The complicated structure of the neutron cannot be calculated using first-principles calculations due to the large colour charge of quarks and the self-interaction of gluons. Its simplest structure ...observables are the electromagnetic form factors1, which probe our understanding of the strong interaction. Until now, a small amount of data has been available for the determination of the neutron structure from the time-like kinematical range. Here we present measurements of the Born cross section of electron–positron annihilation reactions into a neutron and anti-neutron pair, and determine the neutron’s effective form factor. The data were recorded with the BESIII experiment at centre-of-mass energies between 2.00 and 3.08 GeV using an integrated luminosity of 647.9 pb−1. Our results improve the statistics on the neutron form factor by more than a factor of 60 over previous measurements, demonstrating that the neutron form factor data from annihilation in the time-like regime is on par with that from electron scattering experiments. The effective form factor of the neutron shows a periodic behaviour, similar to earlier observations of the proton form factor. Future works—both theoretical and experimental—will help illuminate the origin of this oscillation of the electromagnetic structure observables of the nucleon.Form factors encode the structure of nucleons. Measurements from electron–positron annihilation at BESIII reveal an oscillating behaviour of the neutron electromagnetic form factor, and clarify a long-standing photon–nucleon interaction puzzle.
We study the e^{+}e^{-}→γωJ/ψ process using 11.6 fb^{-1} e^{+}e^{-} annihilation data taken at center-of-mass energies from sqrts=4.008 GeV to 4.600 GeV with the BESIII detector at the BEPCII ...storage ring. The X(3872) resonance is observed for the first time in the ωJ/ψ system with a significance of more than 5σ. The relative decay ratio of X(3872)→ωJ/ψ and π^{+}π^{-}J/ψ is measured to be R=1.6_{-0.3}^{+0.4}±0.2, where the first uncertainty is statistical and the second systematic (the same hereafter). The sqrts-dependent cross section of e^{+}e^{-}→γX(3872) is also measured and investigated, and it can be described by a single Breit-Wigner resonance, referred to as the Y(4200), with a mass of 4200.6_{-13.3}^{+7.9}±3.0 MeV/c^{2} and a width of 115_{-26}^{+38}±12 MeV. In addition, to describe the ωJ/ψ mass distribution above 3.9 GeV/c^{2}, we need at least one additional Breit-Wigner resonance, labeled as X(3915), in the fit. The mass and width of the X(3915) are determined. The resonant parameters of the X(3915) agree with those of the Y(3940) in B→KωJ/ψ and of the X(3915) in γγ→ωJ/ψ observed by the Belle and BABAR experiments within errors.
Though immensely successful, the standard model of particle physics does not offer any explanation as to why our Universe contains so much more matter than antimatter. A key to a dynamically ...generated matter-antimatter asymmetry is the existence of processes that violate the combined charge conjugation and parity (CP) symmetry
. As such, precision tests of CP symmetry may be used to search for physics beyond the standard model. However, hadrons decay through an interplay of strong and weak processes, quantified in terms of relative phases between the amplitudes. Although previous experiments constructed CP observables that depend on both strong and weak phases, we present an approach where sequential two-body decays of entangled multi-strange baryon-antibaryon pairs provide a separation between these phases. Our method, exploiting spin entanglement between the double-strange Ξ
baryon and its antiparticle
Formula: see text, has enabled a direct determination of the weak-phase difference, (ξ
- ξ
) = (1.2 ± 3.4 ± 0.8) × 10
rad. Furthermore, three independent CP observables can be constructed from our measured parameters. The precision in the estimated parameters for a given data sample size is several orders of magnitude greater than achieved with previous methods
. Finally, we provide an independent measurement of the recently debated Λ decay parameter α
(refs.
). The Formula: see text asymmetry is in agreement with and compatible in precision to the most precise previous measurement
.
