Charged-particle Pseudorapidity Density Research Articles

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.

Measurement of charged-particle pseudorapidity density with ALICE at LHC

The ALICE experiment, designed primarily for studying heavy-ion collisions at the Large Hadron Collider, has several features in its apparatus that allow it to contribute significantly to the first proton–proton physics. ALICE started proton–proton data taking at the end of 2009: data collected at a centre-of-mass energy of 0.9, 2.36 and 7 TeV allowed several first physics studies to be carried out. In this contribution, the first measurements of the charged-particle pseudorapidity density at the three energies will be presented. These measurements have been obtained using mainly the two innermost layers of the inner tracking system. The reconstruction algorithm and the analysis procedure are described and the results are compared to other measurements and model predictions.

First results from the ALICE experiment

We present the results from the first series of minimum-bias measurements with the ALICE detector at the CERN LHC. Charged-particle pseudorapidity density, multiplicity distribution and transverse momentum spectra were measured using data collected in 2009 and 2010 at three different centre-of-mass energies of 0.9, 2.36 and 7 TeV. The results are compared to previous proton-antiproton data and to model predictions. Besides the valuable physics outcome the first ALICE results show also the exceptional operation of both the LHC machine and the ALICE experiment.

Charged-particle multiplicity measurement in proton–proton collisions at $\sqrt{s}=7$ TeV with ALICE at LHC

The pseudorapidity density and multiplicity distribution of charged particles produced in proton-proton collisions at the LHC, at a centre-of-mass energy $\sqrt{s} = 7$ TeV, were measured in the central pseudorapidity region |$\eta$| < 1. Comparisons are made with previous measurements at $\sqrt{s}$ = 0.9 TeV and 2.36 TeV. At $\sqrt{s}$ = 7 TeV, for events with at least one charged particle in |$\eta$| < 1, we obtain dNch/deta = 6.01 $\pm$ 0.01 (stat.) $^{+0.20}_{-0.12}$ (syst.). This corresponds to an increase of 57.6% $\pm$ 0.4% (stat.) $^{+3.6}_{-1.8}$% (syst.) relative to collisions at 0.9 TeV, significantly higher than calculations from commonly used models. The multiplicity distribution at 7 TeV is described fairly well by the negative binomial distribution.

Charged-particle multiplicity measurement in proton–proton collisions at $\sqrt{s}=0.9$ and 2.36 TeV with ALICE at LHC

Charged-particle production was studied in proton–proton collisions collected at the LHC with the ALICE detector at centre-of-mass energies 0.9 TeV and 2.36 TeV in the pseudorapidity range |η|<1.4. In the central region (|η|<0.5), at 0.9 TeV, we measure charged-particle pseudorapidity density $\mathrm{d}N_{\mathrm{ch}}/\mathrm{d}\eta=3.02\pm 0.01(\mathit{stat.})^{+0.08}_{-0.05}(\mathit{syst.})$ for inelastic interactions, and $\mathrm{d}N_{\mathrm{ch}}/\mathrm{d}\eta=3.58\pm0.01(\mathit{stat.})^{+0.12}_{-0.12}(\mathit{syst.})$ for non-single-diffractive interactions. At 2.36 TeV, we find $\mathrm{d}N_{\mathrm{ch}}/\mathrm{d}\eta=3.77\pm0.01(\mathit{stat.})^{+0.25}_{-0.12}(\mathit{syst.})$ for inelastic, and $\mathrm{d}N_{\mathrm{ch}}/\mathrm{d}\eta=4.43\pm0.01(\mathit{stat.})^{+0.17}_{-0.12}(\mathit{syst.})$ for non-single-diffractive collisions. The relative increase in charged-particle multiplicity from the lower to higher energy is $24.7\%\pm0.5\%(\mathit{stat.})^{+5.7}_{-2.8}\%(\mathit{syst.})$ for inelastic and $23.7\%\pm0.5\%(\mathit{stat.})^{+4.6}_{-1.1}\%(\mathit{syst.})$ for non-single-diffractive interactions. This increase is consistent with that reported by the CMS collaboration for non-single-diffractive events and larger than that found by a number of commonly used models. The multiplicity distribution was measured in different pseudorapidity intervals and studied in terms of KNO variables at both energies. The results are compared to proton–antiproton data and to model predictions.

