Study of Small Colliding Systems

Available online at www.sciencedirect.com

Nuclear Physics A 982 (2019) 85–91 www.elsevier.com/locate/nuclphysa

XXVIIth International Conference on Ultrarelativistic Nucleus-Nucleus Collisions (Quark Matter 2018)

Study of Small Colliding Systems Li Yi Shandong University, Qingdao, Shandong 266237, China

Abstract In this proceeding, I review a selection of latest studies on small colliding systems. The novel phenomena in highmultiplicity proton-proton, proton-nucleus and deuteron/helium-nucleus collisions have generated increasing interests in heavy-ion and high energy physics communities. A few selected experimental results, focused on collectivity in small colliding systems will be reviewed. Open questions on the interpretations of collectivity in small systems are discussed. Keywords: quark-gluon plasma, relativistic heavy-ion collisions, collectivity, small systems

1. Introduction Small colliding systems (proton-proton and proton/deuteron-nucleus) were once considered as control experiments for Quark-Gluon-Plasma (QGP) discoveries in relativistic heavy-ion collisions. Those control experiments serve as a proxy to disentangle the initial state cold nuclear matter and final state hot matter effects. Among the evidences for QGP formation, the high-pT jet suppression is one notable example which based on comparison between heavy-ion collisions and small colliding systems. One central theme to study QGP is collectivity phenomena. Collectivity is generally quantified by particle azimuthal correlations. Azimuthal correlation can be measured by, for example, scalar product. In the early years of RHIC run, the finite difference in scalar product of d+Au collision from pp collisions was observed at low transverse momentum (pT ) [1]. The difference cannot be reproduced by HIJING simulation [2]. The observed scalar product in d+Au collisions increases as multiplicity increases, while in Au+Au collisions, the scalar product first increases then decreases as multiplicity increases. There are similarities and differences in azimuthal correlations between d+Au and Au+Au collisions. The question is whether the observed azimuthal correlation behavior in d+Au collisions is an indication of existence of collectivity as in Au+Au collisions. Long lasting debates about collectivity in small colliding systems started by high-mulitplicity pp ridge discovery in year 2010. Ridge is a near-side long-range correlation (at small relative azimuth φ and large relative pseudorapidity η) in two-particle correlations. It was first observed after elliptic flow subtraction in central heavy-ion collisions at both RHIC and the LHC [3, 4, 5, 6, 7, 8]. Ridge in heavy-ion collisions is attributed primarily to triangular flow, while initial event-by-event geometry fluctuations lead to final state

https://doi.org/10.1016/j.nuclphysa.2018.08.028 0375-9474/© 2018 Published by Elsevier B.V. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).

