63 research outputs found
Parity-violating DIS and the flavour dependence of the EMC effect
Isospin-dependent nuclear forces play a fundamental role in nuclear
structure. In relativistic models of nuclear structure constructed at the quark
level these isovector nuclear forces affect the u and d quarks differently,
leading to non-trivial flavour dependent modifications of the nuclear parton
distributions. We explore the effect of isospin dependent forces for
parity-violating deep inelastic scattering on nuclear targets and demonstrate
that the cross-sections for nuclei with N /= Z are sensitive to the flavour
dependence of the EMC effect. Indeed, for nuclei like lead and gold we find
that these flavour dependent effects are large.Comment: 4 pages, 2 figure
Pion distribution amplitude from lattice-QCD
A method is explained through which a pointwise accurate approximation to the
pion's valence-quark distribution amplitude (PDA) may be obtained from a
limited number of moments. In connection with the single nontrivial moment
accessible in contemporary simulations of lattice-regularised quantum
chromodynamics (QCD), the method yields a PDA that is a broad concave function
whose pointwise form agrees with that predicted by Dyson-Schwinger equation
analyses of the pion. Under leading-order evolution, the PDA remains broad to
energy scales in excess of 100 GeV, a feature which signals persistence of the
influence of dynamical chiral symmetry breaking. Consequently, the asymptotic
distribution, \phi_\pi^asy(x), is a poor approximation to the pion's PDA at all
such scales that are either currently accessible or foreseeable in experiments
on pion elastic and transition form factors. Thus, related expectations based
on \phi_\pi^asy(x) should be revised.Comment: 5 pages, 2 figure
The NJL-jet model for quark fragmentation functions
A description of fragmentation functions which satisfy the momentum and
isospin sum rules is presented in an effective quark theory. Concentrating on
the pion fragmentation function, we first explain why the elementary (lowest
order) fragmentation process q --> q \pi is completely inadequate to describe
the empirical data, although the "crossed" process \pi --> q \bar{q} describes
the quark distribution functions in the pion reasonably well. Taking into
account cascade-like processes in a generalized jet-model approach, we then
show that the momentum and isospin sum rules can be satisfied naturally,
without the introduction of ad hoc parameters. We present results for the
Nambu--Jona-Lasinio (NJL) model in the invariant mass regularization scheme and
compare them with the empirical parametrizations. We argue that the NJL-jet
model, developed herein, provides a useful framework with which to calculate
the fragmentation functions in an effective chiral quark theory.Comment: 21 pages, 7 figure
Isovector EMC effect explains the NuTeV anomaly
A neutron or proton excess in nuclei leads to an isovector-vector mean-field
which, through its coupling to the quarks in a bound nucleon, implies a shift
in the quark distributions with respect to the Bjorken scaling variable. We
show that this result leads to an additional correction to the NuTeV
measurement of sin^2(Theta_W). The sign of this correction is largely model
independent and acts to reduce their result. Explicit calculation within a
covariant and confining Nambu--Jona-Lasinio model predicts that this vector
field correction accounts for approximately two-thirds of the NuTeV anomaly. We
are therefore led to offer a new interpretation of the NuTeV measurement,
namely, that it is further evidence for the medium modification of the bound
nucleon wavefunction.Comment: 4 pages, 2 figure
Commentary on rainbow-ladder truncation for excited states and exotics
Ground-state, radially-excited and exotic scalar-, vector- and
flavoured-pseudoscalar-mesons are studied in rainbow-ladder truncation using an
interaction kernel that is consonant with modern DSE- and lattice-QCD results.
The inability of this truncation to provide realistic predictions for the
masses of excited- and exotic-states is confirmed and explained. On the other
hand, its application does provide information that is potentially useful in
proceeding beyond this leading-order truncation, e.g.: assisting with
development of projection techniques that ease the computation of excited state
properties; placing qualitative constraints on the long-range behaviour of the
interaction kernel; and highlighting and illustrating some features of hadron
observables that do not depend on details of the dynamics.Comment: 10 pages, 6 figures, 2 table
Spin dependent parton distributions and structure functions
Nuclear parton distributions and structure functions are determined in an
effective chiral quark theory. We also discuss an extension of our model to
fragmentation functions.Comment: To appear in the proceedings of the 20th European Conference on
Few-Body Problems in Physics, Pisa, September 10-14, 200
On unifying the description of meson and baryon properties
A Poincare' covariant Faddeev equation is presented, which enables the
simultaneous prediction of meson and baryon observables using the leading-order
in a truncation of the Dyson-Schwinger equations that can systematically be
improved. The solution describes a nucleon's dressed-quark core. The evolution
of the nucleon mass with current-quark mass is discussed. A nucleon-photon
current, which can produce nucleon form factors with realistic Q^2-evolution,
is described. Axial-vector diquark correlations lead to a neutron Dirac form
factor that is negative, with r_1^{nu}>r_1^{nd}. The proton electric-magnetic
form factor ratio falls with increasing Q^2.Comment: 5 pages, 4 figures, 1 tabl
Chiral susceptibility and the scalar Ward identity
The chiral susceptibility is given by the scalar vacuum polarisation at zero
total momentum. This follows directly from the expression for the vacuum quark
condensate so long as a nonperturbative symmetry preserving truncation scheme
is employed. For QCD in-vacuum the susceptibility can rigorously be defined via
a Pauli-Villars regularisation procedure. Owing to the scalar Ward identity,
irrespective of the form or Ansatz for the kernel of the gap equation, the
consistent scalar vertex at zero total momentum can automatically be obtained
and hence the consistent susceptibility. This enables calculation of the chiral
susceptibility for markedly different vertex Ansaetze. For the two cases
considered, the results were consistent and the minor quantitative differences
easily understood. The susceptibility can be used to demarcate the domain of
coupling strength within a theory upon which chiral symmetry is dynamically
broken. Degenerate massless scalar and pseudoscalar bound-states appear at the
critical coupling for dynamical chiral symmetry breaking.Comment: 9 pages, 5 figures, 1 tabl
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