Study of two-photon decays of pseudoscalar mesons via J/ψ radiative decays

Abstract

Using a sample of 4.48 × 10 8 ψð3686Þ events collected with the BESIII detector at the BEPCII collider, we study the two-photon decays of the pseudoscalar mesons π 0 , η, η 0 , ηð1405Þ, ηð1475Þ, ηð1760Þ, and Xð1835Þ in J=ψ radiative decays using ψð3686Þ → π þ π − J=ψ events. The π 0 , η, and η 0 mesons are clearly observed in the two-photon mass spectra, and the branching fractions are determined to be BðJ=ψ → γπ 0 → 3γÞ ¼ ð3.57 AE 0.12 AE 0.16Þ × 10 −5 , BðJ=ψ → γη → 3γÞ ¼ ð4.42 AE 0.04 AE 0.18Þ × 10 −4 , and BðJ=ψ → γη 0 → 3γÞ ¼ ð1.26 AE 0.02 AE 0.05Þ × 10 −4 , where the first error is statistical and the second is systematic. No clear signal for ηð1405Þ, ηð1475Þ, ηð1760Þ or Xð1835Þ is observed in the two-photon mass spectra, and upper limits at the 90% confidence level on the product branching fractions are obtained.

The anomalous magnetic moment of the muon in the Standard Model

“…The latter contribution can be interpreted as an estimate for the mixed regions in the (g − 2) µ integral and carries large uncertainties due to limited phenomenological information on the two-photon couplings of these states. In fact, in the chiral limit the pseudoscalar excitations would decouple, but η(1475) [94,690] and η(1760) [689] have been seen in γγ collision, with two-photon couplings in line with the expectation from the Regge models constrained by the HLbL SDCs [24,553]. The matching scale in Eq.…”

“…Finally, the longitudinal SDCs can also be saturated explicitly by a large-N c -inspired Regge model for the corresponding scalar functions [24,553], albeit away from the chiral limit. Using phenomenological input for excited pion and η states as well as their two-photon couplings [94,[686][687][688][689][690], the increase compared to the leading pseudoscalar poles alone is found to be [24,553] where the first error propagates the uncertainty from the input model parameters and the second estimates the systematic uncertainty due to the modeling of the TFFs. This is much smaller than obtained when neglecting the momentum dependence of the second form factor of 23.5 × 10 −11 [18].…”

The anomalous magnetic moment of the muon in the Standard Model

“…The latter contribution can be interpreted as an estimate for the mixed regions in the (g − 2) µ integral and carries large uncertainties due to limited phenomenological information on the two-photon couplings of these states. In fact, in the chiral limit the pseudoscalar excitations would decouple, but η(1475) [94,714] and η(1760) [713] have been seen in γ γ collision, with two-photon couplings in line with the expectation from the Regge models constrained by the HLbL SDCs [24,572]. The matching scale in Eq.…”

“…Finally, the longitudinal SDCs can also be saturated explicitly by a large-N c -inspired Regge model for the corresponding scalar functions [24,572], albeit away from the chiral limit. Using phenomenological input for excited pion and η states as well as their two-photon couplings [94,[710][711][712][713][714], the increase compared to the leading pseudoscalar poles alone is found to be [24,572] ∆a where the first error propagates the uncertainty from the input model parameters and the second estimates the systematic uncertainty due to the modeling of the TFFs. This is much smaller than obtained when neglecting the momentum dependence of the second form factor of 23.5 × 10 −11 [18].…”