The quest for the smallest particles. 21-2-2019 The quest for the smallest particles. How research on elementary particles evolved during the last 40 years. Wim Peeters

0. Introduction. World composed of all kinds of different materials Different properties CHAOS Try to order all these materials Look at the building blocks of these materials molecules 21-2-2019 HST 2001

Look at the building blocks of these molecules atoms! 1800: Millions of different materials are made of thousands of different molecules, in different combinations. How? CHEMISTRY Look at the building blocks of these molecules atoms! Different molecules due to different atoms Mendeleev : elements ( different atoms) organized in his table. The logic of the tabel is both chemical and physical 21-2-2019 HST 2001

Conservation laws lead to the existence of neutrinos and positrons. Different atoms due to different nuclei, which are built of protons and neutrons Niels Bohr. Neutron discovered in 1932 Conservation laws lead to the existence of neutrinos and positrons. 21-2-2019 HST 2001

Anderson: 1937, discovery of new particles in the higher atmosphere Cosmic rays, high energy. More and more « strange » particles. CHAOS again! 21-2-2019 HST 2001

Dealing with this chaos. Many, many particles where discovered. Accelerators were built to organize collisions to look at the properties of the outcoming particles They were looking for some order. First a posteriori attempt: 21-2-2019 HST 2001

This scheme leads us again to building blocks on a deeper level: 3 quarks were needed at first: up, down en strange; later on they discovered more particles, with other properties, to order these too,... 21-2-2019 HST 2001

they had to add 3 more quarks :top, bottom and charm. This is an example of such a scheme These particles are built of 3 quarks. I’ve been predicted by this scheme, and found later 21-2-2019 HST 2001

How are the particles ordered now? Hadronen (q) Leptonen (no q) e, µ,  e, µ,   mesons (2q) baryons (3q) spin 3/2 of 1/2 neutrinos Each particle has an antimatter brother 21-2-2019 HST 2001

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But it is not that simple! There must be a correspondance between the properties of the particles and the properties of quarks. There are 3 families: why? Why is there almost no antimatter found? Why did mass appear at all? How do these particles interact? 21-2-2019 HST 2001

For example: 21-2-2019 HST 2001

Quantum chromo dynamics , QCD, is called the discipline that is doing research to give the answers Questions: How do these quarks glue together to form protons, neutrons and other particles? Why are there only particles with 2 or 3 quarks and not more? Are quarks built of even smaller particles? ... 21-2-2019 HST 2001

Scale. So let’s have one more look at the scales at which we are working: 21-2-2019 HST 2001