PENGUIN · Search for New Physics in Electroweak Penguin Transitions at LHCb

Search for New Physics in Electroweak Penguin Transitions at LHCb

The standard model of particle physics (SM) is the theory that describes the fundamental particles and their interactions. This theory is known to be incomplete, thanks to cosmological observations such as the Dark Matter. There are a number of theories that extend the SM and address its short-comings. Experimental particle physicists are searching for evidence of these alternative theories. There are two main ways to look for physics beyond the SM (BSM). The first one, the direct approach, consists in looking for particle that are nonexistent in the SM. This approach is the most usual. The reach of such searches is limited by the energy that is available to produce the putative new particles, which in turn is limited by the energy that particle collider can provide. If the BSM particles have large masses they cannot be produced in the current colliders. The second approach is the indirect search and is used in this project. The indirect approach consists in looking for the effect of BSM particles in processes that are very rare. BSM particles can remain 'virtual', i.e. they do not have to materialise in such processes, but still affect measurable properties. Because the new particles do not materialise, using the same collider, the indirect approach is sensitive to higher scale of BSM particle than direct searches. Because they are very suppressed in the SM, rare decays are processes in which BSM is more likely to be detected. The decays of the neutral B meson into a K*0 and two muons is such a rare process. The K*0 particle decays immediately into a charged Kaon and a charged pion. Thanks to the four particles in the final states, numerous angular observables can be defined. The decay of a charged B meson into a charged Kaon and two muons is similar, although less observables can be constructed. Finally, the branching fraction of the decays of the neutral B meson or the strange B meson into two muons is very sensitive to contribution from BSM theories. In the course of this Fellowship, the Fellow has worked on these three decays. The direct CP asymmetries of the decays B0 -> K*0 mu+ mu- and B+ -> K+ mu+ mu- are observables that are sensitive to BSM contributions. The Fellow has measured them in the LHCb data with Simon Wright and Harry Cliff. Such measurements are delicate, as different sources of asymmetry (coming from the imperfect detector that favours one charge over the other, or coming from the inherent production asymmetry due to the fact that protons are colliding) have to be accounted for. This has been done using 1 inverse femtobarn (1 fb-1) of pp collision data for the charged decay only, and then using the full dataset (3fb-1) collected by LHCb during the first run of the the Large Hadron Collider for both modes. All our measurements were found to be compatible with the SM prediction, so no evidence for any effect beyond the standard model was detected. Such a situation does not means that BSM models are invalidated, but it allows to understand them better by constraining them. We cannot say what BSM is, but we can say what it is not. The decays of neutral B mesons into two muons were identified as golden mode to discover BSM effect since a long time. Experiments have been searching for them since 1984. The probability, or branching fraction, for a Bs or a B0 to decay into two muons is predicted to be very small in the Standard Model at the level of 4 Bs decays into two muons out of one billion Bs decays and one B0 decays into two muons out of 10 billion B0 decays. In addition, the SM predictions for these branching fractions are precise. This leads to a situation were if an experimental measurement of these quantities is found to be incompatible with the SM predictions, that would represent the proof that there is physics at play that is not described in the SM, and help identifying what alternative model it could be. The first evidence for the Bs -> mu+ mu- decay was published by the LHCb collaboration, the Fellow playing a key role in this analysis. With more data, both CMS and LHCb published results that just fell short to a significance of 5 standard deviations for the observation of the Bs->mumu decay. A combined analysis of the CMS and LHCb dataset was set in motion to optimally exploit the LHC data. This effort was led by the Fellow and resulted in the first observation of the Bs->mumu decay, as well as the first evidence for the B0-> mumu decay. These results were published in Nature. This is the first publication in Nature from any experiment at the LHC. In June 2013, the Fellow has been appointed as convener of the Very Rare Decay working group of LHCb. The competences acquired in the course of the Fellowship have been recognised by the LHCb collaboration, and by the CERN international organisation. In October 2014, the Fellow has been appointed convener of the Rare Decays working group of the LHCb collaboration, taking office in January 2015. In this position, the Fellow supervise the scientific work of about 120 researchers and organise the future research activity for a large part of the physics program of LHCb. The Fellow has been awarded a CERN Research Fellowship at CERN.

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