Basically, if I understand your question properly, you will want to look for another file named “header.php” (or “page-{something}-header.php” for a special header that only applies to a specific page type), which represents what is output by wp-head() (shown in your code).
If you open header.php, you should find something like the following:
<head>
<title> <?php wp_title(); ?></title>
...
Keep in mind, this can get really complex. There are many themes with several conditional statements to modify your page title for any of several different circumstances. See the following example of how complex a fairly simple theme can make it.
<?php
if (function_exists('is_tag') && is_tag()) {
echo 'Tag Archive for "'.$tag.'" - ';
} elseif (is_archive()) {
wp_title(''); echo ' Archive - ';
} elseif (is_search()) {
echo 'Search for "'.wp_specialchars($s).'" - ';
} elseif ( (!(is_404()) && (is_single()) || (is_page()) ) && !(is_front_page()) ) {
wp_title(''); echo ' | ';
} elseif (is_404()) {
echo 'Not Found - ';
}
bloginfo('name');
?>
</title>
If you already like your theme, but just want to customize how the page titles are drawn, you may want to review this brief tutorial for a better way to update page titles throughout your site, using add_filter('document_title_parts','your_customizing_function') (documentation for document_title_parts).