Tevatron_Photons
================

We have studied prompt photon production in Hoeche2009xc and this section serves as a practical guide to the features necessary for these studies. Its main emphasis lies on how to generate samples which include both the direct and fragmentation component and how to apply ME+PS merging.

Traditionally, the direct and fragmentation component are well separated in a parton-shower Monte-Carlo, e.g. for single photon production: The direct component is produced by using the 2->2 matrix element with a photon and a parton in the final state, and the fragmentation component can be generated by using the 2->2 matrix element with two partons in the final state. On top of the LO matrix elements the parton shower would then produce interleaved QCD+QED emissions, where the QED shower emissions from the dijet sample will create the fragmentation component.

Note, that the generation of the fragmentation component in this way is very inefficient, because the shower will only very rarely produce hard isolated photons. To be able to compare to this method at all in Hoeche2009xc, we have implemented an enhancement of the QED splitting functions in the parton shower which is of course corrected for by giving the events appropriate weights, cf. the appendix of that paper.

But the main feature of Hoeche2009xc is the consistent treatment of photons in the context of ME+PS merging. This effectively means that one can split the fragmentation component into two parts by phase space slicing. Hard isolated photons are produced from the exact higher-order tree-level matrix element (e.g. pp -> photon + 2, 3, ... partons) while collinear photons are produced by the parton-shower, taking into account the correct resummation of the quark-photon singularities.

This is not only advisable to become less dependent on uncertain parton-shower approximations in the non-collinear region, but will also help to generate the fragmentation component more efficiently: If the phase space slicing criterion in the ME+PS merging is sufficiently similar (or loose) compared to the photon isolation criterion used in the analysis, one can expect that the second, painful, part of the fragmentation component, i.e. the one from the parton shower, becomes irrelevant for the analysis.

So in the following we describe how to generate single photons and diphotons making use of the default photon slicing criterion which looks like:

$\min(p_\perp^2(\gamma), p_\perp^2(parton)) (\DeltaR(\gamma, parton)/D)^2 > Q^2_cut$

where D=0.3 by default and Q_cut is the merging parameter specified in the CKKW line of the processes section. It might be sufficient to adapt these two parameters, e.g. in the following run cards set

       CKKW sqr(10.0/E_CMS)|0.2

for Q_cut=10.0 and D=0.2. If you notice that your photon isolation criterion is not sufficiently similar (the shower sample does contribute to your analysis or your ME sample still produces many photons which aren't isolated according to your criterion) one could still manually adapt the slicing criterion in Sherpa. Please contact us for assistance in that case.

In the following sections we show and discuss run cards for single- and diphoton production. They have been separated into a matrix-element part (i.e. direct and fragmentation-from-ME component) and a parton-shower part (i.e. fragmentation from the shower) as described above. In all analyses which we have compared to so far we found the contribution of the second part negligible. Please note, that these run cards will generate weighted events with ME enhancements in phase space regions which would otherwise not be filled very efficiently.

Single photon production
------------------------

Photons from ME
...............

     [file Run.single.me.dat not included]

  * In the processes section, matrix elements for pp(bar) -> photon + 1, 2, 3 partons are requested. Only Feynman diagrams with exactly one electroweak coupling are allowed.

     The merging criterion is set to Q_cut=10.0 GeV.

     Enhance_Function's (cf. Enhance_Function) are introduced to produce more hard partons/photons than the steeply falling cross section would imply (appropriately weighted).

  * The selector section allows to specify phase space cuts on the matrix element level, cf. Selectors. Here we demand two jets/photons with pT > 10.0 GeV, cf. Jet finders.

  * Electro-weak splitting functions in the shower are activated in the shower section.

  * The me section enables weighted event generation and switches off the emission of additional soft photons from the hard scattering.

  * The steeply rising enhance functions necessitate a redefinition of the integration error target, see Enhance_Function. The new value is set globally in the integration section.

  * To generate only prompt photons, the hadronisation, hadron decays and underlying event are turned off. These steps of the simulation can be enabled as described in Hadronization parameters and MI_HANDLER.

  * The beam section specifies Tevatron Run 2 conditions in this example but can simply be changed.

Photons from Shower
...................

(which will ideally be irrelevant for the analysis)

     [file Run.single.ps.dat not included]

Here only the differences with respect to above are explained:

  * In the processes section, matrix elements for pp(bar) -> dijet + 0, 1, 2 partons are requested. Only Feynman diagrams without electroweak couplings are allowed (for efficiency reasons).

Diphoton production
-------------------

Photons from ME
...............

     [file Run.diphoton.me.dat not included]

Here only the differences with respect to the single photon example are explained:

  * In the processes section, tree-level matrix elements for pp(bar) -> photon photon + 0, 1, 2 partons are requested. Only Feynman diagrams with exactly two electroweak couplings are allowed (for efficiency reasons).

     In addition, the loop-induced matrix element for the process gg -> photon photon is enabled.

Photons from Shower
...................

(which will ideally be irrelevant for the analysis)

The shower can produce di-photon events either from single-photon events or from di-jet events by producing one or two photons respectively. Here these two contributions are generated in one run, but of course they could be separated as well.

     [file Run.diphoton.ps.dat not included]

Here only the differences with respect to above are explained:

  * In the processes section, matrix elements for pp(bar) -> dijet + 0, 1, 2 partons and pp(bar) -> photon + 1, 2, 3 partons are requested. Only Feynman diagrams with 0 or 1 electroweak couplings respectively are allowed (for efficiency reasons).

Notes for users of release 1.2.1
--------------------------------

Unfortunately we realised too late that one minor change that allows event generation in the way described above did not make it into release 1.2.1. To use the NJetFinder in a context of democratic partons/photons you will have to download this patch and apply it to the 1.2.1 source code as follows:
cd SHERPA-MC-1.2.1
patch -p0 < njetfinder.patch
make install

If this is not possible for you, e.g. because you use an externally installed version of Sherpa, there is a workaround. Replace the NJetFinder lines in the selector sections above with the following cuts:
(selector){
  PT 22 10.0 E_CMS
  PT 93 10.0 E_CMS
}(selector)

This will work fine as long as the pT cut applied here is not higher then the Q_cut value in the ME+PS merging (CKKW line in processes).