Today my collaborators Andrzej Buras, Jennifer Girrbach-Noe, Christoph Niehoff and I released our new paper on the decays $B\to K\nu\bar\nu$ and $B\to K^*\nu\bar\nu$. These two closely related decays are sensitive to physics beyond the Standard Model and will very likely be observed for the first time at the upcoming Belle-II experiment. It was about time to improve on the analysis of five years ago; especially interesting for us was the impact of the recent computation of $B\to K$ and $B\to K^*$ form factors on the lattice as well as the host of new measurements of observables in $b\to s\mu^+\mu^-$ transitions at LHC (remember the $B\to K^*\mu^+\mu^-$ anomaly?).
Here is just a selection of some of the main points of the paper that you might be interested in.
If you’re an experimentalist:
- The central value of the Standard Model prediction for the branching ratio of $B\to K^*\nu\bar\nu$ is now 40% higher – this is good news for Belle-II.
- We show that, even if no new physics is discovered in $b\to s\mu^+\mu^-$ transitions at LHC, $b\to s\nu\bar\nu$ decays are still very interesting to probe new physics.
- Even if there is no new physics, measuring these decays precisely is important to reduce theory uncertainties in other processes.
If you care about precise Standard Model predictions:
- Using the information from the lattice and a recent full NLO calculation of electroweak corrections, we obtain relative uncertainties of about 10% on the branching ratios. (Details on the form factors will be discussed in an upcoming paper with Aoife Bharucha and Roman Zwicky.)
If you are interested in new physics:
- We study the correlations between $b\to s\mu^+\mu^-$ and $b\to s\nu\bar\nu$ on a model-independent basis. Interestingly, if the current tensions in $b\to s\mu^+\mu^-$ are due to new physics, $b\to s\nu\bar\nu$ could help disentangle what kind of new physics is responsible for them.
- We point out that if there is new physics (e.g. a $Z’$ boson) that only affects tau leptons (and tau neutrinos), $B\to K\nu\bar\nu$ and $B\to K^*\nu\bar\nu$ would be the first place to look for it.
Our new physics analysis is summarized in this plot, showing predictions of various scenarios in the plane of the $B\to K\nu\bar\nu$ and $B\to K^*\nu\bar\nu$ branching ratios, normalized to their SM values. If you are curious what it all means, have a look at the paper!