General comments
Left view: is the transverse view, the projection transverse to the colliding beams. Here only the barrel part of the ATLAS detector is visible. Therefore tracks which go in the forward direction appear as short tracks (as they are not being registered by some outer parts of the detector)
Right view: is the side view, the projection along the direction of the colliding beams. All parts of the ATLAS detector (barrel and end-caps) are visible in this view, and all tracks are registered. The line at ρ=0 cm is the place where the protons circulate; therefore there are no detectors close to this line since they would suffer from the radiation environment.
Both views can be zoomed separately using the zoom slide rules.
EXERCISE I : Looking at events
Browse through different events using the "Next Event", "Previous Event" buttons. At this stage observe the number of tracks per event and their distribution in each view.
Question: In which view do you think that the momentum is conserved?
EXERCISE II : Working with single particles - Distinguish electrons from muons
Here you will look at the different kinds of tracks. As explained, the muons should reach the muon detectors (the blue chambers) in at least one of the views and the electrons should stop in the electromagnetic calorimeters (green ring) depositing considerable energy there (yellow marks) in at least one of the views.
Make a table with at least 10 electrons and 10 muons with their event number, pT, sign (which you will find in the second column of the track table) and their angles.
Question: Investigate the effect of the cut button. What happens if you increase the p threshold? Explain why.
EXERCISE III: Discovering/Reconstructing the Z0 particle
Combine decay particles to observe and measure invisible particles
When you see two or more particle traces in the detector, "tracks" in one "event", originating from the same point, which we call "vertex", they may belong to one original particle which decayed into these tracks. In order to check this hypothesis one has to calculate the "invariant mass" of the original particle and investigate if these decay products came from the same particle. In other words to investigate if the products, the two muons or two electrons in our case, belong to the decay of the Z0 particle which has a mass of approximately 91 GeV.
For that purpose, one has to find two muons or two electrons (following the directions of part II) and use the "Insert electron" or "Insert muon" button twice to insert them in the invariant mass table which calculates automatically the invariant mass value, which appears under column mll. Ιf the invariant mass is made from a two electron combination an "e" is marked in the last column, otherwise if it is made from a two muon combination an "μ" is marked in the last column. Note that one muon and one electron combinations are not permitted because they violate lepton number conservation. You can also delete tracks with the "Delete track" button and replace them with others.
Question 1: Calculate the invariant mass of two tracks using the analytic formula for two particles. This can be done by copying the momentum information in an excel file. Check the results with the numbers calculated in the invariant mass table.
Question 2: For reconstructing the mass of the Z0 do you choose tracks with same or opposite sign?
Question 3: How to distinguish the two electron or two muon combinations from the background?
Question 4: How close to 91 GeV has the mll got to be, in order for you to be certain that you have discovered the Z0 particle?
EXERCISE IV: Histograms - Measurement of width
Make histograms of reconstructed Z0 masses separately for those decaying in pairs of electrons (by clicking the mee histogram tab) and to muons (by clicking the mμμ histogram tab) in the histogram table. Write down what the mean value and the rms value of each histogram is.
Question 1: Are the masses (mean value) mee and mμμ equal?
Question 2: Is the mass which you determined close to the nominal one? Look at the Particle Data Booklet or in tables hanging in your school or in the Web to see what the mass and width of the Z0 are.
Question 3: Are the widths of electrons and muons the same? The width you measure can be calculated from the rms of the histogram as Γ= 2.35*rms.
EXERCISE V – Discover the Higgs particle
The easiest decays of the Higgs boson to look for, are those to two Z0 particles, namely to four muons, or four electrons or two muons plus two electrons. For this purpose use the Invariant mass table but insert four tracks .You should pick and insert into this table the four tracks which you think belong to a possible Higgs boson decay. Then the four particle invariant mass will be calculated in the mllll column. If you do not "like" the invariant mass of a particular combination of four tracks you can replace from one to four tracks with another(s). The table will automatically update and give you the new invariant mass under the relevant column. If the Higgs boson exists and if you have picked up the right tracks then in the mllll histogram you will see an accumulation of events around a specific mass, which is hopefully the Higgs boson mass!!!
Question: What should be the sum of charges of the four leptons which you think belong to the Higgs boson?
EXERCISE VI - Event batch processing
Using large samples (constisting of thousands) of events, study the histograms and select the appropriate parameters which will allow you to separate the signal (Higs boson decays) from the background.