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long-range correlations through medium’s hydrodynamics response [9, 10]. Ridge in high multiplicity pp collisions was one of the major discoveries at the beginning of the LHC run [11]. Here no elliptic flow subtraction was done for two-particle correlations, as flow was not expected in pp: the long-range ridge in pp thus includes both the second and higher order harmonics contributions. The debate, triggered by ridge in high-multiplicity pp collisions, is whether the strongly interacting QGP can also be produced in high energy pp collisions with much smaller size and if so, what does it imply for QGP in heavy-ion collisions. Since the ridge discovery in pp collisions, there have been numerous developments in small colliding system studies both theoretically and experimentally, such as the efficiency of single hit dynamics transferring spatial anisotropy to momentum anisotropy [12], the measurements from two-particle correlations in e+ e− collisions at ALEPH [13], and in electron-proton collisions at HERA [14] at this Quark Matter conference. In this report, with the limitation of length, only a few selected latest studies on small colliding systems with focus on collectivity and hard probes will be discussed. 2. Collectivity Phenomena Collectivity is a many-body phenomenon whose effect can be reflected on particle productions and twoparticle correlations. The ridge structure is based on two-particle correlations which, similar to collective flow measurement in heavy-ion collisions, can be decomposed to extract Fourier coefficients vn . Compared to heavy-ion collisions, the nonflow contribution is larger in small colliding systems, meaning that nonflow subtraction becomes important in vn determination [15]. There are two popular nonflow subtraction methods: template fit and scaled low-multiplicity subtraction. The basic assumption for those methods is that the shape of nonflow is the same in low-multiplicity and high-multiplicity collisions. Template fit method assumes V1,1 shape is wholey described by away-side jet. The scaled low-multiplicity subtraction assumes near-side and away-side jet yields to scale together. There is also an improvment for template fit with bias from possible vn multiplicity dependence corrected [16]. One needs to keep in mind that different assumptions could introduce different biases, when interpreting vn from different methods. Although there is large uncertainty in nonflow subtraction, it is clear that there are anisotropies in small systems based on two-particle correlations results. 2.1. Evidence of Collectivity If anisotropies in two-particle correlations come from collectivity, one would expect it to be a multiple particle phenomenon. One way to measure multiple particle correlation is multi-particle cumulant cn {m}. One can further calculate multi-particle vn {m} from cn {m}. At LHC, the first measurement showing nonzero v3 {4} in pPb collisions at 8.16 TeV was reported by CMS collaboration [17]. At RHIC, the first hint of negative c2 {4} in d+Au collisions at 200 GeV and 62.4 GeV was reported √ by STAR collaboration [18], as shown in Fig. 1. A negative c2 {4} suggests the non-zero v2 {4} = 4 −c2 {4}, i.e. the existence of 4particle v2 . Naively one would expect that the existence of 4- and multi-particle vn is a strong evidence of collectivity. However, recent iEBE-VISHNU hybrid simulation of pp collisions at 13 TeV in ref. [19] shows that hydrodynamics does not necessarily give negative c2 {4}. The same model, however, is able to describe the measured integrated and differential v2 from two-particle correlations for all charged and identified hadrons for the same collision system. The lack of simultaneous description of both two- and four-particle cumulants could be caused by limitation of knowledge on proton initial state. There has been progress to study the initial fluctuation of proton substructure by coherent and incoherent exclusive J/ψ production in light diffraction with proton [20], while the precise measurement of proton substructure is one of main goal at future Electron Ion Collider [21]. Hydrodynamics gives a good comparison for the initial state fluctuation n {4}/n {2} with the measured final state vn {4}/vn {2} for both v2 and v3 in pPb collisions at 8.16 TeV [17]. Such consistence is a confirmation of vn driven by initial geometry in hydrodynamics framework. However, the multi-particle correlations are not unique to hydrodynamics. Color-Glass-Condensate (CGC) calculation, when considering coherent multiple scatterings off localized domains of gluon shock wave, offers an alternative description of the multi-particle correlations with v2 {2} > v2 {4} ≈ v2 {6} ≈ v2 {8} seen in pPb collisions [22]. In CGC calculation the final vn is originated from initial momentum anisotropy

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without medium response. Both the initial spatial anisotropy with effective theory hydrodynamics and initial momentum anisotropy with CGC calculation can provide explanations for multi-particle correlations in pA collisions.

Fig. 1. c2 {4} as a function of average mid-rapidity charged multiplicity dNch /dη in d+Au collisions at 200 GeV (panel a) and 62.4 GeV (panel b) measured by STAR collaboration [18].

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While multi-particle cumulants probe the fluctuation of single harmonics, symmetric cumulants, which is mixed-order harmonics cumulant, provide further insight into the correlations between harmonics. The lowest order of correlations between two harmonics can be quantified by 4-particle symmetric cumulants sc(n, m){4} = v2n v2m  − v2n v2m . Although multi-particle symmetric cumulants were thought to be much less affected by nonflow contamination, it was shown in ref. [23] that in small colliding systems nonflow is actually important in sc(n, m). The subevent method was proposed to reduce nonflow contribution in standard sc(n, m) [24]. Larger η acceptance would further help to reduce nonflow, especially at lower multiplicity [25, 26]. Figure 2 shows sc(2, 3){4} and sc(2, 4){4} with three-subevent method as a function of dNch /dη in pp, pPb and PbPb collisions by ATLAS collaboration [25, 16]. Negative correlation between v2 and v3 3 ×10

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and positive correlation between v2 and v4 show a consistence across all collision systems in the overlap multiplicity range. This may suggest the physical processes responsible for the observed correlations to be the same in all collision systems.

