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BSMWLHCB Report Summary

Project ID: 239999
Funded under: FP7-IDEAS-ERC
Country: United Kingdom

Final Report Summary - BSMWLHCB (Advanced techniques to Search for Physics Beyond the Standard Model with the LHCb Detector at CERN)

The origin of the asymmetry between matter and antimatter in the Universe is one of the great mysteries in scientific research today. The cosmological asymmetry must be related to a violation of the so-called CP symmetry between particles and antiparticles that is a part of our theory of fundamental interactions. The standard model of particle physics allows CP violation, but the size of the asymmetry that can be produced by standard model physics is far too small to explain what is observed in the Universe. Hence, additional sources of asymmetry must exist in nature.

Since the discovery of CP violation in 1964, physicists have been striving to better understand this curious behaviour. Important progress was achieved as experiments obtained sufficient samples of particles known as B hadrons (which contain the b quark or antiquark) to observe the large CP violation effects that had been predicted by Kobayashi and Maskawa in the context of the standard model. This achievement led to Kobayashi and Maskawa being awarded the Nobel prize of 2008. Still, however, the connection between CP violation and the matter-antimatter asymmetry of the Universe is not understood.

The aim of this project was to advance the field of CP violation studies by using the so-called Dalitz plot analysis technique. A Dalitz plot is a visualisation of the phase space for the decay of a particle into three final-state particles. Such a decay can often proceed via one of several intermediate states known as resonances, which appear as bands on the Dalitz plot. When these bands overlap, the quantum mechanical amplitudes corresponding to the resonances can interfere, resulting in distinctive patterns of overpopulated or underpopulated regions in the phase space. By studying differences in Dalitz plot interference patterns between particle and antiparticle decays, it is possible to increase the sensitivity to CP violation effects that can occur in such decays.

Working within the context of the LHCb collaboration, collecting data with the LHCb detector at CERN's Large Hadron Collider and developing sophisticated techniques with which to analyse it, the goals of the project have been achieved. The team of the PI, two postdoctoral research associates, and a total of seven Ph.D. students (including some part-funded by the University of Warwick), have obtained a large number of important new results from analysis of multibody B hadron decays. These include not only results on CP violation, which demonstrate the improved sensitivity that can be obtained with the amplitude analysis technique, but also observations of new particles that appear as resonant bands in the Dalitz plots. For example, one important outcome was the first observation of a spin-3 charmed particle, which was the first spin-3 state seen to be produced in B hadron decay. The methods developed for the project have also been used by other members of the LHCb collaboration, with results including the first observation of a pentaquark state.

The results of the project will have a significant impact on the field. The continued use of the Dalitz plot technique with the larger data samples that are currently being collected during Run II of the LHC is likely to lead to further observations of new particles and to improved understanding of previously observed states. The sensitivity to CP violation will also continue to improve, which may allow the discovery of asymmetries caused by new particles, beyond the standard model. Such progress towards answering the big open questions in science would be a fitting outcome of the project.

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United Kingdom
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