Charm quark

J. P. Vigier, Charmed quark discovery in anti-neutrino nucleon scattering Lett. Nuovo Cimento 15(2) (Ser. 2), 41 48 (1976). 

It is believed that the main feature of the interaction between charmed quarks and antiquarks is given by a nonrelativistic confining potential of the form (Eichten et ai, 1978)... 

The six quarks, namely the up quark (u), the down quark (d), the strange quark (s), the charm quark (c), the top quark (t), sometimes also called truth quark, and the bottom quark (b), also dubbed beauty quark, carry a colour charge. The bosons that act on colour, are called gluons, which are the carriers of the colour interaction. The residue of this interaction is the strong nuclear interaction, which is operative between the hadrons (for instance the proton and the neutron within an atomic nucleus). 

In this theory the three original quark-antiquark pairs were supplemented by a fourtii pair - the charmed quark and its antiquark. The psi particle itself is a meson having zero charm as it consists of the charmed pair. However, charmed hadrons do exist they are said to possess naked charm. 

We here enlarge the standard model to include the weak and electromagnetic interactions of hadrons. We encounter serious techniceil problems if we try to restrict ourselves to the original three quarks, u,d,s. In particular we find unwanted neutral strangeness-changing currents in the theory. These difficulties are eliminated by the introduction of a new charm quark c. There is then a very attractive universality between the two lepton doublets (e-) ( - ) quark doublets formed from... 

One of the most interesting outcomes of gauge theory is the impossibility of making a reasonable model using just three quarks within the conventional Cabibbo current approach. Gauge theories require a larger group than SU(3) for hadrons. At the simplest level we require one new quark, the charm quark c. 

Today cheurm particles are produced in abundance and their properties are discussed in Chapters 11-13. It is by now an experimentally well established fact that the charm quark has charge 2/3. This leads to a second quark doublet... 

Because of its historical importance we shall briefly discuss one of the original motivations for introducing a charm quark. 

We see, therefore, that the theory is only compatible with experiment because of the vital role played by the charm quark. 

Given the uncertainties in the experimental calibration one sees that the measured values of R are in reasonable agreement with a scheme based on coloured quarks with the usual fractional charges, and with the existence of a heavy lepton. Note too that the data is compatible with the charge assignment Qc = for the charm quark. 

Fig. 11.1 shows the ratio R = hadrons )/ (discussed in Section 6.2.5) up to about 30 GeV (i.e. where electroweak effects can be neglected). As shown in Fig. 11.1, the new pm-ticles appear as very narrow spikes in the e+e" cross-section. Much effort on the theoretical side has been expended in understanding these particles. The dust has long settled and it seems convincingly demonstrated that the J/ is the first manifestation of particles built out of the heavy quark (mass ci 1.5 GeV/c ), i.e. the charm quark introduced in Chapter 9, and whose existence was demanded for the gauge theory of weak interactions to make sense (Glashow, Iliopoulos and Maiani, 1970). The J/ is visualized as a loosely bound state of cc. Similarly, the T(9.46) particle is interpreted as the first manifestation of a 66 bound state. 

We have repeatedly remarked that the assmnption that the charm quark has electric charge 2/3 makes it the natural candidate to be the missing... 

That the charm quark charge is 2/3 is also borne out by the size of the increase in the ratio R (see Fig. 14.1) on crossing the charm threshold (i.e. much bigger than would be the case for a Q = 1/3 quark). 

Estimates can be made, however, if we use the free quark-parton model and assume that partons convert into hadrons with unit probability. A very rough estimate for the inclusive semi-leptonic D decay can be obtained along the lines given previously [eqn (13.2.2)] if we forget all complications coming from non-spectator diagrams and assume that the light quark behaves purely like a spectator while the charm quark decay proceeds as if it were a free particle. In this case one has... 

The possibility of new quarks in this mass range is made unlikely by the overall qualitative success of the charmonium model (Chapter 11), which assumes that only the charm quarks have come into play in the strong interaction sector, and the next known quark (the b) lies too high up in mass to be of any relevance here. 

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