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New gauge bosons

MSEL = 21, 22, 24
ISUB =
141 $\mathrm{f}_i \overline{\mathrm{f}}_i \to \gamma/\mathrm{Z}^0/\mathrm{Z}'^0$
142 $\mathrm{f}_i \overline{\mathrm{f}}_j \to \mathrm{W}'^+$
144 $\mathrm{f}_i \overline{\mathrm{f}}_j \to \mathrm{R}$

The $\mathrm{Z}'^0$ of subprocess 141 contains the full $\gamma^*/\mathrm{Z}^0/\mathrm{Z}'^0$ interference structure for couplings to fermion pairs. With MSTP(44) it is possible to pick only a subset, e.g. only the pure $\mathrm{Z}'^0$ piece. The couplings of the $\mathrm{Z}'^0$ to quarks and leptons in the first generation can be set via PARU(121) - PARU(128), in the second via PARJ(180) - PARJ(187) and in the third via PARJ(188) - PARJ(195). The eight numbers correspond to the vector and axial couplings of down-type quarks, up-type quarks, leptons and neutrinos, respectively. The default corresponds to the same couplings as that of the Standard Model $\mathrm{Z}^0$, with axial couplings $a_{\mathrm{f}} = \pm 1$ and vector couplings $v_{\mathrm{f}} = a_{\mathrm{f}} - 4 e_{\mathrm{f}} \sin^2 \! \theta_W $. This implies a resonance width that increases linearly with the mass. By a suitable choice of the parameters, it is possible to simulate just about any imaginable $\mathrm{Z}'^0$ scenario, with full interference effects in cross sections and decay angular distributions and generation-dependent couplings. The conversion from the coupling conventions in a set of different $\mathrm{Z}'^0$ models in the literature to those used in PYTHIA can be found in [Cio05].

The coupling to the decay channel $\mathrm{Z}'^0 \to \mathrm{W}^+ \mathrm{W}^-$ is regulated by PARU(129) - PARU(130). The former gives the strength of the coupling, which determines the rate. The default, PARU(129) = 1., corresponds to the `extended gauge model' of [Alt89], wherein the $\mathrm{Z}^0 \to \mathrm{W}^+ \mathrm{W}^-$ coupling is used, scaled down by a factor $m_{\mathrm{W}}^2/m_{\mathrm{Z}'}^2$, to give a $\mathrm{Z}'^0$ partial width into this channel that again increases linearly. If this factor is cancelled, by having PARU(129) proportional to $m_{\mathrm{Z}'}^2/m_{\mathrm{W}}^2$, one obtains a partial width that goes like the fifth power of the $\mathrm{Z}'^0$ mass, the `reference model' of [Alt89]. In the decay angular distribution one could imagine a much richer structure than is given by the one parameter PARU(130).

Other decay modes include $\mathrm{Z}'^0 \to \mathrm{Z}^0 \mathrm{h}^0$, predicted in left-right symmetric models (see PARU(145) and ref. [Coc91]), and a number of other Higgs decay channels, see sections [*] and [*].

The $\mathrm{W}'^{\pm}$ of subprocess 142 so far does not contain interference with the Standard Model $\mathrm{W}^{\pm}$ -- in practice this should not be a major limitation. The couplings of the $\mathrm{W}'$ to quarks and leptons are set via PARU(131) - PARU(134). Again one may set vector and axial couplings freely, separately for the $\mathrm{q}\overline{\mathrm{q}}'$ and the $\ell \nu_{\ell}$ decay channels. The defaults correspond to the $V-A$ structure of the Standard Model $\mathrm{W}$, but can be changed to simulate a wide selection of models. One possible limitation is that the same Cabibbo-Kobayashi-Maskawa quark mixing matrix is assumed as for the standard $\mathrm{W}$.

The coupling $\mathrm{W}' \to \mathrm{Z}^0 \mathrm{W}$ can be set via PARU(135) - PARU(136). Further comments on this channel as for $\mathrm{Z}'$; in particular, default couplings again agree with the `extended gauge model' of [Alt89]. A $\mathrm{W}' \to \mathrm{W}\mathrm{h}^0$ channel is also included, in analogy with the $\mathrm{Z}'^0 \to \mathrm{Z}^0 \mathrm{h}^0$ one, see PARU(146).

The $\mathrm{R}$ boson (particle code 41) of subprocess 144 represents one possible scenario [Ben85a] for a horizontal gauge boson, i.e. a gauge boson that couples between the generations, inducing processes like $\mathrm{s}\overline{\mathrm{d}}\to \mathrm{R}^0 \to \mu^- \mathrm{e}^+$. Experimental limits on flavour-changing neutral currents forces such a boson to be fairly heavy.

A further example of new gauge groups follows right after this.


next up previous contents
Next: Left-Right Symmetry and Doubly Up: Non-Standard Physics Previous: Fourth-generation fermions   Contents
Stephen Mrenna 2007-10-30