With the help of CERN’s Large Hadron Collider (LHC), an international team of researchers have found evidence of something physicists have spent decades hoping for – subatomic particles behaving in a way that defies the Standard Model. In particle physics, the Standard Model is the best theory we have for explaining how particles behave and interact; however, it is incomplete as it does not account for gravity. By using the LHC, researchers hope to observe conditions that violate the standard rules of particle physics.
The team of physicists looked at data collected from the LHC’s first run from 2011-2012 – a run made famous for the discovery of the Higgs boson – and found the evidence they were looking for: Leptons defying the Standard Model. Leptons are a group of subatomic particles comprised of three different varieties: the tau, the electron, and the muon. Electrons are very stable, whereas both the tau and muon decay very rapidly.
In the new study, the researchers combed through data looking for evidence of B mesons decaying into lighter particles such as the tau lepton and the muon. The Standard Model dictates that all leptons shall be treated by all the fundamental forces, a concept known as "lepton universality." This means both the tau and the muon should decay at the same rate, once the difference in mass is accounted for. However the team discovered a miniscule, albeit noticeable, difference in the rates of decay which could indicate the presence of potentially unknown forces or particles interfering with the rates of decay.
"The Standard Model says the world interacts with all leptons in the same way. There is a democracy there. But there is no guarantee that this will hold true if we discover new particles or new forces," one of the lead researchers, Hassan Jawahery, from the University of Maryland in the US, said in a statement. "Lepton universality is truly enshrined in the Standard Model. If this universality is broken, we can say that we've found evidence for non-standard physics."
These results compliment a similar discovery from the 2012 BaBar experiment conducted at Stanford’s Linear Accelerator Center (SLAC). The BaBar experiment also focused on B meson decay; however, unlike the LHC which smashes protons together, the SLAC used colliding electrons to drive their experiment. Despite the different methods, having two experiments with similar results is key, and suggestive of real physics.
Further experimentation is needed to confirm the latest findings. In April of this year, the LHC reopened following a two year hiatus for upgrades. Since the LHC came back online, researchers have observed record-breaking energy levels, and the team is confident that they will have a better chance of observing more particle behavior that defies the Standard Model and corroborates these findings.
"We are planning a range of other measurements. The LHCb experiment is taking more data during the second run right now," Jawahery stated in a statement. "Any knowledge from here on helps us learn more about how the universe evolved to this point. For example, we know that dark matter and dark energy exist, but we don't yet know what they are or how to explain them. Our result could be a part of that puzzle [...] If we can demonstrate that there are missing particles and interactions beyond the Standard Model, it could help complete the picture."
The findings will be published in the September 4 issue of Physical Review Letters.