This is the 7487th most frequent Russian word.
демин
"Демин" is a Russian surname and does not have a specific meaning beyond being a family name.
Here 'Демин' is used as a proper noun, likely a surname of a person, to indicate who went to the library.
Демин сегодня пошёл в библиотеку.
Demin went to the library today.
In this example, 'демина' refers to a derived noun associated with a hypothetical concept or reduction (diminution) in astronomy.
Мы обсуждали гипотезу демина звёзд, которая гласит, что они могут двигаться быстрее.
We discussed the hypothesis of stellar diminution, which states that they might move faster.
Here, 'демин' appears as a possessive form of a proper noun, modifying 'опыт' to indicate ownership or origin of the experiment.