Beginner's Guide

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Identifying Stable Particles

Particles You Can Find
After the reaction of electron and positron inside the CLEO detector, the newly created particles are penetrating the various detector parts. The following stable particles are visible at CLEO (including their anti-particles):

• Electrons. Mass = 0.51 MeV, charged, infinite lifetime.
• Muons. Mass = 106 MeV, charged, stable.
• charged Pions. Mass = 140 MeV, stable.
• charged Kaons. Mass = 494 MeV, stable.
• Protons. Mass = 938 MeV, charged, infinite lifetime.

As we said earlier, each type of particle leaves a trace in some or all parts of the CLEO detector. But since their masses and other properties are different, they do this not in the same way! At the design stage of any HEP detector, these differences are exploited to achieve the optimum discrimination between them.

Lets look at the CLEO detector again, to find all the various information we get, when one of the above charged particle is traveling through it. We start from the outermost ``shell'' of CLEO and work our way inwards.

The Muon detector
shows an event with a muon traveling from the interaction point to the left. How do we know that it is a muon?
In order to provide CLEO with a magnetic field there is a coil through which an electric current is flowing. This produces a large magnetic field that has to be confined to the volume of the detector. You can do this by providing a steel barrel which will ``suck in'' the magnetic field lines. This huge amount of iron can by used for finding muons!
They are the only charged particles that can penetrate a real thick iron layer. The event picture shows detectors buried inside the iron being hit by a penetrating particle. The round circles indicate where the particle hit the detector.
You can also find the track that points to these hits. This particle traveled through more than 70 cm (27'') of iron... This has to be a muon! (Again the important hits are marked red)

In the CLEO analysis environment, the penetration length is measured in the unit ``nuclear interaction length'', that is about 16.8 cm. All data is measured as the ratio of the actual length and this unit.

Beginner's Guide

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