
We evaluate the 'σ' exchange contribution to the K¯N→K¯N scattering within a chiral unitary approach. We show that the chiral transition potentials for ππ→KK¯ in the tchannel lead to a 'σ' contribution that vanishes in the K¯ forward direction and, hence, would produce a null 'σ' exchange contribution to the K− optical potential in nuclear matter in a simple impulse approximation. This is a consequence of the fact that the leading order chiral Lagrangian gives an I=0 ππ→KK¯ amplitude proportional to the squared momentum transfer, q2. This finding poses questions on the meaning or the origin of 'σ' exchange potentials used in relativistic mean field approaches to the K− nuclear selfenergy. This elementary 'σ' exchange potential in K¯N→K¯N is compared to the WeinbergTomozawa term and is found to be smaller than present theoretical uncertainties but will be relevant in the future when aiming at fitting increasingly more accurate data.
