2,571 research outputs found
An Instanton-Induced Contribution to the Decay of the Eta_c into P-Pbar
We compute the decay rate for the process 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 :
Massless perturbative QCD forbids, at leading order, the exclusive
annihilation of proton-antiproton into some charmonium states, which, however,
have been observed in the 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 () and the . The
case of the is considered here and a way of detecting such a state
through its typical angular distribution in the radiative decay
is suggested. Estimates of the branching ratio , 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 , 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 collider at BNL-RHIC. We find that the
single transverse spin asymmetry 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
.Comment: 6 pages, version to be published in Phys.Rev.Let
charmonium state decay into in QCD models including constituent quark mass corrections
Stimulated by the experimental observation, made by the E760 Collaboration at
Fermilab, of the state of charmonium resonantly formed in
annihilation, we perform a calculation of the decay width for the 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 to be
in the range 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
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