Is the building of particle physics really going to collapse? Science magazine reveals the biggest progress in physics in recent decades

On April 7, 2022, the cover article of Science magazine broke a big news that shocked the physics community. This paper, co-authored by nearly 400 scientists, pointed out that they had made an unprecedented high-precision measurement of the mass of the W boson and found that the measured value was nearly 0.1% heavier than the predicted value of the standard model of particle physics. This difference may seem very small, but its significance in the particle physics system is extremely significant. Once the measurement value is confirmed, it may mean the arrival of a new era of physics, and the boundaries of human cognition of the material world are likely to undergo a revolutionary breakthrough.

(Source: Screenshot from Science’s official website)

Most of the research results of particle physics are beyond the comprehension of ordinary people like us. One of its greatest achievements so far is the establishment of the standard model of particle physics. So, what does this standard model say? Is it possible to try to understand it with concepts in life? How to understand this "big news" from the perspective of the standard model of particle physics?

First of all, we must admit that the Standard Model is the most complex and difficult to understand physical theory in human history. Even the descriptions in Wikipedia or Baidu Encyclopedia may be incomprehensible to ordinary people. In order to let everyone get some knowledge that can be "taken away", we can only talk to you about particle physics and basic models in a descriptive and graphic way as much as possible.

What problems does particle physics want to solve?

Particle physics mainly studies the elementary particles that make up matter and the interactions between them. Many types of rays (such as electron beams, neutrino beams, etc.) can still interact with other tangible or intangible substances in various ways even if they are invisible to the naked eye. In fact, they belong to the broad sense of matter, and therefore are also within the scope of particle physics research.

Since many elementary particles cannot exist stably or appear alone in nature, physicists can only use high-energy particle accelerators to collide them with each other so that they can appear in the field of observation and study them. Therefore, particle physics is also called high-energy physics. So the question is, what answers do scientists hope to find from the particle physics research that costs a huge amount of money to implement?

The TeraelectronVolt Accelerator (left) and the Large Hadron Collider (right)

(Source: Wikipedia "Particle accelerator")

There are two main questions that particle physics wants to answer: the first is what matter is made of, and the second is how matter interacts with each other. Matter here includes the vast material world from cosmic stars to the inside of atoms, and the forms of interaction also include the four fundamental forces: strong force, weak force, electromagnetic force and gravity.

In other words, particle physics is dedicated to finding answers to a series of the most basic questions in the material world. It may not be like many applied sciences, which can quickly find a point of convergence with daily life and quickly benefit mankind. But particle physics determines the edge of human vision of understanding the objective world, and actually determines the upper limit of human technology. Even if it seems "useless", only by understanding the laws can we use them. It's like learning a foreign language. People can never say words they have never come into contact with. Exploration and understanding are the prerequisites for application.

What is the Standard Model of Particle Physics?

After more than 100 years of efforts by physicists, human beings have gained a deep understanding of the nucleus, and even decomposed the protons and neutrons that make up the nucleus into more basic particles. In addition, humans have discovered that the four fundamental forces are actually related to certain media particles that transmit these forces. For example, electromagnetic force is closely related to photons. This is like two people who are not in direct contact feeling each other's force by throwing a ball at each other.

In the end, scientists obtained a huge family composed of elementary particles by subdividing the particles that make up matter and looking for various particles responsible for transmitting forces between forces. The family tree of this family is called the Standard Model of Particle Physics.

The elementary particles of the standard model are divided into two types, namely quarks and leptons that are responsible for the composition of matter, and mediators that are responsible for transmitting interactions. These elementary particles also have their own properties, such as mass, charge, spin, flavor, color, etc., which leads to the same type of elementary particles sometimes being subdivided into several types. For example, there are 6 flavors of quarks, each flavor of quarks is divided into 3 colors, plus positive and negative, a total of 36 quarks. In the end, the current standard model contains 61 elementary particles.

The Standard Model of Particle Physics

(Source: Wikipedia "Standard Model")

You will definitely feel that the Standard Model is completely different from your impression of physics. The impression of physics is simple and elegant, while the Standard Model has a feeling of "making up a new word to express sorrow". In fact, many scientists also had this confusion at the beginning. If building the Standard Model is like building a house for elementary particles, the scientists' initial plan was actually to build a yurt, but later they found that they had to build a courtyard house, and then they found that the room was not enough and were forced to build a small apartment, and finally they unknowingly built a skyscraper.

But the most amazing thing is that although the process of establishing the Standard Model was a trial and error process, so far, it has perfectly unified the electromagnetic force, strong force and weak force, and many new particles predicted by it have been discovered one after another. We can still use the analogy of building a building to explain this process. At the beginning, no one had an idea of ​​how to build the building, and they thought about it while building. Some people said that we should add another floor here, and some people said that we should leave a few rooms here. In the end, unexpectedly, the room was just right and the guests were satisfied.

Until the discovery of the Higgs boson in 2012, all 61 rooms of the current standard model of particle physics were occupied. In the process of building this building, many scientists played a key role, and there are dozens of Nobel Prize winners directly related to it. They either made outstanding contributions in the process of finding "guests", such as Ting Zhaozhong's first experimental discovery of the heavy quark charm quark c; or made reasonable opinions and suggestions on the "blueprint of the building", such as Higgs predicted the Higgs boson theoretically by constructing the Higgs mechanism.

How to use "Journey to the West" to compare the Standard Model?

