Mass invarint of ttbar pair by Pythia

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In summary, the conversation discusses a program created to calculate and plot the invariant mass distribution of ttbar pairs in pp collisions using Pythia. The steps taken in the program are described, including booking a histogram and defining t and tbar particles. The issue of the loop not showing anything in the histogram is raised and suggestions are given, such as adding debug output and accounting for the decay of first generation particles. The topic of the invariant mass of the ttbar pair is discussed, with the suggestion to include the energy and momentum of the two W bosons. The conversation ends with the suggestion to use another tool until being more proficient in C++.
  • #1
Hi all,

I'm not so expert in C++, but I try to make a a simple program to calculate
and plot the ttbar pair mass invariant distributtion by pythia in pp collition
(pp -> ttbar), taking the events from .lhe file in pythia exampls.

Now about the program attached here , i made the steps as following but
nothing shown in the Histogram .. may be there is some thing wrong in the loop ..
the steps :

- booked the Histogram
- start the event loop
- defined t , tbar
-start the particle loop
- make a contour to find the pairs
- calculate the invM.

So any help about that ? to know how to make such program
if anyone knows about pythia ..

Thank you,


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  • #2
for (int iEvent = 0; iEvent<100; ++iEvent)
I would expect {}-brackets for this loop.
If that is not the issue, add debug output in the loop (This should always be the first step if you cannot spot the issue). Do you get something meaningful from pythia? Do you get something meaningful for the histogram entry?

The indentation is really messy.

if (N + n > 1) {
This does not check for pairs, two b without a bbar pass this test as well (okay, that is rare).

What happened to the W boson from the top decay?
  • #3

The ask about W boson, i think you mean we should to take into account the decay of the first generation
particles, so i defined before the particle loop a variable "iTop " and inside the loop
" if ( == 6) iTop = i;" , i think that should work,

If : if (N+n >1) { --} dosn't account for the pair , what could be the probable loop ? or how to calculate the mass of these pairs ?

  • #4
Your topic is "invariant mass of ttbar pair", but you calculate a mass based on b-quarks. As far as I can see, you are missing the energy and momentum of the two W.

"if (N+n >1)" -> "if (N >0 && n>0)"
Just check if both a b and a bbar are present in the event.
  • #5

Any way thank you for your comments, i may use another tool to do this calculation until i know better in c++.S.

1. What is the "Mass invarint of ttbar pair" in Pythia?

The "Mass invarint of ttbar pair" refers to the invariant mass of a top-antitop quark pair in Pythia, which is a software package commonly used for simulating high energy particle collisions. This quantity is important for understanding the properties and interactions of top quarks, which are fundamental particles in the Standard Model of particle physics.

2. How is the "Mass invarint of ttbar pair" calculated in Pythia?

The "Mass invarint of ttbar pair" is calculated in Pythia using the four-momenta of the top and antitop quark obtained from the simulation. The invariant mass is then calculated by taking the square root of the sum of the squares of the individual quark masses and their momenta.

3. Why is the "Mass invarint of ttbar pair" important in Pythia?

The "Mass invarint of ttbar pair" is important in Pythia because it provides information about the production and decay of top quarks in high energy collisions. It can also be used to validate the accuracy of the simulation and to compare with experimental data.

4. How does Pythia ensure the "Mass invarint of ttbar pair" is conserved?

In Pythia, the "Mass invarint of ttbar pair" is conserved through the use of conservation laws, such as energy-momentum conservation and charge conservation, which are implemented in the simulation. This ensures that the simulated events follow the laws of physics and produce physically meaningful results.

5. What factors can affect the "Mass invarint of ttbar pair" in Pythia?

The "Mass invarint of ttbar pair" in Pythia can be affected by various factors such as the energy of the collision, the initial state of the particles, and the parameters used in the simulation. Other factors, such as the presence of additional particles in the collision, can also have an impact on the invariant mass of the top-antitop quark pair.

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