Multiplicity dependence of <Emphasis Type="Italic">z</Emphasis> scaling for identified hadrons

The multiplicity dependence of inclusive spectra of identified hadrons (π, K, , K _S ⁰ , and Λ) produced in high-energy p and pp collisions is studied. Analysis of experimental data on high-p _T hadron cross sections obtained at the Tevatron and RHIC is performed in the framework of z scaling. Independence of the scaling function ψ(z) on the multiplicity density dN _ch/dη is established. The multiplicity density measured in nucleon-nucleon collisions reaches much higher values than the corresponding quantity [1/(0.5N _p)]dN _ch/dη obtained in heavy-ion collisions. Its influence on particle production reveals properties of self-similarity and fractality. The relation between the scaling variable z and thermodynamic quantities (“heat capacity” and “temperature”) of the medium associated with the produced inclusive particle is discussed. Search for z-scaling violation in p and pp collisions at very high multiplicities as an indication of new-physics phenomena is suggested. The text was submitted by the authors in English.     

PHOBOS at RHIC: Some global observations

Particle production in Au+Au collisions has been measured in the PHOBOS experiment at RHIC for a range of collision energies for a large span of pseudorapidities, |η| < 5.4. Three empirical observations have emerged from this data set which require theoretical examination. First, there is clear evidence of limiting fragmentation. Namely, particle production in central Au + Au collisions, when expressed as dN/dη′ ( η′ ≡ – y_beam), becomes energy independent at high energy for a broad region of η′ around η′ = 0. This energy-independent region grows with energy, allowing only a limited region (if any) of longitudinal boost-invariance. Second, there is a striking similarity between particle production in e⁺e⁻and Au + Au collisions (scaled by the number of participating nucleon pairs). Both the total number of produced particles and the longitudinal distribution of produced particles are approximately the same in e⁺e⁻and in scaled Au + Au. This observation This presentation is based in large part on the PHOBOS summary talk by M Baker at the16th Int. Conf. on Ultrarelativistic Nucleus- Nucleus Collisions, Quark Matter 2002, Nantes, France was not predicted and has not been explained. Finally, particle production has been found to scale approximately with the number of participating nucleon pairs for (N _part ) > 65. This scaling occurs both for the total multiplicity and for highp _T particles (3 <p _T < 4.5 GeV/c). This presentation is based in large part on the PHOBOS summary talk by M Baker at the16th Int. Conf. on Ultrarelativistic Nucleus-Nucleus Collisions, Quark Matter 2002, Nantes, France     

Scaling features of charged-hadron spectra in AA collisions at RHIC in z presentation

Results of analysis of experimental data on inclusive spectra of charged hadrons produced in heavy-ion collisions at RHIC are shown in z presentation. The data indicate similarity as a characteristic feature of mechanism of hadron production at high energies. It is argued that this property includes structure of the colliding objects, interaction of their constituents, and character of the fragmentation process. Relation of the scaling variable z with the entropy and “specific heat” of the interacting system is discussed. A microscopic scenario of the constituent interactions connected with the momentum distribution of hadrons in the final state is suggested. We estimate energy losses of particles in the produced matter characterized by the multiplicity density dN _ch/dη. The text was submitted by the authors in English.     

z-scaling in heavy ion collisions at the RHIC

Experimental data on transverse particle spectra obtained by the STAR, PHENIX, PHOBOS, and BRAHMS collaborations at the RHIC are analyzed in the framework of the generalized concept of z-scaling. It was developed for analysis of inclusive particle production in proton-(anti)proton collisions at high p _T and high multiplicities. The general scheme of the approach based on the physical principles of self-similarity, locality, and fractality is reviewed. Independence of the scaling function ψ(z) from energy, multiplicity, and atomic weight for h ^±, π ^±,0, K _S ⁰ , and Λ hadrons produced in Au-Au and Cu-Cu collisions at √s = 130 and 200 GeV is discussed. Based on z-scaling, the multiplicity dependence of pion transverse spectra up to p _T = 25 GeV/c in Au-Au collisions at √s = 200 GeV for experiments at the RHIC is predicted. The text was submitted by the author in English.     

