CERN Accelerating science

LHCb searches for CP violation

by Monica Pepe Altarelli

The phenomenon of violation of invariance under CP, the combined action of charge conjugation C and parity P, which is related to the difference between properties of matter and antimatter, is one of the most fundamental problems in physics. CP violation arises in the Standard Model (SM) from a single complex phase in the Cabibbo Kobayashi Maskawa (CKM) matrix that describes the transition between up- and down-type quarks by the emission of a W boson. However, the strength of the effect in the SM is largely insufficient to explain the dominance of matter over antimatter in the present universe. This motivates the search for sources of CP violation beyond the SM, which is one of the main objectives of the LHCb experiment. LHCb has been performing many accurate measurements of CP-violating asymmetries in processes involving beauty- and charm-hadron decays to determine whether they are consistent with the CKM mechanism or whether new physics should be invoked to explain them.

CP violation was initially discovered in K meson decays and subsequently observed in B meson decays, but never in any baryon decays. In a recent LHCb paper [1], a search was made for CP-violating asymmetries in the decay angle distributions of Lb baryons, particles with quark content bud, exploiting their large production rate at the LHC. The processes under study are the four-body decays of the Lb  baryon to pπ-π​+π- and to π π-K+K- . These are decays mediated by the weak interactions, which proceed through two different amplitudes of similar magnitude.  CP violation could arise from the interference of these two amplitudes with relative phases that differ between particle and antiparticle decays, leading to differences in the Land L(bar) decay rates. A powerful tool for displaying CP violation in weak decays is the investigation of triple product asymmetries. Asymmetries of scalar triple products of final-state particle momenta in the Lb centre-of-mass frame were studied to search for CP-violating effects using the whole run 1 data sample, corresponding to an integrated luminosity of 3 fb-1. The results were found to be consistent with CP symmetry for the less abundant Lbaryon decaying  to π πKK-, while evidence for CP violation in Lb baryons decaying to p π-ππ-   was found with a statistical significance corresponding to 3.3 standard deviations including systematic uncertainties. This represents the first evidence for CP violation in the baryon sector.

Reconstructed invariant mass distribution for Λb0  —>p π-π+ π- decays

Moving to charm, charm hadrons provide the only sector involving up-type quarks where CP-violating effects are expected to be observable. This uniqueness makes the study of their properties particularly relevant. CP violation in charm is still unobserved and is predicted by the SM to be below 10-3, a precision that is now within the LHCb’s reach. The decays under study were those of the D0 mesons to K+K- and to p+p-. Charm mesons are produced either directly in the proton-proton collisions or in the decays of heavier beauty particles. Only the first category was used in this analysis, which made use of the charge of the pion from a parent D*+ to D0p+ decay to distinguish between D0 and D0 (bar) mesons at production.

Asymmetry as a function of the proper decay time for the D0—> K+K- decay mode in the two samples with magnetic field vertically upwards and downwards.

The difference of the D0 and D0 (bar) meson decay rates into K+K- and to p+p- pairs, which is sensitive to CP violation, was measured as a function of their decay time t using the whole run 1 data sample. An essential part of the analysis was the accurate, data-driven study of systematic effects that could produce small deviations from an ideally CP-symmetric detector acceptance and mimic the effect of CP violation. The results [2] were found to be consistent with CP conservation, improving by nearly a factor two the previous world-leading measurements, also obtained by LHCb using the 2011 subsample of the current data. These are the most precise measurements of CP violation ever made in the charm sector, and are consistent with no CP violation with a precision of a few parts in 104.




[2] LHCb-CONF-2016-009; LHCb-CONF-2016-010


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