OPAL Events at LEP1

During the first phase of LEP (1989-1995), electron and positron beams collided with enough energy that they usually interact by annihilating into the Z boson . The Z boson usually decays into pairs of fermions. The Z boson arises in the Standard Model of particle physics as a result of the unification of the electromagnetic and weak interactions. A primary purpose of LEP is to measure precisely the production of the Z boson and its decay into different kinds of particles.

You can view displays of some typical events recorded at LEP in the OPAL detector .

The displays show decays of the Z boson into leptons and into quarks.

Tracks measured in the central tracking system are shown in blue. Small green boxes show hits in the time-of-flight system. Clusters of energy in the lead glass electromagnetic calorimeter are shown as yellow boxes, of size proportional to their energy. Similarly, clusters of energy in the hadron calorimeter are drawn in magenta, and energy in the forward luminosity calorimeters in green. Penetrating charged particle tracks, which are candidates for muons, are shown as red arrows.

You can get the pictures in either of two formats, gif (which is smaller and thus faster to display on your screen) or PostScript (which contains higher precision and is thus better for printing).

Decays of the Z boson into muons

In the first event, the decay of a Z boson into a pair of muons is seen. The muons are identified by their penetration right through the detector. The process may be shown diagramatically by a "Feynman diagram".

click on display to get full size picture ( ps file)

Feynman diagram

A similar event is shown here but in this case a photon has been emitted by one of the muons, shown as a cluster in the electromagnetic calorimeter with no associated track.

click on display to get full size picture ( ps file)

Feynman diagram

Occasionally an emitted photon may produce an additional pair of muons, as in this event

click on display to get full size picture ( ps file)

Feynman diagram

Cosmic rays

Cosmic rays (generally muons, because the detector lies 100m underground) sometimes traverse the detector, and may fake muon pairs. This event can be rejected because the tracks do not come from the interaction point in the centre of the detector. Timing information from the time-of-flight system is also useful in rejecting cosmics.

click on display to get full size picture ( ps file)

Bhabha events

In this event, the decay of a Z boson into an electron and positron is seen. The electrons are identified because they deposit all their energy in the electromagnetic calorimeter.

click on display to get full size picture ( ps file)

Feynman diagram

The production of electron-positron pairs can also occur by the exchange of a photon (or Z), and this mechanism tends to dominate when the particles are emitted at small angles to the beams. In this event, a photon is also observed, in the forward calorimeter. A possible Feynman diagram is shown, though the photon could be emitted by any of the electrons or positrons, and the process could also proceed via a Z boson .

click on display to get full size picture ( ps file)

Feynman diagram

Photon pair production

The formation of a pair of photons, as in this event, involves only the electromagnetic interaction, with no contribution from the Z boson. Photons are identified by energy deposits in the electromagnetic calorimeter with no track pointing at them, since only charged particles produce tracks in the central detector.

click on display to get full size picture ( ps file)

Feynman diagram

Decays of the Z boson into tau leptons

The Z boson may also decay into a pair of tau leptons. The tau lepton is unstable, with a lifetime of about 0.3 picoseconds , so we detect only its decay products, (and only some of those, because undetected neutrinos are also formed in the tau decays). In this event, one of the tau leptons has decayed to a penetrating muon, and the other to an electron.

click on display to get full size picture ( ps file)

Feynman diagram

One of the tau leptons has decayed to three charged pions, and the other to one charged pion.

Zoom: the tau->3 pions decay point is displaced from the collision point (the red marker in the centre). From events like this, the lifetime of the tau may be measured.

In the final event, both of the taus have decayed to three charged pions, and a photon has also been emitted.

click on display to get full size picture ( ps file)

Decays of the Z boson into neutrinos

The Z boson may also decay to a pair of neutrinos. These weakly interacting particles are not detected. However, occasionally a photon may be radiated by the initial electron or positron, which we can detect. The signature of such an event is a single photon in the electromagnetic calorimeter, with no associated track. From events like these, the number of neutrino species may be determined.

click on display to get full size picture ( ps file)

Feynman diagram

Decays of the Z boson into quarks

The Z boson may also decay to a pair of quarks. As the quarks move apart, the energy in the field between them caused by their "colour" charge builds up and further quarks and antiquarks are formed. Finally, the quarks are seen in the detector as two collimated back-to-back "jets" of hadrons (bound states of quarks and antiquarks), as in this event:

click on display to get full size picture ( ps file )

Feynman diagram

Sometimes, an energetic gluon (a quantum of the colour field) may be emitted by one of the quarks. In an event like this, a third jet may be seen. The three-jet structure of the event is also nicely shown by a "lego plot". The study of events like these allow us to test the theory of the strong interactions, Quantum ChromoDynamics (QCD).

click on display to get full size picture ( ps file )

Feynman diagram

Decays of the Z into b quarks

The production of b-quarks is of especial interest. Hadrons containing b-quarks have lifetimes of around 1 picosecond, which means they travel a significant distance (typically 2 mm) before decay. This can be used to tag the production of b-quarks. The next event involves b-quark production. The interaction point at which the b-quarks were formed is shown by the red marker in the centre.

A decay of a K0 meson is shown by the magenta tracks:

If we zoom in, we see clearly displaced vertices in both jets.

click on display to get full size picture ( ps file)

The next event is a candidate for the production of a Bs meson (bound state of b and s quarks) decaying into J/psi + phi. The two yellow tracks are probably K+ and K- produced from the decay of a phi meson.

Large view: The magenta tracks are muons from the decay of a J/psi:

If we zoom in, we see that these tracks form a displaced vertex.

click on display to get full size picture ( ps file)

In the next picture, we have a three-jet event. The most energetic jet (the one going to the bottom of the picture) is likely to be the quark which didn't radiate a gluon. The jet moving to the top right may be identified as a b-quark jet - in this case because an energetic muon was produced in the decay of the b-hadron (in other events vertex tagging may be used to tag b-quark jets). Thus the third jet is identified as the gluon jet, and permits the comparison of the properties of quark and gluon jets, which is an important test of QCD.

click on display to get full size picture ( ps file)

Isolated photons in hadronic events

In some events, the final quarks (or the initial electrons) may emit an energetic photon. This is analogous to the emission of gluons, but involves the electromagnetic instead of the strong force.

An example is shown here, where an isolated photon is seen towards the lower right:

A more extreme case, where the photon carries about half the collision energy is shown in this second event:

In our third example the photon is detected in the forward calorimeter:

click on display to get full size picture ( ps file)

The Feynman diagrams look like this

for final state radiation

for initial state radiation.

The OPAL Webweavers 10 May 2001 (original version 1994 by David Ward, update 2001 by Stefan Söldner-Rembold).