The process e^{+}e^{-}→D_{s}^{*+}D_{s}^{*-} is studied with a semi-inclusive method using data samples at center-of-mass energies from threshold to 4.95 GeV collected with the BESIII detector ...operating at the Beijing Electron Positron Collider. The Born cross sections of the process are measured for the first time with high precision in this energy region. Two resonance structures are observed in the energy-dependent cross sections around 4.2 and 4.4 GeV. By fitting the cross sections with a coherent sum of three Breit-Wigner amplitudes and one phase-space amplitude, the two significant structures are assigned masses of (4186.8±8.7±30) and (4414.6±3.4±6.1) MeV/c^{2}, widths of (55±15±53) and (122.5±7.5±8.1) MeV, where the first errors are statistical and the second ones are systematic. The inclusion of a third Breit-Wigner amplitude is necessary to describe a structure around 4.79 GeV.
In these proceedings two preliminary results from the BESIII collaboration on polarized Λ and $\bar \Lambda$ are reported. Both results are first observations and concern the resonant and ...non-resonant processes ${e^ - }{e^ + } \to J/\psi \to \Lambda \bar \Lambda$ and ${e^ - }{e^ + } \to {\gamma ^\ast} \to \Lambda \bar \Lambda$ at $\sqrt s = 2.396\,{\rm{GeV}}$, respectively. For $J/\psi \to \Lambda \bar \Lambda$ the relative phase between the electric and magnetic form factors, ΔΦ, has for the first time been determined, $\Delta \Phi = 42.4{(6)_{{\rm{stat}}}(5)_{_{{\rm{syst}}}^^\circ$. That $\Delta \Phi \ne 0$ allows for a simultaneous measurement also of the asymmetry decay parameters $\Lambda \to p{\pi ^ - }\,({\alpha _ - })$, $\bar \Lambda \to \bar p{\pi ^ + }\,({\alpha _ + })$ and $\bar \Lambda \to \bar n{\pi ^0}\,({\bar \alpha _0})$. The measured value of ${\alpha _ - } = 0.750 \pm 0.009 \pm 0.004$ differs by 17(3)% from the PDG value. The simultaneous measurement for the asymmetry parameters allows for the most precise test of CP violation conducted for Λ decays,
A
CP
= −0.006(12)
stat
(7)
syst
. The reaction ${e^ - }{e^ + } \to \Lambda \bar \Lambda$ at $\sqrt s = 2.396\,{\rm{GeV}}$ is the first complete measurement of the time-like electric (
G
E
) and magnetic (
G
M
) form factor of any baryon as also the ratio
R
= |(
G
E
/(
G
M
)| and ΔΦ have been determined:
R
= 0.96(14)(12) and $\Delta \Phi = 37{(12)_{{\rm{stat}}}(6)_{{\rm{stat}}}^\circ$. The obtained cross-section and effective form factor are
σ
= 119.0(53)
stat
(51)
syst
pb and |
G
| = 0.123(3)
stat
(3)
syst
, respectively.
The quantum entangled J/ψ→Σ^{+}Σover ¯^{-} pairs from (1.0087±0.0044)×10^{10} J/ψ events taken by the BESIII detector are used to study the nonleptonic two-body weak decays Σ^{+}→nπ^{+} and Σover ...¯^{-}→nover ¯π^{-}. The CP-odd weak decay parameters of the decays Σ^{+}→nπ^{+} (α_{+}) and Σover ¯^{-}→nover ¯π^{-} (αover ¯_{-}) are determined to be 0.0481±0.0031_{stat}±0.0019_{syst} and -0.0565±0.0047_{stat}±0.0022_{syst}, respectively. The decay parameter αover ¯_{-} is measured for the first time, and the accuracy of α_{+} is improved by a factor of 4 compared to the previous results. The simultaneously determined decay parameters allow the first precision CP symmetry test for any hyperon decay with a neutron in the final state with the measurement of A_{CP}=(α_{+}+αover ¯_{-})/(α_{+}-αover ¯_{-})=-0.080±0.052_{stat}±0.028_{syst}. Assuming CP conservation, the average decay parameter is determined as ⟨α_{+}⟩=(α_{+}-αover ¯_{-})/2=-0.0506±0.0026_{stat}±0.0019_{syst}, while the ratios α_{+}/α_{0} and αover ¯_{-}/αover ¯_{0} are -0.0490±0.0032_{stat}±0.0021_{syst} and -0.0571±0.0053_{stat}±0.0032_{syst}, where α_{0} and αover ¯_{0} are the decay parameters of the decays Σ^{+}→pπ^{0} and Σover ¯^{-}→pover ¯π^{0}, respectively.
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