Hard Processes

The current PYTHIA contains a much richer selection, with around 240 different hard processes. These may be classified in many different ways.

One is according to the number of final-state objects: we speak of `$2 \to 1$' processes, `$2 \to 2$' ones, `$2 \to 3$' ones, etc. This aspect is very relevant from a programming point of view: the more particles in the final state, the more complicated the phase space and therefore the whole generation procedure. In fact, PYTHIA is optimized for $2 \to 1$ and $2 \to 2$ processes. There is currently no generic treatment of processes with three or more particles in the final state, but rather a few different machineries, each tailored to the pole structure of a specific class of graphs.

Another classification is according to the physics scenario. This will be the main theme of section . The following major groups may be distinguished:

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Hard QCD processes, e.g. $\mathrm{q}\mathrm{g}\to \mathrm{q}\mathrm{g}$.

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Soft QCD processes, such as diffractive and elastic scattering, and minimum-bias events. Hidden in this class is also process 96, which is used internally for the merging of soft and hard physics, and for the generation of multiple interactions.

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Heavy-flavour production, both open and hidden, e.g. $\mathrm{g}\mathrm{g}\to \t\overline{\mathrm{t}}$ and $\mathrm{g}\mathrm{g}\to \mathrm{J}/\psi \mathrm{g}$.

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Prompt-photon production, e.g. $\mathrm{q}\mathrm{g}\to \mathrm{q}\gamma$.

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Photon-induced processes, e.g. $\gamma \mathrm{g}\to \mathrm{q}\overline{\mathrm{q}}$.

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Deeply Inelastic Scattering, e.g. $\mathrm{q}\ell \to \mathrm{q}\ell$.

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$\mathrm{W}/ \mathrm{Z}$ production, such as the $\mathrm{e}^+\mathrm{e}^-\to \gamma^* / \mathrm{Z}^0$ or $\mathrm{q}\overline{\mathrm{q}}\to \mathrm{W}^+ \mathrm{W}^-$.

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Standard model Higgs production, where the Higgs is reasonably light and narrow, and can therefore still be considered as a resonance.

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Gauge boson scattering processes, such as $\mathrm{W}\mathrm{W}\to \mathrm{W}\mathrm{W}$, when the Standard Model Higgs is so heavy and broad that resonant and non-resonant contributions have to be considered together.

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Non-standard Higgs particle production, within the framework of a two-Higgs-doublet scenario with three neutral ($\mathrm{h}^0$, $\H ^0$ and $\mathrm{A}^0$) and two charged ($\H ^{\pm}$) Higgs states. Normally associated with SUSY (see below), but does not have to be.

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Production of new gauge bosons, such as a $\mathrm{Z}'$, $\mathrm{W}'$ and $\mathrm{R}$ (a horizontal boson, coupling between generations).

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Technicolor production, as an alternative scenario to the standard picture of electroweak symmetry breaking by a fundamental Higgs.

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Compositeness is a possibility not only in the Higgs sector, but may also apply to fermions, e.g. giving $\d ^*$ and $\u ^*$ production. At energies below the threshold for new particle production, contact interactions may still modify the standard behaviour.

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Left-right symmetric models give rise to doubly charged Higgs states, in fact one set belonging to the left and one to the right SU(2) gauge group. Decays involve right-handed $\mathrm{W}$'s and neutrinos.

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Leptoquark ( $\L _{\mathrm{Q}}$) production is encountered in some beyond-the-standard-model scenarios.

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Supersymmetry (SUSY) is probably the favourite scenario for physics beyond the standard model. A rich set of processes are allowed, already if one obeys $R$-parity conservation, and even more so if one does not. The supersymmetric machinery and process selection is inherited from SPYTHIA [Mre97], however with many improvements in the event generation chain. Many different SUSY scenarios have been proposed, and the program is flexible enough to allow input from several of these, in addition to the ones provided internally.

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The possibility of extra dimensions at low energies has been a topic of much study in recent years, but has still not settled down to some standard scenarios. Its inclusion into PYTHIA is also only in a very first stage.

This is by no means a survey of all interesting physics. Also, within the scenarios studied, not all contributing graphs have always been included, but only the more important and/or more interesting ones. In many cases, various approximations are involved in the matrix elements coded.