Zum Originaltext der Anfrage:
Dort wird in der Fußnote auf einen Artikel verwiesen, in dem es heißt:
The experimental evidence for pentaquarks, both old an new, is discussed. Constraints due to ...scattering data from previous decades is first reviewed, followed by experiments with positive evidence and those with null results.
[hep-ex/0504027] Experimental Search for Pentaquarks (arxiv.org)
Bedeutung:
In science, a null result is a result without the expected content: that is, the proposed result is absent.It is an experimental outcome which does not show an otherwise expected effect. This does not imply a result of zero or nothing, simply a result that does not support the hypothesis.
Null result - Wikipedia
Demgegenüber scheint zero dann verwendet zu werden, wenn der Wert tatsächlich Null beträgt (siehe den zitierten W.-Artikel).