Measurements Of Two-particle Angular Correlations Research Articles

Long-range near-side correlation in e+e− collisions at 183-209 GeV with ALEPH archived data

The first measurement of two-particle angular correlations for charged particles with LEP-II data is presented. The study is performed using archived hadronic e+e− data collected by ALEPH at center-of-mass energies up to 209 GeV, above the W+W− production threshold, which provide access to unprecedented charged-particle multiplicities and more complex color-string configurations if compared to previous measurements at LEP-I energies. An intriguing long-range near-side excess is observed in the correlation function measured with respect to the thrust axis in the highest multiplicity interval (Ntrk≥50). Such a structure is not predicted by the Monte-Carlo simulation. The harmonic anisotropy coefficients vn, which result from the Fourier expansion of the two-particle correlation functions, were also measured for the first time in e+e− data, and compared to pythia 6 predictions and to the results obtained in proton-proton collisions. The results presented in the Letter provide novel experimental constraints on the formation of collective phenomena in point-like e+e− collisions.

Measurement of two-particle correlations and flow coefficients in high multiplicity e+e− collisions using archived ALEPH data at 91-209 GeV

We present measurements of two-particle angular correlations of charged particles emitted in high-energy collisions using data collected by the ALEPH detector at LEP between 1992 to 2000. The correlation functions are measured over a wide range of pseudorapidity and azimuthal angle as a function of charged particle multiplicity. Previous measurement with LEP1 data at √S = 91 GeV shows no significant long-range correlations in either lab coordinate or thrust coordinate analyses, with associated yield distributions in agreement with predictions from the PYTHIA v6.1 event generator. The use of higher collision energy LEP2 data allows access to not only higher event multiplicity but also additional production channels beyond the e+e- → y* / Z → q¯q process. Notably, the highest multiplicity bin (Ntrk ≥ 50) suggests a tantalizing disagreement with MC and implies the potential to search for collective phenomena in small systems. This measurement is pushing the studies of long-range correlation to the smallest collision system limit and includes the first flow coefficient (vn) measurement in e+e− collisions, which uses a Fourier decomposition analysis to quantify the anisotropy in the azimuthal two-particle correlation as a function of charged particles’ transverse momentum. This work supplements our understanding of small-system references to long-range correlations observed in proton-proton, proton-nucleus, and nucleus-nucleus collisions.

Measurements of Two-Particle Correlations in e^{+}e^{-} Collisions at 91GeV with ALEPH Archived Data.

Measurements of two-particle angular correlations of charged particles emitted in hadronic Z decays are presented. The archived e^{+}e^{-} annihilation data at a center-of-mass energy of 91GeV were collected with the ALEPH detector at LEP between 1992 and 1995. The correlation functions are measured over a broad range of pseudorapidity and full azimuth as a function of charged particle multiplicity. No significant long-range correlation is observed in either the lab coordinate analysis or the thrust coordinate analysis, where the latter is sensitive to a medium expanding transverse to the color string between the outgoing qq[over ¯] pair from Z boson decays. The associated yield distributions in both analyses are in better agreement with the prediction from the pythia v6.1 event generator than from herwig v7.1.5. They provide new insights to showering and hadronization modeling. These results serve as an important reference to the observed long-range correlation in proton-proton, proton-nucleus, and nucleus-nucleus collisions.

Study of Angular Correlations in Monte Carlo Simulations in Pb-Pb Collisions

In two-particle angular correlation measurements, the distribution of charged hadron pairs are evaluated as a function of pseudorapidity ( Δ η ) and azimuthal ( Δ φ ) differences. In these correlations, jets manifest themselves as a near-side peak around Δ η = 0 , Δ φ = 0 . These correlations can be used to extract transverse momentum ( p T ) and centrality dependence of the shape of the near-side peak in Pb-Pb collision. The shape of the near-side peak is quantified by the variances of the distribution. The variances are evaluated from a fit combining the peak and the background. In this contribution, identified and unidentified angular correlations are shown from Pb-Pb collisions at s N N = 2.76 TeV from Monte Carlo simulations (AMPT, PYTHIA 8.235/Angantyr). Results show that transport models in AMPT give better results than PYTHIA 8.235/Angantyr when comparing to the experimental results of the ALICE collaboration.

Azimuthal Anisotropy of Heavy-Flavor Decay Electrons in p-Pb Collisions at sqrt[s_{NN}]=5.02 TeV.

