Revisiting $$\gamma ^*\rightarrow \gamma \pi ^0\eta $$ near the $$\phi (1020)$$ using analyticity and the left-cut structure

Amplitudes of the form $$\gamma ^*(q^2)\rightarrow \gamma P_1P_2$$ γ ∗ ( q 2 ) → γ P 1 P 2 appear as sub-processes in the computation of the muon $$g-2$$ g - 2 . We test a proposed theoretical modelling against very precise experimental measurements by the KLOE collaboration at $$q^2=m^2_\phi $$ q 2 = m ϕ 2 . Starting from an exact, parameter-free dispersive representation for the S-wave satisfying QCD asymptotic constraints and Low’s soft photon theorem we derive, in an effective theory spirit, a two-channel Omnès integral representation which involves two subtraction parameters. The discontinuities along the left-hand cuts which, for timelike virtualities, extend both on the real axis and into the complex plane are saturated by the contributions from the light vector mesons. In the case of $$P_1P_2=\pi \eta $$ P 1 P 2 = π η , we show that a very good fit of the KLOE data can be achieved with two real parameters, using a T-matrix previously determined from $$\gamma \gamma $$ γ γ scattering data. This indicates a good compatibility between the two data sets and confirms the validity of the T-matrix. The resulting amplitude is also found to be compatible with the chiral soft pion theorem. Applications to the $$I=1$$ I = 1 scalar form factors and to the $$a_0(980)$$ a 0 ( 980 ) resonance complex pole are presented.

Eta and eta’ physics in KLOE

We present results obtained by the KLOE Collaboration at the DAфNE e+e− collider located in Frascati Laboratories, Italy. The results concern precise measurements of several η meson decays such as Dalitz slope parameters and charge asymmetries for $ \eta \to \mathop \pi \nolimits^ + \mathop \pi \nolimits^ - \mathop \pi \nolimits^0 $, branching ratio and plane asymmetry for $ \eta \to \mathop \pi \nolimits^ + \mathop \pi \nolimits^ - \mathop {\rm{e}}\nolimits^ + \mathop {\rm{e}}\nolimits^ - $, and first observation of $ \eta \to \mathop {\rm{e}}\nolimits^ + \mathop {\rm{e}}\nolimits^ - \mathop {\rm{e}}\nolimits^ + \mathop {\rm{e}}\nolimits^ - $ along with its branching ratio determination. Also preliminary studies of $ \eta \to \mathop \pi \nolimits^0 \gamma \gamma $ decay channel are presented as well as physics perspectives for many different processes for the ongoing KLOE−2 data taking.

AMADEUS Experiment: Studies on Antikaon Interactions with Nucleons and Nuclei

The understanding of the low-energy strong interaction involving strangeness is a challenging topic due to resonances and predicted kaonic nuclear bound states. The K- nucleon interaction is strongly attractive at low energies verified in kaonic hydrogen studied in the SIDDHARTA experiment at the DA[Formula: see text]NE electron-positron collider of LNF-INFN (Frascati/Italy). Hyperon resonances like the elusive [Formula: see text] in the s-wave impose questions about its nature. According to theoretical studies it can be described as a dynamically generated resonance with two poles or a quasi-bound [Formula: see text]N state, which could lead to kaonic nuclear bound states (e.g. K-pp). An insight in many open facets of the antikaon interactions can be provided by the AMADEUS experiment at DA[Formula: see text]NE based on the analysis of the data collected in 2004/2005 by the KLOE collaboration, and of the dedicated data set collected in 2012 by AMADEUS in collaboration with KLOE. As a first step data from antikaon-induced reactions in the drift chamber of KLOE were analyzed and yielded new results on antikaon absorption on nuclei. Recent results of the experimental studies and an outlook to the future possibilities within AMADEUS are presented.