title: Surprises in threshold antikaon-nucleon physics.
abstract: Low energy KN interactions are studied within unitary chiral perturbation theory at next-to-leading order with ten coupled channels. We pay special attention to the recent precise determination of the strong shift and width of the kaonic hydrogen 1s state by the DEAR Collaboration that has challenged our theoretical understanding of this sector of strong interactions. We typically find two classes of solutions, both of them reproducing previous data, that either can or cannot accommodate the DEAR measurements. The former class has not been previously discussed.
authors.affiliation: Departamento de Física, Universidad de Murcia, E-30071 Murcia, Spain.
authors.forename: José A
authors.initials: JA
authors.lastname: Oller
authors.forename: Joaquim
authors.initials: J
authors.lastname: Prades
authors.forename: Michela
authors.initials: M
authors.lastname: Verbeni