Baryon chiral perturbation theory in manifestly Lorentz invariant form

We show that in the presence of massive particles, such as nucleons: the st andard low-energy expansion, in powers of meson momenta and light-quark mas ses, in general converges in only part of the low-energy region. The expans ion of the scalar form factor sigma(t), for instance, breaks down in the vi cinity of t = 4M(pi)(2). In the language of heavy baryon chiral perturbatio n theory, the proper behaviour in the threshold region results only if the multiple internal-line insertions generated by relativistic kinematics are summed up to all orders. We propose a method that yields a coherent represe ntation throughout the low-energy region, while keeping Lorentz and chiral invariance explicit at all stages. The method is illustrated with a calcula tion of the nucleon mass and of the scalar form factor to order p(4).