(S (S (NP (BASENP (DT The) (NNS papers)) (SBAR (WHNP (WDT that)) (S (VP (VBP introduce) (BASENP (DT this) (NN mechanism)))))) (VP (VP (VBD were) (VP (VP (VBN published) (PP (IN in) (BASENP (DT the) (NN journal) (JJ Physical) (NN Review) (NNS Letters)))) (PP (IN in) (BASENP (NN 1964))))) (CC and) (VP (VBD were) (VP (ADVP (DT each)) (VP (VP (VBN recognized) (PP (IN as) (BASENP (NN milestone) (NNS papers)))) (PP (IN by) (NP (BASENP (NNP PRL) (POS 's)) (BASENP (JJ 50th) (NN anniversary) (NN celebration))))))))) (PERIOD .))