) for the largest (left), medium (centre), and lowest (right) charged particle multiplicity classes in p--Pb collisions at $\sqrt{s_{\rm NN}}=5.02\;\text{TeV}$. The correlator values are not shown in the intervals $|\Delta\eta|<0.1$ and $|\Delta\varphi|<0.09$, which are affected by track merging effects (see text for details).

Figure 3

Longitudinal (left) and azimuthal (right) projections of the two-particle transverse momentum correlations $G_{2}^{\rm CD}$ (top) and $G_{2}^{\rm CI}$ (bottom) for selected charged particle multiplicity classes in pp collisions at $\sqrt{s}=7\;\text{TeV}$. The correlator values are not shown in the intervals $|\Delta\eta|<0.1$ and $|\Delta\varphi|<0.09$, which are affected by track merging effects (see text for details). Vertical bars (mostly smaller than the marker size) and shaded bands represent statistical and uncorrelated systematic uncertainties, respectively. Correlated systematic uncertainties are represented as small boxes at the sides of the panels.

Figure 4

Longitudinal (left) and azimuthal (right) projections of the two-particle transverse momentum correlations $G_{2}^{\rm CD}$ (top) and $G_{2}^{\rm CI}$ (bottom) for selected charged particle multiplicity classes in p--Pb collisions at $\sqrt{s_{\rm NN}}=5.02\;\text{TeV}$. The correlator values are not shown in the intervals $|\Delta\eta|<0.1$ and $|\Delta\varphi|<0.09$, which are affected by track merging effects (see text for details). Vertical bars (mostly smaller than the marker size) and shaded bands represent statistical and uncorrelated systematic uncertainties, respectively. Correlated systematic uncertainties are represented as small boxes at the sides of the panels.

Figure 5

Evolution with the average charged particle multiplicity of the longitudinal (left) and azimuthal (right) widths of the two-particle transverse momentum differential correlation $G_{2}^{\rm CD}$ (top row) and $G_{2}^{\rm CI}$ (bottom row) in pp, p--Pb, and Pb--Pb collisions at $\sqrt{s} = 7\;\text{TeV}$, $\sqrt{s_{\rm NN}} = 5.02\;\text{TeV}$, and $\sqrt{s_{\rm NN}} = 2.76\;\text{TeV}$, respectively. Vertical bars (mostly smaller than the marker size) and filled boxes represent statistical and systematic uncertainties, respectively.

Figure 6

Longitudinal (left) and azimuthal (right) projections of the two-particle transverse momentum correlation $G_{2}^{\rm CI}$ for selected charged particle multiplicity classes in pp collisions at $\sqrt{s}=7\;\text{TeV}$ after subtracting the azimuthal ZYAM base level (see text for details) for each multiplicity class. Vertical bars (mostly smaller than the marker size) and shaded bands represent statistical and uncorrelated systematic uncertainties, respectively.

Figure 7

Longitudinal projections of slices of one azimuthal bin (left) and azimuthal projections of slices of one longitudinal bin (right), for selected bins of the two-particle transverse momentum correlation $G_{2}^{\rm CI}$ and its bidimensional fit using Eq. (\ref{eq:fitfunc}) for the 0--5\% (top) and 70--80\% (bottom) charged particle multiplicity classes in pp collisions at $\sqrt{s}=7\;\text{TeV}$. Vertical bars (mostly smaller than the marker size) and shaded bands represent statistical and uncorrelated systematic uncertainties, respectively.

Figure 8

Evolution with the average charged particle multiplicity of the longitudinal (left) and azimuthal (right) widths of the two-particle transverse momentum correlations $G_{2}^{\rm CD}$ (top row) and $G_{2}^{\rm CI}$ (bottom row) in pp, p--Pb, and Pb--Pb collisions at $\sqrt{s} = 7\;\text{TeV}$, $\sqrt{s_{\rm NN}} = 5.02\;\text{TeV}$, and $\sqrt{s_{\rm NN}} = 2.76\;\text{TeV}$, respectively, compared to models. Statistical and systematic uncertainties of the data points are shown as vertical bars (mostly smaller than the marker size) and filled boxes, respectively, while the thickness of the shaded bands represents statistical uncertainties of the models. The data points and the results of HIJING simulations for Pb--Pb collisions are taken from Ref. .