Clocking hadronization at RHIC

The RHIC accelerator collided Au on Au beams at $\sqrt {s_{NN} } = 130$ GeV in the summer of 2000. To study these collisions, the RHIC experiments have been using a number of observables. One new observable in the field of relativistic heavy ions is the balance function. The balance function can be used to measure the correlation of charged particle pairs in rapidity. The rapidity separation of a particle pair that is created at the same point indicates the time of hadronization for that pair. Preliminary balance function analyses of STAR data are reported. The calculation of a balance function for hadron gas simulations (HIJING) is also discussed in this paper. These preliminary results indicate that the balance function is a useful observable in heavy-ion collisions.     

In-medium hadronization in the deconfined matter at RHIC and LHC

We study the mechanism and probability of in-medium hadronization in the deconfined medium produced in heavy-ion collisions at RHIC and LHC. We show the likelihood of color-neutral objects to be formed inside the partonic fireball and the probability of these states to escape the medium with reduced interaction strength and energy loss. We will suggest specific measurements that are sensitive to the early degrees of freedom and show predictions for these measurements at RHIC and the LHC.     

Parton dynamics and hadronization from the sQGP

The hadronization of an expanding partonic fireball is studied within the Parton-Hadron-String Dynamics (PHSD) approach which is based on a dynamical quasiparticle model (DQPM) matched to reproduce lattice QCD results in thermodynamic equilibrium. Apart from strong parton interactions the expansion and development of collective flow is found to be driven by strong gradients in the parton mean-fields. An analysis of the elliptic flow v₂ demonstrates a linear correlation with the spatial eccentricity ? as in the case of ideal hydrodynamics. The hadronization occurs by quark-antiquark fusion or 3 quark/3 antiquark recombination which is described by covariant transition rates. Since the dynamical quarks become very massive, the formed resonant ‘pre-hadronic’ color-dipole states ( or qqq) are of high invariant mass, too, and sequentially decay to the groundstate meson and baryon octets increasing the total entropy. This solves the entropy problem in hadronization in a natural way. Hadronic particle ratios turn out to be in line with those from a grand-canonical partition function at temperature T≈170 MeV.     

Parton coalescence and the antiproton/pion anomaly at RHIC

Coalescence of minijet partons with partons from the quark-gluon plasma formed in relativistic heavy ion collisions is suggested as the mechanism for production of hadrons with intermediate transverse momentum. The resulting enhanced antiproton and pion yields at intermediate transverse momenta give a plausible explanation for the observed large antiproton to pion ratio. With further increasing momentum, the ratio is predicted to decrease and approach the small value given by independent fragmentations of minijet partons after their energy loss in the quark-gluon plasma.     

Statistical hadronization model and transverse momentum spectra of hadrons in high energy collisions

A detailed analysis of transverse momentum spectra of several identified hadrons in high energy collisions within the canonical framework of the statistical model of hadronization is performed. The study of particle momentum spectra requires an extension of the statistical model formalism used to handle particle multiplicities, which is described in detail starting from a microcanonical treatment of single hadronizing clusters. Also, a new treatment of extra strangeness suppression is presented which is based on the enforcement of fixed numbers of pairs in the primary hadrons. The considered center-of-mass energies range from to 30 GeV in hadronic collisions ( and Kp) and from 15 to 35 GeV in collisions. The effect of the decay chain following hadron generation is accurately and exhaustively taken into account by a newly proposed numerical method. The exact conservation at low energy and the increasing hard parton emission at high energy bound the validity of the presently taken approach within a limited center-of-mass energy range. However, within this region, a clear consistency is found between the temperature parameter extracted from the present analysis and that obtained from fits to average hadron multiplicities in the same collision systems. This finding indicates that in the hadronization process the production of different particle species and their momentum spectra are two closely related phenomena governed by one parameter. Received: 31 October 2001 / Published online: 15 February 2002     

Bounds on transverse momentum dependent distribution functions

When more than one hadron takes part in a hard process, an extended set of quark distribution and fragmentation functions becomes relevant. In this talk, the derivation of Soffer-like bounds for these functions, in the case of a spin?1/2 target [1], is sketched and some of their aspects are discussed.     

On the transverse momentum distribution of massive lepton pairs

The experimentally measured q_t distributions of massive lepton pairs produced in hadron collisions are compared with the predictions of QCD [1]. We demonstrate that a fair agreement may be achieved without the use of large transverse momenta of the constitunt quarks - owing to a specific double-logarithmic form factor which manifests itself in this process. A prediction for the q_t distribution in weak intermediate boson production is also given.     

Parton rearrangement and fast thermalization in heavyion collisions at RHIC and LHC energies

The implications of parton rearrangement processes on the dynamics of ultra-relativistic heavyion collisions have been investigated. A microscopic transport approach, namely the quark gluon string model (QGSM) which has been extended for a locally density-dependent partonic rearrangement and fusion procedure served as the tool for this investigations. The model emulates effectively the dynamics of a strongly coupled quark plasma and final hadronic interactions. Main QGSM results on anisotropic flow components v ₁ and v ₂ at top RHIC energy are compiled. Predictions for the pseudorapidity dependence of directed and elliptic flow in Pb+Pb collisions under LHC conditions are presented.     

QCD prediction of transverse momentum distribution of partons

A general method to compute the development of the transverse momentum distributions of the partons as Q² increases is presented. As an example, the average 〈p_⊥²〉 is computed for the valence, charmed, sea and gluonic partons in the Q² range from 1 GeV² to 120 GeV². The results show a fast increase of the gluonic contribution, a slow increase of the valence contribution and an almost flat sea contribution.     

Pion production at large transverse momentum

A constituent interchange model is used to give a natural explanation of the observed energy dependence of ISR data for π° production at 90°. It is predicted that at large fixed p_T and sufficiently high energy the cross section will become constant, with a p_T dependence of (p_T²)^?2.5. It is also tentatively suggested that the e⁺e^? annihilation structure function $\text{ν}\text{W}{}_2$ should behave like ω^?1.5 near ω = 0.     

Particle production at large transverse momentum

From a simple fragmentation approach to inelastic production at large fixed angles, we obtain an inclusive cross-section which falls as an inverse power of large transverse momentum p_T and which scales according to (p_T/√s), both consistent with ISR data. Predictions for charge ratios and associated multiplicities are also presented and compared with available data.     

Linearly polarized gluons and the Higgs transverse momentum distribution

We study how gluons carrying linear polarization inside an unpolarized hadron contribute to the transverse momentum distribution of Higgs bosons produced in hadronic collisions. They modify the distribution produced by unpolarized gluons in a characteristic way that could be used to determine whether the Higgs boson is a scalar or a pseudoscalar particle.     

Multi-lepton production at high transverse momentum at HERA

A search for events containing two or more high-transverse-momentum isolated leptons has been performed in ep collisions with the ZEUS detector at HERA using the full collected data sample, corresponding to an integrated luminosity of 480 pb⁻¹. The number of observed events has been compared with the prediction from the Standard Model, searching for possible deviations, especially for multi-lepton events with invariant mass larger than 100 GeV. Good agreement with the Standard Model has been observed. Total and differential cross sections for di-lepton production have been measured in a restricted phase space dominated by photon–photon collisions.     

Should particle trajectories comply with the transverse momentum distribution?

The momentum distributions associated with both the wave function of a particle behind a grating and the corresponding Bohmian trajectories are investigated and compared. Near the grating, it is observed that the former does not depend on the distance from the grating, while the latter changes with this distance. However, as one moves further apart from the grating, in the far field, both distributions become identical.