Task 3.2 - Spectroscopy of low-lying scalars, strange mesons and strangeonia

 

Coordinator: Susan Schadmand

Contributors: Jose R. Peláez , Alessandra Filippi, Ulrike Thoma , Sebastian Neubert

 

 

General description:

 

The task T3.2 is dedicated to spectroscopy of low-lying scalars, strange mesons and strangeonia. A new generation of dedicated experiments in hadron physics has started with the aim of uncovering properties of strong interactions and specifically the mysteries of confinement.

ELSA, MAMI, CERN, and Jefferson Lab are providing precise and abundant data on poorly known and understood scalar and strange-rich mesons. Complementary information from experiments that make use of nuclear target can be used to constrain the sophisticated PWA in meson spectroscopy. In the recent months,

 

• high-precision data have been accumulated for the study of glueballs and hybrids
• analysis frameworks have been used novel techniques like machine learning for particle identification and large scale computing
• data preservation has been going along with partial wave analyses which provides synergy with theory
• data mining and new analyses have been performed in a world-wide collaborative effort, involving the main EU and abroad
• further experiment proposals have been approved, In particular, the approval of the proposal for a KLong beam facility (KLF), hosted by HallD at Jefferson Lab, is a significant milestone. A substantial part of its scientific program foresees an extensive study of strange baryonic excitations and of mesons with strange content, complementary to and exceeding present experimental investigations.

 

 

Progress [1-18 months]:

 

In the last 18 months there was a significant progress in spectroscopy of low-lying scalars and strange mesons.

Approval of a KLong beam Facility (KLF), hosted by HallD at Jefferson Lab. A substantial part of its scientific program foresees an extensive study of strange baryonic excitations and of mesons with strange content. Among the latter, focus will be put on the search and characterization of the κ/K*(700) scalar, in its overlap with the σ/f₀(500) and the f₀(980), and of other strange excited resonances belonging to heavier nonets and decaying sizably in Kπ, about which the available information is still considerably scarce. In this respect, the KLF program will be thoroughly complementary to the investigations by CLAS12.

The CLAS6 experiment at Jefferson Lab collected a sizable amount of high resolution data with a real photon beam on a proton target at beam energies up to 5 GeV. The data were used for first exploratory searches for light mesons production with open and hidden strangeness, selecting channels with kaons in the final state. These investigations are deeply intertwined with the search of the lightest exotics of the meson spectrum (glueballs and hybrids). Identification and disentanglement of the states call for large statistics and sophisticated analysis methods. Similar analyses are being carried out with the data collected by the upgraded CLAS12 experiment. Important first outcomes about scalars and their features will be provided by the study of the π⁺π^-, K⁺K^- and K⁰_SK⁰_S decay channels through partial wave analyses.

The primary goal of the GlueX Experiment at Jefferson Lab is to identify and study the spectrum of light quark hybrid mesons through photoproduction. A large and unique data set with great prospect for establishing the liqht-quark hybrid meson is being acquired. The current hybrid searches are focused on several "golden channels" with sufficiently large cross sections and clean signals , including ηπ,η'π,ηππ,ωππ and K^?K. The current focus is on the development and validation of the reaction models needed to reproduce the amplitude. The models are developed in collaboration with several groups of theorists, including the The Joint Physics Analysis Center (JPAC).

A multi-channel analysis of existing data is being started by the BnGa-PWA group where older published data (GAMS, CERN-Munich, Crystal Barrel pbar N, and some BES-data) are used. Presently, there are mainly results on baryons.

ALICE has completed a measurement of f₀(980) spectra in different multiplicity class in pp collisions at 13 TeV. f₀(980) is identified by its decay π⁺π^-. ^.The Catania group has started to work on the identification of hadronic resonances using machine learning techniques. To test procedures they start with a well-known resonance like K*(892)⁰ detected via its main Kπ decay where simulated data of PbPb collisions at sqrt(s_NN) = 5.02 TeV are used. Next comes the improvement of the identification of the Omega(2012) baryon by machine learning techniques. This excited state of the Omega has already been observed with ALICE in pp collisions at 13 TeV via its Ξ⁰K^- and Ξ^- K⁰ decay channels.

From the theory side, a dispersive model-independent analysis of pion-kaon and pion-pion->kaon-antikaon partial-wave scattering data in the low energy region below 1.6 GeV, has been provided by means of coupled forward, fixed-t and hyperbolic dispersion relations. The respective group has delivered precise and ready-to-use partial-wave parameterizations for further final-state interaction studies, as well as a  model-independent determination of the lightest strange meson, the controversial kappa or K*0(700), which should remove its "Needs Confirmation" status in the Review of Particle Physics. Several theoreticians in HadSpec have also helped in writing the KLong beam Facility (KLF) proposal to measure K-pi scattering at low energies and a more precise determination of the kappa.

 

K^*₀(700) pole positions. Selected from the Review of Particle Physics. We also show our results using Roy-Steiner equations, using as input our UFD or CFD parametrizations. Red and blue points use for F− a once subtracted or an unsubtracted dispersion relation, respectively. This illustrates how unstable pole determinations are when using simple fits to data. Only once Roy-Steiner equations are imposed as a constraint (CFD), both pole determinations fall on top of each other.

 

 

Highlights of significant results:

A new generation of dedicated experiments in hadron physics has started with the aim of uncovering properties of strong interactions and specifically the mysteries of confinement. Reported above are

• High-precision data are being accumulated at Jefferson Lab for the study of glueballs and hybrids.

• Analysis frameworks are using novel techniques like machine learning for particle identification and large scale computing.

• Data preservation goes along with partial wave analyses which provides synergy with theory.

• Data mining and new analyses are performed in a world-wide collaborative effort, involving the main EU laboratories (CERN, Mainz, Bonn, GSI) and abroad (TJNAF/US, BESIII/China, J-PARC/Japan, Belle/Japan).

The approval of the proposal a KLong beam Facility (KLF), hosted by HallD at Jefferson Lab, is a significant milestone. A substantial part of its scientific program foresees an extensive study of strange baryonic excitations and of mesons with strange content. Among the latter, focus will be put on the search and characterization of the κ/K*(700) scalar, in its overlap with the σ/f₀(500) and the f₀(980), and of other strange excited resonances belonging to heavier nonets and decaying sizably in Kπ, about which the available information is still considerably scarce. In this respect, the KLF program will be thoroughly complementary to the investigations by CLAS12.