SUSY Processes

## SUSY

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.