2024-07-17 04:00:15
The CMS collaboration explored previously unknown physics through a rare decay of a known particle. It’s like trying to guess the contents of a gift package by inspecting it from every angle.
When receiving a birthday gift, some people rush to unwrap it to see what’s inside. Others prefer to examine the package to try to guess its contents from its shape, size, weight or even the sound it makes when shaken.
Image: CMS/CERN
The analyses conducted by scientists on the datasets obtained from the Large Hadron Collider (LHC), designed to discover new phenomena of physiquelike new particles, generally rely on one of two approaches. Searching directly for a specific type of new particle is like unwrapping your birthday present right away, while adopting a strategy indirect based on the subtleties of the Quantum mechanics is more like a careful study of its packaging in order to guess its contents.
On the occasion of the LHCP Annual Conferencewhich was held last week in Boston, the collaboration CMS presented the method she used to search for new physics via rare decays of a particle called méson B0.
The physics process that causes a particle to decay into lighter particles could be influenced by new particles, which have not yet been 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 with the predictions of the Standard model of the particle physics. Just as it is possible to gain information about the contents of a gift package by inspecting it from every angle, it is also possible to spot a clue to new physics from a deviation from the predictions of the Standard Model.
The decay process of the B0 meson, consisting of a b quark and a d quark, into a K*0 meson (consisting of an s quark and a d quark) and a pair of muons lends itself particularly well to this approach. This decay in fact goes through a rare transition, called “Penguin“, highly sensitive to the influence of new heavy particles.
To conduct this new study, the CMS team relied on all the data collected by the detector between 2016 and 2018, during the second run of the LHC, in order to inspect the “packet” of B0 decay products. This “packet” makes it possible to approach new physics in different ways. By first weighing the packet, that is to say by measuring the frequency at which this decay occurs. We can also take two twin packets, for example, one corresponding to a decay into a pair of muons and the other corresponding to a decay into a pair of electrons, and check whether they have the same mass.
For their new study, the CMS scientists investigated the shape of the bunch by examining the distribution of the energy of the parent B0 meson among the decay particles and by measuring the angles of the decay products. The team then determined a set of parameters from these energies and angles, before comparing the results with two sets of Standard Model predictions.
For most parameters, the results agree with these two sets of predictions. However, for the parameters called P’5 and P2, as well as for some energies of the two muons, the results show a tension with the predictions. Overall, the new CMS results are consistent with previous results from the ATLAS, LHCb and Belle experiments, and even improve their accuracy.
Alas, an ugly, charming penguin came to spoil the party. The presence of a c quark in this rare transition called “penguin” contradicts the predictions of the Standard Model and makes any conclusion difficult. To make progress on this question, scientists are now counting on better predictions, more data and improved analysis techniques.
More information on the website of CMS.
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