T1.2 - Hadron decays


    Coordinator: D. Rodríguez Entem

    Contributors: M. Bertani, N. Brambilla, R. Escribano, E. Hernández Gajate, R. Kaminski, V. Magas, A. Mangoni, S. Pacetti, A. Parreño, À. Ramos, E. Santopinto and J. Soto.





    General description:


    We will investigate decays of heavy hadrons, looking for particular interacting hadron pairs in the final state that might produce resonant states. Using Dalitz-plot based methods, EFT and dispersive techniques, we shall also explore issues such as the exotic nature of resonances, isospin or CP violations.

    Hadron decays and hadron reactions are essential in hadron physics. Heavy hadrons are produced in colliders and they are observed through their decay products in different reactions. Their decay properties due to the electromagnetic, weak and strong interactions are deeply related with their nature so the analysis of hadron decays give us important hints to understand hadron states. There are different pictures to understand hadrons as conventional quark model states, dynamically generated resonances, unconventional quark model states as tetraquarks or pentaquarks, etc. Many hadrons can be accommodated in several pictures but its decay properties are usually the best tool to discriminate between them. Also the deviations from well known conservation laws, as for example isospin conservation, is a way to get signatures on new physics.




    Progress [1-18 months]:


    There has been a significant progress in the project. Results have been presented in a good number of international conferences and workshops, celebrated in person before the Covid pandemic, or remotely in the last months.


    List of achievements:


    1. Electromagnetic interaction


    1.1 The theoretical description of radiative decays in charmonium is analyzed in [Br20d] using an elementary emission model to build the most general electromagnetic transition operator. Accurate results for the widths are obtained from a simple quark potential model reasonably fitting the spectroscopy if the complete form of the operator is used.


    1.2 The mass distribution of the decay  ψ(3770) → γDD is analyzed in [Da20]. Since the DD  scattering amplitude is involved, using a model that generates a bound state, it has been shown to have a mass distribution, which could easily measured in BESIII, very different from phase-space.


    1.3 Using an EFT approach we computed in [Br20e] the electromagnetic decay widths and inclusive decay widths into light hadrons for P-wave quarkonia. We also computed the decay widths of 2S and 3S bottomonium states into lepton pairs and their ratios with the inclusive widths into light hadrons  at relative order ν2.


    1.4 We studied in [Ba19] the decays of vector charmonia in pairs of baryons and anti-baryons belonging to the spin-1/2 flavor SU(3) octet. In the framework of a model, which is based on an effective strong Lagrangian density, we have identified the purely strong, purely electromagnetic and mixed strong-electromagnetic contributions to the total branching ratios of these decays. Of particular interest is the mixed strong-electromagnetic contribution that is proven to be crucial for the correct description of the decay mechanism. As a byproduct, the purely electromagnetic branching ratio can be also exploited to obtain the Born non-resonant cross section of the annihilation processes e+e- into baryon-anti-baryon at the vector charmonium mass.


    1.5 An improved phenomenological model that describes V→Pγ and Pγ→V decays including isospin-symmetry breaking is presented in [Es20]. Statistical fits to the most recent experimental data for the radiative transitions and estimations for the mixing angles amongst the three pseudoscalar states with vanishing third-component of isospin are obtained. The current experimental uncertainties allow for isospin-symmetry violations with a confidence level of approximately 2.5 σ.



    2. Weak Interaction


    2.1 We proposed a model independent parametrization for the form factors in the semileptonic decay BDlνl [Ya20]. Using recent experimental and LQCD data, we determine precisely the Cabibbo-Kobayashi-Maskawa matrix element |Vcb| and the ratio of decays between the τ and μ/e  channels. Strong hints about the existence of at least one bound and one virtual BD S-wave 0+  states are given.


    2.2 The decays D+→νe+ ρ K and D+→νe+ π K* have been analyzed in [Wa20]. The goal is to check the double pole nature of K1(1270) the predicted by unitary extensions of chiral perturbation theory. The analysis shows that these decays are suitable to distinguish experimentally the predicted double pole.



    3. Strong Interaction


    3.1 The strong decays of Y states into open bottom channels have been analyzed in [Li20b] using the 3P0 model. The analysis shows that the Y(4S) state has an abnormally two meson component while the other states are largely conventional  bb states. Some deviations from the experimental ratios could be a warning for the existence of more elaborate components in the state.


    3.2 The reaction e+e→ D*D*  in the region of the ψ(4040) and ψ(4160) resonances have been analyzed using the 3P0  model to relate different channels [Ba20]. The conclusion is that these states are largely cc states been their two-meson components very small.


