arXiv subjects

Joaquim Prades

Publications and source records attributed to Joaquim Prades.

62 records · Page 4Linked to original sources

arXiv1995

The $γγ\toπ^0π^0$ and $η\toπ^0γγ$ Transitions in the Extended NJL Model

We calculate within the Extended Nambu--Jona-Lasinio model the leading in $1/N_c$ contribution to $γγ\toπ^0π^0$ to all orders in the external momenta and quark masses. This result is then combined with the known two-loop Chiral Perturbation Theory and compared with the data and other calculations. A technical difficulty in the same calculation beyond order $p^6$ for $η\toπ^0γγ$ is identified and for this decay results up to order $p^6$ are presented.

hep-ph↗

arXiv1995

Hadronic Light-by-Light Contribution to the Muon $g-2$

Mistake in FORTRAN program corrected. The numerical results and conclusions are changed significantly: We present a calculation of the hadronic light-by-light contributions to the muon $g-2$ in the $1/N_c$ expansion. We have used an Extended NJL model and introduced an explicit cut-off for the high energy region. We have then critically studied the relative size of the high energy contributions. Although we find them large we can give a conservative estimate of the light-by-light contribution to $a_μ$ which is $-(11\pm5) \cdot 10^{-10}$. This is between two and three sigmas the expected experimental uncertainty at the forthcoming BNL experiment.

hep-ph↗

arXiv1995

The $B_K$ Parameter in the $1/N_c$ Expansion

We calculate the $B_K$ parameter within the framework of the $1/N_c$ expansion. We essentially use the technique presented by Bardeen, Buras and Gérard but calculate an off-shell Green function in order to disentangle different contributions. We study this Green function in pure Chiral Perturbation Theory (CHPT) first and afterwards in the $1/N_c$ expansion in the presence of an explicit cut-off to determine $B_K$ and the counterterms appearing in CHPT. The high energy part is done using the renormalization group. For the low-energy contributions we use both CHPT and an Extended Nambu--Jona-Lasinio model. This model has the right properties to match with the high energy QCD behaviour. We then study explicit chiral symmetry breaking effects by calculating with both massless and degenerate quarks together with the real case. A detailed analysis and comparison with the results found within other approaches is done. Consequences for present lattice calculations of this parameter are then obtained. As final result we get $0.60 < \hat B_K < 0.80$. If $m_s=m_d=0$ we get $0.25 < \hat B^χ_K < 0.55$.

hep-ph↗

arXiv1994

Light Quark Masses from QCD

We study the value of the light quark masses combination $m_u+m_d$ in QCD using both Finite Energy Sum Rules and Laplace Sum Rules. We have performed a detailed analysis of both the perturbative QCD and the hadronic parametrization inputs needed in these Sum Rules. As main result, we obtain $m_u\left(1~{\rm GeV}^2\right) + m_d \left(1~{\rm GeV}^2\right) = \left(12\pm2.5\right)~{\rm MeV}$ for the running $\overline{MS}$ masses.

hep-ph↗

arXiv1994

$B_K$ and Explicit Chiral Symmetry Breaking

The $B_K$ parameter is discussed in the general context of calculating beyond the factorization approximation for hadronic matrix elements. A variant of the $1/N_c$ method of Bardeen et al. is used. We present calculations within a low energy approach and a calculation within the Nambu--Jona-Lasinio model. Matching with the QCD behaviour and dependence on non-zero current quark masses is studied.

hep-ph↗

arXiv1994

Two- and Three-Point Functions in the Extended NJL Model

The two-point functions in generalized Nambu--Jona-Lasinio models are calculated to all orders in momenta and quark masses to leading order in $1/N_c$. The use of Ward identities and the heat-kernel expansion allows for a large degree of regularization independence. We also show how this approach works to the same order for three-point functions on the example of the vector-pseudoscalar-pseudoscalar three-point function. The inclusion of the chiral anomaly effects at this level is shown by calculating the pseudoscalar-vector-vector three-point function to the same order. Finally we comment on how (vector-)meson-dominance comes out in the presence of explicit chiral symmetry breaking in both the anomalous and the non-anomalous sectors.

hep-ph↗

arXiv1993

Anomalies, VMD and the Enjl Model

We show how the effective action of the Extended Nambu--Jona-Lasinio model (ENJL) can be defined in the presence of anomalies in a way that reproduces the flavour anomaly. This necessarily breaks Vector Meson Dominance (VMD) in the usual sense. The same method can be used to construct other chiral effective theories involving constituent quarks and spin-1 mesons that give the correct flavour anomaly. We also comment on how standard VMD is the root of this problem and those observed by previous authors in implementing VMD in the anomalous sector.

hep-ph↗

arXiv1993

Massive Spin-1 Field Chiral Lagrangian from an Extended Nambu-Jona-Lasinio Model of QCD

We present a calculation of the coupling constants in the massive spin-1 field chiral Lagrangian to chiral ${\cal O}(p^3)$ in the context of an extended Nambu--Jona-Lasinio cut-off model. Phenomenological applications of this Lagrangian to anomalous and non-anomalous low-energy hadronic transitions involving spin-1 particles are also discussed.

hep-ph↗