The CMS collaboration has explored yet unknown physics through a rare decay of a known particle. It's akin to trying to guess the contents of a gift by thoroughly inspecting its wrapping.
When receiving a birthday gift, some people eagerly unwrap it to see what's inside. Others prefer to examine the package to guess its contents based on its shape, size, weight, or even the noise it makes when shaken.
Illustration of a collision recorded by the CMS detector, whose signature might indicate the decay of a B0 meson into a K*0 meson and a pair of muons (represented by the red lines). The K*0 meson then decays into a K+ meson (represented by the magenta line) and a π- meson (represented by the green line).
Image: CMS/CERN
The analyses conducted by scientists on datasets obtained from the
Large Hadron Collider (LHC) to discover new physical phenomena, such as new particles, typically rely on one of these two approaches. Directly searching for a specific new particle type is akin to immediately unwrapping the birthday gift, while adopting an indirect strategy based on the intricacies of quantum mechanics is more like a meticulous study of the wrapping to guess its contents.
At the
annual LHCP conference held last week in Boston, the
CMS collaboration presented the method they used to search for new physics via the rare decays of a particle called the B
0 meson.
The physics process that drives a particle to decay into lighter particles could be influenced by new particles, not yet observed because they are too heavy to be produced in the LHC. The changes induced by these particles in the decay process could be measured and compared to the predictions of the
Standard Model of particle physics. Just as it is possible to glean information about the contents of a gift by thoroughly inspecting its wrapping, it is also possible to spot a hint of new physics from deviations from the Standard Model's predictions.
The decay process of the B
0 meson, composed of a b quark and a d quark, into a K*
0 meson (composed of an s quark and a d quark) and a pair of muons is particularly well-suited to this approach. This decay involves a rare transition, known as a "
penguin" transition, which is highly sensitive to the influence of new heavy particles.
To conduct this new study, the CMS team relied on the complete dataset collected by the detector between 2016 and 2018, during the LHC's second operational period, to inspect the "package" of B
0 decay products. This "package" allows approaching new physics in various ways. By weighing the package first, i.e., measuring the frequency at which this decay occurs. You can also take two twin packages, for instance, one corresponding to a decay into a pair of muons and the other to a decay into a pair of electrons, and check if they have the same mass.
For their new study, CMS scientists examined the shape of the package by looking at the energy distribution of the parent B
0 meson among the decay products and measuring the angles of the decay products. The team then determined a set of parameters from these energies and angles and compared the results with two sets of Standard Model predictions.
For most parameters, the results match these two sets of predictions. However, for parameters called P'
5 and P
2, as well as certain energies of the two muons, the results show a discrepancy with the predictions. Generally, the new CMS results are in line with previous results from the ATLAS, LHCb, and Belle experiments, even improving their precision.
Unfortunately, a tricky penguin, though charming in its own way, has spoiled the party. The presence of a c quark in this rare "penguin" transition contradicts the Standard Model predictions and makes any conclusion difficult. To progress on this issue, scientists now rely on better predictions, more data, and improved analysis techniques.
More information is available on the CMS website.