Physicists map rare particle decay with unprecedented precision
Chinese researchers have measured how exotic particles decay into pairs of lighter particles with extreme accuracy, revealing details about fundamental forces. The findings could help detect physics beyond current theory and improve understanding of matter itself—work that underpins future particle detector technology and high-energy physics experiments worth billions in research investment.
Originaltitel: Study of η′ → π<sup>+</sup>π<sup>-</sup><em>l</em><sup>+</sup><em>l<sup>-</sup></em> decays at BESIII
<p>With a sample of (10087 +/- 44) x 10(6)J/psi events accumulated with the BESIII detector, we analyze the decays eta ' -> pi(+)pi(-)l(+)l(-)(l = e, mu) via the process J/psi -> gamma eta '. The branching fractions are measured to be B(eta ' -> pi(+)pi(-)e(+)e(-)) = (2.45 +/- 0.02(stat.) +/- 0.08(syst.)) x 10(-3) and B(eta ' -> pi(+)pi(-)mu(+)mu(-)) = (2.16 +/- 0.12(stat.) +/- 0.06(syst.)) x 10(-5), and the ratio is B(eta '->pi(+)pi(-)e(+)e(-))/B(eta '->pi(+)pi(-)mu(+)mu(-)) =113.4 +/- 0.9(stat.) +/- 3.7(syst.). In addition, by combining the eta ' -> pi(+)pi(-)e(+)e(-) and eta ' -> pi(+)pi(-)mu(+)mu(-) decays, the slope parameter of the electromagnetic transition form factor is measured to be b(eta ')=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 pi(+)pi(-) 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)(eta '->pi(+)pi(-)e(+)e(-)) = (-0.21 +/- 0.73(stat.) +/- 0.01(syst.))% and A(CP)(eta '->pi(+)pi(-)mu(+)mu(-)) = (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 eta ' -> pi(+)pi(-)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).</p>