Final Report Summary - PRECISIONFLAVOUR (Particle Physics beyond the Energy Frontier)
The project aims to use precision measurements in the decays B and D mesons (particle containing beauty and charm quarks, respectively) to find physics beyond the Standard Model of particle physics (SM). We are in particular interested in the origin of the asymmetry between matter and antimatter (CP violation). Our approach is based on the detailed study of multibody decays. The multibody decays that we study have unique sensitivity to quantum mechanical phases. These include CP-violating phases, in particular the phase gamma, which is of central importance to our undersanding of CP violation. We use the most precise way to measure gamma, which is based on the decay B->DK, where the D is unstable and decays further. We are particularly interested in four body decays of the D, for example D to four pions (D->4pi), a kaon and three pions (D->K3pi) and decays to two kaons, two pions (D->KKpipi). The CP violating phase gamma that originates in the B->DK decay is "imprinted" on kinematics of the subsequent D decay. Analysing the D decay carefully provides the most powerful method to measure gamma. Understanding the D decay, in particular the (not CP-violating) quantum mechanical phases involved in it, is a prerequisite to measuring gamma. In the past, these phases, which originate from the strong interaction, would be inferred from models. Such models are however unreliable due to the non-perturbative nature of the strong interaction. Our approach removes this leading source of uncertainty by measuring the relevant phases in the D decay directly. We do so by studying D mesons in quantum correlated D-Dbar pairs originating from psi(3770) decays at CLEO-c, and by using a new method we developed in the course of this project which is based on D mixing (a phenomenon that allows a D meson to change into its own antiparticle). Also, D decays themselves provide interesting physics: their amplitude structure is interesting for our understanding of the strong interacton; and there is great interest in studying CP violation in D decays. Another one of our innovations is the analysis of four body D decays in this context (such as D->4pi) rather than more conventional analyses limited to three body decyas (such as D->Kºpipi). Including four body decays increases the potential precision on gamma, but it adds many complications and is very challenging.
Our measurements are carried out at the LHCb experiment at CERN (which provides vast clean data sets of B decays and D decays), and the CLEO-c experiment at Cornell University; CLEO-c data gave us access to unique quantum-correlated D-Dbar pairs, where the decay of one D meson reveals the state of the other - something we were able to exploit to measure quantum mechanical phases in the decay of these D mesons.
During the course of this project, we performed the first measurement that searched for CP violation in the four-body phase space of a four body D decay (or any other four body decay) in a model-independent way. We developed sophisticated ampltitude models for four body D decays and probed for CP violation in D->4pi and D->KKpipi decays at CLEO-c. We made model-independent measurements of the relevant strong-interaction phases in D->4pi and D->K3pi. We were the first to realise that the newly discovered phenomenon of D mixing could be used to measure these strong phases; D mixing is a process that allows a D meson to turn into its own antiparticle. We published our new methods in two phenomenology papers and subsequently measured the phases in D->K3pi more precisely than ever before, using our new method. We also found that our new method, when applied to datasets becoming available in the very near future, could lead to the most precise gamma measurement in an individual decay mode. We also published the first observation of D mixing in D->K3pi, and the 2nd observation of D mixing in any given decay mode. Our results provided important input to the world's most precise gamma measurement, which is based on a combination of our input with other results.
While the improvement in the measurement of gamma over the course of this project (from an uncertainty of about 15º to about 6º) is impressive, it is still nowhere near the sub-1º level the community aims for to probe the description of CP violation in the SM. Currently, LHCb is being upgraded to provide even vaster data sets; as statistical uncertainties go down with increasing datasets, the control of systematic uncertainties becomes ever more important, and the impact of our new methods, which remove key systemtic uncertainties, is set to increase further in future gamma measurements.
We established a close collaboration with theorists in order to make progress on amplitude models. The PI (co-)organised LHCb/theory workshops at CERN and at CBPF in Rio. Our model of co-operation between CLEO-c and LHCb inspired the emerging co-operation between BES III and LHCb; the PI is currently organising a joint BES III - LHCb meeting on the topic of amplitude analyses.
Our measurements are carried out at the LHCb experiment at CERN (which provides vast clean data sets of B decays and D decays), and the CLEO-c experiment at Cornell University; CLEO-c data gave us access to unique quantum-correlated D-Dbar pairs, where the decay of one D meson reveals the state of the other - something we were able to exploit to measure quantum mechanical phases in the decay of these D mesons.
During the course of this project, we performed the first measurement that searched for CP violation in the four-body phase space of a four body D decay (or any other four body decay) in a model-independent way. We developed sophisticated ampltitude models for four body D decays and probed for CP violation in D->4pi and D->KKpipi decays at CLEO-c. We made model-independent measurements of the relevant strong-interaction phases in D->4pi and D->K3pi. We were the first to realise that the newly discovered phenomenon of D mixing could be used to measure these strong phases; D mixing is a process that allows a D meson to turn into its own antiparticle. We published our new methods in two phenomenology papers and subsequently measured the phases in D->K3pi more precisely than ever before, using our new method. We also found that our new method, when applied to datasets becoming available in the very near future, could lead to the most precise gamma measurement in an individual decay mode. We also published the first observation of D mixing in D->K3pi, and the 2nd observation of D mixing in any given decay mode. Our results provided important input to the world's most precise gamma measurement, which is based on a combination of our input with other results.
While the improvement in the measurement of gamma over the course of this project (from an uncertainty of about 15º to about 6º) is impressive, it is still nowhere near the sub-1º level the community aims for to probe the description of CP violation in the SM. Currently, LHCb is being upgraded to provide even vaster data sets; as statistical uncertainties go down with increasing datasets, the control of systematic uncertainties becomes ever more important, and the impact of our new methods, which remove key systemtic uncertainties, is set to increase further in future gamma measurements.
We established a close collaboration with theorists in order to make progress on amplitude models. The PI (co-)organised LHCb/theory workshops at CERN and at CBPF in Rio. Our model of co-operation between CLEO-c and LHCb inspired the emerging co-operation between BES III and LHCb; the PI is currently organising a joint BES III - LHCb meeting on the topic of amplitude analyses.