Hadron Formation Research Articles

Hadron formation in high energy photonuclear reactions

We present a new method to account for coherence length effects in a semiclassical transport model. This allows us to describe photoproduction and electroproduction at large nuclei $(A>12)$ and high energies using a realistic coupled channel description of the final state interactions that goes beyond simple Glauber theory. We show that the purely absorptive treatment of the final state interactions can lead to wrong estimates of color transparency and formation time effects in particle production. As an example, we discuss exclusive ${\ensuremath{\rho}}^{0}$ photoproduction on Pb at a photon energy of 7 GeV as well as ${K}^{+}$ production in the photon energy range 1--7 GeV.

Probing hadronic formation times with antiprotons in p+ A reactions at AGS energies

The production of antiprotons in p+ A reactions is calculated in a microscopic transport approach employing hadronic and string degrees of freedom (HSD). It is found that the abundancies of antiprotons as observed by the E910 Collaboration in p+ A reactions at 12.3 GeV/ c as well as 17.5 GeV/ c can approximately be described on the basis of primary proton–nucleon and secondary meson–baryon production channels for all targets. The transport calculations demonstrate that the antiproton rapidity distributions for heavy targets are sensitive to the p ̄ (or hadron) formation time in the nuclear medium. Within our analysis the data from the E910 Collaboration are reasonably described with a formation time of 0.4–0.8 fm/ c in the hadron rest frame.

Hadron formation in a nuclear environment at HERMES

The influence of the nuclear medium on the production of charged hadrons in semi-inclusive deep inelastic scattering has been studied by the HERMES experiment at DESY using 27.5 GeV positrons. A substantial reduction of the multiplicity of charged hadrons and identified charged pions from nuclei relative to that from deuterium has been measured as a function of the relevant kinematic variables. The preliminary results on krypton show a larger reduction of the multiplicity ratio R h M with respect to the one previously measured on nitrogen. Both the krypton and nitrogen data show that the multiplicity ratio is the same for positive and negative pions, while a significant difference is observed between R h M for positive and negative hadrons. This result can be interpreted in terms of a difference between the formation time of protons and pions. Alternatively, it has been suggested that the observed differences between positive and negative hadrons can be attributed to a different modification of the quark and antiquark fragmentation functions in nuclei.

Investigation of quark–antiquark interaction properties using leading particle measurements in e+e− annihilation

Measurements of heavy quark production in electron-positron collisions are used to analyse the strong interactions between quarks and anti-quarks. A scaling behaviour is observed in distributions of the rapidity change of D*, B*, and B mesons. From these distributions information is obtained on the hadron formation time, effective quark masses, and the potential between quark-antiquark pairs. Predictions for fragmentation functions are presented.

Hadron formation in deep-inelastic positron scattering in a nuclear environment

The influence of the nuclear medium on the production of charged hadrons in semi-inclusive deep-inelastic scattering has been studied by the HERMES experiment at DESY using a 27.5 GeV positron beam. The differential multiplicity of charged hadrons and identified charged pions from nitrogen relative to that from deuterium has been measured as a function of the virtual photon energy $\nu$ and the fraction z of this energy transferred to the hadron. There are observed substantial reductions of the multiplicity ratio $R_M^{h}$ at low $\nu$ and at high z, both of which are well described by a gluon-bremsstrahlung model of hadronization. A significant difference of the $\nu$ -dependence of $R_M^{h}$ is found between positive and negative hadrons. This is interpreted in terms of a difference between the formation times of protons and pions, using a phenomenological model to describe the $\nu$ - and z-dependence of $R_M^{h}$ .

Microscopic coloured quark dynamics in the soft non-perturbative regime - description of hadron formation in relativistic S + Au collisions at CERN

The quark molecular dynamics model is used to study microscopically the dynamics of the coloured quark phase and the subsequent hadron formation in relativistic S+Au collisions at the CERN-SPS. Particle spectra and hadron ratios are compared to both data and the results of hadronic transport calculations. The non-equilibrium dynamics of hadronization and the loss of correlation among quarks are studied.

Quark liberation and coalescence at CERN SPS

The linear coalescence approach to hadronization of quark matter is shown to have problems with strangeness conservation in a baryon rich environment. The simplest correct quark counting is shown to coincide with the non-linear algebraic coalescence rehadronization model, ALCOR. We argue that experimental facts agree with the assumption that quark degrees of freedom are liberated before hadron formation in 158 AGeV central Pb + Pb collisions at CERN SPS.

