Screening, antiscreening and asymptotic freedom

A full understanding of screening, antiscreening and calculation of asymptotic freedom is possible only in the QCD field theory of strong interactions, and we limit ourselves to a qualitative discussion of the results. First we will discuss screening in QED, look e.g. at the diagram 7.6 (a) with quantum fluctuations in QED. The charges in extra e+e- pairs due to fluctuations will align themselves and be polarized so that, as calculations show, they “screen” the “original” charge. The effect is less pronounced at small distances, when the “test” charge is closer to the “original”charge (it is possible due to uncertainty relation in scattering, when there is a larger momentum transfer). The Coulomb interaction strength increases at short distances (e.g. a 2s state in hydrogen is more tightly bound (by 2.2 × 10−17eV) than in a Coulomb potential with α ≈ 1/137. Antiscreening in QCD, asymptotic freedom

In quantum fluctuations in QCD, e.g. In quark–quark scattering, there are two lowest-order vacuum polarization corrections to the one-gluon exchange diagram, they are shown in Figure 7.8. The first 7.8(a) leads to a screening correction, as in QED, and the interaction would go stronger at smaller distances. But, there is also the second diagram of Figure 7.8(b), involving a gluon–gluon pair produced in a gluon self-interaction of the type shown in Figure 7.2(a). This diagram has no counterpart in QED and it leads to an antiscreening effect: it causes the interaction to grow weaker at shorter distances. The antiscreening effect due to 7.8(b) is bigger than the screening correction from Figure 7.8(a), and the net result is that the interaction grows weaker at short distances: there is an asymptotic freedom. Exotic hadrons (again). Glueballs and hybrids

Any hadrons that lie outside the simple quark model (so not having the structure qq¯, qqq or q¯q¯q¯) we will call exotic, whether or not their quantum numbers are exotic, that is whether or not their quantum numbers are forbidden to occur in the simple quark model. Such states are allowed in QCD since they may be also colorless, but the calculations and experiments are very difficult, and often not reliable. This is also an active area of research. Since glueballs (if they exist) composed of gluons alone, they are predicted to be strongly interacting neutral bosons with I = S = C = B =0. Some lattice calculations based numerical predictions in QCD can be found in the next slide.

Exotic hadrons. Glueballs and hybrids (2)

While there are no (M&S 4th edition) firmly established experimentally pure glueballs, there is evidence (see e.g. M&S 4th) for hybrid states, which contain one or more gluons in addition to a qq¯ pair, and for mixed states, which are superpositions of qq¯ and glueball components. Exotic baryons

From time to time, experiments have claimed evidence for the existence of baryons with exotic quantum numbers, but as more experiments have been performed and more data accumulated, the evidence has receded. The present consensus (M&S 4th) is that there is no convincing evidence for such baryons. However, following the discovery of the tetraquark–charmonium states described above, it was natural to search for the analogous pentaquark–charmonium states cc¯qqq, where q = u, d, s, which would not have exotic quantum numbers. The first evidence for such states was published in 2015 by the LHCb Collaboration, these results have been updated since. Exotic baryons (2)

Reaction listed below has been studied by LHCb:

In this reaction the LHCb obtained an evidence for possible existence of the pentaquark–charmonium states (see M&S 4th, and more recent papers. The quark–gluon plasma In ordinary matter, quarks are confined within hadrons. However, as the density and/or temperature is increased, a phase transition can occur to a state in which individual hadrons lose their identities, and quarks and gluons become free. It has not yet been possible to evaluate QCD precisely, to see when this occurs, because perturbation theory breaks down in this region. The approximate considerations suggests that in the low-temperature limit, the transition should occur at densities about five times higher than that at the center of a heavy nucleus, while in the limit of low nucleon density, it should occur at temperatures of order 150–200 MeV, which translates to a conventional temperature about five orders of magnitude greater than the temperature at the center of the Sun. The new state of matter is called a quark–gluon plasma (QGP). A quark–gluon plasma is believed to have existed in the first few microseconds after the big bang, and it may exist today at the center of neutron stars. In experiments, a quark–gluon plasma may be created briefly in collisions between heavy ions, Quark-gluon plasma (2) High-energy collisions between heavy ions have been studied in several experiments, at the Relativistic Heavy Ion Collider (RHIC) at the Brookhaven National Laboratory, starting in 2000, and more recently using ALICE and other detectors (CMS and ATLAS) at the LHC.] In these experiments e.g. it is expected as one of the signes of the quark-gouon plazma that the production of J/ψ and upsilon states would be sup- pressed because the c and c¯,orb and b¯, quarks produced (also from gluon fusion) would be separated by many quarks of other flavours, leading instead to the production of mesons with non-zero charm and bottom quantum numbers (see also next slide)