## Leptoquark Processes

Leptoquarks arise in many scenarios, and can have widely different characteristics, with respect to spin, isospin am d flavour. The current implementation in no sense attempts to exhaust these possibilities, but only to encode one of the simplest possibilities, with a single scalar leptoquark, denoted LQ and assigned PDG code 42. 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 e^- → LQ → d nu_e is not allowed.

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. Therefore, to change from the current u e^- to c mu^+, say, you only need a line
`pythia.readString("42:0:products = 4 -13");`
in your main program, or the equivalent in a command file. 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 and lepton flavours are displayed.

The leptoquark is likely to be fairly long-lived, in which case it could have time to fragment into a mesonic- or baryonic-type state, which would decay later on. Currently this possibility is not handled; therefore the leptoquark is always assumed to decay before fragmentation. For that reason the leptoquark can also not be put stable.

### Production processes

Four production processes have been implemented, which normally would not overlap and therefore could be run together.

`flag` LeptoQuark:all (`default = off`)
Common switch for the group of lowest-order LQ production processes, i.e. the four ones below.

`flag` LeptoQuark:ql2LQ (`default = off`)
Scatterings q l → LQ. Code 3201.

`flag` LeptoQuark:qg2LQl (`default = off`)
Scatterings q g → LQ l. Code 3202.

`flag` LeptoQuark:gg2LQLQbar (`default = off`)
Scatterings g g → LQ LQbar. Code 3203.

`flag` LeptoQuark:qqbar2LQLQbar (`default = off`)
Scatterings q qbar → LQ LQbar. Code 3204.

### Parameters

In the above scenario the main free parameters are the leptoquark flavour content, set as already described, and the LQ mass, set as usual. In addition there is one further parameter.

`parm` LeptoQuark:kCoup (`default = 1.0`; `minimum = 0.0`)
multiplicative factor in the LQ → q l squared Yukawa coupling, and thereby in the LQ width and the q l → LQ and other cross sections. Specifically, lambda^2/(4 pi) = k alpha_em, i.e. it corresponds to the $k$ factor of [Hew88].