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XXXIXth RENCONTRES DE MORIOND
QCD AND HIGH ENERGY HADRONIC INTERACTIONS
http://moriond.in2p3.fr/QCD/2004/
Place :  La Thuile, Aosta valley, Italy    Date : March 28 - April 4, 2004

Overview of the main results presented at the conference

Studying violent collisions ("hard processes") allowed the development of a theory of strong interactions in the 80s. During such a collisions, one may consider that the quarks and the gluons, which constitute the ordinary hadrons (those particles sensitive to nuclear interactions, like the proton, the neutron or the pi meson) are independant of each other : a hard collision between a projectile hadron and a target hadron is interpreted as the one of a quark or a gluon in the projectile with a quark or a gluon in the target. The fact that these quarks and gluons are, in the initial state, confined into hadrons is of negligible importance.
During the 90s, techniques have been set up to increase the precision of the predictions to the per cent level. In parallel, we have learnt to better define the measured quantities in order to have meaningful comparison with precise predictions, avoiding as much as possible that instrumental effects introduce biases and inconsistencies in the comparison with predictions. Many very precise tests of the theory resulted from this effort in the past few years. A few talks at the conference were devoted to this topic.

However, most of the talks were devoted to various aspects of the confinement of quarks and gluons. Despite the fact that the theory of nuclear interactions is not predictive in that field, we need to understand how particles which build up ordinary matter, such as protons and neutrons, are made of elementary particles.
This topic is hot because of the recent discovery of bound states of quarks and gluons with unusual properties, which might be interpreted as a construction with 5 quarks instead of 3 for the usual baryons (proton, neutron). At the conference, the discovery of such a state including a heavy quark (c quark) was announced by the H1 collaboration in Hamburg. This is particularly interesting because there are some hopes to extract significant (but rough) predictions from the theory in the case of sufficiently heavy quarks.

A second way to study confinement is the measurement of the production and decay properties of B mesons (i.e. a bound state of a -heavy- b quark and an ordinary light u, d or s quark. One sits at the boundary of the domain where the theory is predictive. This provides excellent conditions to develop model trying to extend the predictive domain of the theory. The experiments at the "B factories", such as CLEO, Babar and Belle study with an impressive precision even the rarest decay modes of B mesons (with probabilities down to one part in a million). The harvest of results was excellent this year, in particular because exotic states were searched for and most probably identified (see above).

A third way to study confinement uses the collision of heavy ions at high energy, mainly at the RHIC collider in Brookhaven National Lab. in the US, operational since 2001. During such collisions, the nuclear matter, very strongly compressed, gets heated up to the point where quarks and gluons are "deconfined" during a very short time. The colliding nuclei somehow melt up and a new state of matter, the quark and gluon plasma, appears during this very short time. The exercise consists in tracking in the very complex fial state of the collisions, faint evidences that deconfinement indeed occurred. Now, theory has to deal with complex initial and final states, instead of the collision between two quarks or gluons. One has to deal with this complexity, bringing into (adapting to)our theoretical frame considerations from thermodynamics and even hydrodynamics. It is also not simple to identify the relevant variables to be measured in order to prove the existence of the plasma. This topic raised passionate and very interesting discussions.

The final overview was given by Dr. Boaz Klima from Fermi National Lab. (US) for the experimental part, and by Dr. Andrzej Czarnecki from the University of Alberta (Canada) for the theoretical part.