Z-Path 2013 – Event Displays
Examples of l+l- ,  and 4-lepton events
This compilation goes through examples of events the students will be looking at. This
includes events easy to interpret, as well as some difficult ones. The tutors are asked to
keep enough time for discussion at the end of the measurements, even if the students would
not finish their assigned sample. Quality primes against quantity and some extra work
dedicated to difficult (and very interesting) cases should be encouraged. It is also through
such cases that students will learn a lot.
Some recommendations how to tackle the detective work
Briefly:
1. Set a minimum pT cut of 5-10 GeV
2. Study remaining tracks and/or physics objects and classify if compatible with
muon, electron or photon.
3. Zoom in both views to distinguish between single and double tracks
4. Check whether the invariant mass of a double track is consistent with a
converted photon
5. If needed, require at least 2 pixel hits to make sure that single or double tracks
stem from the primary vertex. Requiring at least 7 SCT hits ensures having good,
long tracks.
In more details:
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Set a pT cut of tracks in HYPATIA control window (cuts, InDet): typically >10
GeV/c
o a lower cut of 5 GeV might be useful in order not to miss some 4-lepton
events. Advise students to use 10 GeV cut to proceed faster
If hits in the inner detector (ID) tracker and in the muon system (MS),
o enter particles as muons if they have opposite electric charges
o if no other pairs of leptons, proceed to the next event. If an additional
pair of leptons is found (muons or electrons – see electron identification
below) the event may contain 4 leptons and should be categorised as
such.
To find electrons or photons, start either from “Tracks” or “Physics
Objects” (HYPATIA – track momenta window)
o Starting from “Tracks”
 If hits in the ID and none in the MS, check that there are at least 2
important energy deposits in the electromagnetic calorimeter
(ECAL)
 If 2 oppositely (single) charged tracks are found that clearly
point to one ECAL cluster each, enter them as electrons. Ideally
these electrons will have corresponding “physics objects”. It can
however happen that the procedure to assign a physics object to an
electron fails.
o Starting from “Physics Objects”
 If at least 2 objects are found corresponding to ECAL clusters,
these correspond most probably either to electrons or photons
Check tracks – zoom to distinguish single and double (very
close) tracks – a track might seem to point to a cluster in the
side-view but not in the end-view (or vice versa), so both views
should be checked before classifying as electron/photon:
 If 2 (single) oppositely charged tracks are found pointing
to an ECAL cluster each, enter them as electrons (ideal
case as when starting from “Tracks” above)
 If no tracks are found pointing to 2 ECAL clusters, go back
to “Physics Objects” and enter the objects as photons
o Converted photons lead to close, oppositely charged tracks with
very small invariant mass (compatible with the photon mass 0)
 2 very close tracks pointing to one ECAL cluster may come from a
converted photon (… e+e-). To check this, enter the 2 tracks as
electrons and read the resulting invariant mass.
 In most cases M(ee) is very close to 0, such that you can
go back to the “Physics Object” and enter the object as
photon.
 In some cases the tracks disappear when requiring at
least 2 pixel hits, see below, such that you are left with 2
isolated photons in the event.
 It may happen that a double track, proven to stem from a
conversion, does not have a corresponding object. How to proceed?
The current HYPATIA does not mix tracks and objects to build
invariant mass. Enjoy the event and (unfortunately) ignore it …
A Cut on “Number of Pixel Hits” >=2 may help getting rid of a single track or a
double track (from a converted photon) that do not come from the primary
vertex (main interaction point)
A cut on “Number of SCT hits” >=7 may help having good, long tracks.
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1. e+e- – events
1.1. Event 1
Electrons usually have a calorimeter object and a corresponding charged track. The left
figure shows both tracks and calorimeter objects. In the left figure it is clear that both
tracks point to an energy deposit in the calorimeter.
The 2-electrons are of
opposite charge, have
more than 30 GeV
momentum, and stem
from a Z boson decay:
M(e+e- =91.7 GeV
1.2. Event 2
Another event where a Z decays to a pair of e+e-
The information about the calorimeter
objects is reproduced below. The invariant
mass is compatible with a Z boson M=88 GeV.
One of tracks seems short; in fact it is emitted
in the forward direction, as the z-view shows.
2. events
2.1. Event 1
This event has 2 calorimeter objects
clearly without tracks pointing to them
(left figure). After a pT cut of 5 GeV all
tracks disappear (right).
M()=110 GeV
2.2. Event 2
Another 2-photon event with 2 clear calo
objects: M()=108 GeV. After a pT cut of 5 GeV,
one track remains, which however does not
point to the cluster in the z-view. In fact, by
requiring 2 pixel hits, the track disappears
(right figure). The requirement of pixel hits
helps remove charged particles that weren’t
produced at the collision point
3. A “difficult” event
This event has 2 calorimeter clusters, one
without any track pointing to it and one with a
double track pointing to it (see zoom below).
Assuming these are photons: M()=100.1GeV.
Is this a 2-photon event? Study further!
Let’s look at the 4 double tracks.
The double track pointing to some calorimeter
energy (with numbers 45-46, see below) have
an invariant mass M(e+e-) =0.2GeV compatible
with a converted photon! In fact this pair
disappears when requiring at least 2 pixel hits
(below right).
What about the other pair remaining? It has no
energy deposited in the calorimeter and both
tracks have the same sign and can therefore be
ignored.
4. Is this event  or e+e-?
Left: pT>1GeV; Right pT>5 GeV. The zoom below reveals 2 pairs of e+e-. The left pair
disappears if at least 2 pixel hits are required (see pictures at bottom of page). Each pair
has an invariant mass of ~0.1 GeV, i.e. consistent with a photon conversion! We are in
presence of 2-photons, both converted. The invariant mass of the 4 electrons is ~134
GeV.
Here is the calorimeter object information.
The conclusion is that the 2 objects correspond to 2
photons, which have converted and lead to 4 tracks.
One pair of tracks is
rejected by requiring 3
pixel hits.
5. A event
Muons are the easiest particles to recognise: hits in the muon spectrometer and
in the inner detector, and very little activity in the calorimeter.
This is an example of di-muon event from
Z decay. Two tracks (89 and 117 have
opposite charge) with hits in the muon
spectrometer (both views). M(=84.3
GeV.
6. A 4-lepton event: 
This interesting event has 4 clear muons, stemming from ZZ :
M(=94.1 GeV and M()=89.3 GeV. M(=373.5 GeV. One of the muons
has hits only visible in the z-view.
7. A 4-lepton event: e+e-
Another 4-lepton event stemming from
ZZee with M(=91.1 GeV and
M(ee)=89.6 GeV. There are 2 additional
tracks, see further below.
The 2 additional tracks (4 and31) with invariant mass
Mee=1.2 GeV stem from a converted photon. In most
cases such tracks disappear when requiring 2 or 3 pixel
hits. If not, just ignore them. In fact a higher pT cut of 12
GeV (well justified) will get rid of them.