event.preventdefault() removes question mark from url

I have a get form that loads a php file into a div. However the required event.preventdefault() removes the question mark and the form parameters from the url. How can I prevent this?

$(document).ready(function(){
  $('#df').submit(function(){
    var month_ddchart = $('#dm').val();
    var year_ddchart = $('#dy').val();
    source_ddchart = '/data/ddc-'+month_ddchart+'-'+year_ddchart+'-body.php';
    $('#content_box').load(source_ddchart);
    event.preventDefault();
  });
});