3,079 research outputs found
Isospin breaking in the phases of the Ke4 form factors
Isospin breaking in the Kl4 form factors induced by the difference between
charged and neutral pion masses is studied. Starting from suitably subtracted
dispersion representations, the form factors are constructed in an iterative
way up to two loops in the low-energy expansion by implementing analyticity,
crossing, and unitarity due to two-meson intermediate states. Analytical
expressions for the phases of the two-loop form factors of the K\pm -> pi^+
pi^- e^\pm nu_e channel are given, allowing one to connect the difference of
form-factor phase shifts measured experimentally (out of the isospin limit) and
the difference of S- and P-wave pi-pi phase shifts studied theoretically (in
the isospin limit). The isospin-breaking correction consists of the sum of a
universal part, involving only pi-pi rescattering, and a process-dependent
contribution, involving the form factors in the coupled channels. The
dependence on the two S-wave scattering lengths a_0^0 and a_0^2 in the isospin
limit is worked out in a general way, in contrast to previous analyses based on
one-loop chiral perturbation theory. The latter is used only to assess the
subtraction constants involved in the dispersive approach. The two-loop
universal and process-dependent contributions are estimated and cancel
partially to yield an isospin-breaking correction close to the one-loop case.
The recent results on the phases of K^\pm -> pi^+ pi^- e^\pm nu_e form factors
obtained by the NA48/2 collaboration at the CERN SPS are reanalysed including
this isospin-breaking correction to extract values for the scattering lengths
a_0^0 and a_0^2, as well as for low-energy constants and order parameters of
two-flavour ChPT.Comment: 48 pages, 7 figure
The pionic beta decay in chiral perturbation theory
Within the framework of chiral perturbation theory with virtual photons and
leptons, we present an updated analysis of the pionic beta decay including all
electromagnetic contributions of order e**2 p**2. We discuss the extraction of
the Cabibbo-Kobayashi-Maskawa matrix element |Vud| from experimental data. The
method employed here is consistent with the analogous treatment of the Kl3
decays and the determination of |Vus|.Comment: 8 pages, 1 figure, latex file, uses EPJC macro
Radiative corrections to K_{l3} decays
We present a complete calculation of the K_{l3} decays K^+ --> pi^0 l^+ nu_l
and K^0 --> pi^- l^+ nu_l to O(p^4, (m_d-m_u) p^2, e^2 p^2) in chiral
perturbation theory with virtual photons and leptons. We introduce the concept
of generalized form factors and kinematical densities in the presence of
electromagnetism, and propose a possible treatment of the real photon emission
in K^+_{l3} decays. We illustrate our results by applying them to the
extraction of the Kobayashi--Maskawa matrix element |V_{us}| from the
experimental K^+_{e3} decay parameters.Comment: 13 page
Reanalysis of pion pion phase shifts from K -> pi pi decays
We re-investigate the impact of isospin violation for extracting the s-wave
pion pion scattering phase shift difference delta_0(M_K) - delta_2(M_K) from K
-> pi pi decays. Compared to our previous analysis in 2003, more precise
experimental data and improved knowledge of low-energy constants are used. In
addition, we employ a more robust data-driven method to obtain the phase shift
difference delta_0(M_K) - delta_2(M_K) = (52.5 \pm 0.8_{exp} \pm 2.8_{theor})
degrees.Comment: 8 page
Specific binding of chloride ions to lipid vesicles and implications at molecular scale
Biological membranes composed of lipids and proteins are in contact with electrolytes like aqueous NaCl solutions. Based on molecular dynamics studies it is widely believed that Na(+) ions specifically bind to 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine (POPC) membranes, whereas Cl(−) ions stay in solution. Here, we present a careful comparison of recent data from electrophoresis and isothermal titration calorimetry experiments as well as molecular dynamics simulations suggesting that in fact both ions show very similar affinities. The corresponding binding constants are [Formula: see text] M(−1) for Na(+) and [Formula: see text] M(−1) for Cl(−) ions. This is highlighted by our observation that a widely used simulation setup showing asymmetric affinities of Na(+) and Cl(−) for POPC bilayers overestimates the effect of NaCl on the electrophoretic mobility of a POPC membrane by an order of magnitude. Implications for previous simulation results on the effect of NaCl on polarization of interfacial water, transmembrane potentials, and mechanisms for ion transport through bilayers are discussed. Our findings suggest that a range of published simulations results on the interaction of NaCl with phosphocholine bilayers have to be reconsidered and revised and that force field refinements are necessary for reliable simulation studies of membranes at physiological conditions on a molecular level
Rare decay pi0 -> e+e-: theory confronts KTeV data
Within the dispersive approach to the amplitude of the rare decay pi0 -> e+e-
the nontrivial dynamics is contained only in the subtraction constant. We
express this constant, in the leading order in (m_e/\Lambda)^2 perturbative
series, in terms of the inverse moment of the pion transition form factor given
in symmetric kinematics. By using the CELLO and CLEO data on the pion
transition form factor given in asymmetric kinematics the lower bound on the
decay branching ratio is found. The restrictions following from QCD allow us to
make a quantitative prediction for the branching B(pi0 -> e+e-) =(6.2\pm
0.1)*10^{-8} which is 3\sigma below the recent KTeV measurement. We confirm our
prediction by using the quark models and phenomenological approaches based on
the vector meson dominance. The decays \eta -> l^+l^- are also discussed.Comment: 7 pages, 1 figur
Exploring covalently bonded diamondoid particles with valence photoelectron spectroscopy
We investigated the valence electronic structure of diamondoid particles in
the gas phase, utilizing valence photoelectron spectroscopy. The samples were
singly or doubly covalently bonded dimers or trimers of the lower diamondoids.
Both the bond type and the combination of bonding partners are shown to affect
the overall electronic structure. For singly bonded particles, we observe a
small impact of the bond on the electronic structure, whereas for doubly bonded
particles, the connecting bond determines the electronic structure of the
highest occupied orbitals. In the singly bonded particles a superposition of
the bonding partner orbitals determines the overall electronic structure. The
experimental findings are supported by density functional theory computations
at the M06-2X/cc-pVDZ level of theory.Comment: 7 pages, 7 figure
Orbital-selective Mott transitions in the anisotropic two-band Hubbard model at finite temperatures
The anisotropic degenerate two-orbital Hubbard model is studied within
dynamical mean-field theory at low temperatures. High-precision calculations on
the basis of a refined quantum Monte Carlo (QMC) method reveal that two
distinct orbital-selective Mott transitions occur for a bandwidth ratio of 2
even in the absence of spin-flip contributions to the Hund exchange. The second
transition -- not seen in earlier studies using QMC, iterative perturbation
theory, and exact diagonalization -- is clearly exposed in a low-frequency
analysis of the self-energy and in local spectra.Comment: 4 pages, 5 figure
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