179 research outputs found
A narrow "peanut" pentaquark
We analyse the decay in a non-relativistic Fock space
description using three and five constituent quarks for the nucleon and the
pentaquark, respectively. Following Jaffe and Wilczek \cite{jw}, we assume that
quark-quark correlations in spin-zero state play an important role for the
pentaquark internal structure. Within this scenario, a strong dynamical
suppression of the decay width is shown to be possible only if the pentaquark
has an asymmetric "peanut" structure with the strange antiquark in the center
and the two extended composite diquarks rotating around. In this case a decay
width of 1 MeV may be a natural possibility.Comment: Talk given at the Conference "Quark Confinement and Hadron Spectrum
VI", Villasimius (Cagliari) Sardinia, Italy, 21 - 25 September 200
Dispersion approach to quark-binding effects in weak decays of heavy mesons
Dispersion approach based on the constituent quark picture and its
applications to weak decays of heavy mesons are reviewed. Meson interaction
amplitudes are represented within this approach as relativistic spectral
integrals over the mass variables in terms of the meson wave functions and
spectral densities of the corresponding Feynman diagrams. Various applications
of this approach are discussed:
Relativistic spectral representations for meson elastic and transition form
factors at spacelike momentum transfers are constructed. Form factors at
are obtained by the analytical continuation. As a result of this
procedure, form factors are given in the full range of the weak decay in
terms of the wave functions of the participating mesons.
The 1/m_Q expansion of the obtained spectral representations for the form
factors for the particular limits of the heavy-to-heavy and heavy-to-light
transitions are analysed. Their full consistency with the constraints provided
by long-distance QCD for these limits is demonstrated. Predictions for form
factors for B, B(s), D, and D(s) decays to light mesons are given. The
B\to\gamma l\nu decay and the weak annihilation in rare radiative decays are
considered. Nonfactorizable corrections to the B^0-\bar B^0 mixing are
calculated. Inclusive weak B-decays are analysed and the differential
distributions are obtained in terms of the B-meson wave function.Comment: revtex, 119 pages. Summary of changes made: In Sect. V comparison
with the recent experimental data is added, in Sect. VI discussion of contact
terms in B->V\gamma is extended and comments on the recent publications are
given, in Sect. VII signs of the decay constants are brought in agreement
with other Sections, typoes correcte
Exotic polyquark states and their properties in QCD
We formulate rigorous criteria for selecting diagrams to be taken into
account in the analysis of potential polyquark (tetra or penta) poles in QCD.
The central point of these criteria is the requirement that the Feynman
diagrams for the relevant Green functions contain four-quark (or, respectively,
five-quark in the pentaquark case) intermediate states and the corresponding
cuts. It is shown that some diagrams which "visually" seem to contain the
four-quark cuts, turn out to be free of these singularities and therefore
should not be taken into account when calculating the tetraquark properties. We
then consider large- QCD which in many cases provide qualitatively correct
picture of hadron properties and discuss in detail the tetraquark states. For
the "direct" and the "recombination" four-point Green functions, which may
potentially contain the tetraquark poles, we formulate large- consistency
conditions which strongly restrict the behaviour of the tetraquark-to-ordinary
meson transition amplitudes. In the end, these conditions allow us to obtain
constraints on width of the potential tetraquark states at large . We
report that both flavor-exotic and cryptoexotic (i.e., flavor-nonexotic)
tetraquarks, if the corresponding poles exist, have a width of order
, i.e. they should be parametrically narrower compared to the
ordinary mesons with a width of order . Moreover, for
flavor-exotic states, the large- consistency conditions require two narrow
flavor-exotic states, each of these states coupling dominantly to one specific
meson-meson channel.Comment: Plenary talk given at the XXIII International Workshop "High Energy
Physics and Quantum Field Theory" (QFTHEP 2017), June 26-July 3, 2017,
Yaroslavl, Russi
Advances in QCD sum rule calculations
We review the recent progress in the applications of QCD sum rules to hadron
properties with the emphasis on the following selected problems: (i)
development of new algorithms for the extraction of ground-state parameters
from two-point correlators; (ii) form factors at large momentum transfers from
three-point vacuum correlation functions; (iii) properties of exotic tetraquark
hadrons from correlation functions of four-quark currents.Comment: 12 pages, plenary talk given at the XIth International Conference on
Quark Confinement and the Hadron Spectrum, Saint Petersburg, Russia,
September 8-12, 201
Form factors of exclusive transitions
We present the form factors of the transitions induced by the
quark currents at all kinematically accessible . Our analysis is
based on the spectral representations of the form factors within the
constituent quark picture: we fix the soft meson wave functions and the
constituent quark masses by fitting and to the lattice
results at small recoils (). We then calculate
the transition form factors down to . For the
case the region however does not cover the whole
kinematically accessible range. Due to the smallness of the pion mass the
region of small recoils is close to the nearby resonance. We
develop a parametrization which includes the dominance of the form
factors and at small recoils and numerically reproduces the results
of calculations at . We find
ps and
ps.Comment: revtex, 6 pages, 1 figur
On properties of the exotic hadrons from QCD sum rules
We discuss the specific features of extracting properties of the exotic
polyquark hadrons (tetraquarks, pentaquarks) compared to the usual hadrons by
the QCD sum-rule approach. In the case of the ordinary hadrons, already the
one-loop leading-order correlation functions provide the bulk of the hadron
observables, e.g., of the form factor; inclusion of radiative corrections
modifies already nonzero one-loop contributions. In the case of an exotic
hadron, the situation is qualitatively different: discussing strong decays of
an exotic tetraquark meson, which provide the main contribution to its width,
we show that the disconnected leading-order diagrams are not related to the
tetraquark properties. For a proper description of the tetraquark decay width,
it is mandatory to calculate specific radiative corrections which generate the
connected diagrams.Comment: 6 pages, 3 figures, talk presented at "QCD@Work 2016 - International
Workshop on Quantum Chromodynamics: Theory and Experiment" (27 - 30 June
2016, Martina Franca, Italy
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