136 research outputs found
A new G-parity violating amplitude in the J/ψ decay?
No full textPossibility of proving or disproving a new G-parity violating mechanism in J/psi decay using data from the BESIII experiment
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BaBar Results on E+ E- ---> P Anti-P By Means of ISR
No full textBaBar has measured with unprecedented accuracy the e{sup +}e{sup -} {yields} p{bar p} cross section from the threshold up to Q{sub p{bar p}}{sup 2} {approx} 20 GeV{sup 2}/c{sup 4}, finding out an unexpected cross section, with plateaux and negative steps. Evidence for a ratio |G{sub E}/G{sub M}| > 1 has also been found as well as a sudden variation in |G{sub M}| just above the threshold
Proton electromagnetic form factors: Basic notions, present achievements and future perspectives
No full textInternational audienceThe aim of this report is to give basic notions on electromagnetic hadron form factors(FFs), as they are understood at the present time, to summarize and analyze the presentexperimental results and available theoretical models and to open a view on futureperspectives.FFs are fundamental quantities, which describe the internal, dynamical structure ofhadrons. Although the theoretical formalism was settled in the middle of last century, aswell as the first experiments in electron–proton elastic scattering for which R. Hofstadtergot the Nobel prize in 1961, a renewed activity is due to recent, surprising results andto the opening of new experimental possibilities. An elegant formalism was built on theassumption of a hadron electromagnetic interaction based on the exchange of a virtualphoton of four-momentum q2. In this case FFs are analytic functions of only one variable,q2, and the electromagnetic vertex γ ∗hh (h is any hadron) is defined by two structurefunctions, which, in turn, are expressed in terms of (2S + 1) FFs, S being the hadron spin,assuming parity and time-invariance.Our aim is to anticipate the potentiality contained in the future data, combined withthe present knowledge, to point out the relevant observables and the most significativemeasurements, and to give predictions to be compared to the data when they will beavailable
The cross section of
No full textUnder the aegis of isospin conservation, the amplitudes in Born approximation, i.e., considering the only one-photon-exchange mechanism, of the decay , where is a vector charmonium, and of the reaction at the mass, are parametrized by the same electromagnetic coupling. It follows that, the modulus of such a coupling can be extracted from the data on the two observables: the decay branching fraction and the annihilation cross section. By considering the first two vector charmonia, and , it is found that, especially in the case of , there is a substantial discrepancy between the values of the modulus of the same electromagnetic coupling extracted from the branching ratio and the cross section. We propose, as a possible explanation for such a disagreement, the presence in the decay amplitude of isospin-violating contributions driven by two different mechanisms, that, however, appear to be more favored in the than in the decays
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