# Fastest Way to Calculate Hadronic Cross Sections

Many of the particle physicists may think that CompHEP is not a proper way of making hadronic calculations. Since in numerical calculations CompHEP treats each subprocess separately, calculation of a process with lots of subprocesses (as it happens usually in calculations for hadron colliders) can be a laborious task. In order to make the task simpler and enable non-GUI calculations both symbolic and numerical programs in CompHEP are equipped with the batch PERL scripts **symb batch.pl **and **num batch.pl **correspondingly.

Here’s an example how you can use these scripts:

1 -) Open CompHEP in your working directory as ./comphep

2 -) Enter a scattering process like pb,pb -> t,T.

3 -) Enter C-compiler and complete symbolic calculation.

4 -) You will get the numerical session GUI and see Process, SubProcess, Monte Carlo seesion…etc. Notice that you can only calculate subprocess cross sections here.

5 -) Open a new terminal but don’t close CompHEP numerical session window.

6 -) Open your CompHEP working directory where num_batch.pl and sym_batch.pl are located.

7 -) Write ‘./num_batch.pl -run vegas’

8 -) Write ‘./num_batch.pl -show cs’ and you’ll get the total cross section through all subprocesses.

OR edit process.dat to enter your process and use sym_batch.pl in terminal window to complete symbolic calculations.

Reference: http://arxiv4.library.cornell.edu/pdf/0901.4757v1

# A Simple C++ Simulation For Beginners

Phenomenology in physics, mostly deals with the simulation of events and obtaining data from simulations to compare it with real time event datas. Obviously during event processing, it’s not necessary to give extra effort to visuality. So one should not confuse it with visual simulations. We’r just making event based calculations. Therefore you can ask what exactly do we simulate? or can any calculation be a simulation? Notice that in scientific experiments, you always need a satisfactory amount of statistics. So basically you should have a scenario for gathering statistics in simulations. Here i’d like to present a calculation of “Pi” number as a simulation sample. Here we are collecting statistics via producing random numbers which is included a circle with r=1.

**Calculation:** Pi number

**The method:** Monte Carlo Simulation

**Fundamental Formulas:** (pi)r ^{2} and x^{2}+y^{2} = 1 (Note that radius of circle is unit 1.)

1-) Write below code and compile it writing “g++ pi.cpp -o pi.x”

#include<iostream> #include<math.h> #include<stdlib.h> #include<time.h> using namespace std; int main(){ int jmax=1000; // maximum value of HIT number. (Length of output file) int imax=1000; // maximum value of random numbers for producing HITs. double x,y; // Coordinates int hit; // storage variable of number of HITs srand(time(0)); for (int j=0;j<jmax;j++){ hit=0; x=0; y=0; for(int i=0;i<imax;i++){ x=double(rand())/double(RAND_MAX); y=double(rand())/double(RAND_MAX); if(y<=sqrt(1-pow(x,2))) hit+=1; } //Choosing HITs according to analytic formula of circle cout<<""<<4*double(hit)/double(imax)<<endl; } // Print out Pi number }

2-) To understand the code: We have just 2 loops here. The inner loop produce random number (<1) and uses these numbers for coordinates x,y. “If” condition increases hit number if this (x,y) point locates in the area of quarter circle (Look at the figure below.)

The outer loop resets our variables and print out Pi number according to formula: Area of Quarter Circle/ Area of Square = (1/4)πr^{2}/r^{2} = (1/4)π= accepted hits / total hits = hits / imax.

3-) Run it as “./pi.x > pi.dat”

4-) Draw output file.

I used below “root macro” to read and convert it to a .root file.

{ gROOT->Reset(); ifstream in; in.open("pi.dat"); Float_t x; Int_t nlines = 0; TFile *f = new TFile("pi1.root","RECREATE"); TH1F *h1 = new TH1F("h1","pi_grafik",100,2.5,4.0); TNtuple *ntuple = new TNtuple("ntuple","pi","x"); for (nlines=0; nlines<10000; nlines++) { in >> x; if (!in.good()) {break;} if (nlines < 5) {printf("x=%5f\n",x);} h1->Fill(x); ntuple->Fill(x); nlines++; } in.close(); f->Write(); printf("%d deger bulundu\n",nlines); h1->SetXTitle("pi"); h1->SetYTitle("Olay"); h1->Draw(); }

- Paste above root macro in a C file and name it as “pintuple.c”
- Open your root analysis program in the same directory you saved pintuple.c : “root”
- Execute the file: “root> .x pintuple.c”
- You’ll get an ntuple file called “pi1.root”
- Write “TBrowser g” in root.
- Open pi1.root file and you’ll get the above histogram. Congratulations 🙂

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