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Understanding matter-antimatter asymmetries in B meson decays to three particles

Periodic Reporting for period 1 - DalitzCPviolation (Understanding matter-antimatter asymmetries in B meson decays to three particles)

Reporting period: 2016-10-01 to 2018-09-30

In fundamental particle physics, CP violation is a violation of CP symmetry (charge conjugation parity symmetry). The phenomenon of CP violation arises within the Standard Model (SM) of particle physics through the Cabibbo-Kobayashi-Maskawa (CKM) mechanism, which is highly successful in describing the experimental data within the current precision. However, extra sources of CP violation are needed to understand the matter-antimatter asymmetry of the Universe.

The study of charmless decays of B mesons, i.e. decays to final states that do not contain charm or charmonia hadrons, is a powerful approach to search for new sources of CP violation beyond the SM. Three-body decays are of particular interest as they offer a unique environment to study the variation of the CP violation over the phase space. The phase space of such a decay is described by a Dalitz plot, the two-dimensions of which are given by the invariant masses squared of two of the three final state particles (e.g. m2(AB) vs. m2(AC) for the decay M ->ABC). The Dalitz-plot analysis technique, in which the distribution of decays across the phase space is described by a model of the hadronic dynamics, provides full information about the relative phases of decay amplitudes. The LHCb experiment has collected unprecedented samples of three-body charmless B meson decays, which allows them to be studied with the precision needed to test the SM. Four different decays are studied in the project: B+ -> pi+ pi+ pi-, B+ -> K+ pi+ pi-, B+ -> pi+K+K- and B+ -> K+K+K-.
Studies on the charmless three-body B decays are performed following closely the proposal. Within the framework, the branching fraction measurements of the four charmless B decays and the Dalitz plot analyses of the decay channels B+ -> pi+pi+pi- and B+ -> K+pi+pi- are explored. The Dalitz-plot analysis software Laura++ is updated together with the development of the Dalitz plot analyses. In addition, a complementary system, B(s)0 -> D0K+K-, used as standard model candle, is also studied. The main results achieved so far are summarized in details below:
1. Branching fraction measurements of the four charmless B decays
An internal note of the work (deliverable 1.1) is prepared and is now within the LHCb review process. The study aims at a peer-review publication (additional deliverable).
2. Dalitz plot analysis of B+ -> pi+pi+pi-
Due to complicated strong dynamics in pi+pi- S-waves, the Dalitz plot analysis is performed by three different working groups, using different approaches to describe it, namely the Isobar model by the LHCb colleagues from Centre Brasileiro de Pesquisas Fisicas, the quasi-model-independent approach by the LHCb colleagues from University of Bristol and the K-matrix approach by the beneficiary. An internal note (deliverable 2.1) is delivered for the LHCb review process. The results from the three approaches will merge into two publications (deliverable 2.2 + additional deliverable).
3. Dalitz plot analysis of B+ -> K+pi+pi-
The Dalitz plot analysis is performed in collaboration with the LHCb colleagues from centre Brasileiro de Pesquisas Fisicas. The plan of the beneficiary is to use K-matrix approach to describe pi+pi- S-wave as is done in the work package 2 for B+ -> pi+pi+pi-, while K+pi- S-wave will be modeled by a parameterization from LASS experiment. An internal note (deliverable 3.1) is in preparation. The work will be continued after the termination of the project.
4. Laura++ implementation
The Laura++ package is updated during the Dalitz plot analyses of the proposed B decays by the beneficiary (deliverable 6.1). The document describing the principle and usage of Laura++ is published in Computer Physics Communications (additional deliverable).
5. Branching fraction measurements of B(s)->D0KK
The branching fractions of B(s) -> D0 K+K- decays are measured where the decay Bs -> D0 K+K- is observed for the first time. Studies on B(s) -> D(*)0phi system is also performed and a first observation of Bs -> D*phi and improved upper limits of B0 -> D0phi are achieved. The analysis is currently at the end of the LHCb review process and two publications (extra deliverables) will be submitted imminently.
The project is terminated early due to unexpected constraint and securing a permanent position of the researcher, but the ongoing work packages will be continued after the official termination of the project. Expected publications are described in the previous sections. The project has achieved most of its objectives and milestones for the period, with relatively minor deviations.

Besides scientific impact, the researcher has participated in the national-wide events to advertise project related physics. For example, he was a member of the “Anti-matter matters” team of the “Big Bang Fair 2017” in Birmingham. The exhibition has been well attended by public audience.
Discovery of Bs->D0KK and Bs->D*phi decays, to be used for CPV measurements