New physical dark clouds reappear?
At the beginning of the 20th century, physics had developed to a very perfect degree. But the two small dark clouds floating over the edifice of classical physics eventually developed into a storm that toppling the edifice and contributed to the establishment of relativity and quantum mechanics. A hundred years later, on the morning of April 7, 2021, the Fermilab National Accelerator Laboratory (Fermilab) in the United States announced the first measurement of the mucko's abnormal magnetic moment in the mucil g-2 experimental group, which instantly set off a new discussion about the development of physics. Some people think that this discovery will further unveil the new physics, but there is no shortage of skeptical voices. A full year later, on April 7, 2022, FermiLab released a new experimental result again, which once again triggered a big discussion in the physics community: Is the new physical cloud really going to appear?
Figure 1, April 8 Science magazine cover news
In the early morning of April 8, 2022 (April 7, Chicago local time), FermiLab's CDF international cooperation team released the most accurate results of the W boson mass measurement to date through multiple global media, which is 7 standard deviations higher than the expected value of the Standard Model of Particle Physics. The results of the study were published in the April 8 issue of the journal Science (Figure 1).
Standard Model with W Boson
The Standard Model theory of particle physics describes the 61 elementary particles that make up all matter, and also explains the three fundamental interactions between them — electromagnetic force, weak force, and strong force— and is one of the most fundamental theories of physics. According to the Standard Model, the interaction force is transmitted by elementary particles, such as the electromagnetic force between charged particles, which is transmitted through photons, and its force range is infinitely long. The strong force is transmitted through the gluons between the quarks, and the weak force is transmitted by the particles called the intermediate boson such as W and Z, the force range is extremely small (less than 10^-17 meters), and the force is very weak, only about one ten-thousandth of the electromagnetic force. The W boson is named after the initials of The Wind force.
Figure 2, Standard Model (Credit: TriTertButoxy/Stannered at English Wikipedia)
Another amazing feature of the W boson is that unlike zero-mass photons that transmit electromagnetic forces, it actually has mass. Moreover, the mass of W directly affects the Fermi constant, which determines the rate of fusion in the center of the sun, and if this process is too fast, I am afraid that there will not be enough time on Earth to evolve humans.
In the middle of the last century, Sheldon L. Glashow, Steven Weinberg and Abdus Salam unified weak and electromagnetic forces, for which they won the 1979 Nobel Prize in Physics. At the same time, experimental particle physicists have been hoping to find the W boson in high-energy experiments, but because of its heavy mass, it requires an accelerator with high enough energy to easily observe traces from complex experimental data. This effort continued until 1983, when the UA1 and UA2 collaborations led by the likes of Carlo Rubbia and Simon van der Meer at the Super Proton Synchrotron at CERN finally experimentally found evidence of the existence of the W and Z bosons, and won the Nobel Prize in Physics the following year.
How to measure the quality of W, why is it so difficult to measure?
The mass of the W boson is about 80 times that of the proton, about 80,000 MeV/c^2 (1 MeV/c^2 = 1.783 *10^-30 kg), which is an important parameter of the Standard Model. The precise measurement of its numerical values has always been one of the important means of testing the Standard Model and detecting new physics. The mass of the W boson has been measured by the ALEPH experiment, the DELPHI experiment, the L3 experiment, the OPAL experiment on the Large Positron Collider (LEP) at celebrant, the ATLAS experiment on the Large Hadron Collider (LHC), the LHCb experiment, and the CDF experiment and the D0 experiment on the Tevatron at Fermilab in the United States (Figure 3).
Figure 3, The range of experimental measurements and theoretical predictions of W boson mass
In collider experiments, particle physicists typically measure the mass of high-energy particles by studying their decay products. But the decay of the W boson into a charged lepton is accompanied by the production of an invisible neutrino, which makes it extremely difficult to accurately measure the mass of the W boson. For many years, its measurement accuracy (error) has been in the order of dozens of MeV/c^2 (as shown in Figure 3), and the accuracy of the best individual experiments is also around twenty MeV/c^2. This is in sharp contrast to the mass measurement accuracy (2 MeV/c^2) of the Z boson, the sister particle of the W boson. Experimental particle physics workers have been working on this for a long time.
Figure 4, CDF detector, Fermi's old accelerator, Tevatron, was the highest-energy accelerator before the LHC (1985-2011).
