874 research outputs found
Hadron structure in tau -> KKpi nu_tau decays
We analyse the hadronization structure of both vector and axial-vector
currents leading to tau -> KKpi nu_tau decays. At leading order in the 1/Nc
expansion, and considering only the contribution of the lightest resonances, we
work out, within the framework of the resonance chiral Lagrangian, the
structure of the local vertices involved in those processes. The couplings in
the resonance theory are constrained by imposing the asymptotic behaviour of
vector and axial-vector spectral functions ruled by QCD. In this way we predict
the hadron spectra and conclude that, contrarily to previous assertions, the
vector contribution dominates by far over the axial-vector one in all KKpi
charge channels.Comment: 32 pages, 7 figure
Quantum Loops in the Resonance Chiral Theory: The Vector Form Factor
We present a calculation of the Vector Form Factor at the next-to-leading
order in the 1/N_C expansion, within the framework of Resonance Chiral Theory.
The calculation is performed in the chiral limit, and with two dynamical quark
flavours. The ultraviolet behaviour of quantum loops involving virtual
resonance propagators is analyzed, together with the kind of counterterms
needed in the renormalization procedure. Using the lowest-order equations of
motion, we show that only a few combinations of local couplings appear in the
final result. The low-energy limit of our calculation reproduces the standard
Chiral Perturbation Theory formula, allowing us to determine the resonance
contribution to the chiral low-energy couplings, at the next-to-leading order
in 1/N_C, keeping a full control of their renormalization scale dependence.Comment: 27+1 pages, 9 figure
Contour-improved versus fixed-order perturbation theory in hadronic tau decays
The hadronic decay rate of the tau lepton serves as one of the most precise
determinations of the QCD coupling alpha_s. The dominant theoretical source of
uncertainty at present resides in the seeming disparity of two approaches to
improving the perturbative expansion with the help of the renormalisation
group, namely fixed-order and contour-improved perturbation theory. In this
work it is demonstrated that in fact both approaches yield compatible results.
However, the fixed-order series is found to oscillate around the
contour-improved result with an oscillation frequency of approximately six
perturbative orders, approaching it until about the 30th order, after which the
expansion reveals its asymptotic nature. Additionally, the renormalisation
scale and scheme dependencies of the perturbative series for the tau hadronic
width are investigated in detail.Comment: 20 pages, 5 eps-figures; discussion on scale and scheme dependence
added as compared to published journal version JHEP 09 (2005) 05
Scalar K pi form factor and light quark masses
Recent experimental improvements on K-decay data allow for a precise
extraction of the strangeness-changing scalar K pi form factor and the related
strange scalar spectral function. On the basis of this scalar as well as the
corresponding pseudoscalar spectral function, the strange quark mass is
determined to be m_s(2 GeV) = 92 +- 9 MeV. Further taking into account chiral
perturbation theory mass ratios, the light up and down quark masses turn out to
be m_u(2 GeV) = 2.7 +- 0.4 MeV as well as m_d(2 GeV) = 4.8 +- 0.5 MeV. As a
by-product, we also find a value for the Cabibbo angle |V_{us}| = 0.2236(29)
and the ratio of meson decay constants F_K/F_\pi = 1.203(16). Performing a
global average of the strange mass by including extractions from other channels
as well as lattice QCD results yields m_s(2 GeV) = 94 +- 6 MeV.Comment: 5 pages, 2 figures; comparison with lattice and global average added;
version to appear in Phys. Rev.
Magnetic Moments of Heavy Baryons
First non-trivial chiral corrections to the magnetic moments of triplet (T)
and sextet (S^(*)) heavy baryons are calculated using Heavy Hadron Chiral
Perturbation Theory. Since magnetic moments of the T-hadrons vanish in the
limit of infinite heavy quark mass (m_Q->infinity), these corrections occur at
order O(1/(m_Q \Lambda_\chi^2)) for T-baryons while for S^(*)-baryons they are
of order O(1/\Lambda_\chi^2). The renormalization of the chiral loops is
discussed and relations among the magnetic moments of different hadrons are
provided. Previous results for T-baryons are revised.Comment: 11 Latex pages, 2 figures, to be published in Phys.Rev.
