Scanning Noisy Events

Overview

Teaching: min
Exercises: min

Questions

Objectives

In this section, we will have a look at a few events one does not want to end up with after having applied an analysis selection. These events can be of different natures:

• Cosmics: muons created in the interaction process of cosmic radiation with the earth atmosphere can reach the CMS detector and leave a signal in the muon systems and even in the calorimeters. Those events can be identified due to their peculiar signature: a series of hits in the muon chambers (mostly DTs and RPCs) forming a straight line. This line can also be aligned with an energy deposit in the calorimeters.
• Beam halo: the protons of the LHC beams can scatter off elements inside the pipe. These interactions can create muons travelling alongside the beam that can leave signals in the detector. Again, those events have quite a striking signature: a series of CSCs hits aligned in phi (i.e. aligned with the beam axis) and one or several calorimeter towers located at the same azimuthal angle.
• Reconstruction issues: unlike the two previous sources of undesired events, this is not an instrumental issue but a software matter. It can happen that the particle flow algorithm (responsible for the global reconstruction process) assigns huge momentum to particles, due to a mismatch between the tracks and the calorimeter deposits or muon chambers hits. The error can also be very large but the algorithm keeps it in the reconstruction loop somehow. Those events lead to fake very high energy particles and therefore high missing transverse energy.

Let's now have a look at some of those events:

Go to event 274316:385:698231955 of /store/group/upgrade/visualization/BadEvents_Run2016B1.root

Question 22

What is this event?

Hint

Take a look at the CSC-segments. Due to white background, the CSC view is a bit hard to catch. Change the color to see it a little better. If one hovers the mouse one can see click on them to highlight to make it more visible. Also, Ctrl- click will allow user to click multiple at a time.

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Beam halo – On the Rho-Z view, you observe a series of CSC hits forming a straight line. You can also notice that the HCAL deposit actually consists of several HCAL towers. If you now go to the 3D view, you can see that the HCAL towers lie on the same phi coordinate as the CSC hits. To get even more convinced of the phi correspondence of the calorimeter hits, you can also use the Lego view and zoom on to the area of interest.

Go to event 275074:259:417685155 of /store/group/upgrade/visualization/BadEvents_Run2016B2.root

Question 23

What is this event?

Show Answer

Beam halo – This is again a beam halo event. In this case, you actually have CSC hits on both sides of the detector (forward and backward).

Go to event 276244:428:658649950 of /store/group/upgrade/visualization/BadEvents_Run2016C1.root

Question 24

What is this event? Hint: The DT-Segments are not available in FireworksWeb for now. (Will be supported in the future.) So this one will be more easier with Legacy Fireworks. However, try adding collections (Click Add Collections -> Search "cosmic" -> Select "Muons", "muonsFromCosmics" -> AddCollection

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Cosmics – Looking at the Rho-Phi view, you see that some of the DT hits are forming a straight line. This can also be seen from the Rho-Z view. Going to the 3D view and playing a bit with the point of view, you can observe that the DT line coincides with the ECAL deposits. This is a muon hitting ECAL electronics to create a fake high energy ECAL deposit with no track: a photon (as can be seen from the photon collection).

!FireworksWeb with cosmic muon collection:

Legacy Fireworks screenshot:

Go to event 276437:1940:3426602195 of /store/group/upgrade/visualization/BadEvents_Run2016D1.root

Question 25

What is this event?

Show/Hide

Cosmics – The justification is the same as for the previous event. The straight DT line is quite obvious here. Notice that Fireworks tries to draw tracks coming from the center: it always assumes particles come from the center of the detector.

Go to event 276544:83:153985051 of /store/group/upgrade/visualization/BadEvents_Run2016D2.root

Question 26

What is this event?

Show/Hide

Beam halo – Another halo event.

Go to event 276585:73:117748568 of /store/group/upgrade/visualization/BadEvents_Run2016D3.root

Question 27

What is this event?

Show/Hide

Cosmics – Another cosmics event.

Go to event 276811:2460:3888980376 of /store/group/upgrade/visualization/BadEvents_Run2016D4.root

Question 28

What is this event?

Show/Hide

Cosmics – Another cosmics event.

Go to event 276582:638:1144780530 of /store/group/upgrade/visualization/BadEvents_Run2016D5_AOD.root

The fwc file defines all the parameter of the Fireworks display: the colors, the tables and views, the collections shown, etc. You can always change the parameters while looking at an event and then save your configuration in “File→Save Configuration”.

Question 29

Let's now have a look at this event. Can you spot any peculiar feature?
Hint : Look at the “Muons” table.

Show/Hide

What balances the large MET of 1.9 TeV is the Jets 0, which is made by a single charged hadron track. (i.e. single track line + single hcal/ecal tower).
On the other hand, you notice that a muon (Muon_0) passes right at the same place.
This muon has an insanely large track pT error (much larger than the track pT itself which is already at 2 TeV),
and is not identified by the particle flow algorithm as a muon.
Instead, its inner track is associated with the fake charged hadron we just described above.
This makes up a fake event where a charged hadron balances large MET.
Those are reconstruction issues that happen from time to time and that must be filtered out in order to keep the tail of the MET spectrum clean.

Key Points