SUSY
- SUSY Processes
- Decays of SUSY Particles
Here is collected processes involving supersymmetric particle
production, with the exception of the (extended) Higgs sector.
Since the number of separate but closely related processes is so big,
there will not be switches for each separate process but only for a
reasonable set of subgroups. However, the general switches
SUSY:idA
and SUSY:idB
,valternatively
vectors SUSY:idVecA
and SUSY:idVecB
,
may be used in conjunction with any of these groups to provide
some additional flexibility to concentrate on processes involving
only specific (s)particle final states, see below.
Most of the SUSY implementation in PYTHIA 8 has been written by
N. Desai and is documented in [Des11]. Please give due
credit to external contributions to PYTHIA 8, such as this one, by
including the original work in your list of references when using this
implementation. The cross section formulae are mostly taken from
[Boz07] and [Fuk11].
Since the implementation of SUSY processes was only recently
completed [Des11], case-by-case validations against other codes
are still recommended. A set of default validations have
already been carried out by the authors, comparing to
the PYTHIA 6 SUSY implementation and to the XSUSY code,
using an sps1a spectrum. Explicit
validations of the non-trivial SLHA2-specific extensions have
generally not been carried out yet, with the exception of the R-parity
violating single-sparticle production cross sections. Please report
the results of any user validations you may carry out to the authors.
Important Note on SLHA:
In order to simulate SUSY processes it is required to read in the
couplings and masses relevant for the scenario to be studied. This
is done with the help of the SUSY Les Houches Accord (SLHA), including
the SLHA2 extensions and generalizations. (Internally, the SLHA2
conventions are used. SLHA1 spectra are automatically translated into
SLHA2 notation during initialization.) The
reading of a relevant SLHA file must be set up, as described
on the SLHA page.
Attempting to generate SUSY processes without a properly initialized
SLHA spectrum is strongly discouraged and may lead to unexpected
results. Always check for warnings and errors reported by the SLHA
reader during the initialization stage.
SUSY Processes
Note 1: Decays of SUSY particles are described
separately below.
Note 2: One special possibility is that the gluino or
some squark(s) are sufficiently long-lived to hadronize. See
the R-hadrons page for further details.
Note 3: lepton- and photon-initial states are not yet available.
Only quark/gluon-initiated 2 → 2 and 2 → 1 (RPV)
processes have been implemented.
Note 4: cross sections will be correctly folded with open
branching fractions of cascade decays, but at present any difference between
particle and antiparticle decay tables is not taken into account. This
possibility will be included in a future update.
flag
SUSY:all
(default = off
)
Common switch for production of supersymmetric particles, i.e.
particles with R-parity -1.
mode
SUSY:idA
(default = 0
; minimum = 0
)
Option to limit the sum over possible outgoing states in SUSY
2 → 2 processes to ones including a specific particle
identity code. The default corresponds to summing over all possible
indices. A non-zero value of SUSY:idA
selects only processes
that contain the state corresponding to that particular particle identity
code in the fundamental 2 → 2 scattering process (summed
over particle/antiparticle). It is the user's responsibility to ensure
that (a subset of) the processes to be simulated actually include this
particle at the 2 → 2 level; thus, asking for the lightest
neutralino (code 1000021) to be present in a squark-squark production
process will give no match.
mode
SUSY:idB
(default = 0
; minimum = 0
)
As for SUSY:idA
, but requires an additional particle
with PDG code SUSY:idB
to be present in the 2 → 2
process. Thus, using SUSY:idA
and SUSY:idB
a specific subprocess can be selected. Again only the absolute sign is
used, i.e. the summation over particle and antiparticle is retained.
Also the order of SUSY:idA
and SUSY:idB
is
irrelevant; since both possible orderings are checked for a match
with the two outgoing particles. (Although not recommended, should
SUSY:idA
be zero and SUSY:idB
nonzero
a match is searched for just like in the normal case with
SUSY:idA
nonzero and SUSY:idB
zero.)
mvec
SUSY:idVecA
(default = {0}
; minimum = 0
)
As for SUSY:idA
, but as a vector of PDG codes. Thus, it
selects only processes that have a final-state particle corresponding to
one of the identity codes in this vector. Note that, to activate this,
SUSY:idA
must be equal to zero; if not then the match
to SUSY:idA
takes precedence. Character-string input of
this vector should be as a comma-separated list, without any blanks.
mvec
SUSY:idVecB
(default = {0}
; minimum = 0
)
As for SUSY:idB
, but as a vector of PDG codes. As above,
to activate this, SUSY:idB
must be equal to zero; if not
then the match to SUSY:idB
takes precedence. For the matching,
either of SUSY:idA
and SUSY:idVecA
may be
combined with either of SUSY:idB
and SUSY:idVecB
.
