Small Collision Research Articles

Small collision systems: Theory overview on cold nuclear matter effects

Many observables measured at the Relativistic Heavy Ion Collider and the Large Hadron Collider show a smooth transition between proton-proton and protonnucleus collisions (small systems), and nucleus-nucleus collisions (large systems), when represented versus some variable like the multiplicity in the event. In this contribution I review some of the physics mechanisms, named cold nuclear matter effects, that may lead to a collective-like behaviour in small systems beyond the macroscopic description provided by relativistic hydrodynamics. I focus on the nuclear modification of parton densities, single inclusive particle production and correlations.

System size, collision energy, and rapidity dependence of collective dynamics measured by the PHENIX experiment at RHIC

In high energy collisions, partons produced in the initial stage undergo multiple interactions and yield a collective motion as a whole. Recently, several questions have risen including how small the system can be for producing the collectivity, and how far in rapidity the collectivity extends. PHENIX has measured the particle flow in p + Au, d + Au, and 3He + Au collisions over several energies as well as over wider rapidity range. In this work, we summarize our results on the particle flow and discuss the implications for collectivity in small collision systems.

Systematic study of high pT hadron production in small collision systems by the PHENIX experiment at RHIC

High pT hadrons in the small systems of d/3He+Au and p+Al/Au collisions have been measured at the midrapidity (|ƞ| < 0.35) as well as at the forward and backward rapidities (1.2 < |ƞ| < 2.4) at √sNN =200 GeV. A clear system and centrality ordering in the Rp/d/HeA was observed for midrapidity π0 production. Hadron Rcp as a function of rapidity in forward and backward regions show an opposite trend to what HIJING++ predicted. Together with the difference of Rcp in p+Al/Au collisions, the observation can be understood consistently with the measured long-range two-particle correlations, hinting the possible medium creation similar to A+A collisions.

Rapid resolution of carbohydrate isomers via multi-site derivatization ion mobility-mass spectrometry.

Identifying small sugar isomers can be challenging by ion mobility-mass spectrometry (IM-MS) alone due to their small collision cross section differences. Herein, we report IM-MS results for multi-site, covalent carbohydrate derivatization with 3-carboxy-5-nitrophenylboronic acid (3C5NBA). Following reaction in aqueous solutions at room temperature, 3C5NBA reacts with each mono- or disaccharide molecule to yield products that each have a distinguishable mobility signature. The reaction was rapid and resulted in the detection of products within 5 min after 3C5NBA was mixed with the analyte. Eight disaccharides that varied in linkage, composition, and configuration (α or β) as well as four monosaccharides, d-glucose, d-galactose, d-mannose and d-fructose, were included in this study. The derivatives' drift times showed significant shifts, with up to 3-fold gains in resolution when compared to previous literature reports. Moreover, the specific MS/MS fragmentation information gathered from these sugar derivatives provided further validation of the isomers' structures.

Recent ATLAS measurements of azimuthal anisotropies in pp and p+Pb collisions

The azimuthal anisotropies of particle yields observed in relativistic heavy-ion collisions are considered as an evidence of the formation on a deconfined Quark-Gluon Plasma produced in these collisions. Interestingly, recent measurements in pp and p+Pb systems from ATLAS and other experiments show similar features as those observed in A+A collisions, indicating the possibility of the production of such a deconfined medium in smaller collision systems. This report presents a summary of the recent ATLAS results on azimuthal anisotropies in pp collisions at 5.02 TeV and 13 TeV, p+Pb collisions at 5.02 TeV and 8.16 TeV as well as in peripheral 2.76 TeV Pb+Pb interactions. It includes measurements of two-particle correlations of charged particles as well as correlations of heavy flavor muons and charged particles in Δϕ and Δη, with a template fitting procedure used to subtract the dijet contributions. Additionally, measurements of cumulants of multi-particle correlations, cn{2-8} are presented. The two-particle correlations and cumulants confirm a presence of collective phenomena in these collision systems, but the results on four-particle cumulants for pp collisions do not demonstrate a similar collective behaviour. However, the cumulant measurements in small collision systems can be biased by non-flow correlations. A novel subevent cumulant method that suppresses the contribution of non-flow effects was proposed recently by ATLAS allowing to measure significant azimuthal anisotropies in both pp and p+Pb collisions.

