If you cast your mind back to your high school science days you’ll likely remember being taught certain things about atoms and what they’re made up of. The theories you were taught, things like the strong/weak forces and electromagnetism, form part of what’s called the Standard Model of particle physics. This model was born out of an international collaboration of many scientists who were looking to unify the world of subatomic physics and, for the most part, has proved extremely useful in guiding research. However it has its limitations and the Large Hadron Collider was built in order to test them. Whilst the current results have largely supported the Standard Model there is a growing cache of evidence that runs contrary to it, and the latest findings are quite interesting.
The data comes out of the LHCb detector from the previous run that was conducted from 2011 to 2012. The process that they were looking into is called B meson decay, notable for the fact that it creates a whole host of lighter particles including 2 leptons (called the tau lepton and the muon). These particles are of interest to researchers as the Standard Model makes a prediction about them called Lepton Universality. Essentially this theory states that, once you’ve corrected for mass, all leptons are treated equally by all the fundamental forces. This means that they should all decay at the same rate however the team investigating this principle found a small but significant difference in the rate in which these leptons decayed. Put simply should this phenomena be confirmed with further data it would point towards non-Standard Model particle physics.
The reason why scientists aren’t decrying the Standard Model’s death just yet is due to the confidence level at which this discovery has been made. Right now the data can only point to a 2σ (95%) confidence that their data isn’t a statistical aberration. Whilst that sounds like a pretty sure bet the standard required for a discovery is the much more difficult 5σ level (the level at which CERN attained before announcing the Higgs-Boson discovery). The current higher luminosity run that the LHC is conducting should hopefully provide the level of data required although I did read that it still might not be sufficient.
The results have gotten increased attention because they’re actually not the first experiment to bring the lepton universality principle into question. Indeed previous research out of the Stanford Linear Accelerator Center’s (SLAC) BaBar experiment produced similar results when investigating lepton decay. What’s quite interesting about that experiment though is that it found the same discrepancy through electron collisions whilst the LHC uses higher energy protons. The difference in method with similar results means that this discrepancy is likely universal, requiring either a new model or a reworking of the current one.
Whilst it’s still far too early to start ringing the death bell for the Standard Model there’s a growing mountain of evidence that suggests it’s not the universal theory of everything it was once hoped to be. That might sound like a bad thing however it’s anything but as it would open up numerous new avenues for scientific research. Indeed this is what science is built on, forming hypothesis and then testing them in the real world so we can better understand the mechanics of the universe we live in. The day when everything matches our models will be a boring day indeed as it will mean there’s nothing left to research.
Although I honestly cannot fathom that every occurring.
It was 3 years ago that particle physicists working with CERN at the Large Hadron Collider announced they had verified the existence of the Higgs-Boson. It was a pivotal moment in scientific history, demonstrating that the Standard Model of particle physics fundamental basis is solid. Prior to this announcement the LHC had been shut down for a planned upgrade, one that would see the energy of the resulting collisions doubled from 3.5TeV per beam to 7TeV. This upgrade was scheduled to take approximately 2 years and would open up new avenues for particle physics research. Just last week, almost 3 years to the day after the Higgs-Boson announcement, the LHC began collisions again. The question that’s on my mind, and I’m sure many others, is just what is LHC looking for now?
Whilst the verification of the Higgs-Boson adds a certain level of robustness to the Standard Model many researchers have theorized of physics beyond this model at the energies that the LHC is currently operating at. Of these models one that will be explored by the LHC in its current data collection run is Supersymmetry, a model which predicts that each particle which belongs to one of the two elementary classes (bosons or fermions) has a “superpartner” in the other. An example of this would be an electron, which is a fermion, would have a superpartner called a selectron which would be a boson. These particles share all the same properties with the exception of their spin and so should be easy to detect, theoretically. However no such particles have been detected, even in the same run where the Higgs-Boson was. The new, higher energy level of the LHC has the potential to create some of these particles and could provide evidence to support supersymmetry as a model.
Further to the supersymmetry model is every new particle physicist’s favourite theory: String Theory. Now I’ll have to be honest here I’m not exactly what you’d call String Theory’s biggest fan since, whilst it makes some amazing predictions, it has yet to be supported by any experimental evidence. At its core String Theory theorizes that all point like particles are made up of one-dimensional strings, often requiring the use of multi-dimensional physics (10 or 26 dimensions depending on which model you look at) in order to make them work. However since they’re almost purely mathematical in nature there has yet to be any links made between the model and the real world, precluding it from being tested. Whilst the LHC might provide insight into this I’m not exactly holding my breath but I’ll spin on a dime if they prove me wrong.
Lastly, and probably most excitingly for me, is the prospect of discovering the elusive dark matter particle. Due to its nature, I.E. only interacting with ordinary matter through gravity, we’re unlikely to be able to detect dark matter particles in the LHC directly. Instead, should the LHC generate a dark matter particle, we’ll be able to infer its existence by the energy it takes away from the collision. No such discrepancy was noted at the last run’s energy levels so it will be interesting to see if a doubling of the collision energy leads to the generation of a dark matter particle.
Suffice to say the LHC has a long life ahead of it with plenty of envelope pushing science to be done. This current upgrade is planned to last them for quite some time with the next one not scheduled to take place until 2022, more than enough time to generate mountains of data to either support or refute our current models for particle physics.