A. D. Polosa

Four-quark hadrons: An updated review

The past decade witnessed a remarkable proliferation of exotic charmonium-like resonances discovered at accelerators. In particular, the recently observed charged states are clearly not interpretable as mesons. Notwithstanding the considerable advances on the experimental side, conflicting theoretical descriptions do not seem to provide a definitive picture about the nature of the so-called XYZ particles. We present here a comprehensive review about this intriguing topic, discussing both those experimental and theoretical aspects which we consider relevant to make further progress in the field. At this state of progress, XYZ phenomenology speaks in favor of the existence of compact four-quark particles (tetraquarks) and we believe that realizing this instructs us in the quest for a firm theoretical framework.

The 2-+ assignment for the X(3872)

Very recently the BABAR collaboration has put forward a claim that the X(3872) is not a 1{sup ++} resonance, as most of the phenomenological work on the subject was relying on, but rather a 2{sup -+} one. We examine the consequences of this quantum number assignment for the solution of the X(3872) puzzle. The molecular interpretation appears less likely, and the conventional charmonium interpretation should be reconsidered. There are several well-known difficulties with this interpretation, to which we add a new one: the production cross section at CDF is predicted to be much smaller than that observed. We also confirm, using a relativistic string model, the conclusion from potential models that the mass of the state is not consistent with expectations. In the tetraquark interpretation the 2{sup -+} assignment implies a rich spectrum of partner states, although the X(3872) may be among the few which are narrow enough to be observable.

A

New BES and Belle data show a peak in the Y(4260) decay into J/psi plus one charged pion. We point out that the peak might correspond to a charged resonance at about 3880 MeV predicted time ago within a tetraquark model. The same tetraquark model predicts another peak at about 100 MeV below the observed one. We discuss the possibility of having it in present data. On the other hand we expect that if the molecular picture were the correct one, a peak corresponding to a D* D*bar state should appear at about 4020 MeV.

J^{PG}=1^{++}

The problem of understanding loosely bound hadron molecules prompt production at hadron colliders is still open: how is it possible that meson molecules with binding energy compatible with zero could be formed within the bulk of the hadrons ejected in very high energy collisions? Monte Carlo simulations have been performed in the literature, leading to production cross sections, two orders of magnitude which were smaller than the experimental value. One possible mechanism to reduce this gap could be final state interactions of heavy mesons, but a precise evaluation of such effect is challenged by the presence of pions between the molecular constituents. In this paper, we present a new mechanism by using precisely such comoving pions. Heavy meson pairs can indeed slow down because of elastic scattering with surrounding pions. The number of low-relative-momentum meson pairs increases, thereby enhancing prompt production cross section. In this preliminar simulation, we show that an enhancement of 100 is indeed possible.

Charged Resonance in the

Abstract We examine the Ds→f0(980)π amplitude through a constituent quark–meson model, incorporating heavy quark and chiral symmetries, finding a good agreement with the recent E791 data analysis of Ds→3π via f0(980). The f0(980) resonance is considered at the moment of production as an s s state, later evolving to a superposition of mainly s s and K K . The analysis is also extended to the more frequent process Ds→φπ.We examine the D_s -> f_0(980) pi amplitude through a constituent quark-meson model, incorporating heavy quark and chiral symmetries, finding a good agreement with the recent E791 data analysis of D_s -> 3pi via f_0(980). The f_0(980) resonance is considered at the moment of production as an s sbar state, later evolving to a superposition of mainly s sbar and K Kbar. The analysis is also extended to the more frequent process D_s -> phi pi.

Y(4260) \to \pi^+ \pi^- J/\psi

Quark pair annihilation into gluons is suppressed at large momenta due to the asymptotic freedom. As a consequence, mass eigenvalues of heavy states should be almost diagonal with respect to up and down quark masses, thereby breaking isospin. We suggest the particle observed by the SELEX Collaboration, D{sub sJ}(2632), to be to a good extent a [cd][ds] state, which would explain why its D{sup 0}K{sup +} mode is anomalously suppressed with respect to D{sub s}{sup +}{eta}. Predictions for the rates of the yet unobserved modes D{sub s}{sup +}{pi}{sup 0} and D{sup +}K{sup 0} are given.

Decay?

New BES and Belle data show a peak in the Y(4260) decay into J/psi plus one charged pion. We point out that the peak might correspond to a charged resonance at about 3880 MeV predicted time ago within a tetraquark model. The same tetraquark model predicts another peak at about 100 MeV below the observed one. We discuss the possibility of having it in present data. On the other hand we expect that if the molecular picture were the correct one, a peak corresponding to a D* D*bar state should appear at about 4020 MeV.

A Mechanism for Hadron Molecule Production in pp p Collisions

Abstract In a recent Letter we have proposed a method to estimate the prompt production cross section of X ( 3872 ) at the Tevatron assuming that this particle is a loosely bound molecule of a D and a D ¯ ∗ meson. Under this hypothesis we find that it is impossible to explain the high prompt production cross section found by CDF at σ ( X ( 3872 ) ) ∼ 30 – 70 nb as our theoretical prediction is about 300 times smaller than the measured one. Following our work, Artoisenet and Braaten, have suggested that final state interactions in the D 0 D ¯ ∗ 0 system might be so strong to push the result we obtained for the cross section up to the experimental value. Relying on their conclusions we show that the production of another very narrow loosely bound molecule, the X s = D s D ¯ s ∗ , could be similarly enhanced. X s should then be detectable at CDF with a mass of 4080 MeV and a prompt production cross section of σ ( X s ) ∼ 1 – 3 nb .

The and nature of f0(980) in Ds decays

quark avors bdus, has not been conrmed by LHCb. More data and dedicated analyses are needed to cover a larger mass range. In the tightly bound diquark model, we estimate the lightest bdus, 0 + tetraquark at a mass of about 5770 MeV, approximately 200 MeV above the reported X(5568), and just 7 MeV below the B K threshold. The charged tetraquark is accompanied by I = 1 and I = 0 neutral partners almost degenerate in mass. A bdus, S-wave, 1 + quartet at 5820 MeV is implied as well. In the charm sector, cdus, 0 + and 1 + tetraquarks are predicted at 2365 MeV and 2501 MeV, about 40 50 MeV heavier than Ds0(2317) and Ds1(2460). bdus tetraquarks can be searched in the hadronic debris of a jet initiated by a b. However, some of them may also be produced in Bc decays. The proposed discovery modes of S-wave tetraquarks are Bc! Xb0 + with the subsequent decays Xb0! Bs + , giving rise to nal states such as Bs + 0 .

Is the anomalous decay ratio of DsJ(2632) due to isospin breaking

The new data reported by ALICE on the production of light nuclei with