Hadron Multiplicity Research Articles

Universal hydrodynamics and charged hadron multiplicity at energies available at the CERN Large Hadron Collider

Time evolution of a ``little bang'' created in heavy-ion collisions can be divided into two phases, the pre-equilibrium and hydrodynamic. At what moment does the evolution become hydrodynamic and is there any universality in the hydrodynamic flow? To answer these questions we briefly discuss various versions of hydrodynamics and their applicability conditions. In particular, we elaborate on the idea of ``universal'' (all-order resumed) hydrodynamics and propose a simple new model for it. The model is motivated by results obtained recently via the Anti--de Sitter and conformal field correspondence. Finally, charged hadron multiplicities in heavy-ion collisions at the Relativistic Heavy Ion Collider and Large Hadron Collider are discussed. At the freeze-out, the multiplicities can be related to total entropy produced in the collision. Assuming the universal hydrodynamics to hold, we calculate the entropy production in the hydro stage of the collision. We end up speculating about a connection between the multiplicity growth and the temperature dependence of the quark-gluon plasma viscosity.

Suppression of high pT hadrons in Pb + Pb collisions at 2.76 TeV

The nuclear modification factor RAA(pT) for large pT hadrons in central Pb + Pb collisions at TeV/n is calculated within the next-to-leading order perturbative QCD parton model with medium-modified fragmentation functions and agree well with the new data. The jet transport parameter that controls medium modification is assumed to be proportional to the initial parton density and the coefficient is fixed by the RHIC data. The charged hadron multiplicity dNch/dη = 1584 ± 80 in central Pb + Pb collisions from the ALICE experiment at the LHC is used to determine both the jet transport parameter and the initial condition for (3+1)D ideal hydrodynamic evolution of the bulk matter that is employed for the calculation of RAA(pT).

New results on event-by-event ratio fluctuations in Pb+Pb collisions at CERN SPS energies

Event-by-event fluctuations of produced particle multiplicities are believed to be sensitive to a deconfinement phase transition and the critical point of strongly interacting matter. The NA49 collaboration has conducted a systematic study of various fluctuation observables. In this contribution, recent results on hadron ratio fluctuations in central Pb+Pb collisions at –17.3 GeV are reported. These results are complemented by the centrality dependence of hadron ratio fluctuations at . A universal scaling was found, describing the energy and centrality dependence of the (K+ + K−)/(π+ + π−) and ratio fluctuations purely by a change in the observed average hadron multiplicities. This scaling is broken for the fluctuations of the and K+/p ratios, possibly hinting at a change in the baryon number–strangeness correlation.

Charged hadron multiplicity and transverse energy densities in PbPb collisions from CMS

Measurements of the charged hadron multiplicity and transverse energy are presented for minimum bias PbPb collisions at a centre-of-mass energy of 2.76 TeV per nucleon pair. The number of charged hadrons was obtained by two different methods based on the silicon pixel system of the CMS detector at the LHC. The two methods are in excellent agreement, resulting a charged hadron density of 1612 ± 55 for the 5% most central collisions. For the transverse energy measurement, CMS has almost Hermetic calorimetry coverage with fine granularity and excellent resolution. In addition, for particles near central rapidity, momenta from the tracker can be combined with the calorimeter data to give a significant improvement of the system resolution. A transverse energy density of 2.1 TeV is observed for the most central 2.5% collisions. The measurements are compared with heavy-ion results from earlier experiments, where a smooth dependence on the collision energy and impact parameter is observed.

Identified particle spectra and anisotropic flow in an event-by-event hybrid approach in Pb + Pb collisions atsNN=2.76TeV

The first results from heavy ion collisions at the Large Hadron Collider for charged particle spectra and elliptic flow are compared to an event-by-event hybrid approach with an ideal hydrodynamic expansion. This approach has been shown to successfully describe bulk observables at RHIC. Without changing any parameters of the calculation the same approach is applied to Pb+Pb collisions at $\sqrt{s_{\rm NN}}=2.76$ TeV. This is an important test if the established understanding of the dynamics of relativistic heavy ion collisions is also applicable at even higher energies. Specifically, we employ the hybrid approach with two different equations of state and the pure hadronic transport approach to indicate sensitivities to finite viscosity. The centrality dependence of the charged hadron multiplicity, $p_T$ spectra and differential elliptic flow are shown to be in reasonable agreement with the ALICE data. Furthermore, we make predictions for the transverse mass spectra of identified particles and triangular flow. The eccentricities and their fluctuations are found to be surprisingly similar to the ones at lower energies and therefore also the triangular flow results are very similar. Any deviations from these predictions will indicate the need for new physics mechanisms responsible for the dynamics of heavy ion collisions.

