EW Physics

Overview

Teaching: 60 min
Exercises: 30 min

Questions

Objectives

Open http://fireworks.cern.ch and take a look at /store/group/upgrade/visualization/dy.root.

Once the path is provided click the Load File EOS button. Once it opens you should something like the following:

To get a better sense of the event’s geometry, you can add the ECAL barrel to the event view in the following way:

Before starting to play around, we can move to a more interesting event. You can scan through the events by clicking the event navigation buttons (right arrow), or enter the Run/Lumi/Event number directly. In this exercise, each file has only one Run, you just need to edit the lumi/event number to go to the new event.

Skip to the third event in the file, run/lumi/event is 1/416042/83208204.

Try to click the interesting objects in the 3D view window and answer the below question:

Question 1

How many different kinds of objects are there? What are they?

Try to explore this event with different views with Fireworks. To swap the "main" panel with other view, click the button with the left-pointing arrow in the panel you wish to make your main one. You can look at other available views by clicking Views in the top-left and then the selecting the view you wish to add from the drop-down menu.

Question 2

Using the Table View on the bottom right, from the drop down menu below Choose Collection: choose Muons. How many additional Muons are not being displayed? (i.e. grayed out) Why are they hidden (Hint: press the "pencil-shaped" button in the left panel "Muons" section)?

Solution

In this configuration, the muon collection is filtered with expression “pt()>5 & isLooseMuon()”, meaning that each muon is only displayed if its pT is larger than 5 !GeV, and it passes the Loose ID requirements.

Question 3

Change the filter expression to display only muons with pT lower than 5 GeV (revert the change afterwards)

Solution

You should see this:

Question 4

Use the Table View in the bottom right corner to learn more details about the Muons collection. The table doesn't show dZ, to add it: in the table panel, go to the Muons collection, and click on the "Edit Collection Columns" button; then in the expression text box, write i.track()->dz(), give a title "dz", and define precision "3" and click "Add". Why does the 5.9 GeV muon stand out? Could you have seen this by zooming in on the Rho-Z View?

Solution

You should see this:

Question 5

Go to the 9th event in the file, run/lumi/event is 1/416042/83208226. What kind of event is it?

Solution

Z->ee event

Question 6

Uncheck the "Electrons" box to prevent them from being displayed: are there track pointing to both ECAL deposits?

Solution

Yes.

Question 7

Go to the next event, the 10th event in the file, run/lumi/event is 1/416042/83208227. What kind of event is it? What is the MET in this event? What is the transverse mass (mT) of the muon+MET+jet? What about for the muon+MET+LeadingTrack? This is MC, so you can peak at the GenParticles to check your hypothesis. You will have to add pdgId to the column to get a better understanding of the event. From the Table view once the PrunedGenParticles is selected, edit the table to add "i.pdgId()" expression with "pdgId" title with precision of 0.

NOTE: The legacy fireworks had kinematic variable calculator provided with the app. The feature is being worked on and will be added in the near future.

Show Answer

MET is et = 46.6 !GeV, phi = -0.414
Muon has pt = 11.0 !GeV, eta = 1.292, phi = -0.580
Jet has pt = 39.6 !GeV, eta = 1.453, phi = 2.865
Track has pt = 32.6 !GeV,eta = 1.456, phi = 2.869

mT(met, muon, jet) = 95.2 !GeV
mT(met, muon, track) = 86.4 !GeV

The event is likely a Z->tautau, with one tau decaying leptonically (muon + MET) and another in a one-prong hadronic tau decay. The one prong tau track is clustered in a jet with pT = 39.6 !GeV. When looking at the PrunedGenParticles list, one will find that there are pdgId 15 and -15 particle (grayed out) listed confirming that the event is indeed Z->tautau. The track pT is closest to the origina pion (pdgId=211) pT of 31.8 !GeV and also the eta.

Now let us use event filter by clicking the event filter button called FilterDialog. Search for events with large MET (i.e. >60 GeV). Select only the MET filter to be Active. Then click ApplyFilters button. Once the filter runs the FilterStatus: will report the number of events passing.

Question 8

How many events are selected? Can you guess what is the cause?

Show Answer

In total, six events are selected. All are Z-->TauTau events except events 83208351, 8365, 8506. Event 83208351 is a Z(ee)+jets event, the electrons are reconstructed properly (the invariant mass matches the Z). There is a below-threshold jet (pT=24.2 GeV) in the direction opposite to the MET, which could be mis-measured or coming from pile-up.

Event 83208365 is a Z(mm)+jets event with one muon out of acceptance (pt = 45 GeV, eta = 2.4). The lost muon explains most of the MET, while the rest can be ascribed to jet mis-measurement.

Event 83208506 is a Z(mm)+jets event with both muons out of acceptance and two very forward jets. Lost muons and mis-measured jets lead to the MET.

Key Points