4 research outputs found
On the extraction of electromagnetic properties of the Delta(1232) excitation from pion photoproduction
Several methods for the treatment of pion photoproduction in the region of
the Delta(1232) resonance are discussed, in particular the effective Lagrangian
approach and the speed plot analysis are compared to a dynamical treatment. As
a main topic, we discuss the extraction of the genuine resonance parts of the
magnetic dipole and electric quadrupole multipoles of the electromagnetic
excitation of the resonance. To this end, we try to relate the various values
for the ratio R_{EM} of the E2 to M1 multipole excitation strengths for the
Delta(1232) resonance as extracted by the different methods to corresponding
ratios of a dynamical model. Moreover, it is confirmed that all methods for
extracting resonance properties suffer from an unitary ambiguity which is due
to some phenomenological contributions entering the models.Comment: 22 pages revtex including 7 postscript figure
The Spectral Function in a Relativistic Resonance Model
We calculate the spectral function of the meson in nuclear
matter. The calculation is performed in the {\it low density} approximation,
where the in-medium self energy is completely determined by the
vacuum forward scattering amplitude. This amplitude is derived from a
relativistic resonance model. In comparison to previous non-relativistic
calculations we find a much weaker momentum dependence of ,
especially in the transverse channel. Special attention is paid to
uncertainties in the model. Thus, we compare the impact of different coupling
schemes for the interaction on the results and discuss various
resonance parameter sets.Comment: 47 pages, 19 figures, some discussion and formulae added, minor typos
removed, accepted for publication in Nuclear Physics
Charged current weak electroproduction of Delta resonance
We study the weak production of (i.e. and ) in the intermediate
energy range corresponding to the Mainz and TJNAF electron accelerators. The
differential cross sections are found to be of the order of cm/sr, over a range of angles which increases with energy. The
possibility of observing these reactions with the high luminosities available
at these accelerators, and studying the weak N- transition form factors
through these reactions is discussed. The production cross section of
N in the kinematic region of production is also estimated
and found to be small.Comment: 19 pages, REVTEX, 4 figure
Cold Accretion Disks and Lineless Quasars
The optical-UV continuum of quasars is broadly consistent with the emission
from a geometrically thin optically thick accretion disk (AD). The AD produces
the ionizing continuum which powers the broad and narrow emission lines. The
maximum AD effective temperature is given by Teff=fmax(Mdot/M^2)^1/4, where M
is the black hole mass, Mdot the accretion rate, and fmax is set by the black
hole spin a_*. For a low enough value of Mdot/M^2 the AD may become too cold to
produce ionizing photons. Such an object will form a lineless quasar. This
occurs for a local blackbody (BB) AD with a luminosity Lopt=10^46 erg/s for
M>3.6E9 Msun, when a_*=0, and for M>1.4E10 Msun, when a_*=0.998. Using the AD
based Mdot, derived from M and Lopt, and the reverberation based M, derived
from Lopt and the Hbeta FWHM, v, gives Teff \propto Lopt^-0.13v^-1.45. Thus,
Teff is mostly set by v. Quasars with a local BB AD become lineless for v>
8,000 km/s, when a_*=0, and for v> 16,000 km/s, when a_*=0.998. Higher values
of v are required if the AD is hotter than a local BB. The AD becoming
non-ionizing may explain why line emitting quasars with v>10,000 km/s are rare.
Weak low ionization lines may still be present if the X-ray continuum is
luminous enough, and such objects may form a population of weak emission line
quasars (WLQ). If correct, such WLQ should show a steeply falling SED at
lambda<1000A. Such an SED was observed by Hryniewicz et al. in SDSS
J094533.99+100950.1, a WLQ observed down to 570A, which is well modeled by a
rather cold AD SED. UV spectroscopy of z~1-2 quasars is required to eliminate
potential intervening Lyman limit absorption by the intergalactic medium (IGM),
and to explore if the SEDs of lineless quasars and some additional WLQ are also
well fit by a cold AD SED.Comment: Accepted for publication in MNRA