Top quarks are not just in a double pack

For the first time, researchers have identified individual top quarks formed via weak interactions

At the Tevatron accelerator CDF-II experiment in Chicago, a new top quark production process was discovered © Karlsruhe Institute of Technology
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The top quark is the heaviest of the fundamental building blocks of matter, but extremely difficult to detect because of its short lifespan. This has only been achieved in pairs so far. At the same time, two international teams of scientists have succeeded in detecting individual top quarks that have emerged from the weak interaction.

Quarks are building blocks of matter, from which, among other things, the particles in the atomic nucleus - protons and neutrons - are composed. As the last of the six known quarks, the heaviest of these particles, the top quark, was discovered 14 years ago at the particle accelerator Tevatron of Fermilab near Chicago. In the proven process, top quarks are always generated in pairs of quarks and antiquarks via the so-called strong interaction, the interaction of the core building blocks.

However, the standard model of elementary particle physicists said that single top quarks can also be generated, but then via the weak interaction. This is another fundamental force between elementary particles that works only at very short distances. At the same time, two international research groups at Fermilab have succeeded in experimentally demonstrating this generation mechanism of top quarks.

Breakthrough in elementary particle physics

A team from the Institute for Experimental Nuclear Physics of the Karlsruhe Institute of Technology (KIT) was instrumental in this. The head of this research group, Professor Thomas Müller, was already involved in the first discovery of the top quark. "Since 1995, we have been waiting for proof of the emergence of individual top quarks", says Müller. "This is one of the rare breakthroughs in experimental elementary particle physics."

Many physicists had hoped for deviations from the theoretical predictions. Müller continues: "We know that the so-called standard model has to be incomplete. In extreme conditions, as they ruled shortly after the Big Bang, it is no longer valid. The standard model is only the limiting case of a more general theory. "Display

Standard model describes structure of matter

When physicists speak of the "standard model, " they mean their picture of the world of the smallest particles and the fundamental forces. It describes, among other things, the structure of matter, which is composed of six different quarks and leptons, and the forces between them. While the lightest of these particles build our matter today, the remainder are already decaying in the first billionth of a second after the Big Bang.

Their principle existence - and thus a better understanding of the processes of the earliest universe - can only be proven by the most powerful particle accelerators. These generate pure energy according to Einstein's principle E = mc2, for example, quarks and their antiparticles, ie antiquarks. Their decomposition products are then detected in huge electronic detectors.

Search for the Higgs boson

It is particularly interesting that the reaction that has now been discovered represents an important background for an even rarer particle, namely the Higgs boson, after also being feverishly searched for Fermilab. This particle, whose discovery is still pending, is considered the originator of the mass of all matter. However, whether the performance of the US accelerator will reach this fundamental discovery is highly questionable.

These measurements and, of course, the search for the Higgs boson and possible signals of the mysterious dark matter will continue in the future on the more efficient Large Hadron Collider LHC at CERN in Geneva unexpected compulsory break in autumn of this year will resume its operation.

(idw - Karlsruhe Institute of Technology, 18.03.2009 - DLO)