Suppression of high-pThadrons in Pb+Pb collisions at energies available at the CERN Large Hadron Collider

Nuclear modification factor $R_{AA}(p_{T})$ for large transverse momentum pion spectra in $Pb+Pb$ collisions at $\sqrt{s}=2.76$ TeV is predicted within the NLO perturbative QCD parton model. Effect of jet quenching is incorporated through medium modified fragmentation functions within the higher-twist approach. The jet transport parameter that controls medium modification is proportional to the initial parton density and the coefficient is fixed by the RHIC data on suppression of large $p_{T}$ hadron spectra. Data on charged hadron multiplicity $dN_{ch}/d\eta=1584 \pm 80$ in central $Pb+Pb$ collisions from the ALICE Experiment at the LHC are used to constrain the initial parton density both for determining the jet transport parameter and the 3+1D ideal hydrodynamic evolution of the bulk matter that is employed for the calculation of $R_{PbPb}(p_{T})$ for neutral pions.

Multidimensional study of hadronization in nuclei

Hadron multiplicities in semi-inclusive deep-inelastic scattering were measured on neon, krypton and xenon targets relative to deuterium at an electron(positron)-beam energy of 27.6 GeV at HERMES. These ratios were determined as a function of the virtual-photon energy nu, its virtuality Q2, the fractional hadron energy z and the transverse hadron momentum with respect to the virtual-photon direction pt . Dependences were analysed separately for positively and negatively charged pions and kaons as well as protons and antiprotons in a two-dimensional representation. Compared to the one-dimensional dependences, some new features were observed. In particular, when z > 0.4 positive kaons do not show the strong monotonic rise of the multiplicity ratio with nu as exhibited by pions and negative kaons. Protons were found to behave very differently from the other hadrons.

Dependence on pseudorapidity and on centrality of charged hadron production in PbPb collisions at $ \sqrt {{{s_{\text{NN}}}}} = 2.76 $ TeV

A measurement is presented of the charged hadron multiplicity in hadronic PbPb collisions, as a function of pseudorapidity and centrality, at a collision energy of 2.76 TeV per nucleon pair. The data sample is collected using the CMS detector and a minimum-bias trigger, with the CMS solenoid off. The number of charged hadrons is measured both by counting the number of reconstructed particle hits and by forming hit doublets of pairs of layers in the pixel detector. The two methods give consistent results. The charged hadron multiplicity density dN(ch)/d eta, evaluated at eta=0 for head-on collisions, is found to be 1612 +/- 55, where the uncertainty is dominated by systematic effects. Comparisons of these results to previous measurements and to various models are also presented.

Enhancement of high P T hadrons due to collapsing Z(3) walls in heavy-ion collisions

We discuss enhancement of multiplicities of hadrons at high transverse momentum due to multiple reflections of quarks from collapsing Z(3) interfaces in the QGP produced in relativistic heavy-ion collisions. By modeling the dependence of effective mass of the quarks on the Polyakov loop order parameter, we evaluate the reflection coefficient of quarks from collapsing Z(3) interfaces. We use the effective potential proposed by Pisarski for the Polyakov loop to determine the profile of the Z(3) interfaces and calculate the reflection probability for quarks. We discuss the formation of a network of these Z(3) walls in relativistic heavy-ion collisions, in the QGP phase. We do a numerical simulation to calculate the modifications in the thermal transverse momentum spectra of the quarks/anti-quarks that results from a collapsing wall. We then use the recombination model to calculate the transverse momentum spectrum of final hadrons. Our results show enhancement of high PT hadrons, with the enhancement being stronger for heavier quarks. Further, we find that due to larger reflection coefficient for heavier quarks, the density of strange and charm quarks/anti-quarks increases inside the collapsing walls. This implies enhancement in the multiplicities of multi-strange and multi-charmed hadrons.

