799 research outputs found
Cell–cell membrane fusion during mammalian fertilization
AbstractThe mechanism of sperm–egg fusion in mammals is a research area that has greatly benefited from the use of gene deletion technology. Because fertilization is internal in mammals and the gametes (particularly the eggs) are sparse in number, in vitro studies have considerable limitations. Using gene deletions, a few cell surface proteins in both gametes have been identified as essential for gamete fusion. Ongoing studies are directed at analysis of the function of these proteins and the search for additional proteins that may be involved in this process. So far, no mammalian proteins have been found that also function in sperm–egg fusion of non-mammalian species or in other types of cell–cell fusion
Ultraperipheral photoproduction of vector mesons in the nuclear Coulomb field and the size of neutral vector mesons
We point out a significance of ultraperipheral photoproduction of vector
mesons in the Coulomb field of nuclei as a means of measuring the radius of the
neutral vector meson. This new contribution to the production amplitude is very
small compared to the conventional diffractive amplitude, but because of large
impact parameters inherent to the ultraperipheral Coulomb mechanism its impact
on the diffraction slope is substantial. We predict appreciable and strongly
energy dependent increase of the diffraction slope towards very small momentum
transfer.The magnitude of the effect is proportional to the mean radius squared
of the vector meson and is within the reach of high precision photoproduction
experiments, which gives a unique experimental handle on the size of vector
mesons
On decays of light unflavoured pseudoscalar mesons
The ongoing and planned experimental activities with direct reference to
light unflavoured pseudoscalar mesons motivate a new theoretical study
regarding their properties. An overview including details on new precise
calculations is presented.Comment: 9 pages, 1 fugre, presented at Physics of Fundamental Symmetries and
Interactions - PSI201
Effective Hamiltonian Approach to Hyperon Beta Decay with Final-State Baryon Polarization
Using an effective Hamiltonian approach, we obtain expressions for hyperon
beta decay final-state baryon polarization. Terms through second order in the
energy release are retained. The resulting approximate expressions are much
simpler and more compact than the exact expressions, and they agree closely
with them.Comment: 1 Figure Will appear in Phys Rev D 60 Article 117505 (Dec 1, 1999
K+ to pi-mu+mu+ and doubly-charged Higgs
The rate for the lepton-number-violating decay K+ to pi- mu+mu+ is calculated
in a model which incorporates doubly-charged Higgs bosons. We find that for
reasonable values of the parameters the decay branching ratio may be as large
as 2E-16. Although this is a discouragingly small number, it is of the same
order of magnitude as the rate mediated by massive Majorana neutrinos.Comment: 8 pages, RevTex, Figure1 is P
Precision Measurements of the 7Be(p,gamma)8B Reaction with Radioactive Beams and the 8B Solar Neutrino Flux
The 7Be(p,gamma)8B reaction is one of the major source of uncertainties in
estimating the 8B solar neutrino flux and is critical for undertsanding the
Solar Neutrino Problem and neutrinos. The main source of uncertainty is the
existence of conflicting data with different absolute normalization. Attempts
to measure this reaction rate with 7Be beams are under way by the UConn-LLN
collaboration, and we discuss a newly emerging method to extract this cross
section from the Coulomb dissociation of the radioactive beam of 8B. We discuss
some of the issues relevant for this study including the question of the E2
contribution to the Coulomb dissociation process which was measured to be
small. The Coulomb dissociation appears to provide a viable alternative method
for measuring the 7Be(p,gamma)8B reaction rate, with a weighted average of the
RIKEN1, RIKEN2 and GSI1 published results of S17(0) = 19.4 +/- 1.3 eV-b.Comment: 8 pages, 4 Figures Work Supported by USDOE Grant No.
DE-FG02-94ER40870 To Be published in Prog. Part. Nucl. Phy
Ordinary and radiative muon capture on the proton and the pseudoscalar form factor of the nucleon
We calculate ordinary and radiative muon capture on the proton in an
effective field theory of pions, nucleons and delta isobars, working to third
and second order in the small scale expansion respectively. Preceding
calculations in chiral effective field theories only employed pion and nucleon
degrees of freedom and were not able to reproduce the photon spectrum in the
pioneering experiment of radiative muon capture on the proton from TRIUMF. For
the past few years it has been speculated that the discrepancy between theory
and experiment might be due to Delta(1232) related effects, which are only
included via higher order contact interactions in the standard chiral approach.
In this report we demonstrate that this speculation does not hold true. We show
that contrary to expectations from naive dimensional analysis isobar effects on
the photon spectrum and the total rate in radiative muon capture are of the
order of a few percent, consistent with earlier findings in a more
phenomenological approach. We further demonstrate that both ordinary and
radiative muon capture constitute systems with a very well behaved chiral
expansion, both in standard chiral perturbation theory and in the small scale
expansion, and present some new ideas that might be at the bottom of the still
unresolved discrepancy between theory and experiment in radiative muon capture.
Finally we comment upon the procedure employed by the TRIUMF group to extract
new information from their radiative muon capture experiment on the
pseudoscalar form factor of the nucleon. We show that it is inconsistent with
the ordinary muon capture data.Comment: 22 pp, RevTeX, uses epsf, 8 figs, enlarged version, discussion of
ordinary muon capture on protons substantially enlarged, accepted for
publication in Nucl.Phys.A, FZJ-IKP(TH)-2000-0
Photon-Photon and Photon-Hadron Interactions at Relativistic Heavy Ion Colliders
In central collisions at relativistic heavy ion colliders like the
Relativistic Heavy Ion Collider RHIC/Brookhaven and the Large Hadron Collider
LHC (in its heavy ion mode) at CERN/Geneva, one aims at detecting a new form of
hadronic matter --- the Quark Gluon Plasma. We discuss here a complementary
aspect of these collisions, the very peripheral ones. Due to coherence, there
are strong electromagnetic fields of short duration in such collisions. They
give rise to photon-photon and photon-nucleus collisions with high flux up to
an invariant mass region hitherto unexplored experimentally. After a general
survey photon-photon luminosities in relativistic heavy ion collisions are
discussed. Then photon-photon physics at various gamma-gamma-invariant mass
scales is discussed. The region of several GeV, relevant for RHIC is dominated
by QCD phenomena (meson and vector meson pair production). Invariant masses of
up to about 100 GeV can be reached at LHC, and the potential for new physics is
discussed. Lepton-pair production, especially electron-positron pair production
is copious. Due to the strong fields there will be new phenomena, especially
multiple e+e- pair production.Comment: 10 pages, Proceedings of the Erice Summer School on Heavy Ion Physics
199
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