Primordial hadrosynthesis in the little bang

The present status of soft hadron production in high energy heavy-ion collisions is summarized. In spite of strong evidence for extensive dynamical evolution and collective expansion of the fireball before freeze-out I argue that its chemical composition is hardly changed by hadronic final state interactions. The measured hadron yields thus reflect the primordial conditions at hadronization. The observed production pattern is consistent with statistical hadronization at the Hagedorn temperature from a state of uncorrelated, color deconfined quarks and antiquarks, but requires non-trivial chemical evolution of the fireball in a prehadronic (presumably QGP) stage before hadron formation.

Primordial hadrosynthesis in the Little Bang

The present status of soft hadron production in high energy heavy-ion collisions is summarized. In spite of strong evidence for extensive dynamical evolution and collective expansion of the fireball before freeze-out I argue that its chemical composition is hardly changed by hadronic final state interactions. The measured hadron yields thus reflect the primordial conditions at hadronization. The observed production pattern is consistent with statistical hadronization at the Hagedorn temperature from a state of uncorrelated, color deconfined quarks and antiquarks, but requires non-trivial chemical evolution of the fireball in a prehadronic (presumably QGP) stage before hadron formation.

Hadronic signature of critical behavior and final state randomization

Hadronic observables are examined in search for signatures of critical behavior in quark-hadron phase transition. The 2D Ising model is used to simulate hadronic cluster formation on the surface of the expanding cylinder in a heavy-ion collision. Two measures involving the fluctuations of the produced hadrons in bins of various sizes are investigated. They are found to exhibit the canonical power-law behavior near the critical temperature. Final-state randomization is found not to have significant effect on the critical exponents.

Preface

A baryon anomaly – an increased baryon-to-meson production ratio at intermediate pT in heavyion collisions when compared to pp collisions – is observed at RHIC and the LHC. This effect is usually explained by recombination of constituent quarks during QGP hadronization, or as a consequence of a strong radial flow developed during the heavy-ion collision. In this contribution, an additional mechanism to favour baryon over meson production is proposed: when hadrons are formed in the recombination of nearby quarks and antiquarks, only colour-singlet combinations can be chosen. Hadron formation, in particular the probability to create baryons or mesons, depends on the distribution of colour charges among quarks. If the distribution is random – a reasonable assumption for Quark–Gluon Plasma (QGP) – the baryon-to-meson ratio is nearly twice higher than in the situation where quark colours are pre-arranged to obtain a white hadron in the combination of nearest quarks and antiquarks. The correlation of colour charges in the QGP also influences the distance over which recombination occurs. A study of the dependence of the baryon-to-meson ratio on the size of the colour correlation domain is presented.

Hadron formation time and dilepton mass spectra in heavy ion collisions

We point out that formation time of pions produced in heavy ion collisions modifies the mass spectrum of dileptons produced via pipi -> ee annihilation. Increasing formation time enhances the production of dileptons with lower masses. The effect offers an explanation of a part of the enhanced production of dileptons below the rho-meson mass as observed by the CERES and HELIOS Collaborations at the CERN SPS.

Formula omitted] suppression in nuclear collisions at SPS energies

We have carried out a first calculation of J Ψ production from nuclear collisions within the covariant transport approach HSD, which has been very successful in describing both hadronic and electromagnetic observables from heavy-ion collisions at intermediate and high energies. The production of J Ψ is based on the Lund string fragmentation model, while its interactions with hadrons are included by conventional cascade-type two-body collisions. Adopting 6 mb for the J/Ψ-baryon cross sections and 3 mb for the J/Ψ - meson cross sections above the D D threshold, we find that data on J Ψ suppression from both proton-nucleus and nucleus-nucleus collisions (including Pb+Pb) can be described without assuming the formation of a quark-gluon plasma in these collisions. In particular, mesons are found to absorb half the J Ψ 's in nucleus-nucleus collisions. The smaller absorption by nucleons compared to that found in the Glauber model analysis is due to the finite hadron formation time included in the transport approach. However, an appreciable volume of the hadronic matter in which J Ψ 's are absorbed is found to have an energy density which may be above that for the transition to a quark-gluon plasma.

Heavy hadrons and QCD instantons.

Heavy hadrons are analyzed in a random and dilute gas of instantons. We derive the instanton-induced interactions between heavy and light-quarks at next to leading order in the heavy quark mass and in the planar approximation, and discuss their effects on the hadronic spectrum. The role of these interactions in the formation of exotic hadrons is also discussed. \textcopyright{} 1996 The American Physical Society.

