Chiral expansion of baryon masses and σ–terms

We analyze the octet baryon masses and the pion/kaon–nucleon σ–terms in the framework of heavy baryon chiral perturbation theory. We include all terms up-to-and-including quadratic order in the light quark masses, mq. We develop a consistent scheme to estimate low–energy constants related to scalar–isoscalar operators in the framework of resonance exchange involving one–loop graphs. The pertinent low–energy constants can only estimated up to some finite coefficients. Including contributions from loop graphs with intermediate spin–3/2 decuplet and spin–1/2 octet states and from tree graphs including scalar mesons, we use the octet baryon masses and the pion–nucleon σ–term to fix all but one of these coefficients. Physical results are insensitive to this remaining parameter. It is also demonstrated that two–loop corrections only modify some of the subleading low–energy constants. We find for the baryon mass in the chiral limit, ◦ m= 750 ± 110 MeV. While the corrections of order mq are small for the nucleon, they are still sizeable for the Λ, the Σ and the Ξ. Therefore a definitive statement about the convergence of three– flavor baryon chiral perturbation can not yet be made. The strangeness content of the nucleon is y = 0.24 ± 0.20. We also estimate the kaon–nucleon σ–terms and some two–loops contributions to the nucleon mass. email: [email protected] email: [email protected] #3 Address after Oct. 1, 1996: FZ Jülich, IKP (Theorie), D-52425 Jülich, Germany