Particle multiplicities at energies available at the CERN Large Hadron Collider (LHC) and deviations from limiting fragmentation

The pseudorapidity density of charged particles produced at LHC collisions are predicted by using two complementary production mechanisms with a set of consistent integrated and unintegrated parton distributions. We discuss the limiting fragmentation hypothesis and its possible violation, and we compare our model with other partonic models.

Parton rescattering effect on particle production in ultra-relativistic p+p collisions

The parton rescattering effect on the charged particle production in ultra-relativistic p+p collisions is studied by the parton and hadron cascade model, PACIAE, based on PYTHIA. We have calculated charged particle pseudorapidity density ($dN_{ch}/d\eta$) at mid-rapidity and the pseudorapidity distribution in inelastic and non-single diffractive p+p collisions at $\sqrt s$=200, 900, 5500, and 14000 GeV with the PYTHIA and PACIAE models. The calculated results of $\sqrt s$=900 GeV are well compared with the ALICE data. The calculated $dN_{ch}/d\eta$ as a function of center-of-mass energy well meets with the experimental data as well. Comparing the PYTHIA results (without parton rescattering) with the PACIAE results (with parton rescattering), it turned out that the parton rescattering effect plays an important role and this effect increases with increasing center-of-mass energy.

First proton–proton collisions at the LHC as observed with the ALICE detector: measurement of the charged-particle pseudorapidity density at $\sqrt{s}=900$ GeV

On 23rd November 2009, during the early commissioning of the CERN Large Hadron Collider (LHC), two counter-rotating proton bunches were circulated for the first time concurrently in the machine, at the LHC injection energy of 450 GeV per beam. Although the proton intensity was very low, with only one pilot bunch per beam, and no systematic attempt was made to optimize the collision optics, all LHC experiments reported a number of collision candidates. In the ALICE experiment, the collision region was centred very well in both the longitudinal and transverse directions and 284 events were recorded in coincidence with the two passing proton bunches. The events were immediately reconstructed and analyzed both online and offline. We have used these events to measure the pseudorapidity density of charged primary particles in the central region. In the range |η|<0.5, we obtain dN ch/dη=3.10±0.13(stat.)±0.22(syst.) for all inelastic interactions, and dN ch/dη=3.51±0.15(stat.)±0.25(syst.) for non-single diffractive interactions. These results are consistent with previous measurements in proton–antiproton interactions at the same centre-of-mass energy at the CERN Sp \(\overline{\mathrm{p}}\) S collider. They also illustrate the excellent functioning and rapid progress of the LHC accelerator, and of both the hardware and software of the ALICE experiment, in this early start-up phase.

Particle Multiplicities in Lead-Lead Collisions at the CERN Large Hadron Collider from Nonlinear Evolution with Running Coupling Corrections

We present predictions for the pseudorapidity density of charged particles produced in central Pb-Pb collisions at the LHC. Particle production in such collisions is calculated in the framework of k(t) factorization. The nuclear unintegrated gluon distributions at LHC energies are determined from numerical solutions of the Balitsky-Kovchegov equation including recently calculated running coupling corrections. The initial conditions for the evolution are fixed by fitting Relativistic Heavy Ion Collider data at collision energies square root[sNN]=130 and 200 GeV per nucleon. We obtain dNch(Pb-Pb)/deta(square root[sNN]=5.5 TeV)/eta=0 approximately 1290-1480.

Widths of charged particle pseudorapidity density distributions in high energy [formula omitted] and AA collisions

The paper presents a fit to pseudorapidity distributions of final-state charged particles produced in hadron and heavy ion experiments at different center-of-mass energies. Distributions of charged particles measured in p p ¯ , p ( d ) A , and AA collisions at high energies are presented and good agreement between the fit and the data is found. The basis of the fits is a semi-empirical formula which is obtained from a simple Feynman gas model, namely the two-cylinder gas model. For p p ¯ and AA collisions, the dependence of the width of charged particle pseudorapidity density distributions on logarithmic center-of-mass energy is studied and found to be a linear relationship. Meanwhile, the dependences of the two-cylinder length, single cylinder length, and gap width between the two cylinders on logarithmic center-of-mass energy are found to be linear relationships. As another check and application of the semi-empirical formula, the rapidity distributions of net-protons emitted in AA collisions at high energies are studied. Similar conclusions are obtained.

