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

We compute the decay rate for the process $\eta_c\to p\bar p$ 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

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

Spin effects in the fragmentation of a transversely polarized quark

An azimuthal dependence of pions produced in polarized Deep Inelastic Scattering, gamma^* - p(transv. polarized) -> pion + X, has been recently observed and might be related to the so-called Collins effect. We discuss in details, for a general spin configuration of the nucleon, the kinematics of the process and methods of extracting information on the fragmentation properties of a polarized quark. Assuming that the observed azimuthal dependence is indeed due to Collins effect, we derive a lower bound estimate for the size of the quark analysing power, which turns out to be large.Comment: 18+1 pages, LaTeX, 5 eps figures, uses epsfig.sty; v2: Some technical problems in dimensions of Figs. 2 and 3 remove

Status of Spin Physics

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?

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 : ppˉ→1D2→1P1γp \bar p \to ^1D_2 \to ^1P_1 \gamma

Massless perturbative QCD forbids, at leading order, the exclusive annihilation of proton-antiproton into some charmonium states, which, however, have been observed in the $p\bar p$ 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 $^1S_0$ ($\eta_c$) and the $^1P_1$. The case of the $^1D_2$ is considered here and a way of detecting such a state through its typical angular distribution in the radiative decay $^1D_2 \to$ $^1P_1 \gamma$ is suggested. Estimates of the branching ratio $BR(^1D_2 \to p\bar p)$, 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

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

Positivity constraints on initial spin observables in inclusive reactions

For any inclusive reaction of the type $A_1({spin} 1/2)+ A_2({spin} 1/2) \to B + X$, we derive new positivity constraints on spin observables and study their implications for theoretical models in view, in particular, of accounting for future data from the polarized $pp$ collider at BNL-RHIC. We find that the single transverse spin asymmetry $A_N$ for several processes, in the central region for example jet production, direct photon production, lepton-pair production, must be such that |A_N| \lappeq 1/2, rather than the usual bound $|A_N| \leq 1$.Comment: 6 pages, version to be published in Phys.Rev.Let

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

Stimulated by the experimental observation, made by the E760 Collaboration at Fermilab, of the $^1\!P_1$ state of charmonium resonantly formed in $p\,\bar p$ annihilation, we perform a calculation of the decay width for the $^1\!P_1\to p\,\bar p$ 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 $\Gamma(^1\!P_1\to p\,\bar p)$ to be in the range $1-10$ 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