Predictions on three-particle azimuthal correlations in proton--proton collisions

The ridge signal, which is long-ranged in rapidity, in the di-hadron correlations in high-multiplicity p-p and p-A collisions opened up a whole new research area in high-energy QCD. Although the ridge had been observed in A-A collisions and interpreted as a result of the radial flow of quark-gluon plasma, it had not appeared until recently in the data of small collision systems such as p-p and p-A, nor had it been predicted theoretically or seen in the event generators. There are two competing approaches that attempt to explain the systematics of the di-hadron ridge signal; hydrodynamics and gluon saturation physics (color glass condensate/glasma). In this work, we present predictions for the transverse momentum and rapidity dependence of the three-particle correlation function within the gluon saturation physics. Tri-hadron correlations can be measured, and the data can possibly rule out one of the two alternative approaches.

Measurements of azimuthal anisotropy and charged-particle multiplicity in d + Au collisions at sNN=200 , 62.4, 39, and 19.6 GeV

We present measurements of the elliptic flow, $v_2$, as a function of transverse momentum ($p_T$), pseudorapidity ($\eta$), and centrality in $d$$+$Au collisions at $\sqrt{s_{_{NN}}}=$200, 62.4, 39, and 19.6 GeV. The beam-energy scan of $d$$+$Au collisions provides a testing ground for the onset of flow signatures in small collision systems. We measure a nonzero $v_2$ signal at all four collision energies, which, at midrapidity and low $p_T$, is consistent with predictions from viscous hydrodynamic models. Comparisons with calculations from parton transport models (based on the {\sc ampt} Monte Carlo generator) show good agreement with the data at midrapidity to forward ($d$-going) rapidities and low $p_T$. At backward (Au-going) rapidities and $p_T>1.5$ GeV/$c$, the data diverges from {\sc ampt} calculations of $v_2$ relative to the initial geometry, indicating the possible dominance of nongeometry related correlations, referred to as nonflow. We also present measurements of the charged-particle multiplicity ($dN_{\rm ch}/d\eta$) as a function of $\eta$ in central $d$$+$Au collisions at the same energies. We find that in $d$$+$Au collisions at $\sqrt{s_{_{NN}}}=$200 GeV the $v_2$ scales with $dN_{\rm ch}/d\eta$ over all $\eta$ in the PHENIX acceptance. At $\sqrt{s_{_{NN}}}=$62.4, and 39 GeV, $v_2$ scales with $dN_{\rm ch}/d\eta$ at midrapidity and forward rapidity, but falls off at backward rapidity. This departure from the $dN_{\rm ch}/d\eta$ scaling may be a further indication of nonflow effects dominating at backward rapidity.

Frontal collision of internal solitary waves of first mode

The dynamics and energetics of a frontal collision of internal solitary waves (ISW) of first mode in a fluid with two homogeneous layers separated by a thin interfacial layer are studied numerically within the framework of the Navier–Stokes equations for stratified fluid. It was shown that the head-on collision of internal solitary waves of small and moderate amplitude results in a small phase shift and in the generation of dispersive wave train travelling behind the transmitted solitary wave. The phase shift grows as amplitudes of the interacting waves increase. The maximum run-up amplitude during the wave collision reaches a value larger than the sum of the amplitudes of the incident solitary waves. The excess of the maximum run-up amplitude over the sum of the amplitudes of the colliding waves grows with the increasing amplitude of interacting waves of small and moderate amplitudes whereas it decreases for colliding waves of large amplitude. Unlike the waves of small and moderate amplitudes collision of ISWs of large amplitude was accompanied by shear instability and the formation of Kelvin–Helmholtz (KH) vortices in the interface layer, however, subsequently waves again become stable. The loss of energy due to the KH instability does not exceed 5%–6%. An interaction of large amplitude ISW with even small amplitude ISW can trigger instability of larger wave and development of KH billows in larger wave. When smaller wave amplitude increases the wave interaction was accompanied by KH instability of both waves.

