The mystery of the asymmetry between matter and antimatter
The matter we are familiar with is composed of baryons (such as protons and neutrons) and leptons (such as electrons). According to the Standard Model of particle physics, these particles all have their corresponding antiparticles, and they are almost identical in every way except for opposite charges. These antiparticles, like regular particles, can also be combined to form antimatter.
The existence of matter and antimatter raises many profound questions. For example, one of the great mysteries in physics today is: Why is the universe made up almost entirely of matter, but the amount of antimatter is so small? This problem is called baryon asymmetry. Assuming a slight break in the symmetry of a so-called CPT, this problem can be explained, and the mystery of our existence can be explained.
CPT symmetry is one of the cornerstones of the Standard Model, which tells us that the laws of physics remain unchanged under the combined transformation of charge conjugate (C), cosmic inversion (P), and time reversal (T), and the existence of this invariance means that a particle and its antiparticle should have the same mass, but opposite charge. Any deviation from this symmetry can be detected, no matter how small, it will have a huge impact on physics.
CPT symmetry. | Image Source:[1]
However, such a probe is very difficult because particles and antiparticles do not "mix" well, and once a particle comes into contact with its antiparticle, it will be annihilated in an instant, leaving only a strong light.
A recent study published in the journal Nature showed that physicists on the BASE team at CERN conducted the most rigorous test to date for this CPT symmetry by studying the motion of antiprotons (negatively charged protons) and antihydrogens (negatively charged hydrogen ions). Their measurements also show that antimatter responds to gravity in the same way as matter.
Measure the charge-to-mass ratio of antiprotons with high precision
In this experiment, a large number of antiprotons used by the BASE team's researchers were generated through a device at CERN called the "Antimatter Factory." They would then use a device called Penning Well to make measurements of these antiprotons.
Peng Ningsheng. | Image source: CERN
Penning well is a device that traps charged or antiparticles in a vacuum by carefully setting up magnetic and electric fields. This sophisticated device can prevent the contact of antiprotons with ordinary substances, avoiding the immediate disappearance of antiprotons caused by contact. So with its help, scientists can make multiple high-precision measurements.
The BASE team's measurement focused on the charge-to-mass ratio (the ratio of charge to mass) of the antiproton. The key idea behind this measurement is that when a charged particle makes a circular motion in a constant magnetic field (such as a cyclotron), its rotation frequency is proportional to its charge-to-mass ratio. Inside Penning Trap, these antiprotons move along a circular path. With these movements, the researchers could calculate their gyratory frequencies and then calculate the charge-to-mass ratio of these antiprotons.
Theoretically, the next step is for physicists to make corresponding measurements of protons and then compare the measurements with those of the antiprotons. However, since protons have opposite charges to antiprotons, the measurement of protons requires the use of a reversed electric field, which introduces large system errors.
Therefore, they chose to repeat this measurement with negatively charged hydrogen ions under the same experimental conditions. This antihydrogen particle is obtained by combining two electrons onto a single proton. The effects of this binding on protons can be well understood, and researchers can easily correct these effects in data analysis.
Over a year and a half, BASE's physics made four rounds of such measurements, making a total of 24,000 comparisons, each lasting 260 seconds. Their observations showed that the ratio of the charge-mass ratio of the antiproton to the proton was equal to 1, and the uncertainty was 16 parts per trillion. This value is precise enough to be considered a confirmation of CPT symmetry, which increases the accuracy of the previous best test by a factor of 4.3.
This latest finding represents the most accurate direct verification of the fundamental symmetry between matter and antimatter and plays an important role in the Standard Model.
Verify the principle of weak equivalence
Not only that, but this experiment also verified an important principle of general relativity, the principle of weak equivalence. According to this principle, all objects in the same gravitational field, regardless of their properties, experience the same acceleration. To use a popular example, in a vacuum, feathers and hammers do free fall at the same acceleration.
Test the effect of the Sun's gravitational field on antimatter. On a monthly timescale, a change in the Earth's position relative to the Sun (from nearpoint to apogee) causes a small change in the Sun's gravitational potential on Earth. | Image credit: Borchert, M. J. et al.
To test whether antiprotons would have an abnormal coupling to the gravitational field, violating the principle of weak equivalence, the team used several months to collect cyclotron frequency data. Since the Earth's orbit around the Sun is elliptical, the distance between the Earth and the Sun changes enough on such time scales to change the gravitational potential of Penning's position. This allows researchers to estimate how the ratio of the measured gyratory frequency will change with the gravitational potential.
This is the first such analysis ever made. The results show that at a 3% uncertainty level, the cyclotron frequencies of protons and antiprotons vary in the same way. This means that there is no abnormal interaction between antimatter and gravity, confirming that the principle of weak equivalence applies not only to matter, but also to antimatter.
#创作团队:
Author: No. 2 Beidou
Typography: Wenwen
#参考来源:
[1] https://www.mdpi.com/2073-8994/12/11/1821
[2] https://home.cern/news/news/physics/base-breaks-new-ground-matter-antimatter-comparisons
[4] https://physicsworld.com/a/antimatter-and-matter-respond-to-gravity-in-the-same-way-experiment-reveals/
[5] https://www.riken.jp/en/news_pubs/research_news/pr/2022/20220106_1/index.html
#图片来源:
Cover/Header: Principle