Exotic hadron spectroscopy

Since the discovery of the X(3872), a decade ago, more than 20 new charmonium-like resonances have been registered. Most of them have features which do no match what expected from standard charmonium theory. A few resonances have been found in the beauty sector too. Some authors just claim that most of the so called XYZ states are not even resonances but kind of effects of kinematical or dynamical origin, due to the intricacies of strong interactions. According to them, data analyses are naïvely describing and fitting as resonances what are indeed the footprints of such complicated effects. On the other hand, the X(3872), for example, is an extremely narrow state, Gamma ~ 1 MeV, and it is very difficult, in our understanding, to imagine how this could be described with some sort of strong rescattering mechanism. We do not know of other clear examples of such phenomena in the field of high-energy physics and in this thesis we will give little space to this kind of interpretations, which we can barely follow. We shall assume instead that what experiments agree to be a resonance is indeed a resonance. Moreover, we find very confusing the approach of mixing the methods proper of nuclear theory to discuss what we learned with the observations of XY Z resonances especially at Tevatron and LHC. It is true that X seems to be an extreme version of deuterium as its mass happens to be fine-tuned on the value of the D0 D0*bar threshold, but one cannot separate this observation from the fact that X is observed at CMS after imposing kinematical transverse momentum cuts as large as pT ~ 15 GeV on hadrons produced. Is there any evidence of a comparable prompt production of deuterium within the same kinematical cuts, in the same experimental conditions? The ALICE experiment could provide in the near future a compelling measurement of this latter rate (and some preliminary estimates described in the text are informative of what the result will be). Some of the XYZ, those happening to be close to some threshold, are interpreted as loosely-bound molecules, regardless of the great difficulties in explaining their production mechanisms in high energy hadron collisions. Some of them are described just as bound hadron molecules, once they happen to be below a close-by open flavor meson threshold. Other ones, even if sensibly above the close-by thresholds, have been interpreted as molecules as well: in those cases subtle mistakes in the experimental analysis of the mass have been advocated. As a result the field of the theoretical description of XYZ states appears as an heterogeneous mixture of ad-hoc explanations, mainly post-dictions and contradictory statements which is rather confusing to the experimental community and probably self-limiting in the direction of making any real progress. It is our belief instead that a more simple and fundamental dynamics is at work in the hadronization of such particles. More quark body-plans occur with respect to usual mesons and baryons: compact tetraquarks. The diquark-antidiquark model in its updated version, to be described in Chapter 7, is just the most simple and economical description (in terms of new states predicted) that we could find and we think that the recent confirmation of Z(4430) + especially, and of some more related charged J^PG = 1++ states, is the smoking gun for the intrinsic validity of this idea. The charged Z(4430) was the most uncomfortable state for the molecular interpretation for at least two reasons: i) it is charged and molecular models have never provided any clear and consistent prediction about charged states; ii) it is far from open charm thresholds. However, if what observed (by Belle first and confirmed very recently by LHCb) is not an “effect” but a real resonance, we should find the way to explain and put it in connection to all other ones. The Z(4430) appears extremely natural in the diquark-antidiquark model, which in general was the only approach strongly suggesting the existence of charged states years before their actual discovery. We think otherwise that open charm/bottom meson thresholds should likely play a role in the formation of XY Z particles. We resort to the Feshbach resonance mechanism, as mediated by some classic studies in atomic physics, to get a model on the nature of this role. The core of our preliminary analysis is the postulated existence of a discrete spectrum of compact tetraquark levels in the fundamental strong interaction Hamiltonian. The occurrence of open charm/beauty meson thresholds in the vicinity of any of these levels might result in an enhanced probability of resonance formation.