Nuclear attenuation of fast hadrons produced in charged-currentν and $$\bar \nu $$ interactions in neoninteractions in neon

The production of hadrons in charged-current (anti)neutrino interactions is studied with the bubble chamber BEBC exposed to the CERN (anti)neutrino wide-band beam. Fast-hadron production in a neon target is found to be attenuated as compared to that in a hydrogen target. This feature is discussed within theoretical models based on the idea of a hadron formation length. The experimental results favour the ‘constituent’ over the ‘yo-yo’ length concept, and suggest a quark cross section in the order of 3 mb.

POSSIBLE EXISTENCE OF TOPPED HADRONS

We study possibilities of formation of the topped hadrons, the [Formula: see text], [Formula: see text], tqq, ttq and ttt bound systems. We assume that the inverse of binding energy is the parameter of the formation time of the bound system and take the ratio Γtotal/EB as a criterion for the formation of these hadrons, where EB and Γtotal are a binding energy and a total decay width of a topped system respectively. We take into account the binding force caused by the Higgs scalar exchange as well as the one-gluon exchange. It is shown that there is no possibility of forming the single top quark hadrons before the top quark decay. In the standard model, where only one Higgs doublet contributes, among the topped hadrons only toponium [Formula: see text] and the three-top-quark baryon Ωttt with the top quark mass less than 140 GeV and 130 GeV have a possibility of forming the bound states before their decay. In the case of the two-Higgs-doublet model we show that there is a possibility of forming the topped hadrons, toponium, Ξtt and Ωttt for the larger top quark mass if the ratio of vacuum expectation values of two Higgs scalars is large near to the unitarity bound.

Direct semiexclusive production of energetic mesons.

The formation of color-singlet hadrons in hard-scattering processes is governed by process-independent distribution amplitudes. Such amplitudes may be measured in a class of processes we call semiexclusive, where one hadron is formed directly in hard scattering and the other QCD partons hadronize in jets. These processes can be identified experimentally by requiring a large angular or rapidity gap around a directly produced meson. We present cross sections for such processes, and show that experimental results, especially the ratio of charged to neutral $K$ production, are highly sensitive to the hadronic distribution amplitude. We discuss methods and prospects for thus gaining information about hadronic wave functions at present and future colliders.

Investigation of inclusive production ofK 0 andK * (892)0 mesons inK + A interactions at 11.2 GeV

Inclusive production ofK0 andK* (892)0 mesons inK+A-interactions (A=Be, Cu, Pb) at the energy 11.2 GeV has been investigated to study hadronisation of the leading\(\bar s\)-quark; the results are presented. Double differential cross sections d2σ/dxfdpt2 were measured in the region of incident particle fragmentation (0.4≦xf≦1,pt≦0.5 GeV/c). The experimental data obtained were analysed on the basis of the Lund model FRITIOF and a quark-gluon model that takes into account colour screening and hadron formation length effects. The experimental data confirm the picture of hadronisation of the leading\(\bar s\)-quark developed in the latter model.

Hadron production in deep inelastic lepton nucleus scattering

The comparison of hadron production in deep inelastic lepton scattering on different nuclei allows information to obtained about the reinteraction of the final state hadrons, because the projectile particle has pointlike structure and does not undergo multiple reaction with the target. A reduction in the multiplicity of fast forward produced hadrons was observed, which depends on the size of the target nucleus. This nuclear mainly depends on two kinematical variables, the virtual photon energy (ν) and the fraction of ν carried by the hadron observed ( z h ). The explanation of the characteristic ν-dependence is still an unsolved problem. The results of experiments performed in this field in the past 15 years are reviewed and discussed in the framework of intranuclear reinteraction models. A possible influence of the EMC effect on the hadron production in lepton nucleus scattering will also be considered. More detailed experimental information in the range of ν of 10–30 GeV would be very desirable in order to discriminate between competing predictions for the hadron formation time and thus to develop a more consistent picture of the fragmentation process.

Fragmentation of a finite-mass string.

We study a string fragmentation model in which the probability distribution for the locations of the vertices is parametrized explicitly in terms of their space-time coordinates measured from the point of the onset of separation of the quark and the antiquark. An iterative procedure is presented to fragment a finite-mass string into hadrons. We apply the model to calculate the rapidity distribution of the produced hadrons, which is found to give a good fit to the experimental data for ${e}^{+}\ensuremath{-}{e}^{\ensuremath{-}}$ annihilation at 29 GeV. From the parameters characterizing the probability distribution of the vertices, we determine the average hadron formation time in the fragmentation of the string.