Also in this task, the result is the histograms created by the code.
I list the following masses, which will be useful in the presentation. The numbers are gotten from PDG from the following link: http://pdg.lbl.gov/2010/listings/contents_listings.html. All the numbers are given in units GeV/c².
Mass of π+ and π- : 0.139
Mass of K0: 0.498
Mass of proton: 0.938
Mass of Λ: 1.116
The unit on the x-axis on all the plots is GeV/c².
The first plot is the mass distribution of the tracks in all events. These are extracted with the function GetMass() function. Based on the masses of the particles, I assume that the particles are the following, from lowest to highest mass: foton(at mass zero), muon, pi plus and minus, kaon and proton.
This plot shows the invariant mass of the proton + π- system. As expected, we see a peak at the mass of the Λ. Also, note that no calculations yield an invariant mass under approximately 1.1GeV/c². This is because the minimum invariant mass of the system is the sum of the rest masses. The invariant mass then decreases exponentially towards zero. The convergence against zero is something we would expect.
This plot shows the invariant mass of the π+ + π- systems. This plot exhibits the same features as the previous plot, only this time we see a peak at the mass of the K0, just as expected.
This is the estimated background for the proton + π- systems. Also this has zero contributions from masses under approximately 1.1 GeV/c². We see that it does not show any particular regularity, which is to be expected since the tracks are not correlated. It peaks around 2GeV/c², not the mass of the Λ.
This plot of the estimated background of the invariant mass of the π+ + π- systems has a similar shape as the previous plot. I do not know what to conclude from this. Also in this plot we see that no contributions has a lower invariant mass than the rest mass of the two pions.
In this plot, the estimated background has been subtracted from the proton + π- systems. We see that some of the contributions has a mass lower than zero. This is only because of the comparison of the histograms, which is not perfect, but is based on normalization. We can see that this plot goes to zero faster than the plot with the background. However, this plot has a higher mean and RMS value.
This last plot is the invariant mass of the π+ + π- with the background subtracted. This plot shows the same features of the plot with background. However, also in this case the mean and RMS values are higher than the one with background.
Overall, we see that all the invariant mass plots peak at the value we expected them to peak at. However, the fact that both the mean values with background subtracted are further away from the value from PDG might makes one question the implementation of the event mixing technique.