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This article comes from the official account of Wechat: back to Park (ID:fanpu2019), compiled by Ye Lingyuan

Unless the European large Hadron Collider can bring a surprise, particle physics may come to an end helplessly.

The toroidal instrument (ATLAS), one of the four main detectors of the large Hadron Collider, has been upgraded for a new round of collision experiments. Photo Source: MAXIMILIEN BRICE / CERN Ten years ago, particle physicists excited the world. The large Hadron Collider (LHC) of the European Organization for Nuclear Research (CERN) is the world's largest particle accelerator. On July 4, 2012, more than 6000 researchers working here announced that they had found traces of the Higgs boson. This is an extremely high-mass, very short-lived particle, which is the key to explaining how other elementary particles obtain their mass. The discovery confirmed a 48-year-old theoretical prediction, perfected a physical theory known as the standard model, and pushed physicists into the spotlight.

The existence of Higgs particle was first proposed by Peter Higgs in 1964. For a long time, physicists, including Higgs himself, did not know the physical meaning behind this hypothesis. But with the passage of time, people gradually realized the important role of the Higgs boson in particle physics. It is the last jigsaw puzzle missing from the standard model, and the particle (or more accurately, the Higgs field that excites it) is the reason for the mass of all particles. Mass is not the inherent endowment of particles as originally thought; on the contrary, it is the result of the interaction of particles with the Higgs field scattered throughout the universe. Physicists believed this theory decades ago, but it wasn't really confirmed by experiments until 2012.

The discovery of the Higgs boson is an immortal achievement in particle physics: it marks the end of decades of exploration and opens a new era for the study of this extremely special particle. But then the field fell into a long hangover after the carnival. Long before the 27-kilometer annular large Hadron Collider officially began collecting data in 2010, physicists worried that it might only produce Higgs particles, leaving no clues to new physics that might exist outside the standard model. Now, this nightmarish situation is becoming a reality. "it's kind of disappointing," said Barry Barish, a physicist at the California Institute of Technology. "I thought we were going to find supersymmetry." This is a mainstream physical theory that extends the standard model.

But many physicists say it is too early to despair. After three years of upgrading, the large Hadron Collider is getting ready for the third round of the planned five-round experiment. It produces billions of proton-proton collisions per second, in which new particles may be born. The development of artificial intelligence has also brought new opportunities-a decade ago, most physicists would have scoffed at the idea of using neural networks to analyze data. But with the help of many younger researchers and industrial partners, a special neural network has been built to help physicists search for phenomena worthy of further study in vast amounts of data. The large Hadron Collider will run for another 16 years, and with further upgrades, it will collect 16 times as much data as it has already collected. All of these data may contain subtle traces of new particles and new physics.

However, some researchers also believe that colliding physics experiments are at a dead end. Juan Collar, a physicist at the University of Chicago, looked for traces of dark matter in some small experiments: "if they still don't find anything, the whole field will die." John Ellis, a theoretical physicist at King's College London, said that hopes for a breakthrough in this field had been eroded by long and uncertain prospects of exploration, and that the ultimate failure would be as sudden and painful as a tooth extraction, not as silent as a tooth falling naturally.

Physicists have been wrestling with the standard model of particle physics since the 1970s. According to this model, ordinary matter consists of light-weight particles called upper and lower quarks-which combine every three to form protons and neutrons-as well as electrons and almost massless particles called electron neutrinos. Two groups of heavier particles have been lurking in the vacuum, only fleeting in the impact caused by the collision of the particles. All particles interact by exchanging other particles: photons transmit electromagnetic forces, gluons transmit strong interactions that bind quarks together, and massive W and Z bosons transmit weak interactions.

The standard model describes all the phenomena that scientists have observed so far in the Particle Collider. However, it cannot be the ultimate theory of nature. It cannot describe gravity, nor does it include mysterious, invisible dark matter. In the universe, the mass ratio of dark matter to ordinary matter may be about 6:1. Neutrinos are included in the standard model, but it is still impossible to explain their extremely low mass; it is clear that ordinary matter is also described by the standard model, but it is also unknown how it outperformed antimatter after the Big Bang. There are still many mysteries around the Higgs boson itself.

The large Hadron Collider was supposed to break the deadlock. In its circular structure, two protons cycling in opposite directions collide to produce heavy particles that are not available elsewhere, which is more than seven times as energetic as any previous collider. Ten years ago, many physicists imagined that some new phenomena could be quickly discovered in the large Hadron Collider, including new medium particles that transfer interactions and even mini black holes. Beate Heinemann, director of particle physics at the DESY laboratory in Germany, recalled that people thought they would be submerged in the generated supersymmetric particles. Physicists generally believed at the time that it might take longer to find the Higgs.

But unexpectedly, the Higgs was quickly discovered in just three years. Part of the reason is that its mass is smaller than many physicists expected, about 133 times that of protons. If its mass exceeds the energy limit of the large Hadron Collider or its interaction with other particles is weak, we have no hope of finding it at all. Higgs himself has said that he never expected to find evidence of the existence of the Higgs in his lifetime, which is undoubtedly a milestone in particle physics. But in the 10 years since then, physicists have not found any other new particles.

