961 research outputs found
Proton spin structure and the axial U(1) problem
We emphasise the relation between the spin structure of the proton and the
axial U(1) problem. New experiments motivated by the proton spin problem which
could shed light on the nature of U(1) symmetry breaking in QCD are discussed.Comment: Invited talk at the Workshop on the Spin Structure of the Proton and
Polarized Collider Physics, Trento (July 23-28, 2001), 6 pages, 1 figur
Gluons and the eta' nucleon coupling constant
We derive the effective chiral Lagrangian for low energy eta--nucleon and
eta'--nucleon interactions and show that gluonic degrees of freedom, via the
axial anomaly, induce a contact term in the pp -> pp eta and pp -> pp eta'
reactions. We then discuss the consequences for the extraction of g_{eta'NN}
from experimental data.Comment: 10 pages, LaTe
Description of the resonance as the atom
We discuss the possibility that the recently reported resonance in the spectrum can be described in terms of residual interactions.Comment: 3 pages, 1 figur
Eta bound states in nuclei: a probe of flavour-singlet dynamics
We argue that eta bound states in nuclei are sensitive to the singlet
component in the eta. The bigger the singlet component, the more attraction and
the greater the binding. Thus, measurements of eta bound states will yield new
information about axial U(1) dynamics and glue in mesons. Eta - etaprime mixing
plays an important role in understanding the value of the eta-nucleon
scattering length.Comment: 8 pages, version to appear in PL
Pseudoscalar Meson Decay Constants and Couplings, the Witten-Veneziano Formula beyond large N_c, and the Topological Susceptibility
The QCD formulae for the radiative decays \eta,\eta'\to\c\c, and the
corresponding Dashen--Gell-Mann--Oakes--Renner relations, differ from
conventional PCAC results due to the gluonic axial anomaly. This
introduces a critical dependence on the gluon topological susceptibility. In
this paper, we revisit our earlier theoretical analysis of radiative
pseudoscalar decays and the DGMOR relations and extract explicit experimental
values for the decay constants. This is our main result. The flavour singlet
DGMOR relation is the generalisation of the Witten-Veneziano formula beyond
large , so we are able to give a quantitative assessment of the
realisation of the expansion in the sector of QCD.
Applications to other aspects of physics, including the relation with
the first moment sum rule for the polarised photon structure function g_1^\c,
are highlighted. The Goldberger-Treiman relation is extended to
accommodate SU(3) flavour breaking and the implications of a more precise
measurement of the and -nucleon couplings are discussed. A
comparison with the existing literature on pseudoscalar meson decay constants
using large- chiral Lagrangians is also made.Comment: 27 pages, 4 figure
Axial U(1) dynamics in eta and eta' photoproduction
We discuss the sensitivity of eta and eta' photoproduction near threshold to
the gluonic OZI breaking parameters in the U_A(1)-extended effective chiral
Lagrangian for low-energy QCD. Our coupled-channels analysis hints at a strong
correlation between the gluon-induced contributions to the eta' mass and the
low-energy pp -> pp eta' reaction and the near-threshold behaviour of the gamma
p -> eta p cross-section.Comment: 19 pages, 9 figure
Target Independence of the Emc-SMC Effect
An approach to deep inelastic scattering is described in which the matrix
elements arising from the operator product expansion are factorised into
composite operator propagators and proper vertex functions. In the case of
polarised \m p scattering, the composite operator propagator is identified
with the square root of the QCD topological susceptibility
, while the corresponding proper vertex is a
renormalisation group invariant. We estimate using QCD
spectral sum rules and find that it is significantly suppressed relative to the
OZI expectation. Assuming OZI is a good approximation for the proper vertex,
our predictions, \int_{0}^{1}dx g_1^p (x;Q^2=10\GV^2)= 0.143 \pm 0.005 and
, are in excellent agreement
with the new SMC data. This result, together with one confirming the validity
of the OZI rule in the \hp radiative decay, supports our earlier conjecture
that the suppression in the flavour singlet component of the first moment of
observed by the EMC-SMC collaboration is a target-independent feature
