Search for "Ridge" in d+Au Collisions at RHIC by STAR
NNuclear Physics A 00 (2018) 1–4
NuclearPhysics A
Search for “Ridge” in d + Au Collisions at RHIC by STAR
Li Yi (for the STAR Collaboration) Department of Physics and Astronomy, Purdue University, West Lafayette, IN, 47907, USA
Abstract
Dihadron correlations are measured in d + Au collisions at 200 GeV by the STAR detector. The correlated yields with uniformbackground subtraction are studied in high- and low-multiplicity collisions. The e ff ects of multiplicity selection bias on jet-likecorrelations are discussed. Finite correlated yields are observed on the near-side at large pseudo-rapidity separation in high-multiplicity collisions. Keywords: d + Au collisions, jet correlation, long-range correlation, ridge
1. Introduction
Long-range pseudo-rapidity separation ( ∆ η ) correlations at small azimuthal di ff erence ( ∆ φ ), called the ridge, havebeen observed in high-multiplicity p + p and p + Pb collisions at the LHC [1, 2, 3, 4]. A subtraction of the dihadroncorrelation in low-multiplicity p + Pb collisions from high-multiplicity ones reveals a back-to-back double-ridge struc-ture (at ∆ φ ≈ π ) [3, 4]. A similar double-ridge is also observed by PHENIX in d + Au collisions using the samesubtraction method [5]. Di ff erences between multiplicity-selected d + Au collisions (and p + p collisions) have beenobserved before by STAR [6]. Recent ALICE results show a mass ordering of the proton and pion elliptic anisotropyparameters, v , characterizing the double-ridge correlations in p + Pb collisions [7]. A similar mass ordering is ob-served by PHENIX in d + Au collisions [8]. The hydrodynamic models, where collective flow develops in p / d + Acollisions [9, 10, 11], and the Color Glass Condensate model, where two-gluon production is enhanced, provide twopossible explanations to the ridge in small systems [12]. Measurements from the large acceptance STAR detectorshould shed light on the long-range correlation in d + Au collisions at RHIC.
2. Event and track selections
The data used in this analysis are composed of 4 million d + Au events at nucleon-nucleon center-of-mass energyof √ s NN =
200 GeV, collected by the STAR detector in 2003. Three main detectors used in this analysis are the TimeProjection Chamber (TPC), the Forward Time Projection Chamber (FTPC), and the Zero Degree Calorimeter (ZDC).The minimum bias events were triggered by the ZDC in the Au beam direction (ZDC-Au). The events are requiredto have the reconstructed primary vertex within 50 cm of the TPC center along the beam direction. The chargedtracks reconstructed in the TPC and FTPC are required to satisfy the following conditions: the distance of the closestapproach to the primary vertex less than 3 cm to remove secondary tracks from particle decays; the number of fit A list of members of the STAR Collaboration and acknowledgements can be found at the end of this issue. a r X i v : . [ nu c l - e x ] O c t . Yi / Nuclear Physics A 00 (2018) 1–4 points greater than 25 (5) in the TPC (FTPC) for good track reconstruction, and larger than 0.51 times the maximumnumber of possible fit points to avoid split tracks. The track pseudo-rapidity cuts are | η | < . < | η | < .
8) in theTPC (FTPC) [13].
3. Data analysis
Two sets of dihadron correlations are analyzed in this study: TPC-TPC correlations with both the trigger andassociated particles from the TPC ( − < ∆ η < < | ∆ η | < . < p T < / c . The ∆ η - ∆ φ dihadron correlation ( ∆ η = η assoc − η trig and ∆ φ = φ assoc − φ trig ) is given by1 N trig d Nd ∆ η d ∆ φ = N trig S ( ∆ η, ∆ φ ) /(cid:15) assoc B ( ∆ η, ∆ φ ) / (cid:104) B ( ∆ η | , ∆ φ ) (cid:105) . (1)Here S = N trig d N same d ∆ η d ∆ φ is the raw dihadron correlation for pairs in the same event; and B = N trig d N mix d ∆ η d ∆ φ is for triggerand associated particles from di ff erent (mixed) events. The mixed event background serves as the correction for thedetector two-particle acceptance; (cid:104) B (cid:105) is the average B at fixed ∆ η | , where the two-particle acceptance is 100%; | ∆ η | = .
3) for TPC-TPC (TPC-FTPC) correlations. The mixed events are required to have primary verticeswithin 1 cm of each other in the beam direction to resemble similar acceptance, and have similar event characteristics.The yields are corrected for the associated particle tracking e ffi ciencies, (cid:15) assoc =
85% for TPC tracks and 70%for FTPC tracks. The underlying event background is further subtracted by ∆ η -dependent Zero-Yield-At-Minimum(ZYAM) method [14].The systematic uncertainties are estimated by varying the width of the ZYAM normalization ∆ η range from 0.4 to0.2 and 0.6 radian. An additional 5% tracking systematic uncertainty is applied.
