Quark theory

When Gell-Mann and Zweig proposed their theories, quarks had never been observed in nature, so many physicists were skeptical of their existence. Their skepticism deepened when extensive experimental searches failed to turn up any evidence of free quarks in nature. The quark theory seemed to work, but the doubters viewed... 

Quarks carry a fractional charge of Vj or Fy Six flavors or types of quarks make up all subatomic particles. Each flavor of quark can be fiufher classified as having one of three colors. These are not colors or flavors as commonly thought of, but part of a classification scheme used to explain how matter behaves. The language of quarks makes them seem like some creation of fantasy, but the quark theory can be used to explain many properties of subatomic particles. For example, a proton can be considered to be made of two up quarks and a down quark, and a neutron of two down quarks and an up quark (Figure 4.8). Quark flavors and charges are given in Table 4.5. 

A brief review of the complexities to which the quark theory is addressed is in order. Particles which can interact via the strong nuclear force arc called hadrons. Hadrons can be divided into two main classes—the mesous (with baryon number zero) and the baryons (with nonzero baryon number). Within each of the classes there are small subclasses. The subclass of baryons which has been known ihe longest consists of those particles with spin j and even parity. The members of this class are the proton, the neutron, the A0 hyperon, the three hyperons and the two 3 hyperons. There are no baryons with spin 4 and even parity (or, to the usual notation, Jp = i+). The next family of baryons has ten members, each with Jp = l+. The mesons can be grouped into similar families. One of the first successes of the quark model was to explain just why there should be eight baryons with Jp = 1, ten with 1, etc., and why the various members of these families have the particular quantum numbers observed. 

In order to reach the goal of a comprehensive yet simple theory of the composition of all matter, the properties of the neutrinos and the quark theory must be considered. 

Though many attempts have been made to unify all particles into one simple theory, this has not succeeded until recently when the quark theory was developed. To explain this we have to go back somewhat in time. 

This represents only a fragment of the storylike structure of science (cf. p. 67), one of its most intriguing features. It makes science work otherwise, when considering the genetics of peas in biology, we would have to struggle with the quark theory of matter. 

Despite the successes, even with its generalizations, difficulties in thermal field theory remain to be overcome in order to deal with experimental and theoretical demands. In fact, numerous studies, in particular using quantum chromodynamics (A. Smilga, 2001), have been carried out in an attempt to understand, for instance, the quark-gluon plasma at finite temperature and in this common effort, some underlying aspects have been identified. For example, the coupling constants for 7r,a,w and p mesons decrease to zero at a certain critical temperature, which are, respectively, given by = 360 MeV, Tj = 95... 

At very high densities the equilibrium lowest energy state corresponds to quarks. However because of large uncertainties in the theory the condition... 

The formation of quark (strange) stars does not follow unambiguously from the theory, which may be compatible either with the existence or nonexistence of these objects. 

At zero temperature and quark chemical potential the simplest effective Lagrangian describing a relevant part of the nonperturbative physics of the Yang-Mills (YM) theory is the glueball Lagrangian whose potential is [1-4] ... 

A new class of effective Lagrangians have been constructed to show how the information about the center group symmetry is efficiently transferred to the actual physical states of the theory [12-15] and will be reviewed in detail elsewhere. Via these Lagrangians we were also able to have a deeper understanding of the relation between chiral restoration and deconfinement [15] for quarks in the fundamental and in the adjoint representation of the gauge group. 

A color superconducting phase is a reasonable candidate for the state of strongly interacting matter for very large quark chemical potential [16-20], Many properties of such a state have been investigated for two and three flavor QCD. In some cases these results rely heavily on perturbation theory, which is applicable for very large chemical potentials. Some initial applications to supemovae explosions and gamma ray bursts can be found in [21] and [22] respectively, see also [27], The interested reader can find a discussion of the effects of color superconductivity on the mass-radius relationship of compact stars in [45]... 

It is also interesting to note that the explicit dependence on the quark chemical potential is communicated to the Goldstone excitations via the coefficients of the effective Lagrangian (see [31] for a review). For example is proportional to fj, in the high chemical potential limit and the low energy effective theory is a good expansion in the number of derivatives which allows to consistently incorporate in the theory the Wess-Zumino-Witten term [32] and its corrections. 

At low temperature or energy, most degrees of freedom of quark matter are irrelevant due to Pauli blocking. Only quasi-quarks near the Fermi surface are excited. Therefore, relevant modes for quark matter are quasi-quarks near the Fermi surface and the physical properties of quark matter like the symmetry of the ground state are determined by those modes. High density effective theory (HDET) [7, 8] of QCD is an effective theory for such modes to describe the low-energy dynamics of quark matter. 

At low energy, the typical momentum transfer by quarks near the Fermi surface is much smaller than the Fermi momentum. Therefore, similarly to the heavy quark effective theory, we may decompose the momentum of quarks near the Fermi surface as... 

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