MSEL = 1, 2, 4, 5, 6, 7, 8
An (almost) real photon has both a point-like component and a hadron-like one. This means that several classes of processes may be distinguished, see section .
For events, we believe that the best description can be obtained when three separate event classes are combined, one for direct, one for VMD and one for GVMD/anomalous events, see the detailed description in [Sch93,Sch93a]. These correspond to MSTP(14) being 0, 2 and 3, respectively. The direct component is high- only, while VMD and GVMD contain both high- and low- events. The option MSTP(14) = 1 combines the VMD and GVMD/anomalous parts of the photon into one single resolved photon concept, which therefore is less precise than the full subdivision.
When combining three runs to obtain the totality of interactions, to the best of our knowledge, it is necessary to choose the cut-offs with some care, so as to represent the expected total cross section.
The processes in points 1 and 2 can be simulated with a photon beam, i.e. when 'gamma' appears as argument in the PYINIT call. It is then necessary to use option MSTP(14) to switch between a point-like and a resolved photon -- it is not possible to simulate the two sets of processes in a single run. This would be the normal mode of operation for beamstrahlung photons, which have but with a nontrivial energy spectrum that would be provided by some external routine.
For bremsstrahlung photons, the and spectrum can be simulated internally, with the 'gamma/lepton' argument in the PYINIT call. This is the recommended procedure, wherein direct and resolved processes can be mixed. An older -- now not recommended -- alternative is to use a parton-inside-electron structure function concept, obtainable with a simple 'e-' (or other lepton) argument in PYINIT. To access these quark and gluon distributions inside the photon (itself inside the electron), MSTP(12) = 1 must then be used. Also the default value MSTP(11) = 1 is required for the preceding step, that of finding photons inside the electron. Also here the direct and resolved processes may be generated together. However, this option only works for high- physics. It is not possible to have also the low- physics (including multiple interactions in high- events) for an electron beam. Kindly note that subprocess 34 contains both the scattering of an electron off a photon and the scattering of a quark (inside a photon inside an electron) off a photon; the former can be switched off with the help of the KFIN array.
Also, a warning about the usage of PDFLIB/LHAPDF for photons. So long as MSTP(14) = 1, i.e. the photon is not split up, PDFLIB is accessed by MSTP(56) = 2 and MSTP(55) as the parton distribution set. However, when the VMD and anomalous pieces are split, the VMD part is based on a rescaling of pion distributions by VMD factors (except for the SaS sets, that already come with a separate VMD piece). Therefore, to access PDFLIB for MSTP(14) = 10, it is not correct to set MSTP(56) = 2 and a photon distribution in MSTP(55). Instead, one should put MSTP(56) = 2, MSTP(54) = 2 and a pion distribution code in MSTP(53), while MSTP(55) has no function. The anomalous part is still based on the SaS parameterization, with PARP(15) as main free parameter.
Currently, hadrons are not defined with any photonic content. None of the processes are therefore relevant in hadron-hadron collisions. In collisions, the electron can emit an almost real photon, which may interact directly or be resolved. In collisions, one may have direct, singly-resolved or doubly-resolved processes.
The equivalent to the description involves six different event classes, see section . These classes can be obtained by setting MSTP(14) to 0, 2, 3, 5, 6 and 7, respectively. If one combines the VMD and anomalous parts of the parton distributions of the photon, in a more coarse description, it is enough to use the MSTP(14) options 0, 1 and 4. The cut-off procedures follows from the ones used for the ones above.
As with events, the options MSTP(14) = 10 or MSTP(14) = 30 give a mixture of the six possible event classes. The same complications and restrictions exist here as already listed above.
Process 54 generates a mixture of quark flavours; allowed flavours are set by the gluon MDME values. Process 58 can generate both quark and lepton pairs, according to the MDME values of the photon. Processes 84 and 85 are variants of these matrix elements, with fermion masses included in the matrix elements, but where only one flavour can be generated at a time. This flavour is selected as described for processes 81 and 82 in section , with the exception that for process 85 the `heaviest' flavour allowed for photon splitting takes to place of the heaviest flavour allowed for gluon splitting. Since lepton KF codes come after quark ones, they are counted as being `heavier', and thus take precedence if they have been allowed.
Process 80 is a higher twist one. The theory for such processes is rather shaky, so results should not be taken too literally. The messy formulae given in [Bag82] have not been programmed in full, instead the pion form factor has been parameterized as , with in GeV.