Charged-particle pseudorapidity distributions in Au+Au collisions atsNN=62.4GeV

The charged-particle pseudorapidity density for Au+Au collisions at {radical}(s{sub NN})=62.4 GeV has been measured over a wide range of impact parameters and compared to results obtained at other energies. As a function of collision energy, the pseudorapidity distribution grows systematically both in height and width. The midrapidity density is found to grow approximately logarithmically between BNL Alternating Gradient Synchrotron (AGS) energies and the top BNL Relativistic Heavy Ion Collider (RHIC) energy. There is also an approximate factorization of the centrality and energy dependence of the midrapidity yields. The new results at {radical}(s{sub NN})=62.4 GeV confirm the previously observed phenomenon of ''extended longitudinal scaling'' in the pseudorapidity distributions when viewed in the rest frame of one of the colliding nuclei. It is also found that the evolution of the shape of the distribution with centrality is energy independent, when viewed in this reference frame. As a function of centrality, the total charged particle multiplicity scales linearly with the number of participant pairs as it was observed at other energies.

Energy Dependence of Directed Flow over a Wide Range of Pseudorapidity inAu+AuCollisions at the BNL Relativistic Heavy Ion Collider

We report on measurements of directed flow as a function of pseudorapidity in Au + Au collisions at energies of square root of SNN = 19.6, 62.4, 130 and 200 GeV as measured by the PHOBOS detector at the BNL Relativistic Heavy Ion Collider. These results are particularly valuable because of the extensive, continuous pseudorapidity coverage of the PHOBOS detector. There is no significant indication of structure near midrapidity and the data surprisingly exhibit extended longitudinal scaling similar to that seen for elliptic flow and charged particle pseudorapidity density.

Charge particle universal rapidity scaling in and Au + Au collisions at relativistic energies and its partonic origin

The BRAHMS and PHOBOS collaborations recently revisited the limiting fragmentation shown in the charged particle pseudorapidity density distribution in Au + Au collisions at RHIC energies. The PHOBOS collaboration especially emphasized that the observations of the charged particle universal (pseudo)rapidity scaling (limiting fragmentation) in and Au + Au collisions at relativistic energies may play an important role in the study of reaction dynamics. We use a parton and hadron cascade model, PACIAE, to investigate the charged particle universal (pseudo)rapidity scaling in the above collisions. It is pointed out that, because the universal scaling was observed in the tail region of the (pseudo)rapidity distributions, the discrepancies among distributions may not be properly visible. Therefore, this universal scaling may not have much to do with reaction dynamics, especially quark–gluon plasma. In addition, the charged particle universal scaling observed in the hadronic final state seems to arise from the partonic initial state.

The PHOBOS perspective on discoveries at RHIC

This paper describes the conclusions that can be drawn from the data taken thus far with the PHOBOS detector at RHIC. In the most central Au + Au collisions at the highest beam energy, evidence is found for the formation of a very high energy density system whose description in terms of simple hadronic degrees of freedom is inappropriate. Furthermore, the constituents of this novel system are found to undergo a significant level of interaction. The properties of particle production at RHIC energies are shown to follow a number of simple scaling behaviors, some of which continue trends found at lower energies or in simpler systems. As a function of centrality, the total number of charged particles scales with the number of participating nucleons. When comparing Au + Au at different centralities, the dependence of the yield on the number of participants at higher p T ( ∼ 4 GeV / c ) is very similar to that at low transverse momentum. The measured values of charged particle pseudorapidity density and elliptic flow were found to be independent of energy over a broad range of pseudorapidities when effectively viewed in the rest frame of one of the colliding nuclei, a property we describe as “extended longitudinal scaling”. Finally, the centrality and energy dependences of several observables were found to factorize to a surprising degree.

Transverse energy per charged particle at relativistic energies from a statistical model with expansion

Transverse energy and charged particle pseudorapidity densities at midrapidity and their ratio, $dE_{T}/d\eta\mid_{mid} / dN_{ch}/d\eta|_{mid}$, are evaluated in a statistical model with longitudinal and transverse flows for the wide range of colliders, from AGS to RHIC at $\sqrt{s_{NN}}=200$ GeV. Evaluations are done at freeze-out parameters established independently from fits to observed particle yields and $p_{T}$ spectra. Decays of hadron resonances are treated thoroughly and are included in derivations of $dE_{T}/d\eta|_{mid}$ and $dN_{ch}/d\eta|_{mid}$. The predictions of the model agree well with the experimental data. However, some (explicable) overestimation of the ratio has been observed.

