The root schism was between the Sthaviras and the Mahāsāṅghikas. The fortunate survival of accounts from both sides of the dispute reveals disparate traditions. The Sthavira group offers two quite distinct reasons for the schism. The Dipavamsa of the Theravāda says that the losing party in the Second Council dispute broke away in protest and formed the Mahasanghika. This contradicts the Mahasanghikas' own vinaya, which shows them as on the same, winning side. The Mahāsāṅghikas argued that the Sthaviras were trying to expand the vinaya and may also have challenged what they perceived were excessive claims or inhumanly high criteria for arhatship. Both parties, therefore, appealed to tradition.
The Dipavamsa says that the losing party broke away in protest and formed what?
Mahasanghika