The CMS (Compact Muon Solenoid) detector uses a huge solenoid magnet to bend the paths of particles from collisions in the Large Hadron Collider.
The CMS team gave us an exciting update from team members Joel and Vichayanun:
The CMS collaboration has seen evidence of a rare mechanism to produce 4 top quarks!
The simultaneous production of 4 top quarks is predicted by the standard model. As a bonus, the process is sensitive to many hypothetical undiscovered particles and forces that could subtly change how often 4 quarks are made (and how they behave). The new result has a significance of 3.9 standard deviations. That means that the chances of the result being a statistical fluke are very small – only about 1 in 20,000!
Top quarks are elusive particles. It is so rare that, on average, just 1 out of hundreds of millions of collisions produces a top quark. And they’re special: top quarks could be the key to understanding how and why the Higgs boson has the mass it has, an important puzzle in our understanding of elementary particles.
Searching for events with 4 top quarks is challenging, but definitely worth it. New undiscovered particles, or even just the interaction between the top quark and the Higgs boson being different from what we expect, would change how often 4 top quarks are made.
At the same time, 4 top quark production leads to spectacular events. Each top quark breaks up into a W boson and a bottom quark, and every quark creates a distinctive spray of particles called a jet. The W boson can produce a charged lepton and a neutrino, or 2 quark jets. This means that the detector signature that physicists use to identify 4 top quark events varies drastically, and can contain anything from 0 up to 4 charged leptons, such as muons or electrons, and up to 12 jets. The search for 4 top quark events focuses on one of the busiest signatures that we study at the LHC. It is very challenging, especially in the case of the signatures with a small number of charged leptons.
In the new result, CMS focuses for the first time on the scenario where only jets are produced. A new machine-learning algorithm has been used, not only to distinguish the 4 top quark events from other kinds of collisions (uninteresting events that we call background), but also (for the first time at the LHC) to predict the behaviour of that background using the measured data, after having been trained to predict the background properties using simulated events.
For the signatures where only 1 or 2 opposite charge electrons or muons are expected, the main challenge is to identify the background coming from top quark pairs, sometimes produced simultaneously with Higgs, W, and Z bosons. Advances in experimental techniques and background evaluation, typically using more machine learning, improve our insight and understanding of these kinds of LHC events.
When all these different CMS searches for 4 top events are combined, the expected sensitivity to see standard model production of 4 top quarks already exceeds the three standard deviations that particle physicists usually require to claim evidence for a new production mechanism. The number of collisions seen to be consistent with 4 top quark production slightly exceeds the prediction of the standard model, but is still fully compatible with it within the large measurement uncertainties. The precision of the analysis will greatly improve as more data are added, so that 4 top quark production is one of the exciting topics to be studied during the Run 3 of the LHC, as well as with the High-Luminosity LHC, which should start operation in the late 2020s and produce 20 times more data than we have now.
This update is based on a blog post by the CMS team. Challenge yourself and read their blog post in full!
