CMS-HIN-14-008

CMS-HIN-14-008 ; CERN-EP-2016-042
Pseudorapidity dependence of long-range two-particle correlations in pPb collisions at $\sqrt{\smash[b]{s_{_{\mathrm{NN}}}}} =$ 5.02 TeV
CMS Collaboration
18 April 2016
Phys. Rev. C 96 (2017) 014915
Abstract: Two-particle correlations in pPb collisions at a nucleon-nucleon center-of-mass energy of 5.02 TeV are studied as a function of the pseudorapidity separation ( $\Delta \eta$ ) of the particle pair at small relative azimuthal angle ( $\| \Delta \phi \| < \pi/3$ ). The correlations are decomposed into a jet component that dominates the short-range correlations ( $\| \Delta \eta \| <$ 1), and a component that persists at large $\Delta \eta$ and may originate from collective behavior of the produced system. The events are classified in terms of the multiplicity of the produced particles. Finite azimuthal anisotropies are observed in high-multiplicity events. The second and third Fourier components of the particle-pair azimuthal correlations, $V_2$ and $V_3$ , are extracted after subtraction of the jet component. The single-particle anisotropy parameters $v_2$ and $v_3$ are normalized by their lab frame mid-rapidity value and are studied as a function of $\eta_{\text{cm}}$ . The normalized $v_2$ distribution is found to be asymmetric about $\eta_{\text{cm}} =$ 0, with smaller values observed at forward pseudorapidity, corresponding to the direction of the proton beam, while no significant pseudorapidity dependence is observed for the normalized $v_3$ distribution within the statistical uncertainties.
Links: e-print arXiv:1604.05347 [nucl-ex] (PDF) ; CDS record ; inSPIRE record ; CADI line (restricted) ;

