Measurements Of Two-particle Correlations Research Articles

Measurement of in-medium modification of energy-space structure of jets via γ and π0 triggered hadrons in Au+Au collisions at RHIC

Since the discovery of the jet quenching at RHIC, the in-medium interaction of hard scattered partons with the nuclear medium created by highenergy heavy-ion collisions has been an excellent tool to understand not only the transport properties of the medium but also its time evolution towards hadronization. The multi-differential measurement of the high momentum twoparticle correlations can probe a particular space-time window as a function of energy transfer. Comparing the correlations with the prompt photon-triggered hadron spectra, one can extract the property of the medium from various aspects and contribute to distinct models. The PHENIX experiment at RHIC has collected its highest statistics of the γ and π0 triggered hadron events in Au+Au collisions at √SNN = 200 GeV in the RHIC Year-2014 run, and measured not only the inclusive spectra of the triggered hadrons but also the angle and energy dependent IAA and DAA. We will discuss the in-medium modification of the energy-space structure of the jets at the RHIC energies with the results obtained.

Two-particle Correlation and Jet measurements in \(e^{+}e^{-}\) Collisions at 91 GeV with Archived ALEPH Data

Two-particle Correlation and Jet measurements in \(e^{+}e^{-}\) Collisions at 91 GeV with Archived ALEPH Data

Projections of two-particle correlations onto transverse rapidity in Au+Au collisions at sNN=200 GeV at STAR

Two-particle correlation measurements projected onto two-dimensional, transverse rapidity coordinates (yT1,yT2) provide an independent, orthogonal view of the multidimensional correlation distribution that is most often studied via angular projections. As such, these independent transverse projections allow access to manifestations of dynamical fluctuations in relativistic heavy-ion collisions that angular-correlation measurements may not be sensitive to. We report nonidentified charged-particle correlations for Au+Au minimum-bias collisions at sNN=200 GeV taken by the STAR experiment at the Relativistic Heavy-Ion Collider (RHIC). Correlations are presented as two-dimensional functions of transverse rapidity for like-sign, unlike-sign, and all charged-particle pairs, as well as for particle pairs whose relative azimuthal angles lie on the near-side, the away-side, or at all relative azimuth. The correlations are constructed using charged particles with transverse momentum pT≥0.15 GeV/c, pseudorapidity from −1 to 1, and azimuthal angles from −π to π. The significant correlation structures that are observed evolve smoothly with collision centrality. The major correlation features include a saddle shape plus a broad peak with maximum near yT≈3, corresponding to pT≈1.5 GeV/c. The broad peak is observed in both like- and unlike-sign charge combinations and in near- and away-side relative azimuthal angles. The all-charge, all-azimuth correlation measurements are compared with the predictions of hijing and epos to provide theoretical context for these new measurements. The results indicate that the correlations for peripheral to mid-central collisions can be approximately described as a superposition of nucleon + nucleon collisions with minimal effects from the quantum chromodynamics medium. Strong medium effects are indicated in mid- to most-central collisions.1 MoreReceived 22 April 2022Accepted 28 September 2022DOI:https://doi.org/10.1103/PhysRevC.106.044906©2022 American Physical SocietyPhysics Subject Headings (PhySH)Research AreasParticle correlations & fluctuationsRelativistic heavy-ion collisionsNuclear Physics

Implications of correlations and fluctuations in small systems

The small collision systems like p+p and p+A collisions have shown new features like A+A collisions in the relativistic regime. These new aspects in small systems which have altered our research and understanding on the two-particle correlation measurements have been provided. Additionally, a critical observation of the fluctuation measurements provides new ways to infer such novel happening in the small collision systems. The ongoing and future endeavors towards the new measurements are also discussed.

