The Roper Resonance has been a subject of many studies because its mass is actually lower than three-quark states with radial quantum number N = 1.
Only in the late 2000s was the lower-than-expected mass explained by theoretical calculations, revealing a quark core shielded by a dense cloud of mesons.
[1] The Roper resonance was discovered in 1963 by a computer fit of particle-scattering theory to large amounts of pion-nucleon scattering data.
The analysis was done on computers at Lawrence Livermore National Laboratory for Ph.D. thesis work of L. David Roper at Massachusetts Institute of Technology under the direction of Bernard Taub Feld at MIT and Michael J. Moravcsik at LLNL.
The computer code was developed by Richard Allen Arndt and Robert M. Wright.