jQuery Post什么都没有

I created a simple jQuery post that returns data every time an user types in a character in an input field. The problem is that it is not working. Firebug returns no errors. When I type in a keyword FireBug Net shows no requests made. My code is almost a cut and paste from another piece of code that does work. I have no idea what to do and this is not the first time something like this has happened.

Here is the original file.

<input type="text" id="search_tags" />

<div id="tag_results"></div>

  <script type="text/javascript" >

  $(document).ready(function() {

  $('#search_tags').keyup(function() {
    var search_term = $(this).val();
$.post('ajax_file.php', {search_term: search_term}, function (data) {
$('#tag_results').html(data);       
    });     
});

  $('#search_tags').bind('keydown', function(e) {
    if(e.keyCode==13){
    e.preventDefault();
    };});  
    });

  </script>

Here is the ajax file.

$search_term = sanitize($_POST['search_term']); 

    if (!empty($search_term)) {

$search = mysql_query("SELECT DISTINCT `tag_name` FROM `tags` WHERE `tag_name` LIKE 

    '%$search_term%' LIMIT 0, 15");

$y = 1;

         while ($results_row = mysql_fetch_assoc($search)) 

        {

        echo '<br><br>' . $results_row['tag_name'] . '<br><br><hr>'; 

                    $y++;

             }}

Well:

$('#search_tags_').keyup(...

should be:

$('#search_tags').keyup(...

because I can't see an element with id="search_tags_" in your example which has:

<input type="text" id="search_tags" />