Searcharxiv⌕ Search

arXiv subjects

M. Nielsen

Publications and source records attributed to M. Nielsen.

At least 91 records · Page 5Linked to original sources
arXiv2005

Diquark-Antidiquark with open charm in QCD sum rules

Using the QCD sum rule approach we investigate the possible four-quark structure of the recently observed charmed scalar mesons $D_0^{0}(2308)$ (BELLE) and $D_0^{0,+}(2405)$ (FOCUS) and also of the very narrow $D_{sJ}^{+}(2317)$, firstly observed by BABAR. We use diquak-antidiquark currents and work to the order of $m_s$ in full QCD, without relying on $1/m_c$ expansion. Our results indicate that a four-quark structure is acceptable for the resonances observed by BELLE and BABAR: $D_0^{0}(2308)$ and $D_{sJ}^{+}(2317)$ respectively, but not for the resonances observed by FOCUS: $D_0^{0,+}(2405)$.

hep-ph↗
arXiv2005

Disentangling two- and four-quark state pictures of the charmed scalar mesons

We suggest that the recently observed charmed scalar mesons $D_0^{0}(2308)$ (BELLE) and $D_0^{0,+}(2405)$ (FOCUS) are considered as different resonances. Using the QCD sum rule approach we investigate the possible four-quark structure of these mesons and also of the very narrow $D_{sJ}^{+}(2317)$, firstly observed by BABAR. We use diquak-antidiquark currents and work to the order of $m_s$ in full QCD, without relying on $1/m_c$ expansion. Our results indicate that a four-quark structure is acceptable for the resonances observed by BELLE and BABAR: $D_0^{0}(2308)$ and $D_{sJ}^{+}(2317)$ respectively, but not for the resonances observed by FOCUS: $D_0^{0,+}(2405)$.

hep-ph↗
arXiv2005

Hadronic form factors and the $J/ψ$ secondary production cross section: an update

Improving previous calculations, we compute the $D + \bar{D} \to J/ψ+ π$ cross section using the most complete effective lagrangians available. The new crucial ingredients are the form factors on the charm meson vertices, which are determined from QCD sum rules calculations. Some of them became available only very recently and the last one, needed for our present purpose, is calculated in this work.

hep-ph↗
arXiv2005

Pentaquark Decay Width in QCD Sum Rules

In a diquark-diquark-antiquark picture of the pentaquark we study the decay $Θ\rightarrow K^{+} n$ within the framework of QCD sum rules. After evaluation of the relevant three-point function, we extract the coupling $g_{ΘnK}$ which is directly related to the pentaquark width. Restricting the decay diagrams to those with color exchange between the meson-like and baryon-like clusters reduces the coupling constant by a factor of four. Whereas a small decay width might be possible for a positive parity pentaquark, it seems difficult to explain the measured width for a pentaquark with negative parity.

hep-ph↗
arXiv2004

A QCD Sum Rule Study of $Θ^+$ in Nuclear Matter

We consider a $[ud]^2\bar{s}$ current, in the finite-density QCD sum rule approach, to investigate the scalar and vector self-energies of the recently observed pentaquark state $Θ^+(1540)$, propagating in nuclear mater. We find that, opposite to what was obtained for the nucleon, the vector self-energy is negative, and the scalar self-energy is positive. There is a substantial cancellation between them resulting in an attractive net self-energy of the same order as in the nucleon case.

nucl-th↗
arXiv2004

J/psi D*D* vertex from QCD sum rules

We calculated the strong form factor and coupling constant for the $J/ψD^* D^*$ vertex in a QCD sum rule calculation. We performed a double Borel sum rule for the three point correlation function of vertex considering both $J/ψ$ and $D^*$ mesons off--shell. The form factors obtained are very different, but they give the same coupling constant.

hep-ph↗
arXiv2004

A Comparative Study of Pentaquark Interpolating Currents

In a diquark-diquark-antiquark picture of pentaquarks, we use two interpolating currents to calculate the mass of the recently measured $Ξ^{--}$ state in the framework of QCD sum rules. We show that, even though yielding similar values for $m_{Ξ^{--}}$ (and close to the experimental value), these currents differ from each other in what concerns the strength of the pole, convergence of the OPE and sensitivity to the continuum threshold parameter.