4. Multiplicity selection bias
High multiplicity is required for event selection in order to observed the ridge. In this analysis, we use the rawcharged particle multiplicity in − . < η < − . η range used by PHENIX. We also use the neutral particle energy deposited in ZDC-Au. The correlationbetween the FTPC-Au multiplicity and the ZDC-Au neutral energy is positive but broad, thus these two measurementsselect significantly di ff erent event samples. In this contribution, the FTPC multiplicity selection is used for the TPC-TPC correlations, while ZDC energy selection for the TPC-FTPC correlations. ZYAM=0.1578(4)ZYAM=0.3546(6) fD d hD N / d ) d t r i g ( / N fD |<0.3 hD STAR Preliminary (a)
ZYAM=0.1471(6) fD d hD N / d ) d t r i g ( / N fD ZYAM=0.3514(8) |<0.7 hD STAR Preliminary |<0.7 hD (b) ZYAM=0.1324(7)ZYAM=0.3468(10) fD d hD N / d ) d t r i g ( / N fD |<1.8 hD STAR Preliminary (c)
Figure 1. Dihadron ∆ φ correlations for (a) 0 < | ∆ η | < .
3, (b) 0 . < | ∆ η | < . . < | ∆ η | < . + Au collisions at √ s NN =
200 GeVfor charged particles of 1 < p T < / c . Both the trigger and associated particles are from the TPC. FTPC-Au multiplicity is used for eventselection. The red open circles represent the high-multiplicity (0-20%) collisions. The blue solid dots represent the low-multiplicity (40-100%)collisions. The ZYAM backgrounds are listed on the plot. Fig. 1 shows the TPC-TPC ∆ φ correlations at three | ∆ η | ranges for high (0-20%) and low (40-100%) FTPC-Au multiplicity collisions. The near-side correlated yield in high-multiplicity collisions is larger than that in low-multiplicity ones. At large | ∆ η | , the high-multiplicity data have an excess correlated yield on the near-side ( | ∆ φ | ≈ . Yi / Nuclear Physics A 00 (2018) 1–4 -2 -1 0 1 200.050.1 (a) < 3 GeV/c T FTPC cent. 40-100%, 1 < p hD fD d hD N / d ) d t r i g ( / N N = 0.0459(10) = 0.336(6) s C = 0.0019(4)/ndf = 19/25 c /16 p |< min fD - fD /3 - | p |< fD | /16 p |< min fD - fD /3 - | p |< p - fD | STAR Preliminary -2 -1 0 1 200.050.1 (b) < 3 GeV/c T FTPC cent. 0-20%, 1 < p hD fD d hD N / d ) d t r i g ( / N N = 0.0594(18) = 0.382(9) s C = 0.0070(8)/ndf = 19/25 c /16 p |< min fD - fD /3 - | p |< fD | /16 p |< min fD - fD /3 - | p |< p - fD | STAR Preliminary
Figure 2. Dihadron ∆ η correlations in FTPC-Au multiplicity selected (a) 40-100% and (b) 0-20% d + Au collisions at √ s NN =
200 GeV forcharged particles of 1 < p T < / c . The red open points are near-side correlations, while the blue solid for away-side correlations. TheGausian + pedestal fit results are listed as N for the Guassian area, σ for Gaussian width, C for the pedestal constant. Fig. 2 shows the TPC-TPC ∆ η correlations in low- and high-multiplicity events. The correlations are larger inhigh- than low-multiplicity collisions.Dihadron correlations in d + Au collisions are dominated by jet-like correlations. To characterize jet-like correla-tions, the near-side correlations are fit with a Gaussian + constant pedestal. The fit results are shown in Fig. 2. Theratio of the high- and low-multiplicity Gaussian areas is found to be α = . ± .
05. This would represent the ratioof the jet-like correlated yields if the Gaussians represent jets (and the pedestals represent non-jet contributions). Thenon-unity α parameter suggests an event selection bias on the jet population. The high-multiplicity events appear toselect jets with larger yield and wider ∆ η distribution.Since the away-side jet spreads over a wide ∆ η , it cannot be isolated. Because of momentum conservation, theaway-side correlated yield likely scales with the near-side yield. The open circles in Fig. 3 represent the di ff erencebetween high- and low-multiplicity events, with the latter first multiplied by the α parameter from the fit. This scalingwould be a first order correction to the multiplicity selection bias on jet-like correlations, such that the away-side jetcontributions would be subtracted. Indeed, the away-side yields are approximately zero for all | ∆ η | ranges shown inFig. 3. This suggests that the di ff erence in the away-side long-range correlations between high- and low-multiplicityevents is mostly from jet-like correlations. fD d hD N / d ) d t r i g ( / N fD |<0.3 hD STAR Preliminary Cent.-Peri.Peri. ·a Cent.- (a) fD d hD N / d ) d t r i g ( / N fD |<0.7 hD STAR Preliminary Cent.-Peri.Peri. ·a Cent.- (b) fD d hD N / d ) d t r i g ( / N fD |<1.8 hD STAR Preliminary Cent.-Peri.Peri. ·a Cent.- (c)
Figure 3. Dihadron ∆ φ correlation di ff erence between high- and low-multiplicity collisions in (a) 0 < | ∆ η | < .
