Rare Decay Searches

Rare particle decays have a long tradition in particle physics and are interesting because usually they follow a common scheme: they are substantially suppressed in the standard model (SM) by some SM inherent mechanism, like for example Flavor Conservation of Neutral Currents, and therefore called rare, there exist precise theoretical predictions, and they usually have a very clean signature. Observing a rare decay much above (or actually below) its predicted value would therefore make it a clear sign for physics contributions beyond the standard model.

In addition, rare decays can probe new physics effects at energy scales far beyond those directly accessible at the LHC. This sensitivity arises because hypothetical heavy particles can contribute indirectly through quantum loop (virtual) effects, modifying decay rates, angular distributions, and CP violating observables even if the particles cannot be produced directly at colliders. As a result, precise measurements of rare decays, combined with equally precise Standard Model predictions, provide stringent tests of the Standard Model and place strong constraints on physics beyond it, complementary to direct searches.

Scale of new physics probed by selected measurements. Left to right: proton decay, neutrino oscillations, electron EDM, lepton flavor violation in muon decay, kaon mixing, neutron EDM, B-meson mixing, electron (g-2), beta decay, Higgs to tau tau.

Rare B Decays

One of the major challenges in studying rare decays is that processes with very precise theoretical predictions are often experimentally very difficult to measure. Conversely, experimentally accessible channels are frequently hard to predict with comparable precision. The decays $B_S\to\mu\mu$ and $B^0\to\mu\mu$ are a rare exception: they combine precise Standard Model predictions with a clean, well-defined experimental signature.

We have measured how often the rare decay $B_S\to\mu\mu$ happens (its branching fraction) and how long the Bs meson effectively lives in this decay mode (the effective lifetime). We also report the results of a search for the even rarer decay $B^0\to\mu\mu$ using CMS Run 2 data. These are the most precise single measurements to date and they show good agreement with the Standard Model expectation.

For Run 3, our main goals are to improve precision on the Bs → μμ rate, lifetime and possible CP violation effects, and search for the $B^0\to\mu\mu$ decay. With the full Run 3 dataset, we expect to be sensitive to the Standard Model rate and to obtain the first evidence for $B^0\to\mu\mu$ decay.

Rare D Decays

Rare charm decays driven by the c to u quark transition have received less attention than rare decays of bottom and strange hadrons. They are harder to predict in the Standard Model due to loop effects involving lighter quarks, which are difficult to calculate reliably. This theoretical precision is not an issue when searching for deviations that are significantly larger than the associated uncertainty.

$B^0\to\mu\mu$ is one of such interesting cases. The Standard Model branching fraction is expected to be about 3 × 10^-13, while the strongest current limit from LHCb is $B(D^0\to\mu\mu)<3.5\times10^{-9}$ at 95% C.L., leaving roughly three orders of magnitude between present sensitivity and the Standard Model rate.

Our first attempt to search for this decay lowered the upper lit to $B(D^0\to\mu\mu)<2.6\times10^{-9}$ at 95% C.L., using only data from the first two years of Run 3. We plan to improve this measurement using the full Run 3 dataset.

Rare Kaon Decays

Rare K meson decays are another interesting area of research. The field is typically driven by fixed target experiments, but collider experiments can also play an important role. We are working on a search for the decay $K_S\to\mu\mu$ decays, which can probe new physics contributions by measuring the CP-violating contribution extracted from the $K_S$ to $K_L$ interference in a tagged, time dependent analysis.