Importance of non-flow in mixed-harmonic multi-particle correlations in small collision systems

Recently CMS Collaboration measured mixed-harmonic four-particle azimuthal correlations, known as symmetric cumulants SC(n,m), in pp and p+Pb collisions, and interpreted the non-zero SC(n,m) as evidence for long-range collectivity in these small collision systems. Using the PYTHIA and HIJING models which do not have genuine long-range collectivity, we show that the CMS results, obtained with standard cumulant method, could be dominated by non-flow effects associated with jet and dijets, especially in pp collisions. We show that the non-flow effects are largely suppressed using the recently proposed subevent cumulant methods by requiring azimuthal correlation between two or more pseudorapidity ranges. We argue that the reanalysis of SC(n,m) using the subevent method in experiments is necessary before they can used to provide further evidences for a long-range multi-particle collectivity and constraints on theoretical models in small collision systems.

Nonlinear dynamics of a cold collisional electron plasma

We study the influence of collisions on the dynamics of a cold non-relativistic plasma. It is shown that even a comparatively small collision frequency can significantly change the large amplitude wave solution.

Fast and Effective Ion Mobility-Mass Spectrometry Separation of d-Amino-Acid-Containing Peptides.

Despite often minute concentrations in vivo, d-amino acid containing peptides (DAACPs) are crucial to many life processes. Standard proteomics protocols fail to detect them as d/l substitutions do not affect the peptide parent and fragment masses. The differences in fragment yields are often limited, obstructing the investigations of important but low abundance epimers in isomeric mixtures. Separation of d/l-peptides using ion mobility spectrometry (IMS) was impeded by small collision cross section differences (commonly ∼1%). Here, broad baseline separation of DAACPs with up to ∼30 residues employing trapped IMS with resolving power up to ∼340, followed by time-of-flight mass spectrometry is demonstrated. The d/l-pairs coeluting in one charge state were resolved in another, and epimers merged as protonated species were resolved upon metalation, effectively turning the charge state and cationization mode into extra separation dimensions. Linear quantification down to 0.25% proved the utility of high resolution IMS-MS for real samples with large interisomeric dynamic range. Very close relative mobilities found for DAACP pairs using traveling-wave IMS (TWIMS) with different ion sources and faster IMS separations showed the transferability of results across IMS platforms. Fragmentation of epimers can enhance their identification and further improve detection and quantification limits, and we demonstrate the advantages of online mobility separated collision-induced dissociation (CID) followed by high resolution mass spectrometry (TIMS-CID-MS) for epimer analysis.

Multiplicity dependence of identified particle production in proton-proton collisions with ALICE

The study of identified particle production as a function of transverse momentum (pT) and event multiplicity in proton-proton (pp) collisions at different center-of-mass energies (s) is a key tool for understanding similarities and differences between small and large collisions systems. We report on the production of π±, K±, KS0, p(p‾), Λ(Λ‾), Ξ± and Ω± measured in pp collisions in a wide range of center-of-mass energies with ALICE [1]. The multiplicity dependence of identified particle yields is presented for s=7 and 13 TeV and discussed in the context of the results obtained in protonlead (p-Pb) and lead-lead (Pb-Pb) collisions, unveiling remarkable and intriguing similarities. The production rates of strange hadrons are observed to increase more than those of non-strange particles, showing an enhancement pattern with multiplicity which does not depend on the collision energy. Even if the multiplicity dependence of spectral shapes can be qualitatively described by commonly-used Monte Carlo (MC) event generators, the evolution of integrated yield ratios is poorly described by these models.

