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Institute of Nuclear Physics Krakow, Poland PL9801710 The Henryk Niewodniczanski Institute of Nuclear Physics Krakow, Poland REPORT No. 1760/PH MESON CLOUD IN THE NUCLEON AND ITS CONSEQUENCES IN VARIOUS PHENOMENA Rozprawa habilitacyjna Address: Main site: 29-29 High Energy Department: ul. Radzikowskiego 152, ul. Kawiory 26 A, 31-342 Krak6w, Poland 30-055 Krakbw, Poland e-mail: [email protected] I e-mail: [email protected] REPORT No. 1760/PH MESON CLOUD IN THE NUCLEON AND ITS CONSEQUENCES IN VARIOUS PHENOMENA Rozprawa habilitacyjna Antoni Szczurek Krakow, June 1997 PRINTED AT THE HENRYK NIEWODNICZANSKI INSTITUTE OF NUCLEAR PHYSICS KRAK6W, UL. RADZIKOWSKIEGO 152 Xerocopy: INP Krakow To Magda, Krysia, Alik and Mateusz ACKNOWLEDGMENTS The results presented in this report have been obtained in collaboration with physicists from different research and educational institutes. I am particularly indebted to Prof. Josef Speth from the Forschungszentrum Jiilich (KFA) for interesting me in physics of mesons and baryons and his continuous collaboration on the issues presented below. I am also particularly indebted to my younger collaborator Harald Holtmann (Forschungszentrum, Julich) who shared with me the en- thusiasm during all stages of our common work. I have gained a lot by discussing various technical problems with Kolya Nikolaev (Landau Institute, Moscow). I am also indebted to Zbigniew Dziem- bowski (Temple University, Philadelphia) for collaboration on the subject of electromagnetic form factors. In addition I am indebted to many experimental and theoretical physicists and colleagues for discussions and exchange of helpful information: Barbara Badelek, Steve Bass, Jerry Blazey, Arie Bodek, Gerard Bosted, Stan Brodsky, Wojciech Broniowski, Antje Brull, Alfons Buchmann, An- drzej Budzanowski, Nicolo Cartiglia, Tomir Coghen, Alex Dieperink, John Durso, John Ellison, Magda Ericson, Torleif Ericson, Amand Faessler, Gerry Garvey, Krzysztof Golec-Biernat, Dieter von Harrach, Johann Heidenbaur, Karl Holinde, Pauchy Hwang, P. de Witt Huberts, Sergo Boriso- vich Gerasimov, J. Guy, Gunnar Ingelman, Boris Ioffe, Jiirg Jourdan, Misha Khankasayev, Fritz Klein, Siggi Krewald, Stefan Kretzer, Shunzu Kumano, Jan Kwiecinski, Gerry Levman, Lev Lipa- tov, Tim Londergan, Bo-Qiang Ma, Lech Mankiewicz, Alan Martin, Wally Melnitchouk, Joel Moss, Piet Mulders, Franz Osterfeld, Mariusz Przybycien, E. Reya, James Stirling, Marc Strikman, Peter Sutton, A.G. Tenner, Anthony Thomas, Jochen Wambach, Slava Zakharov, Piotr Zenczykowski and Volodya Zoller. I am very much indebted to Profs. Andrzej Budzanowski and Stanislaw Drozdz for their encouragement and continuous interest in this work. I also thank all my colleagues from the H. Niewodniczanski Institute of Nuclear Physics and Forschungszentrum Julich, for stimulating and friendly atmosphere. I am obliged to H. Niewodniczariski Institute of Nuclear Physics and Forschungszentrum Julich, where most of the work has been done, for financial support and providing me with excellent working conditions. The research presented below has been partially supported by the Polish State Committee for Scientific Research (KBN) under Grant No. 2 2409 9102. One stay of A.S. in Julich was supported by KFK Karlsruhe grant X088124. ABSTRACT We discuss several consequences of the meson cloud in the nucleon and search for its phe- nomenological evidences in various phenomena in both soft and hard processes. The cut-off parameters of the form factors for meson-baryon vertices are determined from high- energy particle production data. We find an universal cut-off parameter for processes involving octet baryons. Relativistic calculations of the effects of the pion cloud on the electromagnetic properties of the nucleon are presented. Light-cone formalism is used to construct the physical nucleon wave function that contains two components: a bare nucleon and a pion associated by recoil baryon with the nucleon or delta quantum numbers. Both sectors are coupled by a relativistic light-cone interaction Hamiltonian. A perturbative treatment is used to solve for the physical nucleon state. The elastic electromagnetic nucleon form factors Gp^n(Q2) and GPj^{Q2) are computed in terms of matrix elements of current operator and the nucleon wave function. The Q2-dependence of contributions to the nucleon form factor from the various sectors of the model space is calculated and compared. The nature of the observed deviations from form factor scaling and dipole parameterization is discussed. We present a complete set of formulae for longitudinal momentum distribution functions (split- ting functions) of mesons in the nucleon. It can be applied in the framework of convolution formal- ism to the deep-inelastic structure functions (quark distributions) of