Identification and rejection of fake reconstructed jets from a fluctuating heavy ion background in ATLAS

Full jet reconstruction in relativistic heavy ion collisions provides new and unique insights to the physics of parton energy loss. Because of the large underlying event multiplicity in A+A collisions, random and correlated fluctuations in the background can result in the reconstruction of fake jets. These fake jets must be identified and rejected to obtain the purest jet sample possible. A large but reducible fake rate of jets reconstructed using an iterative cone algorithm on HIJING events is observed. The absolute rate of fake jets exceeds the binary-scaled p+p jet rate below 50 GeV and is not negligible until 100 GeV. The variable Σj _T , the sum of the jet constituent’s E _T perpendicular to the jet axis, is introduced to identify and reject fake jets at by a factor of 100 making it negligible. This variable is shown to not strongly depend on jet energy profiles modified by energy loss. By studying azimuthal correlations of reconstructed di-jets, the fake jet rate can be evaluated in data. All results presented use modified versions of ATLAS software and should be considered “ATLAS preliminary”.     

Systematic study of photon production at forward rapidity in STAR

We present the measurement of multiplicity and pseudorapidity distributions of photons production at forward rapidity (−2.3 ≤ η ≤ −3.8) for Cu + Cu collisions at $ \sqrt {S_{NN} } $ \sqrt {S_{NN} } = 62.4 and 200 GeV. Photons are measured using the Photon Multiplicity Detector (PMD) in STAR experiment at RHIC. The distributions of (η−η _beam), where η _beam is beam rapidity is presented. Pseudorapidity distributions of photons at forward rapidities, when ob-served in the frame of one of the colliding particles show the signature of energy independent scaling at forward rapidities known as Limiting Fragmentation. Measurement of inclusive photons reflect the distribution for π ⁰ as most of the photons come from π ⁰ decay. Gluon saturation at initial conditions of the collisions is one of the possible explanations for this scaling. Study of system-size and incident energy dependence of this phenomenon help us understand the mechanisms better. The system-size and energy dependence of the widths of scaling in pseudorapidity and comparison with model predictions are presented.     

High-pT spectra of charged hadrons in Au + Au collisions at √sNN = 9.2 GeV in STAR

The production of hadrons in heavy-ion collisions at high-p _T provides an important information on mechanism of particle formation and constituent energy loss in medium. Such information is needed for search of a Critical Point and signatures of phase transition. Measurements by the STAR Collaboration of charged hadron production in Au + Au collisions at √s _NN = 9.2 GeV over a wide transverse momentum p _T = 0.2−4 GeV/c and at mid-rapidity range are reported. It allows for a first measurement of the spectra for charged hadrons at high p _T at this energy. The spectra demonstrate the dependence on centrality which enhances with p _T . The constituent energy loss and its dependence on transverse momentum of particle, and centrality of collisions are estimated in the z-scaling approach.     

Azimuthal correlations with high-pT multi-hadron cluster triggers in Au+Au collisions at ?{sNN } \sqrt {s_{NN} } = 200 GeV from STAR

Di-hadron correlation measurements have been used to probe di-jet production in heavy ion collisions at RHIC. A strong suppression of the away-side high-p _T yield in these measurements is direct evidence that high-p _T partons lose energy as they traverse the strongly interacting medium. However, since the momentum of the trigger particle is not a good measure of the jet energy, azimuthal di-hadron correlations have limited sensitivity to the shape of the fragmentation function. We explore the possibility to better constrain the initial parton energy by using clusters of multiple high-p _T hadrons in a narrow cone as the ‘trigger particle’ in the azimuthal correlation analysis. We present first results from this analysis of multi-hadron triggered correlated yields in Au+Au collisions at $ \sqrt {s_{NN} } $ \sqrt {s_{NN} } = 200 GeV from STAR. The results are compared with measurements in d + Au collisions and Pythia calculations, and the implications for energy loss and jet fragmentation are discussed.     