Hard processes in small systems

These proceedings from the Quark Matter 2017 conference give an overview of the latest experimental results on hard processes in small collision systems at RHIC and the LHC, discuss their implications, and consider several prospects for future measurements.

New insights of multi-particle azimuthal correlations with symmetric cumulants in p-p, p-Pb, and Pb-Pb collisions

The first comparison of anisotropy harmonics (vn, n = 2–4) and event-by-event correlations of different orders between p–p (13 TeV), p–Pb (5.02 and 8.16 TeV) and Pb–Pb (5.02 TeV) as a function of multiplicity is presented. The vn coefficients are extracted via long-range (|Δη|>2) two-particle correlations reaching a very-high-multiplicity region. Event-by-event correlations among v2, v3 and v4 are measured using the four-particle symmetric cumulant (SC(n,m), n = 2, m = 3, 4). For high-multiplicity (more than 100 tracks) events, v2 is found to have a negative correlation with the v3, while the v2 and v4 are positively correlated. Normalized by the two-particle vn, the SC(2,3) are quantitatively similar for p–Pb and Pb–Pb data, while a strong system size dependence is observed for SC(2,4). These new data provide important insights to the origin of collectivity observed in small collision systems.

PHENIX results on collective behavior in small systems from geometry-controlled experiments at [formula omitted

Recent measurements in small collisions systems at LHC and RHIC indicate that the particles produced in high-multiplicity collisions exhibit collective behavior very similar to that observed in large systems where QGP is formed. In large systems, it is well established that the final-state particle correlations arise from anisotropic pressure gradients in the initial state of the collisions that drive a near-perfect fluid evolution. Whether QGP is also formed in small collision systems is presently under intense investigation. To study the origin of the collective behavior in small systems, the PHENIX experiment performed a series of geometry-controlled experiments using p+Al, p+Au, d+Au, and 3He+Au collisions at sNN=200 GeV. The elliptic (v2) flow coefficients are measured as a function of pT for inclusive and identified charged hadrons. Mass dependence is observed in v2(pT) indicative of hydrodynamic evolution. The distinct initial geometries provide discriminating power against the models.

Revealing long-range multiparticle collectivity in small collision systems via subevent cumulants

Multi-particle azimuthal cumulants, often used to study collective flow in high-energy heavy-ion collisions, have recently been applied in small collision systems such as $pp$ and $p$+A to extract the second-order azimuthal harmonic flow $v_2$. Recent observation of four-, six- and eight-particle cumulants with "correct sign" $c_2\{4\}<0, c_2\{6\}>0, c_2\{8\}<0$ and approximate equality of the inferred single-particle harmonic flow, $v_2\{4\}\approx v_2\{6\}\approx v_2\{8\}$, have been used as strong evidence for a collective emission of all soft particles produced in the collisions. We show that these relations in principle could be violated due to the non-Gaussianity in the event-by-event fluctuation of flow and/or non-flow. Furthermore, we show, using $pp$ events generated with the PYTHIA model, that $c_2\{2k\}$ obtained with standard cumulant method are dominated by non-flow from dijets. An alternative cumulant method based on two or more $\eta$-separated subevents is proposed to suppress the dijet contribution. The new method is shown to be able to recover a flow signal as low as 4\% imposed on the PYTHIA events, independently of how the event activity class is defined. Therefore the subevent cumulant method offers a more robust way of studying collectivity based on the existence of long-range azimuthal correlations between multiple distinct $\eta$ ranges. The prospect of using the subevent cumulants to study collective flow in A+A collisions, in particular its longitudinal dynamics, is discussed.

