6,468 research outputs found
New bounds on lepton flavor violating decays of vector mesons and the Z0 boson
We give an estimate for the upper bounds on rates of lepton flavor violating
(LFV) decays M to mu(pm) + e(mp) of vector mesons M = rho0, omega, phi, J/psi,
Upsilon and the Z0 boson in a model independent way, analyzing the
corresponding lowest dimension effective operators. These operators also
contribute to nuclear mu-e-conversion. Based on this observation and using the
existing experimental limits on this LFV nuclear process, we show that the
studied two-body LFV decays of vector bosons are strongly suppressed
independent on the explicit realization of new physics. The upper limits on the
rates of some of these decays are significantly more stringent than similar
limits known in the literature. In view of these results experimental
observation of the two-body LFV decays of vector bosons looks presently
unrealistic.Comment: 4 pages, 1 figur
Electron-Positron Annihilation into Hadron-Antihadron Pairs
The reactions of electron-positron to nucleon-antinucleon pairs are studied
in a non-perturbative quark model. The work suggests that the two-step process,
in which the primary quark-antiquark pair forms first a vector meson which in
turn decays into a hadron pair, is dominant over the one-step process in which
the primary quark-antiquark pair is directly dressed by additional
quark-antiquark pairs to form a hadron pair. To reproduce the experimental data
of the reactions of electron-positron to proton-antiproton and
electron-positron to neutron-antineutron a D-wave omega-like vector meson with
a mass of around 2 GeV has to be introduced.Comment: 15 pages, 6 figure
Direct Test of the Scalar-Vector Lorentz Structure of the Nucleon- and Antinucleon-Nucleus Potential
Quantum Hadrodynamics in mean field approximation describes the effective
nucleon-nucleus potential (about -50 MeV deep) as resulting from a strong
repulsive vector (about 400 MeV) and a strong attractive scalar (about -450
MeV) contribution. This scalar-vector Lorentz structure implies a significant
lowering of the threshold for photoproduction on a nucleus by about
850 MeV as compared to the free case since charge conjugation reverses the sign
of the vector potential contribution in the equation of motion for the
states. It also implies a certain size of the photon induced
pair creation cross section near threshold which is calculated for a
target nucleus Pb. We also indicate a measurable second signature of
the photoproduction process by estimating the increased cross
section for emission of charged pions as a consequence of
annihilation within the nucleus.Comment: 18 pages latex, 5 PS figure
Nucleon QCD sum rules in nuclear matter including four-quark condensates
We calculate the nucleon parameters in nuclear matter using the QCD sum rules
approach in Fermi gas approximation. Terms up to 1/q^2 in the operator product
expansion (OPE) are taken into account. The higher moments of the nucleon
structure functions are included. The complete set of the nucleon expectation
values of the four-quark operators is employed. Earlier the lack of information
on these values has been the main obstacle for the further development of the
approach. We show that the four-quark condensates provide the corrections of
the order 20% to the results obtained in the leading orders of the OPE. This is
consistent with the assumption about the convergence of the OPE. The nucleon
vector self-energy \Sigma_v and the nucleon effective mass m^* are expressed in
terms of the in-medium values of QCD condensates. The numerical results for
these parameters at the saturation value of the density agree with those
obtained by the methods of nuclear physics.Comment: 38 pages, 5 figure
Nucleophilic Functionalization of the Calix[6]arene Para- and Meta-Position via p‑Bromodienone Route
It is here demonstrated that the p-bromodienone route,
previously reported for calix[4]arenes, is also effective for the functionalization
of the calix[6]arene macrocycle. Thus, alcoholic O-nucleophiles can be
introduced at the calix[6]arene exo rim. In addition, the reaction of a
calix[6]arene p-bromodienone derivative with an actived aromatic substrate,
such as resorcinol, led to the first example of a meta-functionalized,
inherently chiral calix[6]arene derivativ
The neutron electric dipole form factor in the perturbative chiral quark model
We calculate the electric dipole form factor of the neutron in a perturbative
chiral quark model, parameterizing CP-violation of generic origin by means of
effective electric dipole moments of the constituent quarks and their
CP-violating couplings to the chiral fields. We discuss the relation of these
effective parameters to more fundamental ones such as the intrinsic electric
and chromoelectric dipole moments of quarks and the Weinberg parameter. From
the existing experimental upper limits on the neutron EDM we derive constraints
on these CP-violating parameters.Comment: 20 pages, 3 figure
Dependence of nucleon properties on pseudoscalar meson masses
We discuss the sensitivity of nucleon properties (mass, magnetic moments and
electromagnetic form factors) on the variation of the pseudoscalar meson masses
in the context of the perturbative chiral quark model. The obtained results are
compared to data and other theoretical predictions.Comment: 18 pages, 12 figures, accepted for publication in J. Phys.
