Electroweak Processes

This page contains processes involving Prompt-photon, gamma^*/Z^0 and W^+- production, plus a few with t-channel boson exchange.

Prompt photon processes

This group collects the processes where one or two photons are produced by the hard process. Additional sources of photons include parton showers and hadron decays. A pT cut is required to stay away from the unphysical low-pT region. An eikonalized description, intended to be valid at all pT, is included as part of the multiparton-interactions framework.

flag  PromptPhoton:all   (default = off)
Common switch for the group of all prompt photon processes, as listed separately in the following.

flag  PromptPhoton:qg2qgamma   (default = off)
Scattering q g → q gamma. Code 201.

flag  PromptPhoton:qqbar2ggamma   (default = off)
Scattering q qbar → g gamma. Code 202.

flag  PromptPhoton:gg2ggamma   (default = off)
Scattering g g → g gamma.
Note: This is a box graph. The full quark-mass dependence in the loop leads to very complicated expressions. The current implementation is based on assuming five massless quarks (see below), and thus is questionable at small (pT < m_b) or large (pT > m_t) transverse momenta. Code 203.

flag  PromptPhoton:ffbar2gammagamma   (default = off)
Scattering q qbar → gamma gamma. Code 204.

flag  PromptPhoton:gg2gammagamma   (default = off)
Scattering g g → gamma gamma.
Note: This is a box graph. The full quark-mass dependence in the loop leads to very complicated expressions. The current implementation is based on assuming five massless quarks (see below), and thus is questionable at small (pT < m_b) or large (pT > m_t) transverse momenta. Code 205.

mode  PromptPhoton:nQuarkLoop   (default = 5; minimum = 3; maximum = 6)
Number of quark flavours included in the box graphs responsible for g g → g gamma and g g→ gamma gamma processes. Owing to the complexity if the massive expressions, quarks are treated as massless. The default value should be applicable in the range of transverse momenta above the b mass but below the t one.

Weak boson processes

Under this heading we group processes involving the production of a single electroweak gauge boson, i.e. a gamma^*/Z^0 or a W^+-, or a pair of them, or one of them in combination with a parton. Since the three sets are partly conflicting, each is associated with its own group flag. In addition, t-channel exchange of such a boson between two fermions form a separate group.

There is one flag that can be used to influence the gamma^*/Z^0 structure in all the processes below where it is produced, unless otherwise stated.

mode  WeakZ0:gmZmode   (default = 0; minimum = 0; maximum = 2)
Choice of full gamma^*/Z^0 structure or not in relevant processes.
option 0 : full gamma^*/Z^0 structure, with interference included.
option 1 : only pure gamma^* contribution.
option 2 : only pure Z^0 contribution.
Note: irrespective of the option used, the particle produced will always be assigned code 23 for Z^0, and open decay channels is purely dictated by what is set for the Z^0.

Boson exchange

The two processes in this subgroup is included as part of the multiparton-interactions framework.

flag  WeakBosonExchange:all   (default = off)
Common switch for the group of gamma^*/Z^0 or W^+- exchange between two fermions.

flag  WeakBosonExchange:ff2ff(t:gmZ)   (default = off)
Scattering f f' → f f' via gamma^*/Z^0 t-channel exchange, with full interference between the gamma^* and Z^0. Code 211.

flag  WeakBosonExchange:ff2ff(t:W)   (default = off)
Scattering f_1 f_2 → f_3 f_4 via W^+- t-channel exchange. Code 212.

Single boson

flag  WeakSingleBoson:all   (default = off)
Common switch for the group of a single gamma^*/Z^0 or W^+- production.

flag  WeakSingleBoson:ffbar2gmZ   (default = off)
Scattering f fbar → gamma^*/Z^0, with full interference between the gamma^* and Z^0. Code 221.

flag  WeakSingleBoson:ffbar2W   (default = off)
Scattering f fbar' → W^+-. Code 222.

flag  WeakSingleBoson:ffbar2ffbar(s:gm)   (default = off)
Scattering f fbar → gamma^* → f' fbar'. Subset of process 221, but written as a 2 → 2 process, so that pT can be used as ordering variable, e.g. in multiparton interactions. Hardcoded for the final state being either of the five quark flavours or three lepton ones. Not included in the WeakSingleBoson:all set, but included in the multiparton-interactions framework. Code 223.

flag  WeakSingleBoson:ffbar2ffbar(s:gmZ)   (default = off)
Scattering f fbar → gamma^*/Z^0 → f' fbar'. Equivalent to process 221, but written as a 2 → 2 process, so that pT could be used as cut or ordering variable. Final-state flavour selection is based on the Z^0 allowed decay modes, and the WeakZ0:gmZmode options are implemented. Not included in the WeakSingleBoson:all set. Code 224.

