Currently according to the SLFP Constitution, the serving head of State has to assume leadership of the Party, in this case Maithripala Sirisena.
However according to a resolution by the JHU's Ven. Athureliya Rathana Thera and his Pivithuru Hetak organization, the President of the country does not have to be the head of the SLFP wherein even a member can hold the post.
It was also the same agreement reached between Chandrika and UNP leader Ranil Wickremasinghe when Maithripala Sirisena was nominated as Common Opposition candidate.
Source - mirror.lk