arXiv subjects
Publications and source records attributed to Marina Nielsen.
Using the QCD spectral sum rule approach we investigate different currents with $J^{PC}=1^{++}$, which could be associated with the $X(3872)$ meson. Our results indicate that, with a four-quark or molecular structure, it is very difficult to explain the narrow width of the state unless the quarks have a special color configuration.
In the past years there has been a revival of hadron spectroscopy. Many interesting new hadron states were discovered experimentally, some of which do not fit easily into the quark model. This situation motivated a vigorous theoretical activity. This is a rapidly evolving field with enormous amount of new experimental information. In the present report we include and discuss data which were released very recently. The present review is the first one written from the perspective of QCD sum rules (QCDSR), where we present the main steps of concrete calculations and compare the results with other approaches and with experimental data.
Using the QCD sum rules we test if the new narrow structure, the X(4350) recently observed by the Belle Collaboration, can be described as a $J^{PC}=1^{-+}$ exotic $D_s^*D_{s0}^*$ molecular state. We consider the contributions of condensates up to dimension eight, we work at leading order in $α_s$ and we keep terms which are linear in the strange quark mass $m_s$. The mass obtained for such state is $m_{D_s^*{D}_{s0}^*}=(5.05\pm 0.19)$ GeV. We also consider a molecular $1^{-+}, D^{*}{D}_0^{*}$ current and we obtain $m_{D^*{D}_0^*}=(4.92\pm 0.08)$ GeV. We conclude that it is not possible to describe the X(4350) structure as a $1^{-+} D_s^*{D}_{s0}^*$ molecular state.
We use the QCD sum rules to evaluate the mass of a possible scalar mesonic state that couples to a molecular $D_{s}^{*}\bar{D}_s^{*}$ current. We find a mass $m_{D_s^*D_s^*}=(4.14\pm 0.09)$ GeV, which is in a excellent agreement with the recently observed Y(4140) charmonium state. We consider the contributions of condensates up to dimension eight, we work at leading order in $α_s$ and we keep terms which are linear in the strange quark mass $m_s$. We also consider a molecular $D^{*}\bar{D}^{*}$ current and we obtain $m_{D^*{D}^*}=(4.13\pm 0.10)$, around 200 MeV above the mass of the Y(3930) charmonium state. We conclude that it is possible to describe the Y(4140) structure as a $D_s^*\bar{D}_s^*$ molecular state.
We describe a new exact relation for large $N_c$ QCD for the long-distance behavior of baryon form factors in the chiral limit. This model-independent relation is used to test the consistency of the structure of several baryon models. All 4D semiclassical chiral soliton models satisfy the relation, as does the Pomarol-Wulzer holographic model of baryons as 5D Skyrmions. However, remarkably, we find that the holographic model treating baryons as instantons in the Sakai-Sugimoto model does not satisfy the relation.
I discuss some recent discoveries in the spectroscopy of the charmonium-like resonances, and some of their theoretical interpretations are reviewed.
We use QCD sum rules to study the recently observed resonance-like structures in the $π^+χ_{c1}$ mass distribution, $Z_1^+(4050)$ and $Z_2^+(4250)$, considered as $D^{*+}\bar{D}^{*0}$ and $D_1^+\bar{D}^0+ D^+\bar{D}_1^0$ molecules with the quantum number $J^P=0^+$ and $J^P=1^-$ respectively. We consider the contributions of condensates up to dimension eight and work at leading order in $α_s$. We obtain $m_{D^*D^*}=(4.15\pm0.12) \GeV$, around 100 MeV above the $D^*D^*$ threshold, and $m_{D_1D}=(4.19\pm 0.22) \GeV$, around 100 MeV below the $D_1D$ threshold. We conclude that the $D^{*+}\bar{D}^{*0}$ state is probably a virtual state that is not related with the $Z_1^+(4050)$ resonance-like structure. In the case of the $D_1D$ molecular state, considering the errors, its mass is consistent with both $Z_1^+(4050)$ and $Z_2^+(4250)$ resonance-like structures. Therefore, we conclude that no definite conclusion can be drawn for this state from the present analysis.
We investigate the widths of the recently observed charmonium like resonances X(3872), Z(4430) and Z_2(4250) using QCD sum rules. Extending previous analyses regarding these states as diquark-antiquark states or molecules of D mesons, we introduce the Breit-Wigner function in the pole term. We find that introducing the width increases the mass at small Borel window region. Using the OPE up to dimension eight, we find that the sum rules based on interpolating current with molecular components give a stable Borel curve from which both the masses and widths of these resonances can be well obtained. Thus the QCD sum rule approach strongly favors the molecular description of these states.
We use QCD sum rules to study the possible existence of a $D_s\bar{D}^*+ D_s^*\bar{D}$ molecule with the quantum number $J^P=1^+$. We consider the contributions of condensates up to dimension eight and work at leading order in $α_s$. We obtain $m_{D_sD^*}=(3.96\pm0.10) \GeV$ around 100 MeV above the mass of the meson X(3872). The proposed state is a natural generalized state to the strangeness sector of the X(3872), which was also found to be consistent with a multiquark state from a previous QCD sum rule analysis.