Figures & Tables	Summary	References	CMS Publications

Figures
png pdf	Figure 1: Efficiency-corrected 2D associated yields with Pb-side trigger particle ( $-2.4 < {\eta _\text {lab}^{\text {trig}}} < -2.0$ , left panels) and p-side trigger particle (2.0 $< {\eta _\text {lab}^{\text {trig}}} <$ 2.4, right panels) in low-multiplicity (2 $\leq {N_\text {trk}^\text {offline}} <$ 20, upper panels) and high-multiplicity (220 $\leq {N_\text {trk}^\text {offline}} <$ 260, lower panels) are shown for pPb collisions at ${\sqrt {\smash [b]{s_{_{NN}}}}} =$ 5.02 TeV. The associated and trigger particle ${p_{\mathrm {T}}}$ ranges are both 0.3 $< {p_{\mathrm {T}}} <$ 3 GeV/ $c$ .
png pdf	Figure 1-a: Efficiency-corrected 2D associated yields with Pb-side trigger particle ( $-2.4 < {\eta _\text {lab}^{\text {trig}}} < -2.0$ ) in low-multiplicity (2 $\leq {N_\text {trk}^\text {offline}} <$ 20) and are shown for pPb collisions at ${\sqrt {\smash [b]{s_{_{NN}}}}} =$ 5.02 TeV. The associated and trigger particle ${p_{\mathrm {T}}}$ ranges are both 0.3 $< {p_{\mathrm {T}}} <$ 3 GeV/ $c$ .
png pdf	Figure 1-b: Efficiency-corrected 2D associated yields with p-side trigger particle (2.0 $< {\eta _\text {lab}^{\text {trig}}} <$ 2.4) in low-multiplicity (2 $\leq {N_\text {trk}^\text {offline}} <$ 20) and are shown for pPb collisions at ${\sqrt {\smash [b]{s_{_{NN}}}}} =$ 5.02 TeV. The associated and trigger particle ${p_{\mathrm {T}}}$ ranges are both 0.3 $< {p_{\mathrm {T}}} <$ 3 GeV/ $c$ .
png pdf	Figure 1-c: Efficiency-corrected 2D associated yields with Pb-side trigger particle ( $-2.4 < {\eta _\text {lab}^{\text {trig}}} < -2.0$ ) in high-multiplicity (220 $\leq {N_\text {trk}^\text {offline}} <$ 260) are shown for pPb collisions at ${\sqrt {\smash [b]{s_{_{NN}}}}} =$ 5.02 TeV. The associated and trigger particle ${p_{\mathrm {T}}}$ ranges are both 0.3 $< {p_{\mathrm {T}}} <$ 3 GeV/ $c$ .
png pdf	Figure 1-d: Efficiency-corrected 2D associated yields with p-side trigger particle (2.0 $< {\eta _\text {lab}^{\text {trig}}} <$ 2.4) in high-multiplicity (220 $\leq {N_\text {trk}^\text {offline}} <$ 260) are shown for pPb collisions at ${\sqrt {\smash [b]{s_{_{NN}}}}} =$ 5.02 TeV. The associated and trigger particle ${p_{\mathrm {T}}}$ ranges are both 0.3 $< {p_{\mathrm {T}}} <$ 3 GeV/ $c$ .
png pdf	Figure 2: Examples of the distribution of the associated yields after ZYAM subtraction for both low-multiplicity (2 $\leq {N_\text {trk}^\text {offline}} <$ 20, blue triangles) and high-multiplicity (220 $\leq {N_\text {trk}^\text {offline}} <$ 260, red circles) are shown for pPb collisions at ${\sqrt {\smash [b]{s_{_{NN}}}}} =$ 5.02 TeV. The results for Pb-side (left panels) and p-side (right panels) trigger particles are both shown; small ${\Delta \eta}$ in the upper panels and large ${\| {\Delta \eta} \|}$ in the lower panels. The trigger and associated particle ${p_{\mathrm {T}}}$ ranges are both 0.3 $< {p_{\mathrm {T}}} <$ 3 GeV/ $c$ .
png pdf	Figure 3: The near-side ( $\| {\Delta \phi} \| < \pi /3$ ) correlated yield after ZYAM subtraction in low-multiplicity 2 $\leq {N_\text {trk}^\text {offline}} <$ 20 (upper panels) and high-multiplicity 220 $\leq {N_\text {trk}^\text {offline}} <$ 260 (lower panels) are shown for pPb collisions at ${\sqrt {\smash [b]{s_{_{NN}}}}} =$ 5.02 TeV. The trigger and associated particle ${p_{\mathrm {T}}}$ ranges are both 0.3 $< {p_{\mathrm {T}}} <$ 3 GeV/ $c$ . The trigger particles are restricted to the Pb-side ( $-2.4 < {\eta _\text {lab}^{\text {trig}}} < -2.0$ , left panels) and the p-side (2.0 $< {\eta _\text {lab}^{\text {trig}}} <$ 2.4, right panels), respectively. Fit results using Eq.(1) (black solid curves) are superimposed; the red dashed curve and the blue open points are the two fit components, jet and ridge, respectively.
