Calculating Cross Sections Via comphep v.4.4.3 
Note: Documentation is relevant only for cross sections calculations, hence non-relevant menu options are 

Intro: Comphep is very conveniently navigated via a GUI branching menu system, as such i'll organize this 
documentation sequentially from the first menu you see...
I.Models- for our purposes there is really only 1 model worth using
	1.Standard Model Feynman gauge- This differs from the unitary gauge standard model in that it includes 
        UV cancellations between diagrams due to gauge invariance.  We'd like all the information we can get 
        our hands on, so use the Feynman gauge model
II.Process Menu
*Note that all mass and energies within comphep are in GeV unless specified otherwise
	1.Enter scattering process- This is the money making option.  See next section 
	2.Edit Beams Table- This will enter you into a pseudo text editor, the columns are appropriately labeled 
        name/mass/content/str.  functions.  If you need to, or if you have some unwaivering desire, you can edit        
        the scattering beams however you like.
		1.Name: what shows up in the scattering menu
		2.Mass: ...Mass
		3.Content: Quark content of the beam 
		4.Str. functions: Structure functions used which can be edited; by the next menu option   
	3.Edit Str. functions Table- You can change the name and number assigned to the str. function.  While 
        structure functions are a bit over my head, i don't believe you'll need this 
	4.Edit Model- More of the same, you're given name/value/comment columns what you'll see in comphep (a good reference for parameter names as they're not entirely
		2.value: can't do physics without values 
		3.comment: what exactly that name corresponds to 
III.  Scattering process
	1.Entering beams- note the very nice beam table at the top of the screen,  use pg-dwn and pg-up  to scroll 
        the choices, or hit F3 to switch to  a table of particles which are entered the same way.  Also note the 
        antiparticle is given in parenthesis 
*NOTE for our purposes I strongly advise using the proton beam labeled p, not the beam labeled p1 as using p1 
will generally give you less diagrams (the beam does not have full quark content) and among the lost diagrams 
are those with intermediate Higgs resonance
	2.Energies- enter the energy from each beam (total energy * .5) 
		1.ex: LHC at 14 Tev = 2 x 7Tev beams 
	3.PDF- The structure function used for each beam, generally the default value they give you is what you'd 
        like to use (i.e. proton beam necessitates the proton PDF)
	4. Final State- put in the resulting particles you're interested in
*NOTE: comphep does not do 2->1 processes, there will always need to be 2 or more particles in the final state.  
If you want a two by one process you will need to go to the decay states.  
	Example: if you want a proton proton => Higgs collision you'd have to use W+,W-/l,L/b,b-bar..etc as your 
        final states rather than just H and then look at the subprocesses that have an intermediate Higgs resonance 
        (includes background collisions) or you can include only the H diagrams (only the Higgs collisions) 
	5. Include/Exclude diagrams- now this is one of the more useful features in Comphep.  You can choose to 
        include or exclude particles based on the intermediate particles.  
	Example: using the above example, lets say you just want to look at only the collisions that produce Higgs, 
        you would put H in the include field, and viola, you're left with 4 subprocesses which include ONLY the Higgs
*NOTE: be careful with the exclude/include options.  If you want the noise from the background collisions you CAN'T 
use it as it will delete all of the other diagrams, leaving only Higgs.  Meaning you won't be given the other diagrams 
produced by a specific subprocess.  If you want to include all the junk, just run numerical calculations on the 
SUBPROCESSES that produce Higgs

IV.  Feynman Diagrams 
	1. View diagrams- take a gander at all the subprocesses and thus diagrams that your fancy collisions uses
	2.  Squared diagrams
		1.  View diagrams- even more impressive diagrams to look at 
		2.  Symbolic Calculations
			a.  Write results- always write the results so you have something to compile.  write the results in c, 
                        fortran, or mathemaica code for use in another event generator or for fancy math in mathematica  
 			b.  c-compiler- compile your results to do numerical calculations via comphep.  Hit enter.

A new window will pop up allowing you lots more high energy fun 

I.  Numerical Calculation menu 
	1.  Subproces- Here you choose which subprocess you would like to run a monte carlo session for.  Every 
        subprocess requires a  different monte carlo session.  
*NOTE:The results for a session are written to a file name prt_n where n is the session number.  Again, to keep 
your results from being cleaned, don't end your numerical calculation session (not monte carlo session, you can 
run as many as you like without fear) until you've copied the results elsewhere, OR you can just kill the window 
and remove the LOCK files to start comphep again
	2.  IN state- you can change the parameters of scattering here 
	3. Model Parameters- change the parameters of your model
*NOTE: I realize the parameter names aren't terribly self explanatory, which is why I mentioned before that the
Edit Model; menu when you first start comphep is a good reference
	4.  Constraints- another menu to edit constraints 
	5.  QCD- Edit QCD Values (i'd explain more if i understood QCD 	more) 
	6.  Cuts- for simple cross section calculations cuts are generally not needed, however if relevant, the manual 
        covers the subject well.
	7.  Kinematics- for simple cross section calculation this menu is not very relevant.  If interested see the linked 
 	8.  Regularization- Again not especially useful when doing simple cross sections.  The manual will be more 
        help than I would.
	9.  Vegas/simpson- see next section 
II.  Vegas/Simpson 
	1.  Vegas- the method you will be using to calculate cross sections 90 percent of the time.  usually the best bet.
		1.  Set distributions- Here you will specify which plots you're looking for.  Simply fill in the parameter 
                name and the bounds.  
*NOTE: There is already a good list and explanation of the different plots.  It lists the phase space functions (which 
you use for the parameter) and the syntax for using them: 
		Example: M34 plots the mass of particle3+particle4 i.e. final mass
 		2.  Start integration- hit enter to your heart's content
*NOTE: If immediately you notice very poor chi^2 fits then hit enter over the clear statistics option.  This will clear the i
ntegration for the process you are doing, however do not clear the grid, that's entirely different. 
**NOTE: More integration = less error, I tend to get reasonably good looking plots from 50 or more integrations
		3. Display distribution- enter and choose the distribution you'd like
 *NOTE: hit any button but esc to enter the plot menu, all of the options are exactly what the name implies other than latex.  
To save your plot you will hit enter on the latex option and choose write to file it will then tell you the filename for your 
plot.  I'll write more docs later on how to make use of these .tex files
		4.  Clear statistics- Clear the integration table for the subprocess you've chosen  	
	2.  Simpson- only available for the most simple collisions, not really that useful from what i've used comphep for 

At this point you have your cross sections and plots.  
Note that the cross sections for a subprocess are stored in the files name prt_n and the plots are of the form plot_n.tex 
-- BruceThayre - 19 Jul 2006
Topic revision: r3 - 2006/08/02 - 23:30:44 - BruceThayre
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