Explore the vast range of titles available.

The BESIII experiment has accumulated millions of spin entangled hyperon–antihyperon pairs from the decays of the J/ψ and ψ (2S ) mesons produced in electron–positron collisions. Using multi-dimensional methods to analyze the angular distributions in the sequential decays the hyperon and antihyperon decay amplitudes are studied and tests of the combined charge-conjugation–parity (CP) symmetry are performed. Implications of the recent results using J/ψ decays into Λ $ \\bar \\Lambda $and Ξ −$ \\bar {\\Xi}^{+} $are presented. For the cascade decay chain, the exclusive measurement allows for three independent CP tests and the first direct determination of the weak phases.

I summarize recent experimental results for two photon production of mesons. These processes include the neutral pion, η and η ′ transition form factors and two photon production of pseudoscalar meson pairs. In addition I discuss the related hadronic and radiative processes. All these processes are attracting attention of experiment and theory due their relevance for the hadronic light-by-light contribution to the muon anomalous magnetic moment.

Baryon-antibaryon decays of J /ψ and ψ ‘ produced at an electron-positron collider are presented as a tool to study baryon properties and to test discrete symmetries. We focus on a novel aspect of this process: the transverse polarization of the produced baryons. The polarization was observed for first time by the BESIII Collaboration in e + e → J /ψ → ΛΛ̅ . The observed spin polarization and correlations were used for a direct determination the Λ and Λ ̅ decay asymmetries. The result for Λ → p π − , the well known Λ polarimeter process, differs by 17(3)% from the established value of 0.642(13). Finally the prospects of the method for studies of weak decays of hyperons and for CP tests in baryon sector are discussed.

Structure, interactions and decays of hyperons can be studied at an electron-positron collider in annihilations to a hyperon-antihyperon pair. This method already has huge impact on hyperon physics since it was used in a first modern measurement of decay asymmetry in the weak Λ→pπ− decay and the result calls for reinterpretation of all polarization results for the Λ hyperon. In the experiment by the BESIII Collaboration nearly half million e+e−→ψ→ΛΛ¯ events were used. The ΛΛ¯ pair production process involves two (complex) hadronic form factors. The observed large relative phase ΔΦ between the form factors, (42.4±0.6±0.5)°, makes it possible to use both transverse polarization and the spin correlations of the ΛΛ¯pair to simultaneously measure decay parameters for Λ and Λ¯ hyperons. Of particular importance is the result for the decay asymmetry parameter α− in Λ→pπ− the most probable Λ decay used to determine Λ polarization. The new α− value from BESIII, 0.750±0.009±0.004, is 17(3)% larger than the world average of 0.642 ± 0.013 adopted by all experiments between 1978 and 2019 to determine Λ polarization from the measured asymmetry in the decay proton distribution. A comparison of the concurrently measured at BESIII α− and α+, the parameter for the charge conjugated process Λ¯→p¯π+, results in a direct CP symmetry test for the hyperon. The e+e−→ΛΛ¯ reaction was studied at BESIII also outside the J/ψ resonance using 66.9 pb−1 collected at s=2.396 GeV. The result is the first complete determination of the time-like elastic form factors GM and GE for any baryon including the relative phase ΔΦ = arg(GE/GM) of (37 ± 12 ± 6)°. The absolute normalization of the form factors is set by the determined Born cross section of σBorn= 119.0 ± 5.3 ± 5.1 pb. In a separate run at s=2.2324 GeV, 1.0 MeV above threshold for e+e−→ΛΛ¯, a surprisingly large cross section for of 305 ± 45−36+66pb was measured.

An angular analysis of the decay Bs0 -> phi e+e- is presented, using proton-proton collision data collected with the LHCb detector between 2011 and 2018 at centre-of-mass energies of 7, 8 and 13 TeV. The combined dataset corresponds to an integrated luminosity of 9 fb-1. Observables are determined by fitting time-integrated projections of the angular distribution in three bins of dielectron mass squared, q2, corresponding to [0.1, 1.1], [1.1, 6.0] and [15.0, 19.0] GeV2/c4. The results are compatible with predictions based on the Standard Model of particle physics.

A study of the charmless baryonic decays B 0 (s) → K 0 pp<over bar> is presented, where B 0 (s ) denotes either a B 0 or a B 0 s meson. The analysis is based on proton-proton collision data collected by the LHCb experiment at centre-of-mass energies of 7, 8, and 13 TeV, corresponding to an integrated luminosity of 9 fb -1 . The decay B 0 s → K 0 pp<over bar> is observed for the first time, with a measured branching fraction of (9.14 ± 1.69 ± 0.90 ± 0.33 ± 0.20) x 10 -7 and a significance of 5.6σ. The uncertainties respectively account for statistical and systematic contributions, the precision of the branching fraction of the normalisation channel B 0 → K 0 π + π - and the fragmentation fraction ratio f s /f d . The branching fraction determined for B 0 → K 0 pp<over bar> is (2.82 ± 0.08 ± 0.12 ± 0.10) x 10 -6 , which is the most precise measurement to date.

