First direct measurement of the Λc baryon at the threshold by the BESIII experiment at BEPCII
The charmed baryon Λc+ was first observed in 1979 by the Mark II experiment. Its decays to hadrons proceed only through the weak interaction. Hence, the Λc+ decay branching fractions are key probes to understand the weak interactions inside a baryon. Especially precise measurement of the Λc+ 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 Λc- and Σc-type baryons and b-flavored baryons eventually decay into a Λc+, and studies of these baryons are directly connected to understanding the ground state Λc+.
However, measurements of the ground state Λc+ were mostly performed by experiments more than 20 years ago. Most decay rates of the Λc+ are measured relative to the decay mode Λc+→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 Λc+Λc- pair mass threshold. At this energy, no additional hadrons accompanying the Λc+Λc- are produced. The BESIII collaboration measures hadronic branching fractions at the Λc+Λc- threshold using a double tagging technique which relies on fully reconstructed Λc+Λc- decays. This technique obviates the need for knowledge of the luminosity or the Λc+Λc- 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 Λc+→pK-π+ is determined to be B(Λc+→pK-π+) =(5.84±0.27±0.23)%. This first measurement of the absolute branching fraction of Λc+→pK-π+ at the threshold is made after 30 years since the discovery of the Λc+. 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, BESIII also measured the absolute branching fraction of the semi-leptonic decay Λc+→Λe+ve, using a missing-neutrino technique based on this data set. A future larger Λc+ threshold sample will help to improve our understandings on the Λc+ properties.