2,189 research outputs found

    An Instanton-Induced Contribution to the Decay of the Eta_c into P-Pbar

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    We compute the decay rate for the process ő∑c‚ÜíppňČ\eta_c\to p\bar p using an effective helicity-flipping proton-antiproton-gluon vertex which incorporates nonperturbative chiral symmetry breaking effects induced by instantons. We fix the strength of the vertex by requiring it to account for the screening of the proton's axial charge observed in deep-inelastic scattering, and we estimate the size of the instanton effects by assuming them to depend linearly on the instanton density. We find that, despite a large suppression, the instanton-induced process occurs with a sizable rate comparable to the observed one, whereas the process is forbidden in perturbative QCD and not understood using standard methods.Comment: plain TeX with harvmac (b mode); 15 pages, 1 figure (included as a plain uncompressed postscript file); DFTT 73/9

    Off-diagonal helicity density matrix elements for heavy vector mesons inclusively produced in N-N, gamma-N, l-N interactions

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    Final state interactions in quark fragmentation may give origin to non zero values of the off-diagonal element rho_(1,-1) of the helicity density matrix of vector mesons V produced in current jets, with a large energy fraction x_E; the value of rho_(1,-1)(V) is related to the hard constituent dynamics and tests unusual properties of it. Some recent data on phi, K^* and D^* produced in e^+ e^- annihilations at LEP show such effects. Predictions are given here for rho_(1,-1) of heavy mesons produced in nucleon-nucleon, gamma-nucleon and lepton-nucleon interactions.Comment: LaTeX, 10 pages, 1 postscript figure, uses epsfig.sty. Revised version, to be published on Phys. Lett. B. Some statements added to clarify tex

    Status of Spin Physics

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    Fundamental spin physics has made striking progresses in the last years; new ideas, experiments and data interpretations have been proposed and keep emerging. A review of some of the most important issues in the spin structure of nucleons is made and prospects for the future are discussed.Comment: Few corrections and additions in reference

    What do we know about the proton spin structure?

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    A brief summary of the theoretical and experimental knowledge of the spin structure of the proton is presented. The helicity distributions of quark and gluons are discussed, together with their related sum rules. The transversity distribution is also introduced with possible strategies for its measurement. Novel spin dependent and \bfk_\perp unintegrated distribution and fragmentation functions are discussed, in connection with a new and rich phenomenology of transverse single spin asymmetries.Comment: 8 pages, 5 figures, talk delivered at GDH02, July 3-6 2002, Genova, Ital

    An Interesting Charmonium State Formation and Decay : ppňČ‚Üí1D2‚Üí1P1ő≥p \bar p \to ^1D_2 \to ^1P_1 \gamma

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    Massless perturbative QCD forbids, at leading order, the exclusive annihilation of proton-antiproton into some charmonium states, which, however, have been observed in the ppňČp\bar p channel, indicating the significance of higher order and non perturbative effects in the few GeV energy region. The most well known cases are those of the 1S0^1S_0 (ő∑c\eta_c) and the 1P1^1P_1. The case of the 1D2^1D_2 is considered here and a way of detecting such a state through its typical angular distribution in the radiative decay 1D2‚Üí^1D_2 \to 1P1ő≥^1P_1 \gamma is suggested. Estimates of the branching ratio BR(1D2‚ÜíppňČ)BR(^1D_2 \to p\bar p), as given by a quark-diquark model of the nucleon, mass corrections and an instanton induced process are presented.Comment: 9 pages (no figures), Plain TeX, CBPF-NF-014/94, INFNCA-TH-94-

    A strategy towards the extraction of the Sivers function with TMD evolution

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    The QCD evolution of the unpolarized Transverse Momentum Dependent (TMD) distribution functions and of the Sivers functions have been discussed in recent papers. Following such results we reconsider previous extractions of the Sivers functions from semi-inclusive deep inelastic scattering data and propose a simple strategy which allows to take into account the Q^2 dependence of the TMDs in comparison with experimental findings. A clear evidence of the phenomenological success of the TMD evolution equations is given, mostly, by the newest COMPASS data off a transversely polarized proton target.Comment: 15 pages, 7 figures, 2 table

    1P1^1P_1 charmonium state decay into ppňČp \bar p in QCD models including constituent quark mass corrections

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    Stimulated by the experimental observation, made by the E760 Collaboration at Fermilab, of the 1‚ÄČ‚Ā£P1^1\!P_1 state of charmonium resonantly formed in p‚ÄČpňČp\,\bar p annihilation, we perform a calculation of the decay width for the 1‚ÄČ‚Ā£P1‚Üíp‚ÄČpňČ^1\!P_1\to p\,\bar p process. To this end, we employ a phenomenological model which adds constituent quark mass corrections to the usual massless QCD models for exclusive processes. For massless models, in fact, the process under consideration is forbidden by the so-called helicity selection rules, while it is allowed in our extended model. We find őď(1‚ÄČ‚Ā£P1‚Üíp‚ÄČpňČ)\Gamma(^1\!P_1\to p\,\bar p) to be in the range 1‚ąí101-10 eV. We also compare our results with previous, indirect estimates, based on QCD multipole expansion models.Comment: 17 pages, REVTeX 3.0, 1 postscript figure included. Revised version, to be published in Physical Review D. Brief discussion of the role of other nonperturbative effects and three references added in the introduction; Comparison with QCD multipole expansion model slightly reformulated; Some sentences added in the conclusions; Results unchange

    Accessing transversity via J/Psi production in polarized p-pbar interactions

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    We discuss the possibility of a direct access to transversity distributions by measuring the double transverse spin asymmetry A_TT in p pbar --> J/Psi + X --> l^- l^+ + X processes at future GSI-HESR experiments with polarized protons and anti-protons. In the J/Psi resonance production region, with 30 < s < 45 GeV^2, both the cross-section and A_TT are expected to be sufficiently large to allow a measurement of h_1^q(x,M^2); numerical estimates are given.Comment: 10 pages, 2 figures. Version accepted for publication in PLB; comments and references adde

    New proton polarized structure functions in charged current processes at HERA

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    Estimates for longitudinal spin asymmetries which single out new polarized nucleon structure functions in deeply inelastic charged current interactions at HERA energies are given, exploiting their interpretation in terms of polarized quark distributions. These asymmetries turn out to be large and allow a measurement of the new polarized structure functions g1Wg_1^{W} and g5Wg_5^{W}, which would add valuable tests and information on the spin content of quarks inside a polarized proton. We also show that single spin asymmetries in neutral current interactions are very small.Comment: LaTeX, 8 pages, 6 figure
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