( ( This theory ) ( ( ( ( ( is unusual ) ( because ( ( individual ( ( -LRB- ( free -RRB- ) ) quarks ) ) ( ( can not ) ( ( be observed ) ( -LRB- ( ( see ( color confinement ) ) -RRB- ) ) ) ) ) ) ) , ) ( unlike ( ( ( the situation ) ( with ( composite atoms ) ) ) ( where ( ( ( electrons and ) nuclei ) ( can ( be ( isolated ( by ( ( transferring ( ionization energy ) ) ( to ( the atom ) ) ) ) ) ) ) ) ) ) ) ) . ) )
(ROOT (S (NP (DT This) (NN theory)) (VP (VBZ is) (ADJP (JJ unusual)) (SBAR (IN because) (S (NP (JJ individual) (PRN (-LRB- -LRB-) (ADJP (JJ free)) (-RRB- -RRB-)) (NNS quarks)) (VP (MD can) (RB not) (VP (VB be) (ADJP (JJ observed)) (PRN (-LRB- -LRB-) (S (VP (VB see) (NP (JJ color) (NN confinement)))) (-RRB- -RRB-))))))) (, ,) (PP (IN unlike) (NP (NP (DT the) (NN situation)) (PP (IN with) (NP (JJ composite) (NNS atoms))) (SBAR (WHADVP (WRB where)) (S (NP (NNS electrons) (CC and) (NNS nuclei)) (VP (MD can) (VP (VB be) (VP (VBN isolated) (PP (IN by) (S (VP (VBG transferring) (NP (NN ionization) (NN energy)) (PP (TO to) (NP (DT the) (NN atom))))))))))))) (. .)))
This theory is unusual because individual (free) quarks cannot be observed (see color confinement), unlike the situation with composite atoms where electrons and nuclei can be isolated by transferring ionization energy to the atom.
(ROOT (S (S (NP (NP (DT The) (NNS quarks)) (PP (IN in) (NP (DT a) (NN particle)))) (VP (VBP are) (VP (VBN confined)))) (, ,) (NP (VBG meaning) (JJ individual) (NNS quarks)) (VP (MD can) (RB not) (VP (VB be) (ADJP (RB directly) (JJ observed)))) (. .)))