retrive workItems who's Owner name is Unassigned.

This is my code .I've no idear about Unassinged.
How can i get the IContributorHandle for Unassigned?
Thans for your help!

IQueryableAttribute contributorAttribute = findAttribute(projectArea
.getProjectArea(), IWorkItem.OWNER_PROPERTY, monitor);
AttributeExpression contributorExpression = new AttributeExpression(
contributorAttribute, AttributeOperation.EQUALS, Unassinged);
term.add(contributorExpression);