of QCD related to the anomaly and is not a property of the proton
structure. As a corollary, we extract the magnitude of higher twist effects
from the neutron and Bjorken sum rules.Comment: 22 pages, 8 figures available on request
Medium modifications of the nucleon-nucleon elastic cross section in neutron-rich intermediate energy HICs
Several observables of unbound nucleons which are to some extent sensitive to
the medium modifications of nucleon-nucleon elastic cross sections in
neutron-rich intermediate energy heavy ion collisions are investigated. The
splitting effect of neutron and proton effective masses on cross sections is
discussed. It is found that the transverse flow as a function of rapidity, the
as a function of momentum, and the ratio of halfwidths of the
transverse to that of longitudinal rapidity distribution are very
sensitive to the medium modifications of the cross sections. The transverse
momentum distribution of correlation functions of two-nucleons does not yield
information on the in-medium cross section.Comment: 14 pages, 5 figure
Jet quenching by (pre--)hadronic final state interactions at RHIC
Within a hadron-string dynamical transport approach (HSD) we investigate the
attenuation of high transverse momentum (\pT) hadrons as well as the
suppression of 'near-side' and 'far-side' jets in collisions at
invariant energies = 200 GeV and = 62.4 GeV in comparison
to the data available from the Relativistic Heavy-Ion Collider (RHIC). From our
transport studies we find that a significant part of the high \pT hadron
attenuation seen experimentally can be attributed to inelastic interactions of
'leading' pre-hadrons with the dense hadronic environment. In addition, we also
show results of 'near-side' and 'far-side' angular correlations of high \pT
particles from Au+Au collisions at = 200 GeV and = 62.4
GeV within this (pre-)hadronic attenuation scenario. It turns out that the
'near-side' correlations are unaltered -- in accordance with experiment --
whereas the 'far-side' correlations are suppressed by up to 60% in
central collisions. Since a much larger suppression is observed experimentally
for these reactions in central reactions we conclude that there should be
strong additional (and earlier) partonic interactions in the dense and possibly
colored medium created in Au+Au collisions at RHIC.Comment: 23 pages, 7 figures, final version as accepted for publication in NP
Fragment Formation in Central Heavy Ion Collisions at Relativistic Energies
We perform a systematic study of the fragmentation path of excited nuclear
matter in central heavy ion collisions at the intermediate energy of . The theoretical calculations are based on a Relativistic
Boltzmann-Uehling-Uhlenbeck () transport equation including stochastic
effects. A Relativistic Mean Field () approach is used, based on a
non-linear Lagrangian, with coupling constants tuned to reproduce the high
density results of calculations with correlations.
At variance with the case at Fermi energies, a new fast clusterization
mechanism is revealed in the early compression stage of the reaction dynamics.
Fragments appear directly produced from phase-space fluctuations due to
two-body correlations. In-medium effects of the elastic nucleon-nucleon cross
sections on the fragmentation dynamics are particularly discussed. The
subsequent evolution of the primordial clusters is treated using a simple
phenomenological phase space coalescence algorithm.
The reliability of the approach, formation and recognition, is investigated
in detail by comparing fragment momentum space distributions {\it and
simultaneously} their yields with recent experimental data of the
collaboration by varying the system size of the colliding system, i.e. its
compressional energy (pressure, radial flow). We find an excellent agreement
between theory and experiment in almost all the cases and, on the other hand,
some limitations of the simple coalescence model. Furthermore, the temporal
evolution of the fragment structure is explored with a clear evidence of an
earlier formation of the heavier clusters, that will appear as interesting
of the high density phase of the nuclear Equation of State ().Comment: 21 pages, 8 figures, Latex Elsart Style, minor corrections in p.7,
two refs. added, Nucl.Phys.A, accepte
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