A study of exclusive quarkonium decays within the effective field theory framework

• Applicant: Dr. Nikolay Kivel, Ph.D.
• Subject Area: Nuclear and Elementary Particle Physics, Quantum Mechanics, Relativity, Fields
• Term: from 2020 to 2023
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 445769443

This proposal is dedicated to investigation of exclusive quarkonium decays within the effective theory frameworks. Many new precise data about various exclusive decays of charmonium and bottomonium states have been obtained during the last years by the CLEO, BABAR, BELLE, BES and LHCb collaborations. At present, the BES III, BELLE II and LHCb experiments continue to collect the data. Many accurate measurements are also expected in the near future from the PANDA@FAIR experiments. Although the spectrum of low-lying quarkonia is at present reasonably well understood, many of it decays are not yet well understood or are puzzling. A successful description of exclusive decays will shed light on the complicated underlying QCD dynamics involving different scales.In the first part of the project I plan to study the baryon-antibaryon decays of $S$- and $P$-wave charmonia. These decays are sufficiently well measured for many various final states and include $p, n, \Lambda, \Sigma$ and $\Xi$ mesons. A description of these decays depends on the wave functions of charmonium and baryons. For a long time is was accepted that the $3$-quark leading-twist distribution amplitude (DA), which is associated with the baryon wave functions, provides a reasonable estimate of the decay widths. However, the new calculations of various baryon DAs, which were made in QCD sum rules and on the lattice, indicate that the effect from higher twist DAs can be sufficiently large. This strongly affect the existing description of charmonia decays. This point is also supported by the large values of the branching ratios for various helicity suppressed decays. The latter have not been even systematically considered. The goal of this project is to study the $S$- and $P$- charmonia decays taking into account the higher twist DAs of the baryons. The second part of the project is devoted to a study of the colour-octet contributions in decays of $^3S_1$ charmonia into $PV$ final states, with $PV=\rho\pi, K^*\bar K, \dots$. The measured ratio of the $\Psi(2S)$ and $J/\Psi$ branching ratios is about two order of magnitude smaller than one expected from the leading-order description. A similar effect is also observed for many other two-body hadronic channels. The colour-octet contribution is often proposed to be responsible for an explanation of this effect. However, a systematic description of such a contribution for exclusive decays have not yet been suggested. The goal of this project is to study the colour-octet mechanism in the $PV$-decays in a systematic way using the effective field theory framework. It is planned to define the relevant colour-octet operators, to study their properties and the mixing with the colour-singlet operators, to study colour-octet matrix elements and their role in the description of the decay width.

• DFG Programme: Research Grants