Investigations of quark fragmentation universality

We propose event cuts in deep inelastic scattering, suitable for an examination of quark fragmentation universality. We compare the current hemisphere of the Breit frame with the hemispheres in annihilation events, using Monte Carlo simulations. The agreement between the two processes is improved after the suggested event cuts are introduced. A method to study the scale evolution in the quark hemispheres using data from fixed-energy experiments is presented. This makes it possible to use the high statistics from LEP1 also at scales below the mass. We also discuss observables which are sensitive to the dynamics of regions closer to the remnant. The observables probe the relatively clean region on the current side of the hardest emission in the event, and can be used to distinguish between different assumptions about remnant effects and mechanisms for the parton evolution. Received: 3 November 1998 / Revised version: 25 January 1999 / Published online: 30 June 1999     

A semiclassical model for quark jet fragmentation

A semiclassical model is presented for the way the energy of a fast quark is transformed into observable hadrons. It reproduces the features of 1+1 dimensional QED (the Schwinger model) concerning a flat rapidity distribution in the central region. It also reproduces results from phenomenological considerations, which, based upon scaling, predict that meson formation in the fragmentation region can be described by an iterative scheme, implying a set of coupled integral equations. In particular the model predicts that the probability to find a meson containing the leading quark is independent of the Feynman scaling variablez. The iterative structure corresponds to a Brownian motion with relevance both to the confinement problems and to the distribution of mass in the quark jet.     

Study of highp ⊥ quark and gluon jet fragmentation

The fragmentation properties of highp _⊥ jets are investigated using new data from the ISR and the SPS collider. Effects from gluon radiation are clearly demonstrated by comparison with a state-of-the-art model inlcuding QCD parton cascade evolution and string hadronization, which gives in general good agreement with the data. Differences between quark and gluon jets are discussed as well asQ ₂-dependent scaling violation effects.     

Energy dependence of the differences between the quark and gluon jet fragmentation

Three jet events arising from decays of the Z boson, collected by the DELPHI detector, were used to measure differences in quark and gluon fragmentation. Gluon jets were anti-tagged by identifying b quark jets. Unbiased quark jets came from events with two jets plus one photon. Quark and gluon jet properties in different energy ranges were compared for the first time within the same detector. Quark and gluon jets of nearly the same energy in symmetric three jet event topologies were also compared. Using three independent methods, the average value of the ratio of the mean charged multiplicities of gluon and quark jets is $$< r >=1.241 pm 0.015 (stat.)pm 0.025 (syst.).$$ Gluon jets are broader and produce fragments with a softer energy spectrum than quark jets of equivalent energy. The string effect has been observed in fully symmetric three jet events. The measured ratio R_γ of the charged particle flow in the qq? inter-jet region of the qq?g and qq?γ samples agrees with the perturbative QCD expectation. The dependence of the mean charged multiplicity on the hadronic center-of-mass energy was analysed in photon plus n-jet events. The value for α_s(M _Z) determined from these data using a QCD prediction with corrections at leading and next-to-leading order is $$←pha_s(M_Z)=0.116pm 0.003 (stat.)pm 03009 (syst.).$$     

On the T-old quark fragmentation function and on transversal handedness

The first probe of the correlation of the T-odd one-particle fragmentation function responsible for the left-right asymmetry of fragmentation of a transversely polarized quark is done by using the 1991–95 DELPHI data for Z→2-jet decay. Integrated over the fraction of longitudinal and transversal momenta, this correlation is of 1.5% order, which means order of 13% for the analyzing power.A rather large (≈ 10%) handedness transversal to the production plane was observed in the diffractive production of (π^?+π⁺π^?) triples from nuclei by the 40 GeV/c π^?-beam. It was shown that the phenomenon has a clear dynamic origin and resembles the single spin asymmetry behavior. All this makes us hope to use this effects in polarized DIS experiments for transversity measurement.     