    3.3  The reactions Kp→YKKπ, with Y = Σ,Λ  in the region of KKπ  invariant masses of 1200-1550 MeV have been studied in [Li20c] . The strong coupling of the f1(1285) resonance to K*K makes the mechanism based on K* exchange very efficient to produce this resonance. An associated peak at 1420 MeV is also observed.

    3.4 Assuming that the χc1 state is a singlet of SU(3), the experimental decay ratio into ηf2(1270)  and η'f'2(1525) s described in [Ik20b] assuming f2  and f2' states to be vector-vector dynamically generated states. Six more ratios have been predicted that can be tested in future experiments.



    4. Unconventional states


    4.1 A double beauty tetraquark with quantum numbers JP=1+ has been calculated in [He20]. It is below the strong and electromagnetic threshold, while the semileptonic and non-leptonic weak decays have been evaluated giving a lifetime of 7.6 ps.


    4.2 The weak decay Λb→J/ψ φ Λ  is studied in [Ma20b]. This process may present signatures for exotic hadrons in all three final two-body channels. In the J/ψϕ invariant mass spectrum the interplay between the X(4140) and the X(4160) states is investigated. In the J/ψΛ mass spectrum we conclude that a strange partner of the hidden-charm pentaquark may be identified if it is present in the range between 4450-4500 MeV. Finally, we find that a resonance at around 2160 MeV predicted by some coupled-channels dynamical models has very little chance to be experimentally detected in the φ Λ spectrum.


    4.3 Bottom-charmed meson masses in the region energies in which decay meson-meson thresholds are opened have been computed in [Or20], looking for the analogs to the X(3872) in the spectroscopy. No extra states are found in the  JP=0+ and 1sectors. However, for JP=2+, we found an additional state very close to the DB threshold.


    4.4 The adiabatic formalism, first introduced in molecular physics, is applied to the description of heavy-quark mesons. A spectral study of heavy mesons containing cc  with masses below 4.1 GeV is carried out in [Br20f] within this framework.






    • [Br20d] R. Bruschini and P. González, Phys. Rev. D 101 (2020) 014027.

    • [Da20] L. Dai, G. Toledo and E. Oset., Eur. Phys. J. C 80 (2020) 510

    • [Br20e] N. Brambilla, H.S. Chung, D. Müller and A. Vairo, JHEP 04 (2020), 095.

    • [Ba19] Rinaldo Baldini Ferroli, Monica Bertani, Alessio Mangoni, Simone Pacetti, et al., Chin. Phys. C 43 (2019) 023103.; Rinaldo Baldini Ferroli, Alessio Mangoni, Simone Pacetti, Kai Zhu, Phys. Lett. B 799 (2019) 135041; Rinaldo Baldini Ferroli, Alessio Mangoni, Simone Pacetti, [arXiv:2007.12380 [hep-ph]], EPJC, to be published.

    • [Es20] R. Escribano, E. Royo. Phys. Lett. B 807 (2020) 135534.

    • [Ya20] D.L. Yao, P. Fernandez-Soler, F.K. Guo and J. Nieves, Phys. Rev. D 101 (2020) no.3, 034014.

    • [Wa20] G.Y. Wang, L. Roca, E. Wang, W.H. Liang and E. Oset. Eur. Phys. J. C 80 (2020) 388.

    • [Li20b] W.H. Liang, N. Ikeno and E. Oset, Phys. Lett. B 803 (2020) 135340.

    • [Ba20] M. Bayar, N. Ikeno and E. Oset. Eur. Phys. J. C 80 (2020) 222.

    • [Li20c] W.H. Liang and E. Oset. Eur. Phys. J. C 80 (2020) 407.

    • [Ik20b] N. Ikeno, J.M. Dias, W.H. Liang and E. Oset, Phys. Rev. D 100 (2019) 114011.

    • [He20] E. Hernández, J. Vijande, A. Valcarce and J.-M. Richard, Phys. Lett. B 800, 135073 (2020).

    • [Ma20b] V. Magas, A. Ramos, R. Somasundaram, J. Tena-Vidal., e-Print: 2004.01541 [hep-ph] Phys. Rev. D, (accepted)

    • [Or20] P.G.Ortega, J.Segovia, D.R. Entem and F.Fernandez., Eur Phys J. C (2020) 80:223.

    • [Br20f] R. Bruschini, P. González, e-Print: arXiv:2007.07693 [hep-ph].


    This project has received funding from the European Union's Horizon 2020 research and innovation programme under grant agreement No 824093.

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