After the discovery of the Higgs boson, what else are particle physicists looking forward to?

Ten years ago, on July 4, 2012, the European Organization for Nuclear Research (CERN) announced the discovery of the Higgs boson. This fundamental substance, commonly known as the "God particle," has had a huge impact on both the academic community and the public. Its final discovery confirmed the theoretical predictions of 60 years ago and was considered to have filled the last blank in the standard model of particle physics. It also made headlines around the world, announcing another victory for fundamental physics. However, the imperfections of the standard model have left many questions unanswered. In the past decade, particle physicists have not seen such a victory again, but have only seen some "cracks." They are still looking forward to more answers from the Higgs boson.

Written by Qu Lijian

On July 4, 2012, particle physicists gathered at CERN to listen to an extraordinary academic report - a report on the experimental results of the discovery of the Higgs boson. When the experimental results were presented on the slides, the audience burst into applause. The participants were so excited that they seemed to feel the Higgs field, and suddenly felt heavy, and were excited to be able to participate in creating such a historical moment.

Scientists cheered loudly in the academic lecture hall of CERN

Outside the scene, particle physicists around the world witnessed this moment through live video broadcast.

Physicists at the German Electron Research Center Hamburg watching the live stream

Physicists at Fermilab in the United States gave up their Independence Day holiday and watched the live broadcast at midnight.

It's not just scientists who are excited about this purely scientific discovery, but also the general public.

Public response

After the report, CERN held a press conference to disclose this pure physics news to the public, and major newspapers reported it on the front page. Other media also flocked to this topic, publishers launched a variety of related popular science books, many of which were listed on bestseller lists, TV stations invited scientists from all walks of life to the studio to answer questions and resolve doubts for the audience, and many related documentaries were broadcast on large and small screens.

CERN collected reports from major newspapers around the world on the discovery of the Higgs boson

The Higgs boson became the hottest public topic of the year, with science and popular science complementing each other. The following year, the two proposers of the Higgs boson theory, British physicist Peter Higgs and Belgian physicist François Englert, jointly won the Nobel Prize in Physics, once again bringing the feast of the "God particle" to the public.

Why is there such a grand occasion? This has a lot to do with the "layout" and preheating by scientists and popular science writers many years ago.

As early as 1993, Nobel Prize winner Leon Lederman and his co-authors published the popular science book "The God Particle", which made people mention the God Particle whenever they talked about the Higgs boson.

The God Particle, first edition, 1993

Chinese version of God Particle, 2003

There was another incident that captured the public's imagination.

In 1993, the British Minister of Science solicited the best explanation of the Higgs field and the Higgs particle for the public, and the reward was a bottle of champagne. In the end, Professor David Miller of University College London won the bottle of champagne. Professor Miller compared the Higgs field to a group of cocktail party attendees who were shoulder to shoulder. An ordinary person can easily pass through the crowd, but when a big shot comes, it will instantly attract people to gather around, and the big shot will have a hard time passing through the crowd. The effect of the Higgs field and particles is similar: the particle will attract the Higgs field and slow down its own speed. The more it slows down, the greater the mass given to the particle by the Higgs field.

The Higgs field can be compared to cocktail party attendees

This analogy may make ordinary people feel that they also understand the Higgs mechanism.

However, these things can only explain the enthusiasm of Westerners for Higgs, and cannot explain the enthusiasm of people in other parts of the world, such as China, for scientific news related to the discovery of the Higgs particle. After all, the Chinese version of "The God Particle" is not a bestseller, there is no skewers at cocktail parties, and there is no interest in "God". This shows one thing: the public's curiosity and enthusiasm for basic science exceeds our stereotypes.

The Higgs boson research has reached a high point in the academic and public circles. Is this the end of the related research?

Not really.

The study of the Higgs boson is like a treasure hunt. Finding the knowledge of the Higgs boson is like finding the treasure, but the treasure has not yet been unearthed. In the past decade, particle physicists have maintained their initial excitement and carried out treasure hunting activities. Particle physicists hope to find treasures that can solve some problems that the standard model cannot give standard answers to, such as what is dark matter? Why is there more matter than antimatter? How did the universe originate and what fate will it lead to?

