Hyperon Physics at BESIII

Search for CP violation in hyperon decays

Radiative corrections to semileptonic decays of charged hyperons

Flavor changing weak decays of hyperons and hypernuclei are reviewed in the context of possible experiments for the new high intensity proton facilities discussed at PILAC, KAON, AGS, etc. Both semileptonic and mesonic decays of hyperons are reviewed in the context of the Δ S=Δ Q, Δ S<2, and ΔI=1/2 rules. The nonmesonic hyperon decay modes available for study in hypernuclei are described, together with a comparison of recent data and calculations. The physics motivation for some specific experiments at these high flux facilities is discussed.

In this report the weak nonleptonic decays of kaons and hyperons are examined with the hope of gaining insight into a recently proposed mechanism for the ΔI = 1/2 rule. The effective Hamiltonian for ΔS = 1 weak nonleptonic decays and that for $K^{o} - \overline{K}^{o}$ mixing are calculated in the six-quark model using the leading logarithmic approximation. These are used to examine the CP violation parameters of the kaon system. It is found that if Penguin-type diagrams make important contributions to K→ππ decay amplitudes then upcoming experiments may be able to distinguish the six-quark model for CP violation from the superweak model. The weak radiative decays of hyperons are discussed with an emphasis on what they can teach us about hyperon nonleptonic decays and the ΔI = 1/2 rule.

BESIII is designed to study physics in the τ-charm energy region utilizing the high luminosity BEPCII. For collision physics experiments like the BESIII experiment, particle identification (PID) is one of the most important and commonly used tools for physics analysis. The effective µ/π identification performance is of great significance for most of BESIII physics analysis. However, due to the close masses of these two particles, as well as the intrinsic correlation between multiple detector information, traditional methods at BESIII is facing challenges in µ/π identification. In recent decades, machine learning (ML) techniques have been rapidly developed and have shown successful applications in HEP experiments. The PID based on ML provides powerful capability of combining more detection information from all sub-detectors with the data-driven approach. In this article, targeting at the µ/π identification problem at the BESIII experiment, we have developed a new PID algorithm based on the gradient boosted decision tree (BDT) model. Preliminary results show that the XGBoost classifier provides obviously higher discrimination power than traditional methods. In addition, based on the substantial amount of high-quality data taken by the BESIII detector, a method of evaluating and suppressing the systematical error of the ML model is also introduced, which is critical for applying the model to physics studies.

HyperCP: A high-rate spectrometer for the study of charged hyperon and kaon decays

Particle-identification techniques and performance at LHCb in Run 2

In this paper, we explicate a method of quantum hydrodynamics (QHD) for the study of the quantum evolution of a system of polarized particles. Although we focused primarily on the two-dimensional (2D) physical systems, the method is valid for three-dimensional (3D) and one-dimensional (1D) systems too. The presented method is based upon the Schr\"odinger equation. Fundamental QHD equations for charged and neutral particles were derived from the many-particle microscopic Schr\"odinger equation. The fact that particles possess the electric dipole moment (EDM) was taken into account. The explicated QHD approach was used to study dispersion characteristics of various physical systems. We analyzed dispersion of waves in a two-dimensional ion and hole gas placed into an external electric field, which is orthogonal to the gas plane. Elementary excitations in a system of neutral polarized particles were studied for 1D, 2D, and 3D cases. The polarization dynamics in systems of both neutral and charged particles is shown to cause formation of a new type of waves as well as changes in the dispersion characteristics of already known waves. We also analyzed wave dispersion in 2D exciton systems, in 2D electron-ion plasma, and in 2D electron-hole plasma. Generation of waves in 3D-system neutral particles with EDM by means of the beam of electrons and neutral polarized particles is investigated.

A comparison is made of Cabibbo theory to the experimental data on semileptonic hyperon decays. With a confidence level of 89%, the results are that the Cabibbo angle $\ensuremath{\theta}$ is 0.206\ifmmode\pm\else\textpm\fi{}0.009, the weak $\frac{D}{F}$ ratio $\ensuremath{\alpha}$ is 0.662\ifmmode\pm\else\textpm\fi{}0.018, and the strangeness-changing axial-vector renormalization $\ensuremath{\xi}$ is 1.42\ifmmode\pm\else\textpm\fi{}0.10.