Centrality Dependence of Charged-Particle Pseudorapidity Distributions fromd+AuCollisions atsNN=200 GeV

Charged-particle pseudorapidity densities are presented for the d + Au reaction at sqrt[s(NN)] = 200 GeV with -4.2 < or = eta < or = 4.2. The results, from the BRAHMS experiment at BNL Relativistic Heavy-Ion Collider, are shown for minimum-bias events and 0%-30%, 30%-60%, and 60%-80% centrality classes. Models incorporating both soft physics and hard, perturbative QCD-based scattering physics agree well with the experimental results. The data do not support predictions based on strong-coupling, semiclassical QCD. In the deuteron-fragmentation region the central 200 GeV data show behavior similar to full-overlap d+Au results at sqrt[s(NN)] = 19.4 GeV.

Pseudorapidity distribution of charged particles in d+Au collisions at sqrt[sNN]=200 GeV.

The measured pseudorapidity distribution of primary charged particles in minimum-bias d+Au collisions at sqrt[s(NN)]=200 GeV is presented for the first time. This distribution falls off less rapidly in the gold direction as compared to the deuteron direction. The average value of the charged particle pseudorapidity density at midrapidity is <dN(ch)/d eta>|eta|< or =0.6)=9.4+/-0.7(syst) and the integrated primary charged particle multiplicity in the measured region is 82+/-6(syst). Estimates of the total charged particle production, based on extrapolations outside the measured pseudorapidity region, are also presented. The pseudorapidity distribution, normalized to the number of participants in d+Au collisions, is compared to those of Au+Au and p+(-)p systems at the same energy. The d+Au distribution is also compared to the predictions of the parton saturation model, as well as microscopic models.

First results and future perspectives of the NA60 experiment

The NA60 experiment at the CERN SPS studies the production of open charm and prompt dimuons in collisions induced by proton and heavy ion beams on nuclear targets. The experimental setup includes a silicon vertex telescope, placed inside a dipole magnetic field, to match charged particle tracks with the muon tracks measured by a muon spectrometer. In p-A collisions the vertex telescope is mostly made of silicon microstrip planes, whereas in A-A interactions the higher multiplicity imposes the use of high granularity silicon pixel detectors. The high interaction rate required for the study of rare processes imposes the use of radiation tolerant pixel detectors. NA60 took its first data in the year 2002, with 400 GeV protons and with Pb ions of 20 and 30 GeV per nucleon. The proton data have been used for the study of dimuon production, while the heavy ion data were devoted to the commissioning of the pixel detectors and to the study of charged particle pseudorapidity densities. After an overview of the detector concept, this paper presents the first results obtained with the 2002 data.

Nucleon participants or quark participants?

We show that centrality dependence of charged particle pseudorapidity density at midrapidity in Au+Au collisions at RHIC is well described as proportional to We show that centrality dependence of charged particle pseudorapidity density at midrapidity in Au+Au collisions at RHIC is well described as proportional to the number of participating constituent quarks. In this approach there is no need for an additional contribution from hard processes usually considered in the models based on the number of the nucleon participants.

Charged-particle multiplicity at mid-rapidity in Au-Au collisions at relativistic heavy-ion collider

The particle density at mid-rapidity is an essential global variable for the characterization of nuclear collisions at ultra-relativistic energies. It provides information about the initial conditions and energy density reached in these collisions. The pseudorapidity densities of charged particles at mid-rapidity in Au+Au collisions at roots(NN) 130 and 200 GeV at RHIC (relativistic heavy ion collider) have been measured with the PHENIX detector. The measurements were performed using sets of wire-chambers with pad readout in the two central PHENIX tracking arms. Each arm covers one quarter of the azimuth in the pseudorapidity interval eta < 0.35. Data is presented and compared with results from proton-proton collisions and nucleus-nucleus collisions at lower energies. Extrapolations to LHC energies are discussed.