9,344 research outputs found
The hidden-charm pentaquark and tetraquark states
In the past decade many charmonium-like states were observed experimentally.
Especially those charged charmonium-like states and bottomonium-like
states can not be accommodated within the naive quark model. These
charged states are good candidates of either the hidden-charm tetraquark
states or molecules composed of a pair of charmed mesons. Recently, the LHCb
Collaboration discovered two hidden-charm pentaquark states, which are also
beyond the quark model. In this work, we review the current experimental
progress and investigate various theoretical interpretations of these
candidates of the multiquark states. We list the puzzles and theoretical
challenges of these models when confronted with the experimental data. We also
discuss possible future measurements which may distinguish the theoretical
schemes on the underlying structures of the hidden-charm multiquark states.Comment: Review accepted by Physics Reports, 152 pages, 66 figures, and 29
table
Isospin symmetry breaking of K and K* mesons
We use the method of QCD sum rules to investigate the isospin symmetry
breaking of K and K* mesons. The electromagnetic effect, difference between up
and down current-quark masses and difference between up and down quark
condensates are important. We perform sum rule analyses of their masses and
decay constant differences, which are consistent with experimental values. Our
results yield Delta f_K = f_{K^0} - f_{K^\pm} = 1.5 MeV.Comment: 9 pages, 7 figures, one reference adde
Light Scalar Mesons in the QCD Sum Rule
We study the light scalar mesons in the QCD sum rule. We find that there are
five independent scalar tetraquark currents in the local form, and we perform
QCD sum rule analysis using both these currents and their combinations.
Compared with the sum rules by conventional \bar{q} q currents, our result
supports a tetraquark structure for low-lying scalar mesons.Comment: 4 pages, 1 figure. Talk given at the Yukawa International Seminar
(YKIS) 2006, "New frontiers in QCD", Kyoto, Japan, 20 Nov. - 8 Dec. 200
Mass spectra of Zc and Zb exotic states as hadron molecules
We construct charmonium-like and bottomonium-like molecular interpolating
currents with quantum numbers J(PC)=1(+-) in a systematic way, including both
color singlet-singlet and color octet-octet structures. Using these
interpolating currents, we calculate two-point correlation functions and
perform QCD sum rule analyses to obtain mass spectra of the charmonium-like and
bottomonium-like molecular states. Masses of the charmonium-like q_bar c c_bar
q molecular states for these various currents are extracted in the range
3.85-4.22 GeV, which are in good agreement with observed masses of the Zc
resonances. Our numerical results suggest a possible landscape of hadronic
molecule interpretations of the newly-observed Zc states. Mass spectra of the
bottomonium-like q_bar b b_bar q molecular states are similarly obtained in the
range 9.92-10.48 GeV, which support the interpretation of the Zb(10610) meson
as a molecular state within theoretical uncertainties. Possible decay channels
of these molecular states are also discussed.Comment: 15 pages, 3 figures, 2 tables, accepted by PR
The Possible J^{PC}=0^{--} Exotic State
In order to explore the possible existence of the exotic state, we
have constructed the tetraquark interpolating operators systematically. As a
byproduct, we notice the tetraquark operators without derivatives do
not exist. The special Lorentz structure of the currents forbids the
four-quark correction to the spectral density. Now the gluon condensate is the
dominant power correction. Within the framework of the finite energy sum rule,
none of the seven interpolating currents supports a resonant signal. Therefore
we conclude that the exotic state does not exist below 2 GeV, which is
consistent with the current experimental observations.Comment: 12 pages, 27 figure
Light vector meson and heavy baryon strong interaction
We calculate the coupling constants between the light vector mesons and heavy
baryons within the framework of the light-cone QCD sum rule in the leading
order of heavy quark effective theory. Most resulting sum rules are stable with
the variations of the Borel parameter and the continuum threshold. The
extracted couplings will be useful in the study of the possible heavy baryon
molecular states
Establishing low-lying doubly charmed baryons
We systematically study the -wave doubly charmed baryons using the method
of QCD sum rules. Our results suggest that the recently
observed by LHCb can be well identified as the -wave state of
. We study its relevant state, whose mass is
predicted to be around 3.7 GeV. We also systematically study the -wave
doubly charmed baryons, whose masses are predicted to be around 4.1 GeV.
Especially, there can be several excited doubly charmed baryons in this energy
region, and we suggest to search for them in order to study the fine structure
of the strong interaction.Comment: 6 pages, 2 figures, 1 table; A mistake was found when evaluating
decay constants of the S-wave charmed baryons. The conclusion is not change
Light Scalar Meson sigma(600) in QCD Sum Rule with Continuum
The light scalar meson sigma(600) is known to appear at low excitation energy
with very large width on top of continuum states. We investigate it in the QCD
sum rule as an example of resonance structures appearing above the
corresponding thresholds. We use all the possible local tetraquark currents by
taking linear combinations of five independent local ones. We ought to consider
the pi-pi continuum contribution in the phenomenological side of the QCD sum
rule in order to obtain a good sum rule signal. We study the stability of the
extracted mass against the Borel mass and the threshold value and find the
sigma(600) mass at 530 MeV +- 40 MeV. In addition we find the extracted mass
has an increasing tendency with the Borel mass, which is interpreted as caused
by the width of the resonance.Comment: 9 pages, 7 figures, comments and suggestions welcom
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