First CMS results from PbPb collisions

Abstract We present the first results on PbPb collisions at S N N = 2.76 TeV with the CMS experiment at the LHC. This is a huge energy jump of a factor of 14 over RHIC creating the hottest matter ever produced in a laboratory. The prime goal of this research is to study the fundamental theory of the strong interaction (QCD) in extreme conditions of temperature, density and parton momentum fraction. We give an overview of the measurements by CMS in PbPb collisions both in “soft” and “hard” regimes. The first results on the reconstruction of jets, Z and quarkonia are described. These signals are the probes of the hottest and densest phases of the reaction. Also, an overview of the bulk observables such as charged hadron multiplicity, low p T inclusive hadron identified spectra and elliptic flow which provide information on the collective properties of the system is given.

THE INTERNAL STRUCTURE OF JETS AT COLLIDERS: LIGHT AND HEAVY QUARK INCLUSIVE HADRONIC DISTRIBUTIONS

In this paper, we report our results on charged hadron multiplicities of heavy quark initiated jets produced in high energy collisions. After implementing the so-called dead cone effect in QCD evolution equations, we find that the average multiplicity decreases significantly as compared to the massless case. Finally, we discuss the transverse momentum distribution of light quark initiated jets and emphasize the comparison between our predictions and CDF data.

Gluon saturation and energy dependence of hadron multiplicity inppandAAcollisions at the LHC

The recent results in \sqrt{s}=2.76 TeV Pb+Pb collisions at the Large Hadron Collider (LHC) reported by the ALICE collaboration shows that the power-law energy-dependence of charged hadron multiplicity in Pb+Pb collisions is significantly different from p+p collisions. We show that this different energy-dependence can be explained by inclusion of a strong angular-ordering in the gluon-decay cascade within the Color-Glass-Condensate (or gluon saturation) approach. This effect is more important in nucleus-nucleus collisions where the saturation scale is larger than 1 GeV. Our prescription gives a good description of the LHC data both in p+p and Pb+Pb collisions.

Investigation of high p t events in nucleus-nucleus collisions using the Hijing event generator

In recent years lot of interest has been observed in the nucleus-nucleus collisions at RHIC energies in phenomena related to high $p_{t}$ physics \cite{ref1}. The suppression of high $p_{t}$ particles and disappearance of back-to-back jets compared to the scaling with number of binary nucleon-nucleon collisions indicates that a nearly perfect liquid is produced in these collisions. Results on self shadowing of high $p_{t}$ events are presented using hadron multiplicity associated to high $p_{t}$ and unbiased events in nucleus-nucleus collisions \cite{ref2} obtained from the hijing event generator.

Gluon shadowing and hadron production in heavy-ion collisions at LHC

The recently published first measurement of charged hadron multiplicity density at mid-rapidity dNch/dη=1584±4(stat.)±76(sys.) in central Pb+Pb collisions at s=2.76 TeV by the ALICE experiment at LHC is in good agreement with the HIJING2.0 prediction within the experimental errors and theoretical uncertainties. The new data point is used to carry out a combined fit together with the RHIC data to reduce the uncertainty in the gluon shadowing parameter sg which controls the overall magnitude of gluon shadowing at small fractional momentum x in HIJING2.0 model. Predictions on the centrality dependence of charged hadron multiplicity density at mid-rapidity with reduced uncertainties are given for Pb+Pb collisions at s=2.76 and 5.5 TeV. The centrality dependence is surprisingly independent of the colliding energy similar to that in Au+Au collisions at RHIC for most of centralities starting at Npart=50 (100) at s=2.76 (7) TeV. However, it becomes stronger in peripheral collisions at higher colliding energies.

Significant in-mediumη′mass reduction insNN = 200 GeV Au+Au collisions at the BNL Relativistic Heavy Ion Collider

In high energy heavy ion collisions a hot and dense medium is formed, where the U_A(1) or chiral symmetry may temporarily be restored. As a consequence, the mass of the eta'(958) mesons may be reduced to its quark model value, and the abundance of eta' mesons at low p_T may be enhanced by more than a factor of 10. The intercept parameter lambda_* of the charged pion Bose--Einstein correlations provides a sensitive observable of the possibly enhanced eta' abundance. We have analyzed lambda_*(m_T) data from sqrt(s(NN))=200 GeV central Au+Au reactions measured at the BNL Relativistic Heavy Ion Collider (RHIC), using extensive Monte Carlo simulations based on six popular models for hadronic multiplicities. Based on the combined STAR and PHENIX data set, and on various systematic investigations of resonance multiplicities and model parameters, we conclude that in sqrt(s(NN))=200 GeV central Au+Au reactions the mass of the eta' meson is reduced by Delta m > 200 MeV, at the 99.9% confidence level in the considered model class. Such a significant eta' mass modification may indicate the restoration of the U_A(1) symmetry in a hot and dense hadronic matter and the return of the ninth "prodigal" Goldstone boson. A similar analysis of NA44 S+Pb data at top CERN Super Proton Synchrotron (SPS) energies showed no significant in-medium eta' mass modification.