The barren nature of the new phenomenon challenges several principles cherished by physicists. The naturalness principle means that in a theory, the dimensionless ratio of physical constants should be of the same order as 1. Accordingly, the lower mass of the Higgs particle more or less ensures the existence of new unknown particles within the energy range that the large Hadron Collider can achieve. According to the principles of quantum mechanics, any virtual particle wandering in a vacuum will interact with real particles and affect their properties-this is how the virtual Higgs boson gives mass to other particles. The mass of the Higgs particle should have been significantly increased by other standard model particles in a vacuum, especially the top quark, but this is not the case. So theorists infer that at least one new particle with similar mass and just right physical properties-especially different spins-exists in a vacuum to counteract the effects of the top quark "naturally".

Supersymmetry theory can provide the basis for the existence of such particles: for every known standard model particle, it assumes that there is a partner particle with different spins and heavier masses. These companion particles can not only ensure that the mass of the Higgs particle is not too high, but also help explain how the Higgs field is generated.

But in the past decade, only small differences have been found between experimental observations and standard model predictions, and these anomalies do not point to the desired new particles. In 2017, for example, physicists who experimented with the bottom quark detector (LHCb, one of the four main particle detectors at the large Hadron Collider) found that B mesons (a particle containing heavy bottom quarks) had a higher probability of decaying into electrons and positrons than into muons and antimicrons-both probabilities should be the same according to the standard model. Similarly, some experiments have shown that the magnetism of μ mesons may be slightly stronger than that predicted by the standard model.

The Higgs particle itself provides other directions of exploration. In August 2020, a team of physicists working on the large Hadron Collider Torus instrument (ATLAS) and the Compact μ Coil (CMS) detector announced that they had discovered the decay of the Higgs into μ and antimuon pairs. Marcela Carena, a theoretical physicist at Fermi National Accelerator Lab, shows that if this rare decay has a different rate than theoretical predictions, the deviation may indicate new particles hidden in a vacuum.

Physicists will explore these phenomena in the next three-year experiment at the large Hadron Collider. However, these explorations may not lead to the dramatic "Eureka!" time. "the experiment is now moving towards measuring subtle phenomena with extremely high precision," Heinemann said. " However, Carena said, "I highly doubt that in 20 years' time, I will say,'Oh, son, we haven't learned anything new since the discovery of the Higgs particle.'"

If we think of the discovery of the Higgs boson as climbing a mountain, when Higgs first came up with his theory, we didn't even know where the mountain was, or how high it might be-the standard model of particle physics was not even complete. People are only vaguely aware that there is a Higgs particle somewhere on a mountain peak, which can really confirm the existence of the entire standard model structure. It was not until the late 1990s that we had the slightest sense of the height of the mountain; it was not until 2012 that we finally climbed it.

But it's different now. We have to go down from the other side of the mountain and across the barren plain. The plain stretches forward, perhaps touching the Planck scale (the smallest scale of space in the universe). If our current predictions are correct, there must be other mountains somewhere in the plain, marking another peak in physics. Maybe we can find new particles, such as leptoquarks (which may be the key to explaining the abnormality of B and μ mesons mentioned earlier), or even supersymmetric particles or dark matter particles; maybe we can solve more mysteries about the Higgs particle-is the Higgs particle itself an elementary particle or a composite particle? Can it interact with dark matter? If so, can we learn more about dark matter through it? Does the Higgs field give the Higgs particle its own mass through self-interaction? Many scientists are optimistic that we can solve these problems (though it sounds like a pie). But at the very least, there is no clear indication of how far we must cross the plains to see these new mountains-that is the difference between where we are now and in the past few decades.

Others are less optimistic about the opportunities for the large Hadron Collider experimenters. Marvin Marshak, a physicist at the University of Minnesota in Twin cities, said: "they are facing a desert, and they don't know how lush it is." To solve these problems, we will probably need the ability to produce large quantities of Higgs particles, which is not available at the large Hadron Collider today, or even two decades later. CERN is planning the next higher-energy collider, the Future Ring Collider (Future Circular Collider), as the future "Higgs plant." But even optimists believe that if the large Hadron Collider finds nothing new, it will be harder to persuade governments around the world to build the next bigger and more expensive collider to keep the field going.

Today, many physicists at the large Hadron Collider are just excited to get back to work on proton collisions. In the past three years, scientists have upgraded the detector and redesigned the low-energy accelerator part of the Collider. Mike Lamont, director of accelerator and particle beam at CERN, said: now, the large Hadron Collider should have a more stable collision rate and effectively increase the amount of data by as much as 50%. For months, accelerator physicists have been slowly adjusting the beam of particles produced by the large Hadron Collider. After the particle beam is stable enough, they will turn on the detector, resume data collection, conduct a new round of experiments, and continue to march on the dark plain.

references

[1] https://www.science.org/doi/10.1126/science.372.6538.113

[2] https://www.scientificamerican.com/article/how-the-higgs-boson-ruined-peter-higgss-life/

[3] https://www.scientificamerican.com/article/10-years-after-the-higgs-physicists-are-optimistic-for-more-discoveries/

[4] https://home.cern/news/press-release/physics/higgs-boson-ten-years-after-its-discovery

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