hep-ph↗
arXiv2004

$J/ψ$ couplings to open charm mesons from QCD sum rules

We employ QCD sum rules to calculate the $J/ψ$ form factors and coupling constants with charmed mesons, by studying three-point correlation functions. In particular we consider the $J/ψD^*D$ and $J/ψDD$ vertices. We determine the momentum dependence of the form factors for kinematical conditions where J/ψ, D or D^* are off-shell. Extrapolating to the pole of each one of the so obtained form factors, we determine the coupling constants. For both couplings our results (within the errors) are compatible with estimates based on constituent quark model.

hep-ph↗
arXiv2004

The $J/ψ-K^*$ dissociation cross section in a meson exchange model

In this work we study the energy dependence of the $J/ψ-K^*$ dissociation cross section, using a meson exchange model based on effective hadronic Lagrangian that includes couplings between pseudoscalar and vector mesons. We also consider anomalous parity terms. Off-shell effects at the vertices were handled with QCD sum rule estimates for the running coupling constants, and we compare the results with and without form factors. We also study the $J/ψ-ρ$ cross section with form factors. The total $J/ψ-K^*$ cross section was found to be $1.6 \sim1.9$ mb for $4.2\leq\sqrt{s}\leq5~\GeV$.

nucl-th↗
arXiv2004

Meson loop effects on the pion electromagnetic form factor

In this work we calculate the meson loop effects on the pion electromagnetic form factor and perform a comparison with experimental data. We show that, even though the meson cloud is not the only nonperturbative process to be considered, its contribution is significant.

nucl-th↗
arXiv2004

Meson loops and the $g_{D^* D π}$ coupling

The $D^*Dπ$ form factor is evaluated at low and moderate $Q^2$ in a hadronic loop model, for off-shell $D$ mesons. The results contain arbitrary constants, which are fixed so as to match previous QCD sum rule calculations valid at higher $Q^2$. The form factor obtained in this way was used to extract the coupling constant, which is in very good agreement with the experimental value.

hep-ph↗
arXiv2003

$J/ψ$-kaon cross section in meson exchange model

We calculate the cross section for the dissociation of $J/ψ$ by kaons within the framework of a meson exchange model including anomalous parity interactions. Off-shell effects at the vertices were handled with QCD sum rule estimates for the running coupling constants. The total $J/ψ$-kaon cross section was found to be $1.0 \sim1.6$ mb for $4.1\leq\sqrt{s}\leq5 \GeV$.

nucl-th↗
arXiv2003

Are $Θ^+$ and the Roper resonance diquark-diquark-antiquark states?

We consider a $[ud]^2\bar{s}$ current in the QCD sum rule framework to study the mass of the recently observed pentaquark state $Θ^+(1540)$, obtaining good agreement with the experimental value. We also study the mass of the pentaquark $[ud]^2\bar{d}$. Our results are compatible with the interpretation of the $[ud]^2\bar{d}$ state as being the Roper resonance N(1440), as suggested by Jaffe and Wilczek.

hep-ph↗
arXiv2003

The $J/ψDD^*$ vertex

We employ QCD sum rules to calculate the $J/ψDD^*$ form factors and coupling constant by studying the three-point $J/ψD^*D$ correlation function. We find that the momentum dependence of the form factor depends on the off-shell meson. We get a value for the coupling which is in agreement with estimates based on constituent quark model.

hep-ph↗
arXiv2003

A model for $J/ψ$ - kaon cross section

We calculate the cross section for the dissociation of $J/ψ$ by kaons within the framework of a meson exchange model. We find that, depending on the values of the coupling constants used, the cross section can vary from 5 mb to 30 mb at $\sqrt{s}\sim5$ GeV.

nucl-th↗
arXiv2002

The $J/ψD D$ Vertex in QCD Sum Rules

The $J/ψD D$ form factor is evaluated in a QCD sum rule calculation for both $D$ and $J/ψ$ off-shell mesons. We study the double Borel sum rule for the three point function of two pseudoscalar and one vector meson current. We find that the momentum dependence of the form factors is different if the $D$ or the $J/ψ$ meson is off-shell, but they lead to the same coupling constant in the $J/ψD D$ vertex.

hep-ph↗
arXiv2002

Semileptonic D decay into scalar mesons: a QCD sum rule approach

Semileptonic decays of D-mesons into scalar hadronic states are investigated. Two extreme cases are considered: a) the meson decays directly into an uncorrelated scalar state of two two mesons and b) the decay proceeds via resonance formation. QCD sum rules including instanton contributions are used to calculate total and differential decay rates under the two assumptions.

hep-ph↗