Selection criteria in experimental particle physics I often read in particle physics articles, "relaxed selection criteria" and "nominal selection criteria" of events.
what is the meaning of relaxed and nominal ? 
PS : Links and details would be deeply appreciated.
 A: Data analysis is basically statistical in nature, and like any other statistical measure you have two goals in selecting the study population


*

*To get as large a population as possible.

*Get only the population that you want, with no contamination from populations you don't mean to study.


Selection criteria are chosen to pick out the study population from the mass of data, but the more selective you make them the more events that might be part of your population or might be something else you exclude.
That is the desires (1) and (2) are generally in conflict.
When a paper or talk indicates two different selections they simply mean that they've tried two (or more) different choices in the trade-off between population size and population purity. The particular name present here suggest that the 'nominal selection' is biased in favor of purity while the second one is biased in favor of size. This is a pretty normal arrangement as you want to validate the analysis on a pure sample and then might try for better overall significance by getting more data at the possible cost of some contamination.
