2,189 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 $\eta_c\to p\bar p$ 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

### 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 : $p \bar p \to ^1D_2 \to ^1P_1 \gamma$

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

### $^1P_1$ charmonium state decay into $p \bar p$ 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

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

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

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 $g_1^{W}$ and $g_5^{W}$,
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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