Particles in the Standard Model. The purple part is quarks. Quarks have 6 flavors, each flavor has 3 colors, and each quark has an antiquark, for a total of 36 quarks. The green part is leptons. Leptons have 6 flavors, no color, and have antiparticles, for a total of 12 leptons. In the picture, quarks and leptons are arranged in 3 columns, and each column constitutes a generation of matter. The column to the right of quarks and leptons is the gauge boson, of which gluons have 8 colors and no antiparticles, photons and Z bosons have only one each, and W bosons have antiparticles, for a total of 2. The particle on the far right is the Higgs boson, of which there is only one. The Standard Model has a total of 61 elementary particles

Now, let's see what big news there will be in the future.

coupling

The Higgs particle gives mass to other particles through its interaction with particles, which physicists call "coupling". Different coupling strengths give particles different masses. For example, all measurements so far are consistent with the standard model, which brings physicists more confusion than comfort. Why do particles have their respective masses?

The values ​​of the masses of particles in the Standard Model are carefully assigned by physicists, not derived from theory. Particle physics is a very "ambitious" field, which aims to reveal the most basic laws of the world and get a theory of everything. As a result, the important parameters are carefully selected. Isn't it ironic? It's like if you want to develop a theory of gravity, apples, celestial bodies, and people each satisfy different laws of attraction. This is not a theory of gravity, this is a unique theory of gravity.

Taking electrons, tauons, and muons as examples, let’s talk about the questions that puzzle physicists.

In the standard model, the only difference between these three particles is their mass, which means that their coupling strength with the Higgs particle is different. Some physicists speculate that particles have a deeper structure, and that careful measurements of the coupling between the Higgs particle and other particles are expected to reveal results that cannot be explained by the standard model, and then follow this clue to develop a more fundamental theory.

The experimental way to measure the coupling is to observe the creation and decay of Higgs particles.

When the Higgs boson was discovered, the coupling of the Higgs boson to other bosons was confirmed. In 2016, the coupling of the Higgs boson to the tau was experimentally confirmed. None of these experiments gave unexpected results.

However, hope is not completely lost.

In 2018, physicists experimentally coupled top quarks and anti-top quarks to the Higgs particle. The top quark is the heaviest elementary particle and has the strongest coupling to the Higgs particle. Significant deviations from the standard model are more likely to occur here. Unfortunately, the experimental results in 2018 still did not bring any surprises.

It's frustrating.

There are no unexpected results. One possible reason is that the error in the experimental results is still relatively large. If the experimental accuracy is improved, perhaps unexpected phenomena will be discovered.

However, the experimental results have also made physicists increasingly feel that a bold idea is very reliable: there may be more than one type of Higgs particle, or it may have an internal structure.

Lone Particle

Particle physicists have a particularly promising solution to the shortcomings of the Standard Model: supersymmetry. In supersymmetric theory, every particle has a partner particle. Before the LHC was turned on, physicists hoped that the LHC would be able to find such a partner particle. However, so far, nothing has been found. Although supersymmetry has not been completely ruled out, there is not much hope left for it.

There are more theories beyond the Standard Model than just supersymmetry, and it is not clear which theory is most likely to win. Physicists hope that measuring the properties of the Higgs particle will reveal something beyond the Standard Model and point the way to new physics.

One property of the Higgs particle that physicists are investigating is whether it is unique.

The Higgs particle appears lonely

All other elementary particles have spin, but the Higgs boson has spin zero. Bosons with spin zero are called "scalar particles."

Other particles have close relatives, so should the Higgs boson also have a scalar relative? Supersymmetry theory and some other theories predict that there are multiple Higgs particles. Some particle physicists speculate that the Higgs particle may be just the first scalar particle we have discovered, and there may be a family of scalar particles waiting for us to discover.

