4,373 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
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.
Syntactic structure and artificial grammar learning : The learnability of embedded hierarchical structures
Embedded hierarchical structures, such as ‘‘the rat the cat ate was brown’’, constitute a core generative property of a natural language theory. Several recent studies have reported learning of hierarchical embeddings in artificial grammar learning (AGL) tasks, and described the functional specificity of Broca’s area for processing such structures. In two experiments, we investigated whether alternative strategies can explain the learning success in these studies. We trained participants on hierarchical sequences, and found no evidence for the learning of hierarchical embeddings in test situations identical to those from other studies in the literature. Instead, participants appeared to solve the task by exploiting surface distinctions between legal and illegal sequences, and applying strategies such as counting or repetition detection. We suggest alternative interpretations for the observed activation of Broca’s area, in terms of the application of calculation rules or of a differential role of working memory. We claim that the learnability of hierarchical embeddings in AGL tasks remains to be demonstrated
Decay of pseudoscalars into lepton pairs and large-Nc QCD
The counterterm combination that describes the decay of pseudoscalar mesons
into charged lepton pairs at lowest order in chiral perturbation theory is
considered within the framework of QCD in the limit of a large number of
colours Nc. When further restricted to the lowest meson dominance approximation
to large-Nc QCD, our results agree well with the available experimental data.Comment: 5 pages, 2 figure
Pion-Pion Phase-Shifts and the Value of Quark-Antiquark Condensate in the Chiral Limit
We use low energy pion-pion phase-shifts in order to make distinction between
the alternatives for the value of the quark-antiquark condensate in the
chiral limit. We will consider the amplitude up to and including contributions within the Standard and Generalized Chiral Perturbation
Theory frameworks. They are unitarized by means of Pad\'e approximants in order
to fit experimental phase-shifts in the resonance region. As the best fits
correspond to , we conclude that pion-pion phase-shift
analysis favors the standard ChPT scenario, which assumes just one, large
leading order parameter .Comment: 5 pages, 3 figures and 1 tabl
Contributions of order to form factors and unitarity of the CKM matrix
The form factors for the semileptonic decay are computed to
order in generalized chiral perturbation theory. The main difference
with the standard expressions consists in contributions quadratic in
quark masses, which are described by a single divergence-free low-energy
constant, . A new simultaneous analysis is presented for the CKM matrix
element , the ratio , decay rates and the
scalar form factor slope . This framework easily accommodates the
precise value for deduced from superallowed nuclear -decays
Hadronic Contributions to the Muon Anomaly in the Constituent Chiral Quark Model
The hadronic contributions to the anomalous magnetic moment of the muon which
are relevant for the confrontation between theory and experiment at the present
level of accuracy, are evaluated within the same framework: the constituent
chiral quark model. This includes the contributions from the dominant hadronic
vacuum polarization as well as from the next--to--leading order hadronic vacuum
polarization, the contributions from the hadronic light-by-light scattering,
and the contributions from the electroweak hadronic vertex.
They are all evaluated as a function of only one free parameter: the
constituent quark mass. We also comment on the comparison between our results
and other phenomenological evaluations.Comment: Several misprints corrected and a clarifying sentence added. Three
figures superposed and two references added. Version to appear in JHE
- …