png pdf	Figure 4: Fourier coefficients, $V_2$ (upper) and $V_3$ (lower), of two-particle azimuthal correlations in high-multiplicity collisions (220 $\leq {N_\text {trk}^\text {offline}} <$ 260) with (circles) and without (triangles) subtraction of low-multiplicity data, as a function of ${\eta _{\text {lab}}}$ . Left panel shows data for Pb-side trigger particles and the right panel for the p-side. Statistical uncertainties are mostly smaller than point size; systematic uncertainties are 3.9% and 10% for $V_2$ and $V_3$ without low-multiplicity subtraction, 5.8% and 15% for $V_2$ and $V_3$ with low-multiplicity subtraction, respectively. The systematic uncertainties are shown by the shaded bands.
png pdf	Figure 5: Self-normalized anisotropy parameters, $v_2({\eta _{\text {lab}}})/v_2({\eta _{\text {lab}}} =0)$ (left panel) and $v_3({\eta _{\text {lab}}})/v_3({\eta _{\text {lab}}} =0)$ (right panel), as a function of ${\eta _{\text {lab}}}$ . Data points (curves) are results with (without) low-multiplicity data subtraction; filled circles and solid lines are from the Pb-side trigger. Open circles and dashed lines are from the p-side trigger. The bands show systematic uncertainties of $\pm$ 5.7% and $\pm$ 14% for $v_2({\eta _{\text {lab}}})/v_2({\eta _{\text {lab}}} =0)$ and $v_3({\eta _{\text {lab}}})/v_3({\eta _{\text {lab}}} =0)$ , respectively. The systematic uncertainties in $v_n({\eta _{\text {lab}}})/v_n({\eta _{\text {lab}}} =0)$ without subtraction are similar. Error bars indicate statistical uncertainties only.
png pdf	Figure 5-a: Self-normalized anisotropy parameter $v_2({\eta _{\text {lab}}})/v_2({\eta _{\text {lab}}} =0)$ as a function of ${\eta _{\text {lab}}}$ . Data points (curves) are results with (without) low-multiplicity data subtraction; filled circles and solid lines are from the Pb-side trigger. Open circles and dashed lines are from the p-side trigger. The bands show systematic uncertainties of $\pm$ 5.7% for $v_2({\eta _{\text {lab}}})/v_2({\eta _{\text {lab}}} =0)$ . Error bars indicate statistical uncertainties only.
png pdf	Figure 5-b: Self-normalized anisotropy parameter $v_3({\eta _{\text {lab}}})/v_3({\eta _{\text {lab}}} =0)$ as a function of ${\eta _{\text {lab}}}$ . Data points (curves) are results with (without) low-multiplicity data subtraction; filled circles and solid lines are from the Pb-side trigger. Open circles and dashed lines are from the p-side trigger. The bands show systematic uncertainties of $\pm$ 14% for $v_3({\eta _{\text {lab}}})/v_3({\eta _{\text {lab}}} =0)$ . Error bars indicate statistical uncertainties only.
png pdf	Figure 6: $v_2({\eta _{\text {c.m.}}})/v_2(- {\eta _{\text {c.m.}}})$ , as a function of ${\eta _{\text {c.m.}}}$ in the center-of-mass frame. The data points are results from $V_n^\text {sub}$ with low-multiplicity data subtracted. The bands show the systematic uncertainty of $\pm$ 5.7%. Error bars indicate statistical uncertainties only.
png pdf	Figure 7: Self-normalized $v_2({\eta _{\text {c.m.}}})/v_2({\eta _{\text {c.m.}}} =-0.465)$ distribution with low-multiplicity subtraction from Pb-side (filled circles) and p-side (open circles) triggers, and $< {p_{\mathrm {T}}} > ({\eta _{\text {c.m.}}})/< {p_{\mathrm {T}}} > ({\eta _{\text {c.m.}}} = - 0.465)$ of 0 $< {p_{\mathrm {T}}} <$ 6 GeV/ $c$ range from minimum-bias events (solid line) and 0.3 $< {p_{\mathrm {T}}} <$ 3 GeV/ $c$ range from high-multiplicity (220 $\leq {N_\text {trk}^\text {offline}} <$ 260) events (dotted line) as functions of ${\eta _{\text {c.m.}}}$ . Dashed curve is the hydrodynamic prediction for $< {p_{\mathrm {T}}} > ({\eta _{\text {c.m.}}})/< {p_{\mathrm {T}}} > ({\eta _{\text {c.m.}}} = -0.465)$ distribution.

