460 research outputs found
Ion-lithium collision dynamics studied with an in-ring MOTReMi
We present a novel experimental tool allowing for kinematically complete
studies of break-up processes of laser-cooled atoms. This apparatus, the
'MOTReMi', is a combination of a magneto-optical trap (MOT) and a Reaction
Microscope (ReMi). Operated in an ion-storage ring, the new setup enables to
study the dynamics in swift ion-atom collisions on an unprecedented level of
precision and detail. In first experiments on collisions with 1.5 MeV/amu
O-Li the pure ionization of the valence electron as well as
ionization-excitation of the lithium target has been investigated
Breit Equation with Form Factors in the Hydrogen Atom
The Breit equation with two electromagnetic form-factors is studied to obtain
a potential with finite size corrections. This potential with proton structure
effects includes apart from the standard Coulomb term, the Darwin term,
retarded potentials, spin-spin and spin-orbit interactions corresponding to the
fine and hyperfine structures in hydrogen atom. Analytical expressions for the
hyperfine potential with form factors and the subsequent energy levels
including the proton structure corrections are given using the dipole form of
the form factors. Numerical results are presented for the finite size
corrections in the 1S and 2S hyperfine splittings in the hydrogen atom, the
Sternheim observable and the 2S and 2P hyperfine splittings in muonic
hydrogen. Finally, a comparison with some other existing methods in literature
is presented.Comment: 24 pages, Latex, extended version, title change
Z^* Resonances: Phenomenology and Models
We explore the phenomenology of, and models for, the Z^* resonances, the
lowest of which is now well established, and called the Theta. We provide an
overview of three models which have been proposed to explain its existence
and/or its small width, and point out other relevant predictions, and potential
problems, for each. The relation to what is known about KN scattering,
including possible resonance signals in other channels, is also discussed.Comment: 29 pages, uses RevTeX4; expanded version (published form
History of exotic Meson (4-quark) and Baryon (5-quark) States
I briefly review the history of exotic meson (4-quark) and baryon (5-quark)
states, which is rooted in the formalism of Regge pole and duality. There are
robust model-independent predictions for the exchange of 4-quark (Baryonium)
Regge trajectories in several processes, which are strongly supported by
experiment. On the other hand the predictions for the spectroscopy of 4-quark
resonances are based on specific QCD inspired models, with some experimental
support. The corresponding predictions for the recently discovered exotic
baryon (Pentaquark) state are briefly discussed.Comment: 14 pages Latex including 4 eps figures, final version to appear as a
topical review in J. Phys.
Quasi-Elastic Scattering in the Inclusive (He, t) Reaction
The triton energy spectra of the charge-exchange C(He,t) reaction
at 2 GeV beam energy are analyzed in the quasi-elastic nucleon knock-out
region. Considering that this region is mainly populated by the charge-exchange
of a proton in He with a neutron in the target nucleus and the final proton
going in the continuum, the cross-sections are written in the distorted-wave
impulse approximation. The t-matrix for the elementary exchange process is
constructed in the DWBA, using one pion- plus rho-exchange potential for the
spin-isospin nucleon- nucleon potential. This t-matrix reproduces the
experimental data on the elementary pn np process. The calculated
cross-sections for the C(He,t) reaction at to triton
emission angle are compared with the corresponding experimental data, and are
found in reasonable overall accord.Comment: 19 pages, latex, 11 postscript figures available at
[email protected], submitted to Phy.Rev.
Quantum time scales in alpha tunneling
The theoretical treatment of alpha decay by Gamow is revisited by
investigating the quantum time scales in tunneling. The time spent by an alpha
particle in front of the barrier and traversing it before escape is evaluated
using microscopic alpha nucleus potentials. The half-life of a nucleus is shown
to correspond to the time spent by the alpha knocking in front of the barrier.
Calculations for medium and super heavy nuclei show that from a multitude of
available tunneling time definitions, the transmission dwell time gives the
bulk of the lifetime of the decaying state, in most cases.Comment: LaTex, 1 figure, new comments and references adde
Coherent pion production in neutrino nucleus collision in the 1 GeV region
We calculate cross sections for coherent pion production in nuclei induced by
neutrinos and antineutrinos of the electron and muon type. The analogies and
differences between this process and the related ones of coherent pion
production induced by photons, or the (p,n) and reactions are
discussed. The process is one of the several ones occurring for intermediate
energy neutrinos, to be considered when detecting atmospheric neutrinos. For
this purpose the results shown here can be easily extrapolated to other
energies and other nuclei.Comment: 13 pages, LaTex, 8 post-script figures available at
[email protected]
A Naturally Narrow Positive Parity Theta^+
We present a consistent color-flavor-spin-orbital wave function for a
positive parity Theta^+ that naturally explains the observed narrowness of the
state. The wave function is totally symmetric in its flavor-spin part and
totally antisymmetric in its color-orbital part. If flavor-spin interactions
dominate, this wave function renders the positive parity Theta^+ lighter than
its negative parity counterpart. We consider decays of the Theta^+ and compute
the overlap of this state with the kinematically allowed final states. Our
results are numerically small. We note that dynamical correlations between
quarks are not necessary to obtain narrow pentaquark widths.Comment: 10 pages, 1 figure, Revtex4, two-column format, version to be
published in Phys. Rev. D, includes numerical estimates of decay width
Characterisation of Inactivation Domains and Evolutionary Strata in Human X Chromosome through Markov Segmentation
Markov segmentation is a method of identifying compositionally different subsequences in a given symbolic sequence. We have applied this technique to the DNA sequence of the human X chromosome to analyze its compositional structure. The human X chromosome is known to have acquired DNA through distinct evolutionary events and is believed to be composed of five evolutionary strata. In addition, in female mammals all copies of X chromosome in excess of one are transcriptionally inactivated. The location of a gene is correlated with its ability to undergo inactivation, but correlations between evolutionary strata and inactivation domains are less clear. Our analysis provides an accurate estimate of the location of stratum boundaries and gives a high–resolution map of compositionally different regions on the X chromosome. This leads to the identification of a novel stratum, as well as segments wherein a group of genes either undergo inactivation or escape inactivation in toto. We identify oligomers that appear to be unique to inactivation domains alone
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