Electroweak Penguins
Flavour-changing neutral currents (FCNCs) are rare processes in the standard model (SM) that are forbidden at the so-called tree level and instead proceed via quantum loops such as those shown in the figure below. In the figure below, the plot on the left is referred to as an electroweak penguin diagram, also simply said a penguin diagram.
Rare and suppressed processes in the SM are of interest not only because they provide information on the flavour structure of the SM itself but also because they can be used to search for new physics. These types of transitions have had a central role in the development of the field in recent years due to a number of anomalies that have emerged in experimental measurements. Some anomalies have been understood with more precise analyses, while others persist.
New heavy particles, such as new vector bosons or other particles, might be exchanged in the loops that mediate the decay and modify the decay properties, such as the rate or the angular distribution of the final states with respect to the theoretical prediction. The weak Decay B^+\to K^+\nu\bar\nu is an example of FCNC where a large amount of missing energy (due to the neutrinos) is present in the final state. In addition to a very low expected decay rate, the presence of the neutrinos in the final state makes the search for this process extremely challenging, and so far, only B-factories have been able to search for this process, and a very interesting tension has now appeared between the observed branching fraction and its theoretical prediction.
I use an inclusive reconstruction technique to investigate this process using data collected at the Belle experiment, and I am preparing a new set of measurements to be performed at Belle II. Confirming the tension in this channel might indicate the existence of new physics, which may be related to a new dark sector.
One of my phenomenology studies investigating the possibility of light new physics in this and related decays can be found here.
