This article describes a stateless authentication protocol designed for sensor networks. A mutual authentication between a sensor and a sink can be useful in many applications such as the monitoring of electricity meters or surveillance and monitoring of industrial plants. The authentication protocol we propose can counter usual attacks on sensor networks. First, as it is based on a PUF function, it is efficient against physical node capture. An attacker can not get into the hardware of the sensor or the sink node to obtain the secret keys of the system even if the node is captured physically. Secondly, this protocol can also counter replay attacks since the authentication uses a time-stamp, and the time interval during which a replay attack could be launched can be controlled and greatly reduced. Thirdly, this protocol is stateless, and so the sink can be authenticated to many co-located sensors using a single authentication message. Moreover, the same message can combine the authentication with the transmission of encrypted or unencrypted data.