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2.2. Collectivity Discussions With many persuasive pieces of evidence for collectivity in small colliding systems, there have been ample theoretical efforts to uncover its origin. Here a few selected models are discussed: models based on hydrodynamics, low-density models and CGC. Many new developments are not discussed here, including but not limited to multi-parton interaction with color reconnection, dynamical core-corona picture and so on. Hydrodynamics is a high density system evolution. In hydrodynamics framework, the initial spatial anisotropy is translated to the final momentum anisotropy by the medium pressure gradient. In low density models, such as the effective single hit kinetic transport model and the “escape model” AMPT, the initial spatial anisotropy is transferred into momentum anisotropy, by only one or few scatterings. In CGC, momentum anisotropy starts from the initial color field. A fundamental question is how can we distinguish between models. When comparing hydrodynamics with low density models, one leading difference is whether there is pressure gradient. Pressure gradient is needed to translate the anisotropy from initial geometry to final momentum space in hydrodynamics. While there is little pressure gradient in low density systems, radial flow shall also be not expected. If there is radial flow feature present in low density model, it would also be interesting to ask how does it form with a few scatterings. 0.2

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One primary difference between hydrodynamics and CGC is whether the collectivity is driven by initial geometry. RHIC has performed the initial geometry scan with p+Au, d+Au and 3 He+Au at 200 GeV. The spatial eccentricities given by Monte Carlo (MC) Glauber model have the relationships of 2p+Au < 2d+Au ≈ 3 3 2He+Au and 3p+Au < 3d+Au ≈ 3He+Au [27]. Hydrodynamics provide a clear prediction for the ordering of v2 and v3 signals, following the n relationships. PHENIX collaboration has reported v2 (pT ) and v3 (pT ) for three collision systems [27]. Figure 3 shows a simultaneous description of these two observables in three collision systems with a common initial geometry model and the same η/s = 0.08 by two hydrodynamics models with different hadronic re-scattering packages. The apparent agreement of hydrodynamics on elliptic and triangular harmonics ordering in three systems with data suggests a geometry driven picture, which may further imply the possibility of small QGP droplet if hydrodynamics is the solely explanation. However, the latest CGC calculation of initial state gluon correlations also qualitatively share the feature of vn (pT ) system size dependence [28]. The system size dependence in CGC arises from the interplay between saturation scale Q s and transverse momentum resolution pT . When pT > Q s , the magnitude of correlation follows the scaling of inverse number of color domains, as gluons can resolve individual color domains. Therefore, at high pT , v2 in 3 He +Au shall be smaller than that in p+Au, as 3 He +Au has more color domains. When pT < Q s , gluons cannot resolve individual color domains, but interact with them simultaneously. The latter is the case for low pT v2 in p+Au and 3 He +Au collisions. In simple color domain models, anisotropies come from chromoelectric fields, which scale with Q2s for simultaneous interactions. In small colliding systems, the multiplicity also scales with Q2s . The 0-5% 3 He+Au collisions have higher multiplicity than the same 0-5% p+Au collisions. Therefore, 0-5% 3 He+Au collisions will have larger anisotropies than 0-5% p+Au

