How many colors does a quark come in?

The exchange particle of the electroweak force is the photon. It couples to particles with an electromagnetic charge. This charge can either be positive or negative. The exchange particle of the strong force, the gluon, couples just like that to particles that carry a so called color charge.

Today we want to experimentally determine the number of possible color charges (or simply colors) with data of the Belle II experiment.

Eine Wissenschaftlerin und ein Wissenschaftler arbeiten hinter einer Glasfassade und mischen Chemikalien mit Großgeräten.
© By Quark67 - Own work, CC BY-SA 3.0,

What is the R-value?

To that end we should first approach the subject from the theoretical side. To start with we
should take a look at the so called R-value.

The so called R-value is directly linked to the number of existing quark-colors – so the number of different charges of the strong force put together.

You can learn how exactly that connection is build and what the R-value is in the following worksheet.

Measurement with data from the Belle II experiment

After that we’re ready to look at the experimental measurement, to determine the number of color charges.

To experimentally determine the R-value – and from that the number of quark-colors – we want to assign the events from the Belle II detector to the different physical processes. First look at the example events before testing your newly gained knowledge in the practice task. If that works out well, you should be ready for the main task where you’ll measure the actual R-value.

Event in the Belle II detector
© Belle II / KEK

Example events

How do the different particle-antiparticle pairs look in the detector?

Practice task

Now you can look at pictures of particle collisions in the SuperKEKb and try to find out yourself, which particle-antiparticle pair was created by the annihilation of one electron and one positron.

Main task

Now you can experimentally determine the number of colors a quark comes in!

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