556 research outputs found
Effects of Quark Spin Flip on the Collins Fragmentation Function in a Toy Model
The recent extension of the NJL-jet model to hadronization of transversely
polarized quarks allowed the study of the Collins fragmentation function. Both
favored and unfavored Collins fragmentation functions were generated, the
latter purely by multiple hadron emissions, with 1/2 moments of opposite sign
in the region of the light-cone momentum fraction accessible in current
experiments. Hints of such behavior has been seen in the measurements in
several experiments. Also, in the transverse momentum dependent (TMD) hadron
emission probabilities, modulations of up to fourth order in sine of the polar
angle were observed, while the Collins effect describes just the linear
modulations. A crucial part of the extended model was the calculation of the
quark spin flip probability after each hadron emission in the jet. Here we
study the effects of this probability on the resulting unfavored and favored
Collins functions by setting it as a constant and use a toy model for the
elementary single hadron emission probabilities. The results of the Monte Carlo
simulations showed that preferential quark spin flip in the elementary hadron
emission is needed to generate the favored and unfavored Collins functions with
opposite sign 1/2 moments. For the TMD hadron emission modulations, we showed
that the model quark spin flip probabilities are a partial source of the higher
rode modulations, while the other source is the Collins modulation of the
remnant quark from the hadron emission recoil.Comment: 7 pages, 6 figures. To appear in proceedings of HITES 2012,
Conference in Honor of Jerry P. Draayer, Horizons of Innovative Theories,
Experiments, and Supercomputing in Nuclear Physics, New Orleans, Louisiana,
June 4-7, 201
Color Transparency via Coherent Exclusive rho Production
We examine the potential of the COMPASS experiment at CERN to study color
transparency via exclusive coherent vector meson production in hard
muon-nucleus scattering. It is demonstrated that COMPASS has high sensitivity
to test this important prediction of perturbative QCD.Comment: Feasibility study for COMPASS collaboration, 3 pages, no figures, 1
table. This contribution is based on talk presented at the Workshop on Spin
Physics, Trento, Italy, July 2001. The conference www site is
http://ECTstar.ect.it/contents.html The proceedings of the workshop will be
published as a special issue of Nuclear Physics B (proc suppl), eds. S Bass,
A De Roeck and A Deshpande. A more complete 26 page feasibility study with 7
figures and 3 tables is available as A. Sandacz et al., hep-ex/0106076
Revision of 26 Sept. for Ref. 11 modificatio
Extraction of the coupling constant from NN scattering data
We reexamine Chew's method for extracting the coupling constant from
np differential cross section measurements. Values for this coupling are
extracted below 350 MeV, in the potential model region, and up to 1 GeV. The
analyses to 1~GeV have utilized 55 data sets. We compare these results to those
obtained via mapping techniques. We find that these two methods give
consistent results which are in agreement with previous Nijmegen
determinations.Comment: 12 pages of text plus 2 figures. Revtex file and postscript figures
available via anonymous FTP at ftp://clsaid.phys.vt.edu/pub/n
The evolution of H{\sc ii} galaxies: Testing the bursting scenario through the use of self-consistent models
We have computed a series of realistic and self-consistent models of the
emitted spectra of H{\sc ii} galaxies. Our models combine different codes of
chemical evolution, evolutionary population synthesis and photoionization. The
emitted spectrum of H{\sc ii} galaxies is reproduced by means of the
photoionization code CLOUDY, using as ionizing spectrum the spectral energy
distribution of the modelled H{\sc ii} galaxy, which in turn is calculated
according to a Star Formation History (SFH) and a metallicity evolution given
by a chemical evolution model that follows the abundances of 15 different
elements. The contribution of emission lines to the broad-band colours is
explicitly taken into account.
The results of our code are compared with photometric and spectroscopic data
of H{\sc ii} galaxies. Our technique reproduces observed diagnostic diagrams,
abundances, equivalent width-colour and equivalent width-metallicity relations
for local H{\sc ii} galaxies.Comment: 13 figures and 2 tables, accepted for publication in MNRAS Main
Journa
The ionization mechanism of NGC 185: how to fake a Seyfert galaxy?
NGC 185 is a dwarf spheroidal satellite of the Andromeda galaxy. From
mid-1990s onwards it was revealed that dwarf spheroidals often display a varied
and in some cases complex star formation history. In an optical survey of
bright nearby galaxies, NGC 185 was classified as a Seyfert galaxy based on its
emission line ratios. However, although the emission lines in this object
formally place it in the category of Seyferts, it is probable that this galaxy
does not contain a genuine active nucleus. NGC 185 was not detected in radio
surveys either in 6 or 20 cm, or X-ray observations, which means that the
Seyfert-like line ratios may be produced by stellar processes. In this work, we
try to identify the possible ionization mechanisms for this galaxy. We
discussed the possibility of the line emissions being produced by planetary
nebulae (PNe), using deep spectroscopy observations obtained with GMOS-N, at
Gemini. Although the fluxes of the PNe are high enough to explain the
integrated spectrum, the line ratios are very far from the values for the
Seyfert classification. We then proposed that a mixture of supernova remnants
and PNe could be the source of the ionization, and we show that a composition
of these two objects do mimic Seyfert-like line ratios. We used chemical
evolution models to predict the supernova rates and to support the idea that
these supernova remnants should be present in the galaxy.Comment: 9 pages, 7 figures, accepted for publication in MNRA
Cosmic Supernova Rates and the Hubble Sequence
We compute the type Ia, Ib/c and II supernova (SN) rates as functions of the
cosmic time for galaxies of different morphological types. We use four
different chemical evolution models, each one reproducing the features of a
particular morphological type: E/S0, S0a/b, Sbc/d and Irr galaxies. We
essentially describe the Hubble sequence by means of decreasing efficiency of
star formation and increasing infall timescale. These models are used to study
the evolution of the SN rates per unit luminosity and per unit mass as
functions of cosmic time and as functions of the Hubble type. Our results
indicate that: (i) the observed increase of the SN rate per unit luminosity and
unit mass from early to late galaxy types is accounted for by our models. Our
explanation of this effect is related to the fact that the latest Hubble types
have the highest star formation rate per unit mass; (ii) By adopting a Scalo
(1986) initial mass function in spiral disks, we find that massive single stars
ending their lives as Wolf-Rayet objects are not sufficient to account for the
observed type Ib/c SN rate per unit mass. Less massive stars in close binary
systems can give instead a significant contribution to the local Ib/c SN rates.
On the other hand, with the assumption of a Salpeter (1955) IMF for all galaxy
types, single massive WR stars are sufficient to account for the observed type
Ib/c SN rate. (iii) Our models allow us to reproduce the observed type Ia SN
rate density up to redshift z~1. We predict an increasing type Ia SN rate
density with redshift, reaching a peak at redshift z >= 3, because of the
contribution of massive spheroids.Comment: ApJ, accepted for publication. 17 pages, 11 figure
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