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collisions at low pT , as observed experimentally. If comparing these collision systems at same multiplicity, CGC predicts identical vn for high-multiplicity events. STAR collaboration has reported similar v2 (pT ) for p+Au and d+Au collisions with similar multiplicity [18], which is consistent with CGC expectation. Event shape engineering (ESE) may serve as possible discriminator for further investigation: various event shapes could be selected while keeping the multiplicity to be the same. vn from hydrodynamics consideration shall depend on the geometries selected, while CGC is not expected to depend on initial geometry with the same multiplicity requirement. Nevertheless ESE is selecting on final particle momenta, which makes it hard to tell whether ESE is actually related to initial spatial geometry selection. System size scan at RHIC yields interesting results, while how to discriminate between hydrodynamics and CGC may still be buried in the quantitive details. 3. Hard Probes Another sets of QGP formation observables are hard probes, which are generated early in collisions by hard scatterings. Jet nuclear modification factor and heavy flavor anisotropy could provide information on how hard probes interact with possible QGP in small colliding systems. 3.1. Jet Quenching? One intriguing question on testing smallest QGP droplet is whether there is jet quenching. If the hot dense medium is formed in small systems, high pT jet would lose energies when transversing through it. Although the path length jet transversed L shall be shorter in small system, the temperature could be higher in small systems at similar multiplicity, which leads to a larger transport coefficient qˆ for energy loss. The strong correlation between high pT and low pT hadron v2 in PbPb collisions implies a common geometry dependence origin. The non-zero high pT v2 in PbPb collisions arises from jet path length dependence. In high-multiplicity pPb, the non-zero v2 at high pT up to 10 GeV/c reported by ATLAS [29] could be explained by possible jet quenching. However, ALICE result is consistent with no reconstructed jet suppression in top 20% high event activity class of pPb collisions [30]. The latest peripheral PbPb jet measurement by ALICE further shows no jet suppression, as in pPb collisions at similar multiplicity [31]. Pushing toward even higher multiplicity with careful handling of auto-correlation from event class selection and jet may be a path forward. 3.2. Heavy Flavor Anisotropies CMS Preliminary

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Heavy flavors are primarily produced via hard scatterings at early time and could probe the entire evolution of the possible QGP dynamics. While collectivity for D0 meson has both charm quark and light quark contribution, J/ψ’s collective behavior if any will provide a strong evidence for charm quark collectivity in