3, (b) 0 . < | ∆ η | < . . < | ∆ η | < . + Au collisions at √ s NN =
200 GeV for charged particles of 1 < p T < / c . Both the trigger and associatedparticles are from the TPC. FTPC-Au multiplicity is used for event selection. The solid dots represent “Cent. − Peri.”. The open circles represent“Cent. − α × Peri.”, where α is near-side Gaussian area ratio in high- and low-multiplicity collisions. The error bars are statistical errors. The solid dots in Fig. 3 show the simple di ff erence between high- and low-multiplicity data, as done by PHENIX[5]. The peak magnitudes on the near-side and away-side turn out to be similar, resembling a double-ridge. As thelarge acceptance STAR data show, the resulting double-ridge structure may well be due to residual jet correlationswhich remain after the simple subtraction of the low- from the high-multiplicity correlation distributions.From the di ff erent yields and widths shown in Fig. 2 for the near-side ∆ η dihadron correlations, we conclude thatthe jet population is a ff ected by the multiplicity selection of the low- and high-multiplicity event classes. Using low-multiplicity data to subtract the jet contributions from the high multiplicity event dihadron correlations is thus only a3 . Yi / Nuclear Physics A 00 (2018) 1–4 rough approximation to the desired correction. The interpretation of the double-ridge structure resulting from such asubtraction in d + Au collisions at RHIC should be taken with caution.
5. Near-side long range correlation
ZYAM=0.0978(2)ZYAM=0.1776(3) fD d hD N / d ) d t r i g ( / N fD <-2 hD -4.5< STAR Preliminary (a)
ZYAM=0.0361(1)ZYAM=0.0438(1) fD d hD N / d ) d t r i g ( / N fD <4.5 hD STAR Preliminary (b)
Figure 4. Dihadron ∆ φ correlations in (a) − . < ∆ η < − . . < ∆ η < . + Au collisions at √ s NN =
200 GeV for charged particlesof 1 < p T < / c . The trigger particle is from TPC and the associated particle from FTPC. ZDC-Au neutral energy is used for event selection.The red open circles represent the high-multiplicity (0-20%) collisions. The blue solid dots represent the low-mulitiplicity (40-100%) collisions.The subtracted background values are listed on the plot. As seen in Fig. 1 (c), the TPC-TPC correlation shows a near-side peak at | ∆ η | ≈ . ∆ η , Fig. 4 shows the TPC-FTPC ∆ φ correlations for associated particles in the Auand deuteron beam directions. Here the ZDC-Au energy is used for event selection to avoid self-correlations. There isa near-side long-range correlation in FTPC-Au at ∆ η ≈ − ∆ η in the Au beam direction is under investigation.
6. Conclusions
Dihadron ∆ η - ∆ φ correlations in d + Au collisions at √ s NN =
200 GeV are presented. The multiplicity selection ofevents a ff ects or biases the jet correlations. Simple subtraction of low-multiplicity data as a technique to remove jetcontributions in high-multiplicity events at RHIC is problematic. A finite near-side correlated yield above a uniformbackground is observed at | ∆ η | ≈ . ∆ η ≈ − References [1] V. Khachatryan et al. (CMS collaboration), J. High Energy Phys. (2010) 091.[2] S. Chatrchyan et al. (CMS collaboration), Phys. Lett. B (2013) 795814.[3] B. Abelev et al. (ALICE collaboration), Phys. Lett. B (2013) 29.[4] G. Ada et al. (ATLAS collaboration), Phys. Rev. Lett. et al. (PHENIX collabration), Phys. Rev. Lett. et al. (STAR collaboration), Phys. Rev. C et al. (ALICE collaboration), Phys. Lett. B (2013) 164.[8] A. Adare et al. (PHENIX collaboration), arXiv:1404.7461 [nucl-ex][9] P. Bozek and W. Broniowksi, Phys. Rev. C , 014903 (2013).[10] A. Bzdak, B. Schenke, P. Tribedy, and R. Venugopalan, Phys. Rev. C , 064906 (2013).[11] G.-Y. Qin and B. Mueller, Phys. Rev. C , 044902 (2014).[12] K. Dusling and R. Venugopalan, Phys. Rev. D , 094034 (2013).[13] J. Adams, et al. (STAR collaboration), Phys. Rev. Lett. et al. , Phys. Rev. C011902 (2005).