Fast radio bursts (FRBs) are highly dispersed radio bursts prevailing in the universe [1][2][3] . The recent detection of FRB 200428 from a Galactic magnetar [4][5][6][7][8] suggested that at least some FRBs originate from magnetars, but it is unclear whether the majority of cosmological FRBs, especially the actively repeating ones, are produced from the magnetar channel. Here we report the detection of 1863 polarised bursts from the repeating source FRB 20201124A 9 during a dedicated radio observational campaign of Five-hundred-meter Aperture Spherical radio Telescope (FAST). The large sample of radio bursts detected in 88 hr over 54 days indicate a significant, irregular, short-time variation of the Faraday rotation measure (RM) of the source during the first 36 days, followed by a constant RM during the later 18 days. Significant circular polarisation up to 75% was observed in a good fraction of bursts. Evidence suggests that some low-level circular polarisation originates from the conversion from linear polarisation during the propagation of the radio waves, but an intrinsic radiation mechanism is required to produce the higher degree of circular polarisation. All of these features provide evidence for a more complicated, dynamically evolving, magnetised immediate environment around this FRB source. Its host galaxy was previously known 10-12 . Our optical observations reveal that it is a Milky-Way-sized, metal-rich, barred-spiral galaxy at redshift z = 0.09795 ± 0.00003, with the FRB source residing in a low stellar density, interarm region

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Using a 478 pb −1 data sample collected with the BESIII detector operating at the BEPCII storage ring at a center-of-mass energy of √ s = 4.009 GeV, the production of e + e − → ηJ/ψ is observed for the first time with a statistical significance of greater than 10σ. The Born cross section is measured to be (32.1 ± 2.8 ± 1.3) pb, where the first error is statistical and the second systematic. Assuming the ηJ/ψ signal is from a hadronic transition of the ψ(4040), the fractional transition rate is determined to be B(ψ(4040) → ηJ/ψ) = (5.2 ± 0.5 ± 0.2 ± 0.5) × 10 −3 , where the first, second, and third errors are statistical, systematic, and the uncertainty from the ψ(4040) resonant parameters, respectively. The production of e + e − → π 0 J/ψ is searched for, but no significant signal is observed, and B(ψ(4040) → π 0 J/ψ) < 2.8 × 10 −4 is obtained at the 90% confidence level. 3PACS numbers: 13.25. Gv, 13.40.Hq, 14.40.Pq The properties of excited J P C = 1 −− charmonium states above the DD production threshold is of great interest but not well understood, even decades after their first observation [1]. The current experimentally well established structures in the hadronic cross section are the ψ(3770), ψ(4040), ψ(4160), and ψ(4415) resonances [2]. Unlike the low-lying vector cc states J/ψ and ψ(3686), all of these states couple to opencharm final states with large partial widths, and disfavor hidden charm decays.Recently, new vector charmonium-like states, the Y (4260), the Y (4360) and the Y (4660) have been discovered via their decays into exclusive π + π − J/ψ and π + π − ψ(3686) final states [3]. The common properties of these states are relatively narrow widths and strong couplings to hidden-charm final states. These Y -states cannot be assigned to any of the conventional cc 1 −− ψ family states [4] in any natural way and suggest the existence of a non-conventional meson spectroscopy [5].Hadronic transitions play an important role in understanding the nature of conventional heavy quarkonium. An excess of η over π + π − hidden-bottom transition rates of the Υ(4S) [6] has been explained as an admixture of a fourquark state in the Υ(4S) wave function [7]. A similar picture might be expected in the charm sector but, as of yet, there is no experimental data available for η transitions in the highmass charmonium and charmoniumlike states, except for evidence of ψ(3770) → ηJ/ψ (3.5σ) [8] and ψ(4160) → ηJ/ψ (4.0σ) [9]. Moreover, there are predictions of many new states in various models trying to explain the conventional and unconventional states observed in this mass region [5].In this Letter, we report cross section measurements for e + e − → ηJ/ψ and π 0 J/ψ at the center-of-mass energy √ s = (4.009 ± 0.001) GeV. The analysis is performed with a 478 pb −1 data sample collected with the BESIII detector located at the BEPCII storage ring [10]. The integrated luminosity of this data sample was measured using Bhabha events, with an estimated uncertainty of 1.1%. In order to control systematic errors, an accompanying d...

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