lookrotation only on x axis

 Vector3 relativePos = target.position - transform.position;          Quaternion LookAtRotation = Quaternion.LookRotation( relativePos );            Quaternion LookAtRotationOnly_Y = Quaternion.Euler(transform.rotation.eulerAngles.x, LookAtRotation.eulerAngles.y, transform.rotation.eulerAngles.z);            transform.rotation = LookAtRotationOnly_Y;