New pentaquark searches in beauty decays

Pentaquarks, bound states of five quarks predicted in the first formulation of the quark model in 1964, have had a troubled history. Following disputed claims of the discovery of light-flavour species over 20 years ago, pentaquarks with hidden charm are now well-established members of the hadronic spectrum. The breakthrough was achieved by the LHCb experiment in 2015 with the observation of P_c⁺ states in the J/ψ p system.

The P_c⁺ quark content (uudcc) implies that decays to two open-charm hadrons, such as Λ_c⁺ D⁰ or Λ_c⁺ D*⁰, are possible. The rates of such decays are important for understanding more about the nature of the P_c⁺ states, as different models predict rates that differ by orders of magnitude. Distinguishing between the proposed mechanisms by which pentaquarks, and excited hadrons in general, are produced and bound allows a better understanding of the dynamics of the strong interaction in the non-perturbative regime.

A new analysis by LHCb of the open-charm hadrons in Λ_b decays was presented at the International Conference on Meson-Nucleon Physics and the Structure of the Nucleon, held in Mainz in October. It concerns the first observation and measurement of the branching fractions of Λ_b⁰ → Λ_c⁺ D(*)⁰ K^– and Λ_b⁰ → Λ_c⁺ D_s^*– decays using proton–proton collision data collected during LHC Run 2.

All branching fractions are measured relative to the known Λ_b⁰ → Λ_c⁺ D_s^– decay mode, which is reconstructed with the same set of six final-state hadrons: p K^– π⁺ K⁺ π^– K^–. Many systematic uncertainties in the measured ratios therefore cancel out, making the precision on the relative branching fraction of Λ_b⁰ → Λ_c⁺ D⁰ K^– statistically limited. For Λ_b⁰ → Λ_c⁺ D^0* K^– and Λ_b⁰ → Λ_c⁺ D*^– the resulting branching fractions are systematically limited. This is because either a photon or neutral pion is not reconstructed, so their shape in the invariant mass spectrum of the reconstructed particles is more difficult to describe and more affected by the backgrounds (see figure 1, where the components with a missing photon for which a branching fraction is calculated are shown in orange and those with a missing neutral pion in green).

The partially reconstructed Λ_b⁰ → Λ_c⁺ D_s^*– decay cannot be used directly to search for pentaquarks, but it is an important input to model calculations. In addition, as a two-body decay, it is a powerful test of factorisation assumptions in heavy-quark effective theory.

In the Λ_b⁰ → Λ_c⁺ D(*)⁰  K^– decay, the production process of the P_c⁺ pentaquarks is the same as in the discovery channel, Λ_b⁰ → J/ψ p K^–. A comparison between the measured branching fractions and observed signal yields can thus be used to estimate the expected sensitivity for observing P_c⁺ signals in the open-charm channels. In particular, the rate of a Λ_b⁰ decay to Λ_c⁺ D⁰ K^– is about six times greater than to J/ψ p K^–; however, more than 60 times as much data would be needed to match the currently available Λ_b⁰ → J/ψ p K^– signal yield.

A factor of about 24 in this calculation comes from the branching fractions ratio of J/ψ and open-charm hadrons, given their reconstructed decay modes. The rest is from reconstruction and selection inefficiencies, which favour the four-prong μ⁺μ^– p K^– over the fully hadronic six-body final state. With the upgraded Run 3 detector and now triggerless detector readout, a large part of the inefficiency for fully hadronic final states is recoverable, making pentaquark searches in double open-charm final states more favourable compared to the situation in Run 2.