Triangle singularity enhancing isospin violation in B^0_s→ J/ψπ~0f_0(980)

We perform calculations for the Bs0→ J/ψπ0f0（980） and Bs0 → J/ψπ0a0（980） reactions, showing that the first is isospin-suppressed while the second is isospin-allowed. The reaction proceeds via a triangle mechanism, with Bs0 →J/ψK＊ K ＋c.c., followed by the decay K＊ →Kπ and a further fusion of KK into the f0（980） or a0（980）. We show that the mechanism develops a singularity around the π0 f0（980） or π0a0（980） invariant mass of 1420 MeV, where the π0f0 and π0a0 decay modes are magnified and also the ratio of π0f0 to π0a0 production. Using experimental information for the Bs0 → J/ψK＊K ＋c.c. decay, we are able to obtain absolute values for the reactions studied which fall into the experimentally accessible range. The reactions proposed and the observables evaluated, when contrasted with actual experiments, should be very valuable to obtain information on the nature of the low lying scalar mesons.

Triangle mechanism in the decay process B_(0)→J/ψK^(0)f_(0)(980)(a_(0)(980))

The role of the triangle mechanism in the decay processes B_(0)→J/ψK^(0)f_(0)→J/ψK^(0)π^(+)π^(-)and B_(0)→J/ψK^(0)a_(0)→J/ψK^(0)π^(0)ηis probed.In these processes,the triangle singularity appears from the decay of B_(0)intoJ/ψ∅K^(0),and then,∅decays intoK^(0)K^(0)andK^(0)K^(0)merged into f_(0)or a_(0),which finally decay intoπ^(+)π^(-)andπ^(0)η,respectively.We find that this mechanism leads to a triangle singularity around M_(inv)(K^(0)f_(0)(a_(0)))≈1520 MeV and gives sizable branching fractions Br(B_(0)→J/ψK^(0)f_(0)→J/ψK^(0)π^(+)π^(-))=7.67×10^(-7)and Br(B_(0)→J/ψK^(0)a_(0)→J/ψK^(0)π^(0)η)=1.42×10^(-7).This investigation can help us obtain the information of the scalar mesonf_(0)(980)or a_(0)(980).

Precise determination of the pole position of the exotic Z_(c)(3900)

We perform a unified description of the experimental data of the π^(+)π^(-)and J/ψπ^(±)invariant mass spectra for e^(+)e^(-)→J/ψπ^(+)π^(-)and the D^(0)D^(*-)mass spectrum for e^(+)e^(-)→D^(0)D^(*-)π^(+)at e^(+)e^(-)center-of-mass energies 4.23 and 4.26 GeV.The analysis takes into account open-charm meson loops that contain triangle singularities,the J/ψπ-DD^(*)coupled-channel interaction respecting unitarity,and the strong ππ-KK final state interaction using dispersion relations.The analysis leads to a precise determination of the Z_(c)(3900)pole with the pole mass and width(3880.7±1.7_(stat)±22.4_(syst))MeV and(35.9±1.4_(stat)±15.3_(syst))MeV,respectively,and hints at that the DD^(*)molecular and non-molecular components are of similar importance in the Z_(c)(3900)formation.