Example: Find rare top processes

Follow the steps of a real ATLAS analysis using Histogram Analyser

$t\bar t Z \rightarrow \rightarrow bW^+ \bar b W^- \ell^+ \ell^- \rightarrow bq\bar q' \bar b q\bar q' \ell^+ \ell^-$

($\ell$ = lepton = electron or muon)

We are looking for $t \bar t Z$ decays into two leptons, two b-quarks and two $W$ bosons which subsequently decay into quarks.

The major background contributions to the search in this decay mode are top quark pair, and $Z$+jets events.

Use the cursor to implement the following cuts, one by one.
As you apply the cuts, you should see the distributions changing.

Try to understand why each of the distributions change. Think of the physics motivating the cuts.

**To clear your selection on a specific histogram click on the white background within the histogram area. **

To clear all your selections, reload the webpage where you're using the Histogram Analyser

Now let's try to separate the signal from the background using Histogram Analyser

$t \bar t Z$ with 6 jets

We want to select events which contain two leptons, low missing transverse momentum and 6 jets.

Select:

• Number of Jets at least 6 : We have decided this analysis is specifically with at least 6 jets.

• Channel $ee$ and $\mu\mu$ : Throwing away $e\mu$ events throws away a lot of $t \bar t$ but not much $t \bar t Z$.

• Reconstructed Dilepton Mass between 80 and 100 GeV : $t \bar t$ events are a major background in this analysis. The $Z$ boson has a mass of 91 GeV, which is reconstructed from the mass of the two leptons. Requiring Reconstructed Dilepton Mass to be around the $Z$ boson mass removes a large fraction of $t \bar t$ events.

• Number of b-tagged Jets at least 2 : $t \bar t Z$ decays contain similar amounts of events with 1 and 2 b-tagged jets, whereas $Z$ boson decays mostly contain 0 b-tagged jets. Therefore, requiring at least 2 b-tagged jets is a worthwile cut.

The dominant background after all these cuts in the
$t \bar t Z$ with 6 jets channel comes from $t \bar t$ and $Z$+jets.

12 $t \bar t Z$ events are identified, yielding a significance of 0.885

$t \bar t Z$ with 5 jets

Select:

• Number of Jets = 5
• at least 2 b-tagged jets
• $ee$ and $\mu\mu$ channels only
• Reconstructed Dilepton Mass between 80 and 100 GeV

The dominant background after all these cuts in the
$t \bar t Z$ + 5 jets channel comes from $Z$+jets and top pair
production.

7 $t \bar t Z$ events are identified, yielding a significance of 0.329.

Have a go yourself!

Perhaps try the $t \bar t Z$ with 6 jets, but separate it into leptonic channels and tune the cuts to maximise the number of $t \bar t Z$ events with maximum significance.

Or try something completely different...