Measurement of the Electromagnetic Transition Form-factors in the decays η'→π^+π^-l^+l^-
arxiv(2024)
摘要
With a sample of (10087±44)×10^6 J/ψ events accumulated with
the BESIII detector, we analyze the decays
η'→π^+π^-l^+l^-(l=e, μ) via the process
J/ψ→γη'. The branching fractions are measured to be
ℬ(η'→π^+π^-e^+e^-)=(2.45±0.02(stat.)±0.08(syst.))
×10^-3 and
ℬ(η'→π^+π^-μ^+μ^-)=(2.16±0.12(stat.)±0.06(syst.))×10^-5,
and the ratio is
ℬ(η'→π^+π^-e^+e^-)/ℬ(η'→π^+π^-μ^+μ^-)
= 113.4±0.9(stat.)±3.7(syst.). In addition, by combining the
η'→π^+π^-e^+e^- and η'→π^+π^-μ^+μ^-
decays, the slope parameter of the electromagnetic transition form factor is
measured to be b_η'=1.30±0.19 (GeV/c^2)^-2, which is
consistent with previous measurements from BESIII and theoretical predictions
from the VMD model. The asymmetry in the angle between the π^+π^- and
l^+l^- decay planes, which has the potential to reveal the CP-violation
originating from an unconventional electric dipole transition, is also
investigated. The asymmetry parameters are determined to be
𝒜_CP(η'→π^+π^-e^+e^-)=(-0.21±0.73(stat.)±0.01(syst.))%
and
𝒜_CP(η'→π^+π^-μ^+μ^-)=(0.62±4.71(stat.)±0.08(syst.))%,
implying that no evidence of CP-violation is observed at the present
statistics. Finally, an axion-like particle is searched for via the decay
η'→π^+π^-a, a→ e^+e^-, and upper limits of the
branching fractions are presented for the mass assumptions of the axion-like
particle in the range of 0-500 MeV/c^2.
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