Broad Pseudorapidity Range Research Articles

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.

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.

Pseudorapidity Dependence of Particle Production and Elliptic Flow in Asymmetric Nuclear Collisions of p+Al, p+Au, d+Au, and ^{3}He+Au at sqrt[s_{NN}]=200 GeV.

Asymmetric nuclear collisions of p+Al, p+Au, d+Au, and ^{3}He+Au at sqrt[s_{NN}]=200 GeV provide an excellent laboratory for understanding particle production, as well as exploring interactions among these particles after their initial creation in the collision. We present measurements of charged hadron production dN_{ch}/dη in all such collision systems over a broad pseudorapidity range and as a function of collision multiplicity. A simple wounded quark model is remarkably successful at describing the full data set. We also measure the elliptic flow v_{2} over a similarly broad pseudorapidity range. These measurements provide key constraints on models of particle emission and their translation into flow.

System-size dependence of the charged-particle pseudorapidity density at [formula omitted] with ALICE

We present the charged-particle pseudorapidity density in pp, p–Pb, and Pb–Pb collisions at sNN=5.02 TeV over a broad pseudorapidity range. The distributions are determined using the same experimental apparatus and methodologies, thereby minimizing systematic uncertainties, and providing clear and model-independent observations on the system-size dependence of the particle production at relativistic energies. An increase of particle production in Pb–Pb collisions near mid-rapidity, relative to pp collisions, is observed. The relative linearity in p–Pb collisions of the ratio to the smaller system indicate a coherent particle production throughout the longitudinal extend of the collision zone.

HYDRO + JETS (HYDJET+ +) event generator for Pb+Pb collisions at LHC

The Monte Carlo event generator HYDJET++ is one of the few generators, designed for the calculations of heavy-ion collisions at ultrarelativistic energies, which combine treatment of soft hydro-like processes with the description of jets traversing the hot and dense partonic medium. The model is employed to study the azimuthal anisotropy phenomena, dihadron angular correlations and event-by-event (EbyE) fluctuations of the anisotropic flow in Pb+Pb collisions at ^/snn = 2.76 TeV. The interplay of soft and hard processes describes the violation of the mass hierarchy of meson and baryon elliptic and triangular flows at pT > 2 GeV/c, the fall-off of the flow harmonics at intermediate transverse momenta, and the worsening of the number-of-constituent-quark (NCQ) scaling of elliptic/triangular flow at LHC compared to RHIC energies. The cross-talk of v2 and v3 leads to emergence of higher order harmonics in the model and to appearance of the ridge structure in dihadron angular correlations in a broad pseudorapidity range. HYDJET++ possesses also the dynamical EbyE fluctuations of the anisotropic flow. The model results agree well with the experimental data.

Centrality dependence of charged-particle pseudorapidity density distributions in Cu–Cu collisions at high energies

We investigate the centrality dependence of charged-particle pseudorapidity density distributions in Cu–Cu collisions at high energy. In a thermal model, the particle-production contributions are classified according to the different origin of sources formed in the collisions. Comparing with the experimental data measured at RHIC energies, it is found that the improved model can describe the pseudorapidity density distributions over a broad pseudorapidity range and the changes of source-location parameters depend on collision centralities.

Measurement of Long-Range Near-Side Two-Particle Angular Correlations in pp Collisions at sqrt[s]=13 TeV.

Results on two-particle angular correlations for charged particles produced in pp collisions at a center-of-mass energy of 13TeV are presented. The data were taken with the CMS detector at the LHC and correspond to an integrated luminosity of about 270 nb^{-1}. The correlations are studied over a broad range of pseudorapidity (|η|<2.4) and over the full azimuth (ϕ) as a function of charged particle multiplicity and transverse momentum (p_{T}). In high-multiplicity events, a long-range (|Δη|>2.0), near-side (Δϕ≈0) structure emerges in the two-particle Δη-Δϕ correlation functions. The magnitude of the correlation exhibits a pronounced maximum in the range 1.0<p_{T}<2.0 GeV/c and an approximately linear increase with the charged particle multiplicity, with an overall correlation strength similar to that found in earlier pp data at sqrt[s]=7 TeV. The present measurement extends the study of near-side long-range correlations up to charged particle multiplicities N_{ch}∼180, a region so far unexplored in pp collisions. The observed long-range correlations are compared to those seen in pp, pPb, and PbPb collisions at lower collision energies.

