LUND6.3 has been the default hadronizer of the JLC physics group. This is partly because the current standard interface between the event generators which generate partons and the hadronization package is a software module called LUHADR based on LUND6.3. The LUHADR program is a module defined in the FULL analysis system and thus can be easily used together with the detector simulator to generate Monte Carlo events. In order to preserve this nice feature, while upgrading to JETSET7.3, the best way is to rewrite LUHADR to accomodate with JETSET7.3. Because of the incompatibility between JETSET6.3 and JETSET7.3, this revision requires a substantial change in the program. In addition, there had been no immediate necessity for JETSET7.3 until recently. These are the two main reasons that delayed the modification.
The situation has changed recently in the course of SUSY studies requiring correct polarization treatments of tau decays. There is a general tau decay program in the market. This program called TAUOLA, however, includes some unstable particles which are supported only in JETSET7.3 or later. Moreover, the implementation of TAUOLA implies a substantial change in the hadronization module anyway. We thus decided to upgrade the LUHADR module in order to implement TAUOLA and JETSET7.3.
Another important point in this upgrade is the treatment of parton-showering of color-non-singlet system, which is important in decays of heavy colored objects. In a top quark decay, for instance, the parton-showering takes place in the b-W+ system: the way gluons are emitted is controlled by the acceleration that the b-quark receives when the top decays and the 4-momentum adjustment should be carried out between the b-quark and the W boson. The 4-momentum of the W boson thus changes after the gluon emissions and the W's daughters momenta also change accordingly. This effect is consistently taken into account in the upgrade.