M.E. Bracco

D* D pi and B* B pi form-factors from QCD sum rules

Abstract The H ∗ Hπ form factor for H = B and D mesons is evaluated in a QCD sum rule calculation. We study the Borel sum rule for the three point function of two pseudoscalar and one vector meson currents up to order four in the operator product expansion. The double Borel transform is performed with respect to the heavy meson momenta. We discuss the momentum dependence of the form factors and two different approaches to extract the H ∗ Hπ coupling constant.

QCD sum rule approach for the light scalar mesons as four-quark states

Abstract 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 K ¯ . We find that the couplings are consistent with existing experimental data, supporting the four-quark structure for the light scalar mesons.

A QCD sum rule calculation for the Y(4140) narrow structure

Abstract We use the QCD sum rules to evaluate the mass of a possible scalar mesonic state that couples to a molecular D s ∗ D ¯ s ∗ current. We find a mass m D s ∗ D s ∗ = ( 4.14 ± 0.09 ) GeV , which is in an 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 ∗ D ¯ ∗ current and we obtain m D ∗ D ∗ = ( 4.13 ± 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 ∗ D ¯ s ∗ molecular state or even as a mixture of D s ∗ D ¯ s ∗ and D ∗ D ¯ ∗ molecular states.

D and ρ mesons: who resolves whom?

Abstract The DDρ form factor is evaluated in a QCD sum rule calculation for both D and ρ off-shell mesons. We study the double Borel sum rule for the three point function of two pseudoscalar and one vector meson currents. We find that the momentum dependence of the form factors is very different if the D or the ρ meson is off-shell, but they lead to the same coupling constant in the DDρ vertex. We discuss two different approaches to extract the DDρ coupling constant.

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

Abstract 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 s J + ( 2317 ) , firstly observed by BaBar. We use diquark–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 s J + ( 2317 ) respectively, but not for the resonances observed by FOCUS: D 0 0 , + ( 2405 ) .

Charm couplings and form factors in QCD sum rules

Abstract We review the calculations of form factors and coupling constants in vertices with charm mesons in the framework of QCD sum rules. We first discuss the motivation for this work, describing possible applications of these form factors to heavy ion collisions and to B decays. We then present an introduction to the method of QCD sum rules and describe how to work with the three-point function. We give special attention to the procedure employed to extrapolate results obtained in the deep euclidean region to the poles of the particles, located in the time-like region. We present a table of ready-to-use parametrizations of all the form factors, which are relevant for the processes mentioned in the introduction. We discuss the uncertainties in our results. We also give the coupling constants and compare them with estimates obtained with other methods. Finally we apply our results to the calculation of the cross section of the reaction J / ψ + π → D + D ∗ ¯ .

ρD*D* vertex from QCD sum rules

We calculate the form factors and the coupling constant in the ρD∗D∗ρD∗D∗ vertex in the framework of QCD sum rules. We evaluate the three point correlation functions of the vertex considering both ρ and D∗D∗ mesons off-shell. The form factors obtained are very different but give the same coupling constant: gρD∗D∗=6.60±0.31gρD∗D∗=6.60±0.31. This number is 50% larger than what we would expect from SU(4) estimates.

The meson Z + ( 4430 ) as a tetraquark state

Abstract We test the validity of the QCD sum rules applied to the meson Z + ( 4430 ) , by considering a diquark–antidiquark type of current with J P = 0 − and with J P = 1 − . We find that, with the studied currents, it is possible to find an acceptable Borel window. In such a Borel window we have simultaneously a good OPE convergence and a pole contribution which is bigger than the continuum contribution. We get m Z = ( 4.52 ± 0.09 ) GeV and m Z = ( 4.84 ± 0.14 ) GeV for the currents with J P = 0 − and J P = 1 − , respectively. We conclude that the QCD sum rules results favors J P = 0 − quantum numbers for the Z + ( 4430 ) meson.

D∗DsK and Ds∗DK vertices in a QCD sum rule approach

Abstract We calculate the strong form factors and coupling constants of D ∗ D s K and D s ∗ D K vertices using the QCD sum rules technique. In each case we have considered two different cases for the off-shell particle in the vertex: the lightest meson and one of the heavy mesons. The method gives the same coupling constant for each vertex. When the results for different vertices are compared, they show that the SU(4) symmetry is broken by around 40%.

J / ψ 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.