Centrality evolution of the charged-particle pseudorapidity density over a broad pseudorapidity range in Pb–Pb collisions at sNN=2.76 TeV

The centrality dependence of the charged-particle pseudorapidity density measured with ALICE in Pb-Pb collisions at $\sqrt{s_{\rm NN}}$ over a broad pseudorapidity range is presented. This Letter extends the previous results reported by ALICE to more peripheral collisions. No strong change of the charged-particle pseudorapidity density distributions with centrality is observed, and when normalised to the number of participating nucleons in the collisions, the evolution over pseudorapidity with centrality is likewise small. The broad pseudorapidity range allows precise estimates of the total number of produced charged particles which we find to range from $162\pm22$ (syst.) to $17170\pm770$ (syst.) in 80-90% and 0-5 central collisions, respectively. The total charged-particle multiplicity is seen to approximately scale with the number of participating nucleons in the collision. This suggests that hard contributions to the charged-particle multiplicity are limited. The results are compared to models which describe $\mbox{d}N_{\mbox{ch}}/\mbox{d}\eta$ at mid-rapidity in the most central Pb-Pb collisions and it is found that these models do not capture all features of the distributions.

Angular dihadron correlations as an interplay between elliptic and triangular flows

The hybrid model HYDJET++, which combines soft and hard components, is employed for the analysis of dihadron angular correlations measured in PbPb collisions at center-of-mass energy sqrt(s_NN) = 2.76 TeV. The model allows for study both individual contributions and mutual influence of lower flow harmonics, v_2 and v_3, on higher harmonics and dihadron angular correlations. It is shown that the typical structure called ridge in dihadron angular correlations in a broad pseudorapidity range could appear just as interplay of v_2 and v_3. Central, semi-central and semi-peripheral collisions were investigated. Comparison of model results with the experimental data on dihadron angular correlations is presented for different centralities and transverse momenta p_T.

Measurement of dN/dη and dET/dη in PbPb collisions with CMS

Abstract The charged particle multiplicity and transverse energy in PbPb collisions at 2.76 TeV nucleon-nucleon center-of-mass energy ( s NN ) has been measured over a broad range of pseudorapidity ( η ) and collision centrality using the CMS detector at the LHC. The transverse energy density per unit pseudorapidity ( d E T / d η ) increases faster with collision energy than the charged particle multiplicity d N / d η . This implies that the mean energy per particle is increasing with collision energy. At all pseudorapidities the multiplicity and transverse energy per participating nucleon increases with the centrality of the collision. The ratio of transverse energy per unit pseudorapidity in peripheral to central collisions varies significantly as the pseudorapidity increases from η = 0 to | η | = 5.0 .

Observation of long-range, near-side angular correlations in pPb collisions at the LHC

Results on two-particle angular correlations for charged particles emitted in pPb collisions at a nucleon–nucleon center-of-mass energy of 5.02 TeV are presented. The analysis uses two million collisions collected with the CMS detector at the LHC. The correlations are studied over a broad range of pseudorapidity, η, and full azimuth, ϕ, as a function of charged particle multiplicity and particle transverse momentum, pT. In high-multiplicity events, a long-range (2<|Δη|<4), near-side (Δϕ≈0) structure emerges in the two-particle Δη–Δϕ correlation functions. This is the first observation of such correlations in proton–nucleus collisions, resembling the ridge-like correlations seen in high-multiplicity pp collisions at s=7 TeV and in AA collisions over a broad range of center-of-mass energies. The correlation strength exhibits a pronounced maximum in the range of pT=1–1.5 GeV/c and an approximately linear increase with charged particle multiplicity for high-multiplicity events. These observations are qualitatively similar to those in pp collisions when selecting the same observed particle multiplicity, while the overall strength of the correlations is significantly larger in pPb collisions.