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small colliding systems. In Fig. 4, CMS collaboration reports non-zero v2 (pT ) for D0 meson and prompt J/ψ up to high pT of 8 GeV/c in pPb collisions at 8.16 TeV. The heavy flavor v2 in pPb is almost as strong as light flavors v2 . Due to their large mass, heavy flavor quarks need more scatterings to generate flow than light quark. Whether such large J/ψ vn can be achieved in small systems in hydrodynamics is challenging and more theoretical inputs are desired. 4. Summary and Outlook Ample evidences of similarity between high-multiplicity small colliding systems and heavy-ion collisions have been accumulated over the years since the first ridge observation in pp collisions. The most recent measurements of heavy flavor flow, subevent anisotropies with nonflow removal, vn massing ordering including multi-strangeness, RHIC system shape scan and many more results, impose new constraints on possible theoretical interpretations. The ability to distinguish between initial state effects, such as CGC, and finial state effects, such as low-density models and hydrodynamics, still need more work. The geometry dependence test for hydrodynamics response in small systems is promising but subject to the lack of precise knowledge for initial state in pp and p/d/He+A collisions. A mid-size symmetric light-ion collision systems would be helpful to scrutinize the hydrodynamic paradigm, where Glauber model is applicable to determine the initial condition with the system size still much smaller compared to heavy-ion collision. A comprehensive study of unified framework including both initial and final state effects cross collision systems could provide more insights on a rich variety of QCD phenomena. References [1] J. Adams, et al., Azimuthal anisotropy in Au+Au collisions at s(NN)**(1/2) = 200-GeV, Phys.Rev. C72 (2005) 014904. arXiv:nucl-ex/0409033, doi:10.1103/PhysRevC.72.014904. [2] M. Gyulassy, X.-N. Wang, HIJING 1.0: A Monte Carlo program for parton and particle production in high-energy hadronic and nuclear collisions, Comput.Phys.Commun. 83 (1994) 307. arXiv:nucl-th/9502021, doi:10.1016/0010-4655(94)90057-4. [3] J. Adams, et al., Distributions of charged hadrons associated with high transverse momentum particles in pp and Au + Au collisions at s(NN)**(1/2) = 200-GeV, Phys.Rev.Lett. 95 (2005) 152301. arXiv:nucl-ex/0501016, doi:10.1103/PhysRevLett.95.152301. [4] B. Abelev, et al., Long range rapidity correlations and jet production in high energy nuclear collisions, Phys.Rev. C80 (2009) 064912. arXiv:0909.0191, doi:10.1103/PhysRevC.80.064912. [5] B. Alver, et al., High transverse momentum triggered correlations over a large pseudorapidity acceptance in Au+Au collisions at s(NN)**1/2 = 200 GeV, Phys.Rev.Lett. 104 (2010) 062301. arXiv:0903.2811, doi:10.1103/PhysRevLett.104.062301. [6] B. Abelev, et al., Three-particle coincidence of the long range pseudorapidity correlation in high energy nucleus-nucleus collisions, Phys.Rev.Lett. 105 (2010) 022301. arXiv:0912.3977, doi:10.1103/PhysRevLett.105.022301. [7] S. Chatrchyan, et al., Multiplicity and transverse momentum dependence of two- and four-particle correlations in pPb and PbPb collisions, Phys.Lett. B724 (2013) 213–240. arXiv:1305.0609, doi:10.1016/j.physletb.2013.06.028. √ [8] K. Aamodt, et al., Higher harmonic anisotropic flow measurements of charged particles in Pb-Pb collisions at (s(NN) ) = 2.76 TeV, Phys.Rev.Lett. 107 (2011) 032301. arXiv:1105.3865, doi:10.1103/PhysRevLett.107.032301. [9] B. Alver, G. Roland, Collision geometry fluctuations and triangular flow in heavy-ion collisions, Phys.Rev. C81 (2010) 054905, erratum-ibid. C82, 039903 (2010). arXiv:1003.0194, doi:10.1103/PhysRevC.82.039903, 10.1103/PhysRevC.81.054905. [10] L. Adamczyk, et al., Third Harmonic Flow of Charged Particles in Au+Au Collisions at sqrtsNN = 200 GeV, Phys. Rev. C 88, 014904. arXiv:1301.2187, doi:10.1103/PhysRevC.88.014904. [11] V. Khachatryan, et al., Observation of Long-Range Near-Side Angular Correlations in Proton-Proton Collisions at the LHC, JHEP 1009 (2010) 091. arXiv:1009.4122, doi:10.1007/JHEP09(2010)091. [12] A. Kurkela, this proceeding. [13] Y. Lee, this proceeding. [14] J. Onderwaater, this proceeding. √ [15] L. Adamczyk, et al., Effect of event selection on jetlike correlation measurement in d+Au collisions at sNN = 200 GeV, Phys. Lett. B743 (2015) 333–339. arXiv:1412.8437, doi:10.1016/j.physletb.2015.02.068. [16] M. Aaboud, et al., Correlated long-range mixed-harmonic fluctuations measured in pp, p+Pb and low-multiplicity Pb+Pb collisions with the ATLAS detectorarXiv:1807.02012. [17] Q. Wang, this proceeding. [18] S. Huang, this proceeding. [19] W. Zhao, Y. Zhou, H. Xu, W. Deng, H. Song, Hydrodynamic collectivity in proton?proton collisions at 13 TeV, Phys. Lett. B780 (2018) 495–500. arXiv:1801.00271, doi:10.1016/j.physletb.2018.03.022. [20] H. M¨antysaari, this proceeding.

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