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A candidate event for a top quark–antiquark pair recorded by the CMS detector. Such an event is expected to produce an electron (green), a muon (red) of opposite charge, two high-energy «jets» of particles (orange) and a large amount of missing energy (purple). Source: CMS/CERN.
The top, t, quark, discovered at Tevatron in 1995, is a 3rd generation up-type quark, and is the heaviest known fundamental particle. Its invariant mass, in natural units, is approximately 173 GeV. Although equivalent to only approximately 0.3 millionth billionth billionths of a gram, this mass is enormous when compared to the mass of the next heaviest SM particle, the Higgs boson, H, at approximately 125 GeV. It is even more mind-boggling when compared to the next heaviest quark, the bottom, b, quark, at around 4.2 GeV. In fact, the difference in mass between the lightest quark, up, u, and the t spans nearly five orders of magnitude! Despite the scientific community being aware of this huge disparity in the invariant masses of fundamental particles for nearly three decades, the origin of these differences remains a mystery.
|A decay diagram of a top quark pair.
The extreme mass of the top quark gives it a few interesting and unique properties. Quarks are the only matter particles that carry colour charge and thus the only matter particles that interact via the nuclear strong force. The strong force is unique among the four fundamental forces in that it grows as the distance between the interacting bodies increases. This is due to the force carrier of the interaction, the gluon, g, also possessing the colour charge, which leads to what is called self-interaction. This results in the property known as confinement - free quarks do not exist in nature, but in turn are confined into colourless bound states called hadrons. The top quark, however, is so massive that, upon its creation in the collisions at the LHC, it decays instantly via the nuclear weak-force. This decay happens on such a short timescale that the strong force does not have the time to act upon the top quark and it does not hadronise. This property means that top is the only quark that can be observed as an almost free quark through a careful analysis of its decay products.
Another reason why studying the top quark is of such interest is its coupling to the Higgs field. The coupling between the Higgs and other fundamental particles is proportional to the mass of the particles in question, thus top has the greatest Higgs coupling strength of all known fundamental particles. This means that careful study of the properties of the top quark, such as its production rate, is a gateway to further studying the properties of the Higgs.
Finally, careful and precise study of the production and decay rates of the top is also a doorway to BSM and New Physics. For example, any enhancement in the top quark production can point to the existence of undetectable and unknown heavy particles, such as the supersymmetric partners of the SM particles.
Our group is involved in multiple top physics analyses at CMS. Dr. Viesturs Veckalns is the lead scientist working on the colour flow analysis in top decays using data collected by the CMS experiment. This analysis studies top decays, where the W boson subsequently decays into two light quarks, which are connected through the colour charge. Such studies are an incredibly important tool for furthering our understanding of processes that lead to the condensation of these fundamental particles into objects that form the macroscopic world around us.