Measurement of the Pseudorapidity and Centrality Dependence of the Transverse Energy Density in Pb-Pb Collisions atsNN=2.76 TeV

The transverse energy (E(T)) in Pb-Pb collisions at 2.76 TeV nucleon-nucleon center-of-mass energy (√(s(NN)) has been measured over a broad range of pseudorapidity (η) and collision centrality by using the CMS detector at the LHC. The transverse energy density per unit pseudorapidity (dE(T)/dη) increases faster with collision energy than the charged particle multiplicity. This implies that the mean energy per particle is increasing with collision energy. At all pseudorapidities, the transverse energy per participating nucleon increases with the centrality of the collision. The ratio of transverse energy per unit pseudorapidity in peripheral to central collisions varies significantly as the pseudorapidity increases from η=0 to |η|=5.0. For the 5% most central collisions, the energy density per unit volume is estimated to be about 14 GeV/fm(3) at a time of 1 fm/c after the collision. This is about 100 times larger than normal nuclear matter density and a factor of 2.6 times higher than the energy density reported at √(s(NN))=200 GeV at the Relativistic Heavy Ion Collider.

Soft QCD from ATLAS and CMS

Measurements of hadron production in pp collisions by the ATLAS and CMS experiments are presented, including charged particle transverse momentum, pseudorapidity and event-by-event multiplicity distributions at sqrt(s) = 0.9, 2.36 and 7 TeV, for NSD and inelastic events. Diffraction is studied with either diffraction enriched or suppressed data samples. Total inelastic cross-section as well as gap cross-section measurements are shown. Measured spectra of identified strange hadrons, reconstructed based on their decay topology, are also discussed. Comparisons to several QCD Monte Carlo models and tunes are exhibited. Results on two-particle angular correlations over a broad range of pseudorapidity and azimuthal angle in pp collisions are presented. Underlying event activity are studied with different hard probes: tracks, trackjets, calorimeter clusters, or in Drell-Yan events.

Dihadron correlations in Pb–Pb collisions at = 2.76 TeV with CMS

Measurements of charged dihadron Δη–Δϕ correlations from the CMS collaboration are presented for Pb–Pb collisions at a center-of-mass energy of 2.76 TeV per nucleon pair over a broad range of pseudorapidity and the full range of azimuthal angle. A significant correlated yield is observed for pairs of particles with small Δϕ but large longitudinal separation Δη, commonly known as the ‘ridge’. The ridge persists up to at least |Δη| = 4 and the dependence of the ridge region shape and yield on collision centrality and transverse momentum has been measured. A Fourier analysis of the long-range two-particle correlation is presented and discussed in the context of higher order flow coefficients.

Ridge correlation structure in high-multiplicity pp collisions with CMS

Results on two-particle angular correlations are presented in proton–proton collisions at center of mass energies of 7 TeV, over a broad range of pseudorapidity and azimuthal angle. In very high-multiplicity events at 7 TeV, a pronounced structure emerges in the two-dimensional correlation function for particle pairs with intermediate pT of 1–3 GeV/c, in the kinematic region 2.0 < |Δη| < 4.8 and small Δϕ. This structure, which has not been observed in pp collisions before, is similar to what is known as the ‘ridge’ in heavy-ion collisions. It is not predicted by commonly used proton–proton Monte Carlo models and is not seen in lower multiplicity pp collisions. Updated studies of this new effect as a function of particle transverse momentum, rapidity and event characteristics are shown.

Correlations and fluctuations measured by the CMS experiment in pp and PbPb

Measurements of charged dihadron angular correlations are presented in proton–proton (pp) and lead–lead (PbPb) collisions, over a broad range of pseudorapidity and azimuthal angle, using the CMS detector at the LHC. In very high multiplicity pp events at center-of-mass energy of 7 TeV, a striking ‘ridge’-like structure emerges in the two-dimensional correlation function for particle pairs with intermediate pT of 1–3 GeV/c, in the kinematic region 2.0 < |Δη| < 4.8 and small Δϕ, which is similar to observations in heavy-ion collisions. Studies of this new effect as a function of particle transverse momentum are discussed. The long-range and short-range dihadron correlations are also studied in PbPb collision at a nucleon–nucleon center-of-mass energy of 2.76 TeV, as a function of transverse momentum and collision centrality. A Fourier analysis of the long-range dihadron correlations is presented and discussed in the context of CMS measurements of higher order flow coefficients.