the nucleon viewed as a system composed of virtual 'mesons' and 'baryons'. Pseudoscalar and vector mesons as well as octet and decuplet baryons are included. In contrast to many approaches in the literature the present ap- proach ensures charge and momentum conservation by the construction. We present not only spin averaged splitting functions but also helicity dependent ones, which can be used to study the spin content of the nucleon. This combined with the information about meson-baryon vertices allows us to calculate the flavour and spin content of the nucleon. The value of the Gottfried Sum Rule obtained from our model (So = 0.224J nicely agrees with that obtained by the NMC. In addition, we calculate the x-dependence of the d — u asymmetry and get an impressive agreement with a recent fit of Martin-Stirling-Roberts. The calculated axial coupling constants for semileptonic decays of the octet baryons agree with the experimental data already with SU(G) wave function for the bare nucleon. As a consequence the Bjorken Sum Rule is nicely reproduced. Although we get improve- ments for the Ellis-Jaffe Sum Rules for the proton and neutron in comparison to the naive quark model, the meson cloud model is not sufficient to reproduce the experimental data. We assume that the quark distributions in bare baryons are related by appropriate flavour sym- metries to quark distributions in the bare proton. The latter are fitted to the muon and charged- current (anti)neutrino deep-inelastic scattering including parameter free meson cloud corrections. We take into account kinematical target mass corrections which become important at large x and lowQ2. We examine to which degree the effect of the inclusion of target mass corrections in the descrip- tion at low-Q2 DIS and meson cloud effects may, after QCD evolution, modify DIS cross section at large x and Q2. This is of interest in the context of recently reported excess of events observed by HI and ZEUS collaborations at HERA. We predict enhanced production of events at large x in comparison to standard sets of quark distributions with rather mild Q2-dependence. Our analysis leaves some room for more exotic effects beyond parton approach and/or Standard Model. Our large-x effects can be verified in the future when the HERA luminosity will be increased. The semi-inclusive cross section for producing slow protons in charged current deep inelastic (anti-)neutrino scattering on protons and neutrons is calculated as a function of the Bjorken x and the proton momentum. The standard hadronization models based upon the colour neutralization mechanism appear to underestimate the rate of slow proton production on hydrogen. The presence of the virtual mesons (pions) in the nucleon leads to an additional mechanism for proton production, referred to as spectator process. It is found that at low proton momenta both mechanisms compete, whereas the spectator mechanism dominates at very small momenta, while the colour neutralization mechanism dominates at momenta larger than 1-2 GeV/c. The results of the calculations are compared with the CERN bubble chamber (BEBC) data. The spectator model predicts a sharp increase of the semi-inclusive cross section at small x due to the sea quarks in virtual mesons. We propose a further possibility to test the concept of the pion cloud at HERA through the analysis of the structure of deep inelastic scattering events induced by pion-exchange. Momentum and energy distributions of outgoing nucleons as well as rapidity and multiplicity distributions are investigated using Monte Carlo simulations. Most observables cannot distinguish this process from ordinary DIS, but in the energy distribution of final neutrons we find a significantly different prediction from the pion cloud model than from the standard hadronization models. Forward neutron calorimeters will be essential to test the concept of pions in the nucleon. The contributions of subleading fi, u), ai and p reggeons to the diffractive structure function 2 F2 (xp,j3,Q ) are estimated. In addition we include the pion exchange discussed in the previous section. The reggeon and pion contributions lead to a violation of the factorization of the diffractive structure function. The diffractive structure function is separated into the contributions with leading p) D proton A( F2 and neutron A^Ff. We predict pronounced increase of the ratio A^F^/A^F^ as a function of xp in the interval 10~2 < xp < 10"1. The effect is due to the exchange of the 2 isovector a<2 and p reggeons at smaller xp and the pion exchange at xp > 10~ . We discuss the present status of the u — d asymmetry in the nucleon and analyze the quantities which are best suited to verify the asymmetry. We find that the Drell- Yan asymmetry is the quantity insensitive
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