PHENIX measurements of open and hidden heavy flavor in p+p, p+Al, and p/d/3He+Au collisions across a wide range of rapidity

Despite intense theoretical and experimental investigation, the physical mechanisms governing the suppression of bound quark-antiquark states in nuclear collisions are not yet fully understood. While color screening in a plasma phase is expected to play a role, there are numerous other possible suppression mechanisms that do not require deconfinement, as well as effects on the heavy quark initial state in the nucleus which can also play a role. To study these effects, the PHENIX collaboration has used the flexibility of the RHIC accelerator complex to observe the evolution of open heavy flavor and quarkonia dynamics as both the projectile and target nuclei size are varied. Open heavy flavor in small collision systems can serve as the baseline for interpreting quarkonia production in the nuclear environment, and comparisons of the ψ(2S) with the ψ(1S) show that in rapidity regions with relatively high hadron density, the larger 2S state is preferentially more suppressed than the more tightly bound ψ(1S). This suggests that late-stage mechanisms may be at least partially responsible for quarkonia suppression in nuclear collisions. In this talk, we will present results on excited-state quarkonia in p+p, p+Al, and p/d/3He+Au collisions and open heavy flavor in small systems, and discuss how these measurements impact our understanding of heavy quark behavior in the quark-gluon plasma.

Investigations of anisotropic collectivity using multi-particle correlations in pp, p–Pb and Pb–Pb collisions

Two- and multi-particle azimuthal correlations have proven to be an excellent tool to probe the properties of the strongly interacting matter created in heavy-ion collisions. Recently, the results obtained for multi-particle cumulants have been interpreted as evidence for collectivity in the small collision systems (pp and p–Pb) providing new insights into the systems' fluctuating initial conditions. In this article, first ALICE results on two- and multi-particle cumulants as a function of charged hadron multiplicity produced at midrapidity (|η| < 1.0) in pp collisions at s=13 TeV are reported. The results are compared with measurements in p–Pb collisions at sNN=5.02 TeV and in Pb–Pb collisions at sNN=2.76 TeV. A new method for the 4-particle cumulant with an η gap denoted as cn{4,|Δη|} will be presented, which is capable of further suppressing non-flow effects in multi-particle cumulants. The results allow for further understanding of the origin of multi-particle correlations in pp collisions.

Two- and Multi-particle Azimuthal Correlations in Small Collision Systems with the ATLAS Detector

The recent ATLAS results on two- and multi-particle azimuthal correlations of charged particles are presented for s=5.02 TeV and 13 TeV pp, sNN=5.02 TeV p+Pb and sNN=2.76 TeV low-multiplicity Pb+Pb collisions. To suppress the “non-flow” contribution from the correlations, a template fitting procedure is used in the two-particle correlations (2PC) measurements, while for multi-particle correlations the cumulant method is applied. The correlations are expressed in the form of Fourier harmonics vn(n=2,3,4) measuring the global azimuthal anisotropy of produced particles. The measurements presented hereafter confirm the evidence for collective phenomena in high-multiplicity p+Pb and low-multiplicity Pb+Pb collisions. For pp collisions the results on four-particle cumulants do not demonstrate a similar collective behaviour.

Collective behavior in small systems

The presence of correlations between particles significantly separated in pseudorapidity in proton-proton and proton-nucleus collisions has raised questions about whether collective effects are observed in small collision systems as well as in heavy-ion collisions. The quantification of these long-range correlations by vn coefficients is of particular interest. A selection of the latest vn measurements is presented, including results from the recent d+Au beam energy scan at RHIC where a significant non-zero v2 is measured down to low center-of-mass energies (sNN=39GeV). Results from a collision system scan – comprising p+Au, d+Au, and 3He+Au collisions – are also shown to address the role of the initial nuclear geometry in the final state anisotropy. Finally, the challenge of measuring multi-particle cumulants, particularly c2{4}, in p+p collisions is discussed, and new methods for reducing the effects of non-flow are shown to produce a more robust measurement of v2{4} in p+p collisions.