On the Anomalous Discrete Symmetry
We examine an interesting scenario to solve the domain wall problem recently
suggested by Preskill, Trivedi, Wilczek and Wise. The effective potential is
calculated in the presence of the QCD axial anomaly. It is shown that some
discrete symmetries such as CP and Z_2 can be anomalous due to a so-called
-term induced by instantons. We point out that Z_2 domain-wall problem in
the two-doublet standard model can be resolved by two types of solutions: the
CP-conserving one and the CP-breaking one. In the first case, there exist two
Z_2-related local minima whose energy splitting is provided by the instanton
effect. In the second case, there is only one unique vacuum so that the domain
walls do not form at all. The consequences of this new source of CP violation
are discussed and shown to be well within the experimental limits in weak
interactions.Comment: 10 papges in LaTeX, SFU-Preprint-92-
The NuTeV Anomaly, Lepton Universality, and Non-Universal Neutrino-Gauge Couplings
In previous studies we found that models with flavor-universal suppression of
the neutrino-gauge couplings are compatible with NuTeV and Z-pole data. In this
paper we expand our analysis to obtain constraints on flavor-dependent coupling
suppression by including lepton universality data from W, tau, pi and K decays
in fits to model parameters. We find that the data are consistent with a
variety of patterns of coupling suppression. In particular, in scenarios in
which the suppression arises from the mixing of light neutrinos with heavy
gauge singlet states (neutrissimos), we find patterns of flavor-dependent
coupling suppression which are also consistent with constraints from mu -> e
gamma.Comment: REVTeX4, 25 pages, 10 postscript figures. Updated fits using the new
top mass. Updated figures. Extended discussion on the status of the
determination of B(tau->pi nu
Hard Corrections as a Probe of the Symmetry Breaking Sector
Non-decoupling effects related to a large affecting non-oblique
radiative corrections in vertices () and boxes (- mixing
and ) are very sensitive to the particular mechanism of spontaneous
symmetry breaking. We analyze these corrections in the framework of a chiral
electroweak standard model and find that there is only one operator in the
effective lagrangian which modifies the longitudinal part of the boson
without touching the oblique corrections. The inclusion of this operator
affects the vertex, the - mixing and the CP-violating
parameter , generating interesting correlations among the hard
corrections to these observables, for example, the maximum
vertex correction allowed by low energy physics is about one
percent.Comment: LaTex, 8 pages, 1 postscript figure include
The McCoy-Wu Model in the Mean-field Approximation
We consider a system with randomly layered ferromagnetic bonds (McCoy-Wu
model) and study its critical properties in the frame of mean-field theory. In
the low-temperature phase there is an average spontaneous magnetization in the
system, which vanishes as a power law at the critical point with the critical
exponents and in the bulk and at the
surface of the system, respectively. The singularity of the specific heat is
characterized by an exponent . The samples reduced
critical temperature has a power law distribution and we show that the difference between the values of the
critical exponents in the pure and in the random system is just . Above the critical temperature the thermodynamic quantities behave
analytically, thus the system does not exhibit Griffiths singularities.Comment: LaTeX file with iop macros, 13 pages, 7 eps figures, to appear in J.
Phys.
Charm mass dependence of the weak Hamiltonian in chiral perturbation theory
Suppose that the weak interaction Hamiltonian of four-flavour SU(4) chiral
effective theory is known, for a small charm quark mass m_c. We study how the
weak Hamiltonian changes as the charm quark mass increases, by integrating it
out within chiral perturbation theory to obtain a three-flavour SU(3) chiral
theory. We find that the ratio of the SU(3) low-energy constants which mediate
Delta I=1/2 and Delta I=3/2 transitions, increases rather rapidly with m_c, as
\sim m_c ln (1/m_c). The logarithmic effect originates from "penguin-type"
charm loops, and could represent one of the reasons for the Delta I=1/2 rule.Comment: 20 pages. v2: references and clarifications added, published versio
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