In-Medium Jet Modification Measured by PHENIX Via Two-Particle Correlations and High pT Hadrons in A + A Collisions

The first evidence of jet quenching was observed at RHIC via suppression of single high pT hadron RAA and the disappearance of the away-side jet peak in two-particle correlations. Since then, hadron RAA and two-particle correlations continue to be useful probes of the QGP in heavy ion collisions, since the particles involved are fragments of the jets produced in the initial hard scattering. PHENIX recently improved the width measurements extracted from π0-hadron correlations after removing the higher order flow terms in the underlying event subtraction. Measurements of the away-side jet correlated with high pT neutral pions show an increase in low pT particle production at wide angles consistent with theoretical expectations for energy loss. The system size dependence of energy loss is further investigated at PHENIX by measuring the absolute yield and RAA for various hadron species at high pT in several collision systems including U + U and Cu + Au. These proceedings will present the newest PHENIX RAA and two-particle correlation measurements and their role in our understanding of jet quenching and medium response in heavy ion collisions.

Characterizing the medium created in Pb–Pb collisions at by means of the evolution of two-particle transverse momentum correlations

Two-particle correlations are powerful tools for studying the medium produced in heavy-ion collisions. In particular, two-particle transverse momentum correlations enable measurements of the collision dynamics sensitive to momentum currents. Their evolution with collision centrality, which is related to the system lifetime, provides information about the shear viscosity, ɳ/S, and the system relaxation time, . We report on measurements of two-particle transverse momentum correlations as a function of centrality in Pb-Pb collisions using the ALICE detector at the LHC. The centrality dependence of the near-side peak of the correlation function, particularly in the longitudinal dimension, provides information about the shear viscosity and the system relaxation time of the produced medium. The data are compared to predictions from selected Monte Carlo models. The charge independent momentum correlator exhibits a longitudinal broadening from peripheral to central collisions that is qualitatively consistent with expectations from a model with viscous effects. We will discuss an interpretation of the observed broadening in the context of this model.

Two-Particle Transverse Momentum Correlations in Pb–Pb Collisions at $$\sqrt {{{{\text{s}}}_{{{\text{NN}}}}}} = 2.76$$ TeV

Two-particle correlations are powerful tools for studying the medium produced in heavy-ion collisions. In particular, two-particle transverse momentum correlations enable measurements of the collision dynamics sensitive to momentum currents. Their evolution with collision centrality, which is related to the system lifetime, provides information about the shear viscosity, $$\eta {\text{/}}s$$, and the system relaxation time, $${{\tau }_{\pi }}$$. We report on measurements of two-particle transverse momentum correlations as a function of centrality in Pb‒Pb collisions using the ALICE detector at the LHC. The centrality dependence of the near side peak of the correlation function, particularly its longitudinal width, provides information about the shear viscosity of the produced medium. The data are compared to predictions from selected Monte Carlo models. The charge independent momentum correlator exhibits a longitudinal broadening from peripheral to central collisions that is qualitatively consistent with expectations from a model with viscous effects. An interpretation of the observed broadening in the context of this model will be discussed.

Measurement of long-range two-particle azimuthal correlations in Z-boson tagged pp collisions at sqrt{s}=8 and 13\xa0TeV

Results are presented from the measurement by ATLAS of long-range (|Delta eta |>2) dihadron angular correlations in sqrt{s}=8 and 13 TeV pp collisions containing a Z boson. The analysis is performed using 19.4 mathrm {fb}^{-1} of sqrt{s}=8 TeV data recorded during Run 1 of the LHC and 36.1 mathrm {fb}^{-1} of sqrt{s}=13 TeV data recorded during Run 2. Two-particle correlation functions are measured as a function of relative azimuthal angle over the relative pseudorapidity range 2<|Delta eta |<5 for different intervals of charged-particle multiplicity and transverse momentum. The measurements are corrected for the presence of background charged particles generated by collisions that occur during one passage of two colliding proton bunches in the LHC. Contributions to the two-particle correlation functions from hard processes are removed using a template-fitting procedure. Sinusoidal modulation in the correlation functions is observed and quantified by the second Fourier coefficient of the correlation function, v_{2,2}, which in turn is used to obtain the single-particle anisotropy coefficient v_{2}. The v_{2} values in the Z-tagged events, integrated over 0.5<p_{mathrm {T}}<5 GeV, are found to be independent of multiplicity and sqrt{s}, and consistent within uncertainties with previous measurements in inclusive pp collisions. As a function of charged-particle p_{mathrm {T}}, the Z-tagged and inclusive v_{2} values are consistent within uncertainties for p_{mathrm {T}}<3 GeV.

