Having customer that asked me about how to control snippets for Google search results, i would like make a clarification.
Google follows a completely automated system to build search results snippets for your site, so when you search your site pages on Google you find snippets title that looks different from <title> tag on your pages... well you have no control over this! Google can build snippets for your pages based on a lot of factor.
Of course following good SEO guidelines and well structured data and titles is fundamental.
You find more informations here: https://support.google.com/webmasters/answer/35624?hl=en
Finally, sitemaps are not related to this aspect in any way! Sitemaps are an XML documents containing a list of URLs to your site, nothing more. Google scan your site to grab semantic informations like title, metades, etc.