Test of the universality of naive-time-reversal-odd fragmentation functions

We investigate the "spontaneous" hyperon transverse polarization in e+ e- annihilation and semi-inclusive deep inelastic scattering processes as a test of the universality of the naive-time-reversal-odd transverse momentum dependent fragmentation functions. We find that universality implies definite sign relations among various observables. This provides a unique opportunity to study initial or final state interaction effects in the fragmentation process and test the associated factorization.     

Jets in small-pT hadronic collisions,universality of quark fragmentation,and rising rapidity plateaus

We consider a two-rapidity-chain fragmentation model for small-p_T hadronic collisions based on the dual topological approach. This model, when supplemented by the universality of quark fragmentation functions, accounts for the energy and x-dependence of all available one particle inclusive data. The model provides a novel explanation for the rise of hadronic plateaus with energy.     

Spin dependence of heavy quark fragmentation

We propose that the non-perturbative part of the fragmentation function describing the transition from a heavy quark to a heavy meson is proportional to the square of the produced meson wave function at the origin, taking into account hyperfine interactions. We analyze the effects of this proposal on the number of pseudoscalar mesons compared to the number of vector mesons produced and find a good agreement with experimental data. Finally, we discuss further experimental checks for our hypothesis.     

Coherent versus incoherent quark fragmentation picture

We study the helicity density matrix of vector mesons produced ine ⁺ e ^? interactions at high energies through the two step process \(e^ + e^ - \to q\bar q \to VX\) . Whereas in the usual incoherent fragmentation picture in which each quark decays independently, ρ(V) is predicted to be diagonal, we find that final state interactions (coherent fragmentation) give rise to a non-vanishing ρ_1-1(V). Various corrections to the standard picture such as, e.g., gluon bremsstrahlung, quark masses, transverse momenta, are shown to be small. Therefore, any significant non-zero value of ρ_1-1(V) found experimentally can be considered as a clear measurement of coherence effects.     

Gluon fragmentation functions from quark jets

The analysis of events with a fixed, different from unity, fraction of energy flowing into a solid angle ΔΩ can be used to extract the probability functions related to the scaling violations of deep inelastic scattering. One can obtain from the study of a quark jet, both the gluon and the quark fragmentation functions into hadrons.     

Dynamical connections between quark fragmentation functions

Assuming that the quark kicked by a virtual photon emits pions one by one, integral equations connecting the fragmentation functions D_u^π+ (z) and D_u^π? (z) are obtained. It is shown that they have a plateau the height of which can be determined from the multiplicity difference N_p^π+ (z) ? N_p^π? (z). Comparison with experiment is made.     

Modelling quark distribution and fragmentation functions

The representation of quark distribution and fragmentation functions in terms of non-local operators is combined with a simple spectator model. This allows us to estimate these functions for the nucleon and the pion ensuring correct crossing and support properties. We give estimates for the unpolarized functions as well as for the polarized ones and for subleading (higher twist) functions. Furthermore, we can study several relations that are consequences of Lorentz invariance and of C, P, and T invariance of the strong interactions.     

QGP and modified jet fragmentation

Recent progress in the study of jet modification in hot medium and its consequences in high-energy heavy-ion collisions is reviewed. In particular, I will discuss energy loss for propagating heavy quarks and the resulting modified fragmentation function. Medium modification of the parton fragmentation function due to quark recombination is formulated within finite temperature field theory and the implication for the search for a deconfined quark-gluon plasma is also discussed.Arrival of the final proofs: 20 July 2005PACS: 13.87.Fh, 12.38.Bx, 12.38.Mh, 11.80.La     

Resonance production from jet fragmentation

Short lived resonances are sensitive to the medium properties in heavy-ion collisions. Heavy hadrons have larger probability to be produced within the quark gluon plasma phase due to their short formation times. Therefore heavy mass resonances are more likely to be affected by the medium, and the identification of early produced resonances from jet fragmentation might be a viable option to study chirality. The high momentum resonances on the away-side of a triggered di-jet are likely to be the most modified by the partonic or early hadronic medium. We will discuss first results of triggered hadron-resonance correlations in Cu+Cu heavy ion collisions.