Data-driven subtraction of anisotropic flows in jet-like correlation studies in heavy-ion collisions

Measurements of two-particle azimuthal angle correlations are a useful tool to study the distribution of jet energy loss, however, they are complicated because of the significant anisotropic flow background. We devise a data-driven method for subtracting anisotropic flow background in jet-like correlation analysis. We first require a large recoil momentum ($P_x$) within a given pseudo-rapidity ($\eta$) range from a high-transverse momentum particle to enhance in-acceptance population of away-side jet-like correlations. Then we take the difference of two-particle correlations in the close-region and far-region with respect to the $\eta$ region of $P_x$ to subtract the anisotropic flow background. We use a toy model which contains only anisotropic flow and PYTHIA8 which have jets to demonstrate the validity of our data-driven method. The results indicate that the data-driven method can subtract anisotropic flow effectively.

Measurement of two-particle correlations with respect to second- and third-order event planes in Au + Au collisions at sNN=200 GeV

We present measurements of azimuthal correlations of charged hadron pairs in $\sqrt{s_{_{NN}}}=200$ GeV Au$+$Au collisions for the trigger and associated particle transverse-momentum ranges of $1<p_T^t<10$~GeV/$c$ and $0.5<p_T^a<10$~GeV/$c$. After subtraction of an underlying event using a model that includes higher-order azimuthal anisotropy $v_2$, $v_3$, and $v_4$, the away-side yield of the highest trigger-\pt ($p_T^t>4$~GeV/$c$) correlations is suppressed compared to that of correlations measured in $p$$+$$p$ collisions. At the lowest associated particle $p_T$ ($0.5<p_T^a<1$ GeV/$c$), the away-side shape and yield are modified relative to those in $p$$+$$p$ collisions. These observations are consistent with the scenario of radiative-jet energy loss. For the low-$p_T$ trigger correlations ($2<p_T^t<4$ GeV/$c$), a finite away-side yield exists and we explore the dependence of the shape of the away-side within the context of an underlying-event model. Correlations are also studied differentially versus event-plane angle $\Psi_2$ and $\Psi_3$. The angular correlations show an asymmetry when selecting the sign of the difference between the trigger-particle azimuthal angle and the $\Psi_2$ event plane. This asymmetry and the measured suppression of the pair yield out of plane is consistent with a path-length-dependent energy loss. No $\Psi_3$ dependence can be resolved within experimental uncertainties.

Measurements of two-particle correlations in e+e− collisions at 91 GeV with ALEPH archived data

The first measurements of two-particle angular correlations for charged particles emitted in e+e− collisions at a center-of-mass energy of 91 GeV are presented. The archived data are collected with the ALEPH detector at LEP. The correlation functions are measured over a broad range of pseudorapidity and azimuthal angle of the charged particles. Those results are compared to predictions from the PYTHIA event generator. In contrast to the results from high charged particle multiplicity nucleon-nucleon, nucleon-nucleus and nucleus-nucleus collisions, where long-range correlations with a large pseudorapidity gap are observed, no significant enhancement of long-range correlations is observed with respect to PYTHIA predictions, which do not include additional final state interactions of the outgoing partons.

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.

Constraints on rapidity-dependent initial conditions from charged-particle pseudorapidity densities and two-particle correlations

We study the initial three-dimensional spatial configuration of the quark-gluon plasma produced in relativistic heavy-ion collisions using centrality and rapidity-dependent measurements of charged particle pseudorapidity densities and two-particle correlations. A cumulant-generating function is used to parametrize the rapidity dependence of local entropy deposition and extend arbitrary boost-invariant initial conditions to nonzero beam rapidities. The model is compared to p+Pb and Pb+Pb single-particle distributions and systematically optimized using Bayesian parameter estimation to extract high-probability initial condition parameters. The optimized initial conditions are then compared to a number of experimental observables including two-particle rapidity correlations, the rapidity dependence of anisotropic flow, and event-plane decorrelations.

