# BESIII makes first direct measurement of the Λc at threshold

18 March 2016

The charmed baryon, Λc, was first observed at Fermilab in 1976. Now, 40 years later, the Beijing Spectrometer (BESIII) experiment at the Beijing Electron–Positron Collider II (BEPCII) has measured the absolute branching fraction of Λc→ pΚπ+ at threshold for the first time.

Because the decays of the Λ+c to hadrons proceed only through the weak interaction, their branching fractions are key probes for understanding weak interactions inside of a baryon. In particular, precise measurements of the decays of the Λ+c will provide important information on the final-state strong interaction in the charm sector, thereby improving the understanding of quantum chromodynamics in the non-perturbative energy region. In addition, because most of the excited baryons of the Λc and Σc types, as well as the b-flavoured baryons, eventually decay into a Λ+c, studies of these baryons are directly connected to understanding the ground state, Λ+c.

Most decay rates of the Λ+c are measured relative to the decay mode, Λc→ pΚπ+, but there are no completely model-independent measurements of the absolute branching fraction for this decay mode. Moreover, most measurements of the ground-state Λ+c were made more than 20 years ago.

In 2014, BESIII accumulated a data sample of e+e annihilations with an integrated luminosity of 567 pb–1 at a centre-of-mass energy of 4.599 GeV. This is about 26 MeV above the mass threshold for a Λ+cΛc, so no additional hadrons accompanying the Λ+cΛc are produced.

The BESIII collaboration measures hadronic branching fractions at the Λ+cΛc threshold using a double-tagging technique that relies on fully reconstructed Λ+cΛc decays. This technique obviates the need for knowledge of the luminosity or the Λ+cΛc production cross-section. To improve precision, BESIII combines 12 Cabibbo-favoured decay channels and implements a global least-squares fit by considering their correlations. This leads to a result for the branching fraction for Λ+c → pΚπ+ of B(Λ+c → pΚπ+) = (5.84±0.27±0.23)%.

This is the first measurement of the absolute branching fraction of the decay Λc→ pΚπ+ at threshold, and it has the advantage of incorporating an optimal understanding of model uncertainty. In addition, BESIII has made significantly improved measurements of the other 11 Cabibbo-favoured hadronic-decay modes.

In 2015, based on the same data set, BESIII also measured the absolute branching fraction of the semi-leptonic decay Λc→ Λe+νe, using a missing-neutrino technique. In future, a larger Λc+ threshold sample will help to improve further understanding of the properties of the Λc+.