In the last decade many states in the spectrum of charmonium and bottomonium have been observed experimentally above the lowest open-flavour threshold. Most of these states reside in the vicinity of ...strong thresholds and show properties that cannot be captured by simple quark models. Description and understanding of such exotic states is a challenge for the phenomenology of strong interactions, since it requires building adequate theoretical tools and approaches. In this work, a practical parametrisation for the line shapes of near threshold resonance(s) is derived in the framework of a coupled-channel model which includes an arbitrary number of elastic and inelastic channels as well as of bare pole terms. Parameters of the distribution have a direct relation to phenomenology and the resulting analytical parametrisation is therefore ideally suited to harvest the full information content provided by the measurements and to establish a link between the experimental data and their theoretical interpretation.
The cross sections of the reactions
e
+
e
–
→ ϒ(
nS
)π
+
π
−
(
n
= 1, 2,3) and
e
+
e
–
→
h
b
(
nP
)π
+
π
−
(
n
= 1, 2) are measured as a function of the cms collision energy from their thresholds up ...to 11.02 GeV using the data of the Belle experiment operating at the KEKB
e
+
e
–
collider. The peaks of the ϒ(10 860) and ϒ(11020) resonances are observed in the cross sections with an insignificant contribution of the continuum. The decay ϒ(11020) →
h
b
(
nP
)π
+
π
−
is found to fully proceed through intermediate isovector states
Z
b
(10610) and
Z
b
(10650).
QCD string in light-light and heavy-light mesons Kalashnikova, Yu. S.; Nefediev, A. V.; Simonov, Yu. A.
Physical review. D, Particles and fields,
07/2001, Letnik:
64, Številka:
1
Journal Article
Recenzirano
Odprti dostop
The spectra of light-light and heavy-light mesons are calculated within the framework of the QCD string model, which is derived from QCD in the Wilson loop approach. Special attention is paid to the ...proper string dynamics that allows us to reproduce the straight-line Regge trajectories with the inverse slope being 2{pi}{sigma} for light-light and twice as small for heavy-light mesons. We use the model of the rotating QCD string with quarks at the ends to calculate the masses of several light-light mesons lying on the lowest Regge trajectories and compare them with the experimental data as well as with the predictions of other models. The masses of several low-lying orbitally and radially excited heavy-light states in the D, D{sub s}, B, and B{sub s} mesons spectra are calculated in the einbein (auxiliary) field approach, which has proven to be rather accurate in various calculations for relativistic systems. The results for the spectra are compared with the experimental and recent lattice data. It is demonstrated that an account of the proper string dynamics encoded in the so-called string correction to the interquark interaction leads to an extra negative contribution to the masses of orbitally excited states that resolves the problem of the identification of the D(2637) state recently claimed by the DELPHI Collaboration. For heavy-light system we extract the constants {bar {Lambda}}, {lambda}{sub 1}, and {lambda}{sub 2} used in heavy quark effective theory and find good agreement with the results of other approaches.
We compare axial charges of excited nucleons, as predicted by the chiral symmetry restoration picture, with the traditional, moderately successful for the ground-state baryons
SU
(
6
)
symmetry. The ...axial charges of excited nucleons can (and will) be measured in lattice QCD simulations, and comparison of the lattice results with the two different symmetry schemes will give an insight on the origins of the excited hadron masses as well as on interrelations of chiral symmetry and confinement.
Recently the LHCb Collaboration announced the first observation of nontrivial structures in the double-\(J/\psi\) mass spectrum in the mass range 6.2-7.2 GeV, and a theoretical coupled-channel ...analysis of these data performed in Phys. Rev. Lett. 126, 132001 (2021) evidenced the existence of a new state \(X(6200)\) close to the double-\(J/\psi\) threshold. Although its molecular interpretation seems the most plausible assumption, the present data do not exclude an admixture of a compact component in its wave function, for which a fully-charmed compact tetraquark is the most natural candidate. It is argued in this work that the QCD string model is compatible with the existence of a compact \(cc\bar{c}\bar{c}\) state bound by QCD forces just below the double-\(J/\psi\) threshold. A nontrivial interplay of the quark dynamics associated with this compact state and the molecular dynamics provided by soft gluon exchanges between \(J/\psi\) mesons is discussed and the physical \(X(6200)\) is argued to be a shallow bound state, in agreement with the results of the aforementioned coupled-channel analysis of the LHCb data.
We re-examine one-pion exchange as a possible binding mechanism in the
X
(3872) charmonium-like state and argue it to be not sufficiently binding for this purpose. We conclude therefore that other ...short-range dynamics are responsible for the
X
formation.
Multiple lattice evidences support the existence of a confining but chirally symmetric regime of QCD above the chiral symmetry restoration crossover at Tch ~ 155 MeV. This regime is characterised by ...an approximate chiral spin symmetry of the partition function, which is a symmetry of the colour charge and the confining electric part of the QCD Lagrangian. It is traditionally believed that confinement should automatically induce spontaneous breaking of chiral symmetry, which would preclude the existence of a confining but chirally symmetric regime of QCD at high temperatures. We employ a well-known solvable quark model for QCD in 3+1 dimensions that is chirally symmetric and manifestly confining and argue that while confinement indeed induces dynamical breaking of chiral symmetry at T=0, a chiral restoration phase transition takes place at some critical temperature Tch. Above this temperature, the spectrum of the model consists of chirally symmetric hadrons with approximate chiral spin symmetry.
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