What is the relationship between 'Syriza' and 'Alexis Tsipras' in the context of the input sentence.
Options: distributed by, chairperson, owned by, product/material produced

But prime minister Alexis Tsipras will have to fight down a rebellion in his Leftist Syriza party, a week after more than 38 of his MPs rejected a raft of tax hikes and pension cuts .
chairperson