flag  WeakSingleBoson:ffbar2ffbar(s:W)   (default = off)
Scattering f_1 fbar_2 → W+- → f_3 f_4. Almost equivalent to process 222, but written as a 2 → 2 process, so that pT could be used as cut or ordering variable. Final-state flavour selection is based on the W allowed decay modes. There are two simplifications relative to the implementation in process 222. Firstly, it is not possible to set different decay modes for the W^+ and the W^-; instead the allowed W^+ ones will be used throughout, with charge conjugation for the W^-. Secondly, quark mass corrections are neglected in the decay angular distribution. Not included in the WeakSingleBoson:all set. Code 225.

Boson pair

Note that, in the decay of the two vector bosons produced by an f fbar → V V process, the full four-fermion correlations from the leading-order f fbar → V V → 4f matrix elements are included [Gun86] (with some extensions by the authors). The matrix elements are provided in the double-resonant approach, i.e. excludes graph like f fbar → V → f fbar → f fbar V → 4f.

flag  WeakDoubleBoson:all   (default = off)
Common switch for the group of pair production of gamma^*/Z^0 and W^+-.

flag  WeakDoubleBoson:ffbar2gmZgmZ   (default = off)
Scattering f fbar' → gamma^*/Z^0 gamma^*/Z^0. Code 231.

flag  WeakDoubleBoson:ffbar2ZW   (default = off)
Scattering f fbar' → Z^0 W^+-. Note that here the gamma^* contribution is not (currently) included. Code 232.

flag  WeakDoubleBoson:ffbar2WW   (default = off)
Scattering f fbar → W^+ W^-. Code 233.

Boson and parton

flag  WeakBosonAndParton:all   (default = off)
Common switch for the group of production of a single electroweak gauge boson, i.e. a gamma^*/Z^0 or a W^+-, in association with a parton, i.e. a quark, gluon, photon or lepton. These processes give first-order corrections to the ones in the WeakSingleBoson class, and both sets cannot be used simultaneously without unphysical double-counting. The current class should only be used to study the high-pT tail of the gauge-boson production processes (for LHC applications at least pT > 20 GeV), while the ones in WeakSingleBoson should be used for inclusive production.

flag  WeakBosonAndParton:qqbar2gmZg   (default = off)
Scattering q qbar → gamma^*/Z^0 g. Code 241.

flag  WeakBosonAndParton:qg2gmZq   (default = off)
Scattering q g → gamma^*/Z^0 q . Code 242.

flag  WeakBosonAndParton:ffbar2gmZgm   (default = off)
Scattering f fbar → gamma^*/Z^0 gamma. Code 243.

flag  WeakBosonAndParton:fgm2gmZf   (default = off)
Scattering f gamma → gamma^*/Z^0 f. Code 244.

flag  WeakBosonAndParton:qqbar2Wg   (default = off)
Scattering q qbar → W^+- g. Code 251.

flag  WeakBosonAndParton:qg2Wq   (default = off)
Scattering q g → W^+- q. Code 252.

flag  WeakBosonAndParton:ffbar2Wgm   (default = off)
Scattering f fbar → W^+- gamma. Code 253.

flag  WeakBosonAndParton:fgm2Wf   (default = off)
Scattering f gamma → W^+- f. Code 254.

Photon Collision Processes

A few electroweak two-photon production processes are available. To use them, photon PDFs have to be defined for the incoming beam particles. For proton beams an appropriate set would be MRST QED 2004 [Mar05], available in the LHAPDF library.

flag  PhotonCollision:all   (default = off)
Common switch for the group of six processes presented below.

flag  PhotonCollision:gmgm2qqbar   (default = off)
Scatterings gamma gamma → q qbar, where q is a light quark (u, d, s) . Code 261.

flag  PhotonCollision:gmgm2ccbar   (default = off)
Scattering gamma gamma → c cbar. Code 262.

flag  PhotonCollision:gmgm2bbbar   (default = off)
Scattering gamma gamma → b bbar. Code 263.

flag  PhotonCollision:gmgm2ee   (default = off)
Scattering gamma gamma → e+ e-. Code 264.

flag  PhotonCollision:gmgm2mumu   (default = off)
Scattering gamma gamma → mu+ mu-. Code 265.

flag  PhotonCollision:gmgm2tautau   (default = off)
Scattering gamma gamma → tau+ tau-. Code 266.