Multiplicity dependence of jet-like two-particle correlations in pp collisions at [formula omitted] and 13 TeV with ALICE

Two-particle correlations in relative azimuthal angle (Δϕ) and pseudorapidity (Δη) have been used to study heavy-ion collision dynamics, including medium-induced jet modification. Further investigations also showed the importance of Multiple Parton Interactions (MPI) in high-multiplicity pp collisions, which are often described by pQCD-inspired models. In these proceedings the latest ALICE measurements of two-particle correlations as a function of multiplicity in pp collisions are presented using the data from Run I and Run II at the LHC.

Measurements of long-range azimuthal anisotropies and associated Fourier coefficients for pp collisions at s=5.02 and 13 TeV and p+Pb collisions at sNN=5.02 TeV with the ATLAS detector

ATLAS measurements of two-particle correlations are presented for $\sqrt{s} = 5.02$ and $13$ TeV $pp$ collisions and for $\sqrt{s_{\mathrm{NN}}} = 5.02$ TeV $p$+Pb collisions at the LHC. The correlation functions are measured as a function of relative azimuthal angle $\Delta \phi$, and pseudorapidity separation $\Delta \eta$, using charged particles detected within the pseudorapidity interval $|\eta|{<}2.5$. Azimuthal modulation in the long-range component of the correlation function, with $|\Delta\eta|{>}2$, is studied using a template fitting procedure to remove a "back-to-back" contribution to the correlation function that primarily arises from hard-scattering processes. In addition to the elliptic, $\cos{(2\Delta\phi)}$, modulation observed in a previous measurement, the $pp$ correlation functions exhibit significant $\cos{(3\Delta\phi)}$ and $\cos{(4\Delta\phi)}$ modulation. The Fourier coefficients $v_{n,n}$ associated with the $\cos{(n\Delta\phi)}$ modulation of the correlation functions for $n =$ $2$-$4$ are measured as a function of charged-particle multiplicity and charged-particle transverse momentum. The Fourier coefficients are observed to be compatible with $\cos{(n\phi)}$ modulation of per-event single-particle azimuthal angle distributions. The single-particle Fourier coefficients $v_n$ are measured as a function of charged-particle multiplicity, and charged-particle transverse momentum for $n {=} $ $2$-$4$. The integrated luminosities used in this analysis are, $64$ $\mathrm{nb^{-1}}$ for the $\sqrt{s}=13$ TeV $pp$ data, $170$ $\mathrm{nb^{-1}}$ for the $\sqrt{s}=5.02$ TeV $pp$ data and $28$ $\mathrm{nb^{-1}}$ for the $\sqrt{s_{\mathrm{NN}}}=5.02$ TeV $p$+Pb data.

π0-hadron correlations in pp and Pb–Pb collisions and π0 elliptic flow in Pb–Pb collisions measured at the ALICE experiment

The study of the azimuthal correlation and anisotropy with π0 offers a powerful way to investigate the properties of strongly-interacting matter created in ultra-relativistic heavy-ion collisions. In this talk, we present the measurement of two-particle correlations of π0 trigger particles with associated charged particles versus azimuthal angle difference (Δφ) in pp and central Pb–Pb collisions at sNN=2.76 TeV, as well as the measurement of the elliptic flow (v2) of π0 in Pb-Pb collisions at sNN=2.76 TeV at the ALICE experiment. π0 are detected by the ALICE electromagnetic calorimeter EMCal for transverse momenta 8<pT<16 GeV/c, and charged particles are measured by the ALICE main tracking detectors ITS and TPC for 0.5<pT<10 GeV/c, both at mid-rapidity. For π0-hadron correlations, the per-trigger yield modification factors (IAA) are extracted on the near and away side and are compared with results from models and other experiments. For π0v2, the current status of measurement will be presented. Both measurements serve as important investigations of the medium modified fragmentation pattern and the path-length dependence of energy loss in QGP, as well as the corresponding physics interpretation.

