I think the dilemma here would have been to use "that" or "which" and "that" is right usage here as is limits the focus to neutrinos. Here author does not simply wish to add information about neutrinos but is giving focussed information about them.

ivyctor2010 wrote:I think the dilemma here would have been to use "that" or "which" and "that" is right usage here as is limits the focus to neutrinos. Here author does not simply wish to add information about neutrinos but is giving focussed information about them.

Thanks for your valuable reply but my question is :

Isn't "that" is a singular pronoun?

Ankit,

that is not a singular or plural pronoun.

Example

I read an article that referred back to a particular controvery.
She read a number of articles that were controversial.

Regards
Anup

"That" has more than one meaning. The one that you are referring to is when it is used as an adjective:
e.g. Was it THAT man who was shooting?
You are right that it can be used to modify only singular nouns.

However, in the sentence "As a result of a supernova explosion, every human being on Earth was bombarded on February 23, 1987, by about 100 billion neutrinos; fortunately, neutrinos are harmless elementary particles that are produced in nuclear reactions and that interact very weakly with matter." "that" acts as a function word introducing a relative clause modifying "neutrinos." When used for such a purpose, "that" can modify both singular and plural nouns.