Partial-wave analysis of the radiative decay of J/psi into p pbar

The Standard Model describes the smallest building blocks of our universe, so-called quarks and leptons, and the forces that act on them. The building blocks can bundle into larger and heavier particles. These composite particles are called hadrons. One of the best-known hadrons is the proton. The Higgs boson, which was first detected in 2012, explains how the smallest building blocks get their mass. However, the Higgs mechanism only explains 1% of the mass of the proton. The other 99% arises from internal strong forces that are not yet well-understood. To better understand the 99%, it is important to study the production of hadrons in different processes, and to obtain information from as many different decay reactions as possible. Hadrons of the charmonia group, such as J/psi and the ground state eta_c, are highly suitable particles for this, and result in a relatively easy-to-interpret spectrum of narrow peaks that do not overlap. In this thesis the radiative decay of J/psi into a proton (p) and antiproton (pbar) is studied. The aim of this study was to gain a better understanding of the full spectrum of the proton-antiproton invariant mass, and especially to gain more insight into the poorly-understood properties of the intermediate resonance eta_c. In this study, a special multidimensional analysis, a so-called partial-wave analysis, was applied for the first time to the radiative decay from J/psi to eta_c. This study is based on data taken by the Beijing Spectrometer (BES) III, which has collected world's largest dataset of J/psi reactions

Study of ψ (3686) → Λ Λ ¯ ω

Based on a data sample of (448.1±2.9)×106 ψ(3686) events collected with the BESIII detector at the BEPCII collider, the branching fraction of ψ(3686)→ΛΛ¯ω is measured to be (3.30±0.34(stat)±0.29(syst))×10-5 for the first time. In addition, the Λω (or Λ¯ω) invariant mass spectra is studied and the potential presence of excited Λ states has been investigated

B(s)→eμ: search for lepton flavour violation at LHCb

The existence of three flavors of fundamental fermions is one of the great mysteries of the standard model of particle physics. At the same time it is at the basis of a variety of exciting phenomena, such as CP violation and neutrino flavor oscillations. Hints are mounting that the assumption of lepton universality and charged lepton flavour conservation, as currently incorporated in the standard model, may not be valid. We report on our test of charged lepton flavour conservation in the decay of B mesons, as measured at LHCb

Search for a CP-odd light Higgs boson in J/ψ →γA⁰

Using J/ψ radiative decays from 9.0 billion J/ψ events collected by the BESIII detector, we search for di-muon decays of a CP-odd light Higgs boson (A0), predicted by many new physics models beyond the Standard Model, including the next-to-minimal supersymmetric Standard Model. No evidence for the CP-odd light Higgs production is found, and we set 90% confidence level upper limits on the product branching fraction B(J/ψ→γA0)×B(A0→μ+μ-) in the range of (1.2-778.0)×10-9 for 0.212≤mA0≤3.0 GeV/c2. The new measurement is a 6-7 times improvement over our previous measurement, and is also slightly better than the BABAR measurement in the low-mass region for tanβ=1

Oscillating features in the electromagnetic structure of the neutron

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

Measurement of the branching fraction of leptonic decay D⁺_s → τ⁺ν_τ via τ⁺→ π⁺π⁰¯ν_τ

By analyzing 6.32  fb−1 of e+e− annihilation data collected at the center-of-mass energies between 4.178 and 4.226 GeV with the BESIII detector, we determine the branching fraction of the leptonic decay D+s→τ+ντ, with τ+→π+π0¯ντ, to be BD+s→τ+ντ=(5.29±0.25stat±0.20syst)%. We estimate the product of the Cabibbo-Kobayashi-Maskawa matrix element |Vcs| and the D+s decay constant fD+s to be fD+s|Vcs|=(244.8±5.8stat±4.8syst)  MeV, using the known values of the τ+ and D+s masses as well as the D+s lifetime, together with our branching fraction measurement. Combining the value of |Vcs| obtained from a global fit in the standard model and fD+s from lattice quantum chromodynamics, we obtain fD+s=(251.6±5.9stat±4.9syst)  MeV and |Vcs|=0.980±0.023stat±0.019syst. Using the branching fraction of BD+s→μ+νμ=(5.35±0.21)×10−3, we obtain the ratio of the branching fractions BD+s→τ+ντ/BD+s→μ+νμ=9.89±0.71, which is consistent with the standard model prediction of lepton flavor universality

Study of the decay D⁺_s → π⁺π⁺π⁻η and observation of the W-annihilation decay D⁺_s→ a₀(980)⁺ρ⁰

The decay D+s→π+π+π−η is observed for the first time, using e+e− collision data corresponding to an integrated luminosity of 6.32  fb−1, collected by the BESIII detector at center-of-mass energies between 4.178 and 4.226 GeV. The absolute branching fraction for this decay is measured to be B(D+s→π+π+π−η)=(3.12±0.13stat±0.09syst)%. The first amplitude analysis of this decay reveals the substructures in D+s→π+π+π−η and determines the relative fractions and the phases among these substructures. The dominant intermediate process is D+s→a1(1260)+η,a1(1260)+→ρ(770)0π+ with a branching fraction of (1.73±0.14stat±0.08syst)%. We also observe the W-annihilation process D+s→a0(980)+ρ(770)0, a0(980)+→π+η with a branching fraction of (0.21±0.08stat±0.05syst)%, which is larger than the branching fractions of other measured pure W-annihilation decays by 1 order of magnitude

Measurement of J/ψ → Ξ(1530)⁻¯Ξ⁺ and evidence for the radiative decay Ξ(1530)⁻→ γΞ⁻

The SU(3)-flavor violating decay J/ψ→Ξ(1530)−¯Ξ++c.c. is studied using (1310.6±7.0)×106 J/ψ events collected with the BESIII detector at BEPCII, and the branching fraction is measured to be B(J/ψ→Ξ(1530)−¯Ξ++c.c.)=(3.17±0.02stat±0.08syst)×10−4. This result is consistent with previous measurements with an order of magnitude improved precision. The angular parameter for this decay is measured for the first time and is found to be α=−0.21±0.04stat±0.06syst. In addition, we report evidence for the radiative decay Ξ(1530)−→γΞ− with a significance of 3.9σ, including the systematic uncertainties. The 90% confidence level upper limit on the branching fraction is determined to be B(Ξ(1530)−→γΞ−)≤3.7%