Measurement of charged particle pseudorapidity density in Pb+Pb collisions at TeV with the ATLAS detector at the LHC

We present a study of charged particle pseudorapidity density distribution over a broad range of pseudorapidity and centrality in Pb+Pb collisions at TeV with the ATLAS detector at the LHC. The global shapes of multiplicity density distributions over four units of pseudorapidity from −2 to 2 are presented. Centrality and collision energy dependence of mid-rapidity density per participating nucleon pair is shown and compared to results of previous measurements.

Observation of long-range, near-side angular correlations in proton-proton collisions at the LHC

Results on two-particle angular correlations for charged particles emitted in proton-proton collisions at center-of-mass energies of 0.9, 2.36, and 7 TeV are presented, using data collected with the CMS detector over a broad range of pseudorapidity (eta) and azimuthal angle (phi). Short-range correlations in Delta(eta), which are studied in minimum bias events, are characterized using a simple "independent cluster" parametrization in order to quantify their strength (cluster size) and their extent in eta (cluster decay width). Long-range azimuthal correlations are studied differentially as a function of charged particle multiplicity and particle transverse momentum using a 980 inverse nb data set at 7 TeV. In high multiplicity events, a pronounced structure emerges in the two-dimensional correlation function for particle pairs with intermediate transverse momentum of 1-3 GeV/c, 2.0< |Delta(eta)| <4.8 and Delta(phi) near 0. This is the first observation of such a long-range, near-side feature in two-particle correlation functions in pp or p p-bar collisions.

Forward calorimeter upgrades in PHENIX: Past and future

Over the past few years, the PHENIX detector has undergone several upgrades in the forward region (1<|η|≤4). Initially covered only by the muon arms, the addition of the MPC and the future FOCAL expands on the physics capabilities of the PHENIX detector. The focus of these upgrades is toward a better understanding of the Color-Glass Condensate (CGC) and the interplay between the different components of the proton's spin valence/sea quark and gluon contributions. These proceedings highlight the latest results from the MPC related to the CGC as well the newly proposed forward calorimeter detector, FOCAL. Both detectors aim to constrain the current view of gluon saturation at small x in the Color-Glass condensate framework, through correlations using π0's (MPC and FOCAL) and isolated direct photons (FOCAL) at high-pT over a broad range of pseudorapidity.

System size dependence of cluster properties from two-particle angular correlations inCu+CuandAu+Aucollisions atsNN=200GeV

We present results on two-particle angular correlations in Cu+Cu and Au+Au collisions at a center of mass energy per nucleon pair of 200 GeV over a broad range of pseudorapidity ($\eta$) and azimuthal angle ($\phi$) as a function of collision centrality. The PHOBOS detector at RHIC has a uniquely-large angular coverage for inclusive charged particles, which allows for the study of correlations on both long- and short-range scales. A complex two-dimensional correlation structure in $\Delta \eta$ and $\Delta \phi$ emerges, which is interpreted in the context of a cluster model. The effective cluster size and decay width are extracted from the two-particle pseudorapidity correlation functions. The effective cluster size found in semi-central Cu+Cu and Au+Au collisions is comparable to that found in proton-proton collisions but a non-trivial decrease of the size with increasing centrality is observed. Moreover, a comparison between results from Cu+Cu and Au+Au collisions shows an interesting scaling of the effective cluster size with the measured fraction of total cross section (which is related to the ratio of the impact parameter to the nuclear radius, $b/2R$), suggesting a geometric origin. Further analysis for pairs from restricted azimuthal regions shows that the effective cluster size at $\Delta\phi \sim 180^{\circ}$ drops more rapidly toward central collisions than the size at $\Delta\phi \sim 0^{\circ}$. The effect of limited $\eta$ acceptance on the cluster parameters is also addressed, and a correction is applied to present cluster parameters for full $\eta$ coverage, leading to much larger effective cluster sizes and widths than previously noted in the literature. These results should provide insight into the hot and dense medium created in heavy ion collisions.