Shares of sea and valence quarks in the proton spin puzzle

The first moments of the polarized valence parton distribution functions truncated to the wide Bjorken x region 0.004 < x < 0.7 are directly (without any fitting procedure) extracted in NLO QCD from the combined COMPASS and HERMES semi-inclusive deep inelastic scattering data. Applying the proposed original procedure to these results, we estimate the contributions of light sea quarks to the proton spin, which occur just zero within the errors.     

On the recombination mechanism of quarks into a baryon in proton fragmentation

The similarity and difference between our quark cascade model with recombination mechanism and Van Hove's quark fragmentation-recombination model are discussed. Van Hove's fragmentation-recombination model is extended to the model which describesx-dependence of the hadron spectra as well as the baryon multiplicities by using our diffusion equation. The recombination probability in Van Hove's model can be related with the recombination factor λ′ in our diffusion equation. Thex-dependence of the hadron spectra causes another restriction on the recombination probability in Van Hove's model. The inclusive spectra of mesons, proton andΔ ⁺⁺ in proton fragmentation and baryon multiplicities except∑ ^? multiplicity are explained by both our model with recombination mechanism and the model with recombination mechanism a la Van Hove. But the ration(∑ ^?)/n(∑ ^+;) may not be explained by naive quark-parton model with recombination mechanism without considering quark spin.     

Chiral quarks and proton decay

In this paper we calculate the hadronic matrix elements of baryon decay operators using a chiral effective lagrangian with quarks, gluons and Goldstone boson fields. We study in detail the cases where the ΔB = 1 operators arise from supersymmetric SU(5) GUT as well as the minimal SU(5) GUT model. In each model the results depend on two parameters. In particular there is a range of values for the two parameters, where the dominant decay modes in the minimal SU(5) GUT are: p → ηe⁺ and n → π^? e⁺.     

Soliton with valence quarks in the chiral invariant σ-model

The chiral invariant σ-model is used to obtain bound states (solitons) of valence quarks. A hedgehog shape is assumed for the pion field. A soluble model is presented and compared to a self-consistent calculation. The results are discussed in the light of the possible effective baryon number carried by the chiral fields.     

Measuring the transverse polarization of quarks in the proton

The transverse spin polarization distribution of quarks in a proton contributes at leading twist to the angular distribution of lepton pair production in doubly polarized proton-proton collisions. Although this distribution is straightforward to define in the QCD-improved parton model and represents a fundamental quark degree of freedom, it has never been measured. Moreover, deeply inelastic electroproduction experiments are only sensitive to this distribution at the higher twist level, and then mix this contribution with others, including the one coming from multiparton initial states. Reviewing this, we conclude that the Drell-Yan process is the best way to measure the quark transverse spin polarization distribution.Unité propre du CNRS     

On the fragmentation functions of heavy quarks into hadrons

An explicit form for the charmed quark fragmentation function D^C_c(z) into hadrons has been obtained with the help of the “reciprocity relation” and the c-quark distribution function in charmed mesons (the function calculated in terms of the Kuti-Weisskopf model). D^C_c(z) turns out to peak mainly at z close to 1. The analysis of new data on muon pair production in neutrino reactions points to such a behaviour of the D^C_c(z) function. The obtained fragmentation function, contrary to those, used earlier, leads to a charmed particle (average) multiplicity in e⁺e^?-annihilation independent of energy.     

Left-right asymmetry and orbiting valence quarks in polarized protons and antiprotons

It is shown that the asymmetry observed in the recent single-spin large-x _F inclusive pion production experiments at high-energies should be considered as a strong indication for the existence of orbiting valence quarks in polarized protons and antiprotons. The underlying physical picture in which orbiting valence quarks give rise to flavor-currents is discussed. The close connection between these currents and the measured quantities — in particular baryon's magnetic moment and the abovementioned asymmetry — is pointed out.     

Fast mesons are the fragmentation products of the dressed quarks

Using the Kuti-Weisskopf model we calculate dressed quark fragmentation function from proton. We show that the fragmentation spectrum of such a quark correctly fits to the fast meson spectrum.     

Continuum limit of overlap valence quarks on a twisted mass sea

We study a lattice QCD mixed action with overlap valence quarks on two flavours of Wilson maximally twisted mass sea quarks. Employing three different matching conditions to relate both actions to each other, we investigate the continuum limit by using three values of the lattice spacing ranging from a≈0.05 fm$a\approx 0.05\text{fm}$ to 0.08 fm$0.08\text{fm}$. A particular emphasis is put on the effect on physical observables of the topological zero modes appearing in the valence overlap operator. We estimate the region of parameter space where the contribution from these zero modes is sufficiently small such that their effects can be safely controlled and a restoration of unitarity of the mixed action in the continuum limit is reached.     

Distribution of valence quarks in hadrons at small x in QCD

Valence u- and d-quark distributions in nucleons and pions at small x corresponding to ? and ω Regge pole exchanges have been obtained on the basis of the QCD sum rule method.     

The role of fluctuations in mixed valence compounds

Previous renormalization group results for a simple spinless mixed valence model yielded large intersite ?-electron correlations if extended coherent hybridization mixed valence states are assumed. We simulate these correlations by an ?-level energy distribution and analyze its consequences. It is seen that a relatively small distribution width already smears the coherent hybridization gap leading to a lattice of essentially independent resonant levels.     

A study of the fragmentation of quarks in et-p collisions at HERA

Deep inelastic scattering (DIS) events, selected from 1993 data taken by the H1 experiment at HERA, are studied in the Breit frame of reference. The fragmentation function of the quark is compared with those of e⁺e⁻ data. It is shown that certain aspects of the quarks emerging from within the proton in e⁻p interactions are essentially the same as those of quarks pair-created from the vacuum in e⁺e⁻ annihilation. The measured area, peak position and width of the fragmentation function show that the kinematic evolution variable, equivalent to the e⁺e⁻ squared centre of mass energy, is in the Breit frame the invariant square of the four-momentum transfer. We comment on the extent to which we have evidence for coherence effects in pArton showers.     

Proton–proton interaction in constituent quarks model at LHC energies

In this paper we consider soft processes at LHC energies in the framework of the constituent quark model. We show that this rather naive model is able to describe all available soft process data at lower energies and to predict the behavior of the total cross section, the elastic and diffractive cross sections at LHC energy. It turns out that the “input” pomeron that has been used in this approach has parameters that are close to the so called “hard” pomeron with rather large intercept Δ≈0.12 and small value of the slope α^′ _P≈0.08 GeV^-2. We show that the elastic amplitude has a minimum at impact parameter b=0 and a maximum at b≈2 GeV^-1. Such a behavior is the result of overlapping of the parton clouds that belong to the different quarks in the hadron.     

Probabilistic quark-model approach to proton fragmentation

A probabilistic approach is formulated to study the behaviour of the incident hadron quark flavours in the fragmentation process of high energy protons in lowp _T reactions. Analysis of available data onpp collisions, mainly on hyperon and antibaryon multiplicities, leads to estimates of the probabilities for the different ways in which the incident valence quarks recombine into final hadrons. We find that all three incident quarks emerge in one and the same outgoing nucleon (or nucleon resonance) with probabilityA ₃=0.35–0.4, that two of them emerge in one baryon and the third in another hadron (mostly a meson or meson resonance) with probabilityA ₂=0.6–0.5, and that they emerge in three distinct hadrons (mostly mesons or meson resonances) with probabilityA ₁=0.05–0.1. We find good support for a very simple probabilistic picture of the fragmentation process.