As above one of the two outgoing SUSY particles must match one of the
particles in SUSY:id(Vec)A
and the other one of the
particles in SUSY:id(Vec)B
when both are nonzero.
Character-string input of this vector should be as a comma-separated
list, without any blanks.
mode
SUSY:sin2thetaWMode
(default = 2
; minimum = 1
; maximum = 3
)
The value of sin2(thetaW) should be taken from
option
1 : SM value, defined at M_Z, taken from
PYTHIA's StandardModel:sin2thetaW
parameter.
option
2 : SUSY value, defined at M_SUSY, derived from the
running gauge couplings in BLOCK GAUGE
in the SLHA
file. Note: if no such block is present in the input file, this option
will default back to option 1 above, i.e., the SM value.
option
3 : Pole value, defined by 1 - M_W^2/M_Z^2,
using the pole masses stored in the SLHA BLOCK MASS
, or,
alternatively, PYTHIA's internal pole masses if no such block is
present.
Gluino Pair Production
flag
SUSY:gg2gluinogluino
(default = off
)
Pair production of gluinos by gluon-gluon initial states.
flag
SUSY:qqbar2gluinogluino
(default = off
)
Pair production of gluinos by quark-antiquark annihilation and
t-channel squark exchange. The cross section
expression follows [Fuk11] and include the possibility
of non-minimal flavour violation through misalignment of quarks with
squarks. Only the MFV case has been explicitly validated.
Associated Squark-Gluino Production
flag
SUSY:qg2squarkgluino
(default = off
)
Associated production of a squark with a gluino. The cross section
expression follows [Fuk11] and include the possibility
of non-minimal flavour violation through misalignment of quarks with
squarks. Only the MFV case has been explicitly validated.
Squark Pair Production
flag
SUSY:gg2squarkantisquark
(default = off
)
Pair production of a scalar quark together with a scalar antiquark by
gluon annihilation via s-channel gluon exchange, t- and
u-channel squark exchange, and the direct 4-point coupling.
The cross section expression follows [Boz07].
Only the MFV case has been explicitly validated.
flag
SUSY:qqbar2squarkantisquark
(default = off
)
Pair production of a scalar quark together with a scalar antiquark
by quark-antiquark annihilation.
For same-isospin ~q~q* production (i.e., ~u~u*,
~u~c*, ...), the s-channel gluon, photon, and
Z and t-channel gluino contributions have so far
been implemented (i.e., the t-channel neutralino contributions
are neglected). For opposite-isospin ~q~q* production
(~u~d*, ~u~s*, ...), the s-channel W
and t-channel gluino contributions have been implemented
(i.e., the t-channel neutralino contributions are neglected).
The cross section expressions follow [Boz07].
Only the MFV case has been explicitly validated.
(Note to PYTHIA 6 users:
in older PYTHIA 6 versions, a bug caused the ~t1~t2* cross to be
overcounted by a factor of 2. Starting from version 6.4.24, that
generator now agrees with the implementation here.)
flag
SUSY:qqbar2squarkantisquark:onlyQCD
(default = off
)
When switched on
this flag switches off all but the
s-channel gluon contribution in the
calculation of same-isospin squark-antisquark production cross
sections. Intended for reference only. For the most
accurate physics simulation, leave this flag in the off
position.
flag
SUSY:qq2squarksquark
(default = off
)
Pair production of scalar quarks (squark-squark and its charge
conjugate process; for squark-antisquark production see above)
by t- and u-channel gluino, neutralino, and
chargino exchange. The cross section expressions follow [Boz07].
Only the MFV case has been explicitly validated.
(Note to PYTHIA 6 users: PYTHIA 6 only included the gluino exchange
contribution, which typically dominates due to the size of the strong
coupling; for counterchecks,
the flag SUSY:qq2squarksquark:onlyQCD
below can be switched on to eliminate the chargino and neutralino
contributions.)
flag
SUSY:qq2squarksquark:onlyQCD
(default = off
)
When switched on
this flag causes the t- or
u-channel neutralino and chargino contributions to be
ignored in the calculation of squark pair production cross sections.
