This paper presents a new event generator, ALPGEN, dedicated to the study of multiparton hard processes in hadronic collisions. The code performs, at the leading order in QCD and EW interactions, the calculation of the exact matrix elements for a large set of parton-level processes of interest in the study of the Tevatron and LHC data. The current version of the code describes the following final states: (W → ff ′ )QQ + N jets (Q being a heavy quark, and f = ℓ, q), with N ≤ 4; (Z/γ * → ff ) QQ + N jets (f = ℓ, ν), with N ≤ 4; (W → ff ′ ) + charm + N jets (f = ℓ, q, N ≤ 5); (W → ff ′ ) + N jets (f = ℓ, q) and (Z/γ * → ff ) + N jets (f = ℓ, ν), with N ≤ 6; nW + mZ + lH + N jets, with n + m + l + N ≤ 8, N ≤ 3, including all 2-fermion decay modes of W and Z bosons, with spin correlations; QQ + N jets, with t → bff ′ decays and relative spin correlations included if Q = t, and N ≤ 6; QQQ ′ Q ′ + N jets, with Q and Q ′ heavy quarks (possibly equal) and N ≤ 4; HQQ + N jets, with t → bff ′ decays and relative spin correlations included if Q = t and N ≤ 4; N jets, with N ≤ 6. Parton-level events are generated, providing full information on their colour and flavour structure, enabling the evolution of the partons into fully hadronised final states.

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We propose that the Y (4260) particle recently announced by BaBar is the first orbital excitation of a diquark-antidiquark state ([cs][cs]). Using parameters recently determined to describe the X(3872) and X(3940) we show that the Y mass is compatible with the orbital excitation picture. A crucial prediction is that Y (4260) should decay predominantly in DsDs. The Y (4260) should also be seen in B non-leptonic decays in association with one kaon. We consider the full nonet of related four-quark states and their predicted properties. Finally, we comment on a possible narrow resonance in the same channel.ROMA1 In a series of exciting experiments, BELLE and BaBar have discovered several states that, although decaying in charmonium plus pions, do not seem to fit the cc picture, in particular the X(3872) and X(3940) states.In a recent paper [1] we have pointed out that the properties of the new states can be well explained if they are S-wave diquark-antidiquark bound states with the composition (q = u, d): [(cq)(cq)] S−wave . An alternative scenario is the molecular picture where the X(3872) would be a D 0 D * 0 bound state. A crucial difference between the two alternatives is that colored objects in a rising confining potential, such as diquarks, should exhibit a series of orbital angular momentum excitations. This is clearly at variance with the molecular picture. Colorless objects bound by a short range potential should have a very limited spectrum, possibly restricted to S-wave states only.In this note we would like to propose that the first orbital excitation of a diquark-antidiquark state may have indeed been found in the state Y (4260) recently announced by the BaBar collaboration [2]. We discuss the properties of the new state in this framework and spell out a few distinctive predictions. The most revealing among them is that the dominant decay mode of Y (4260) should be in D sDs pairs. We shall also briefly discuss other states implied by the scheme and their properties. We comment on the possibility of an additional narrow state.The Y (4260) is observed by BaBar in e + e − annihilation, in association with an Initial-State-Radiation pho- * Electronic address: luciano.maiani@roma1.infn.it † Electronic address: veronica.riquer@cern.ch ‡ Electronic address: fulvio.piccinini@pv.infn.it § Electronic address: antonio.polosa@cern.ch ton, which implies J P C = 1 −− . The particle has a width of about 90 MeV and it is seen to decay in J/ψ π + π − . The π + π − mass distribution peaks around 1 GeV, consistently with a decay into J/ψ f 0 (980). BaBar reports the value [2]:(1) The diquark-antidiquark assumption together with the negative parity call for at least one unit of orbital angular momentum. In addition, the decay into f 0 (980), which fits the (

Heavy-light diquarks can be the building blocks of a rich spectrum of states which can accommodate some of the newly observed charmonium-like resonances not fitting a pure cc assignment. We examine this possibility for hidden and open charm diquark-antidiquark states deducing spectra from constituent quark masses and spin-spin interactions. Taking the X(3872) as input we predict the existence of a 2 ++ state that can be associated to the X(3940) observed by Belle and re-examine the state claimed by SELEX, X(2632). The possible assignment of the previously discovered states Ds(2317) and Ds (2457) is discussed. We predict X(3872) to be made of two components with a mass difference related to mu − m d and discuss the production of X(3872) and of its charged partner X ± in the weak decays of B +,0 .

Multiquark resonances are undoubtedly experimentally observed. The number of states and the amount of details on their properties have been growing over the years. It is very recent the discovery of two pentaquarks and the confirmation of four tetraquarks, two of which had not been observed before. We mainly review the theoretical understanding of this sector of particle physics phenomenology and present some considerations attempting a coherent description of the so called X and Z resonances. The prominent problems plaguing theoretical models, like the absence of selection rules limiting the number of states predicted, motivate new directions in model building. Data are reviewed going through all of the observed resonances with particular attention to their common features and the purpose of providing a starting point to further research.

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Following the recent confirmation of the Z + (4430) resonance with J P G = 1 ++ , we have re-examined the model of S and P wave tetra--quarks. We propose a 'type-II' diquark-antidiquark model which shows to be very effective at producing a simple and comprehensive picture of the J P G = 1 ++ and 1 −− sectors of the recently discovered charged tetraquarks and of the observed Y resonances. The model is still faced with the unresolved difficulty of explaining why some states seem to have incomplete isospin multiplets.

Light scalar mesons are found to fit rather well a diquark-antidiquark description. The resulting nonet obeys mass formulas which respect, to a good extent, the Okubo-Zweig-Iizuka (OZI) rule. OZI allowed strong decays are reasonably reproduced by a single amplitude describing the switch of a qq pair, which transforms the state into two colorless pseudoscalar mesons. Predicted heavy states with one or more quarks replaced by charm or beauty are briefly described; they should give rise to narrow states with exotic quantum numbers.