Angular correlations between heavy-flavor decay electrons and charged particles at midrapidity (|η|<0.8) are measured in p-Pb collisions at sqrt[s_{NN}]=5.02 TeV. The analysis is carried out for the 0%-20% (high) and 60%-100% (low) multiplicity ranges. The jet contribution in the correlation distribution from high-multiplicity events is removed by subtracting the distribution from low-multiplicity events. An azimuthal modulation remains after removing the jet contribution, similar to previous observations in two-particle angular correlation measurements for light-flavor hadrons. A Fourier decomposition of the modulation results in a positive second-order coefficient (v_{2}) for heavy-flavor decay electrons in the transverse momentum interval 1.5<p_{T}<4 GeV/c in high-multiplicity events, with a significance larger than 5σ. The results are compared with those of charged particles at midrapidity and those of inclusive muons at forward rapidity. The v_{2} measurement of open heavy-flavor particles at midrapidity in small collision systems could provide crucial information to help interpret the anisotropies observed in such systems.

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.

Measurement of emission-angle anisotropy via long-range angular correlations with high- pT hadrons in d+Au and p+p collisions at sNN=200 GeV

We present measurements of two-particle angular correlations between high-transverse-momentum ($2<p_T<11$ GeV/$c$) $\pi^0$ observed at midrapidity ($|\eta|<0.35$) and particles produced either at forward ($3.1<\eta<3.9$) or backward ($-3.7<\eta<-3.1$) rapidity in $d$$+$Au and $p$$+$$p$ collisions at $\sqrt{s_{_{NN}}}=200$ GeV. The azimuthal angle correlations for particle pairs with this large rapidity gap in the Au-going direction exhibit a ridge-like structure that persists up to $p_T{\approx}6$ GeV/$c$ and which strongly depends on collision centrality, which is a similar characteristic to the hydrodynamical particle flow in A+A collisions. The ridge-like structure is absent in the $d$-going direction as well as in $p$$+$$p$ collisions, in the transverse-momentum range studied. The results indicate that the ridge-like structure is shifted in the Au-going direction toward more central collisions, similar to the charged-particle pseudorapidity distributions.

Anomalous Evolution of the Near-Side Jet Peak Shape in Pb-Pb Collisions at sqrt[s_{NN}]=2.76 TeV.

The measurement of two-particle angular correlations is a powerful tool to study jet quenching in a p_{T} region inaccessible by direct jet identification. In these measurements pseudorapidity (Δη) and azimuthal (Δφ) differences are used to extract the shape of the near-side peak formed by particles associated with a higher p_{T} trigger particle (1<p_{T,trig}<8 GeV/c). A combined fit of the near-side peak and long-range correlations is applied to the data allowing the extraction of the centrality evolution of the peak shape in Pb-Pb collisions at sqrt[s_{NN}]=2.76 TeV. A significant broadening of the peak in the Δη direction at low p_{T} is found from peripheral to central collisions, which vanishes above 4 GeV/c, while in the Δφ direction the peak is almost independent of centrality. For the 10% most central collisions and 1<p_{T,assoc}<2 GeV/c, 1<p_{T,trig}<3 GeV/c a novel feature is observed: a depletion develops around the center of the peak. The results are compared to pp collisions at the same center of mass energy and ampt model simulations. The comparison to the investigated models suggests that the broadening and the development of the depletion is connected to the strength of radial and longitudinal flow.

Evolution of the longitudinal and azimuthal structure of the near-side jet peak in Pb-Pb collisions at sNN=2.76 TeV

In two-particle angular correlation measurements, jets give rise to a near-side peak, formed by particles associated to a higher $p_{\mathrm{T}}$ trigger particle. Measurements of these correlations as a function of pseudorapidity ($\Delta\eta$) and azimuthal ($\Delta\varphi$) differences are used to extract the centrality and $p_{\mathrm{T}}$ dependence of the shape of the near-side peak in the $p_{\mathrm{T}}$ range $1 < p_{\mathrm{T}} <$ 8 GeV/c in Pb-Pb and pp collisions at $\sqrt{s_{\mathrm{NN}}}$ = 2.76 TeV. A combined fit of the near-side peak and long-range correlations is applied to the data and the peak shape is quantified by the variance of the distributions. While the width of the peak in the $\Delta\varphi$ direction is almost independent of centrality, a significant broadening in the $\Delta\eta$ direction is found from peripheral to central collisions. This feature is prominent for the low $p_{\mathrm{T}}$ region and vanishes above 4 GeV/c. The widths measured in peripheral collisions are equal to those in pp in the $\Delta\varphi$ direction and above 3 GeV/c in the $\Delta\eta$ direction. Furthermore, for the 10\% most central collisions and $1 < p_{\mathrm{T, assoc}} <$ 2 GeV/c, $1 < p_{\mathrm{T, trig}} <$ 3 GeV/c a departure from a Gaussian shape is found: a depletion develops around the centre of the peak. The results are compared to AMPT model simulations as well as other theoretical calculations indicating that the broadening and the development of the depletion is connected to the strength of radial and longitudinal flow.

