3,837 research outputs found
The Photon Structure Function at Small-x
It is shown that recent small-x measurements of the photon structure function
F_2^{\gamma}(x,Q^2) by the LEP-OPAL collaboration are consistent with
parameter-free QCD predictions at all presently accessible values of Q^2.Comment: 7 pages, LaTeX, 2 figure
Spectroscopy of Po
Prompt, in-beam rays following the reaction Yb + 142 MeV
Si were measured at the ATLAS facility using 10 Compton-suppressed Ge
detectors and the Fragment Mass Analyzer. Transitions in Po were
identified and placed using -ray singles and coincidence data gated on
the mass of the evaporation residues. A level spectrum up to
J10 was established. The structure of Po is more
collective than that observed in the heavier polonium isotopes and indicates
that the structure has started to evolve towards the more collective nature
expected for deformed nuclei.Comment: 8 pages, revtex 3.0, 4 figs. available upon reques
Has the QCD RG-Improved Parton Content of Virtual Photons been Observed?
It is demonstrated that present and DIS ep data on the structure of
the virtual photon can be understood entirely in terms of the standard `naive'
quark--parton model box approach. Thus the QCD renormalization group (RG)
improved parton distributions of virtual photons, in particular their gluonic
component, have not yet been observed. The appropriate kinematical regions for
their future observation are pointed out as well as suitable measurements which
may demonstrate their relevance.Comment: 24 pages, LaTeX, 5 figure
The QCD Pomeron in Ultraperipheral Heavy Ion Collisions: IV. Photonuclear Production of Vector Mesons
The photonuclear production of vector mesons in ultraperipheral heavy ion
collisions is investigated within the QCD color dipole picture, with particular
emphasis on the saturation model. The integrated cross section and the rapidity
distribution for the A + A --> V + A + A (V = rho, omega, phi, J/Psi) process
are computed and theoretical estimates for scattering on both light and heavy
nuclei are given for energies of RHIC and LHC. A comparison with the recent
STAR data on coherent production of rho mesons is also presented. Furthermore,
we calculate the photoproduction of vector mesons in proton-proton collisions
at RHIC, Tevatron and LHC energies.Comment: 19 pages, 6 figures and 2 tables. Version to be published in Eur.
Phys. J.
The QCD Pomeron in ultraperipheral heavy ion collisions: III. Photonuclear production of heavy quarks
We calculate the photonuclear production of heavy quarks in ultraperipheral
heavy ion collisions. The integrated cross section and the rapidity
distribution are computed employing sound high energy QCD formalisms as the
collinear and semihard approaches as well as the saturation model. In
particular, the color glass condensate (CGC) formalism is also considered using
a simple phenomenological parameterization for the color field correlator in
the medium, which allow us to obtain more reliable estimates for charm and
bottom production at
LHC energies.Comment: 15 pages, 2 figures. Extended version to be published in Eur. Phys.
J.
Parton distributions in the virtual photon target up to NNLO in QCD
Parton distributions in the virtual photon target are investigated in
perturbative QCD up to the next-to-next-to-leading order (NNLO). In the case
, where () is the mass squared of the
probe (target) photon, parton distributions can be predicted completely up to
the NNLO, but they are factorisation-scheme-dependent. We analyse parton
distributions in two different factorisation schemes, namely and
schemes, and discuss their scheme dependence. We show that
the factorisation-scheme dependence is characterised by the large-
behaviours of quark distributions. Gluon distribution is predicted to be very
small in absolute value except in the small- region.Comment: 28 pages, 5 figures, version to appear in Eur. Phys. J.
Virtual photon structure functions and positivity constraints
We study the three positivity constraints among the eight virtual photon
structure functions, derived from the Cauchy-Schwarz inequality and which are
hence model-independent. The photon structure functions obtained from the
simple parton model show quite different behaviors in a massive quark or a
massless quark case, but they satisfy, in both cases, the three positivity
constraints. We then discuss an inequality which holds among the unpolarized
and polarized photon structure functions , and
, in the kinematic region , where is the mass squared of the probe (target) photon, and we examine
whether this inequality is satisfied by the perturbative QCD results.Comment: 24 pages, 13 eps figure
Relative spins and excitation energies of superdeformed bands in 190Hg: Further evidence for octupole vibration
An experiment using the Eurogam Phase II gamma-ray spectrometer confirms the
existence of an excited superdeformed (SD) band in 190Hg and its very unusual
decay into the lowest SD band over 3-4 transitions. The energies and dipole
character of the transitions linking the two SD bands have been firmly
established. Comparisons with RPA calculations indicate that the excited SD
band can be interpreted as an octupole-vibrational structure.Comment: 12 pages, latex, 4 figures available via WWW at
http://www.phy.anl.gov/bgo/bc/hg190_nucl_ex.htm
Next-to-next-to-leading order QCD corrections to the photon's parton structure
The next-to-next-to-leading order (NNLO) corrections in massless perturbative
QCD are derived for the parton distributions of the photon and the deep
inelastic structure functions F_1^gamma and F_2^gamma. We present the full
photonic coefficient functions at order alpha alpha_s and calculate the first
six even-integer moments of the corresponding O(alpha alpha_s^2) photon-quark
and photon-gluon splitting functions together with the moments of the alpha
alpha_s^2 coefficient functions which enter only beyond NNLO. These results are
employed to construct parametrizations of the splitting functions which prove
to be sufficiently accurate at least for momentum fractions x >= 0.05. We also
present explicit expressions for the transformation from the MS_bar to the
DIS_gamma factorization scheme and write down the solution of the evolution
equations. The numerical impact of the NNLO corrections is discussed in both
schemes.Comment: 39 pages, LaTeX, 9 eps-figures. A few minor typos and a misprint in
Eq. (5.9) correcte
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