Standard model
What is the universe made of?
After more than two thousand years of searching, we already have a general picture:
The image above contains all known elementary particles. Elementary particles refer to those particles that are inseparable, such as the electrons that make up atoms, upper quarks and lower quarks, which belong to elementary particles. Theories that describe the properties of these elementary particles, and the interactions between them, are called the Standard Model. This is one of the most precise theories to date.
Although the Standard Model is extremely successful, it still has shortcomings. For example, it explains three fundamental forces in nature—electromagnetic, strong, and weak—but does not include gravity. In addition, it does not contain dark matter, an invisible substance that accounts for 85% of the material in the universe. As a result, physicists have always hoped to find loopholes in the Standard Model.
Last year, the Muzi G-2 experiment observed that the Muzi behaved inconsistently with what the Standard Model expected, much to the excitement of physicists, but the results are yet to be further confirmed. Today, the protagonist we are going to talk about is not the muse, but the W boson in the Standard Model. For some physicists, the W boson is the best object to look for new physics and deviate from the predictions of the Standard Model.
W boson
The W boson was predicted in the 1960s and was discovered in 1983. There are two kinds of W bosons, positively charged or negatively charged, and together with the neutral Z boson, they are both carrier particles responsible for transmitting weak forces, similar to the exchange of photons involved in every electromagnetic interaction. In particle decay and solar luminescence, weak forces play an important role.
Electromagnetic forces are transmitted by photons, weak forces are transmitted by W and Z bosons, and strong forces are transmitted by gluons. Some physicists think that gravity may be transmitted by hypothetical gravitons. | Image source: Typoform
In one of the most accurate measurements to date of the W boson, the Collider Detector (CDF) from Fermilab found that the mass of the W boson deviated from the Standard Model's predictions. This is the latest result that physicists have come up with in a decade.
In a collider, when protons and antiprotons collide, a large number of particles, including the W boson, are produced. The W boson can only exist briefly, and it rapidly decays into electrons or muses, as well as neutrinos. Neutrinos are undetectable, they escape the detector without a trace, while electrons or muse leave obvious traces.
Since the discovery of the W boson in 1983, experiments have calculated that its mass is about 85 times that of a proton. The collision opportunity produces the W boson through the collision of high-energy particles. The W boson then decays rapidly.
During decay, most of the original mass of the W boson is converted into the energy of new particles. If physicists could measure the energy and path of all decaying particles, they could immediately calculate the mass of the W boson that produced those particles. But because neutrinos can't be traced, it makes it impossible for researchers to determine which part of the energy of the electron or muse comes from the mass of the W boson and which part comes from the momentum of the W boson. This can make measurements extremely difficult.
Heavier than theory predicts
In the latest work, the data used by the researchers was collected before Fermilab's megawael-volt accelerator shut down. They looked at 4.2 million W bosons produced by CDF detectors between 2002 and 2011, a dataset four times larger than the data they used when they first measured it in 2012. The researchers calculated its energy by measuring how much the trajectory of each decaying electron bends in the magnetic field.
After plotting the distribution of electron energies, the researchers calculated the W boson mass that best matched the data: 80433.5MeV, with a margin of error of only 9.4 MeV, which was heavier than theory expected. Although this is a new result taken from old data, the latest measurements were not possible until 2012. That's because data analysis techniques are constantly improving, and particle physicists are getting a deeper understanding of how protons and antiprotons behave in collisions.
Image source: [1]
The ACCURACy of cdf's latest measurements is twice as accurate as the previous best measurements (from atlas). As shown in the figure above, some of the earlier, less accurate measurements are closer to those predicted by theory (80357MeV).
New physics?
If the new measurements hold true, how do we explain the difference between theory and measurements?
One possible explanation is that this may imply that there are entirely new particles in the universe predicted by supersymmetry theory, which assumes that every known elementary particle has a heavier partner. These particles constantly appear and disappear in the vacuum around the W boson, making the W boson heavier. Intriguingly, supersymmetric particles could also explain the results reported by the Muzi g-2 experiment.
Another possible explanation is related to the Higgs boson, which was discovered in 2012 at the Large Hadron Collider and is the last piece of the puzzle for the Standard Model. If the properties of the Higgs boson differ from what existing theories predict, such as whether it may be a composite particle rather than an elementary particle, or if there are multiple versions of the Higgs boson, it will affect the mass of the W boson.
Whenever we see the results of an experiment that could subvert an existing theory, we need to keep in mind that "extraordinary claims require extraordinary evidence." So before we claim that this is a major discovery, we will have to wait for other experiments to confirm it.
#创作团队:
Author: Hara
Design: Wenwen
#参考来源:
[1] https://www.science.org/doi/10.1126/science.abk1781
#图片来源:
Cover/Cap: Institute of New Principles