物理科学学院 Physics

First Direct Measurement of the Lc Baryon at the Threshold in the BESIII Experiment

The UCAS group lead by Prof. Yangheng Zheng and Prof. Xiao-Rui Lyu, as a member of the BESIII collaboration, recently carried out direct measurements on the hadronic decay rates of the Lc baryon based on the threshold production in the BESIII experiment at BEPCII in Beijing. The work was published as a letter in issue 5, Vol. 116 in Physical Review Letters in 2016.

The charmed baryon L_c⁺ was first observed in 1979 by the Mark II experiment. Its decays to hadrons proceed only through the weak interaction. Hence, the L_c⁺ decay branching fractions are key probes to understand the weak interactions inside a baryon. Especially precise measurement of the L_c⁺ decays will provide important information on the final state strong interaction in the charm sector, thereby improving our understanding of Quantum Chromodynamics in the non-perturbative energy region. In addition, most excited L_c− and Σ_c−type baryons and b-flavored baryons eventually decay into a L_c⁺, and studies of these baryons are directly connected to understanding the ground state L_c⁺.

However, measurements of the ground state L_c⁺ were mostly performed by experiments more than 20 years ago. Most decay rates of the L_c⁺ are measured relative to the decay mode L_c⁺ → pK⁻π⁺. There are no completely model-independent measurements of the absolute branching fraction for this decay mode.

In 2014, the Beijing Spectrometer (BESIII) experiment accumulated a data sample of e⁺e⁻ annihilations with an integrated luminosity of 567pb⁻¹ at the center-of-mass energy of 4.599 GeV, about 26 MeV above the L_c⁺L_c⁻ pair mass threshold. At this energy, no additional hadrons accompanying the L_c⁺L_c⁻ are produced. The UCAS group measures hadronic branching fractions at the L_c⁺L_c⁻ threshold using a double tagging technique which relies on fully reconstructed L_c⁺L_c⁻ decays. This technique obviates the need for knowledge of the luminosity or the L_c⁺L_c⁻ production cross section. To improve precision, BESIII combines twelve Cabibbo-favored decay channels and implements a global least-squares fit by considering their correlations. The branching fraction of the L_c⁺ → pK⁻π⁺ is determined to be (5.84±0.27±0.23)%. This first measurement of the absolute branching fraction of L_c⁺→pK⁻π⁺ at the threshold is made after 30 years since the discovery of the L_c⁺. It has the advantage of optimal understanding of model uncertainty. In addition, measurements of the other eleven Cabibbo-favored hadronic decay modes are significantly improved.

In 2015, the UCAS group also measured the absolute branching fraction of the semi-leptonic decay L_c⁺→Λe⁺v_e, using a missing-neutrino technique based on this data set. A future larger L_c⁺ threshold sample will help to improve our understandings on the L_c⁺ properties.

Further reading

BESIII collaboration 2016, Phys. Rev. Lett. 116, 052001

BESIII collaboration 2015, Phys. Rev. Lett. 115, 221805

Written by Lv Xiaorui