Development of DIRC counters for the PANDA experiment at FAIR

Some unsatisfactory features of the GIM model are noted and SU(3) u, d, c symmetry is employed to discuss weak hadronic and radiative decays. Comparing the NN pi weak vertex in the u, d, s and u, d, c quark sectors, we estimate the decay rates and asymmetry parameters of some charmed baryons.

Parity violating radiative weak decays and the quark model

We explore the strangeness-changing decays of the lightest hyperons into another baryon plus missing energy within and beyond the standard model (SM). In the SM these processes arise from the loop-induced quark transition sto dnu overline{nu} and their branching fractions are estimated to be less than 10−11. In the presence of new physics (NP) the rates of these hyperon decays with missing energy could increase significantly with respect to the SM expectations because of modifications to the SM process or contributions from additional modes with new invisible particles. Adopting a model-independent approach and taking into account constraints from the kaon sector, we find that the current data on Kto pi nu overline{nu} do not permit sizable NP impact on the hyperon decays via underlying operators having mainly parity-even quark parts. In contrast, NP operators with primarily parity-odd quark parts are much less restricted by the existing bounds on K → invisible and Kto pi pi nu overline{nu} and consequently could produce substantially amplifying effects on the hyperon modes. Their NP-enhanced branching fractions could reach levels potentially observable in the ongoing BESIII experiment.

Hyperons constitute a unique diagnostic tool to shed light on various unresolved puzzles in contemporary physics. Prominent examples are the matter–antimatter asymmetry of the universe and how the strong interaction confines quarks into hadrons. The weak, parity violating decay of hyperons make their spin properties experimentally accessible. This can be exploited e.g. in searches for CP violation and in the decomposition of the inner, electromagnetic structure of hyperons. The BESIII experiment at BEPC-II, Beijing, China is excellently suited for hyperon physics. Recently collected large data samples have been analysed and a plethora of new results have emerged. In these proceedings, we discuss the virtues of polarised and entangled hyperons, and present a collection of recent results from BESIII.

Eighty-four interactions of protons and neutrons were located in a 22 liter stack of nuclear emulsion by tracing back showers of minimum-ionizing particles to their origins. The distribution of the number of shower particles, and the number of heavily ionizing prongs are presented for 57 events with dip angles 17\ifmmode^\circ\else\textdegree\fi{}. The average energy of these events is 3.5\ifmmode\times\else\texttimes\fi{}${10}^{12}$ ev. The average number of shower particles emitted in nucleon-nucleon collisions at this energy is 15\ifmmode\pm\else\textpm\fi{}5, as estimated from 8 interactions without heavy prongs. The angular distributions of the shower particles are presented for the 57 events. They can be transformed into a system in which the angular distribution is roughly symmetric. This is true even for the collisions with heavy target nuclei (${N}_{h}5$). The degree of anisotropy of the angular distributions is in disagreement with a hydrodynamical model of nucleon-nucleus collisions. A lower limit for the collision mean free path of the primary particles of 20 cm in emulsion was obtained. By scanning the forward cone of the primary interactions, 76 secondary interactions of charged and neutral shower particles were found. The distribution of the prong numbers, of the energy, and the characteristics of their angular distribution are presented. The best estimate of the ratio of secondary collisions produced by neutral particles, and the number produced by charged particles is: $\frac{{N}_{n}}{{N}_{\mathrm{ch}}}=0.40\ifmmode\pm\else\textpm\fi{}0.11$. Adding this result to other published data, it is concluded that 30\ifmmode\pm\else\textpm\fi{}6% of the particles produced in collisions having a primary energy of several Tev are not $\ensuremath{\pi}$ mesons. A collision mean free path of 41\ifmmode\pm\else\textpm\fi{}8 cm was found for the forwardcone shower particles.