Amazing! This is how scientists ‘found’ new particles

In March 2021, CERN announced that its Large Hadron Collider (LHC) had discovered four completely new particles, four different tetraquark states.

Amazing! This is how scientists 'found' new particles

The Large Hadron Collider (LHC) is buried 100 meters deep underground in a tunnel with a total length of 27 kilometers.

So far, the LHC has discovered a total of 59 new hadrons, including this latest discovery of a tetraquark state and many other particles. So, how did the scientists determine that they had found a “new particle”?

Announcing new discoveries is not easy

In the 1970s, scientists created the Standard Model of particle physics, which gives clear information about the fundamental particles that make up the world, including their species, mass, charge, spin, and their interactions. The Standard Model is like a “manual” for a hotel suite, which clearly details the name, type, quantity, and location of each item so that guests can stay there very comfortably.

For nearly half a century, all the particles discovered by scientists in their experiments have found their identities in the Standard Model, and their properties match the predictions in the “manual”.

“A central task for scientists to experimentally verify and expand the theoretical framework is to find new phenomena and particles that are predicted by the Standard Model but not yet discovered, and that are beyond the scope of the theory’s explanation.” Such new discoveries are crucial in the scientific study of mankind’s step-by-step efforts to unravel the fundamental laws of the material world, Yusheng Wu, Distinguished Professor in the Department of Modern Physics at the University of Science and Technology of China, told reporters.

This is evidenced by the many breakthroughs in basic science since the last century: from the confirmation of the existence of photons through the photoelectric effect in 1905 to the discovery of the Higgs particle in 2012, all the elementary particles predicted by the Standard Model theory have been discovered. The research on elementary particles predicted by the Standard Model has also led mankind to explore the origin of the universe and the nature of matter further and further: predicting the existence of antimatter, discovering that neutrino mass is not zero, observing black holes and even detecting gravitational waves. “Each new discovery is like throwing a stone into a quiet, deep pool, causing ripples and advancing the birth of a series of new sciences and technologies.” Wu Yusheng said.

“In particle physics experiments, whether the discovery of a new phenomenon is conclusive and plausible or not is generally characterized by the degree of significance in statistics. Significance is expressed as a multiple of the standard deviation of the Gaussian distribution, i.e., several times σ (sigma).” The larger the multiple, the more credible it is, Wu Yusheng told reporters. “If the significance is 5σ, this new finding is considered conclusive and the probability of falsification is even less than one in a million, while the experimental result with 3σ significance is also more credible, corresponding to a probability of falsification of only one in a thousand.” Wu Yusheng said.

Wu Yusheng told reporters that 5σ and 3σ correspond to the scientific conclusion “discovered a new phenomenon” and “found signs of the existence of new phenomena”, which is an important basis for scientists to announce new discoveries. For fundamental discoveries, the scientific community is extremely rigorous, requiring not only the statistical significance of experimental findings to reach 5σ, but also to be able to withstand the test of time. “In addition, other experiments conducted independently are required to repeat the results to verify them.” Wu Yusheng said.

The testing of the standard model never stops

“The testing of the standard model has never stopped since its inception.” Wu Yusheng said that exploring the wonders of the microscopic world requires reliable experimental means to observe all kinds of physical phenomena, and these experimental means need to be able to produce microscopic particles and carry out reactions, and to record and analyze the results of the reactions so that they can be compared with theoretical predictions. “A large number of sophisticated experiments in different scenarios and under different conditions can test the standard model in all directions and have the potential to discover new phenomena that are rare, inside and outside the model.” He said.

The best-known particle in the particle physics leptons family is the electron, which is a key component of matter. But the electron is not the only member of the leptons family; it has two heavier siblings, the muon and the τ leptons, which together are known as the three lepton flavors or “flavors. According to the Standard Model of particle physics, the only difference between these siblings should be mass: the muon is about 200 times heavier than the electron, while the τ leptons are about 17 times heavier than the muon.

