PARIS — The head of French carmaker PSA Peugeot, Philippe Varin, said on Tuesday that the firm had no choice but to close its historic Aulnay plant north of Paris as he defended a shock announcement to cut 8,000 French jobs. In an interview with French newspaper Le Monde, Varin said PSA Peugeot Citroen had studied all other options before finally deciding on shutting down the plant and cutting jobs in France. “We looked at all alternatives. We cannot have factories that work at half their capacity, in the long term, when the European market has shrunk 25 percent in five years,” Varin said. PSA, the biggest French carmaker and second in Europe to Germany’s Volkswagen, announced last week it would cease production at Aulnay, which employs 3,000 people, with 1,400 jobs also going at its Rennes plant and about 3,600 jobs to be cut across the corporate structure. The announcement sparked union anger and underlined the difficulty for French industry in competing in international markets, a key factor in the stagnation of its economy.