Recent results from PHOBOS on particle production at high p T

A selection of experimental results from the PHOBOS Collaboration relevant for probing high-energy nuclear collisions with high transverse momentum particles is presented. The inclusive yields of charged particles and comparisons between nuclear and elementary collisions already reveal a large amount of parton energy loss in the hot and dense medium created in heavy ion collisions. Remarkable scaling and factorization features are observed, unifying the data taken at various collision energies, centralities and nuclear sizes. To further analyze the nature of the energy loss, a measurement of pseudorapidity (Δη) and azimuthal angle (Δφ) correlations between high transverse momentum charged hadrons (p _T >2.5 GeV/c) and all associated charged particles is presented at both short-range (small Δη) and long-range (large Δη) over a continuous detector acceptance covering −4<Δη<2. Various near- and away-side features of the correlation structure are discussed as a function of centrality in Au + Au collisions at  GeV. The results provide new information about the longitudinal (Δη) extent of the near-side ‘ridge’ structure, first observed by the STAR Collaboration over a narrower η range. In central Au + Au collisions the ridge structure extends to at least Δη=4, and its strength completely diminishes as collisions become more peripheral. Presenter of the paper on the 3rd International Conference on Hard and Electromagnetic Probes of High-Energy Nuclear Collisions, 8–14 June 2008, Illa da Toxa (Galicia-Spain).     

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 AuAu collisions at √^s _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 |η| < 035. Data is presented and compared with results from proton-proton collisions and nucleus-nucleus collisions at lower energies. Extrapolations to LHC energies are discussed.     

Cosmic-ray Monte Carlo predictions for forward particle production in p-p, p-Pb, and Pb-Pb collisions at the LHC

We present and compare the predictions of various cosmic-ray Monte Carlo models for the energy (dE/dη) and particle (dN/dη) flows in p-p, p-Pb and Pb-Pb collisions at $ \sqrt {s_{NN} } $ \sqrt {s_{NN} } = 14, 8.8, and 5.5 TeV respectively, in the range covered by forward LHC detectors like CASTOR or TOTEM (5.2 < |η|<6.6) and ZDC or LHCf (|η| ≥ 8.1 for neutrals).     

Surface versus volume emissions in photon–hadron correlations

High-p _T photon–hadron correlations are studied within the next-to-leading order (NLO) perturbative QCD parton model with modified parton-jet fragmentation functions due to jet quenching in high-energy A+A collisions. In central A+A collisions, the away-side hadrons with large z _T=p _T ^h /p _T ^γ are controlled mainly by the surface emission of the gamma-jet events, while a small z _T region will be volume emission bias. In other words, gamma jets for a small-z _T region probe the dense matter deeper than those gamma jets for a large-z _T region, so the small-z _T gamma jets are found to be slightly more sensitive to the properties of the dense matter than the large-z _T gamma jets.     

Measurement of π0 and η mesons with PHENIX in ?[`(sNN )]\surd \overline {s_{NN} } 200 GeV Au+Au collisions at RHIC

The π ^o meson has been a crucial probe for observing jet quenching in ultrarelativistic heavy-ion collisions at RHIC. Measurements of the η meson in the same collisions have also shed light on a possible dependence of the observed suppression on the particle species. The preliminary π ⁰ nuclear modification factor R _AA from the 2004 RHIC run allowed a first systematic comparison between a precise measurement with high statistics and theoretical calculations, constraining model parameters such as the initial gluon density dN^g/dy, and the transport coefficient [^(q)]\hat q. The final π ⁰ spectra and R _AA are shown as well as the first η results obtained with both PHENIX electromagnetic calorimeters.     

Neutral-meson production in <Emphasis Type="Italic">pp</Emphasis> collisions at RHIC and QCD test of <Emphasis Type="Italic">z</Emphasis> scaling

New experimental data on inclusive cross section of neutral-vector-meson (ω ⁰, ϕ, K _S ⁰) production in proton-proton collisions at = 200 GeV obtained at RHIC are analyzed in the framework of z scaling. Properties of z-presentation are used to predict hadron yields over a wide range of transverse momentum p _T and collisions energy . The next-to-leading-order perturbative QCD calculations of inclusive cross sections of π ⁰ mesons produced in pp and collisions over a wide range of and p _T are compared with ISR, S pS, RHIC data and predictions of z scaling. The dependence of the spectra in p _T and z presentations for different parton distribution and fragmentation functions is studied. The sensitivity of the obtained results to the choice of the renormalization (μ _R ), initial-state factorization (μ _F ), and final-state factorization (fragmentation) (μ _H ) scales is verified. It is shown that self-similar features of particle production dictated by the z scaling give strong restriction on the asymptotic behavior of the inclusive spectra in high-p _T region. The text was submitted by the author in English.     

