pɹˈaɪɚ tə ðə bˈɪldɪŋ ʌvðə mˈɑːɹk ˈaɪ, ɪt wʌz θˈɔːt ðæt mˈoʊst pˈɑːɹɾɪkəl dᵻkˈeɪ pɹˈɑːdʌkts wʊdhɐv ɹˈɛlətˌɪvli lˈoʊ tɹænsvˈɜːs moʊmˈɛntəm ˈaɪ.ˈiː. moʊmˈɛntəm pˌɜːpəndˈɪkjʊlɚ tə ðə bˈiːmlaɪn, sˌoʊ ðæt dɪtˈɛktɚz kʊd kˈʌvɚ ðɪs ˈɛɹiə ˈoʊnli.
Prior to the building of the Mark I, it was thought that most particle decay products would have relatively low transverse momentum (i.e. momentum perpendicular to the beamline), so that detectors could cover this area only.