ML Helps in "X" Particle Detection

Courtesy: CERN Photolab

Physicists have found evidence of mysterious particles known as "X" particles, which were first thought to form just after the Big Bang. "X" particles, called so because of their mysterious unknown inner structure, were created millionths of a second after the Big Bang. In a trillion degree sea of quarks and gluons that randomly collided, "X" particles were formed before the plasma cooled down and such stable particles as protons and neutrons were created. Today, X particles are extremely rare. 

X (3872) was first discovered in 2003 by the Belle experiment, a particle collider in Japan that smashes together high-energy electrons and positrons. Within this environment, however, the rare particles decayed too quickly for scientists to examine their structure in detail. 

Evidence of X particles in the quark-gluon plasma produced in the Large Hadron Collider (LHC) at CERN based near Geneva, Switzerland has now been found. The LHC's 2018 dataset included more than 13 billion lead-ion collisions, each of which released quarks and gluons that scattered and merged to form more than a quadrillion short-lived particles before cooling and decaying. 

To sift through the tonnes of data points, the research team used a machine-learning algorithm which they trained to pick out decay patterns characteristic of X particles. Immediately after particles form in quark-gluon plasma, they quickly break down into “daughter” particles that scatter away. For X particles, this decay pattern, or angular distribution, is distinct from all other particles. The team identified key variables that describe the shape of the X particle decay pattern. They trained a machine-learning algorithm on a labelled training set and then fed the algorithm actual data from the LHC’s collision experiments. The algorithm was able to sift through the extremely dense and noisy dataset to pick out the observations that were likely a result of decaying X particles. Out of billions upon billions of collisions, 100 of them were found to be X (3872).

Now that the team has shown X particles can be detected in quark-gluon plasma, they plan to probe this particle with quark-gluon plasma in more detail, to pin down the X particle’s structure. Research advancements such as these would redefine the kinds of particles that were abundant in the early universe and give cosmologists something more to be excited about.

Here is the original research paper published in the Physical Review Letters (Warning: Highly technical on the physics side)