Charmless baryonic B decays

The LHCb collaboration has made the first sightings of the decay of B mesons into two baryons containing no charm quarks. While the collaboration has previously reported on multibody baryonic B decays, these are its first results on the rare two-body charmless modes and will help to address open questions concerning baryon formation in B decays.

Baryonic decays of B mesons were studied extensively by the BaBar and Belle experiments at SLAC and KEK, respectively. The measured branching fractions are typically in the range 10^–6–10^–4, with charmless modes at the low end of this range and those with charm having larger branching fractions. Decays with double-charm final states have branching fractions up to 10^–3 in some cases, which is a surprisingly large value. The channel B⁺ → ppK⁺ was the first charmless baryonic B-meson decay mode to be seen, in 2002 (Belle collaboration 2002). Soon after, Belle struck gold again with the first observation of a two-body baryonic B decay, B⁰ → Λcp, which manifestly has charm (Belle collaboration 2003). However, there were no signs of charmless two-body baryonic decays of B mesons until now.

The suppression of low-multiplicity compared with higher-multiplicity decay modes is a striking feature of B decays to baryons that is not replicated by their two-body and three-body decays to mesons. It is also a key to the theoretical understanding of the dynamics behind these types of decays.

The LHCb collaboration used the 1.0 fb^–1 data sample collected in 2011 to study the proton–antiproton spectra with or without an extra light meson – a pion or a kaon. Figure 1 shows the invariant mass distribution of ppK⁺ candidates in the pK⁺ mass window 1.44–1.585 GeV/c², where a B⁺ → ppK⁺ signal is visible. The inset shows the pK⁺ invariant mass distribution near the threshold for B-signal candidates weighted to remove the non-B⁺ → ppK⁺ decay background.

The analysis reveals a clear Λ(1520) resonance, with the branching fraction for the decay chain B⁺→ pΛ(1520) → ppK⁺ measured to be close to 4 × 10^–7 (LHCb collaboration 2013a). With a statistical significance exceeding 5σ, the result constitutes the first observation of a two-body charmless baryonic B decay, B⁺ → pΛ(1520).

Figure 2 shows a fit from a related analysis, searching for B → pp decay (LHCb collaboration 2013b). An excess of B⁰ → pp candidates with respect to background expectations is observed with a statistical significance of 3.3σ, giving a measurement of the branching fraction for B⁰ → pp = (1.47^+0.71_–0.53) × 10^–8. No significant signal is observed for B⁰_s → pp but the current analysis improves the previous bound on the branching fraction by three orders of magnitude.