Standard Model

The Standard Model (SM) of particle interactions is one of the major achievements of fundamental science, recently validated also with the discovery of the Higgs boson. It is the most successful theory and for many years it has been probed and systematically confirmed in collider experiments. A complete understanding of this paradigm is of fundamental importance to assess how current and future experimental results, in particular in the flavor sector, can hint at or constrain new physics scenarios. The group consists of UAB Profs. Rafel Escribano, Pere Masjuan, Joaquim Matias, Santi Peris and Antonio Pineda, postdocs Aritra Biswas, Bernat Capdevila, Alejandro Miranda and Emilio Royo as well as PhD students Cristian Alarcón, Camilo Rojas. The group activities are mainly in the Standard Model and Flavour Physics.

Assessment of the a2(1320) tensor-meson contribution to η/η’ → π0γγ decays

Rafel Escribano

In light of the recent measurement of the η → π0γγ decay by the KLOE-2 Collaboration, we have extended a previous analysis including vector- and scalar-meson exchange contributions using the vector meson dominance and linear sigma model frameworks, respectively, to incorporate the effects of the a2(1320) tensor meson within a chiral context. Although the individual contribution of the a2 is negligible, its destructive interference with the vector-meson resonances is found to be significant, representing approximately 18% of the total signal and substantially affecting the diphoton invariant-mass distribution, especially at low γγ invariant mass values. The result obtained for the corresponding branching ratio is approximately 5σ below the reported value of the particle data group, while it is in very good agreement with the KLOE-2 measurement. In contrast, the total contribution of the a2 is found to be negligible in η’ → π0γγ, as this process is completely dominated by the exchange of an on-shell ω vector resonance.