Part II Particle Physics : Question 14
Shown below are a selection of events recorded by the OPAL detector
at LEP. For each event try to answer the questions. The events are
divided into two section. The first set, LEP 1 events, were recorded in 1994
when LEP operated with a centre-of-mass equal to the Z0 mass, i.e. 91.2 GeV.
The second set, LEP 2 events, were recorded in 2000
when LEP operated at the highest ever e+e- centre-of-mass energy, 206 GeV.
Don't spend too much time on this question.
However,
if you manage to make it to the end you
should have a good feel for the physics/measurements at LEP
LEP 1 EVENTS
EVENT 1
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The basics and hints:
- The BLUE lines emanating from the centre are tracks left by charged
particles. The curvature of the track due to the magnetic field
provides a measure of the particle momentum.
- The YELLOW blocks represent energy deposits in the
electromagnetic calorimeter (ECAL). The signal in the ECAL
is proportional to the energy deposited.
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The measurments:
- Particle 1 : p = 46.0 ± 3.2 GeV, E = 44.7 ± 1.2 GeV
- Particle 2 : p = 49.5 ± 3.5 GeV, E = 46.0 ± 1.2 GeV
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The questions:
- What is the event ?
The event is e+e- -> Z0 -> e+e-
- Draw the Feynman diagram for the process.
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EVENT 2
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The basics and hints:
- The MAGENTA blocks indicate energy deposits in the hadron
calorimeter (HCAL)
- The ARROWS indicate hits in the MUON chambers.
- The small size of the YELLOW blocks indicate small energy deposits
in the ECAL.
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The measurments:
- Particle 1 : p = 44.6 ± 2.6 GeV, E = 1.2 GeV
- Particle 2 : p = 43.2 ± 2.4 GeV, E = 0.7 GeV
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The questions:
- What is the event ?
The event is e+e- -> Z0 -> mu+mu-
- Draw the Feynman diagram for the dominant process.
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EVENT 3
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The basics and hints:
- Note that in the zoomed view two of the tracks (tracks 2 and 4)
form a vertex beyond the beam pipe.
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The measurments:
- Particle 1 : p = 4.1 GeV, E = 4.0 GeV
- Track 2 : p = (-4.73, +6.09, -7.18) GeV
- Track 3 : p = (-9.11, +12.98, -15.59) GeV
- Track 4 : p = (-0.65, +0.98, -1.10) GeV
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The questions:
- What particle produces the two tracks which form the vertex displaced
from the beam pipe ?
A photon converting to e+e- in the material
surrounding the tracking chamber.
- What is the event ?
The event is e+e- -> Z0 -> tau+tau-. One tau decays to an electron and the other decays to
a hadron (in this case it is tau -> rho nutau)
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EVENT 4
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The measurments:
- TOTAL ECAL energy = 40.3 GeV
- TOTAL TRACK momentum = 42.6 GeV
- TOTAL HCAL energy = 27.8 GeV
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The questions:
- What is the event ?
The event is e+e- -> Z0 -> q qbar.
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EVENT 5
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The questions:
- What is the event ?
This 3-jet event is e+e- -> Z0 -> q qbar gluon.
- Draw the Feynman diagram.
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EVENT 6
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The questions:
- What is the event ?
This is a 4-jet event e.g. e+e- -> Z0 -> q qbar gluon gluon.
- Draw two possible Feynman diagrams for this process.
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LEP 2 EVENTS
EVENT 7
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The measurments:
- Particle 1 : E = 96.2 ± 2.8 GeV
- Particle 2 : E = 100.6 ± 3.0 GeV
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The questions:
- What is the event ?
This is e+e- -> gamma gamma.
- Draw the Feynman diagram for the process.
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EVENT 8
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The measurments:
- Particle 1 : p = 95.2 ± 13.3 GeV
- Particle 2 : p = 100.0 ± 15.9 GeV
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The questions:
- What is the event ?
The event is e+e- -> mu+mu-
- Draw 2 possible Feynman diagrams for the process.
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EVENT 9
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The measurments:
- Particle 1 : p = 68.4 ± 16.1 GeV
- Particle 2 : p = 39.7 ± 2.7 GeV
- Mass (12) : M12 = sqrt[(E1+E2)2-(p1+p2)2] = 81.3 +-9.6 GeV
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The questions:
- What is the event ?
The event is e+e- -> gamma Z0 -> mu+mu-. This process is called radiative return. The incoming
electron/positron radiates a photon such that the invariant mass of
the e+e- equals MZ. The cross section
is enhanced due to the Z0 resonance.
Note that typically the photon is radiated in the direction of the incoming
electron and positron and hence goes down the beam pipe and is not observed.
- Draw the Feynman diagrams for the process.
