Rare hyperon-decay anomaly under the spotlight

The LHCb collaboration has shed light on a long-standing anomaly in the very rare hyperon decay Σ⁺ → pµ⁺µ^– first observed in 2005 by Fermilab’s HyperCP experiment. The HyperCP team found that the branching fraction for this process is consistent with Standard Model (SM) predictions, but that the three signal events observed exhibited an interesting feature: all muon pairs had invariant masses very close to each other, instead of following a scattered distribution.

This suggested the existence of a new light particle, X⁰, with a mass of about 214 MeV/c², which would be produced in the Σ⁺ decay along with the proton and would decay subsequently to two muons. Although this particle has been long sought in various other decays and at several experiments, no experiment other than HyperCP has so far been able to perform searches using the same Σ⁺ decay mode.

The large rate of hyperon production in proton–proton collisions at the LHC has recently allowed the LHCb collaboration to search for the Σ⁺ → pµ⁺µ^– decay. Given the modest transverse momentum of the final-state particles, the probability that such a decay is able to pass the LHCb trigger requirements is very small. Consequently, events where the trigger is activated by particles produced in the collisions other than those in the decay under study are also employed.

This search was performed using the full Run 1 dataset, corresponding to an integrated luminosity of 3 fb^–1 and about 10¹⁴ Σ⁺ hyperons. An excess of about 13 signal events is found with respect to the background-only expectation, with a significance of four standard deviations. The dimuon invariant- mass distribution of these events was examined and found to be consistent with the SM expectation, with no evidence of a cluster around 214  eV/c². The signal yield was converted to a branching fraction of (2.1^+1.6_–1.2) × 10^–8 using the known Σ⁺ → pπ⁰ decay as a normalisation channel, in excellent agreement with the SM prediction. When restricting the sample explicitly to the case of a decay with the putative X⁰ particle as an intermediate state, no excess was found. This sets an upper limit on the branching fraction at 9.5 × 10^–9 at 90% CL, to be compared with the HyperCP result (3.1^+2.4_–1.9 ± 1.5) × 10^–8.

This result, together with the recent search for the rare decay K_S → μ⁺μ^– shows the potential of LHCb in performing challenging measurements with strange hadrons. As with a number of results in other areas reported recently, LHCb is demonstrating its power not only as a b-physics experiment but as a general-purpose one in the forward region. With current data, and in particular with the upgraded detector thanks to the software trigger from Run 3 onwards, LHCb will be the dominant experiment for the study of both hyperons and K_S mesons, exploiting their rare decays to provide a new perspective in the quest for physics beyond the SM.