CMS-PAS-HIN-15-008

CMS-PAS-HIN-15-008
Differential flow harmonic $v_n$ in pPb and PbPb collisions
CMS Collaboration
September 2015

Abstract: Previous CMS measurements have demonstrated the collective nature of multiparticle correlations in high-multiplicity pPb collisions at the LHC. This collectivity is consistent with a hydrodynamic flow origin. However, it can also be interpreted in terms of initial state effects arising from gluon saturation. The pseudorapidity dependence of the azimuthal Fourier coefficients ( $v_n$ ) is expected to be sensitive to the underlying mechanism with, in the hydrodynamic picture, the longer lifetime of the fireball on the Pb-going side expected to lead to a larger flow signal than found on the p-going side. To investigate the detailed properties of the observed collectivity, differential $v_n$ values in transverse momentum ( $p_T$ ) and pseudorapidity ( $\eta$ ) are presented over the full range of the CMS tracker detector ( $-2.4 < \eta < 2.4$ ) for pPb collisions at a nucleon-nucleon center-of mass energy of 5.02 TeV. Results based on multiparticle analyses involving four or more particles are shown. An event plane analysis is presented where the influence of recently demonstrated event-plane decorrelation is considered. Comparisons are made with peripheral PbPb collisions measured at similar mid-rapidity particle multiplicities. The results will be discussed in the context of current models of the longitudinal dependence of the multiparticle correlations.
Links: CDS record (PDF) ; CADI line (restricted) ; Figures are also available from the CDS record. These preliminary results are superseded in this paper, PRC 98 (2018) 044902. The superseded preliminary plots can be found here.

Figures
png ; pdf	Figure 1-a: $v_2$ as a function of $p_T$ in pPb (a) and PbPb (b) collisions for different ${N_\mathrm {trk}^\mathrm {offline}}$ ranges. $v_2\{2, \|\Delta \eta \|>2\}$ and $v_2\{4\}$ are extracted from Ref [26].
png ; pdf	Figure 1-b: $v_2$ as a function of $p_T$ in pPb (a) and PbPb (b) collisions for different ${N_\mathrm {trk}^\mathrm {offline}}$ ranges. $v_2\{2, \|\Delta \eta \|>2\}$ and $v_2\{4\}$ are extracted from Ref [26].
png ; pdf	Figure 2: Top: Comparison of $v_{2}(p_{\rm T})$ distributions based on $HF$ event planes located on the p-going ( $v_2^{\rm p}$ ) and Pb-going ( $v_2^{\rm Pb}$ ) sides of the tracker region, with $\eta _{\rm C} = 0$ . Bottom: Same, but with $\eta _{\rm C} = \eta _{\rm POI}$
png ; pdf	Figure 3: $v_2\{EP\}$ as a function of $\eta$ in pPb collisions for different ${N_\mathrm {trk}^\mathrm {offline}}$ ranges with (top) $\eta _{\rm C}=0$ and (bottom) $\eta _{\rm C} = \eta _{\rm POI}$ . The pseudorapidities are given in the laboratory frame. Non-flow effects are evident for points where the pseudorapidity gap between the particles of interest and the respective event plane is small.
png ; pdf	Figure 4: $v_2\{EP\}$ as a function of $\eta$ in PbPb collisions for different ${N_\mathrm {trk}^\mathrm {offline}}$ ranges with (top) $\eta _{\rm C}=0$ and (bottom) $\eta _{\rm C} = \eta _{\rm POI}$ . Non-flow effects are evident for points where the pseudorapidity gap between the particles of interest and the respective event plane is small.
png ; pdf	Figure 5-a: $v_2$ corresponding to event plane, cumulant, and LYZ methods as a function of $\eta$ in pPb (a) and PbPb (b) collisions for different ${N_\mathrm {trk}^\mathrm {offline}}$ ranges. The $v_2\{\rm EP\}$ results are based on the furthest $HF$ event plane in pseudorapidity. The pseudorapidities are given in the laboratory frame.
png ; pdf	Figure 5-b: $v_2$ corresponding to event plane, cumulant, and LYZ methods as a function of $\eta$ in pPb (a) and PbPb (b) collisions for different ${N_\mathrm {trk}^\mathrm {offline}}$ ranges. The $v_2\{\rm EP\}$ results are based on the furthest $HF$ event plane in pseudorapidity. The pseudorapidities are given in the laboratory frame.
png ; pdf	Figure 6: Role of fluctuations in the pPb and PbPb systems as a function of pseudorapidity for the indicated ${N_\mathrm {trk}^\mathrm {offline}}$ ranges.
png ; pdf	Figure 7: Comparison of event-plane ( $v_2\{\rm EP\}$ ) and cumulant ( $v_2\{4\}$ ) results for the ratio $v_{2}(\eta )/v_{2}(\eta =0)$ with the two-particle correlation results from [49] for pPb collisions at $\sqrt {s_{\rm NN}} = 5.02{\rm TeV}$ and with $220\leq N_{\rm trk}^{\rm offline}< 260$ . The [49] results are shown without the peripheral $v_{2}$ component subtraction, a correction for non-flow effects which increases the $v_{2}$ harmonics. The pseudorapidities are given in the laboratory frame.
png ; pdf	Figure 8-a: (a) Ratio p-going to Pb-going side $v_2$ coefficients at comparable $\eta _{\rm CM}$ values for pPb collisions. (b) Ratio of $+\eta$ to $-\eta$ $v_2$ values at comparable $\eta _{\rm CM}$ values for PbPb collisions.
png ; pdf	Figure 8-b: (a) Ratio p-going to Pb-going side $v_2$ coefficients at comparable $\eta _{\rm CM}$ values for pPb collisions. (b) Ratio of $+\eta$ to $-\eta$ $v_2$ values at comparable $\eta _{\rm CM}$ values for PbPb collisions.
png ; pdf	Figure 9: Triangular flow (n=3) harmonic from the event-plane method for pPb collisions at $\sqrt {s_{\rm NN}} =$ 5.02 TeV with (top) $\eta _{\rm C} = 0$ and (bottom) $\eta _{\rm C} = \eta _{\rm POI}$ . The legends indicate the direction of the respective HF event planes. Non-flow effects are evident for points where the pseudorapidity gap between the particles of interest and the respective event plane is small.
png ; pdf	Figure 10: Triangular flow (n=3) harmonic from the event-plane method for PbPb collisions at $\sqrt {s_{\rm NN}} =$ 2.76 TeV with (top) $\eta _{\rm C} = 0$ and (bottom) $\eta _{\rm C} = \eta _{\rm POI}$ . The legends indicate the direction of the respective HF event planes. Non-flow effects are evident for points where the pseudorapidity gap between the particles of interest and the respective event plane is small.
png ; pdf	Figure 11: Elliptic (n=2) and triangular flow (n=3) harmonics for pPb collisions at $\sqrt {s_{\rm NN}} =$ 5.02 TeV and PbPb at $\sqrt {s_{\rm NN}} =$ 2.76 TeV with $\eta _{C}=\eta _{POI}$ . The $v_2\{\rm EP\}$ results are based on the furthest $HF$ event plane in pseudorapidity.

Tables
png ; pdf	Table 1: Systematic uncertainty

Compact Muon Solenoid
LHC, CERN