4,934 research outputs found

### Electromagnetic Corrections to Charged Pion Scattering at Low Energies

The electromagnetic corrections to the low energy scattering amplitude
involving charged pions only are investigated at leading and next-to-leading
orders in the two-flavour chiral expansion. As an application, the
corresponding variation in the strong $2S-2P$ level shift is evaluated. The
relative variation is of the order of 5%.Comment: LateX2e, 10 pages, 2 figure

### An example of resonance saturation at one loop

We argue that the large-Nc expansion of QCD can be used to treat a Lagrangian
of resonances in a perturbative way. As an illustration of this we compute the
L_10 coupling of the Chiral Lagrangian by integrating out resonance fields at
one loop. Given a Lagrangian and a renormalization scheme, this is how in
principle one can answer in a concrete and unambiguous manner questions such as
at what scale resonance saturation takes place.Comment: 9 pages, 5 figures. Enlarged discussion, results unchanged. To be
published in Phys. Rev.

### On the holomorphic factorization for superconformal fields

For a generic value of the central charge, we prove the holomorphic
factorization of partition functions for free superconformal fields which are
defined on a compact Riemann surface without boundary. The partition functions
are viewed as functionals of the Beltrami coefficients and their fermionic
partners which variables parametrize superconformal classes of metrics.Comment: 5 pages, LATEX, MPI-Ph/92-7

### 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

### V0 particle production studies at LHCb

Although QCD is firmly established as the fundamental theory of strong interactions, the fragmentation process from partons into hadrons is still poorly understood. Phenomenological models tuned to Tevatron data show significant differences when extrapolated to LHC energies. The hadronization process can be probed at the LHC by studying V0 production, i.e. the production of KS mesons and Lambda hyperons. The LHCb experiment, with a rapidity range complementary to that of the other LHC detectors, offers a particularly interesting environment, covering the forward region where the existing models are very tunable but lack predictive power. The first 100 millions minimum bias events at LHCb will already provide a high-statistics and high-purity V0 sample. Measurements will include differential cross sections and production ratios for different strange particles as a function of rapidity and transverse momentum. The analysis can naturally be extended to cover heavier hyperons as well, and eventually lead, with larger data sets obtained with a J/psi trigger, to b-baryon spectroscopy with J/psi-hyperon final states

### Chiral Perturbation Theory with Virtual Photons and Leptons

We construct a low-energy effective field theory which allows the full
treatment of isospin-breaking effects in semileptonic weak interactions. To
this end, we enlarge the particle spectrum of chiral perturbation theory with
virtual photons by including also the light leptons as dynamical degrees of
freedom. Using super-heat-kernel techniques, we determine the additional
one-loop divergences generated by the presence of virtual leptons and give the
full list of associated local counterterms. We illustrate the use of our
effective theory by applying it to the decays pi -> l nu_{l} and K -> l nu_{l}.Comment: 22 pages, no figure

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