Intended for reference only. For the most accurate physics simulation,
leave this flag in the off
position.
Neutralino and Chargino Pair Production
flag
SUSY:qqbar2chi0chi0
(default = off
)
Pair production of neutralinos by quark-antiquark annihilation. With
four neutralino species this gives ten separate processes, codes
1201 - 1210. The cross section expressions follow [Boz07].
Only the MFV case has been explicitly validated.
flag
SUSY:qqbar2chi+-chi0
(default = off
)
Associated chargino-neutralino production by quark-antiquark
annihilation. With four neutralino species, two chargino ones, and
maintaining charge conjugate processes separate, this gives 16
separate processes, codes 1221 - 1236. The cross section expressions
follow [Boz07].
Only the MFV case has been explicitly validated.
flag
SUSY:qqbar2chi+chi-
(default = off
)
Pair production of charginos by quark-antiquark annihilation. With
two chargino species and maintaining mutually charge conjugate
processes separate, this gives four separate processes, codes
1241 - 1244. The cross section expressions follow [Boz07].
Only the MFV case has been explicitly validated.
Associated Neutralino/Chargino + Squark/Gluino Production
flag
SUSY:qg2chi0squark
(default = off
)
Pair production of neutralinos from quark-gluon initial states.
The cross section expressions follow [Boz07].
Only the MFV case has been explicitly validated.
flag
SUSY:qg2chi+-squark
(default = off
)
Associated chargino-squark production from quark-gluon initial states.
annihilation. The cross section expressions
follow [Boz07].
Only the MFV case has been explicitly validated.
flag
SUSY:qqbar2chi0gluino
(default = off
)
Associated neutralino-gluino production by quark-antiquark
annihilation.
The cross section expressions follow
[Fuk11]. Only the MFV case has been explicitly validated.
flag
SUSY:qqbar2chi+-gluino
(default = off
)
Associated chargino-gluino production by quark-antiquark
annihilation. The cross section expressions follow
[Fuk11]. Only the MFV case has been explicitly
validated. (Note to PYTHIA 6 users: small differences between
this implementation and PYTHIA 6 arise due
to slightly different treatments of the weak mixing angle,
which is fixed in PYTHIA 6, while it is computed from the SLHA input
in PYTHIA 8; see SUSY:sin2thetaWMode
above.)
Slepton Production
flag
SUSY:qqbar2sleptonantislepton
(default = off
)
Pair production of slepton-antislepton via s-channel W, Z and gamma
exchange. Includes both charged sleptons and sneutrinos but right
handed sneutrinos currently not supported.
R-parity violating squark production
flag
SUSY:qq2antisquark
(default = off
)
Resonant squark production via R-parity violating UDD couplings. The
couplings must be input using the SLHA2 structure.
Decays of SUSY Particles
Based on the parameters read in from the SLHA, PYTHIA 8 will normally
compute the decay modes of SUSY particles automatically, using the
SusyResonanceDecays
class(es). Essentially all tree-level
2-body decays in the MSSM
have been implemented this way, excepting so far only those involving
Higgs bosons (either in the in- or out-state) or gravitinos.
Available channels so far include:
- ~q → q + ~chi
- ~q → ~q + W/Z
- ~q → q + q (RPV UDD)
- ~q → l + q (RPV LQD)
- ~g → ~q + q
- ~chi → ~chi + Z/W
- ~chi → ~q + q
- ~chi → ~l/~nu + l/nu
- ~chi0 → q + q + q (RPV UDD)
- ~l/~nu → l/nu + ~chi
- ~l/~nu → ~l/~nu + W/Z
All channels are still undergoing validation, so this
implementation should be considered preliminary.
Still missing but to be included in a forthcoming update
are: 3-body decays of charginos (via RPV), and 2-body decays of squarks and
gauginos with Higgs as one of the decay products.
Some 3-body decays have been implemented with Matrix Element weighting.
In particular, those for a neutralino to a lighter neutralino and a
fermion pair can be enabled.
flag
SUSYResonance:3BodyMatrixElement
(default = off
)
When "on", the spin-averaged, squared matrix element is used
to sample the phase space for resonance decay. Currently, only
possible for a heavy neutralino decay to a light neutralino
and a fermion-antifermion pair.