BESIII makes first direct measurement of the Λ_c at threshold

The charmed baryon, Λ_c, 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 Λ⁺ _c→ pΚ^–π⁺ 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 Λ_c and Σ_c 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, Λ⁺ _c→ pΚ^–π⁺, 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⁺_cΛ^–_c, so no additional hadrons accompanying the Λ⁺_cΛ^–_c are produced.

The BESIII collaboration measures hadronic branching fractions at the Λ⁺_cΛ^–_c threshold using a double-tagging technique that relies on fully reconstructed Λ⁺_cΛ^–_c decays. This technique obviates the need for knowledge of the luminosity or the Λ⁺_cΛ^–_c 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 → pΚ^–π⁺ of B(Λ⁺_c → pΚ^–π⁺) = (5.84±0.27±0.23)%.

This is the first measurement of the absolute branching fraction of the decay Λ⁺ _c→ pΚ^–π⁺ 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 Λ_c⁺ → Λe⁺ν_e, using a missing-neutrino technique. In future, a larger Λ_c⁺ threshold sample will help to improve further understanding of the properties of the Λ_c⁺.