Recent discoveries from the research we have been conducting at the Large Hadron Collider (LHC) in Cologne, Geneva, suggest that we may be on the verge of a sign of undiscovered physics.
If confirmed, these hints would overturn the theory known as the Standard Model, which has dominated particle physics for 50 years. This finding suggests that how certain subatomic particles behave within the LHC is inconsistent with the Standard Model.
Fundamental particles are the most fundamental building blocks of matter and are subatomic particles that cannot be divided into smaller units. Four fundamental forces control the interactions of these particles: gravity, electromagnetism, the weak force, and the strong force.
The LHC is a giant particle accelerator built in a 27km-long circular tunnel beneath the border between France and Switzerland. Its main purpose is to find cracks in the standard model.
This theory is our best understanding of fundamental particles and forces, but we know it’s not the whole story. It doesn’t explain gravity or dark matter, an invisible and previously unmeasured type of matter that makes up about 25% of the universe.
At the LHC, beams of proton particles traveling in opposite directions collide to reveal hints of undiscovered physics. The new results come from LHCb, an experiment at the Large Hadron Collider where these collisions are analyzed.
The results came from studies of the decay (a type of transformation) of subatomic particles called B mesons. We investigated how these B mesons decay into other particles and found that the particular way this happens is inconsistent with the predictions of the Standard Model.
elegant theory
The Standard Model is built on two of the most transformative advances in 20th century physics. Quantum mechanics and Einstein’s special theory of relativity.
Physicists can rigorously test theories by comparing measurements made at facilities such as the LHC with predictions based on the Standard Model.
Despite knowing that the Standard Model is incomplete, particle physicists have yet to find any cracks in the theory, despite more than 50 years of increasingly rigorous testing. That is, it is possible until now.
Our measurement results have been accepted for publication physical review letterindicating a tension that is 4 standard deviations from the standard model’s expected value.
In real-world terms, this means that after accounting for uncertainties from experimental results and theoretical predictions, there is only a 1 in 16,000 chance that such extreme random fluctuations in the data would occur if the standard model were correct.
Although this falls short of science’s gold standard, so-called five sigma, or 5 standard deviations (about a 1 in 1.7 million chance), the evidence is starting to mount. Adding to this compelling story are the results of an independent LHC experiment, CMS, published in early 2025.
Although the CMS results are not as precise as the LHCb results, they are in good agreement and strengthen the case. Our new results were discovered in the study of a particular type of process known as electroweak penguin decay.
unusual event
The term “penguin” refers to a specific type of decay (transformation) of short-lived particles. In this case, we study how the B meson decays into four other elementary particles: a kaon, a pion, and two muons.
With a little imagination, you can visualize that the arrangement of particles involved looks like a penguin. Importantly, this decay measurement allows us to study how one type of fundamental particle, the beauty quark, transforms into another strange quark.
This penguin collapse is incredibly rare in the Standard Model. Only one B meson in a million will decay in this way. We carefully analyzed the angles and energies at which these particles are produced in their collapse to determine exactly how often the process occurs. We find that the measurements of these quantities do not match the predictions of the standard model.
The precise study of such decays is one of the main objectives of the LHCb experiment, and has been since its inception in 1994. The Penguin process is uniquely sensitive to the effects of potentially very heavy new particles that cannot be produced directly at the LHC.
Such particles can still have a measurable impact on these decays, beyond the small contribution of the Standard Model. This type of indirect observation is not new. For example, radioactivity was discovered 80 years before the fundamental particle responsible for it (the W boson) was directly observed.
Future direction
Our study of rare processes will allow us to explore parts of nature that may otherwise be inaccessible using particle colliders, which are planned for the 2070s. There is a wide range of new theories that could potentially explain our findings. Many of them contain new particles called leptoquarks, which combine two different types of matter: leptons and quarks.
Other potential theories involve particles that are heavier analogs of particles already found in the Standard Model. The new results constrain the form of these models and direct the search for future models.
Despite our excitement, there remain unresolved theoretical questions that prevent us from definitively claiming that physics beyond the Standard Model has been observed. The most serious problem arises from the so-called “charming penguin”, a set of processes present in the standard model, but whose contributions are very difficult to predict. Recent estimates of these attractive penguins suggest that their effects are not large enough to explain our data.
Furthermore, combining the theoretical model with experimental data from LHCb suggests that the attractive penguin (and thus the standard model) struggles to explain the anomalous results.
New data already collected will allow us to see the situation in the coming years. To find these penguin decays, the current study studied approximately 650 billion B meson decays recorded between 2011 and 2018. Since then, the LHCb experiment has recorded three times as many B mesons.
Further advances are planned for the 2030s, taking advantage of future upgrades to the LHC to once again accumulate a dataset 15 times larger. This ultimate step could allow definitive claims to be made and unlock new understanding of how the universe works at the most elementary level.
