Study of charm mixing and CP violation with the LHCb experiment

Lead Research Organisation: University of Oxford
Department Name: Oxford Physics

Abstract

The primary purpose of the thesis research is to perform a measurement of mixing and a search for indirect CP violation in the neutral charm meson ('D') system. This goal will be achieved through a time-dependent Dalitz plot study of multibody decays, initially focusing on the channel D->K^0_S KK. The analysis requires knowledge of the variation of the strong phase over Dalitz space, and this information will be taken from measurements performed by experiments at charm threshold (i.e. CLEO-c and BESIII). This approach will enable the study to avoid any model-dependence, and will allow the results to be interpreted in a clean and straightforward manner.

The aims of the measurement are of very high scientific importance. Charm mixing is described by two parameters, x_D and y_D, that are both still poorly known. Indeed it is not yet established whether x_D is even non zero. Indirect CP violation in charm is predicted to be very small in the Standard Model, and any observation made with current experimental precision would have a strong claim to be interpreted as a New Physics driven effect. LHCb has accumulated samples of charm decays much larger than at any previous facility, and hence the opportunities presented for analysis are unprecedented and very exciting.

There have been very few previous mixing studies performed with D0->K^0_S KK, and none on LHCb. The resonance structure of the decay is very different to that of D0->K^0_S pipi, which is itself the subject of an ongoing analysis within LHCb, making the two channels complementary in their physics reach. (Indeed the student will collaborate closely with the team working on K^0_S pipi, which will allow ideas and methods to be exchanged between the two analyses). No previous study of D0->K^0_S KK has adopted the model independent approach that will be pursued in the thesis analysis.

The possibility exists to extend the measurement programme to other D decay channels, again pursuing the model-independent approach. One good candidate is the mode D->Kpipipi, which has a rich structure of intermediate resonances, and where expertise exists in the host institute from previous studies performed at LHCb and CLEO-c.

As stated, the student will work together with a team of other physicists (from Oxford and CERN), who are already embarked on a similar analysis with a sister decay. He will also benefit from the association that Oxford has recently made with the BESIII experiment in Beijing. 'Engineering measurements' performed at BESIII will be invaluable in harnassing the full statistical power of the LHCb data set.

Finally, it is possible that the student will develop calibration, reconstruction and analysis tools suitable for analysing first data with the LHCb Upgrade. This experiment will begin data taking in 2021, and it may be feasible for the student to perform one of the first physics measurements with the early data from the Upgrade. A decision will be made late in the second year of the studentship whether this path will be pursued, of whether more studies will be made with the existing data set. This work would count as a secondary project alongside the major thesis goal, described above.

Publications

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Studentship Projects

Project Reference Relationship Related To Start End Student Name
ST/S505638/1 01/10/2018 30/09/2022
2114891 Studentship ST/S505638/1 01/10/2018 31/03/2022 Kamil Fischer