Top Physics

Overview

Teaching: 60 min
Exercises: 30 min

Questions

Objectives

The LHC has a very large ttbar production cross section, it is often called a top factory. Understanding ttbar events would be an important task for standard model measurement or searches for new physics. Now let us scan the ttbar file /store/group/upgrade/visualization/ttjets.root

Since events start to become complicated, a few hints before we start:

Hint 1: Top quarks decay into W+b, so you can look for b-tagged jets. To do this, create a table for the jet collection and add the b-tag discriminator method, i.bDiscriminator(\"pfCombinedInclusiveSecondaryVertexV2BJetTags\"). A "medium" b-tagging working point requires that the discriminator value is larger than 0.8. Alternatively, add a new Jet collection ("Add Collection") with a bDiscriminator filter applied.

NOTE: Please do not forget the escape character "\".

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Hint 2 (MC only): use the PrunedGenParticles collection to make sense of the event: filter the collection with "!isHardProcess" to get the relevant particles (rather than the default filter).

First, go to the first event, 36/24518.

Question 9

What is this event? Can you find the combination of jets from Ws and from tops?

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This is a particularly difficult ttbar event, with one W decaying to leptons (muon+neutrino), and other to hadrons.

The transverse mass (mT) of the muon+MET is 98 GeV, very high for a W, presumably due to MET resolution. To make things worse, one of the quarks from the hadronic W is very low pT and does not form a jet.

Next, let's see event 2744/1876862

Question 10

What are the objects in the event? Are the muons close to jets?

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This is a nice dilepton (e+mu) ttbar event. There are three muons, but two of them are close to jets. Looking at the truth record, we confirm that those two jets are coming from the b quarks. The truth record says that there are following two b-quarks:

pt = 87.4 !GeV, eta = -0.49, phi = 2.39,
pt = 52.6 !GeV, eta = 1.96, phi = 1.71

While the two jets are
pt = 66.5 !GeV, eta = -0.721, phi = 2.287
pt = 38.4 !GeV, eta = 2.012, phi = 1.622

The eta / phi directions show good agreement indicating that these indeed are the b-jets.

Key Points