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EVENT 10
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The measurments:
- Particle 1 : p = 97.1 ± 5.0 GeV, E = 103.2 ± 5.0 GeV
- Particle 2 : p = 102.2 ± 7.4 GeV, E = 98.5 ± 5.4 GeV
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The questions:
- What is the event ?
The event is e+e- -> e+e-
- Why are events of this type seen much frequently
than events such as EVENT 8
In addition to the diagrams of EVENT 8 there is another possibility:
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EVENT 11
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The measurments:
- The isolated ECAL cluster (large YELLOW block) : E = 74.1 GeV
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The questions:
e+e- -> gamma Z0 -> qq gamma.
Radiative return again, but in this case the photon is observed.
The invariant mass of the Z0 may be reconstructed
from the photon energy and knowledge of the beam energy.
- Draw the Feynman diagram for the process.
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Hint:
- Express the invariant mass of the jet-jet system in terms of
the centre-of-mass energy, 206 GeV, and the energy of the
isolated ECAL cluster.
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EVENT 12
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Hint:
- The arrow in this event indicates the direction of the missing
momentum in the event, i.e. minus the summed momenta of the
electron and jets. This is a clear signature for a neutrino.
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The measurments:
- Electron energy : 45.0 ± 2.1 GeV
- Missing energy : 58.0 ± 10.5 GeV
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The questions:
- What is the event.
e+e- -> W+W- -> qq e nue.
- Draw the possible Feynman diagrams
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EVENT 13
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The questions:
- What is this event ?
e+e- -> W+W- -> e+ nue mu- nubarmu.
- What is the approximate production rate compared to that for EVENT 12 ?
1/6 (2 q-qbar flavours ud and cs multiplied by 3 colours)
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EVENT 14
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Hint:
- This is a clear 4 jet event. This event could comes from the process
e+e- -> X Y where both X and Y decay to a
quark/anti-quark pair. In this case there are 3 possible associations
of 4 jets to the two particles X and Y. To identify the process
it is useful to reconstruct the masses of particles X and Y for
each jet combination.
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The measurments:
- Jet pairing (12)(34) : MX = 71.0 ± 1.7 GeV,
MY = 70.1 ± 2.0 GeV.
- Jet pairing (13)(24) : MX = 79.1 ± 1.8 GeV,
MY = 79.8 ± 1.9 GeV.
- Jet pairing (14)(23) : MX = 102.8 ± 5.9 GeV,
MY = 101.5 ± 5.8 GeV.
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The questions:
- Suggest the possible origins of the event.
- e+e- -> W+W- -> qqqq
- e+e- -> Z0Z0 -> qqqq
- e+e- -> Z0H0 -> qqqq
- Based on MX and MY what is the most likely
explanation for the event ?
e+e- -> W+W- -> qqqq
since jet pairing (13)(24) gives the W mass for both jet pairs
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EVENT 15
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The questions:
This event is e+e- -> Z0Z0 -> e+e-e+e-.
- Draw the Feynman diagram.
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EVENT 16
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The measurments:
- Jet pairing (12)(34) : MX = 95.5 ± 2.2 GeV,
MY = 77.1 ± 2.4 GeV.
- Jet pairing (13)(24) : MX = 78.7 ± 1.8 GeV,
MY = 88.1 ± 2.3 GeV.
- Jet pairing (14)(23) : MX = 86.2 ± 2.2 GeV,
MY = 90.5 ± 2.3 GeV.
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The questions:
- Suggest the possible origins of the event.
- e+e- -> W+W- -> qqqq
- e+e- -> Z0Z0 -> qqqq
- e+e- -> Z0H0 -> qqqq
Based on MX and MY what is the most likely
explanation for the event ?
e+e- -> Z0Z0 -> qqqq
since jet pairing (14)(23) gives the Z mass for both jet pairs
(within errors)
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EVENT 17
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The questions:
- The invariant mass of the two electrons in this event is consistent
with the Z0 mass. What is the event ?
e+e- -> Z0Z0 -> qq e+e-
- Compare the production rate for this process with that of EVENT 15.
1/40 (partial width for Z->qq is 20 times greater than Z->ee,
plus two combinations qqee eeqq )
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EVENT 18
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Health warning:
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The measurments:
- Energy of isolated ECAL cluster : E = 102.0 ± 3.0 GeV
- Invariant mass of the tracks : M = 3.3 ± 0.4 GeV
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The questions:
- What are the two particles produced in this process ?
photon and two muons. The two muons are from the decay of a J/psi as
can be seen from the invariant mass.
- Draw the Feynman diagram.
Radiative return to the J/psi diagram similar to Q11 with the Z0
replaced by a photon and the qq replace with a the bound cc state J/psi
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MAT, 14-Feb-2003