Tables
png pdf	Table 1: Summary of fit parameters for low- and high- ${N_\text {trk}^\text {offline}}$ ranges in pPb collisions.
png pdf	Table 2: Summary of systematic uncertainties in the second and third Fourier harmonics in pPb collisions. The label "low-mult sub'' indicates the low-multiplicity subtracted results, while "no sub" indicates the results without subtraction.

Summary

Two-particle correlations as functions of

$\Delta \phi$ and

$\Delta \eta$ are reported in pPb\ collisions at

$\sqrt{\smash[b]{s_{_{\mathrm{NN}}}}} = 5.02$ TeV by the CMS experiment. The trigger particle is restricted to narrow pseudorapidity windows. The combinatorial background is assumed to be uniform in

$\Delta \phi$ and normalized by the ZYAM procedure as a function of

$\Delta \eta$ . The near-side jet correlated yield is fitted and found to be greater in high-multiplicity than in low-multiplicity collisions. The ridge yield is studied as a function of

$\Delta \phi$ and

$\Delta \eta$ and it is found to depend on pseudorapidity and the underlying background shape ZYAM(

$\Delta \eta$ ). The pseudorapidity dependence differs for trigger particles selected on the proton and the Pb sides.

The Fourier coefficients of the two-particle correlations in high-multiplicity collisions are reported, with and without subtraction of the scaled low-multiplicity data. The pseudorapidity dependence of the single-particle anisotropy parameters,

$v_2$ and

$v_3$ , is inferred. Significant pseudorapidity dependence of

$v_2$ is found. The distribution is asymmetric about

$\eta_{\text{cm}} =$ 0 with an approximate (20

$\pm$ 4 )% decrease from

$\eta_{\text{cm}} =$ 0 to

$\eta_{\text{cm}}\approx$ 1.5, and a smaller decrease towards the Pb-beam direction. Finite

$v_3$ is observed, but the uncertainties are presently too large to draw conclusions regarding the pseudorapidity dependence.

The self-normalized

$v_2(\eta_{\text{cm}})/v_2(\eta_{\text{cm}}=-0.465)$ distribution is compared to the

$< {p_{\mathrm{T}}} > (\eta_{\text{cm}})/ < {p_{\mathrm{T}}} > (\eta_{\text{cm}}= -0.465)$ distribution as well as from hydrodynamic calculations. The

$< {p_{\mathrm{T}}} > (\eta_{\text{cm}})/ < {p_{\mathrm{T}}} > (\eta_{\text{cm}}= -0.465)$ distribution shows a decreasing trend towards positive

$\eta_{\text{cm}}$ . The

$v_2(\eta_{\text{cm}})/v_2(\eta_{\text{cm}} = -0.465)$ distribution also shows a decreasing trend towards positive

$\eta_{\text{cm}}$ , but the decrease is more significant in the case of the

$v_2$ measurement. This indicates that physics mechanisms other than the change in the underlying particle spectra, such as event plane decorrelation over pseudorapidity, may influence the anisotropic flow.

