Leptoquarks

MSEL = 25
ISUB =

145	$\mathrm{q}_i \ell_j \to \L _{\mathrm{Q}}$
162	$\mathrm{q}\mathrm{g}\to \ell \L _{\mathrm{Q}}$
163	$\mathrm{g}\mathrm{g}\to \L _{\mathrm{Q}} \overline{\L }_{\mathrm{Q}}$
164	$\mathrm{q}_i \overline{\mathrm{q}}_i \to \L _{\mathrm{Q}} \overline{\L }_{\mathrm{Q}}$

Several processes that can generate a leptoquark have been included. Currently only one leptoquark has been implemented, as particle 42, denoted $\L _{\mathrm{Q}}$. The leptoquark is assumed to carry specific quark and lepton quantum numbers, by default $\u$ quark plus electron. These flavour numbers are conserved, i.e. a process such as $\u\mathrm{e}^- \to \L _{\mathrm{Q}} \to \d\nu_{\mathrm{e}}$ is not allowed. This may be a bit restrictive, but it represents one of many leptoquark possibilities. The spin of the leptoquark is assumed to be zero, i.e. its decay is isotropical.

Although only one leptoquark is implemented, its flavours may be changed arbitrarily to study the different possibilities. The flavours of the leptoquark are defined by the quark and lepton flavours in the decay mode list. Since only one decay channel is allowed, this means that the quark flavour is stored in KFDP(MDCY(42,2),1) and the lepton one in KFDP(MDCY(42,2),2). The former must always be a quark, while the latter could be a lepton or an antilepton; a charge-conjugate partner is automatically defined by the program. At initialization, the charge is recalculated as a function of the flavours defined; also the leptoquark name is redefined to be of the type 'LQ_(q)(l)', where actual quark (q) and lepton (l) flavours are displayed.

The $\L _{\mathrm{Q}} \to \mathrm{q}\ell$ vertex contains an undetermined Yukawa coupling strength, which affects both the width of the leptoquark and the cross section for many of the production graphs. This strength may be changed in PARU(151). The definition of PARU(151) corresponds to the $k$ factor of [Hew88], i.e. to $\lambda^2/(4\pi\alpha_{\mathrm{em}})$, where $\lambda$ is the Yukawa coupling strength of [Wud86]. Note that PARU(151) is thus quadratic in the coupling.

The leptoquark is likely to be fairly long-lived, in which case it has time to fragment into a mesonic- or baryonic-type state, which would decay later on. This is a bit tedious to handle; therefore the leptoquark is always assumed to decay before fragmentation. This may give some imperfections in the event generation, but should not be off by much in the final analysis [Fri97].

Inside the program, the leptoquark is treated as a resonance. Since it carries colour, some extra care is required. In particular, it is not allowed to put the leptoquark stable, by modifying either MDCY(42,1) or MSTP(41): then the leptoquark would be handed undecayed to PYTHIA, which would try to fragment it (as it does with any other coloured object), and most likely crash.