In this paper

we explore a secure downlink transmission system and propose a simultaneously transmitting and reflecting reconfigurable intelligent surface (STAR-RIS) assisted cooperative jamming scheme denoted by STAR-CJ

where a multi-antenna base station (BS) maintains secure communications with a single-antenna user via the STAR-RIS with the help of a jammer in the presence of an eavesdropper (Eve). Notably

since the BS and the jammer are positioned on opposite sides of the STAR-RIS

the STAR-RIS operates in dual-sided mode. We devise a secrecy rate maximization problem by jointly designing the transmission and reflection coefficients of the STAR-RIS and the beamforming vector of the BS and the jammer. In order to solve this problem

we utilise an alternating optimization (AO) algorithm

which effectively decouples the problem into two sub-problems

subsequently employing the successive convex approximation (SCA) method for resolution. Simulation results demonstrate that the proposed STAR-CJ scheme is superior to benchmark schemes including conventional RISs scheme (C-RISs) and without jammer scheme (STAR-WJ) in terms of secrecy rate.