Smart collections do not use the Headline field, it does use the title field. You might want to consider transferring the headline to the title. This can be done either manually, or you can do it globally via the freeware called ExifTool.
To build on dj_paige's reply: LR's "Searchable IPTC" smart-collection or filter criterion will search most IPTC fields, including Headline. But it will search other fields as well as Headline.
If you decide to use another field as dj_paige suggests, you can use either Title or Caption (which is what LR calls the IPTC Description field). Which you choose depends on the downstream use of your photos' metadata. If this is just for your own personal catalog use, either will do, though if you have large headlines, Caption might be a little easier to work with, since LR provides the Large Caption metadata tagset that lets you enter multi-line captions. If you expect to be uploading your pics to an online service, you should check out the service to see how it handles Title and Caption (there is no consistency). If you are sending pics to services and organizations that strictly follow the IPTC metadata standards, you might consider what the standards say about the fields:
Headline: A brief synopsis of the caption. Headline is not the same as Title. Enter a brief publishable synopsis or summary of the contents of the image.
Title: A shorthand reference for the item. Title provides a short human readable name which can be a text and/or numeric reference. It is not the same as Headline. Enter a short verbal and human readable name for the image, this may be the file name. Many use the Title field to store the filename of the image, though the field may be used in many ways. Formal identifiers are provided by the Digital Image Id, or the Registry Entry property of the IPTC Extension.
Description (what LR calls Caption): A textual description, including captions, of the item's content, particularly used where the object is not text. A textual description, including captions, of the image. Enter a "caption" describing the who, what, and why of what is happening in this image, this might include names of people, and/or their role in the action that is taking place within the image.