If you still can't understand the standard model, let's take Journey to the West as an example. In the world of Journey to the West, the Dragon King and the land god are in charge of the mountains, rivers and oceans, the demons and monsters are each causing trouble in their own area, the gods in heaven are carefree, the Buddha is in the West saving all living beings, and the King of Hell is secretly recording the book of life and death in the underground.

If you want to know what connects them, it is the four people and one horse of the pilgrim group. If the standard model is "Journey to the West", then each force contains several members, such as various quarks are like small drill wind, white bone spirit, jade face fox, collectively known as demons and monsters; and the four people of the pilgrim group can communicate with the heaven and the underworld, so they play the role of intermediaries.

What exactly is the “big news”?

Now that we have a general understanding of the Standard Model, let’s take a look at the major discoveries that have “shocked” the world of physics.

The W boson is at the core of the Standard Model, and scientists have been trying to determine its mass with the highest possible accuracy since it was observed in 1983. In fact, previous experimental results have all yielded a W boson mass that is highly consistent with the Standard Model's predictions, which is one of the decisive evidences for the Standard Model's correctness.

But in the latest measurement, the mass of the W boson is 80433.5±9MeV (CDF Ⅱ in the lower right of the figure below), while the previous standard model predicted a value of 80357MeV (gray column in the figure). Interestingly, the data from this measurement overturned the conclusion obtained at the same collider in 2012 (the blue CDF2012 in the lower figure below), and also greatly improved the accuracy of another measurement (CDF I in the upper figure below). Due to the extremely high accuracy and significant statistical differences, the results of this measurement can be said to be the biggest discovery in particle physics in the past three decades. So, what measurement method did the scientists use, and is this method reliable?

The mass of the W boson measured by experimental data from different accelerators

(Source: Science Magazine)

Determining the mass of the W boson is no easy task

W bosons are produced in high-energy collisions and then decay rapidly. During their decay, they may produce electrons, muons, or antineutrinos. Since neutrinos cannot cause the detectors currently used to react, it seems impossible to know the actual amount of neutrinos produced.

Normally, we can determine the mass of elementary particles by measuring the energy and momentum of decay products, but since it is impossible to directly measure the neutrino products produced by W bosons, the above classical method is not applicable.

Therefore, scientists have taken a compromise approach. Instead of measuring individual particles produced in a single collision event, they measure the overall result of the entire collision event. That is, the overall loss of energy and momentum should be attributed to the neutrino, and then the mass of the W boson can be inferred from this.

However, if the entire collision event is measured, there are also certain difficulties - particle detectors cannot measure very small momentum changes. However, scientists have a compromise. Based on the momentum distribution of particles after the collision, they can calculate the proportion of particles whose momentum cannot be measured, and then correct the momentum measurement results accordingly.

It can be seen that the precise determination of the mass of the W boson is by no means an easy task. Nearly 400 scientists spent ten years to finally make this conclusion public. Especially considering that the experimental conclusion is bound to have a strong impact on existing theories, this paper was published only after repeated verification.

The list of authors of the paper takes up one page

(Source: Science Magazine)

Is the standard model building really going to collapse?

One of the researchers involved in the project said, "The method we used is definitely correct...but such a big difference can only mean that there is something new in nature that is not covered by the original standard model." So how should we understand his words using the analogy of building a house?

First of all, we must admit that the Standard Model is not perfect, and it still has many unanswered questions, such as the nature of gravity, the mystery of dark matter and dark energy in the universe, etc. This shows that the Standard Model building needs to be further expanded, and there are still residents who are homeless and locked out of the building, unable to find their own room.

The position of the Standard Model of Particle Physics in the system of physics

(Source: Wikipedia "Theory of Everything")

Secondly, within the previous standard model, we still believe that it is sufficiently self-consistent. As mentioned above, although the construction of the building took decades, the final result is still perfect. Whether from the perspective of the "61 residents" or the "building structure" itself, it looks just right, and the residents get along harmoniously with each other.

However, the new discovery suggests that the registration information of one of the "residents" does not match the actual situation. We still do not know whether there is a problem with the design of the room or there are some "mysterious new residents". Of course, it is not impossible that there is something wrong with the experimental results themselves. The current priority is to use data obtained from other accelerators for independent verification.

If the conclusion of this experiment is proven to be correct, then similar predictions made by the Standard Model theory, such as the mass of the Z boson, the Weinberg angle, the mass of the top quark, and the mass of the Higgs boson, need to be further verified with high precision. Once there is a general deviation between the measured data and the theoretical predictions of the Standard Model, the existing Standard Model may have to be revised.

In this sense, this achievement is attributed to the large number of experiments conducted in the field of particle physics over the past two or three decades, and can be regarded as the culmination of the work of countless researchers. The "new physics" that theoretical physicists have been looking forward to for the past few decades may really be entering the human field of vision.

(Source: Freepik.com)

References:

Challenging the Standard Model? The latest W boson mass measurement is 7 standard deviations higher than the theoretical value

https://www.163.com/dy/article/H4GVDGNA05327918.html

High-precision measurement of the W boson mass with the CDF II detector

https://www.science.org/doi/10.1126/science.abk1781

The quality of W Boson is...

https://www.163.com/dy/article/H4GVDGNA05327918.html

Fermilab's W boson mass experiment contradicts theory, featured on the cover of Science

https://baijiahao.baidu.com/s?id=1729525697847274262&wfr=spider&for=pc

This article is produced by Science Popularization China Frontier Technology, produced by Zhang Hao, and supervised by China Science Popularization Expo. "Science Popularization China" is an authoritative scientific brand that the China Association for Science and Technology and all sectors of society use information technology to carry out scientific communication.