Particle production at high energy and large transverse momentum: “The hybrid formalism” revisited

We revisit the "`hybrid formalism"' for particle production used recently to study saturation effects in single hadron multiplicities at forward rapidities at RHIC and LHC. We point out that at leading twist there is an extra contribution to the formulae used so far, which corresponds to particle production via inelastic scattering of the projectile partons on the target fields. This contribution is expected to be small due to kinematics at very forward rapidities/very high transverse momenta, but should be significant at high momenta and very high energies. This contribution is expected to be most affected by saturation effects and is therefore an interesting object of study in the context of possible onset of saturation at high energies.

The heavy-ion program of the CMS experiment

Abstract We present the capabilities of the CMS experiment to explore the heavy-ion physics program of the CERN Large Hadron Collider (LHC). Collisions of lead nuclei at energies up to s N N = 5.5 TeV will probe quark and gluon matter at unprecedented values of energy density. The prime goal of this research is to study the fundamental theory of the strong interaction (QCD) in extreme conditions of temperature, density and parton momentum fraction. We give an overview of the potential of CMS to carry out a full set of representative Pb-Pb measurements both in “soft” and “hard” regimes. Measurements include “bulk” observables — charged hadron multiplicity, low- p T inclusive identified hadron spectra and elliptic flow — which provide information on the collective properties of the system; as well as perturbative processes — such as quarkonia, heavy quarks, jets, γ–jet and high p T hadrons — which yield “tomographic” information on the hottest and densest phases of the reaction. In addition, reference measurements that have been performed on early p+p collision data will be reviewed.

Hadronization in the Nuclear Media

The influence of the nuclear environment on the production of charged pions, kaons and (anti)protons in semi-inclusive deep inelastic scattering has been studied by the HERMES experiment at DESY using a 27.6 GeV positron beam. Identified hadron multiplicities have been measured for helium, neon, krypton and xenon relative to that of deuterium as a function of ν, z and \({p_{t}^{2}}\) . Dependences have been extracted in a one and two-dimensional representation, i.e. in the form of detailed binning over one variable and integrating over all other variable or three slices over the other variable. The most prominent feauteres compared to a one-dimensional analysis are changes of the ν-, \({p_{t}^{2}}\) two-dimensional dependences of ratios, in particular in case of protons. In general pions and negative kaons show similar dependences, however, positive kaons, protons and antiprotons behave quite differently.

Hadron production inp+p,p+Pb, andPb+Pbcollisions with the hijing 2.0 model at energies available at the CERN Large Hadron Collider

The Heavy-Ion Jet Interaction Generator (hijing) Monte Carlo model is updated with the latest parton distributions functions and a new set of the parameters in the two-component minijet model that controls the total $p+p$ cross section and the central pseudorapity density. We study hadron spectra and multiplicity distributions using the hijing$2.0$ model and compare to recent experimental data from $p+p$ collisions at the Large Hadron Collider (LHC) energies. We also give predictions of hadron production in $p+p$, $p+\text{Pb}$, and $\text{Pb}+\text{Pb}$ collisions at the full LHC energy.

Charged particle multiplicities in pp interactions at $ \sqrt {s} = 0.9 $ , 2.36, and 7 TeV

Measurements of primary charged hadron multiplicity distributions are presented for non-single-diffractive events in proton-proton collisions at centre-of-mass energies of sqrt(s) = 0.9, 2.36, and 7 TeV, in five pseudorapidity ranges from |eta|<0.5 to |eta|<2.4. The data were collected with the minimum-bias trigger of the CMS experiment during the LHC commissioning runs in 2009 and the 7 TeV run in 2010. The multiplicity distribution at sqrt(s) = 0.9 TeV is in agreement with previous measurements. At higher energies the increase of the mean multiplicity with sqrt(s) is underestimated by most event generators. The average transverse momentum as a function of the multiplicity is also presented. The measurement of higher-order moments of the multiplicity distribution confirms the violation of Koba-Nielsen-Olesen scaling that has been observed at lower energies.