Detected an elusive subatomic particle which has been sought for over a decade: the single top quark.
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What is the world made of?
This question is as old as mankind and scientists have long aimed to find the basic building blocks that all matter is composed of. What they have been looking for is something that they call ‘fundamental’, meaning that it cannot be further divided. Initially, atoms were believed to be fundamental, but physicists soon discovered that atoms are made of electrons and a nucleus. The nucleus itself is made of protons and neutrons. And protons and neutrons are made of quarks.
As far as we know today, quarks and electrons are fundamental. They are also called ‘elementary particles’ because (so far) there’s nothing smaller.
Image 1: Atom model (Image provided by: The Particle Adventure from the Particle Data Group (LBNL))
How many elementary particles exist in total?
The standard model of particle physics states that there are 6 quarks, 6 leptons, and several force-carrying particles (bosons). The electron is one of the leptons. The photon, the particle associated with light, is one of the bosons.
How do physicists study elementary particles?
Most of the elementary particles exist in isolation only at very high energy. To reach these energies, physicists use particle accelerators. These are large, often circular, underground tunnels in which particles, such as electrons and protons, are brought up to enormous speed using high vacuum and high-powered superconducting magnets, and then they are made to collide with each other. In these high-energy collisions, elementary particles are created. They can be observed in detectors that surround the collision site.
The collision takes place in the center causing particles to fly off in all directions spreading throughout the different layers of the detector. The moving particles trigger electronic signals that are recorded and analyzed by powerful computers. Each elementary particle gives rise to different characteristic signals.
Image 2: Schematic of a particle detector, the ATLAS detector at CERN
Image 3: Photograph of the ATLAS detector
By 2006, experimental evidence for most elementary particles had been found with the help of particle colliders. The top quark, however, had been observed only in pairs. Theory predicted that it could also be produced in isolation, as the single top quark. An experiment called DZero, which collected data at a particle accelerator located in Batavia, Illinois, was specially designed to find the single top quark.
Initially, however, physicists could not find any evidence of the single top quark in their detectors. This was because the signal that the single top quark emits was very weak and was hidden among background noise. New computing techniques were necessary to differentiate between signal and noise.
O’Neil overcame this challenge by designing a computer program called a ‘boosted decision tree’.
Decision trees are programs that separate things based on distinguishing features. For example, imagine you had a pile of apples and you wanted to separate Galas from Granny Smiths and Red Delicious. The most distinguishing feature among the three -given you bought them at a store- is their label, so you would choose to read the labels to separate them.
Now imagine you (the detector) cannot read labels. This means that you have to find other features you can use for separation. You might come up with distinguishing traits such as color, shape, and taste, which will also allow you to eventually separate the apples. This is exactly what O’Neil’s decision trees had to do. They had to sort out signal from noise without a very strong distinguishing feature, but rather using many weak distinguishing traits.
This was still not good enough to solve the problem, though. A single decision tree gave a first description of what might be noise and what might be signal, but there was a lot of data coming from the detector that the first tree could not classify. So O’Neil and his group took the data that the first tree had most difficulties with and fed it into a second tree, a tree that used different distinguishing features. And the data that still proved problematic after the second tree was fed into a third tree and so on. In the end, the physicists had a little grove of 20 decision trees that succeeded in identifying the single top quark signal. This technique is called a ‘boosted’ decision tree and O’Neil and his co-workers were the first to prove that it can be successfully used to identify a weak signal in a particle physics detector.
To learn more, click on the links below:
CERNland (link to https://project-cernland.web.cern.ch/project-CERNland/)
Masterclasses (link to http://physicsmasterclasses.org/neu/)
A New Hope
The Particles Strike Back
Today O’Neil is using boosted decision trees to count tau leptons, elementary particles that were first discovered in the 1960s. So finding their signal will be straightforward this time. The challenge will be to count their exact numbers. O’Neil hopes to find a higher number of taus than expected relative to other particles, as this could be the signature of a new direction in physics. It could be a sign of the Higgs boson, for example, a particle that would explain the origin of mass and the only elementary particle that has not yet been observed. Physicists do not know if the Higgs boson exists, or if there are several different types of Higgs Bosons, so these are all mysteries waiting to be uncovered. In addition to the origin of mass such discoveries could explain the existence of dark matter and dark energy which make up 95% of the universe based on current theories. That is: 95% of what is out there is a mystery to modern science. We simply don't know what it is.