We use QCD sum rules to study the recently observed meson $Z^+(4430)$, considered as a $D^*D_1$ molecule with $J^{P}=0^{-}$. We consider the contributions of condensates up to dimension eight and work at leading order in $α_s$. We get $m_Z=(4.40\pm0.10) \GeV$ in a very good agreement with the experimental value. We also make predictions for the analogous mesons $Z_{s}$ and $Z_{bb}$ considered as $D_s^*D_1$ and $B^*B_1$ molecules respectively. For $Z_{s}$ we predict $m_{Z_{s}}= (4.70\pm 0.06) {\rm GeV}$, which is above the $D_s^*D_1$ threshold, indicating that it is probably a very broad state and, therefore, difficult to be experimentally seen. For $Z_{bb}$ we predict $m_{Z_{bb}}= (10.74\pm 0.12) {\rm GeV}$, in agreement with quark model predictions.
We use QCD sum rules to study the masses of the baryons $Ξ_c$ and $Ξ_b$. We work with a current where the strange and the light quarks are in a relative spin zero, at leading order in $α_s$. We consider the contributions of condensates up to dimension six. For $Ξ_b$ we get $m_{Ξ_{b}}= (5.75\pm 0.25) {\rm GeV}$, and for $Ξ_c$ we get $m_{Ξ_{c}}= (2.5\pm 0.2) {\rm GeV}$, both in excelent agreement with the experimental values. We also make predictions to the state $Ω_b(ssb)$ obtaining m_{Ω_{b}}= (5.82\pm 0.23) GeV.
We use QCD sum rules to study the possible existence of $QQ-\bar{u}\bar{d}$ mesons, assumed to be a state with $J^{P}=1^{+}$. For definiteness, we work with a current with an axial heavy diquark and a scalar light antidiquark, at leading order in $α_s$. We consider the contributions of condensates up to dimension eight. For the $b$-quark, we predict $M_{T_{bb}}= (10.2\pm 0.3) {\rm GeV}$, which is below the $\bar{B}\bar{B}^*$ threshold. For the $c$-quark, we predict $M_{T_{cc}}= (4.0\pm 0.2) {\rm GeV}$, in agreement with quark model predictions.
We use diquark-antidiquark currents to investigate the masses and partial decay widths of the recently observed mesons $D_{sJ}^{+}(2317)$, $D_0^{*0}(2308)$ and X(3872), considered as four-quark states, in a QCD sum rule approach. In particular we investigate the coupling constant $g_{D_{sJ}DK}$. We found that the $g_{D_{sJ}DK}$ obtained in this four-quark scenario is smaller than the coupling constant obtained when $D_{sJ}^{+}(2317)$ is considered as a conventional $c\bar{s}$ state.
We use the QCD sum rules to analize the hadronic decay $D_{sJ}^{+}(2317)\to D_s^+π^0$, in the hypotesis that the $D_{sJ}^{+}(2317)$ can be identified as a four-quark state. We use a diquak-antidiquark current and work to the order of $m_s$ in full QCD, without relying on $1/m_c$ expansion. We find that the partial decay width of the hadronic isospin violating mode is proportional to the isovector quark condensate, $\langle0|\bar{d}d-\bar{u}u|0\rangle$. The estimated partial decay width is of the order of 6 keV.
We use the meson cloud model, including the kaon and the $K^*$ contributions, to estimate the electric and magnetic strange form factors of the nucleon. We compare our results with the recent measurements of the strange quark contribution to parity-violating asymmetries in the forward G0 electron-proton scattering experiment. We conclude that it is not possible to explain the data using this model.
We study the two point-function for the scalar mesons $σ, κ, f_0(980)$ and $a_0(980)$ as diquak-antidiquark states. We also study the decays of these mesons into $ππ$, $Kπ$ and $K\bar{K}$. We found that the couplings are consistent with existing experimental data, pointing in favor of the four-quark structure for the light scalar mesons.
We use the QCD sum rules to evaluate the form factors associated with the semileptonic decays of $D_s$ and $D$ mesons into $f_0(980)$. We consider the $f_0(980)$ meson as a quark-antiquark state with a mixture of strange and light components. The decay rates are evaluated in terms of the mixing angle. Using the same form factors to evaluate nonleptonic decays in the framework of the factorization approximation we conclude that the importance of the light quarks in $f_0(980)$ is not negligible.
We compute the $J/ψπ\to {charmed mesons}$ cross section using QCD sum rules. This cross section is important to distinguish the suppression in the production of $J/ψ$ through the dissociation by comoving pions and through the formation of quark-gluon plasma. Our sum rules for the $J/ψπ\to \bar{D} D^*$, $D \bar{D}^*$, ${\bar D}^* D^*$ and ${\bar D} D$ hadronic matrix elements are constructed by using vaccum-pion correlation functions, and we work up to twist-4 in the soft-pion limit. After doing a thermal average we get $<σ^{πJ/ψ} v>\sim 0.2 - 0.4$ mb at $T=150\MeV$