The production of prompt D+ and D + s mesons is studied in proton-lead collisions at a centre-of-mass energy of √s NN = 5.02TeV. The data sample corresponding to an integrated luminosity of (1.58 ± 0.02)nb -1 is collected by the LHCb experiment at the LHC. The differential production cross-sections are measured using D + and D + s candidates with transverse momentum in the range of 0 < p T < 14 GeV/c and rapidities in the ranges of 1.5 < y* < 4.0 and -5.0 < y* < -2.5 in the nucleon-nucleon centre-of-mass system. For both particles, the nuclear modification factor and the forward-backward production ratio are determined. These results are compared with theoretical models that include initial-state nuclear effects. In addition, measurements of the cross-section ratios between D + , D + s and D 0 mesons are presented, providing a baseline for studying the charm hadronization in lead-lead collisions at LHC energies.

The first measurement of the Z boson production cross-section at centre-of-massenergy √s = 5.02 TeV in the forward region is reported, using pp collision data collected bythe LHCb experiment in year 2017, corresponding to an integrated luminosity of 100 ± 2 pb−1.The production cross-section is measured for final-state muons in the pseudorapidity range2.0 < η < 4.5 with transverse momentum pT > 20 GeV/c. The integrated cross-sectionis determined to be σZ→μ+μ− = 39.6 ± 0.7(stat) ± 0.6(syst) ± 0.8(lumi) pb for the di-muon invariant mass in the range 60 < Mμμ < 120 GeV/c2. This result andthe differential cross-section results are in good agreement with theoretical predictions atnext-to-next-to-leading order in the strong coupling constant.Based on a previous LHCb measurement of the Z boson production cross-section in pPbcollisions at √sNN = 5.02 TeV, the nuclear modification factor RpPb is measured for the firsttime at this energy. The measured values are 1.2+0.5−0.3(stat) ± 0.1(syst) in the forward region(1.53 < y∗μ < 4.03) and 3.6+1.6−0.9(stat) ± 0.2 (syst) in the backward region (−4.97 < y∗μ < −2.47),where y∗μ represents the muon rapidity in the centre-of-mass frame.

This paper reports the observation of the decays B-(s)(0) -> D-s1(2536)K--/+(+/-) using proton-proton collision data collected by the LHCb experiment, corresponding to an integrated luminosity of 9 fb(-1). The branching fractions of these decays are measured relative to the normalisation channel B-0 -> (D) over bar (K+K-)-K-0. The D-s1(2536)(-) meson is reconstructed in the (D) over bar*(2007)K-0(-) decay channel and the products of branching fractions are measured to be B(B-s(0) -> D-s1(2536)K--/+(+/-)) x B(D-s1(2536)(-) -> D ($) over bar*(2007)K-0(-)) = (2.49 +/- 0.11 +/- 0.12 +/- 0.25 +/- 0.06) x 10(-5), B(B-0 -> D-s1(2536)K--/+(+/-) ) x B(D-s1(2536)(-) -> (D) over bar*(2007)K-0(-)) = (0.510 +/- 0.021 +/- 0.036 +/- 0.050) x 10(-5). The first uncertainty is statistical, the second systematic, and the third arises from the uncertainty of the branching fraction of the B-0 -> (D) over bar (K+K-)-K-0 normalisation channel. The last uncertainty in the B-s(0) result is due to the limited knowledge of the fragmentation fraction ratio, f(s)/f(d). The significance for the B-s(0) and B-0 signals is larger than 10 sigma. The ratio of the helicity amplitudes which governs the angular distribution of the D-s1(2536)(-) -> (D) over bar*(2007)K-0(-) decay is determined from the data. The ratio of the S- and D-wave amplitudes is found to be 1.11 +/- 0.15 +/- 0.06 and the phase difference between them 0.70 +/- 0.09 +/- 0.04 rad, where the first uncertainty is statistical and the second systematic.

Evidence for the decays B-0 -> (D) over bar (0)phi and B-0 -> (D) over bar (*0) phi is reported with a significance of 3.6 sigma and 4.3 sigma, respectively. The analysis employs pp collision data at centre-of-mass energies root s = 7, 8 and 13TeV collected by the LHCb detector and corresponding to an integrated luminosity of 9 fb(-1). The branching fractions are measured to be B(B-0 -> (D) over bar (0)phi) = (7.7 +/- 2.1 +/- 0.7 +/- 0.7) x 10(-7), B(B-0 -> (D) over bar (*0)phi) = (2.2 +/- 05 +/- 0.2 +/- 0.2) x 10(-6). In these results, the first uncertainty is statistical, the second systematic, and the third is related to the branching fraction of the B-0 -> (D) over bar K-0(+) K- decay, used for normalisation. By combining the branching fractions of the decays B-0 -> (D) over bar ((*)0)phi and B-0 -> (D) over bar ((*)0)omega, the omega-phi mixing angle delta is constrained to be tan(2)delta = (3.6 +/- 0.7 +/- 0.4) x 10(-3), where the first uncertainty is statistical and the second systematic. An updated measurement of the branching fractions of the B-s(0) -> (D) over bar ((*)0).phi decays, which can be used to determine the CKM angle gamma, leads to B(B-s(0) -> (D) over bar (0)phi) = (2.30 +/- 0.10 +/- 0.11 +/- 0.20) x 10(-5), B(B-s(0) -> (D) over bar (*0)phi) = (3.17 +/- 0.16 +/- 0.17 +/- 0.27) x 10(-5).