The chargino and neutralino masses and their mixing angles (that is, their gaugino and Higgsino composition) are determined by the SM gauge boson masses ( and ), , two soft SUSY-breaking parameters (the gaugino mass and the gaugino mass ), together with the Higgsino mass parameter , all evaluated at the electroweak scale . PYTHIA assumes that the input parameters are evaluated at the `correct' scale. Obviously, more care is needed to set precise experimental limits or to make a connection to higher-order calculations.

Explicit solutions of the chargino and neutralino masses and
their mixing angles (which appear in Feynman rules)
are found by diagonalizing the chargino
and
neutralino mass matrices:

is written in the basis, in the basis, with the notation and . Different sign conventions and bases sometimes appear in the literature. In particular, PYTHIA agrees with the ISASUSY [Bae93] convention for , but uses a different basis of fields and has different-looking mixing matrices.

In general, the soft SUSY-breaking parameters can be
complex valued, resulting in CP violation in some sector of the theory, but
more directly expanding the possible masses and mixings of sparticles.
Presently, the consequences of arbitrary phases are
only considered in the chargino and neutralino sector,
though it is well known that they can have a significant
impact on the Higgs sector. A generalization of the
Higgs sector is among the plans for the future development
of the program. The chargino and neutralino
input parameters are `RMSS(5)` (),
`RMSS(1)` (the modulus of
) and `RMSS(30)` (the phase of ),
`RMSS(2)` and `RMSS(31)` (the modulus and
phase of ), and `RMSS(4)` and `RMSS(33)`
(the modulus and phase of ). To simulate the
case of real parameters (which is CP-conserving),
the phases are zeroed by default. In addition,
the moduli parameters can be signed, to make a simpler
connection to the CP-conserving case. (For example,
`RMSS(5) = -100.0` and `RMSS(30) = 0.0` represents
GeV.)

The expressions for the production cross sections
and decay widths
of neutralino and chargino pairs contain the phase dependence,
but ignore possible effects of the phases in the sfermion
masses appearing in propagators.
The production cross sections have been updated to include
the dependence on beam polarization through the
parameters `PARJ(131,132)` (see Sect. ).
There are several approximations made for three-body decays.
The numerical expressions for three-body decay widths
ignore the effects of finite
fermion masses in the matrix element, but include them
in the phase space. No
three-body decays
are simulated,
nor
.
Finally, the effects of mixing between the third generation interaction
and mass eigenstates for sfermions is ignored, except that the
physical sfermion masses are used.
The kinematic distributions of the decay products are spin-averaged,
but include the correct matrix-element weighting.
Note that for the -parity-violating decays (see below), both
sfermion mixing effects and masses of , , and are fully
included.

In some corners of SUSY parameter space, special decay modes must be
implemented to capture important phenomenology. Three different
cases are distinguished here: (1) In the most common
models of Supergravity-mediated SUSY breaking, small values of
, , and can lead to a neutralino spectrum with
small mass splittings. For this case, the radiative decays
can be relevant, which are the SUSY analog
of
with two particles switched to superpartners
to yield
, for example.
These decays are calculated approximately for all neutralinos
when
, or the decay
can be forced using `IMSS(10) = 1`; (2) In models of gauge-mediated
SUSY breaking (GMSB), the gravitino
is light and phenomenologically
relevant at colliders. For `IMSS(11) = 1`, the two-body
decays of sparticle to particle plus gravitino are allowed. The most
relevant of these decay modes are likely
,
with
or a Higgs boson; (3) In models of anomaly-mediated
SUSY breaking (AMSB), the wino mass parameter is much smaller
than or . As a result, the lightest chargino and neutralino
are almost degenerate in mass. At tree level, it can be shown
analytically that the chargino should be
heavier than the neutralino, but this is hard to achieve numerically.
Furthermore, for this case, radiative corrections are important in
increasing the mass splitting more. Currently,
if ever the neutralino is heavier than the chargino
when solving the
eigenvalue problem numerically, the chargino mass is set to the
neutralino mass plus 2 times the charged pion mass, thus allowing the
decay
.

Since the symmetry of the SM is not broken, the
gluinos have masses determined by the gaugino mass
parameter , input through the parameter `RMSS(3)`.
The physical gluino mass is shifted from the value
of the gluino mass parameter because of radiative corrections.
As a result, there is an indirect dependence on the squark masses.
Nonetheless, it is sometimes convenient to input the physical
gluino mass, assuming that there is some choice of which
would be shifted to this value. This can be accomplished through
the input parameter `IMSS(3)`.
A phase for the gluino mass can be set using `RMSS(32)`,
and this can influence the gluino decay width (but no effect
is included in the
production).
Three-body decays of
the gluino to and and
plus the appropriate neutralino or chargino are allowed and
include the full effects of sfermion mixing. However, they
do not include the effects of phases arising from complex
neutralino or chargino parameters.