References
1	STAR Collaboration	Experimental and theoretical challenges in the search for the quark gluon plasma: The STAR Collaboration's critical assessment of the evidence from RHIC collisions	NP A 757 (2005) 102	nucl-ex/0501009
2	PHENIX Collaboration	Formation of dense partonic matter in relativistic nucleus-nucleus collisions at RHIC: Experimental evaluation by the PHENIX collaboration	NP A 757 (2005) 184	nucl-ex/0410003
3	PHOBOS Collaboration	The PHOBOS perspective on discoveries at RHIC	NP 757 (2005) 28	nucl-ex/0410022
4	STAR Collaboration	Distributions of charged hadrons associated with high transverse momentum particles in pp and Au+Au collisions at $\sqrt{s_{NN}} =$ 200 GeV	PRL 95 (2005) 152301	nucl-ex/0501016
5	PHOBOS Collaboration	High transverse momentum triggered correlations over a large pseudorapidity acceptance in Au+Au collisions at $\sqrt{s_{NN}}=$ 200 GeV	PRL 104 (2010) 062301	0903.2811
6	STAR Collaboration	Long range rapidity correlations and jet production in high energy nuclear collisions	PRC 80 (2009) 064912	0909.0191
7	STAR Collaboration	Three-particle coincidence of the long range pseudorapidity correlation in high energy nucleus-nucleus collisions	PRL 105 (2010) 022301	0912.3977
8	CMS Collaboration	Centrality dependence of dihadron correlations and azimuthal anisotropy harmonics in PbPb collisions at $\sqrt{s_{NN}}=$ 2.76 TeV	EPJC 72 (2012) 2012	CMS-HIN-11-006 1201.3158
9	CMS Collaboration	Observation of long-range near-side angular correlations in proton-proton collisions at the LHC	JHEP 09 (2010) 091	CMS-QCD-10-002 1009.4122
10	CMS Collaboration	Observation of long-range near-side angular correlations in proton-lead collisions at the LHC	PLB 718 (2013) 795	CMS-HIN-12-015 1210.5482
11	ALICE Collaboration	Long-range angular correlations on the near and away side in $p$ -Pb collisions at $\sqrt{s_{NN}} =$ 5.02 TeV	PLB 719 (2013) 29	1212.2001
12	ATLAS Collaboration	Observation of Associated Near-side and Away-side Long-range Correlations in $\sqrt{s_{NN}} =$ 5.02 TeV Proton-lead Collisions with the ATLAS Detector	PRL 110 (2013) 182302	1212.5198
13	PHENIX Collaboration	Measurement of long-range angular correlation and quadrupole anisotropy of pions and (anti)protons in central d+Au collisions at $\sqrt{s_{NN}} =$ 200 GeV	PRL 114 (2015) 192301
14	STAR Collaboration	Long-range pseudorapidity dihadron correlations in d+Au collisions at $\sqrt{s_{NN}}=$ 200 GeV	PLB 747 (2015) 265
15	PHENIX Collaboration	Measurements of elliptic and triangular flow in high-multiplicity $^{3}$ He $+$ Au collisions at $\sqrt{s_{_{NN}}}=$ 200 GeV	PRL 115 (2015), no. 14, 142301	1507.06273
16	J.-Y. Ollitrault	Anisotropy as a signature of transverse collective flow	PRD 46 (1992) 229
17	R. Andrade et al.	Examining the necessity to include event-by-event fluctuations in experimental evaluations of elliptical flow	PRL 97 (2006) 202302	nucl-th/0608067
18	B. Alver and G. Roland	Collision geometry fluctuations and triangular flow in heavy-ion collisions	PRC 81 (2010) 054905	1003.0194
19	C. Gale, S. Jeon, and B. Schenke	Hydrodynamic modeling of heavy-ion collisions	Int. J. Mod. Phys. A 28 (2013) 1340011	1301.5893
20	U. Heinz and R. Snellings	Collective flow and viscosity in relativistic heavy-ion collisions	Ann. Rev. Nucl. Part. Sci. 63 (2013) 123	1301.2826
21	L. He et al.	Anisotropic parton escape is the dominant source of azimuthal anisotropy in transport models	PLB 753 (2016) 506	1502.05572
22	A. Dumitru et al.	The Ridge in proton-proton collisions at the LHC	PLB 697 (2011) 21	1009.5295
23	K. Dusling and R. Venugopalan	Comparison of the color glass condensate to di-hadron correlations in proton-proton and proton-nucleus collisions	PRD 87 (2013) 094034	1302.7018
24	S. Gavin, L. McLerran, and G. Moschelli	Long Range Correlations and the Soft Ridge in Relativistic Nuclear Collisions	PRC 79 (2009) 051902	0806.4718
25	K. Dusling and R. Venugopalan	Azimuthal collimation of long range rapidity correlations by strong color fields in high multiplicity hadron-hadron collisions	PRL 108 (2012) 262001	1201.2658
26	CMS Collaboration	Measurement of long-range near-side two-particle angular correlations in $pp$ collisions at $\sqrt{s}=13\text{}$ TeV	PRL 116 (Apr, 2016) 172302
27	ATLAS Collaboration	Observation of long-range elliptic azimuthal anisotropies in $\sqrt{s}=$ 13 and 2.76 $TeV pp$ collisions with the ATLAS detector	PRL 116 (2015) 172301	1509.04776
28	ALICE Collaboration	Forward-central two-particle correlations in p-pb collisions at $\sqrt{s_{NN}} =$ 5.02 TeV	Physics Letters B 753 (2016) 126
