Surreal illustration of rare particle decays at the LHCb experiment.

Unlocking the Secrets of the Universe: How Rare Particle Decays Could Rewrite Physics

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Nico Varela in Science & Nature December 2025 • 5 min read.

"LHCb experiment data provides key insights into rare decays, offering a glimpse beyond the Standard Model and potential new physics scenarios."


The Standard Model (SM) of particle physics is our best current understanding of the fundamental building blocks of the universe and how they interact. However, there are phenomena it simply can't explain, motivating physicists to search for what lies beyond. One promising avenue is the study of rare decays of beauty and charm hadrons—unstable subatomic particles containing heavy quarks. These decays, suppressed or forbidden within the SM, offer a sensitive probe for new physics.

Flavour Changing Neutral Currents (FCNCs) are processes that change the flavor of a quark without changing its electric charge. The Standard Model forbids FCNCs at the tree level, so any observation of FCNCs can provide a good basis to find out new particles, thus hinting at new physics.

The LHCb (Large Hadron Collider beauty) experiment at CERN is designed specifically to study these rare decays. By meticulously analyzing the debris of high-energy proton-proton collisions, LHCb physicists can identify fleeting instances of these decays and compare their properties to theoretical predictions. Any significant deviations could point towards new particles or interactions not accounted for in the Standard Model.

Rare Decay Insights: Challenging the Standard Model

Surreal illustration of rare particle decays at the LHCb experiment.

The LHCb experiment has achieved significant milestones in the study of rare decays, providing a wealth of data that either confirms the Standard Model or hints at potential new physics. The experiment focuses on measuring branching fractions (the frequency of a specific decay) and analyzing the angular distributions of the decay products. Deviations from theoretical predictions in these measurements can signal the presence of new particles influencing the decay process.

One of the most notable achievements has been the observation of the very rare decay Bs → μ⁺μ¯ (a Bs meson decaying into two muons). This decay is heavily suppressed in the Standard Model, making it a sensitive probe for new physics. The LHCb's measurement, combined with results from the CMS experiment, provided the first definitive observation of this decay, with a branching fraction close to, but slightly above, the SM prediction.

  • Bs → μ⁺μ¯ Decay: First observation, branching fraction close to SM but potential hints of enhancement.
  • Dº → μ⁺μ¯ Decay Search: Setting new limits on this decay, probing for R-parity violating SUSY theories.
  • B+ → K+μ⁺μ¯ Decay Analysis: Observation of a new resonance, ψ(4160), impacting understanding of exotic charm states.
Another important area of investigation is the search for lepton flavour violation (LFV). According to the Standard Model, the decays that don't conserve leptons are forbidden, if scientists observe this the Standard Model can be replaced by new models that allow it. The LHCb experiment has been searching for the decay Bs → eµ, where a Bs meson decays into an electron and a muon (different lepton flavours). While no evidence for this decay has been found, the experiment has set stringent limits on its branching fraction, constraining models that predict LFV.

The Future of Rare Decay Physics: A Glimpse Beyond

The study of rare decays is a vibrant and ongoing field, with the potential to revolutionize our understanding of fundamental physics. The LHCb experiment, with its dedicated focus and increasing data samples, is poised to play a central role in this quest. Future analyses, incorporating new data and refined techniques, will further test the Standard Model and probe for signs of new physics.

While no definitive evidence for new physics has yet emerged from rare decay studies, the hints and tensions with Standard Model predictions are tantalizing. The slightly elevated branching fraction of Bs → μ⁺μ¯ and the anomalies observed in angular analyses continue to motivate further investigation and theoretical model building.

As the LHC continues to deliver more data and experimental techniques improve, the search for rare decays will continue to push the boundaries of our knowledge, with the exciting possibility of uncovering the next layer of fundamental particles and interactions that govern our universe.

About this Article -

This article was crafted using a human-AI hybrid and collaborative approach. AI assisted our team with initial drafting, research insights, identifying key questions, and image generation. Our human editors guided topic selection, defined the angle, structured the content, ensured factual accuracy and relevance, refined the tone, and conducted thorough editing to deliver helpful, high-quality information.See our About page for more information.

This article is based on research published under:

DOI-LINK: 10.1051/epjconf/20147100053, Alternate LINK

Title: Rare Decays At Lhcb

Subject: General Medicine

Journal: EPJ Web of Conferences

Publisher: EDP Sciences

Authors: Sam Hall

Published: 2014-01-01

Everything You Need To Know

1

What is the Standard Model, and why is it relevant to the study of rare decays?

The Standard Model (SM) is our current best theory describing the fundamental particles and forces of the universe. However, it doesn't explain everything, like dark matter or the matter-antimatter asymmetry. Scientists study rare decays because these decays are very sensitive to new physics that isn't part of the SM. Observing these rare decays allows physicists to understand what is beyond our current understanding of the universe, and provide a path for new models that address the SM limitations.

2

What is the LHCb experiment, and how does it contribute to understanding rare decays?

The LHCb experiment is designed to study rare decays of beauty and charm hadrons. The experiment analyzes data from high-energy proton-proton collisions. By meticulously analyzing the debris from these collisions, physicists can identify rare decay events and measure their properties. Analyzing the data allows scientists to find out deviations from the Standard Model, which could point to the existence of new particles or interactions.

3

What are Flavour Changing Neutral Currents (FCNCs), and why are they important in this context?

Flavour Changing Neutral Currents (FCNCs) involve quarks changing their flavor without changing their electric charge. The Standard Model forbids FCNCs at a certain level, making them a good place to look for new physics. The observation of FCNCs would be a clear indication that the Standard Model needs to be updated. Studying FCNCs can help identify new particles or interactions that are not accounted for in the Standard Model.

4

What is the significance of the Bs → μ⁺μ¯ decay?

The Bs → μ⁺μ¯ decay is a very rare decay where a Bs meson decays into two muons. This decay is heavily suppressed in the Standard Model, meaning it happens very infrequently. The LHCb experiment, along with the CMS experiment, has provided the first definitive observation of this decay. The branching fraction, or the frequency of this decay, is close to, but slightly above, the Standard Model prediction. This slight enhancement hints at the possibility of new physics influencing this decay process.

5

What is Lepton Flavour Violation (LFV), and why is it being investigated?

Lepton Flavour Violation (LFV) refers to decays where the type of lepton (electron, muon, or tau) is not conserved. According to the Standard Model, decays that don't conserve leptons are forbidden. The LHCb experiment is searching for decays like Bs → eµ, where a Bs meson decays into an electron and a muon. No evidence of this decay has been found, but the experiment has set limits on how often it can occur. If LFV is observed, it would be a significant discovery, indicating the need for new models beyond the Standard Model.

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