We have extended the metadata fields provided by arXiv.org.

Field	Type	Description
id	String	ArXiv identifer of the form yymm.nnnn or arch-ive/yymmnnn.
version	String	A literal v followed by a version number of 1 or more digits starting at v1.
published	ISODate	Published date in the form yyyy-mm-ddThh:mm:ssZ.
updated	ISODate	Last updated date in the form yyyy-mm-ddThh:mm:ssZ.
title	String	Queries for the title field are converted to regular expressions by default.
authors	Array::String	You should use the exact match for the author name.
comment	String	Queries for the comment field are converted to regular expressions by default.
journal	String	Queries for the journal field are converted to regular expressions by default.
doi	String	Only available for the article that has been published on a journal.
categories	Array::String	You should use the exact match for the category provided by arXiv.
abstract	String	Queries for the abstract field are converted to regular expressions by default.
subjects	Array::String	Codes for the PACS, the MSC-class, the ACM-class, or the JEL-class.
themes	Array::Object	A theme consits of a category and an array of topics.
keywords	Array::String	Keywords summarize the key points of the article.
tags	Array::String	Labels that classify eprints in a non-hierarchical manner.
feedback	Object	Contains the statistics of reviews, bookmarks, readers, ratings, etc.
publication	Object	Contains the information on publisher, journal, volume, number, etc.
note	Object	Contains the information on typesetting, language, license, status, etc.