29	CMS Collaboration	Multiplicity and transverse momentum dependence of two- and four-particle correlations in pPb and PbPb collisions	PLB 724 (2013) 213	CMS-HIN-13-002 1305.0609
30	ATLAS Collaboration	Measurement with the ATLAS detector of multi-particle azimuthal correlations in p+Pb collisions at $\sqrt{s_{NN}} =$ 5.02 TeV	PLB 725 (2013) 60
31	CMS Collaboration	Evidence for collective multi-particle correlations in pPb collisions	PRL 115 (2015) 012301
32	P. Bo\.ek	Collective flow in $p$ -pb and $d$ -pb collisions at tev energies	PRC 85 (2012) 014911
33	CMS Collaboration	Long-range two-particle correlations of strange hadrons with charged particles in pPb and PbPb collisions at LHC energies	PLB 742 (2015) 200
34	ALICE Collaboration	Long-range angular correlations of $\pi$ , k, and p in p-pb collisions at $\sqrt{s_{NN}} =$ 5.02 tev	PLB 726 (2013) 164
35	P. Bozek	Elliptic flow in proton-proton collisions at $\sqrt{s_{\rm NN}} =$ 7 TeV	EPJC 71 (2011) 1530	1010.0405
36	P. Bozek and W. Broniowski	Correlations from hydrodynamic flow in p-Pb collisions	PLB 718 (2013) 1557	1211.0845
37	P. Bozek, A. Bzdak, and V. Skokov	The rapidity dependence of the average transverse momentum in p+Pb collisions at the LHC: the Color Glass Condensate versus hydrodynamics	PLB 728 (2014) 662
38	F. O. Dur\ aes, A. V. Giannini, V. P. Goncalves, and F. S. Navarra	Rapidity dependence of the average transverse momentum in hadronic collisions	PRC 94 (2016) 024917	1510.04737
39	T. Pierog et al.	EPOS LHC: Test of collective hadronization with data measured at the CERN Large Hadron Collider	PRC 92 (2015) 034906	1306.0121
40	P. Bozek, A. Bzdak, and G. Ma	Rapidity dependence of elliptic and triangular flow in proton-nucleus collisions from collective dynamics	PLB 748 (2015) 301
41	P. Bo\.zek, W. Broniowski, and J. Moreira	Torqued fireballs in relativistic heavy-ion collisions	PRC 83 (2011) 034911
42	K. Xiao, F. Liu, and F. Wang	Event-plane decorrelation over pseudorapidity and its effect on azimuthal anisotropy measurements in relativistic heavy-ion collisions	PRC 87 (2013) 011901
43	L.-G. Pang et al.	Longitudinal decorrelation of anisotropic flows in heavy-ion collisions at the CERN Large Hadron Collider	PRC 91 (2015) 044904
44	CMS Collaboration	Evidence for transverse-momentum- and pseudorapidity- dependent event-plane fluctuations in PbPb and $p$ Pb collisions	PRC 92 (2015) 034911	nucl-ex/1503.01692
45	CMS Collaboration	Long-range and short-range dihadron angular correlations in central PbPb collisions at a nucleon-nucleon center of mass energy of 2.76 TeV	JHEP 07 (2011) 076	CMS-HIN-11-001 1105.2438
46	L. Xu, C.-H. Chen, and F. Wang	Event mixing does not reproduce single particle acceptance convolutions for nonuniform pseudorapidity distributions	PRC 88 (2013) 064907	1304.8120
47	ALICE Collaboration	Harmonic decomposition of two-particle angular correlations in Pb-Pb collisions at $\sqrt{s_{_{NN}}} =$ 2.76 TeV	PLB 708 (2012) 249	1109.2501
48	CMS Collaboration	The CMS experiment at the CERN LHC	JINST 3 (2008) S08004	CMS-00-001
49	Geant4 Collaboration	GEANT4 --- a simulation toolkit	NIMA 506 (2003) 250
50	CMS Collaboration	Luminosity calibration for the 2013 proton-lead and proton-proton data taking	CMS-PAS-LUM-13-002	CMS-PAS-LUM-13-002
51	CMS Collaboration	Tracking and vertexing results from first collisions	CDS
52	M. Gyulassy and X.-N. Wang	HIJING 1.0: A Monte Carlo program for parton and particle production in high-energy hadronic and nuclear collisions	CPC 83 (1994) 307	nucl-th/9502021
53	S. Porteboeuf, T. Pierog, and K. Werner	Producing Hard Processes Regarding the Complete Event: The EPOS Event Generator		1006.2967
54	S. M. T. Sjostrand and P. Skands	PYTHIA 6.4 physics and manual	JHEP 05 (2006) 025
55	N. N. Ajitanand et al.	Decomposition of harmonic and jet contributions to particle-pair correlations at ultra-relativistic energies	PRC 72 (2005) 011902	nucl-ex/0501025
56	J. Jia and S. Mohapatra	Disentangling flow and nonflow correlations via bayesian unfolding of the event-by-event distributions of harmonic coefficients in ultrarelativistic heavy-ion collisions	PRC 88 (2013) 014907
57	CMS Collaboration	Nuclear effects on the transverse momentum spectra of charged particles in pPb collisions at $\sqrt{s_\mathrm{NN}}=$ 5.02 TeV	EPJC 75 (2015) 237
58	CMS Collaboration	Measurement of the elliptic anisotropy of charged particles produced in PbPb collisions at $\sqrt{s_{NN}} =$ 2.76 TeV	PRC 87 (2013) 014902