Charmless decays

Experimental evidence suggests that matter dominates the Universe whilst no appreciable amount of primordial antimatter is observed. Although the main ideas to explain such an asymmetry are incorporated within the Standard Model (SM) of particle physics, additional mechanisms are required to explain the large size of the observed imbalance between matter and antimatter. In particular, the search for new sources of CP violation (the charge conjugation and its combination with parity transformation)is paramount in extensions of the SM.

Studies of non-leptonic charmless multi-body decays of B mesons or beauty baryons are of great interest not only for the search for CP violation effects but also for improving the understanding of hadronic interactions.

Our group is leading amplitude analyses at LHCb (using the so-called Dalitz-plot analysis technique) of three-body decay processes involving long-lived particles in the final state. This approach provides both more sensitivity and more observables than the “quasi-two-body” approach. This methodology has recently revealed surprisingly large CP violation effects in the phase-space distribution of charged B mesons to combinations of three charged kaons or pions.

[CP asymmetries in B -> pipipi Dalitz plot]
Measurement of CP asymmetry over the phase space distribution of B±→π±π+π- decays using full Run I LHCb dataset. 

Although significant effort has been dedicated to studies of B meson decays, the b-baryon sector is still almost unexplored. One interesting possibility that has been led by our group is the search for the first observation of CP violating asymmetries in the b-baryon sector. First evidence for CP violation at the three standard deviations level has been observed in the decay mode Λ0bpπ-π+π-. Further studies of similar decay modes providing even better sensitivity are compelling. A prime example is the decay Λ0bpK*- via a Dalitz-plot analysis of Λ0bpK0π-, which is being pursued by our group. Asymmetries in this decay mode are predicted to be as large as 20 % and by exploiting the sensitivity of the Dalitz-plot approach, it could be the golden mode for a first observation of CP violation in b-baryons.

[CP asymmetries in Lambdab -> p3pi phase-space bins][CP asymmetries in Lambdab -> p3pi angular bins]
Asymmetry measurements in bins of the phase space (left) or the angles between the two planes of the decay products (right) of Λ0bpπ-π+π-.