It is also possible that the Higgs particle is not a fundamental particle, but is composed of more fundamental particles. Some particles can be combined to form zero-spin particles, such as alpha particles composed of two protons and two neutrons, and pions composed of quark particles.

Some of the most puzzling recent experimental results in particle physics may be related to the properties of the Higgs particle.

In 2021, Fermilab reported experimental results on the magnetic properties of muons that were inconsistent with the Standard Model predictions. In April 2022, Fermilab reported that the mass of the W boson was greater than the Standard Model predictions.

The answers to these puzzles may lie in the Higgs particle, the latest discovered particle in the Standard Model, the least studied particle, and our hope for unlocking the ultimate mysteries of the material world.

Self-coupling

Does the Higgs particle interact with itself, that is, is it self-coupled?

Higgs self-coupling has not yet been experimentally measured. Theoretical physicists are looking forward to related measurements and believe that they will bring new physics.

Higgs self-coupling is closely related to the Higgs potential. The Higgs potential is a function that describes the energy of the Higgs field. The graph of this function looks like a Mexican hat.

The Higgs potential is shaped like a Mexican hat. In the early universe, the universe was at the top of the hat, and then it slowly evolved into the groove of the hat brim. In this process, each elementary particle gained mass.

In the early universe, when the Higgs field was first generated, the Higgs potential determines how elementary particles acquire mass. The study of how particles go from being massless to having mass helps us understand why there is more matter than antimatter in the universe.

What role did the Higgs field play in the early days of the universe? The answer lies in the Higgs potential.

According to the standard model, the first-order approximation of the Higgs potential can be determined from the masses of the Higgs particle and the top quark. From the current results, billions of years ago, the universe entered a local minimum point of the Higgs potential, not a global minimum point, which means that the universe is metastable; in the future, the universe will enter another smaller minimum point, that is, a phase transition will occur, and the mass of elementary particles will change accordingly, and the universe we are familiar with now will be completely different.

However, there is no need to be pessimistic now. Some theories beyond the standard model predict some particles and their interactions, which make the Higgs field present a different appearance and save the fate of the universe.

The stability of the universe obtained by the standard model. The black dot in the middle is the current measurement result, and the areas of the three ellipses correspond to 1, 2, and 3 standard deviations respectively.

The key clue to understanding the past and future of the universe lies in the Higgs potential. The Higgs particle self-coupling experiment will give us a more accurate understanding of the Higgs potential.

To conduct a Higgs particle self-coupling experiment, it is necessary to produce pairs of Higgs particles. In the LHC, it is possible to produce a set of Higgs particle pairs by producing 1,000 single Higgs particles. Self-coupling experiments are extremely difficult.

The LHC is being upgraded to the High Luminosity Large Hadron Collider (HL-LHC), but this only increases the amount of data by 10 times. For self-coupling experiments, the error is still large, but it is possible to produce some related phenomena.

The high-energy physics community is pushing for more powerful collider projects, such as China's Circular Electron Positron Collider (CEPC), Japan's International Linear Collider (ILC), Europe's Future Circular Collider (FCC) and the Compact Linear Collider (CLIC). If any of these colliders can be built, it will be able to measure the properties of the Higgs boson and other particles in detail, and may discover clues to new physics beyond the Standard Model.

It will take at least ten years to see one of them completed.

Schematic diagram of the site selection and size of the future circular collider

This is particle physics, which requires not only ingenuity and money, but also patience.

References

https://www.sciencenews.org/article/higgs-boson-particle-physics-standard-model-discovery-anniversary

https://www.symmetrymagazine.org/article/five-mysteries-the-standard-model-cant-explain

https://www.symmetrymagazine.org/article/four-things-physicists-still-wonder-about-the-higgs-boson

https://www.quantamagazine.org/the-physics-still-hiding-in-the-higgs-boson-20190304/

https://home.cern/science/physics/higgs-boson/ten-years

https://physicstoday.scitation.org/do/10.1063/PT.6.4.20220630b/full/

Produced by: Science Popularization China

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