Decays of Zb(+) and Zb'(+) as hadronic molecules
The two newly observed hidden-bottom mesons Zb(10610) and Zb'(10650) with
quantum numbers J(P) = 1(+) are considered as hadronic molecules composed of
BB(*) and B(*)B(*), respectively. We give predictions for the widths of the
strong two-body decays Zb(+) to Upsilon(nS) + pi(+) and Zb'(+) to Upsilon(nS) +
pi(+) in a phenomenological Lagrangian approach.Comment: 9 page
Strong CP violation and the neutron electric dipole form factor
We calculate the neutron electric dipole form factor induced by the CP
violating theta-term of QCD, within a perturbative chiral quark model which
includes pion and kaon clouds. On this basis we derive the neutron electric
dipole moment and the electron-neutron Schiff moment. From the existing
experimental upper limits on the neutron electric dipole moment we extract
constraints on the theta-parameter and compare our results with other
approaches.Comment: 18 pages, 2 figures, accepted for publication in Phys. Atom. Nuc
Size-Matched Radical Multivalency
Persistent π-radicals such as MV^+• (MV refers to methyl viologen, i.e., N,Nꞌ-dimethyl-4,4ꞌ-bipyridinum) engage in weak radical-radical interactions. This phenomenon has been utilized recently in supramolecular chemistry with the discovery that MV+• and [cyclobis(paraquat-p-phenylene)]2(+•) (CBPQT2(+•)) form a strong 1:1 host-guest complex [CBPQT⊂MV]3(+•). In this full paper, we describe the extension of radical-pairing-based molecular recognition to a larger, square-shaped diradical host, [cyclobis(paraquat-4,4ꞌ-biphenylene)]2(+•) (MS2(+•)). This molecular square was assessed for its ability to bind an isomeric series of possible diradical cyclophane guests, which consist of two radical viologen units that are linked by two ortho-, meta-, or para-xylylene bridges to provide different spacings between the planar radicals. UV-Vis-NIR measurements reveal that only the m-xylylene-linked isomer (m-CBPQT2(+•)) binds strongly inside of MS2(+•), resulting in the formation of a tetra-radical complex [MS⊂m-CBPQT]4(+•). Titration experiments and variable temperature UV-Vis-NIR and EPR spectroscopic data indicate that, relative to the smaller tris-radical complex [CBPQT⊂MV]3(+•), the new host-guest complex forms with a more favorable enthalpy change that is offset by a greater entropic penalty. As a result, the association constant (Ka = (1.12+/- 0.08) x 10^5 M^(-1)) for [MS⊂m-CBPQT]4(+•) is similar to that previously determined for [CBPQT⊂MV]3(+•). The (super)structures of MS2(+•), m-CBPQT2(+•), and [MS⊂m-CBPQT]4(+•) were examined by single-crystal X-ray diffraction measurements and DFT calculations. The solid-state and computational structural analyses reveal that m-CBPQT2(+•) is ideally sized to bind inside of MS2(+•). The solid-state superstructures also indicate that localized radical-radical interactions in m-CBPQT2(+•) and [MS⊂m-CBPQT]4(+•) disrupt the extended radical-pairing interactions that are common in crystals of other viologen radical cations. Lastly, the formation of [MS⊂m-CBPQT]4(+•) was probed by cyclic voltammetry, demonstrating that the radical states of the cyclophanes are stabilized by the radical-pairing interactions
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