According to the Standard Model, each flavor could have the same interaction with a W boson. “The flavors of leptons are also called ‘generations,’ and electrons, muons, and τ leptons belong to three different generations in the Standard Model. There may be a ‘generation gap’ between each human generation, but in the Standard Model, the corresponding leptons of different generations, although fat and thin, i.e., with different masses, must all participate in the same face in all kinds of gatherings in the microscopic world, i.e., the reaction processes of elementary particles, which is the prediction of the universality of leptonic flavors. ” Wu Yusheng said.

“It’s a wonderful prediction, and many physicists want to look for new phenomena that defy this universality to explore the possibility of going beyond the existence of the Standard Model theory.” One recent experimental work at the LHCb, reported in the media, is focused on testing a fundamental prediction of the Standard Model, namely possible signs of phenomena that violate leptonic universality, Wu Yusheng said. “The results of this experiment have attracted a lot of attention, but are not yet significant enough to claim a new discovery and are subject to the test of time and the validation of other experiments to follow.”

“The pervasiveness of leptonic flavors has been explored with high precision over a wide range of processes and energies. Although the principle of lepton-flavor universality has passed the latest tests and examinations, the LHC experiments are continuing until many of the anomalies found are explicitly detected.” Wu Yusheng said.

Gas pedals and colliders join forces to find the mystery

What do new elementary particles rely on to find them? The answer is through gas pedals and colliders.

“Simply put, a gas pedal is a charged particle lending energy to an electric field, and scientific research is conducted through charged particles with energy, such as going to a target or conducting collisions. The television set in our homes, for example, is one of the simplest linear gas pedals.” Wu Yusheng said, the electron will get a certain amount of energy after the acceleration of the TV set, the unit of energy is called electron volt. The electric field is used to accelerate the charged particles, that is, to provide it with energy and increase the speed.

Gas pedals can be roughly divided into two categories: a class called linear gas pedal, a class is a circular gas pedal. A linear gas pedal is where the particles go in a straight line, and a ring gas pedal is where the particles run in circles all through a magnetic field.

“With a gas pedal, you can use it for scientific research, using charged particles to hit atomic nuclei or collisions.” Wu Yusheng said the particles hitting the human eye are invisible, so a detector is needed, which is equivalent to replacing the human eye to see the type, amount and characteristics of the particles produced after the collision.

Colliders are also divided into two categories: one is a linear collider, where particles move in opposite directions and collide at a collision point. The other is a ring collider, where positive and negative electrons are traveling relative to each other, accelerating in a circle without stopping, and then colliding.

“The gas pedal is both a physical science and also technology and engineering, which is a very important feature of particle gas pedals.” Wu Yusheng said.

The world’s first positron collider was built in Italy in the 1960s. It was followed by the world’s largest positron collider, the LEP, at CERN in Geneva, after which scientists dismantled the LEP and built the world’s largest proton collider, the LHC, in its tunnel.

“Because the deepest layers of matter we want to explore are getting smaller and smaller in size, and the deeper the level of matter being probed and the smaller the things being looked at, the smaller the wavelength of ‘light’ is needed and the higher the energy must be. Therefore the high energy of the collider is a necessary need.” The world’s largest proton collider, the LHC, is also the highest-energy particle gas pedal, buried 100 meters deep underground in a tunnel with a total length of 27 kilometers (including the ring tunnel), Wu Yusheng said.

“The LHC is so large that it can accelerate particles to near the speed of light, which is the result of the strong magnetic field that makes the particles orbit around the acceleration ring. The strength of its magnetic field is so strong that if it is placed on the ground, the presence of other facilities and people is not allowed within a large perimeter.” The LHC is also buried deep underground to exclude other interference and to obtain a purer experimental environment, Wu Yusheng said.

“The LHC energy state is comparable to that of the universe shortly after the Big Bang. Scientists use the products of proton collisions to explore physical phenomena, such as finding the Higgs particle predicted by the Standard Model and exploring new physics beyond the Standard Model such as supersymmetry and extra dimensions.” Wu Yusheng said.

In fact, in the process of building the LHC, scientists have obtained many scientific results that have improved our lives. The Internet, for example, was originally invented by CERN scientists to solve the problem of data transmission, and other achievements such as cancer treatment, destroying nuclear waste and helping scientists study climate change have also been made possible by the LHC.

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