Mass vs Low Mass

Mass and Low Mass both are nouns.

Word	Noun	Adjective	Verb	Adverb
Mass	Yes	No	No	No
Low Mass	Yes	No	No	No

As nouns, Low Mass is a hyponym of Mass; that is, Low Mass is a word with a more specific, narrower meaning than Mass:

Mass: (Roman Catholic Church and Protestant Churches) the celebration of the Eucharist
Low Mass: a Mass recited without music

Other hyponyms of Mass include High Mass, Requiem.

Mass (noun)	Low Mass (noun)
(Roman Catholic Church and Protestant Churches) the celebration of the Eucharist	a Mass recited without music
a musical setting for a Mass
a sequence of prayers constituting the Christian Eucharistic rite