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Long-lived coloured sparticles

In SUSY scenarios, the coloured sparticles -- squarks and gluinos -- typically have widths of several GeV, and thus decay well before they would have had time to hadronize. There are specific cases in which one of them is more long-lived or even (quasi-)stable, however. A recent example is the split SUSY scenarios, wherein the gauginos are rather light and the sfermions very heavy [Ark05]. Then the gluino decay is strongly suppressed, since it has to go via a virtual squark. In such cases, hadronic states may have time to form around them. These are called $R$-hadrons, since they carry negative $R$-parity.

In the long-lived-gluino case, the set of possible $R$-hadrons include `gluino-balls' $\tilde{\mathrm{g}}\mathrm{g}$, `gluino-mesons' $\tilde{\mathrm{g}}\mathrm{q}\overline{\mathrm{q}}$ and `gluino-baryons' $\tilde{\mathrm{g}}\mathrm{q}\mathrm{q}\mathrm{q}$, in multiplets similarly to normal hadrons, but of course with differences in the mass spectra and other properties. In the case of a long-lived squark, such as the stop, there would be `squark-mesons' $\tilde{\mathrm q}\overline{\mathrm{q}}$ and `squark-baryons' $\tilde{\mathrm q}\mathrm{q}\mathrm{q}$. When they pass through a detector, they may undergo charge- and baryon-number-changing interactions [Kra04], giving rise to quite spectacular and characteristic experimental signals [Kra04a].

Owing to the somewhat special nature of their production, these $R$-hadrons do not fit exactly into the current PYTHIA event generation chain, although all the separate tools exist. Therefore the production of gluino- and stop-hadrons is simulated by two separate add-on programs, available on the PYTHIA webpage, where the calling sequence is modified as appropriate. In future versions with a modified administrative structure, the intention is to include this capability in the standard distribution.

Finally, we note that a numbering scheme for $R$-hadrons is under development, for eventual inclusion in the PDG standard. The abovementioned programs comply with the draft proposal, but the absence of an approved standard is another reason why the programs have not yet been fully integrated into PYTHIA.


next up previous contents
Next: Polarization Up: Supersymmetry Previous: NMSSM   Contents
Stephen Mrenna 2007-10-30