Near-side jet peak broadening in Pb–Pb collisions at [formula omitted

Two-particle angular correlation measurements are sensitive probes of the interactions of particles with the medium formed in heavy-ion collisions. Such measurements are done by determining the distribution of the relative pseudo-rapidity (Δη) and azimuthal angle (Δϕ) of particles with respect to a higher pT trigger particle (1<pT,trig<8 GeV/c). The near-side peak is fitted with a function, which includes both the near-side jet peak and also accounts for the Δη-independent long-range correlations. The centrality evolution of the width (variance) of the fitted distribution is investigated. In Pb–Pb collisions a significant broadening of the near-side peak in the Δη direction is observed from peripheral to central collisions, while in the Δφ direction the peak is almost independent of centrality. For the 10% most central events, a departure from the Gaussian shape is also observed at low transverse momentum (1<pT,assoc<2 GeV/c,1<pT,trig<3 GeV/c). In this contribution the results obtained by the ALICE experiment in Pb–Pb and pp collisions at sNN=2.76 TeV are shown, and they are interpreted in terms of radial and elliptic flow by comparing them to AMPT model simulations.

Azimuthal anisotropy harmonics from long-range correlations in high multiplicity pp collisions at CMS

Measurements of two-particle angular correlations in pp collisions at s=7 TeV are presented as a function of charged-particle multiplicities. The data, corresponding to an integrated luminosity of about 6.2 pb−1, were collected during the 2010 LHC pp run using the CMS detector. In high-multiplicity events, a long-range (|Δη|>2), near-side (Δϕ≈0) structure is found in the two-particle Δη−Δϕ correlation functions. The second-order (v2) and third-order (v3) azimuthal anisotropy harmonics of charged particles, KS0 and Λ/Λ‾ particles are extracted from long-range two-particle correlations as a function of particle multiplicity and transverse momentum, after correcting for the contribution of back-to-back jet correlations. A v2 and v3 value of about 4% and 1%, averaging over 0.3<pT<3 GeV/c, are observed in the high-multiplicity region, and are found to be smaller than values obtained in pPb and PbPb collisions at similar multiplicities.

Forward-central two-particle correlations in p–Pb collisions with ALICE at the LHC

We present the results of measurements of two-particle angular correlations between trigger particles reconstructed in the ALICE Forward Muon Spectrometer (−4.0<η<−2.5) and associated particles reconstructed in the central barrel detectors (|η|<1.0) in p–Pb collisions at a nucleon-nucleon center-of-mass energy of 5.02 TeV. At low transverse momentum (pT), the reconstructed trigger particles mainly originate from weak decays of primary pions and kaons, whereas at high pT, they originate mainly from the decay of heavy-flavor particles. The ridge structure elongated in the η-direction, discovered recently in p–Pb collisions, is found to persist to the pseudorapidity ranges studied here. The second-order Fourier coefficients of muons are extracted after subtracting the correlations obtained in low-multiplicity events from those in high-multiplicity events. The ratio of coefficients in the Pb-going and p-going directions is presented as a function of pT, and the coefficients are observed to have a similar pT dependence in both directions with the Pb-going coefficients larger by 16±6%. The results are compared with calculations from a multi-phase transport model.

Long-range two-particle correlations of strange hadrons with charged particles in pPb and PbPb collisions at LHC energies