In the Tevatron, the world's largest collider, protons and antiprotons are accelerated to 1,000 times their rest mass, and then collide, resulting in a large number of W bosons. CDF (Collider Detector at Fermilab) is a general-purpose particle detector on Tevatron where particle physics experimenters calculate the mass of a W boson by studying the signals of charged leptons produced by the decay of the W boson detected by the CDF. After a decade of tireless efforts, they developed a new data analysis method that, using all the data collected during the CDF Phase II operation, for the first time reduced the accuracy of the mass of the W boson to a single digit , 9 MeV /c^2. The accuracy of this result reached 0.01%, surpassing the accuracy of any previous experiment and the weighted comprehensive accuracy of all previous experimental results, and the test of the Standard Model reached a new milestone.
Is the new physics really coming?
Why do scientists think that deviations in the mass of the W boson suggest the existence of new physics?
In the Standard Model of particle physics, the mass of the W boson is closely linked by internal symmetry and other parameters in the Standard Model. Particle theorists can calculate the mass of the W boson from the measured mass of the Higgs boson, the mass of the Z boson, the mass of the top quark, and the lifetime of the muse. The latest calculations give the mass of the W boson to be 80357±6 MeV/c^2 (as shown in the gray part of Figure 3). The latest measurements of the CDF collaboration group (the most accurate result yet) show that its W mass measurement is 80433.5±9.4 MeV/c^2 (the results shown by CDF II in Figure 3). There are 7 standard deviations between the two. That is to say, if the Standard Model's prediction is correct, the probability of the CDF experiment observing such experimental results is only about 10^-12. If the CDF's latest results are correct, then the mass of the W boson and the mass of the previously measured Z boson, the mass of the top quark, the mass of the Higgs boson, and the lifetime of the muse are incompatible within the framework of the Standard Model.
This means that the Standard Model of Particle Physics is incomplete and requires the introduction of new physical corrections. But this new physical correction tends to have a lot of possibilities. Therefore, we need further experiments to test these new physical models.
Figure 5, LHC's ATLAS detector. (Image courtesy of CERN)
It should be noted that from Figure 3 we can see that the latest CDF results and ATLAS measurements also have a deviation of about 3 standard deviations, while the results of ATLAS and the results of the Standard Model are consistent within one standard deviation. Therefore, whether the Standard Model's prediction of the mass of the W boson is biased also needs to be further tested by other experiments. The ATLAS experiment, CMS experiment and LHCb experiment on the Large Hadron Collider in which China participated are conducting related research. The planned ring positron collider (CEPC) and the future ring collider (FCC) will be able to make more detailed measurements of the mass of the W boson and further detect whether the Standard Model calculations need to be modified or extended.
Figure 6, SCHEMATIC DIAGRAM OF THE CEPC DESIGN
Reference Links:
https://www.science.org/doi/10.1126/science.abk1781
https://news.fnal.gov/2022/04/cdf-collaboration-at-fermilab-announces-most-precise-ever-measurement-of-w-boson-mass/
https://www.science.org/content/article/mass-rare-particle-may-conflict-standard-model-signaling-new-physics
http://hep.tsinghua.edu.cn/news/20220408wmass.html
About the author
Wang Qing (2nd from right) is a professor at the Department of Physics of Tsinghua University, the director of the Institute of Particle Physics, Nuclear Physics, astrophysics, Department of Physics, Tsinghua University, and the director of the Center for High Energy Physics Research at Tsinghua University.
Yi Kai (3rd from left) is a professor at Nanjing Normal University and a visiting professor at Tsinghua University. He is currently a member of the CDF, CMS, belle II cooperation group, and the head of the CMS group of Tsinghua-South Division, and has long been engaged in B physics, hadron physics and new physics research.
Chen Xin (1st from right), Associate Professor, Department of Physics, Tsinghua University, focuses on Higgs physics, hadron physics, long-lived particles and dark matter particle search. Member of the ATLAS and FASER collaboration group.
An Haipeng (1st from the left), an associate professor in the Department of Physics at Tsinghua University, is mainly engaged in theoretical research in particle physics and cosmology.
Hu Zhen (2nd from the left), associate professor of the Department of Physics, Tsinghua University, whose main research areas are hadron physics, new physics search, and detector electronics research and development. He is currently a member of the CMS and FASER Cooperation Group.
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Source: Journal of Modern Physics Knowledge
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