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Search for New Physics in Charm at LHCb

Final Report Summary - CHARM@LHCB (Search for New Physics in Charm at LHCb)

The goal of the project was to search for phenomena that cannot be described by the Standard Model of elementary particles (SM). The hunt for these effects, commonly known as New Physics (NP), is motivated by the fact that the SM, although widely successful in describing current experimental data, fails to give answers to the fundamental questions of structure of the Universe. Open questions include the discrepancy in strength between gravity and the other interactions, the nature of dark matter, thought to constitute about a quarter of the universe, and nature’s apparent preference for matter over antimatter, without which we would not exist. . A powerful approach to probe for New Physics is to perform ‘indirect searches’, in which precise measurements are made of processes that can be reliably calculated within the SM. Any discrepancy between measurement and prediction would then indicate the contribution of new particles or interactions.

One important class of indirect search is to test the predictions of the Cabibbo-Kobayashi-Maskawa (CKM) matrix, which governs the decay of hadrons by the weak interaction, and CP violation (the difference in behaviour between matter and antimatter). Such measurements are the principal focus of the LHCb experiment. The project focuses on CKM-related measurements involving charm hadrons, where the experimental sensitivity has been until recently rather limited. The large charm samples that can be collected at LHCb allow for precise studies of ‘D0 mixing’ (the oscillation of a neutral charm meson into its antiparticle), and to search for CP violation, a phenomenon that has yet to be observed in the charm sector. The project focused on the measurement of charm mixing and the search for CP violation in D0->K0S h+ h- decays. Central to the analysis was an innovative approach to describe the 3-body phase space of the D0 decay which made use of measurements available from the CLEO experiment parameters. Another critical feature of the study was the use of data-driven methods to assess the detector acceptance for the decay time of the meson, thereby avoiding assumptions taken from simulation studies. All elements of this challenging and important analysis are in place, and a publication based on the run-1 LHC data set will soon appear. The indications are that the results will be of world-leading sensitivity. Furthermore, the techniques developed for the study are applicable to many other multi-body D0 decays.

In parallel to the experimental studies, the project has encompassed a strong phenomenological component. Here the global analysis of D0 mixing results has been updated to include the latest experimental input. The new analysis interprets the current experimental results in terms of the underlying SM parameters, which is valuable for comparing the status of the charm sector with the better understood beauty hadron system. This analysis was then incorporated into a global flavour fit within the context of the SM, and a study of generic NP scenarios. It was concluded that the data allow for the existence of NP particles that could be directly produced and observed at the LHC.