Measurements of two-particle angular correlations between an identified strange hadron (KS0 or Λ/Λ‾) and a charged particle, emitted in pPb collisions, are presented over a wide range in pseudorapidity and full azimuth. The data, corresponding to an integrated luminosity of approximately 35 nb−1, were collected at a nucleon–nucleon center-of-mass energy (sNN) of 5.02 TeV with the CMS detector at the LHC. The results are compared to semi-peripheral PbPb collision data at sNN=2.76 TeV, covering similar charged-particle multiplicities in the events. The observed azimuthal correlations at large relative pseudorapidity are used to extract the second-order (v2) and third-order (v3) anisotropy harmonics of KS0 and Λ/Λ‾ particles. These quantities are studied as a function of the charged-particle multiplicity in the event and the transverse momentum of the particles. For high-multiplicity pPb events, a clear particle species dependence of v2 and v3 is observed. For pT<2 GeV, the v2 and v3 values of KS0 particles are larger than those of Λ/Λ‾ particles at the same pT. This splitting effect between two particle species is found to be stronger in pPb than in PbPb collisions in the same multiplicity range. When divided by the number of constituent quarks and compared at the same transverse kinetic energy per quark, both v2 and v3 for KS0 particles are observed to be consistent with those for Λ/Λ‾ particles at the 10% level in pPb collisions. This consistency extends over a wide range of particle transverse kinetic energy and event multiplicities.

Long range two-particle correlations with Ks0 and Λ in pPb collisions

Abstract Measurements of two-particle angular correlations between identified strange hadrons ( K s 0 and Λ / Λ ¯ ) and unidentified charged particles, produced in pPb collisions, are presented over a wide range in pseudorapidity and full azimuth. Results are based on an integrated luminosity of approximately 35 nb − 1 collected during the 2013 LHC pPb run at a nucleon–nucleon center-of-mass energy of 5.02 TeV by the CMS experiment. K s 0 and Λ / Λ ¯ are reconstructed via the topology of their secondary decays. The observed long-range correlations of K s 0 and Λ / Λ ¯ are characterized by the second-order ( v 2 ) and third-order ( v 3 ) anisotropy harmonics, which are presented as a function of particle multiplicity and transverse momentum. Particle-species dependence of v 2 and v 3 is observed in a wide range of multiplicities in pPb events. Constituent quark scaling is observed within 10–15% for each multiplicity class for the v 2 and v 3 harmonics of K s 0 and Λ / Λ ¯ as a function of kinetic energy per quark.

Hadron Correlations Measured with ALICE

Angular particle correlations are a powerful tool to study collective effects and in-medium jet modification as well as their interplay in the hot and dense medium produced in central heavy-ion collisions. We present measurements of two-particle angular correlations of inclusive charged and identified particles performed with the ALICE detector. The near-side peak in the short-range correlation region is quantitatively analyzed: while the rms of the peak in $\phi$-direction is independent of centrality within uncertainties, we find a significant broadening in $\eta$-direction from peripheral to central collisions. The particle content of the near-side peak is studied finding that the $p/\pi$ ratio of particles associated to a trigger particle is much smaller than the one in the bulk of the particles and consistent with fragmentation of a parton in vacuum.

Measurement of inclusive two-particle angular correlations in pp collisions with the ATLAS detector at the LHC

Abstract We present a measurement of two-particle angular correlations in proton- proton collisions at $ \sqrt {s} = 900 $ GeV and 7 TeV. The collision events were collected during 2009 and 2010 with the ATLAS detector at the Large Hadron Collider using a single-arm minimum bias trigger. Correlations are measured for charged particles produced in the kinematic range of transverse momentum p T &gt; 100 MeV and pseudorapidity |η| &lt; 2.5. A complex structure in pseudorapidity and azimuth is observed at both collision energies. Results are compared to pythia 8 and herwig++ as well as to the AMBT2B, DW and Perugia 2011 tunes of pythia 6. The data are not satisfactorily described by any of these models.

Nonflow correlations in a cluster model

We derive analytical forms for nonflow contributions from cluster correlation to the measurement of two-particle elliptic flow (v2{2}). We estimate nonflow contribution from rho->pi+pi decays and find it is negative but not a major contributor to the nonflow effect in v2{2}. We also estimate nonflow contribution from the recent STAR measurement of two-particle angular correlations.

Effects of cluster particle correlations on local parity violation observables

We investigate effects of cluster particle correlations on two- and three-particle azimuth correlator observables sensitive to local strong parity violation. We use two-particle angular correlation measurements as input and estimate the magnitudes of the effects with straightforward assumptions. We found that the measurements of the azimuth correlator observables by the STAR experiment can be entirely accounted for by cluster particle correlations together with a reasonable range of cluster anisotropy in non-peripheral collisions. Our result suggests that new physics, such as local strong parity violation, may not be required to explain the correlator data.

Two-particle correlations in e+e− annihilation

We present measurements of two-particle angular correlations in hadron jets produced in e+e− annihilation between 7.7 and 31.6 GeV c.m. energy. The data are compared to predictions of high order perturbative QCD calculations.