Search results for "PERTURBATION"
showing 10 items of 811 documents
The pionic beta decay in chiral perturbation theory
2003
Within the framework of chiral perturbation theory with virtual photons and leptons, we present an updated analysis of the pionic beta decay including all electromagnetic contributions of order e**2 p**2. We discuss the extraction of the Cabibbo-Kobayashi-Maskawa matrix element |Vud| from experimental data. The method employed here is consistent with the analogous treatment of the Kl3 decays and the determination of |Vus|.
The nucleon mass and pion-nucleon sigma term from a chiral analysis ofNf= 2 lattice QCD world data
2014
We investigate the pion-mass dependence of the nucleon mass within the covariant SU (2) baryon chiral perturbation theory up to order p 4 with and without explicit Δ (1232) degrees of freedom. We fit lattice QCD data from several collaborations for 2 and 2+1 flavor ensembles. Here, we emphasize our N f = 2 study where the inclusion the Δ (1232) contributions stabilizes the fits. We correct for finite volume and spacing effects, set independently the lattice QCD scale by a Sommer-scale of r 0 = 0.493(23) fm and also include one σ π N lQCD data point at M π ≈ 290 MeV. We obtain low-energy constants of natural size which are compatible with the rather linear pion-mass dependence observed in la…
Δ(1232)resonance in theγ→p→pπ0reaction at threshold
2016
We calculate the neutral pion photoproduction on the proton near threshold in covariant baryon chiral perturbation theory, including the $\Delta(1232)$ resonance as an explicit degree of freedom, up to chiral order $p^{7/2}$ in the $\delta$ counting. We compare our results with recent low-energy data from the Mainz Microtron for angular distributions and photon asymmetries. The convergence of the chiral series of the covariant approach is found to improve substantially with the inclusion of the $\Delta(1232)$ resonance.
WEAK DECAY OF Λ–HYPERNUCLEI
2005
The nonmesonic weak decay of L hypernuclei is studied in a shell model framework. A complete strangeness-changing weak LN!NN transition potential, based on one boson exchange, is constructed by including the exchange of the pseudoscalar mesons p, K, h as well as the vector mesons r,v, and K*, whose weak-coupling constants are obtained from soft meson theorems and SU~6! w . General expressions for nucleons in arbitrary shells are obtained. The transition matrix elements include realistic LN short-range correlations and NN final state interactions based on the Nijmegen baryon-baryon potential. The decay rates are found to be especially sensitive to the inclusion of the strange mesons K and K*…
Virtual Compton Scattering—Generalized Polarizabilities of Nucleons and Pions
1999
Virtual Compton scattering off nucleons and pions at low energies is discussed. Predictions for the generalized polarizabilities of the nucleon are presented within the framework of heavy-baryon chiral perturbation theory and the linear sigma model. First results for the generalized polarizabilities of the charged pion in chiral perturbation theory at O(p 4) are shown.
Masses of doubly charmed baryons in the extended on-mass-shell renormalization scheme
2016
In this work, we investigate the mass corrections of the doubly charmed baryons up to $N^2LO$ in the extended-on-mass-shell (EOMS) renormalization scheme, comparing with the results of heavy baryon chiral perturbation theory. We find that the terms from the heavy baryon approach are a subset of those obtained in the EOMS scheme. By fitting the lattice data, we can determine the parameters $\tilde{m}$, $\alpha$, $c_1$ and $c_7$ from the Lagrangian, while in the heavy baryon approach no information on $c_1$ can be obtained from the baryons mass. Correspondingly, the masses of $m_{\Xi_{cc}}$ and $m_{\Omega_{cc}}$ are predicted, in the EOMS scheme, extrapolating the results from different value…
Molecular interpretation of the XYZ states
2012
We study the vector – vector system including all the possible channels with quantum numbers charm = 0, strangeness = 0 around the energy region of 4000 MeV. New states with hidden charm around 4000MeV have been discovered by the B factories. They are intriguingly close to the D*D¯*${D^*}{ar D^*}$ and Ds*D¯s*$D_s^*ar D_s^*$ thresholds and do not have the properties of the charmonium states.We study the possible formation of D*D¯*${D^*}{ar D^*}$ and Ds*D¯s*$D_s^*ar D_s^*$ bound states in the framework of the Hidden Gauge formalism and discuss some of the models that favor the molecular assumption of some XYZ states: The hidden gauge formalism, Heavy baryon Chiral Perturbation Theory and the …
The Two-Nucleon System beyond the Pion Threshold
2003
A model is developed for the hadronic and electromagnetic interaction in the two-nucleon system above pion threshold in the framework of meson, nucleon and Δ degrees of freedom. It is based on time-ordered perturbation theory and includes full meson retardation in potentials and exchange currents as well as loop contributions to the nucleonic one-body current. Results for some selected processes are discussed.
Vector form factor of the pion in chiral effective field theory
2015
The vector form factor of the pion is calculated in the framework of chiral effective field theory with vector mesons included as dynamical degrees of freedom. To construct an effective field theory with a consistent power counting, the complex-mass scheme is applied.
NNLL momentum-space resummation for stop-pair production at the LHC
2013
If supersymmetry near the TeV scale is realized in Nature, the pair production of scalar top squarks is expected to be observable at the Large Hadron Collider. Recently, effective field-theory methods were employed to obtain approximate predictions for the cross section for this process, which include soft-gluon emission effects up to next-to-next-to-leading order (NNLO) in perturbation theory. In this work we employ the same techniques to resum soft-gluon emission effects to all orders in perturbation theory and with next-to-next-to-logarithmic (NNLL) accuracy. We analyze the effects of NNLL resummation on the stop-pair production cross section by obtaining NLO+NNLL predictions in pair inv…