Multiplicity and transverse momentum evolution of charge-dependent correlations in pp, p-Pb, and Pb-Pb collisions at the LHC

We report measurements of two-particle charge-dependent correlations in pp, p-Pb, and Pb-Pb collisions at , and 2.76 TeV, respectively, as a function of pseudorapidity and azimuthal angle differences. These measurements, carried out using the balance function, probe the charge creation time and the development of collectivity in the produced systems. The balance function is studied as a function of the event multiplicity as well as the transverse momentum (pT) of charged particles detected in the range |η| < 0.8. In the low transverse momentum region, 0.2 < pT < 2.0 GeV/c, the balance function becomes narrower in both Δη and Δφ directions in all three systems for events with higher multiplicity. For higher values of transverse momenta, the balance function becomes even narrower but exhibits no significant multiplicity dependence, indicating that the observed narrowing with increasing multiplicity at low pT is a feature of bulk particle production.

Kaon femtoscopy at the STAR experiment

The properties of the quark-gluon plasma have been extensively studied in high-energy nuclear collisions at RHIC. Femtoscopic measurements of two-particle correlations at small relative momenta reveal the space-time characteristics of the system at the moment of particle emission. In comparison to analyses using the most abundant pions, like-sign kaons provide a cleaner probe of the emission source as they less frequently result from resonance weak decays. Additionally, kaons contain strange quarks so these measurements can be sensitive to different effects and earlier collision stages. Pairs of like-sign kaons exhibit correlations due to Coulomb interactions and Bose-Einstein quantum statistics. The system of unlike-sign kaons contains a narrow φ(1020) resonance in the final state. Femtoscopic measurements have been predicted to be particularly sensitive to the source size and momentum-space correlations in the region of this resonance.In this proceedings, we present the STAR preliminary results on the like-sign and unlike-sign kaon femtoscopic correlation functions in 200 GeV Au+Au collisions at RHIC. Collision centrality and the transverse pair momentum kT dependence of the radius parameters will be discussed. The results from unlike-sign kaon correlation functions will be compared with model predictions [1].

Minimizing statistical and systematic bias in transverse momentum correlations for relativistic heavy-ion collisions

Two-particle correlation measurements and analysis are an important component of the relativistic heavy-ion physics program. In particular, particle pair-number correlations on two-dimensional transverse momentum ($p_t$) allow unique access to soft, semi-hard and hard-scattering processes in these collisions. Precise measurements of this type of correlation are essential for understanding the dynamics in heavy-ion collisions. However, transverse momentum correlation measurements are especially vulnerable to statistical and systematic biases. In this paper the origins of these large bias effects are explained and mathematical correlation forms are derived from mean-$p_t$ fluctuation quantities in the literature. Monte Carlo simulations are then used to determine the conditions, e.g. multiplicity and collision centrality bin widths, where each correlation form is minimally biased. The ranges of applicability for each correlation quantity are compared. Several are found to reproduce the assumed input correlations with reasonable fidelity over a wide range of conditions encountered in practical analysis of data.

Recent highlights from the ATLAS heavy-ion program

These proceedings report on recent measurements performed with the ATLAS Detector at the LHC presented at the Quark Matter 2015 Conference. These include measurements of two-particle correlations in pp collisions at s=2.76 and 13 TeV. These results are the first to show a second harmonic modulation in the azimuthal dependence of the particle yields in pp collisions, a signature closely associated with collective flow in ion-ion collisions. Measurements of p+Pb collisions at sNN=5.02 TeV are presented including charged particle multiplicities, Bose-Einstein correlations and yields of jets and electroweak bosons that provide insight into the initial stages of these collisions including the nuclear geometry. Finally, measurements of dijet asymmetry and jet structure in Pb+Pb and pp collisions at sNN=2.76 TeV are presented that further illuminate the phenomenon of partonic energy loss.