A four component picture for jet induced correlations in heavy ion collisions?

These proceedings review recent results from two and three particle correlations in heavy ion collisions. In particular we discuss the modified structure of the away side jet correlations. Under the assumption that the away side can be decomposed into a punch through component at Δ φ=π and a shoulder component with a peak displaced from π many similar properties are observed between the ridge and shoulder. The particle ratios, yields and p _T spectra are in near agreement. We also highlight important future measurements, including investigating if the decomposition of the away side jet correlations remains reasonable with high p _T triggers and technical improvements to the extraction of jet induced correlations.     

Central jet production as a probe of the perturbative formalism for exclusive diffraction

We propose a new variable, R_j, in order to identify exclusive double-diffractive high E_T di-jet production. The variable R_j is calculated using the transverse energy E_T and pseudorapidity of the jet with the largest E_T. For a purely exclusive event the value of R_j→1, if we were to neglect hadronisaton and the detector resolution effects. To illustrate the expected R_j distribution we also compute exclusive three-jet production; and, moreover, we include jet smearing effects. By studying the predictions as a function of the size of the rapidity interval, δη, which allows for additional gluon radiation, one can probe the QCD radiation effects which are responsible for the Sudakov suppression of the exclusive amplitude. In this way we may check and improve the formalism used to predict the cross sections of exclusive double-diffractive Higgs boson (and/or other new physics) production.     

Quasi-particle model for lattice QCD: quark–gluon plasma in?heavy ion collisions

We propose a quasi-particle model to describe the lattice QCD equation of state for pure SU(3) gauge theory in its deconfined state, for T≥1.5T _c. The method involves mapping the interaction part of the equation of state to an effective fugacity of otherwise non-interacting quasi-gluons. We find that this mapping is exact. Using the quasi-gluon distribution function, we determine the energy density and the modified dispersion relation for the single particle energy, in which the trace anomaly is manifest. As an application, we first determine the Debye mass, and then the important transport parameters, viz., the shear viscosity, η, and the shear viscosity to entropy density ratio, h/S\eta/{\mathcal{S}}. We find that both η and h/S\eta/{\mathcal{S}} are sensitive to the interactions, and that the interactions significantly lower both η and h/S\eta/\mathcal{S}.     

Jet quenching pattern at LHC in PYQUEN model

The first LHC data on high transverse momentum hadron and dijet spectra in PbPb collisions at center-of-mass energy 2.76 TeV per nucleon pair are analyzed in the frameworks of PYQUEN jet quenching model. The presented studies for the nuclear modification factor of high-p _T hadrons and the imbalance in dijet transverse energy support the supposition that the intensive wide-angular (“out-of-cone”) medium-induced partonic energy loss is seen in central PbPb collisions at the LHC.     

Generalized z-scaling for charged hadrons and jets

Generalization of z-scaling observed in the inclusive high-p _T-charged hadron and jet production is proposed. Scaling function ψ(z) describing both charged hadrons and jets produced in proton-(anti)proton colisions for various multiplicity densities and collision energies is constructed. Anomalous fractal dimensions and parameters characterizing an associated medium for both classes of events are established. The basic features of the scaling established in minimum bias events are shown to be preserved up to the highest multiplicity densities measured in the experiments UA1, E735, CDF, and STAR. The obtained results are of interest for use of z-scaling as a tool for searching for new physics phenomena of particle production at a high transverse momentum and in the high-multiplicity region at U70, Tevatron, RHIC, and LHC. The text was submitted by the authors in English.     

Recent high-<Emphasis Type="Italic">p</Emphasis><Subscript><Emphasis Type="Italic">T</Emphasis></Subscript> results from STAR

We present selected recent results of multi-hadron correlation measurements in azimuth and pseudorapidity at intermediate and high p _T in Au+Au collisions at , from the STAR experiment at RHIC. At intermediate p _T , measurements are presented that attempt to determine the origin of the associated near-side (small Δφ) yield at large pseudo-rapidity difference Δη that is found to be present in heavy ion collisions. In addition, results are reported on new multi-hadron correlation measures at high-p _T that use di-hadron triggers and multi-hadron cluster triggers with the goal to constrain the underlying jet kinematics better than in the existing measurements of inclusive spectra and di-hadron correlations.